Nitride semiconductor device

The present invention relates to a nitride semiconductor device, and more specifically, a nitride semiconductor light emitting device such as a laser diode (LD) device and a light emitting diode (LED) device, and a solar cell. Such as light receiving devices, especially nitride semiconductor light emitting devices.

[0002] Nitride semiconductors can have a bandgap energy of 1.95 to 6.0 eV depending on their composition, and therefore semiconductors such as light emitting diode (LED) devices and laser diode (LD) devices. It has been attracting attention as a material for light emitting devices. Recently, high-brightness blue LED devices and green LED devices have been put into practical use using this nitride semiconductor material. These LED devices have a double heterostructure with a pn junction and both have an output of over 1 mW.

[0003] Conventional LED devices basically have a double heterostructure in which an active layer made of InGaN is sandwiched between an n-type and a p-type clad layer made of AlGaN. An n-type contact layer made of GaN is formed in the n-type clad layer, and a p-type contact layer made of GaN is formed in the p-type clad layer. This laminated structure is provided on a substrate made of, for example, sapphire.

[0004] The LD device may basically have the same structure as the LED device. However, especially in the case of LD devices, a separate confinement type structure in which light and carriers are confined separately is often used. A separate confinement type LD device for a nitride semiconductor is disclosed in, for example, Japanese Patent Application Laid-Open No. 6-21511. In this publication, an InGaN active layer sandwiched between two optical guide layers, one of which is made of n-type GaN and the other of which is made of p-type GaN, is provided, and a carrier made of n-type AlGaN is provided on the n-type optical guide layer. A light emitting device having a separate confinement structure in which a confinement layer is formed and another carrier confinement layer made of p-type AlGaN is formed on the p-type optical guide layer is disclosed.

[0005] By the way, in a semiconductor device having a normal double hetero structure, a first clad layer having a bandgap energy larger than that of the active layer is provided in contact with the active layer, and is in contact with the first clad layer. , A second clad layer having a bandgap energy larger than that of the first clad layer is provided. This is to ensure that electrons and holes are efficiently injected into the active layer according to the energy level.

Similarly, in the case of a nitride semiconductor LD device, clad layers such as an optical guide layer and a carrier confinement layer (optical confinement layer) are sequentially formed so that the bandgap energy gradually increases in contact with the active layer. (For example, see the above publication).

[0007] However, it has been found that conventional nitride semiconductor devices having an active layer containing indium, particularly LD devices, have low luminous efficiency in the above structure. In particular, it was found that when the device temperature rises as the current applied to the device increases, the luminous efficiency drops sharply.

[0008] Therefore, an object of the present invention is to provide a nitride semiconductor device having an active layer including a nitride semiconductor containing indium and having high luminous efficiency. Another object of the present invention is to provide a nitride semiconductor device in which the luminous efficiency does not decrease even if the device temperature rises.

[Means for Solving the Problems] In order to solve the above problems, the present invention relates to a separation and confinement type nitride semiconductor laser diode in which an optical guide layer is formed between an active layer and a clad layer. The active layer has a quantum well structure having a well layer made of a nitride semiconductor containing In, and the p side of the active layer is made of a nitride semiconductor containing Al, and the film thickness is 500 angstroms or less.<u style="single">In contact with the active layer</u>The first nitride semiconductor layer and In_xGa_1-xIt consists of a second nitride semiconductor layer consisting of N (0 x 1) and acts as an optical guide layer, and a nitride semiconductor containing Al, which acts as a clad layer.<u style="single">, In contact with the second nitride semiconductor layer</u>Provided is a nitride semiconductor laser diode characterized in that a third nitride semiconductor layer is sequentially laminated.

[0010] In the nitride semiconductor device of the present invention, the active layer is sandwiched between a layer structure that should ultimately contact the positive electrode and a layer structure that should ultimately contact the negative electrode. In the following description, the side on which the layer structure that should ultimately contact the positive electrode is formed is referred to as the p side, and the side on which the layer structure that should ultimately contact the negative electrode is formed may be referred to as the n side. ..

[0011] The first nitride semiconductor layer preferably has a thickness of 0.1 μm or less in order to have a thickness sufficiently thin so that carriers can tunnel, but from the viewpoint of good laser oscillation, 500 It shall be less than or equal to Angstrom. Generally, the first nitride semiconductor layer preferably has a thickness of at least 10 angstroms or more.

[0012] Also<u style="single">As a separate invention from this case,</u>First clad layer made of n-type nitride semiconductor; provided on the first clad layer, made of a nitride semiconductor containing indium and gallium, having a thickness of 70 angstrom or less, and above the base layer. It is an active layer of a quantum well structure including at least one well layer provided in a lattice-mismatched state with respect to the formation, and the well layer is an active layer containing a plurality of indium-rich regions and indium-poor regions. And there are nitride semiconductor devices provided on the active layer and provided with a second clad layer made of a nitride semiconductor doped with acceptor impurities.

[0013] In the present invention, when a nitride semiconductor is broadly referred to, a nitride of a Group 3 element of the periodic table, more specifically, the formula In._x Al_y Ga_1-xy It refers to a nitride semiconductor represented by N (0 x 1, 0 y 1, 0 x + y 1).

[0014] The present inventors have studied a decrease in luminous efficiency with increasing temperature, particularly in a nitride semiconductor device having an active layer containing indium. As a result, the main cause of the decrease in luminous efficiency is that indium-containing nitride semiconductors, especially InGaN, have the property of being less likely to grow than aluminum-containing nitride semiconductors or gallium nitride (GaN). It turned out to be in. That is, InN and GaN constituting InGaN have significantly different decomposition temperatures, and InGaN tends to be phase-separated into InN and GaN during growth, and an active layer having a uniform composition can be obtained by increasing the indium content. Hateful. Therefore, in the conventional nitride semiconductor having an InGaN active layer, the indium content tends to be kept low.

