JP4541318B2 - Nitride semiconductor light emitting / receiving device - Google Patents

Description

  The present invention relates to a nitride semiconductor light emitting device applicable to, for example, a short wavelength light emitting diode or a blue-violet semiconductor laser, or a nitride semiconductor light emitting device such as a nitride semiconductor light receiving device capable of receiving visible light or ultraviolet light.

  GaN-based III-V nitride semiconductors (hereinafter simply referred to as nitride semiconductors) have a large band gap at room temperature of 3.4 eV, for example, and can form a pn junction or double heterojunction structure relatively easily. Therefore, it can be applied to light emitting devices such as visible light emitting diodes and short wavelength semiconductor lasers. GaN-based light-emitting devices have already been put into practical use, and research and development for further device characteristics improvement has been actively conducted. Already, blue, green and white light emitting diodes have been commercialized, and blue-violet semiconductor lasers have been commercialized for next-generation optical disks. In order to further expand the application fields of these GaN-based light emitting devices, it can be said that one of the future issues is to increase the emission wavelength from the green to the red region.

  Looking back on the history of high-performance GaN-based light-emitting devices so far, the progress of crystal growth technology centered on metal organic chemical vapor deposition (MOCVD) contributes greatly. Specifically, the establishment of elemental technologies such as heteroepitaxial growth with low-temperature buffer layer on sapphire substrate, active layer growth with InGaN multiple quantum well structure, and low-resistance p-type GaN growth by activation annealing technology improves device characteristics Has contributed greatly. In the future, it is indispensable to improve the quality of the quantum well structure or to propose a new active layer structure in order to increase the wavelength.

  Hereinafter, a blue light-emitting diode, which is one of nitride semiconductor light-emitting elements using a conventional sapphire substrate, will be described with reference to FIGS. 10 and 11 (see, for example, Patent Document 1).

  FIG. 10 is a cross-sectional view showing a structure of a light emitting diode using a nitride semiconductor in a conventional example. FIG. 11 is a configuration diagram showing a band diagram around the quantum well active layer of the light emitting diode in the conventional example. In FIG. 10, 301 is a C-plane sapphire substrate, 302 is an n-type GaN layer, 303 is an n-type AlGaN cladding layer, 304 is an InGaN / GaN multiple quantum well active layer, 305 is a p-type AlGaN cladding layer, and 306 is a Ti / Al Electrode, 307 is a Ni / Au transparent electrode, and 308 is an Au electrode.

A method for manufacturing the light emitting diode shown in FIG. 10 will be described below. First, an n-type GaN layer 302, an n-type AlGaN cladding layer 303, an InGaN / GaN multiple quantum well active layer 304, and a p-type AlGaN cladding layer 305 are formed in this order on the sapphire substrate 301, for example, by MOCVD. Subsequently, in order to extract an electrode on the n layer side of the wafer, etching of the p-type AlGaN cladding layer 305, the InGaN / GaN multiple quantum well active layer 304, and the n-type AlGaN cladding layer 303 is performed using, for example, Cl ₂ gas. Performed by dry etching. Subsequently, a Ti / Al electrode 306 is formed on the n-layer side so as to be in contact with the n-type AlGaN cladding layer 303, while a Ni / Au transparent electrode 307 is formed on the p-layer side. Here, the thickness of the Ni / Au transparent electrode 307 needs to be 10 nm or less in order to function as a transparent electrode. Subsequently, an Au electrode 308 is selectively formed on the Ni / Au transparent electrode 307 in order to form a bonding pad on the p layer side. By using a transparent electrode, most of the blue light emission of, for example, 470 nm from the InGaN / GaN multiple quantum well active layer 304 is transmitted through the Ni / Au transparent electrode 307 and extracted outside. The band diagram of the active layer portion of the light emitting diode structure shown in FIG. 10 is as shown in FIG. In FIG. 11, the left side is the surface side, and the right side is the substrate side. When the active layer shown in FIG. 11 is formed on the C-plane ((0001) plane), spontaneous polarization occurs in GaN, while in InGaN occurs due to crystal distortion due to spontaneous polarization and lattice mismatch. Piezoelectric polarization occurs (see, for example, Non-Patent Document 1). Due to this polarization, an internal electric field is generated in the active layer, and as a result, the active layer is configured to accumulate electrons on the surface side of the InGaN layer and holes on the substrate side of the InGaN layer.
JP-A-6-314822 SFChichibu et al., Applied.Physics.Letters 73 (1998) p.2006-2008

