Optical pickup and optical disk apparatus using the same

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION:

0001The present invention relates to an optical pickup which utilizes coherent light for use in the field of optical information processing, and also to an optical disk apparatus incorporating the optical pickup.

2. DESCRIPTION OF THE RELATED ART:

0002DVDs (digital video disks), which define one species of optical disks, can record 4.7 GB of data per disk medium with a diameter of 12 cm by utilizing a red semiconductor laser. Thus, signals equivalent to about 2 hours of motion pictures in the NTSC format can be recorded/reproduced on a single disk. A semiconductor laser which is employed as a light source in such a DVD apparatus typically generates red laser light with a wavelength of about 635 nm to about 650 nm, and has output power of about 5 mW. The relative intensity of noise (RIN) is generally -130 dB/Hz, thus satisfying the DVD requirement of -126 dB/Hz or smaller RIN.

0003However, the following problem may occur in a conventional DVD apparatus incorporating the above-described red semiconductor laser when reproducing motion pictures of the HDTV (high definition TV) class, which is one type of high quality TVs that requires high density recording/reproduction as necessitated by increase in the amount of information to be processed.

0004Specifically, the use of a red semiconductor laser, i.e., a semiconductor laser having a relatively long wavelength, as light source permits data equivalent to only about 40 minutes of motion pictures to be recorded on one DVD medium. Therefore, it is impossible to record or reproduce a relatively long program, e.g., a movie, on one disk. In addition, since the amount of information to be recorded/reproduced increases with the enhancement in the quality of images, a higher transfer rate may be required at the time of recording/reproduction. This calls for a great increase in the rotation speed of the disk.

SUMMARY OF THE INVENTION

0005The optical pickup of the present invention includes: a laser light source having an oscillating wavelength of 435 nm or less; an optical system providing light emitted from the laser light source with a prescribed optical path; and an optical detector for detecting light from the optical system, wherein the laser light source is provided with wavelength locking by optical feedback.

0006The laser light source may be a GaN type semiconductor laser.

0007The optical feedback of the laser light source may be provided by a grating structure.

0008In one embodiment, the laser light source is a semiconductor laser having a DBR structure, and the optical feedback thereof is provided by the DBR structure. The DBR structure may be formed in the vicinity of an end face opposite to a light emitting portion of the semiconductor laser. A member for absorbing laser light may be provided on an end face opposite to a light emitting portion of the semiconductor laser.

0009A tapered beam-shaping portion may be provided on the side of a light emitting portion of the laser light source.

0010The laser light source may be operated with RF superimposition.

0011According to another aspect of the present invention, the optical pickup includes: a laser light source; an optical system providing light emitted from the laser light source with a prescribed optical path; an optical detector for detecting light from the optical system; and a control member for controlling a light amount of an incident light onto the optical detector.

0012In one embodiment, the controlling member controls the light amount of the incident light onto the optical detector by controlling a polarization direction of the incident light.

0013Specifically, the controlling member may change a polarization direction of a laser light emitted from the laser light source, thereby controlling the polarization direction of the incident light onto the optical detector. For example, the laser light source may be a semiconductor laser, and the controlling member may apply pressure to the semiconductor laser, thereby changing the polarization direction of the laser light emitted from the semiconductor laser.

0014Alternatively, the controlling member may be a variable wavelength plate disposed between a polarizing optical element and a focus lens in the optical system.

0015The incident light onto the optical detector may be a signal light from an optical disk medium.

0016The optical disk apparatus according to the present invention includes: a laser light source having an oscillating wavelength of 435 nm or less; an optical system providing light emitted from the laser light source with a prescribed optical path; an optical detector for detecting light from the optical system; and an optical disk medium having pits with a length of 0.3 µm or less, wherein the laser light source is provided with wavelength locking by optical feedback.

0017The laser light source may be a GaN type semiconductor laser.

0018The optical feedback of the laser light source may be provided by a grating structure.

0019In one embodiment, the laser light source is a semiconductor laser having a DBR structure, and the optical feedback thereof is provided by the DBR structure. The DBR structure may be formed in the vicinity of an end face opposite to a light emitting portion of the semiconductor laser. A member for absorbing laser light may be provided on an end face opposite to a light emitting portion of the semiconductor laser.

0020A tapered beam-shaping portion may be provided on the side of a light emitting portion of the laser light source.

0021The laser light source may be operated with RF superimposition.

0022Preferably, the laser light source is a semiconductor laser having a level of a relative intensity of noise (RIN) of -135 dB/Hz or less.

0023The apparatus may perform optical recording onto the optical disk medium by using the light emitted from the laser light source.

0024According to another aspect of the present invention, the optical disk apparatus includes: a laser light source; an optical system providing light emitted from the laser light source with a prescribed optical path; an optical detector for detecting light from the optical system; an optical disk medium having pits with a length of 0.3 µm or less; and a control member for controlling a light amount of a signal light incident onto the optical detector from the optical disk medium.

0025In one embodiment, the controlling member controls the light amount of the signal by controlling a polarization direction of the signal light.

0026Specifically, the controlling member may change a polarization direction of a laser light emitted from the laser light source, thereby controlling the polarization direction of the signal light. For example, the laser light source may be a semiconductor laser, and the controlling member may apply pressure to the semiconductor laser, thereby changing the polarization direction of the laser light emitted from the semiconductor laser.

0027Alternatively, the controlling member may be a variable wavelength plate disposed between a polarizing optical element and a focus lens in the optical system.

0028Thus, the invention described herein makes possible the advantage of providing, in the implementation of an optical pickup and an optical disk apparatus utilizing a short-wavelength laser as a light source, a high-performance optical pickup capable of high-density recording/reproduction and an optical disk apparatus incorporating such an optical pickup.

0029The optical pickup according to the present invention, and the optical disk apparatus incorporating the same, utilize light having a relatively short wavelength, e.g., light of blue and other colors on the shorter end of the spectrum. As a result, it is possible to realize a high-performance, high definition DVD (HD-DVD) apparatus which is capable of recording/reproducing information in pits of an optical disk that are far more minute than those of an optical disk for a conventional DVD apparatus.

0030Furthermore, by performing wavelength locking of the semiconductor laser used as a light source, an optical pickup and an optical disk apparatus which are immune to returned light and capable of excellent recording/reproduction performance with a low noise level can be provided.

0031This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

0032<ul id="ul0001" list-style="none"><li>Figure <b>1</b> is a graph illustrating the relationship between the wavelength of a light source and recording density.</li><li>Figure <b>2</b> is a graph illustrating the relationship between the wavelength of a semiconductor laser (a light source) and its RIN level.</li><li>Figure <b>3</b> is a graph illustrating the shift amount of the focal point across the wavelengths of the light source responsive to a change in the light source wavelength of 2 nm between recording and reproduction.</li><li>Figure <b>4</b> is a schematic diagram illustrating a configuration of an optical pickup and an optical disk apparatus according to Example 1 of the present invention.</li><li>Figure <b>5</b> is a cross-sectional view illustrating a configuration of a GaN type semiconductor laser employed in Example 1 of the invention.</li><li>Figure <b>6</b> is a cross-sectional view illustrating a configuration of a GaN type semiconductor laser employed in Example 1 of the invention.</li><li>Figure <b>7</b> is a plan view illustrating a configuration of a GaN type semiconductor laser employed in Example 1 of the invention.</li><li>Figures <b>8A-1</b> to <b>8F-1</b> and Figures <b>8A-2</b> to <b>8F-2</b> are schematic diagrams illustrating manufacturing steps of a semiconductor laser employed in Example 1 of the invention.</li><li>Figure <b>9</b> is a cross-sectional view illustrating another configuration of a GaN type semiconductor laser employed in Example 1 of the invention.</li><li>Figures <b>10A</b> to <b>10C</b> are graphs illustrating the operational characteristics of a GaN semiconductor laser employed in Example 1 of the invention.</li><li>Figure <b>11</b> is a cross-sectional view illustrating a configuration of a GaN type semiconductor laser employed in Example 3 of the invention.</li><li>Figure <b>12</b> is a schematic diagram illustrating a configuration of an optical pickup and an optical disk apparatus according to Example 4 of the present invention.</li><li>Figure <b>13</b> is a cross-sectional view illustrating a configuration of a II-VI group type semiconductor laser employed in Example 5 of the invention.</li><li>Figure <b>14A</b> is a cross-sectional view illustrating a configuration of a II-VI group type semiconductor laser employed in Example 5 of the invention.</li><li>Figure <b>14B</b>, <b>14C</b>, <b>14D-1, 14D-2</b>, <b>14E-1</b>, and <b>14E-2</b> are schematic diagrams illustrating a configuration and manufacturing steps of a II-VI group type semiconductor laser employed in Example 5 of the invention.</li><li>Figure <b>15</b> is a schematic diagram illustrating a configuration of a molecular beam epitaxy apparatus used for the manufacture of the semiconductor laser shown in Figure <b>13</b>.</li><li>Figure <b>16</b> is a schematic diagram illustrating a configuration of an optical pickup according to Example 5 of the invention.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0033Hereinafter, the experiments and studies conducted by the inventors which led to accomplishing the present invention will be described prior to the descriptions of specific examples of the invention.