[0015] When an optical guide layer made of GaN is formed in contact with an InGaN active layer having such a low indium content, the band offset between the active layer and the optical guide layer becomes extremely small. This will be described with reference to FIG. 6, which is a diagram showing an energy band corresponding to a conventional nitride semiconductor light emitting device. As shown in FIG. 6, in the conventional nitride semiconductor device, the bandgap energy of the optical guide layer (GaN) that directly sandwiches the InGaN active layer is not so large as compared with the bandgap energy of the active layer (InGaN). (Because the content of In in InGaN is low, the InGaN composition is close to the GaN composition). Therefore, when the temperature of the device rises as the current value applied to the semiconductor device increases, the electrons and holes injected into the active layer from the n-layer and p-layer, respectively, are recombined due to the influence of the thermal energy. Before emitting light (hν), the electrons and holes overflow the active layer to the guide layer (GaN) on the opposite side of the injection side, that is, the electrons are in the p-type optical guide layer and the holes are in the p-type optical guide layer. It reaches the n-type optical guide layer. As a result, the luminous efficiency is low in the conventional structure, and the efficiency is particularly lowered when the temperature rises.

[0016] Therefore, in the nitride semiconductor device of the present invention, two first layers (first p-side layer and first p-side layer) formed in contact with an active layer including a nitride semiconductor containing indium and sandwiching the active layer. The n-side layer) is formed of a nitride semiconductor having a bandgap energy larger than that of the active layer. These first layers need only have a bandgap energy larger than that of the active layer, preferably the two first layers have a bandgap energy 0.01 to 4.05 eV greater than that of the active layer. Have. The presence of the first layer with such a large bandgap energy prevents electrons or holes injected into the active layer from overflowing the active layer. Then, each first layer preferably has a bandgap energy smaller than the bandgap energy of the first layer in contact with the first layer, but preferably has a bandgap energy larger than the bandgap energy of the active layer. Two second layers (second p-side layer and second n-side layer) formed of a nitride semiconductor are provided. These second layers need only have a bandgap energy smaller than the bandgap energy of the first layer, and preferably has a bandgap energy 0.01 to 4.05 eV smaller than the bandgap energy of the first layer. In addition, on each second layer<u style="single">, Ko</u>In contact with this, two third layers (third p-side layer and third n-side layer) formed of a nitride semiconductor having a bandgap energy larger than that of the second layer are formed. Provide. These third layers need only have a bandgap energy larger than the bandgap energy of the second layer, preferably 0.01 to 4.05 eV higher bandgap energy than the bandgap energy of the second layer. Has. Thus, the electrons or holes injected from the third layer side are efficiently injected into the second layer, which has a smaller bandgap energy, but are active due to the higher bandgap energy of the first layer. Injection into the layer tends to be blocked by the first layer. Therefore, in the present invention, the first layer is formed thin enough that electrons or holes can penetrate the first layer by the tunnel effect (tunneling). In this way, electrons or holes are efficiently injected from the third layer to the active layer. Thus, in the device of the invention, electrons and holes are efficiently injected from the third layer into the active layer, and electrons or holes are blocked by the first layer opposite the injection side, even if The active layer does not overflow even if the device temperature rises. Regarding the bandgap energy of the nitride semiconductor, the bandgap energy of AlN is 6.0 eV, the bandgap energy of GaN is 3.4 eV, and the bandgap energy of InN is 1.95 eV.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below with reference to FIGS. 1 to 5. Throughout these figures, the same elements / members are indicated by the same reference numerals. FIG. 1 is a schematic cross-sectional view showing a structure of an LD device according to the first aspect of the present invention. In this LD device, the three-layer structure of the present invention is provided on the p side.

In the LD device shown in FIG. 1, an n-type contact layer 13, an n-type carrier confinement layer (optical confinement layer) 14, an n-type optical guide layer 15, and an active layer are placed on a substrate 11 via a buffer layer 12. 16, the first p-side nitride semiconductor layer 101 having a larger band gap energy than the active layer 16, the second p-side nitride semiconductor layer 102 having a smaller band gap energy than the first p-side nitride semiconductor layer, A nitride semiconductor laminated structure including a third p-side nitride semiconductor layer 103 and a p-type contact layer 17 having a band gap larger than that of the second p-side nitride semiconductor layer 102 is provided. A current constriction layer 18 provided with a contact hole 18a is provided on the p-type contact layer 17. A negative electrode 19 is provided on the exposed surface of the n-type contact layer 13, and a positive electrode 20 is provided on the current constriction layer 20. The positive electrode 20 is in contact with the p-type contact layer 17 through the contact hole 18a of the current constriction layer 18.

The substrate 11 is a spinel (MgAl).₂ O₄ ), Sapphire (Al₂ O₃ , A-plane, R-plane, C-plane included), SiC (including 6H, 4H, 3C), ZnS, ZnO, GaAs, GaN, etc. ..

[0021] The buffer layer 12 can be formed of AlN, GaN, AlGaN, or the like, and can be formed at a temperature of 900 ° C. or lower and having a thickness of several tens of angstroms to several hundreds of angstroms. The buffer layer 12 is formed to alleviate the lattice constant mismatch between the substrate 11 and the nitride semiconductor layer formed on the substrate 11. Therefore, when using a substrate lattice-matched with the nitride semiconductor, a substrate having a lattice constant close to the lattice constant of the nitride semiconductor, or the like, it may be omitted depending on the growth method of the nitride semiconductor or the like.

The n-type contact layer 13 is made of a nitride semiconductor, particularly GaN, In._a Ga_1-a It is preferably formed with N (0 &lt;a &lt;1). (In this specification, In_a Ga_1-a Nitride semiconductors represented by N (0 &lt;a &lt;1) or similar expressions are sometimes referred to simply as InGaN). In particular, when the n-type contact layer 13 is formed of Si-doped GaN, an n-type layer having a high carrier concentration can be obtained, and preferable ohmic contact with the negative electrode 19 can be obtained, so that the threshold current of the laser element is lowered. be able to. The thickness of the n-type contact layer 13 is not particularly limited, but it can usually be formed with a thickness of 0.1 μm to 5 μm.

[0023] Since the negative electrode 19 formed on the exposed surface of the n-type contact layer 13 can obtain preferable ohmic contact with the n-type contact layer 13, Al, Ti, W, Cu, Zn, Sn, It is preferably formed of a metal such as In or an alloy thereof.

[0024] The n-type carrier confinement layer 14 and the n-type optical guide layer 15 formed on the n-type carrier confinement layer 14 are each formed of an n-type nitride semiconductor. In the embodiment shown in FIG. 1, the n-type optical guide layer 15 has a bandgap energy larger than the bandgap energy of the active layer 16, and the n-type carrier confinement layer 14 has the bandgap energy of the n-type optical guide layer 15. Has greater bandgap energy than. It is desirable that the n-type carrier confinement layer 14 is usually formed with a thickness of 0.1 μm to 1 μm, and the n-type optical guide layer 15 is usually formed with a thickness of 100 angstroms to 1 μm.