  However, in the conventional light emitting diode shown in FIG. 10, when realizing a longer wavelength such as green light emission (550 nm), the In composition of the InGaN layer used as the well layer constituting the active layer is about 35%, for example. The crystallinity of the active layer deteriorates due to crystal growth problems such as crystal defects due to lattice mismatch between the InGaN well layer and the GaN layer or segregation of In in the InGaN well layer. However, there is a problem that there is a limit in improving the luminous efficiency. Another problem is that due to the internal electric field, electrons and holes are spatially separated by the thickness of the InGaN well layer, resulting in a decrease in luminous efficiency.

  In view of the above-described technical problems, the present invention reduces the lattice mismatch in a semiconductor element as much as possible and keeps the In composition of the nitride semiconductor constituting the semiconductor element relatively small in a semiconductor element using a nitride semiconductor. However, an object of the present invention is to provide an active layer having a quantum well structure capable of realizing a light emitting element having a longer wavelength and higher efficiency. Another object of the present invention is to provide a visible light receiving element using the active layer as a light receiving layer.

In order to solve the above problems, in the nitride semiconductor light emitting device and the nitride semiconductor light receiving device of the present invention, an active layer in which an InAlGaN quaternary mixed crystal layer and an AlGaN layer that are lattice-matched to the GaN layer are formed periodically The InAlGaN quaternary mixed crystal layer has a higher electron affinity than the AlGaN layer and the InAlGaN quaternary mixed crystal layer and the AlGaN layer have almost the same forbidden band width. And the composition of the AlGaN layer. Thus, the active layer is configured to be a so-called type II quantum well active layer in which electrons and holes are quantized and confined in separate layers in the periodic structure. The type II quantum well active layer can emit light with a wavelength longer than the forbidden band width of the InAlGaN quaternary mixed crystal layer and the AlGaN layer constituting the active layer. The InAlGaN quaternary mixed crystal layer can be formed so as to lattice match with the GaN layer by setting the composition of the InAlGaN quaternary mixed crystal layer to a desired composition, so that the lattice strain is reduced as much as possible. It is possible. The high electron affinity of the InAlGaN quaternary mixed crystal layer is the knowledge obtained by the inventors' experiment, that is, when an ohmic electrode is formed on the InAlGaN quaternary mixed crystal layer, the potential barrier is small and 1 × 10 ⁻ The findings suggest that contact resistances below ⁶ Ωcm ² can be achieved.

  By adopting such a configuration, the composition of the InAlGaN quaternary mixed crystal layer and the AlGaN layer is set so as to be a type II quantum well active layer, so that the emission wavelength can be increased without increasing the In composition as in the prior art. Since the wavelength can be realized, light emission with a longer wavelength can be realized with a smaller In composition without degrading the crystallinity of the active layer. In addition, since the polarization difference between the InAlGaN quaternary mixed crystal layer and the AlGaN layer can be made smaller than the polarization difference between the conventional InGaN layer and the GaN layer, the internal electric field generated in the active layer is smaller, The concentration of electrons and holes that can be confined can be increased, and more efficient light emission can be realized. In addition, by creating a type II quantum well active layer on a nonpolar surface such as the a-plane ((11-20) plane), the active layer is not affected by polarization and achieves more efficient light emission. It becomes possible to do.