0034An optical disk for the high-quality TV standard HDTV is required to have a recording capacity of at least about 10 GB or more even when the data is compressed at the maximum rate. This is because of the increased amount of information to be recorded corresponding to the increased number of pixels, which in turn is necessitated by the need for higher quality of images.

0035Figure <b>1</b> illustrates the relationship between the wavelength of a light source and the recording density (recording capacity) of an optical disk. Specifically, the change in the recording density (recording capacity) responsive to the change in the wavelength of the light source is described as a ratio (capacity ratio) based on the recording density (recording capacity) of a conventional DVD in the case of employing a red semiconductor laser (having a wavelength of about 650 nm) as a light source, which was defined as 1.

0036Figure <b>1</b> shows that it would be technically possible to increase the recording density to about 2.2 times that of the current DVD or higher (e.g., 10 GB) by selecting a wavelength of 435 nm or lower.

0037The experiments and studies by the inventors also indicated that a semiconductor laser for use in such a HD (high definition)-DVD apparatus is required not only to be capable of generating a light beam with a small radius but also to have a RIN level which is equal to or lower than a predetermined level. This is because the sensitivity of a Si detector lowers in the blue region against the broad range of HDTV signals. Specifically, the inventors discovered through experiments and studies that the jitter must be reduced to less than 7%, which in turn requires a RIN level of -135 dB/Hz or less. The jitter will increase when the RIN level requirement is not met, thereby preventing sufficient signal reproduction.

0038Figure <b>2</b> is a graph illustrating the relationship between the wavelength of a semiconductor laser and its typical RIN level, as obtained through experiments and studies by the inventors.

0039As seen from Figure <b>2</b>, the RIN level of a usual Fabry-Pérot semiconductor laser increases as the wavelength becomes smaller. For example, the RIN level at a light source wavelength of 400 nm is about -120 dB/Hz, far from meeting the above-mentioned RIN level requirement.

0040Thus, the experiments and studies by the inventors have shown that achieving high-density recording for an optical disk apparatus such as a DVD apparatus requires more than selecting a short-wavelength semiconductor laser as a light source in order to attain recording/reproduction at a sufficiently high quality.

0041Therefore, the present invention not only employs a short-wavelength semiconductor laser as a light source but also achieves wavelength locking of the short-wavelength semiconductor laser by optical feedback, thereby attaining a low noise level, as shown in Figure 2. Specifically, a RIN level of about -140 dB/Hz is attained by the present invention, which is significantly lower than the above-mentioned RIN level requirement. Such low noise is a result of stabilizing the longitudinal mode by optical feedback, thereby reducing the generation of mode hop noise and/or mode competitive noise.

0042Furthermore, the present inventors have confirmed that the wavelength locking by optical feedback also provides other advantages as set forth below.

0043An optical lens typically employed as an element constituting a convergence lens in a conventional configuration is susceptible to a shift in the focal point between a recording operation and a reproduction operation. This is due to the change in refractive index that is experienced by the light, which in turn is due to the slight change in the wavelength of the light source. Such shift in the focal point is about 0.4 µm in the recording/reproduction in the red wavelength region and therefore is negligible in practice use. However, the experiments and studies of the inventors revealed that since the refractive index of glass has a larger wavelength dependency in the blue wavelength region, the focal point greatly shifts when switching operation from recording to reproduction, thereby preventing sufficient recording/reproduction in the blue wavelength region.

0044The above point will be explained with respect to an exemplary optical lens BaCD6 (manufactured by HOYA Corporation), which is a common choice in the field. Figure <b>3</b> is a graph illustrating the shift amount of the focal point across the wavelengths of the light source responsive to a change in the light source wavelength of 2 nm between recording and reproduction, obtained by the present inventors. As shown by the graph, the refractive index (i.e., amount of change in the focal point) has small wavelength dependency in the red wavelength region such that only a 0.4 µm shift of the focal point results responsive to the 2 nm shift in the light source wavelength, as stated above.

0045On the other hand, in the blue wavelength region, the refractive index (i.e., amount of change in the focal point) has large wavelength dependency, so that a substantially threefold shift in the focal point results, i.e., 1.2 µm. Thus, the desired signal reproduction operation becomes difficult to attain.

0046The experiments and studies of the inventors revealed a similar phenomenon in other glass materials which are commonly employed for a convergence lens in the red wavelength region. Thus, it has been confirmed that the optical glass materials typical for the red wavelength region may not be appropriate choices for the blue wavelength region as they are.

0047On the other hand, according to the present invention, a short-wavelength semiconductor laser is wavelength locked by optical feedback, thereby reducing the variation of the oscillation wavelength of the semiconductor laser used as the light source to a lower level. Thus, it has been confirmed that, according to the present invention, a typical conventional optical glass material for the red wavelength region, refractive index of which has large wavelength dependency, can also be employed for the blue wavelength region while reducing the shift amount in the focal point responsive to changes in the refractive index because the shift in the light source wavelength is in itself minimized.

0048Hereinafter, several examples of the present invention achieved through the above discussion will be described with reference to the accompanying figures.

(Example 1)

0049An optical pickup and an optical disk apparatus <b>100</b> according to Example 1 of the present invention will be described with reference to Figure <b>4</b>.

0050The optical disk apparatus <b>100</b> at least includes a semiconductor laser <b>50</b>, an optical system including a lens, an optical disk medium <b>55</b>, and an optical detector <b>57</b>. A GaN type semiconductor laser <b>50</b> is used as a semiconductor laser <b>50.</b> The semiconductor laser <b>50</b> has a DBR (distributed Bragg reflector) structure with grating formed in its interior, and is subject to optical feedback for wavelength locking.

0051The semiconductor laser has an oscillation wavelength of 420 nm for 2.4 times density as compared with that of conventional DVDs. Thus, the optical disk apparatus is capable of reproducing HDTV at a transfer rate of 12 Mpbs from 11.5 GB of information recorded on an optical disk medium (having a 12 cm diameter),

0052Next, the operation of the optical pickup and the optical disk apparatus <b>100</b> is described.

0053Laser light emitted from the GaN semiconductor laser <b>50</b> is collimated through a collimation lens <b>51,</b> led through a polarization beam splitter <b>58</b> and a 1/4 wavelength plate <b>59</b>, and converged by a focus lens <b>52</b> so as to be radiated on pits <b>56</b> formed in an optical disk medium <b>55</b>. The signal light from the optical disk medium <b>55</b> is collimated by the focus lens <b>52</b>, and has its polarization direction turned by the 1/4 wavelength plate <b>59</b> by 90° relative to its polarization direction before being returned from the optical disk medium <b>55</b>. As a result, the signal light is reflected from the polarization beam splitter <b>58</b> so as to be converged on the optical detector <b>57</b> by the focus lens <b>53</b>. The focus lens <b>52</b> for converging light onto the optical disk medium <b>55</b> has a numerical aperture (NA) of 0.6. The pits <b>56</b> formed in the optical disk medium <b>55</b> have a length of 0.26 µm and a track pitch of 0.49 µm.

0054While an optical pickup incorporating a red semiconductor laser can only read pits that are 0.4 µm long or more, the optical pickup according to the present invention can read pits <b>56</b> that are 0.26 µm long. The experiments by the present inventors have shown that excellent HDTV images free of noise can be reproduced. This is in stark contrast with a conventional Fabry-Pérot type semiconductor laser which is not wavelength locked; an optical pickup incorporating such a laser is incapable of signal reproduction due to its high noise level.

