A semiconductor photo-diode.

Field of the Invention

This invention relates to a semiconductor avalanche photo-diode (referred to hereinafter as APD). More particularly, it relates to an APD capable of low noise as well as high speed operation.

Description of the Related Art

Long-distance, high-capacity optical transmission using a wave length of 1.0 to 1.7 µm, in which the optical transmission loss of a silica optical fiber becomes minimum, has been increasing in practical use. As an optical detector for this application, many types of APD devices have been developed. A typical one was the object of U.S. Patent aplication No. 037,000. In this prior application, as schematically illustrated in FIG. 1, where the numeral 2 denotes an n⁺-type GaSb substrate; the numeral 2 denotes an n-type GaSb light absorbing layer; the numeral 3 denotes an n-type Al_xGa_1-xSb avalanche multiplication layer, where x=0.065; the numeral 4A denotes an n⁻-type Al_xGa_1-xSb window layer 3; and the numeral 5 denotes a p⁺-type region doped in the window layer for forming a pn-junction, while some thickness of the window layer 4A is left between the pn junction 5′ and the avalanche multiplication layer 3; and a light to be detected is injected through the p⁺-type region 5 into the light absorbing layer 2. The above-described material of the avalanche multiplication layer 3 generates a high ionization rate of positive holes to that of electrons by a resonant impact ionization phenomena, resulting in an achievement of low noise and high speed operation of the APD. The resonant impact ionization is disclosed by O. Hilderbrand et al. in IEEE Journal of Quantum Electronics, vol. QE-17, No.2, p284-288, Feb. 1981.

Problems of the cited first prior art APD are hereinafter described. In FIG. 2, characteristics of quantum efficiency, which is an average number of electrons or positive holes photoelectrically emitted in a photoelectric material per incidental photon (i.e. a unit intensity of light) versus the incidental wave length, is presented. The incidental light having 1.3 µm wavelength produces both carriers, i.e. electrons and positive holes, in the avalanche multiplication layer 3 having x of 0.065, as indicated by a mark "M", even though the light should pass therethrough without producing carriers therein. The most important feature of employing the resonant impact ionization that reduces the noise generation is enjoyed when only the positive carriers are injected from the light absorbing layer 2 into the avalanche multiplication layer 3. Accordingly, the undesirable generation of both carriers i.e. electrons and positive holes, in the multiplication layer 3 deteriorates the pure generation of the resonant impact ionization, accordingly, deteriorates the low noise characteristics of the APD. The same problem still exists for a 1.55 µm wavelength incidental light as indicated by a mark "M'", through the amount of the produced carriers is much less.

Another prior art APD disclosed by the same assignee in Japanese unexamined patent publication Sho 60-105281 is schematically illustrated in FIG. 3, where the numeral 3-11 denotes a p⁺-type GaSb substrate; the numeral 3-12 denotes a p-type Ga_0.935Al_0.065Sb avalanche multiplication layer which generates a resonant impact ionization phenomena; the numeral 3-13 denotes a p-type Ga_0.6Al_0.4Sb window layer; the numeral 3-14 denotes an n⁺-type region doped in the window layer for forming a pn junction; the numeral 3-18 denotes an anti-reflection layer; and the numeral 3-19 and 3-20 denote electrodes for applying a bias voltage to the APD. The light to be detected is injected through the n⁺ region 3-15 into the light absorbing layer 3-13, and accordingly, does not have to pass through the avalanche multiplication layer 3-12. Therefore, the problem of the first prior art APD is avoided. However, other problems are thereby created. Because the pn junction is located far from the avalanche multiplication layer 3, fluctuations of the impurity density or the thickness, etc., of the layers therebetween in the fabrication process seriously affect the APD characteristics. Furthermore, there are difficulties in designing and fabricating the n⁺ region; for instance, with currently available diffusion source materials, such as sulfur or selenium, for forming the n⁺ region, it is difficult to achieve an adequate diffusion depth.