  Specifically, in the nitride semiconductor device of the present invention, the first nitride semiconductor layer and the second nitride semiconductor layer having a composition different from the composition of the first nitride semiconductor layer are periodically formed. A first nitride semiconductor layer having a conduction band lower end energy lower than a conduction band lower energy of the second nitride semiconductor layer, and a valence electron of the first nitride semiconductor layer. The band top energy is lower than the valence band top energy of the second nitride semiconductor layer.

With such a configuration, the active layer includes a so-called type II quantum well active layer in which electrons are confined in the first nitride semiconductor layer and holes are confined in the second nitride semiconductor layer. Therefore, it is possible to emit light with energy smaller than the energy of the forbidden band width of the first and second nitride semiconductor layers constituting the type II quantum well active layer, and more easily realize light emission with a long wavelength. Is possible.

  In the nitride semiconductor device of the present invention, the forbidden band width of the first nitride semiconductor layer and the forbidden band width of the second nitride semiconductor layer are further equal.

  With such a configuration, the confinement of electrons and holes in the type II quantum well active layer is improved, and more efficient light emission can be realized.

  In the nitride semiconductor device of the present invention, the active layer is further formed above the base layer formed on the substrate, and the first nitride semiconductor layer or the second nitride semiconductor layer is latticed on the base layer. It is the structure formed so that it may match.

  With such a configuration, crystal distortion in the type II quantum well active layer is suppressed, and more efficient light emission can be realized without deteriorating the crystallinity of the active layer.

  The nitride semiconductor device of the present invention further has a configuration in which the polarization of the first nitride semiconductor layer is different from the polarization of the second nitride semiconductor layer.

  By adopting such a configuration, for example, when a type II quantum well active layer is formed on the C-plane ((0001) plane), an internal electric field is generated in the type II quantum well active layer, and a longer wavelength Light emission can be realized.

The nitride semiconductor device of the present invention, the further first nitride semiconductor layer has the general formula _{In x Al y Ga 1-xy} N (0 <x <1, 0 <y <1) a compound represented by The quaternary mixed crystal layer is formed.

  By adopting such a configuration, for example, the first nitride semiconductor layer is an InAlGaN quaternary mixed crystal layer lattice-matched to the GaN layer, and the second nitride semiconductor layer is an AlGaN layer or an InGaN layer. A type II quantum well active layer with a multilayer structure of InAlGaN quaternary mixed crystal layer and AlGaN layer or a multilayer structure of InAlGaN quaternary mixed crystal layer and InGaN layer can be realized, realizing longer wavelength and higher efficiency light emission It becomes possible.

  In the nitride semiconductor device of the present invention, the composition of the quaternary mixed crystal layer is such that the quaternary mixed crystal layer is lattice-matched with the GaN layer.

  By adopting such a configuration, similar to the above-described nitride semiconductor light emitting device, a type having a multilayer structure of an InAlGaN quaternary mixed crystal layer and an AlGaN layer, or a multilayer structure of an InAlGaN quaternary mixed crystal layer and an InGaN layer. It is possible to realize an II quantum well active layer, and to realize light emission with a longer wavelength and higher efficiency.

  The nitride semiconductor device of the present invention is further configured to satisfy a range where the y / x value is 3.5 or more and 3.7 or less.

  By adopting such a configuration, the lattice matching between the InAlGaN quaternary mixed crystal layer and the GaN layer can be ensured, so that the InAlGaN quaternary mixed crystal layer and the AlGaN layer are similar to the above-described nitride semiconductor light emitting device. A type II quantum well active layer having a multilayer structure with a layer or a multilayer structure of an InAlGaN quaternary mixed crystal layer and an InGaN layer can be realized, and light emission with a longer wavelength and higher efficiency can be realized.

  In the nitride semiconductor device of the present invention, the lattice constant of the first nitride semiconductor layer and the lattice constant of the second nitride semiconductor layer are further different.