0055Figure <b>5</b> is a cross-sectional view of the semiconductor laser <b>50</b> taken at line <b>5-5</b> in Figure <b>6</b> (the cross section being parallel to the cavity end faces). Figure <b>6</b> is a cross-sectional view of the semiconductor laser <b>50</b> taken at line <b>6-6</b> in Figures <b>5</b> and <b>7</b> (the cross section being parallel to the cavity length direction). Figure <b>7</b> is a plan view of the semiconductor laser <b>50</b>.

0056A substrate <b>1</b> is an n-SiC substrate. SiC is a suitable material for the substrate <b>1</b> of the GaN type semiconductor laser <b>50</b> because it has a similar lattice constant to those of GaN and AlGaN (lattice constant difference: about 3.4%) and allows formation of oscillation end faces through cleavage, with appropriate electrical conductivity and good thermal conduction.

0057Sapphire (lattice constant difference: about 13.8%), which has a large difference in lattice constant from GaN or AlGaN, is also frequently employed as the material for the substrate 1 of the GaN type semiconductor laser <b>50.</b> However, since sapphire has poor electrical conductivity, an n-electrode <b>10</b> (see Figure <b>5</b>) must be provided on an n-GaN layer <b>2</b> (see Figure <b>5</b>), as shown in Figure <b>9</b>.

0058Figure <b>9</b> is a cross-sectional view of a semi-conductor laser <b>60</b> incorporating such a sapphire substrate, the cross section taken in parallel to the cavity end faces. In Figure <b>9</b>, the constituent elements which also appear in the semiconductor laser <b>50</b> illustrated in Figures <b>5</b> to <b>7</b> are indicated by the same reference numerals as used therein, with their descriptions omitted.

0059Other materials for the substrate <b>1</b> include Si, GaAs, ZnO, LiAlO₂, LiGaO₂, MgAl₂O₄, and the like. A p-type substrate can be employed instead of an n-type substrate. It will be appreciated that, when using a p-type substrate, the conductivity types (i.e., p or n) of the respective layers in the multilayer structure formed on such a substrate must be reversed from their exemplary conductivity types in the following description.

0060As shown in Figure <b>5</b>, on an n-SiC substrate 1, the following layers are formed in this order: <ul id="ul0002" list-style="none" compact="compact"><li>an n-GaN layer <b>2</b> (Si doped, having a carrier density of 1×10¹⁸ cm^-3 and a thickness of 3 µm);</li><li>an n-AlGaN cladding layer <b>3</b> (Al content 10%, Si doped, having a carrier density of 1×10¹⁸ cm^-3 and a thickness of 0.5 µm);</li><li>an n-GaN optical waveguide layer <b>4</b> (Si doped, having a carrier density of 1×10¹⁸ cm^-3 and a thickness of 0.1 µm);</li><li>an InGaN multiple quantum well (MQW) layer <b>5</b> (including 7 alternating layers of 4 nm thick In_0.02Ga_0.98N barrier layers and 3 nm thick In_0.15Ga_0.85N well layers;</li><li>a p-GaN optical waveguide layer <b>6</b> (Mg doped, having a carrier density of 1×10¹⁸ cm^-3 and a thickness of 0.1 µm);</li><li>a p-AlGaN cladding layer <b>7</b> (Al content 10%, Mg doped, having a carrier density of 1×10¹⁸ cm^-3 and a thickness of 0.5 µm); and</li><li>a p-GaN layer <b>8</b> (Mg doped, having a carrier density of 1×10¹⁹ cm^-3 and a thickness of 0.3 µm).</li></ul>

0061A buffer layer (not shown) is provided at the interface between the substrate <b>1</b> and the n-GaN layer <b>2</b>. An AlN layer (thickness: 20 nm) is employed as the buffer layer in the case where the substrate <b>1</b> is a SiC substrate, and a GaN layer (thickness: 30 nm) in the case where the substrate 1 is a sapphire substrate, thereby minimizing the lattice constant difference between the n-GaN layer 2 and the substrate 1 so as to obtain an excellent crystal with a low defect density.

0062A mesa ridge is formed from the upper face of the p-GaN layer <b>8</b> to about the middle of the p-AlGaN layer <b>7</b>. The width of the mesa ridge varies along the cavity length direction as shown in Figure <b>7</b>, thereby forming a tapered beam shaping portion. In the narrowest portion (the left end in Figure <b>7</b>) of the mesa ridge adjoining the cavity end face, the bottom width of the mesa ridge is 2 µm, and it gradually increases into the interior of the crystal (oscillator) to a maximum value of 5 µm. As a result, the horizontal spread angle of the laser light emitted from the end face (adjoining the narrow portion of the mesa ridge) becomes relatively broad (about 20°), so that a laser beam which is substantially circular with respect to the vertical spread angle (about 23°) is obtained.

0063With reference to Figure <b>6</b>, the cavity length of the semiconductor laser <b>100</b> is typically 1 mm. In a region <b>100a</b> which accounts for 0.6 mm, an MQW active layer <b>5</b> is provided. Neither the active layer <b>5</b> nor the upper and lower optical waveguide layers <b>4</b> and <b>6</b> are formed in the remaining region <b>100b;</b> instead, a GaN layer <b>12</b> including a diffraction grating (distributed Bragg reflector: DBR) <b>13</b> with a period of 158 nm is provided in the region <b>100b.</b>

0064The GaN layer <b>12</b> does not absorb laser light with a wavelength of 410 nm, and the semiconductor laser <b>50</b> oscillates in a single longitudinal mode with the oscillation spectrum selected by the DBR <b>13</b>. A Si film <b>14</b> (thickness: 500 nm) is provided on the end face adjacent the DBR <b>13</b> so as to absorb the light component (accounting for about 10%) that was not reflected by the DBR <b>13,</b> thereby preventing the Fabry-Pérot cavity mode. On the other hand, four layers of alternating SiO₂ films <b>15</b> (thickness: 71 nm) and TiO₂ films <b>16</b> (thickness: 47 nm) are provided on the end face adjacent the MQW so as to increase the reflectance to 76%. As a result, instability in the laser spectrum due to returned light can be prevented, whereby low noise characteristics can be realized.

0065Figures <b>8A-1</b> to <b>8F-1</b> and Figures <b>8A-2</b> to <b>8F-2</b> illustrate the manufacturing steps of the semiconductor laser <b>50</b> according to the present invention. Figures <b>8A-1</b> to <b>8F-1</b> are cross-sectional views in parallel with the cavity end faces of the semiconductor laser <b>50;</b> Figures <b>8A-2</b> to <b>8F-2</b> are cross-sectional views along the cavity length direction of the semiconductor laser <b>50.</b>

0066First, the respective layers from the n-GaN layer <b>2</b> to the p-GaN layer <b>6</b> are formed on the n-SiC substrate <b>1</b> by MOVPE (metal organic vapor phase epitaxy), with an AlN buffer layer (not shown) interposed therebetween (Figures <b>8A-1</b> to <b>8A-2</b>). During the growth process, TMG, TMI, TMA, and NH₃ are used as a Ga source, an In source, an Al source, and a N source, respectively; and H₂ or N₂ is used as a carrier gas to thermally decompose each material gas at a high temperature in the range of about 800°C to about 1100°C. thereby growing the respective layers on the substrate. In addition, SiH₄ or Cp₂Mg is flowed to impart the layers with the p-type conductivity or the n-type conductivity.

0067Next, after the SiO₂ layer <b>17</b> is deposited by CVD (chemical vapor deposition), a mask pattern is formed by photolithography and etching, and the p-GaN layers <b>6</b> to the n-GaN layer <b>4</b> are processed so as to leave predetermined patterns (Figures <b>8B-1</b> and <b>8B-2</b>). In the case where milling technique is employed, a mask pattern of a resist layer is employed instead of the SiO₂ layer <b>17</b>.