A third prior art APD is schematically illustrated in FIG. 4, where the same or like numerals denote the same or corresponding devices of FIG. 1. A substrate 1, and a light absorbing layer 2 are the same as those of FIG. 1. An avalanche multiplication layer 3′ is formed of the same material in the same way as in FIG. 1, however in a mesa shape. A peripheral portion of the mesa is filled with the same material as that of the window layer 4A, so as to form a guard-ring 4B. Onto the surface of the mesa-shaped window layer 3′ and an adjacent part of the guard-ring 4B, zinc is diffused approximately 1 um deep to form a p⁺-type region 5. The p⁺-type region 5 forms a pn junction 5′ in the avalanche multiplication layer 3′ as well as in the guard-ring 4B. A light to be detected is injected through the p⁺-type region 5 into the light absorbing layer 2. The problem of the third prior art APD is the same as that of the first prior art APD shown in FIG. 1.

SUMMARY OF THE INVENTION

It is a general object of the invention to provide an APD having low noise and high speed characteristics.

It is another object of the invention to provide an APD having an improved quantum efficiency.

It is still another object of the invention to provide a wide margin in designing an APD structure.

According to the present invention, a semiconductor substrate formed of an n⁺-type Al_xGa_1-xSb or Al_xGa_1-x­Sb_yAs_1-y, having x of 0.1 to 0.3 and 0 < y < 1, allows a light to pass therethrough, without producing carriers therein, into a light absorbing layer formed on the semiconductor substrate of an n-type GaSb, where the injected light produces carriers. The produced positive carriers are injected into an avalanche multiplication layer formed of an n-type Al_xGa_1-xSb or Al_xGa_1-xSb­_yAs_1-y, having x of approximately 0.065 and 0 < y < 1, on the light absorbing layer. A P⁺-type region is doped in the avalanche multiplication layer or in a surface layer formed of an n-type Al_xGa_1-xSb or Al_xGa_1-xSb_yAs_1-y, having x of 0.1 to 0.3 and 0< y < 1, on the avalanche multiplication layer, so as to form a pn junction.

Aluminum content of the material of the avalanche multiplication layer is chosen so as to produce a high ionization rate ratio, by a resonant impact ionization phenomena, generated with only the positive carrier by an electric field induced from the pn junction being located close (i.e. directly or only via the surface layer) to the avalanche multiplication layer; therefore, resulting in a low noise and high speed operation of the APD.

The above-mentioned features and advantages of the present invention, together with other objects and advantages which will become apparent, will be more fully described hereinafter, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none"><li>FIG. 1 schematically illustrates a prior art APD.</li><li>FIG. 2 shows quantum efficiency versus incidental wave length for various alumnium content ration of Al_xGa_1-xSb.</li><li>FIG. 3 schematically illustrates a second prior art APD.</li><li>FIG. 4 schematically illustrates a third prior art APD.</li><li>FIG. 5 schematically illustrates a cross-sectional side view of a preferred embodiment of the present invention.</li><li>FIG. 6 is a graph of the effect of aluminum content on ionization rate ratio.</li><li>FIG. 7 schematically illustrates a cross-sectional side view of a second preferred embodiment of the present invention.</li><li>FIG. 8 shows a comparison of noise figures achieved by an APD of the present invention with a prior art APD.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 5, a preferred embodiment of the present invention is hereinafter described in detail. A semiconductor substrate 1A is formed approximately 350 um thick n⁺-type Al_xGa_1-xSb (aluminum gallium antimony), where x is approximately from 0.1 to 0.3, doped with, for example, tellurium having a concentration, typically 10¹⁹cm⁻³, of an impurity such as LPE (liquid phase epitaxy), for which a method disclosed in U.S. Patent No. 4,620,897, "Method for Growing Multi-Component Compound Semiconductor Crystals" dated November 4, 1986 by K. Nakajima, is preferably used. With this method, a source can be automatically supplemented into the melt to form a thick substrate. A light absorbing layer 2 is formed of an n-type GaSb (gallium antimony) doped typically with tellurium having concentrate of an impurity such as LPE, typically of approximately 5 x 10¹⁵cm⁻³, as thick as approximately from 2 to 3 µm, on the semiconductor substrate 1A by a widely known method. An avalanche multiplication layer 3 is formed of an n-type Al_xGa_1-xSb, where x is approximately from 0.02 to 0.1, typically approximately 0.065, doped typically with tellurium having a concentration of an impurity such as LPE, typically 5 x 10¹⁵cm⁻³, as thick as approximately 1 µm on the light absorbing layer 2 by a widely known method. A surface layer 4A is formed of an n⁻-type Al_xGa_1-xSb, where x is from 0.1 to 0.3, doped typically with tellurium having a concentration of an impurity such as LPE of 5 x 10¹⁵cm⁻³ maximum, typically 2 x 10¹⁵cm⁻³, as thick as approximately from 2 to 3 µm on the avalanche multiplication layer 3 by a widely known method. At the central surface of the surface layer 4A, a p⁺-type region 5 having an impurity concentration, typically approximately 10¹⁹cm⁻³ is formed approximately 1 um deep for forming a pn junction 5′ in the surface layer 4A, while an approximately 1 µm thickness of the surface layer 4A is left between the pn junction 5′ and the avalanche multiplication layer 3. The p⁺-type region 5 is formed by a widely known method, such as diffusion of zinc or ion implantation of beryllium. An electrode 6 is formed by depositing typically Au/Zn/Au (gold, zinc and gold) as thick as 360/60/360 Å for each material on the p⁺-type region 5, and further depositing thereon typically Au/Ti (gold and titanium) as thick as 2700/300 Å for each material, by a widely known method, such as sputtering, for applying a negative side of a bias voltage to the APD. A ring shaped electrode 7 is formed by depositing typicaly Au/AuGe (gold and gold-germanium alloy) as thick as 2700/300 Å for each material, on a surface, opposite to the light absorbing layer 2, of the substrate 1A, for connecting a return of the bias voltage. Through an opening at the centre portion of the electrode 7, the incidental light is injected into a portion to receive the light on the substrate 1A.