  With such a configuration, for example, the first nitride semiconductor layer is an InAlGaN quaternary mixed crystal layer lattice-matched to the GaN layer, and the second nitride semiconductor layer is a lattice constant smaller than the lattice constant of the GaN layer. A multi-layer structure of an InAlGaN quaternary mixed crystal layer and an AlGaN layer, similar to the above-described nitride semiconductor light-emitting device, by making an AlGaN layer having an InGaN layer having a lattice constant larger than the lattice constant of the GaN layer, Alternatively, a type II quantum well active layer having a multilayer structure of an InAlGaN quaternary mixed crystal layer and an InGaN layer can be realized, and light emission with a longer wavelength and higher efficiency can be realized.

In the nitride semiconductor device of the present invention, the second nitride semiconductor layer further comprises a binary mixed crystal layer or a compound crystal layer made of a compound represented by the general formula Al _x Ga _1-x N (0 ≦ x ≦ 1). The original mixed crystal layer, or a binary mixed crystal layer or a ternary mixed crystal layer made of a compound represented by the general formula In _x Ga _1-x N (0 <x ≦ 1). .

  By adopting such a configuration, similar to the above-described nitride semiconductor light emitting device, a type having a multilayer structure of an InAlGaN quaternary mixed crystal layer and an AlGaN layer, or a multilayer structure of an InAlGaN quaternary mixed crystal layer and an InGaN layer. It is possible to realize an II quantum well active layer, and to realize light emission with a longer wavelength and higher efficiency.

  In the nitride semiconductor device of the present invention, the first nitride semiconductor layer and the second nitride semiconductor layer are further laminated on a nonpolar surface containing the same number of nitrogen atoms and group III atoms. .

  By adopting such a configuration, a type II quantum well active layer can be formed without generating an internal electric field due to polarization, so that localization of electrons and holes confined in the active layer at the heterointerface is possible. It is possible to suppress and realize more efficient light emission.

The nitride semiconductor device of the present invention further substrate is sapphire, SiC, or the general formula consists of _{In x Al y Ga 1-xy} N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) compound represented by the The underlayer is made of GaN.

  By adopting such a configuration, it is possible to form a type II quantum well active layer in which the first and second nitride semiconductor layers having higher crystallinity are stacked, so that light emission with a longer wavelength and higher efficiency can be achieved. It can be realized.

  In the nitride semiconductor device of the present invention, the surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer are (0001) planes.

  By adopting such a configuration, an internal electric field is generated in the type II quantum well active layer as in the case where the type II quantum well active layer is formed on the C-plane ((0001) plane) described above. Light emission with a wavelength can be realized.

  In the nitride semiconductor device of the present invention, the surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer are (11-20) plane or (1-100) plane.

  By adopting such a configuration, a type II quantum well active layer can be formed without generating an internal electric field due to polarization, as in the case where a type II quantum well active layer is formed on the nonpolar surface described above. Therefore, it is possible to suppress the localization of electrons and holes confined in the active layer at the heterointerface and to realize more efficient light emission.

  In the nitride semiconductor device of the present invention, the surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer are (11-20) planes, and the first nitride semiconductor layer and the second nitride semiconductor layer The physical semiconductor layer is configured to be formed above the sapphire substrate having a (1-102) surface.

  With this configuration, a nitride semiconductor layer having a surface (a (11-20) surface) on the R surface ((1-102) surface) sapphire substrate can be easily formed. As in the case where a type II quantum well active layer is formed on the active surface, a type II quantum well active layer can be formed without generating an internal electric field due to polarization, so electrons and holes confined in the active layer can be formed. Thus, it is possible to suppress the localization at the heterointerface and to realize more efficient light emission.

  In the nitride semiconductor device of the present invention, a cladding layer having a refractive index smaller than the refractive index of the portion having the maximum refractive index in the active layer providing light emission is formed above and below the active layer. ing.

  With this configuration, when a type II quantum well active layer is applied to a semiconductor laser, it is possible to improve the confinement of light in the active layer and realize a semiconductor laser with a smaller threshold current. Become.