0068Next, the GaN layer <b>12</b> is grown by again using MOVPE. It should be noted that the GaN layer <b>12</b> does not grow in portions covered with the SiO₂ layer <b>17</b>. Since the GaN layer <b>12</b> is not doped with any impurities, the GaN layer <b>12</b> acquires high resistance so as to ensure that a current does not flow in the DBR region. After the crystal growth, a diffraction grating <b>13</b> (with a pitch of 158 nm) is formed on the GaN layer <b>12</b> by laser light interference exposition and etching (Figures <b>8C-1</b> and <b>8C-2</b>).

0069After removing the SiO₂ layer <b>17</b>, the p-type layers <b>7</b> and <b>8</b> are grown by using MOVPE (Figures <b>8D-1</b> and to <b>8D-2</b>). Next, a mesa ridge is formed from the upper face of the layer <b>8</b> to about the middle of the layer <b>7</b> by photolithography and etching (Figures <b>8E-1</b> and <b>8E-2</b>).

0070Furthermore, Ni/Au is deposited on the mesa ridge so as to form the p-electrode <b>9,</b> and Ti/Au is deposited on the lower face of the substrate <b>1</b> so as to form the n-electrode <b>10</b>. As a result, the semiconductor laser <b>50</b> is formed.

0071Figures <b>10A</b> to <b>10C</b> show the operational characteristics of the semiconductor laser <b>50</b>. Specifically, Figure <b>10A</b> shows the current-optical output characteristics; Figure <b>10B</b> shows the beam spreading distribution; and Figure <b>10C</b> shows the oscillation spectrum.

0072It will be seen from these figures that the semiconductor laser <b>50</b> has a threshold current of 45 mA with a beam aspect ratio (defined as the vertical spread angle/the horizontal spread angle) of 1.15, indicative of a single longitudinal mode oscillation. Since the aspect ratio is very close to 1, light can be radiated at an efficiency of 40% from a laser with an output level of 2 mW onto the surface of an optical disk medium, without even using a beam shaping element. As a result, the size and cost of the optical pickup can be minimized. Since the reflectance at the front end face of the cavity is increased while ensuring that the unreflected light at the rear end face of the cavity is absorbed, the laser can stably operate with low noise even under the condition of the 4% returned light.

0073As described above, the optical pickup according to Example 1 of the present invention is capable of reproducing/recording data in pits provided at higher densities than conventional pit densities. As a result, an optical disk apparatus capable of high-density reproduction/recording is realized. By employing a DBR semiconductor laser as the light source semiconductor laser <b>50</b>, the noise level can be reduced to a RIN level of about -140 dB/Hz.

0074By forming a tapered portion at the outgoing end face of the semiconductor laser <b>50</b> so as to bring the beam aspect ratio to the proximity of 1, the effective transmission ratio can be greatly improved. As a result, a low-output laser can be used as the semiconductor laser <b>50</b>, thereby providing the device with a prolonged lifetime. Specifically, the conventional art techniques employ a 5 mW semiconductor laser, for example, with the peripheral portions of the emitted beam being cut off. In contrast, the present example requires the semiconductor laser <b>50</b> to have an output power of only 2 mW, implying a threefold increase in lifetime.

0075According to the present example, an absorption film is provided on the rear end face of the DBR semiconductor laser <b>50</b> to prevent the occurrence of returned light. Alternatively, an absorption layer can be provided in the vicinity of the rear end face of the cavity (e.g., an upper portion in the figure).

0076While the semiconductor laser <b>50</b> in the above example has an oscillation wavelength of about 420 nm, an oscillation at 380 nm is possible with a different composition of the material of the semiconductor laser <b>50</b>. A resultant optical disk apparatus would provide for a recording density 2.9 times as compared with that of a conventional red DVD, resulting in a disk capacity of about 13.8 GB. A disk capacity of about 20 GB becomes possible by further increasing the NA of the lens to 0.7. In such cases, a 2 hour HDTV program can be recorded/reproduced on a single disk at a transfer rate of 20 Mbps. A disk capacity of about 40 GB can be provided by a disk with a two layer structure.

0077It is also applicable to induce optical feedback in the semiconductor laser <b>50</b> by employing a DFB (distributed feedback) structure, instead of a DBR structure as set forth above. However, a DBR laser is more preferable than a DFB laser in terms of production yield.

0078According to the present example, the oscillation wavelength of the semiconductor laser <b>50</b> has a very small variation owing to the locking of the oscillation wavelength due to optical feedback. This substantially eliminates the constraints on the lens material (optical glass) due to wavelength variation. As a result, it becomes possible to employ high refractive index materials such as fluorine type glass material, thereby facilitating the realization of a lens having a high NA value. Furthermore, the oscillation wavelength locking configuration of the present example also makes it possible to employ a diffraction type lens, etc., which was not available to the conventional art techniques because of the variation in the focal point responsive to variation in the oscillation wavelength.

(Example 2)

0079Next, an embodiment will be described where recording is performed for an optical disk medium by employing an optical pickup incorporating the semiconductor laser according to the present invention as a light source.

0080The fundamental structure of the optical pickup of the present example is similar to that described in Example 1. Specifically, a GaN type semiconductor laser having an oscillation wavelength of 400 nm and a beam aspect ratio of 1.5 is employed. However, in the present example, optical feedback is attained by means of an external grating.

0081The present example also attains an aspect ratio which is very close to 1. As a result, light can be radiated at a 40% efficiency from a 2 mW laser onto the surface of an optical disk medium, without even using a beam shaping element. As a result, excellent recording can be performed on the surface of an optical disk medium with a recording power of about 8 mW. In contrast, an optical pickup incorporating a conventional semiconductor laser having an aspect ratio of 3 would only provide a light utility efficiency of 20%, so that a laser output of about 40 mW would be required to achieve a similar recording performance; this would reduce the lifetime of the apparatus and therefore is impractical.

0082Moreover, according to the present example, RF superimposition at 600 MHz is performed for the semiconductor laser. Although this brings down the RIN level to about -137 dB/MHz, it also provides for increased tolerance against returned light. Such an RF superimposition technique is effective because it neither diversifies the oscillation mode nor substantially degrades the noise characteristics.

0083In accordance with the optical disk apparatus of the present example, 12.5 GB of data can be recorded on an optical disk medium.

0084Also according to the present example, the oscillation wavelength of the semiconductor laser <b>50</b> has a very small variation owing to the locking of the oscillation wavelength due to optical feedback. This substantially eliminates the constraints on the lens material (optical glass) due to wavelength variation. It also becomes possible to employ a diffraction type lens, etc., which was not available to the conventional art techniques because of the variation in the focal point responsive to variation in the oscillation wavelength.

(Example 3)

0085Polycarbonate is generally employed as a protection layer on an optical disk medium due to its low cost and high reliability. However, the birefringence of polycarbonate induces wavelength dispersion. In this connection, the present inventors performed experiments and studies to discover that the birefringence or wavelength dispersion of polycarbonate induce substantial problems in the blue wavelength region.

0086Specifically, a polycarbonate protection layer on an optical disk medium may act in a similar manner to a 1/4 wavelength plate, especially in the wavelength region of 435 nm or below. It has been confirmed that the magnitude of such an action varies from the inner periphery to the outer periphery of the optical disk medium and becomes particularly problematic in the case of employing a polarization beam splitter or a polarization hologram device.

0087The aforementioned problems were not recognized under the conventional art.

0088In order to overcome the aforementioned problems, the present example controls polarization of a laser light emitted from a semiconductor laser <b>70</b> (corresponding to the semiconductor laser <b>50</b> in Example 1) in an optical pickup structure essentially similar to that shown in Figure <b>4</b>.

0089Figure <b>11</b> is a schematic cross-sectional view illustrating a configuration of the semiconductor laser <b>70</b> (GaN type semiconductor laser) employed in the present example.

0090In an active layer <b>30</b> of a GaN laser chip <b>34</b> in the illustrative structure, a DBR structure is formed (defining a DBR section <b>32</b>) in the vicinity of end face of the laser chip <b>34</b> opposite the end face from which laser light <b>P</b> goes out. The remaining portion of the active layer <b>30</b> defines an active section <b>31</b>. Furthermore, after being bonded to a Si mount <b>33</b>, a pressure is applied to the semiconductor laser chip <b>34</b> on the side of the active section <b>31</b> via its substrate (not shown) or opposite the Si mount <b>33</b>. By varying the applied pressure, the polarization direction of the laser light <b>P</b> from the semiconductor laser chip <b>34</b> is rotated. In the present example, a piezo element <b>35</b> is used for the application of pressure.