As represented in FIG. 2 by curve (1) for the material having x of 0.3 used for the substrate, the quantum efficiency of the wave length now in consideration for a practical communication use, i.e. mainly 1.3 and 1.55 µm, is essentially zero, in other words, the incidental light passes through the substrate while producing essentially no carriers therein. The light having passed through the substrate 1A enters into the light absorbing layer 2, where sufficient carriers are produced for a wide range of wave lengths, i.e. for both 1.3 and 1.55 µm wave lengths, as represented by curve (2) having sufficient quantum efficiency in FIG. 2. The carriers produced in the light absorbing layer 2 are both the positive holes ph and electrons e as denoted in FIG. 3. The produced positive hole ph is accelerated by the electric field induced by the applied bias voltage into the avalanche multiplication layer 3.

For the avalanche multiplication layer, the aluminum content x is particularly chosen so that an ionization rate ratio β/α indicates an ionization rate of the positive holes produced therein and α indicates an ionization rate of the electrons produced therein) is remarkably increased. This is known as resonant impact ionization.

General idea of the resonant impact ionization is further explained hereinafter. Resonant impact ionization is a phenomena in which the spin orbit splitting Δ of the valence band becomes equal (thus called resonant) to the band gap energy E_g between the conduction band and the valence band. A large value of the ionization rate ratio β/α is obtained even at a lower voltage than that of the conventional avalanche break down which does not employ the resonant impact ionization. As is well known, the greater the value of the ionization rate ratio the greater the reduction in the fluctuation noise produced in the APD, and the faster the operation of the APD.

The particular aluminum content ratio optimum for the maximum ionization rate ratio varies from 0.02 ato 0.1, typically 0.065 as shown in FIG. 6.

Owing to a sufficient electric field intensity, such as approximately 4 x 10⁴ V/cm, in the avalanche multiplication layer 3 and induced by an applied voltage as low as, for example, 12.5 V on the pn junction 5′ located close the the avalanche multiplication layer (in other words, separated only by a 1 um thick layer of the surface layer 4A), and owing to the resonant impact ionization, the ionization rate ratio can be as high as 20 or more.

Band gap energy of the substrate 1A is more the 1 eV. Band gap energy of the light-absorbing layer is typically 0.72 eV. Band gap energy of the avalanche multiplication layer 3 is typically 0.8 to 0.83 eV. Because an absorption edge of the quantum efficiency curve is inversely proportional to the band gap energy of the material as shown in FIG. 2, the high band gap energy of the substrate 1A allows the light to pass therethrough while generating no carriers therein. The band gap energy of the avalanche multiplication layer 3 allows for the generation of the resonant impact ionization.