The active layer 16 formed on the n-type optical guide layer 15 has a quantum well structure (that is, a single quantum well structure or a multiple quantum well structure), and this quantum well structure is n-type optical. An indium-containing nitride semiconductor having a bandgap smaller than the bandgap of both the guide layer 15 and the first p-side nitride semiconductor layer 101, that is, In._d Al_e Ga_1-de It has a well layer consisting of N (0 &lt;d 1, 0 e 1, 0 &lt;d + e 1). Preferably, the well layer is a ternary mixed crystal In._f Ga_1-f It is formed by N (0 &lt;f &lt;1). Since the ternary mixed crystal InGaN provides a layer having better crystallinity than the quaternary mixed crystal one, the emission output is improved.

[0026] Among them, the active layer 16 has a multiple quantum well structure (minimum three-layer structure) in which a well layer made of InGaN and a barrier layer made of a nitride semiconductor having a bandgap energy larger than that of the well layer are alternately laminated. ) Is particularly preferable. In the present invention, the multiple quantum well structure includes a well layer as a lowermost layer provided directly on an n-type layer such as an n-type optical guide layer 15, and a p-type layer such as the first p-side nitride semiconductor 101 described below. Even if the structure has a well layer as the uppermost layer in direct contact with the n-type optical guide layer 15, a barrier layer as the lowermost layer provided directly on the n-type layer such as the n-type optical guide layer 15 and a first p-side nitride semiconductor. The structure may have a barrier layer as the uppermost layer that is in direct contact with the p-type layer such as layer 101. The nitride semiconductor forming the barrier layer includes GaN, AlGaN and the like. However, the barrier layer is a ternary mixed crystal In, similar to the well layer._f'Ga_1-f'It is particularly preferable to form N (0 &lt;f'&lt;1, but f'&lt;f). When the active layer 16 has a multiple quantum well structure in which InGaN layers having different bandgap energies are laminated in this way, the indium mole fraction of the active layer 16 is changed, or the first or third n-side or p-side nitride described below is used. By changing the aluminum mole fraction of the semiconductor layer, it is possible to realize a high-power LD device with a wavelength of about 365 nm to 660 nm by quantum level emission. Further, when the InGaN barrier layer is laminated on the well layer, the crystal of the InGaN barrier layer is softer than that of AlGaN, so that the thickness of, for example, the AlGaN layer formed on the InGaN barrier layer is increased without causing cracks. Therefore, excellent laser oscillation can be realized.

[0027] In a multiple quantum well structure, it is particularly desirable that the well layer has a thickness of 70 angstroms or less and the barrier layer has a thickness of 150 angstroms or less. On the other hand, it is particularly desirable that the active layer having a single quantum well structure composed of one quantum well layer has a thickness of 70 angstroms or less. The lower limit of the thickness of both the well layer and the barrier layer is preferably 5 angstroms.

[0028] The active layer 16 may not be doped with impurities (non-doped), or the well layer and / or the barrier layer may be doped with impurities (acceptor impurities and / or donor impurities). Particularly preferable active layers 16 are non-doped active layers and silicon or germanium-doped active layers, and among the impurity-doped active layers, particularly preferable ones are silicon-doped active layers. In particular, when the active layer is doped with silicon, the threshold current tends to decrease in the LD device. During the growth of the nitride semiconductor that should form the active layer, the silicon dope can be used as a raw material gas, for example, an organic silicon gas such as tetraethylsilane, a hydrided silicon gas such as silane, or a halogenated silicon gas such as silicon tetrachloride. Etc. can be added.

[0029] The first p-side nitride semiconductor layer 101 provided in contact with the active layer 16 is formed of a nitride semiconductor having a bandgap energy larger than that of the active layer 16 (more strictly, its well layer). Has been done. Particularly preferably, the first nitride semiconductor layer is a nitride semiconductor containing Al, that is, In._g Al_h Ga_1-gh It is formed by N (0 g 1, 0 &lt;h 1, 0 &lt;g + h 1), and is particularly preferably a ternary mixed crystal Al._j Ga_1-j It is formed by N (0 &lt;j &lt;1). (In this specification, Al_j Ga_1-j Nitride semiconductors represented by N (0 &lt;j &lt;1) or similar expressions are sometimes referred to simply as AlGaN).

[0030] The first p-side nitride semiconductor layer 101 is preferably i-type or p-type. In particular, AlGaN can easily obtain a p-type having a high carrier concentration, and by forming it in contact with an active layer 16 including a well layer containing InGaN, an element having a high emission output can be obtained.

[0031] In the present invention, in order to make a nitride semiconductor (including a nitride semiconductor in an active layer) p-type, acceptor impurities such as Mg, Zn, C, Be, Ca, and Ba are added during crystal growth. Obtained by doping. Acceptor impurity concentration is 1x10¹⁷~1×10²²/cm³ Is preferable. Especially when the acceptor impurity is magnesium, its concentration is 1x10.¹⁸~1×10²⁰/cm³ , Especially 1x10¹⁹~1×10²⁰/cm³ Is particularly preferable. In any case, in order to obtain a p-layer having a high carrier concentration, it is more desirable to dope the acceptor impurities and then annealing (heat treatment) at 400 ° C. or higher in an inert gas atmosphere. Annealing usually results in 1 × 10 for Mg-doped p-type AlGaN.¹⁷~1×10¹⁹/cm³ Carrier concentration is obtained. To obtain an i-type nitride semiconductor, for example, Al_j Ga_1-j By growing a nitride semiconductor having a value of 0.5 or more in N, an i-type nitride semiconductor can be obtained without doping with acceptor impurities. Further, the i-type nitride semiconductor is an acceptor that either doping the p-type nitride semiconductor layer with a donor impurity sufficient to compensate for its hole carrier concentration or compensating the n-type nitride semiconductor layer with its electron carrier concentration. It can also be obtained by doping with impurities.