0091With a pressure of 400 g/cm² applied to the semiconductor laser chip <b>34</b> in the above structure, the polarization direction of the laser light <b>P</b> from the semiconductor laser chip <b>34</b> is rotated by about 60°. By thus controlling the polarization of the laser light <b>P</b>, it becomes possible to maintain substantially a constant amount of signal light being reflected from an optical disk medium and entering an optical detector. Specifically, the light amount of the signal light is monitored, and the monitoring results are fed back to the voltage applied to the piezo element <b>35</b> so as to vary the polarization direction of the laser light <b>P</b>. Consequently, the amount of incident signal light is kept constant.

0092The apparatus according to the present example realizes excellent reproduction operation. Without performing such control for the signal light, the incident light amount would decrease, thereby making it difficult to correctly reproduce signals from the optical disk medium.

0093It is preferable to restrain the application of pressure onto the semiconductor laser chip <b>34</b> (with the piezo element <b>35</b>, etc.) within the vicinity of the active section <b>31</b> because any pressure applied on the DBR section <b>32</b> might induce undesirable wavelength variation.

0094Instead of the piezo element <b>35</b>, a torque motor or the like may also be used for the application of pressure.

(Example 4)

0095In the present example, the birefringence problem of the optical disk medium as set forth in Example 3 is addressed by employing a variable wavelength plate <b>61</b> interposed between the focus lens <b>52</b> and the polarization beam splitter <b>58,</b> instead of applying a pressure onto the semiconductor laser chip.

0096Figure <b>12</b> is a schematic diagram illustrating a configuration of an optical pickup and an optical disk apparatus <b>200</b> according to the present example.

0097The laser light emitted from the GaN semiconductor laser <b>50</b> is collimated through a collimation lens <b>51,</b> led through a polarization beam splitter <b>58</b> and the variable wavelength plate <b>61</b>, and converged by a focus lens <b>52</b> so as to be radiated on pits <b>56</b> formed in an optical disk medium <b>55</b>. The signal light from the optical disk medium <b>55</b> is collimated by the focus lens <b>52,</b> led through the variable wavelength plate <b>61</b>, and reflected from the polarization beam splitter <b>58</b> so as to be converged on the optical detector <b>57</b> by the focus lens <b>53</b>.

0098As the variable wavelength plate <b>61,</b> a LiNbO₃ plate is employed, a refractive index of which in a particular direction can be varied by an applied voltage through electro-optical effects. By applying an electric field over the entire surface of the variable wavelength plate (the LiNbO₃ plate) <b>61</b>, its birefringence difference is controlled so that the variable wavelength plate <b>61</b> has a variable function ranging from that of a 0 wavelength plate to a 1/4 wavelength plate. As a result, the amount of signal light entering the optical detector <b>57</b> is always maintained at the maximum level when reproducing data both from the inner periphery and the outer periphery of the optical disk medium. By thus inserting a variable wavelength plate <b>61</b> between the focus lens <b>52</b> and the polarization element (polarization beam splitter) <b>58,</b> polarization is controlled so as to substantially fix the amount of signal light entering the optical detector <b>57</b> at the maximum level.

(Example 5)

0099In the present example, a semiconductor laser for an optical pickup is constructed by using a II-VI group material, which provides an oscillation wavelength of 435 nm or below.

0100Figure <b>13</b> is a cross-sectional view illustrating a configuration of a distributed reflection type semiconductor laser <b>80</b> composed essentially of a II-VI group compound semiconductor, the cross-section being perpendicular to the optical axis of the semiconductor laser <b>80</b>.

0101Specifically, the following layers are formed on an n-GaAs (100) substrate <b>101</b> in this order: an n-GaAs buffer layer <b>102</b>; an n-ZnSe buffer layer <b>103</b>; an n-ZnSSe cladding layer <b>104</b>; an n-ZnMgSSe cladding layer 105; an n-ZnMgSSe optical guide layer <b>106;</b> a ZnMgSSe multiple quantum well active layer 107; a p-ZnMgSSe optical guide layer <b>108</b>; a p-ZnMgSSe cladding layer <b>109</b>; a p-ZnSSe cladding layer <b>110;</b> a p-ZnSe cap layer <b>111;</b> a p-ZnTe/ZnSe pseudo slant layer <b>112;</b> and a p-ZnTe contact layer <b>113.</b> A p-electrode <b>114</b> is provided on the p-ZnTe contact layer <b>113.</b> A portion of the p-ZnMgSSe cladding layer <b>109</b> and the layers thereabove are etched in a mesa stripe shape, where the mesa ridge has a bottom width of 4 µm, for example. A current constriction layer <b>115</b> is formed over portions of the p-ZnMgSSe cladding layer <b>109</b> other than the mesa stripe, thereby forming an index-guided structure. Above the current constriction layer <b>115,</b> an electrode <b>116</b> is formed so as to be in contact with the stripe-shaped p-electrode <b>114.</b> On the other hand, an n-electrode <b>117</b> is formed on the lower face of the n-GaAs substrate <b>101</b>.

0102Figure <b>14A</b> schematically illustrates an exemplary cross-section of the semiconductor laser <b>80,</b> the cross sectional being parallel to the optical axis of the semiconductor laser <b>80</b> (taken at line <b>14-14</b> in Figure <b>13</b>). The semiconductor laser <b>80</b> includes an active region <b>201</b> and a distributed reflector region <b>202</b>. The distributed reflector is formed directly above the p-ZnMgSSe optical guide layer <b>108</b>, for example.

0103Crystals which can be employed for the substrate <b>101</b> include semiconductors of the sphalerite structure, e.g., GaAs, InGaAs, GaP, ZnSe, ZnS; and semiconductors of the diamond structure, e.g., Si and Ge. Although an n-substrate <b>101</b> is employed in the present example, it is also applicable to form a laser structure on a p-substrate. Although the (100) face is used for the substrate <b>101</b>, it is also applicable to use a face which is tilted from the (100) face in the [111]A direction or the [111]B direction. For example, the (511)B face (which is tilted by 15.8° in the [111]B direction) or the (811)A face (which is tilted by 10.0° in the [111]A direction) can be used.

0104It is preferable to select the composition of the II-VI group compound semiconductor epitaxially grown on the substrate <b>101</b> so as to have a substantial lattice match with the substrate <b>101</b>, thereby preventing misfit defects from occurring at the interface with the substrate <b>101</b>. For a substrate of GaAs, InGaAs, ZnSe, Ge, or the like, a II-VI group compound semiconductor of a ZnMgSSe type or the ZnMgBeSe type can be selected, for example. On the other hand, for a substrate of GaP, ZnS, Si, or the like, a II-VI group compound semiconductor of a ZnMgBeSSe type can be selected, for example. In the case of selecting a composition with a large lattice mismatch with the substrate 101, it is preferable to prescribe the thickness of the layer at a critical thickness with such a composition or smaller.

0105As for the composition of the cladding layers of a semiconductor laser with an oscillation wavelength lower than 435 nm, Zn_0.6Mg_0.4S_0.5Se_0.5 can be selected for a ZnMgSSe type, for example, and Zn_0.6Mg_0.3Be_0.1Se can be selected for a ZnMgBeSe type, for example. Although the present example will discuss laser structures employing a ZnMgSSe type material, it will be appreciated that a semiconductor laser with similar characteristics can be constructed from a ZnMgBeSe type material.

0106The GaAs buffer layer <b>102</b> is provided in order to provide a flat surface on the atom alignment level and to reduce the density of crystal defects in the II-VI group compound semiconductor formed thereon, so that a high-quality semiconductor laser crystal is obtained. The thickness of the GaAs buffer layer <b>102</b> can be 0.3 µm, for example. As an n-type impurity for doping the GaAs buffer layer <b>102</b>, Si can be used, for example. As a p-type impurity for doping the GaAs buffer layer <b>102</b>, Zn can be used, for example.