In the nitride semiconductor device of the present invention, the cladding layer further includes a binary mixed crystal layer or a ternary mixed crystal layer made of a compound represented by the general formula Al _x Ga _1-x N (0 <x ≦ 1). It is the structure formed.

  By adopting such a configuration, in the semiconductor laser to which the present invention is applied, by using an AlGaN layer having a larger Al composition as a cladding layer, light confinement in the active layer can be further improved, and more A semiconductor laser having a smaller threshold current can be realized.

  As described above, according to the nitride semiconductor light emitting device and the nitride semiconductor light receiving device of the present invention, the InAlGaN quaternary mixed crystal layer and the AlGaN layer are set by setting the composition of the InAlGaN quaternary mixed crystal layer and the AlGaN layer. The active layer having the periodic structure can be formed to be a so-called type II quantum well active layer, so that longer wavelength light emission can be realized without increasing the In composition as in the conventional example. Furthermore, for example, by setting the composition of the InAlGaN quaternary mixed crystal layer and the AlGaN layer to be a type II quantum well active layer while achieving lattice matching between the GaN layer and the InAlGaN quaternary mixed crystal layer, the active layer Thus, it is possible to realize long wavelength light emission without degrading the crystallinity. In a light emitting diode using an active layer having this type II quantum well structure, for example, a green light emitting diode with high luminous efficiency can be realized. In addition, in a nitride semiconductor light receiving element using an active layer having a type II quantum well structure as a light receiving layer, for example, a visible light receiving element with high light receiving efficiency can be realized.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional view showing the structure of a nitride semiconductor light-emitting device according to the first embodiment of the present invention. FIG. 2 is a configuration diagram showing the forbidden band width of the InAlGaN quaternary mixed crystal lattice-matched with GaN used in the present invention, and the relationship between the lattice constant and the forbidden band width in the nitride semiconductor. Figure 3 is a _{In x Al y Ga 1-xy} N 4 -element mixed in various compositions to the present inventors actually on GaN by crystal growth, in the _{In x Al y Ga 1-xy} N 4 mixed crystal FIG. 6 is a diagram in which y / x values are classified and plotted for each crystal strain amount in each In _x Al _y Ga _1-xy N quaternary mixed crystal. FIG. 4 is a diagram showing the dependence of the forbidden band width of the InAlGaN quaternary mixed crystal on the In composition when it is formed under the lattice matching condition that lattice matches with GaN and the bowing parameter is 2.6 eV. FIG. 5 is a diagram showing the relationship between the barrier height and the work function of the metal when Schottky electrodes are formed on AlGaN / GaN, GaN, and InAlGaN. FIG. 6 is a graph showing the In composition dependence of the polarization of the InAlGaN quaternary mixed crystal formed under the lattice matching condition for lattice matching with GaN. FIG. 7 is a configuration diagram showing a band diagram of the active layer portion of the nitride semiconductor light emitting device according to the first embodiment of the present invention. In FIG. 1, 101 is a C-plane sapphire substrate, 102 is an n-type GaN layer, 103 is an n-type Al _0.1 Ga _0.9 N cladding layer, and 104 is an In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N multiple quantum well active layer. 105 is a p-type Al _0.1 Ga _0.9 N cladding layer, 106 is a Ti / Al / Ni / Au electrode, 107 is a Ni / Au transparent electrode, and 108 is an Au electrode.

A method of manufacturing a light emitting diode using an active layer having a periodic structure of the In _0.07 Al _0.33 Ga _0.6 N layer and the Al _0.04 Ga _0.96 N layer will be described below with reference to FIG. First, an n-type GaN layer 102, an n-type AlGaN cladding layer 103, an In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N multiple quantum well active layer 104, and a p-type are formed on a C-plane sapphire substrate 101 by, for example, MOCVD. An AlGaN cladding layer 105 is formed in this order.