[0032] The thickness of the first nitride semiconductor layer 101 is preferably thin enough for carriers to tunnel the first nitride semiconductor layer 101. More specifically, it is desirable that the semiconductor layer 101 has a thickness of 0.1 μm or less, more preferably 0.05 μm (500 angstroms) or less, and most preferably 0.03 μm (300 angstroms) or less. When the thickness of the semiconductor layer 101 is reduced in this way, cracks can be prevented from occurring in the first p-side nitride semiconductor layer 101, and a nitride semiconductor layer having good crystallinity can be grown. Further, the thinner the AlGaN having a larger ratio of Al, the easier it is for laser oscillation. For example, Al with a value of j greater than or equal to 0.2_j Ga_1-j When N is used, it is desirable to form the semiconductor layer 101 with a thickness of 500 angstroms or less. The lower limit of the thickness of the first p-side nitride semiconductor layer 101 is not particularly limited, but it is desirable to form the first p-side nitride semiconductor layer 101 with a film thickness of 10 angstroms or more.

The second p-side nitride semiconductor layer 102 has a bandgap energy smaller than the bandgap energy of the first p-side nitride semiconductor layer 101, and the first p-side nitride semiconductor layer 101 It is located farther from the active layer than the active layer, and most preferably formed in contact with the first p-side nitride semiconductor layer 101 as shown in FIG. The second p-side nitride semiconductor layer 102 is preferably In._k Ga_1-k It is formed by N (0 k 1), and is particularly preferably formed by GaN or InGaN. When the second p-side nitride semiconductor layer 102 is formed of GaN or InGaN, a second semiconductor layer 102 having good crystallinity with less cracking even if it is formed relatively thick can be obtained. The second p-side nitride semiconductor layer 102 preferably has a thickness of 0.01 μm to 5 μm, more preferably 0.02 μm to 1 μm, and can act as a preferable optical guide layer in a thickness in this range, for example. The second p-side nitride semiconductor layer 102 contains acceptor impurities and is preferably p-type.

[0034] Further, the second p-side nitride semiconductor layer 102 particularly formed of InGaN or GaN also acts as a buffer layer for growing the third p-side nitride semiconductor layer 103, which will be described later. InGaN or GaN has a softer crystal than AlGaN. Therefore, a second p-side nitride semiconductor layer made of InGaN or GaN between the first p-side nitride semiconductor layer 101 having a bandgap larger than that of the active layer and the third p-side nitride semiconductor layer 103. The presence of 102 prevents cracks from occurring in the third p-side nitride semiconductor layer 103, thereby making the third p-side nitride semiconductor layer 103 into the first p-side nitride semiconductor layer. It can be formed thicker than 101.

[0035] The third p-side nitride semiconductor layer 103 has a bandgap energy larger than the bandgap energy of the second p-side nitride semiconductor layer 102, and is more than the second p-side nitride semiconductor layer 102. Is also located away from the active layer and most preferably formed in contact with the second p-side nitride semiconductor layer 102 as shown in FIG. The third p-side nitride semiconductor layer 103 is a nitride semiconductor containing Al, that is, In._m Al_n Ga_1-mn It is preferably formed with N (0 m 1, 0 &lt;n 1, 0 &lt;m + n 1), and particularly preferably with a ternary mixed crystal AlGaN.

[0036] The third p-side nitride semiconductor layer 103 is required to have a bandgap energy larger than the bandgap energy of the second p-side nitride semiconductor layer 102. This is because the third p-side nitride semiconductor layer 103 acts as a carrier confinement layer and a light confinement layer. The third p-side nitride semiconductor layer 103 preferably has a thickness of 0.01 μm or more and 2 μm or less, more preferably 0.05 μm or more and 1 μm or less, and has good crystallinity within this thickness range. It can act as a carrier confinement layer. The third p-side nitride semiconductor layer 103 contains acceptor impurities and is preferably p-type.

[0037] The p-type contact layer 17 formed on the third p-side nitride semiconductor layer 103 is formed of a p-type nitride semiconductor. In particular, when the p-type contact layer 103 is formed of InGaN or GaN, especially Mg-doped p-type GaN, the p-type layer with the highest carrier concentration is obtained, achieving good ohmic contact with the positive electrode, thereby achieving a threshold. The current can be reduced.

[0038] The positive electrode 20 is preferably formed of a metal having a relatively high work function such as Ni, Pd, Ir, Rh, Pt, Ag, Au, or an alloy thereof, preferably, in order to obtain ohmic contact. ..

[0039] The current constriction layer 18 is formed of an insulating material, preferably silicon dioxide. This current constriction layer 18 can be omitted. By the way, in FIG. 1, the n-type carrier confinement layer 14 is formed on the n-type contact layer 13 via the crack prevention layer 30.

[0040] That is, the nitride semiconductor containing aluminum has a property that cracks are likely to occur in the grown crystal when the thickness is increased. In particular, it is difficult to grow an n-type aluminum-containing nitride semiconductor directly thick without causing cracks on the GaN layer or AlGaN layer. For example, a nitride semiconductor containing aluminum is formed on an n-type contact layer 13 formed of n-type GaN or the like by forming an n-type layer having a thickness of, for example, 0.1 μm or more, such as an n-type carrier confinement layer 14. In particular, it is difficult to form with AlGaN. Therefore, a nitride semiconductor containing indium, preferably In, is used as the crack prevention layer 30 on the n-type contact layer 13._p Ga_1-p After forming an n-type layer made of N (0 &lt;p 1), an n-type carrier confinement layer 14 made of an n-type aluminum-containing nitride semiconductor is formed. Due to the presence of the crack prevention layer 30, the n-type carrier confinement layer 14 can be grown to a desired thickness (for example, 0.1 μm or more) without cracking. The crack prevention layer 30 preferably has a thickness of 100 angstroms or more and 0.5 μm or less. The crack prevention layer 30 has the same effect even inside the n-type contact layer 13 layer.

FIG. 2 is a cross-sectional view schematically showing a nitride semiconductor LD device according to the second aspect of the present invention, and the same reference numerals as those in FIG. 1 indicate the same members. Referring to FIG. 2, on the substrate 11, the n-type contact layer 13, the crack prevention layer 30, the third n-side nitride semiconductor layer 203, and the second n-side nitride semiconductor layer are placed on the substrate 11 via the buffer layer 12. 202, the first n-side nitride semiconductor layer 201, the active layer 16, the p-type optical guide layer 31, the p-type carrier confinement layer (optical confinement layer) 32, the p-type contact layer 17, and the current constriction layer 18 are sequentially formed. ing. A negative electrode 19 is electrically connected to the n-type contact layer 13, and a positive electrode 20 is electrically connected to the p-type contact layer 17.