0107The n-ZnSe buffer layer <b>103</b> is provided in order to enhance the generation and growth of a two-dimensional nucleus and suppress the generation of lamination defects in the incipient growth stage of the n-ZnSSe cladding layer <b>104</b> to be grown thereupon. Since ZnSe has a lattice mismatch of about +0.28% with GaAs, its critical thickness is about 150 nm. Therefore, the thickness of the ZnSe buffer layer <b>103</b> is prescribed at 30 nm, for example. The effective donor density is 8×10¹⁷ cm^-3, for example, and Cl can be used as a n-type impurity, for example. It is preferable not to add an impurity in a region spanning about 2 nm to about 3 nm adjoining the GaAs buffer layer <b>102</b> in order to minimize defects occurring due to the dispersion of the impurity into the GaAs interface.

0108A conduction band is discontinuous with a gap of about 0.6 eV between the n-GaAs substrate <b>101</b> and the n-ZnMgSSe cladding layer <b>105</b> to create a barrier against the electron injection. Therefore, the n-ZnSSe cladding layer <b>104</b> is provided to lower the barrier. As a result, the discontinuous gap in the conduction band is divided into about 0.3 eV each, thereby substantially eliminating the barrier against the electron injection. The ZnSSe cladding layer <b>104</b> has a composition with a S mole fraction of 0.06. The ZnSSe material having such a composition has a substantial lattice match with GaAs, and a band gap energy of 2.75 eV at room temperature. The n-ZnSSe cladding layer <b>104</b> can have a thickness of about 0.2 µm, for example, and an effective donor density of about 5×10¹⁷ cm^-3, for example.

0109The n-ZnMgSSe cladding layer 105 and the p-ZnMgSSe cladding layer <b>109</b> have a composition with a Mg mole fraction of 0.4 and a S mole fraction of 0.5. The ZnMgSSe material having such a composition has a substantial lattice match with GaAs, and a band gap energy of 3.3 eV at room temperature. The band gap energy of the ZnMgSSe can be varied from about 2.7 eV to about 3.4 eV by appropriately selecting its composition, while maintaining a substantial lattice match with GaAs. The n-ZnMgSSe cladding layer <b>105</b> can have a thickness of about 0.8 µm, for example, and an effective donor density of about 3×10¹⁷ cm^-3, for example. The p-ZnMgSSe cladding layer <b>109</b> can have a thickness of about 0.4 µm, for example, and an effective acceptor density of about 3× 10¹⁶ cm^-3, for example. As a p-type impurity, N can be used, for example.

0110The n-ZnMgSSe optical guide layer <b>106</b> and the p-ZnMgSSe optical guide layer <b>108</b> have a composition with a Mg mole fraction of 0.3 and a S mole fraction of 0.4. The ZnMgSSe material having such a composition has a substantial lattice match with GaAs, and a band gap energy of 3.2 eV at room temperature. The composition of each optical guide layer is selected so that its refractive index at the laser oscillation wavelength is larger than the refractive index of the cladding layers and smaller than the refractive index of the active layer, and that its band gap energy is smaller than the band gap energy of the cladding layers and the larger than the band gap energy of the active layer. The thickness of each optical guide layer can be about 0.1 µm, for example. It is preferable not to add impurities (whether it is p-type or n-type) in portions of the optical guide layers <b>106</b> and <b>108</b> that are in the vicinity of their interfaces with the ZnMgSSe MQW active layer <b>107</b> in order to prevent the formation of an impurity level in the ZnMgSSe MQW active layer <b>107</b>. Alternatively, it is also applicable not to dope the entire optical guide layers <b>106</b> and <b>108</b> with any impurities at all (p-type or n-type). The optical guide layers <b>106</b> and <b>108</b> have an effective donor density of about 4×10¹⁷ cm³, for example, and an effective acceptor density of about 6×10¹⁶ cm^-3, for example.

0111The ZnMgSSe MQW active layer <b>107</b> includes, for example, alternating layers of three well layers (thickness: 4 nm) having a Mg mole fraction of 0.1 and a S mole fraction of 0.1, and two barrier layers (thickness: 6 nm) having the same composition as that of the optical guide layers. The well layers have a band gap energy of 2.9 eV at room temperature. The ZnMgSSe material having such a composition has a substantial lattice match with GaAs. The composition of the well layers is selected so as to have a band gap energy which is smaller than the band gap energy of the cladding layers and the band gap energy of the optical guide layers.

0112Although a lattice matched system is adopted for both the well layers and the barrier layers in the present example, it is also applicable to adopt a strained quantum well structure or a strain-compensated quantum well structure by controlling the amount of strain. It is preferable not to add impurities (whether it is p-type or n-type) in the well layers or the barrier layers in order to prevent the formation of an impurity level in the active layer.

0113The p-ZnSSe cladding layer <b>110</b> is provided in order to reduce the thermal resistance and serial resistance of the semiconductor laser <b>80</b> and to reduce the discontinuous gap in valence band. This provides an advantage of reducing the thickness of the p-ZnMgSSe cladding layer <b>109</b> having a large mole fraction and an insufficient effective acceptor density. The p-ZnSSe cladding layer <b>110</b> has a thickness of about 0.5 µm, for example, and an effective donor density of about 4x10¹⁷ cm^-3, for example.

0114The p-ZnSe cap layer <b>111</b>, the p-ZnTe/ZnSe pseudo slant layer <b>112</b>, and the p-ZnTe contact layer <b>113</b> are provided for achieving p-type ohmic contact. The p-ZnSe cap layer <b>111</b> has a thickness of about 80 nm, for example, and an effective acceptor density of about 8×10¹⁷cm^-3, for example. The p-ZnTe contact layer <b>113</b> has a thickness of about 50 nm, for example, and an effective acceptor density of about 1×10¹⁹ cm^-3, for example. Although an ohmic electrode can be easily formed upon the p-ZnTe contact layer <b>114,</b> a discontinuous gap in the valence band of about 0.8 eV exists between ZnSe and ZnTe to create a barrier against the hole injection. Therefore, the p-ZnTe/ZnSe pseudo slant layer <b>112</b> is provided in order to eliminate this barrier. The p-ZnTe/ZnSe pseudo slant layer <b>112</b> can have a 12-layer structure with a period of 2.1 nm, for example, including a first layer (adjoining the p-ZnSe cap layer <b>111</b>) composed of a 0.3 nm thick p-ZnTe layer and a 1.8 nm thick p-ZnSe layer, a second layer composed of a 0.4 nm thick p-ZnTe layer and a 1.7 nm thick p-ZnSe layer, a third layer composed of a still thicker p-ZnTe layer and a still thinner p-ZnSe layer, ...., and a twelfth layer (adjoining the p-ZnTe contact layer <b>113</b>) composed of a 1.8 nm thick p-ZnTe layer and a 0.3 nm thick p-ZnSe layer. Alternatively, any structure in which the band structure macroscopically transits from ZnSe to ZnTe gradually, or any structure in which holes can flow through the barrier between ZnTe and ZnSe owing to a tunnel effect, or a combination of both, can be adopted. For example, a mixed crystal such as ZnSeTe can be used; or, p-ZnTe quantum well structure can be provided closer to the p-ZnSe cap layer <b>111</b>.

0115The p-ohmic contact layer can also be formed of BeTe. Such a p-ohmic contact layer is applicable to a II-VI group semiconductor device by being combined with a p-BeTe/ZnSe pseudo slant layer, a BeSeTe mixed crystal, a BeTe quantum well structure, etc., of similar materials. BeTe has about -0.47% lattice mismatch with GaAs, and is advantageous in that it is capable of achieving ohmic contact with high crystal quality at a thickness below the critical thickness, unlike in the case of ZnTe.

0116As the p-type electrode <b>114</b>, an AuPd electrode can be used, for example, in which Pd films (thickness: 10 nm) and Au films (thickness: 600 nm) are stacked alternately. As the n-type electrode <b>117</b>, an AuGeNi electrode can be used, for example.

0117The material and composition of the current constriction layer <b>115</b> are selected in accordance with the refractive index of the cladding layers. For example, ZnMgSSe, ZnMgBeSe, ZnO, and the like are preferable materials.

0118As the method for growing a laser structure of the semiconductor laser <b>80</b> described above, a molecular beam epitaxy method, an organic metal vapor epitaxy method, or the like can be used. In the case where accurate control on a molecular layer level or an atomic layer level is required, an atomic layer epitaxy method or a migration enhanced epitaxy method can be used.

0119Hereinafter, a manufacture process by a molecular beam epitaxy method will be described.

0120Figure <b>15</b> is a conceptual diagram representing a molecular beam epitaxy apparatus.