Because the surface layer 4A, formed of Al_xGa_1-xSb, where x is from 0.1 to 0.3, has a lower impurity concentration as well as a lower ionization rate than the avalanche multiplication layer 3, resulting in a relatively high avalanche breakdown voltage, for example, more than 50 V, and also in the reduction of the concentration of electric field at peripheral portion 5˝ of the pn junction, the surface layer 4A contributes to reduce a tunnel current flowing therethrough, i.e. a dark current of the APD, even though a guard-ring, which has been widely employed in APD's in order to reduce the peripheral electric field concentration, is not provided in the APD of the embodiment shown in FIG. 4.

A second preferred embodiment of the present invention is hereinafter described, referring to FIG. 7, where the same or like numerals denote the same or corresponding devices. The substrate 1A and the light absorbing layer 2 are essentially the same and act respectively the same as those of the first preferred embodiment shown in FIG. 5. An avalanche multiplication layer 3˝ is formed of the same material as that of the first embodiment and as thick as, for example, 0.5 to 1.0 um on the light absorbing layer 2 in essentially the same way as that of FIG. 5. Next, a peripheral portion of the deposited avalanche multiplication layer 3˝ is removed by a chemical etching so as to leave a mesa of the avalanche multiplication layer 3˝, while the centre portion is masked with a resist mask (not shown in the figure). Next, a guard-ring 4B is formed on the etched peripheral portion by filling with an n⁻-type Al_xGa_1-xSb having x of approximately 0.3 doped with, for example, tellurium of as low impurity concentration as 5 x 10¹⁵cm⁻³ maximum, typically 2 x 10¹⁵cm⁻³, typically by a widely used LPE technique, while the mask is left on the mesa. Next, after removing the mask from the top of the mesa, a positive dopant is doped into the surfaces of the avalanche multiplication layer 3˝ as well as the adjacent part of the guardring 4B approximately 1 um deep typically by diffusing beryllium or ion implanting zinc, to a concentration of as much as typically 10¹⁹cm⁻³, as denoted with the hatched area 5. The p⁺ region 5 forms a pn junction 5′ in the avalanche multiplication layer 3˝ and the guard-ring 4B. However, because the peripheral portion 5˝ of the pn junction is located in the low impurity-doped guard-ring 4B having a low ionization rate, the undesirably low voltage avalanche breakdown caused by an electric field concentration at the peripheral portion can be prevented. Accordingly, the avalanche breakdown takes place only at the centre portion having no undesirable electric field concentration. The direct contact of the pn junction with the avalanche multiplication layer 3˝ is advantageous for inducing a high electric field intensity in the avalanche multiplication layer 3˝.

As shown in FIG. 8, at the multiplication factor of 10, an APD of the present invention achieved a noise as low as 3.8 db compared with 6 to 9 of a prior art APD.

Mismatching of lattice constant of the crystals of the Al_xGa_1-xSb family with the GaSb light absorbing layer 2 increases slightly with the increased x value. Accordingly, the Al_xGa_1-xSb for the layers 1A, 3 and/or 4 may be replaced with Al_xGa_1-xSb_yAs_y-1 (aluminum gallium antimony arsenic) in order to achieve better matching of the lattice constant with the GaSb. Values of y are chosen: for example, 0.01 to 0.03 in the substrate; typically from 0.002 to 0.01 in the avalanche multiplication layer; and typically from 0.01 to 0.03 in surface layer 4A or the guard-ring 4B. The value of the content ratio y has essentially no effect on the above-described characteristics having various values of aluminum content x.

Though in the preferred embodiments of the present invention the electrode 7 is formed in a ring shape so that the light is injected through the opening into the substrate 1A, it is apparent that the structure of the electrode 7 is arbitrarily chosen as long as the bias voltage is properly applied to the APD device.

Owing to the fact that in the APD structure of the invention the light is injected from a side opposite the avalanche multiplication layer through a transparent substrate, a deterioration of quantum efficiency of the APD is prevented, resulting in an achievement of a low noise and fast operation of the APD. Furthermore, owing to the close location of the pn junction to the avalanche multiplication layer, the design of the electric field in the avalanche multiplication layer is facilitated as compared with the prior art APD structures.