[0042] In the LD device shown in FIG. 2, the first n-side nitride semiconductor layer 201, the second n-side nitride semiconductor layer 202, and the third n-side nitride semiconductor layer 203 are excluding the conductive type. , Their band gap energies, their constituent nitride semiconductor materials and their thickness range, respectively, the corresponding first p-side nitride semiconductor layers 101, second p-sides described with respect to FIG. 1, respectively. It is basically the same as the nitride semiconductor layer 102 and the third p-side nitride semiconductor layer 103, and is the first p-side nitride semiconductor layer 101, the second p-side nitride semiconductor layer 102, and the third p. The material preference, thickness preference, etc. described for the side nitride semiconductor layer 103 are also the first n-side nitride semiconductor layer 201, the second n-side nitride semiconductor layer 202, and the third, respectively. It can be applied to the n-side nitride semiconductor layer 203 of.

[0043] To repeat briefly, the first n-side nitride semiconductor layer 201 formed in contact with the active layer 16 has a larger bandgap energy than the active layer 16 (more strictly, its well layer). It is made of a nitride semiconductor. Particularly preferably, the first n-side nitride semiconductor layer 201 is formed of a nitride semiconductor containing Al, and particularly preferably formed of a ternary mixed crystal AlGaN.

[0044] As for the thickness of the first n-side nitride semiconductor layer 201, carriers (electronic carriers) can tunnel the first n-side nitride semiconductor layer 201 in the same manner as in the first p-side nitride semiconductor 101. It has a sufficiently thin thickness. More specifically, the semiconductor layer 201 preferably has a thickness of 0.1 μm or less, more preferably 0.05 μm (500 angstroms) or less, and most preferably 0.03 μm (300 angstroms) or less. It is desirable that the first n-side nitride semiconductor layer 201 also has a thickness of 10 angstroms or more.

[0045] The first n-side nitride semiconductor layer 201 is preferably n-type or i-type. In the present invention, an n-type nitride semiconductor (including the case of an active layer) can be obtained without doping (in a state where impurities are not doped), but in order to obtain a preferable n-type, Si, Ge, Sn, S during crystal growth. It is obtained by doping with donor impurities such as. In that case, the donor impurities are 1x10¹⁶~1×10²²/cm³ It is preferable to dope at the concentration of. Especially silicon is 1x10¹⁷~1×10²¹/cm³ Concentration of 1 × 10¹⁸~1×10²⁰/cm³ Concentration is most preferred.

[0046] The second n-side nitride semiconductor layer 202 has a bandgap energy smaller than the bandgap energy of the first n-side nitride semiconductor layer 201, and the first n-side nitride semiconductor layer 201 It is located farther from the active layer, and most preferably formed in contact with the first n-side nitride semiconductor layer 201, as shown in FIG. The second n-side nitride semiconductor layer 202 is preferably In._k Ga_1-k It is formed by N (0 k 1), and is particularly preferably formed by GaN or InGaN. The second n-side nitride semiconductor layer 202 preferably has a thickness of 0.01 μm to 5 μm, more preferably 0.02 μm to 1 μm, and can act as a preferable optical guide layer in a thickness in this range, for example. The second n-side nitride semiconductor layer 202 is n-type. As described with respect to FIG. 1, the second p-side nitride semiconductor layer 102 serves as a buffer layer for growing the third p-side nitride semiconductor layer 103 formed relatively thick on the second p-side nitride semiconductor layer 102. It is working. Similarly, the second n-side nitride semiconductor layer 202 also acts as a buffer layer when growing the first n-side nitride semiconductor layer 201, but since the first n-side nitride semiconductor layer 201 is thin, , The role as a buffer layer is not so important.

Like the third p-side nitride semiconductor layer 103, the third n-side nitride semiconductor layer 203 also acts as a carrier confinement layer and an optical confinement layer, so that the second n-side nitride semiconductor layer 202 It has a band gap energy larger than the band gap energy, is located farther from the active layer 16 than the second n-side nitride semiconductor layer 202, and most preferably the second n-side nitride as shown in FIG. It is formed in contact with the semiconductor layer 202. The third n-side nitride semiconductor layer 203 is also preferably formed of a nitride semiconductor containing Al, and particularly preferably formed of a ternary mixed crystal AlGaN. The third n-side nitride semiconductor layer 203 also preferably has a thickness of 0.01 μm or more and 2 μm or less, more preferably 0.05 μm or more and 1 μm or less, and has good crystallinity within this thickness range. It can act as a carrier confinement layer and a light confinement layer. The third n-side nitride semiconductor layer 203 is n-type. The third n-side nitride semiconductor layer 203, preferably made of an aluminum-containing nitride semiconductor, is formed on the n-type contact layer 13, preferably formed of n-type GaN, via the crack prevention layer 30. There is.

[0048] The p-type optical guide layer 31 and the p-type carrier confinement layer (optical confinement layer) 32 are each formed of a p-type nitride semiconductor. The bandgap energy of the p-type carrier confinement layer (light confinement layer) 32 is larger than that of the p-type optical guide layer 31.

FIG. 3 shows a nitride semiconductor LD device according to the present most preferable embodiment in which the three-layer laminated structure of the present invention is formed on both sides (p side and n side) of the active layer, respectively. Referring to FIG. 3, on the substrate 11, the n-type contact layer 13, the crack prevention layer 30, the third n-side nitride semiconductor layer 203, and the second n-side nitride semiconductor layer are placed on the substrate 11 via the buffer layer 12. 202, 1st n-side nitride semiconductor layer 201, active layer 16, 1st p-side nitride semiconductor layer 101, 2nd p-side nitride semiconductor layer 102, 3rd p-side nitride semiconductor layer 103 and A nitride semiconductor laminated structure including a p-type contact layer 17 is provided. A current constriction layer 18 provided with a contact hole 18a is provided on the p-type contact layer 17. A negative electrode 19 is provided on the exposed surface of the n-type contact layer 13, and a positive electrode 20 is provided on the current constriction layer 20. The positive electrode 20 is in contact with the p-type contact layer 17 through the contact hole 18a of the current constriction layer 18. The elements that make up the device shown in FIG. 3 are as described with respect to FIGS. 1 and 2.