0121This apparatus includes a load lock chamber <b>301</b>, a III-V group semiconductor growth chamber <b>302</b>, a substrate conveyance chamber <b>303</b>, and a II-VI group semiconductor growth chamber <b>304</b>. A substrate retention/heating mechanism <b>305</b> and heating evaporation sources <b>306</b> are provided in the III-V group semiconductor growth chamber <b>302</b> and the II-VI group semiconductor growth chamber <b>304</b>. In the III-V group semiconductor growth chamber <b>302</b>, a Ga source, an As source, a Si source, and a Zn source, for example, are provided as the heating evaporation sources <b>306.</b> In the II-VI group semiconductor growth chamber <b>304</b>, a ZnSe source, a ZnS source, an Mg source, a ZnTe source or a BeTe source, and a ZnCl₂ source, for example, are provided as the heating evaporation sources <b>306</b>. The II-VI group semiconductor growth chamber <b>304</b> is further equipped with an N₂ radical source <b>307</b> having a radiofrequency discharge tube. It is also applicable to provide a heating evaporation sources filled with Zn, Se, S, Cd, Be, Te, and the like, instead of a ZnSe source, a ZnS source, a ZnTe source.

0122In accordance with the apparatus shown in Figure <b>15</b>, a Zn molecular beam irradiation can be performed immediately after forming the n-GaAs buffer layer <b>102</b> on the substrate in the III-V group semiconductor growth chamber <b>302</b>, so that the surface of the GaAs substrate is terminated with a few layers of Zn atoms. As a result, the formation of Ga-Se bonds and Ga-S bonds can be prevented from occurring at the interface between the II-VI semiconductor and GaAs, thereby preventing lamination defects due to such bonds.

0123The Si-doped n-GaAs (100) substrate <b>101</b> (carrier density: 2×10¹⁸ cm^-3) is prebaked in the load lock chamber <b>301</b> at 350°C, and then conveyed through the substrate conveyance chamber <b>303</b> (maintained at a vacuum of about 1×10^-10 Torr) to the III-V group semiconductor growth chamber <b>302</b>. After removing the native oxidation film by heating the GaAs substrate <b>101</b> to about 600°C while irradiating the substrate with an As molecular beam, Ga, As, and Si molecular beams are radiated so as to grow the Si-doped n-GaAs buffer layer <b>102</b>. Assuming that the intensities of the Ga and As molecular beams are about 5×10^-7 Torr and 1×10^-5 Torr, respectively, for example, and the substrate <b>101</b> is maintained at a temperature of about 590°C, for example, a growth rate of 0.9 µm/h is obtained.

0124After the growth of the n-GaAs buffer layer <b>102</b>, a Zn molecular beam is radiated on the surface of the GaAs buffer layer <b>102</b> while lowering the temperature of the substrate to about 300°C. Thus, the GaAs surface is termininated with Zn. For example, the intensity of the molecular beam from the heating evaporation source can be 1×10^-6 Torr, and the irradiation can be performed for 30 seconds. It can be confirmed by high-speed electron beam diffraction that the restructured surface is thus changed from a (2×4) structure (indicative of an As stabilized face of GaAs) into a (1×4) structure (indicative of the face terminated with Zn).

0125The substrate <b>101</b> on which the n-GaAs buffer layer <b>102</b> (terminated with Zn) has been formed is conveyed through the substrate conveyance chamber <b>303</b> to the II-VI group semiconductor growth chamber <b>304.</b> In the II-VI group semiconductor growth chamber <b>304,</b> the following layers are formed on the n-GaAs buffer layer <b>102</b> in this order: the n-ZnSe buffer layer <b>103</b>; the n-ZnSSe cladding layer <b>104;</b> the n-ZnMgSSe cladding layer <b>105</b>; the n-ZnMgSSe optical guide layer <b>106</b>; the ZnMgSSe MQW active layer <b>107</b>; the p-ZnMgSSe optical guide layer <b>108</b>; the p-ZnMgSSe cladding layer <b>109;</b> the p-ZnSSe cladding layer <b>110;</b> the p-ZnSe cap layer <b>111</b>; the p-ZnTe/ZnSe pseudo slant layer <b>112</b>; and the p-ZnTe contact layer <b>113</b>. As the n-type impurity, ZnCl₂ can be used, for example. As the p-type impurity, active N₂ generated by radiofrequency plasma discharge can be used, for example. The substrate temperature during the crystal growth can be set at 290°C, for example. The growth rate of the ZnMgSSe cladding layer can be 0.7 µm/h, for example.

0126The laser structure of the semiconductor laser <b>80</b> of the present example employs a different composition of ZnMgSSe for each of the quantum well layer, the optical guide layers, and the cladding layers. Therefore, in order to form each layer with mixed crystal of a uniform composition, it is necessary to accurately control the molecular beam intensities of the constituent elements and the ratio between the intensities during the growth of each layer. In general, the molecular beam intensities of the constituent elements and the ratio between the intensities can be controlled by controlling the temperatures of the heating evaporation sources or the opening/closing ratios of the valves of valved heating evaporation sources.

0127On the contrary, by forming each layer with a supper lattice combining binary mixed alloy compounds or quaternary mixed alloy crystals, the complexity of controlling molecular beam intensities will be reduced, so that the laser structure of the present example can be formed simply by opening/closing of the shutters of the heat evaporation sources.

0128For example, the temperatures of the heating evaporation sources of ZnSe and Mg can be fixed at the temperatures required for forming a ZnMgSSe cladding layer having a Mg mole fraction of 0.4 and a S mole fraction of 0.5, and the temperature of the ZnS heating evaporation means can be switched between a temperature required for forming a ZnSSe cladding layer having a S mole fraction of 0.06 and a temperature required for forming a ZnMgSSe cladding layer having a Mg mole fraction of 0.4 and a S mole fraction of 0.5.

0129By providing two ZnS heating evaporation sources, it becomes possible to fix the temperatures of all the heating evaporation sources. Alternatively, by employing a valved ZnS heating evaporation source, it becomes possible to obtain the molecular beam intensities required for growing the above two compositions by appropriately switching the opening/closing ratios of the valve. Such technique provides an advantage in that the growth process can be continuously performed without being interrupted because the ZnS molecular beam intensity can be instantaneously changed.

0130The laser structure according to the present example obtained by using the above-mentioned technique is as follows:

0131The ZnSSe cladding layers and the ZnMgSSe cladding layers are composed of a mixed crystal having a uniform composition including a S mole fraction of 0.06, and a mixed crystal having a uniform composition including a Mg mole fraction of 0.4 and a S mole fraction of 0.5, respectively. The ZnMgSSe optical guide layers are constructed by, for example, stacking 23 periods of a supper lattice where each period includes a ZnSe (thickness: 1.1 nm) and Zn_0.6Mg_0.4S_0.5Se_0.5 (thickness: 3.4 nm). The ZnMgSSe quantum well layer is constructed by, for example, stacking 2 periods of a short period supper lattice (period: about 2 nm) where each period includes 3 molecular layers of ZnSe, 1 molecular layer of ZnS, 3 molecular layers of ZnSe, and 1 molecular layer of MgSe. The ZnMgSSe barrier layer is constructed by, for example, stacking 6 periods of a short period supper lattice (period: about 1 nm) where each period includes 1 molecular layer of ZnSe and 3 molecular layers of Zn_0.6Mg_0.4S_0.5Se_0.5. By adopting this structure, it is possible to form the layered structure from the n-ZnMgSSe cladding layer <b>105</b> through the p-ZnMgSSe cladding layer <b>109</b> shown in Figure <b>13</b> by simply controlling the opening/closing of the shutters of the heating evaporation sources, at least without interrupting the growth.

0132A wafer having the laser structure obtained through the above-described epitaxial growth process is further processed into a distributed reflection type semiconductor laser <b>80</b> shown in Figures <b>13</b> and <b>14A</b> as follows.

0133First, Pd and Au are sequentially vacuum-deposited over the entire surface of the wafer to form the p-electrode <b>114</b>. Then, the p-ZnMgSSe cladding layer <b>109</b> is partially etched away so as to form a mesa stripe having a width of 4 µm and a length of 600 µm, for example.