[0050] The nitride semiconductor layer constituting the nitride semiconductor device of the present invention can be preferably grown by the organic metal vapor phase growth method (MOVPE). However, the nitride semiconductor layer can also be grown by other methods conventionally used for growing nitride semiconductors, such as hydride vapor phase epitaxy (HDVPE) and molecular beam epitaxy (MBE). Can be done.

FIG. 4 schematically shows the energy band of the LD device having the active layer of the multiple quantum well structure of the structure shown in FIG. As shown in FIG. 4, in the LD device having a double heterostructure of the present invention, the first p-side nitride semiconductor layer 101 and the first n-side are in contact with the active layer 16 including the indium-containing nitride semiconductor. A nitride semiconductor layer 201 is provided. That is, it is larger than the bandgap energy of the active layer 16 (more strictly, its well layer), and more than the bandgap energy of the second p-side nitride semiconductor layer 102 and the second p-side nitride semiconductor layer 202. Two first nitride semiconductor layers 101 and 201, which also have a large bandgap energy, are provided in contact with the active layer 16. Moreover, since the film thickness of these two first nitride semiconductor layers is set to be thin, these semiconductor layers 101 and 201 do not act as a barrier to carriers, and the third n-side nitride is used. The electron carriers injected from the semiconductor layer 203 side into the second n-side nitride semiconductor layer 202 and the positive electrons injected into the second p-side nitride semiconductor layer 102 from the third p-side nitride semiconductor layer 103 side. The hole carriers can penetrate the first n-side nitride semiconductor layer 201 and the first p-side nitride semiconductor layer 101 by the tunnel effect, respectively, and efficiently recombine in the active layer 16 to emit light (hν). Emit.

[0052] Since the injected carriers have a large hand gap energy of the first nitride semiconductor layers 101 and 201, the carriers are active layers even if the temperature of the device rises or the injection current density increases. Since 16 is not overflowed and is blocked by the first nitride semiconductor layers 101 and 201, carriers are effectively accumulated in the active layer 16 and light can be efficiently emitted. Therefore, the nitride semiconductor device of the present invention is an LD device in which the luminous efficiency is less likely to decrease even if the device temperature rises and the threshold current is low.

By the way, the present inventors have studied in detail the active layer in the device of the present invention, particularly the active layer having a well layer made of a nitride semiconductor containing indium and gallium. As a result, it was found that, for example, when InGaN was grown, the grown InGaN layer became totally non-uniform in the indium content, thus forming an indium rich region and an indium poor region. Electron carriers and hole carriers are localized in the indium-rich region thus formed, and emit light based on excitons or light emitted based on bi-exciton. That is, the indium-rich region constitutes a quantum dot or a quantum box. In order for the InGaN well layer to form such a quantum dot or quantum box, the well layer is placed on the n-type semiconductor layer, as in the device described with respect to FIGS. 1 to 3, the n-type semiconductor layer (aluminum). It was found that the n-type semiconductor layer must be formed on the contained nitride semiconductors 15,201) in a lattice-mismatched state and its thickness must be 70 angstroms or less. This well layer structure is conveniently formed by allowing a short time, preferably 2 to 20 seconds, after forming the well layer structure, and then forming a semiconductor layer on the well layer structure. Further, the further layer formed on the active layer having the well layer may contain acceptor impurities. An LD device with such a configuration may have a lower threshold current and a higher characteristic temperature than a normal quantum well laser.

[0054] Thus, the present invention is a first clad layer made of an n-type nitride semiconductor; which is provided on the first clad layer and is made of a nitride semiconductor containing indium and gallium and has a thickness of 70 angstroms or less. It is an active layer of a quantum well structure including at least one well layer provided on the base layer in a lattice mismatch state with respect to the base layer, and the well layer has a plurality of indium-rich regions. Also provided is an active layer comprising the indium poor region; and a nitride semiconductor device provided on the active layer and comprising a second clad layer made of a nitride semiconductor doped with an acceptor impurity. .. Here, the base layer is the first clad layer itself such as an n-type semiconductor layer (aluminum-containing nitride semiconductors 15, 201) or the first clad as in the devices described with reference to FIGS. 1 to 3. It shall refer to the barrier layer provided on the layer or the barrier layer itself. FIG. 5 is a cross-sectional view conceptually showing this device. In FIG. 5, the active layer is shown as having a single quantum well structure for convenience. As shown in FIG. 5, the quantum well layer (active layer) 54 formed on the first clad layer 52 made of an n-type nitride semiconductor to a thickness of 70 angstroms or less in a lattice-mismatched state is, for example, overall. Is formed of InGaN, which forms an indium-rich region 54a and an indium-poor region 54b by causing phase separation. More specifically, the indium-rich region 54a and the indium-poor region 54b exist as dots or boxes, which can be 20 to 50 angstroms in size, and each indium-rich region 54a and each indium-poor region 54b is a well layer. They are arranged alternately and almost regularly in the plane direction of. A second clad layer 56 made of a nitride semiconductor doped with acceptor impurities is provided on the active layer 54.

[0055] Of course, the active layer having the well layer constituting the quantum dot or the quantum box preferably constitutes the active layer 16 in the devices described with respect to FIGS. 1 to 3. The bandgap energy of the phase-separated well layer is determined by the average composition of the well layer.

[0056] Doping acceptor impurities and / or donor impurities in an active layer having a well layer that constitutes such a quantum dot or quantum box can further reduce the threshold current.

[0057] That is, the non-uniform indium content in the plane of one well layer means that the InGaN regions (indium rich region and indium poor region) having different band gaps in the plane direction of a single well layer It means that it exists. Therefore, the electrons existing in the conduction band once fall into the indium-rich region, and then recombine with the holes existing in the valence band to release the energy of hν. In other words, electron carriers and hole carriers are localized in the indium-rich region (phase) of the well layer, forming localized excitons, helping to lower the threshold current of the laser and improving the emission output of the laser. Let me.

[0058] When such a well layer is doped with a donor impurity such as silicon and / or an acceptor impurity, an energy level at the impurity level is further formed between the conduction band and the valence band. Therefore, the electron carriers fall to the energy level of the deeper impurity level, and the hole carriers move to the level of the p-type impurity, where the electron carriers and the hole carriers recombine to produce a smaller energy hν. discharge. It is believed that this means that the electron carriers and hole carriers are further localized, and the threshold current of the laser device is lowered by the effect of the excitons formed by the further localization. As the impurities to be doped in the well layer, silicon and germanium are preferable, and silicon is particularly preferable. In particular, doping silicon tends to further reduce the threshold current of the device. Impurities such as silicon and magnesium may be doped not only in the well layer but also in the barrier layer, and in the case of an active layer having a multiple quantum well structure, only one well layer or one barrier layer Only may be doped.