0134Either wet etching or dry etching can be adopted. In the case of wet etching, for example, the p-electrode <b>114</b> is etched with an aqueous solution of potassium iodide, followed by etching the II-VI group compound semiconductor layers with an etchant mixture containing a saturated aqueous solution of potassium dichromate and conc. H₂SO₄ at a volume ratio of 3:2. In the case of dry etching, Ar ion milling, a reactive ion etching with BCl₃, or the like can be employed.

0135Next, the distributed reflector region <b>202</b> having a length of 400 µm, for example, is formed on the p-ZnMgSSe cladding layer <b>109</b> immediately above the p-ZnMgSSe optical guide layer <b>108</b> and adjacent one end of the mesa stripe region (corresponding to the active region <b>201</b> in Figure <b>14A</b>). Figure <b>14B</b> is a plan view illustrating the laser structure shown in Figure <b>14A,</b> where the cladding layer <b>109</b> is in an exposed state. Thereafter, as shown in Figure <b>14C,</b> a diffraction grating is formed on the cladding layer <b>109</b> by performing the wet etching of the p-ZnMgSSe cladding layer <b>109</b> with using a resist mask formed by laser interference exposure. The diffraction grating can have a pitch of 185 nm based on secondary diffraction, for example. Alternatively, the diffraction grating can have a pitch of 277 nm based on tertiary diffraction, for example. The diffraction grating needs to be formed at least in the region <b>202</b>, and in the illustrative structure, the diffraction grating is formed over all the surface of the cladding layer <b>109</b> excluding the stripe including the p-electrode <b>114.</b>

0136Instead of the aforementioned chromic mixture acid, a bromine type etchant such as saturated bromine water or bromomethanol liquid can be used for the etching.

0137Figure <b>14D-1</b> is a cross-sectional view taken at line X-X' in Figure <b>14B,</b> and Figure <b>14D-2</b> is a cross-sectional view taken at line <b>Y-Y'</b> in Figure <b>14B.</b> Each of these figures illustrates the structure in a direction perpendicular to the cavity length direction where the diffraction grating has been formed over the cladding layer <b>109.</b>

0138Furthermore, as shown in Figures <b>14E-1</b> and <b>14E-2</b> which are also cross-sectional views taken at line <b>X-X'</b> and at line <b>Y-Y'</b> in Figure <b>14B,</b> respectively, after the formation of a resist mask on the stripe-shaped p-electrode <b>114,</b> the current constriction layer <b>115</b> (e.g., a ZnO layer) and the electrode <b>116</b> (e.g., an Au electrode) which covers the current constriction layer <b>115</b> are consecutively vacuum deposited. The thickness of the current constriction layer <b>115</b> is made substantially equal to the mesa height, so that the mesa stripe is buried therein. Thereafter, the p-electrode <b>114</b> is exposed by a lift-off technique. As the current constriction layer <b>115</b>, ZnOS or ZnMgSSe can be employed instead of ZnO. By optimizing its refractive index, it becomes possible to realize a high-efficiency waveguide coupling between the active region <b>201</b> and the distributed reflector region <b>202</b>.

0139An electrode for facilitating the mounting of the laser device can be optionally provided on the p-electrode <b>114</b> and/or the electrode <b>116.</b> In addition, Ni, Ge, and Au, for example, are vacuum deposited on the lower face of the n-GaAs substrate <b>101</b> to form the n-electrode <b>117</b>.

0140Thereafter, the wafer is cleaved, and a reflection film <b>203</b> having appropriate reflectance is formed on the outgoing end face (i.e., opposite the end face adjacent the distributed reflector) by vacuum deposition or sputtering. The reflection film <b>203</b> can be composed of a 1/4 wavelength multilayer film of a dielectric material which does not induce absorption even in the short wavelength region of about 400 nm, e.g., silicon oxide or titanium oxide. The reflectance is preferably set to be about 50% to 70% in order to reduce noises of the semiconductor laser induced by the returned light and to obtain an optical output of about 30 mW with low power consumption. Such reflectance can be obtained by, for example, forming one layer of a silicon oxide film having a thickness corresponding to 1/4 wavelength and a titanium oxide film having a thickness corresponding to 1/4 wavelength on the end face of the laser device.

0141On the end face of the laser device adjacent the diffraction grating, a low reflection or absorption film <b>204</b> is formed in order to suppress oscillation at a complex cavity mode. A low reflection film can be composed of a dielectric material such as silicon oxide, aluminum oxide, or titanium oxide. Substantially zero reflectance can be obtained by controlling the refractive index, thickness, and/or multilayer structure of the low reflection film. An absorption film can be composed of a semiconductor film having an appropriate thickness with a band gap sufficiently smaller than that of the waveguide layer of ZnSe, Si, or the like. The thickness of the semiconductor film is selected so that the light transmitted through the distributed reflector region <b>202</b> having a reflectance over 99% is sufficiently attenuated and substantially absorbed by the absorption film. For example, a Si film having a thickness of 400 nm can be formed by chemical vapor deposition.

0142The operation characteristics of the DBR semi-conductor laser <b>80</b> of the present example mounted in an optical pickup are as follows: oscillation wavelength of 427 nm; operation current of 95 mA; operation voltage of 5.6 V; aspect ratio of 2.4; and astigmatic deviation of 5 µm with an output power of 30 mW at an ambient temperature of 25°C. The DBR semiconductor laser <b>80</b> has a RIN level of -137 dB/Hz at an output power of 5 mW. The shift in the oscillation wavelength at an ambient temperature in the range of about -10°C to about 60°C is about 1.5 nm. The laser device stably operates with low noise even under the condition with the 5% returned light.

0143Next, an optical pickup incorporating the above semiconductor laser <b>80</b> is constructed. Figure <b>16</b> schematically shows a partial structure thereof.

0144The optical pickup at least includes a laser, an optical system, and an optical detection system. A polarization hologram <b>58a</b> is employed for the optical system. Specifically, the laser light emitted from the II-VI group semiconductor laser <b>80</b> (wavelength: 427 nm) is collimated through a collimation lens <b>51</b>, deflected by a prism <b>63</b>, led through a polarization hologram <b>58a</b> and a 1/4 wavelength plate <b>59,</b> and converged by a focus lens <b>52</b> so as to be radiated on pits formed in an optical disk medium (not shown). The signal light from the optical disk medium is collimated by the focus lens <b>52</b> and has its polarization direction turned by the 1/4 wavelength plate <b>59</b> by 90° relative to its polarization direction before being returned from the optical disk medium. Thereafter, the signal light passes the polarization hologram <b>58a</b> and deflected by the prism <b>63</b> so as to be converged on optical detectors <b>57</b>.

0145A portion of the laser light is separated by the prism <b>63</b> so as to enter a detector <b>64</b> for output monitoring, thereby controlling the power of the semiconductor laser <b>80</b>. The polarization hologram <b>58a,</b> the focus lens <b>52</b>, and the like are driven by a movable unit <b>62.</b>

0146The pits formed in an optical disk medium have a length of about 0.28 µm and a track pitch of about 0.50 µm. The experiments by the present inventors have shown that excellent HDTV images free of noise can be reproduced.

0147Thus, a low-noise optical pickup for high-density optical recording/reproduction having small wavelength variation can be realized by using the II-VI group semiconductor laser <b>80</b> of the present example. RF superimposition has also been shown to be effective for controlling returned light.

0148As described above, the optical pickup according to the present invention is capable of recording and/or reproducing data in pits provided at higher densities than conventional pit densities. By employing a wavelength-locked semiconductor laser (e.g., a DBR laser) as a light source, the noise of the optical pickup is minimized.

0149When optical recording is performed with a high-power semiconductor laser according to the present invention, the oscillation wavelength of the semiconductor laser has a very small variation due to wavelength locking when switching its operation from recording to reproduction. Therefore, problems due to the wavelength dependency of the optical characteristics (refractive index) of the lens material are prevented. This substantially eliminates the constraints on the lens material. Since the oscillation wavelength is locked, it is also possible to employ a diffraction type lens, etc.

0150By forming a tapered portion at the outgoing end face of the semiconductor laser so as to bring the beam aspect ratio to the proximity of 1, the effective transmission ratio can be greatly improved. As a result, a low-output laser can be used as the semiconductor laser, thereby realizing a prolonged lifetime for the device.

0151Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.