[Examples] Hereinafter, the present invention will be described with reference to Examples. Example 1 In this example, a nitride semiconductor LD device having the structure shown in FIG. 3 was manufactured.

[0060] First, a well-cleaned spinel substrate 11 (MgAl)₂ O₄ ) Was set in the reaction vessel, the inside of the reaction vessel was sufficiently replaced with hydrogen, and then the temperature of the substrate was raised to 1050 ° C while flowing hydrogen to clean the substrate.

Next, the temperature was lowered to 510 ° C., hydrogen was used as the carrier gas, ammonia and trimethylgallium (TMG) were used as the raw material gas, and the GaN buffer layer 12 was placed on the substrate 11 to a thickness of about 200 angstroms. Grow up.

[0062] After the buffer layer grew, the temperature was raised to 1030 ° C while stopping only the TMG flow and flowing ammonia gas. At 1030 ° C, add TMG gas and silane gas (SiH) as a dopant gas.₄ ) Was used to grow a Si-doped n-type GaN layer as the n-type contact layer 13 to a thickness of 4 μm.

[0063] Next, the temperature is lowered to 800 ° C., TMG, TMI (trimethylindium) and ammonia are used as the raw material gas, and silane gas is used as the impurity gas, and Si-doped In._0.1 Ga_0.9 The crack prevention layer 30 consisting of N was grown to a thickness of 500 angstroms.

[0064] Then, the temperature was raised to 1030 ° C., trimethylaluminum (TMA), TMG and ammonia were used as the raw material gas, silane was used as the dopant, and Si-doped n-type Al was used._0.2 Ga_0.8 The third n-type nitride semiconductor layer 203 composed of N was grown to a thickness of 0.5 μm.

Next, the temperature was lowered to 800 ° C., only the TMA flow was stopped, and the second n-type nitride semiconductor layer 202 made of Si-doped n-type GaN was grown to a thickness of 0.2 μm. Then, the temperature was raised to 1050 ° C, TMA, TMG and ammonia were used as the raw material gas, and silane was used as the dopant, and Si-doped n-type Al was used._0.1 Ga_0.9 The first n-type nitride semiconductor layer 201 composed of N was grown to a thickness of 300 angstroms.

Next, the active layer 16 was grown as follows using TMG, TMI and ammonia as raw material gases. First, keep the temperature at 800 ° C and non-dope In_0.2 Ga_0.8 A well layer consisting of N was grown to a film thickness of 25 angstroms. Next, the molar ratio of TMI is changed, and at the same temperature, non-doped In_0.01Ga_0.99A barrier layer consisting of N is grown with a film thickness of 50 angstroms. This operation was repeated twice, and finally the well layers were laminated to grow a total of 7 active layers with multiple quantum well structures.

Next, raise the temperature to 1050 ° C, TMG, TMA, ammonia, cyclopentadienyl magnesium (Cp).₂ Mg dope p-type Al using Mg)_0.1 Ga_0.9 The first p-type nitride semiconductor layer 101 composed of N was grown to a thickness of 300 angstroms.

Subsequently, at 1050 ° C, TMG, ammonia, and Cp.₂ Using Mg, a second p-nitride semiconductor layer 102 made of Mg-doped p-type GaN was grown to a thickness of 0.2 μm.

[0069] Then, at 1050 ° C, TMG, TMA, ammonia, cyclopentadienyl magnesium (Cp).₂ Using Mg), Mg-doped p-type Al_0.2 Ga_0.8 The third p-type nitride semiconductor layer 103 composed of N is grown to a thickness of 0.5 μm.

Finally, at 1050 ° C, a p-type contact layer 17 made of Mg-doped p-type GaN was grown to a thickness of 0.5 μm. After all the reactions were completed, the temperature was lowered to room temperature, the wafer was taken out of the reaction vessel, and the wafer was annealed at 700 ° C. to further reduce the resistance of the p-type layer. Next, the uppermost p-type contact layer 17 to the n-type contact layer 13 were etched in a striped manner until the surface was exposed. After etching, a current constriction layer 18 made of silicon dioxide is formed on the surface of the p-type contact layer 17, and after forming contact holes in the current constriction layer 18, Ni is in contact with the p-type contact layer 17 via the current constriction layer 30. The positive electrode 20 composed of and Au was formed in a striped shape. On the other hand, the negative electrode 19 composed of Ti and Al was formed in a striped shape.

Next, the wafer is cut into a bar shape in a direction perpendicular to the striped electrode, and the cut surface is polished to create a parallel mirror, and then the SiO is formed on the parallel mirror.₂ And TiO₂ And were alternately laminated to form a dielectric multilayer film. Finally, after cutting the bar in the direction parallel to the electrode to make a laser chip with a stripe size of 4 μm × 600 μm, the chip was placed on a heat sink and laser oscillation was attempted at room temperature. Laser oscillation with an oscillation wavelength of 400 nm was confirmed at a duty ratio of 10% per second), and the threshold pulse current density = 2 kA / cm.² , T₀ (Characteristic temperature) = 200K.

Next, the LD device of the present invention was evaluated based on the temperature dependence of the threshold current density of the device. LD threshold current density J_thIs exp (T / T₀ ) (However, T: Operating temperature (K), T₀ : It is proportional to the characteristic temperature (K)). That is, T₀ The larger the value, the lower the threshold current density and the more stable the LD device operates even at high temperatures.

[0073] In the device of Example 1, when neither of the first nitride semiconductor layers 101 and 201 was formed, laser oscillation did not occur. Further, in Example 1, the LD device of the present invention in the case where only one of the first nitride semiconductor layers 101 and 201 is not formed is J._th= 3kA / cm² , T₀ Was 100K. The LD device of Example 1 is an Al of both the first nitride semiconductor layers 101 and 201._j Ga_1-j When the j value of N is 0.1 (Example 1), as described above, J_th= 2kA / cm² , T₀ = 200K, but if the j value is 0.2, then J_th= 1.5kA / cm² , T₀ = 300K, J when j value is 0.3_th= 1.4kA / cm² , T₀ = 400K, indicating that the temperature characteristics of the LD device of the present invention are very good.

[0074] The LD device of the present invention was obtained in the same manner as in Example 1 except that the first n-side nitride semiconductor layer 201 was not grown. This LD has the same structure as the LD device shown in FIG. 1, and the n-type carrier confinement layer (optical confinement layer) 14 in FIG. 1 corresponds to the third n-side nitride semiconductor layer 203, and n The type optical guide layer 15 corresponds to the second n-side nitride semiconductor layer 202. This LD device is J_th= 3kA / cm² Laser oscillation with an oscillation wavelength of 400 nm was confirmed at T.₀ = 100K.

[0075] Example 3 The LD device of the present invention was obtained in the same manner as in Example 1 except that the first p-side nitride semiconductor layer 101 was not grown. This LD has the same structure as the LD device shown in FIG. 2, and the p-type carrier confinement layer (optical confinement layer) 32 in FIG. 2 corresponds to the third p-side nitride semiconductor layer 103, and p. The type optical guide layer 31 corresponds to the second p-side nitride semiconductor layer 102. This LD device is the same as the LD of Example 2, and is J._th= 3kA / cm² Laser oscillation with an oscillation wavelength of 400 nm was confirmed at T.₀ = 100K.

[0076] Example 4 The active layer 16 is non-doped In with a film thickness of 50 angstroms._0.2 Ga_0.8 A single quantum well structure consisting of N well layers, and the first p-type nitride semiconductor layer 101 is Al._0.3 Ga_0.7 The LD device of the present invention was obtained in the same manner as in Example 2 except that it was formed of N. This LD is also J_th= 5kA / cm² Laser oscillation with an oscillation wavelength of 410 nm was confirmed at T.₀ It was = 50K.

Example 5 The second n-type nitride semiconductor layer 202 is Si-doped n-type In._0.01Ga_0.99Formed with N, the second p-type nitride semiconductor 102 is Mg-doped p-type In_0.01Ga_0.99An LD device was produced in the same manner as in Example 1 except that it was formed of N. This LD device showed exactly the same characteristics as the LD device of Example 1.

[0078] Example 6 Silicon as a donor impurity is added to the well layer and the barrier layer of the active layer by 1 × 10, respectively.¹⁹/cm³ The LD device of the present invention was produced in the same manner as in Example 1 except that it was doped at the concentration of. This LD device has a threshold current reduced by about 5% as compared with the LD device of Example 1, and has a T.₀ Improved by about 10%.

[0079] Example 7 Magnesium as an acceptor impurity is added to the well layer and the barrier layer of the active layer by 1 × 10, respectively.¹⁸/cm³ The LD device of the present invention was produced in the same manner as in Example 1 except that it was doped at the concentration of. This LD device showed almost the same characteristics as the LD device of Example 1.

[0080] Example 8 Silicon is 1 × 10 as a donor impurity in the well layer and the barrier layer of the active layer, respectively.¹⁹/cm³ Magnesium at a concentration of 1 x 10 as an acceptor impurity¹⁸/cm³ The LD device of the present invention was produced in the same manner as in Example 1 except that it was doped at the concentration of. This LD device showed almost the same characteristics as the LD device of Example 6.

Example 9 Each non-doped In_0.2 Ga_0.8 After the formation of the N (average composition) well layer, the LD device was prepared in the same manner as in Example 1 except that each barrier layer was formed after holding the well layer as it was for 5 seconds. In this LD device, the well layer is phase-separated into an indium-rich region and an indium-poor region, and the composition of the indium-rich region is almost In._0.4 Ga_0.6 Corresponding to N, the composition of the indium poor region is almost In_0.02Ga_0.98It was equivalent to N. In addition, cross-sectional TEM photographs of the well layer confirmed that indium-rich regions and indium-poor regions, each with an average size of 30 angstroms, were arranged alternately and regularly in the plane direction (see Fig. 5). .. The LD device thus produced has a threshold current density that is approximately 30% lower than that of the LD device of Example 1, and is T.₀ Was improved by 20%.

[0082] Example 10 An LD device was produced in the same manner as in Example 9 except that each well layer was doped with silicon. This LD device has a threshold current density reduced by approximately 40% compared to the LD device of Example 1 and T.₀ Was improved by 30%. In each of the above examples, the impurities whose concentration was not particularly pointed out were also doped within the above-mentioned preferable range.

[0083] In the above-described embodiment, as the most preferable example, the active layer, the first nitride semiconductor layer, the second nitride semiconductor layer, and the third nitride semiconductor layer are formed in contact with each other. However, in the present invention, only the first nitride semiconductor layer is formed in contact with the active layer, and the first nitride semiconductor layer and the second nitride semiconductor layer are formed. In the meantime, another nitride semiconductor layer can be inserted between the second nitride semiconductor layer and the third nitride semiconductor layer. [0084] [Effect of the Invention] As described above, according to the present invention, the concentration of the doped impurity is a nitride semiconductor device having an active layer including a nitride semiconductor containing indium, and the light emission efficiency. A nitride semiconductor device having a high emission efficiency is provided, and a nitride semiconductor device having a small decrease in light emission efficiency even when the device temperature rises is provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view schematically showing an LD device according to a first aspect of the present invention.

FIG. 2 is a cross-sectional view schematically showing an LD device according to a second aspect of the present invention.

FIG. 3 is a cross-sectional view schematically showing an LD device according to a third aspect of the present invention.

FIG. 4 is a diagram showing an energy band corresponding to the device structure shown in FIG.

FIG. 5 is a cross-sectional view schematically showing an LD device according to a fourth aspect of the present invention.

FIG. 6 is a diagram showing an energy band corresponding to a layered structure of a conventional LD device.

[Description of Code] 11 ... Substrate 13 ... n-type contact layer 14 ... n-type carrier confinement layer 15 ... n-type optical guide layer 16,54 ... Active layer 17 ... p-type Contact layer 19 ... Negative electrode 20 ... Positive electrode 30 ... Crack prevention layer 52 ... First clad layer 54a ... Indium rich region 54b ... Indium poor region 56 ... Second Clad layer 101 ... 1st p-side nitride semiconductor layer 102 ... 2nd p-side nitride semiconductor layer 103 ... 3rd p-side nitride semiconductor layer 201 ... 1st n Side nitride semiconductor layer 202 ... Second n-side nitride semiconductor layer 203 ... Third n-side nitride semiconductor layer.