Next, etching of the p-type AlGaN cladding layer 105, the InAlGaN / AlGaN multiple quantum well active layer 104, and the n-type AlGaN cladding layer 103 is performed using, for example, a Cl ₂ gas for taking out the electrode on the n-layer side of the wafer. Etching is performed.

  Subsequently, a Ti / Al / Ni / Au electrode 106 is formed on the n layer side so as to be in contact with the n-type AlGaN cladding layer 103, while a Ni / Au transparent electrode 107 is formed on the p layer side. An Au electrode 108 is selectively formed on the Ni / Au transparent electrode 107 to form a bonding pad on the p-layer side.

The composition of the In _0.07 Al _0.33 Ga _0.6 N layer is a composition that lattice matches with the GaN layer. In addition, the forbidden band width of the In _0.07 Al _0.33 Ga _0.6 N layer is almost equal to the forbidden band width of the Al _0.04 Ga _0.96 N layer, and the energy at the bottom of the conduction band of the In _0.07 Al _0.33 Ga _0.6 N layer is Al _0.04 Ga _0.96. The active layer, which is lower than the energy at the bottom of the N layer conduction band and has a multiple structure of In _0.07 Al _0.33 Ga _0.6 N layer and Al _0.04 Ga _0.96 N layer, has electrons confined in the In _0.07 Al _0.33 Ga _0.6 N layer. As a result, a so-called type II quantum well structure (type II quantum well active layer) is formed in which holes are confined in the Al _0.04 Ga _0.96 N layer.

Here, the forbidden band width of the InAlGaN quaternary mixed crystal constituting the type II quantum well active layer will be described with reference to FIG. In the following description, the case where the InAlGaN quaternary mixed crystal is formed so as to lattice match with GaN will be considered. As shown in FIG. 2, in the general formula _{In x Al y Ga 1-xy} N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) so-called nitride compound semiconductor consisting of compounds expressed by the 0.77eV of InN The band gap can be changed over a wide range of AlN up to 6.2 eV. Depending on the so-called bowing parameter, the band gap ranges from 3.4 eV to 5 eV in the InAlGaN quaternary mixed crystal lattice-matched to GaN. It can be changed over a wide range. Assuming that the lattice constant of this In _x Al _y Ga _1-xy N quaternary mixed crystal is given by linear interpolation,

Given in. Here, a _InN , a _AlN, and a _GaN are lattice constants of InN, AlN, and GaN, respectively, specifically, a _InN = 0.3548 nm, a _AlN = 0.3112 nm, and a _GaN = 0.3189 nm. When the lattice constant of In _x Al _y Ga _1-xy N quaternary mixed crystal is equal to the lattice constant of GaN of 0.3189 nm, the x and y values of In _x Al _y Ga _1-xy N quaternary mixed crystal Is the following [Equation 2]

It becomes the relationship. The forbidden band width of InAlGaN quaternary mixed crystals is shown in [Equation 3] below.

C and c ′ are so-called bowing parameters. Figure 3 is a _{In x Al y Ga 1-xy} N 4 -element mixed in various compositions to the present inventors actually on GaN by crystal growth, in the _{In x Al y Ga 1-xy} N 4 mixed crystal FIG. 6 is a diagram in which y / x values are classified and plotted for each crystal strain amount in each In _x Al _y Ga _1-xy N quaternary mixed crystal. The composition of InAlGaN quaternary mixed crystal is measured by EPMA (Electron Probe Micro-Analysis) method, and the amount of crystal distortion in InAlGaN quaternary mixed crystal is determined by X-ray diffraction pattern and reciprocal lattice mapping between GaN and InAlGaN quaternary mixed crystal. It was calculated by evaluating the deviation of the lattice constant. FIG from crystal distortion quantity of In _x near 0 Al _y Ga _1-xy N composition of quaternary mixed _{crystal, In x Al y Ga 1-} xy N 4 -way x and y values of the mixed crystal Equation 2 Rather than the relationship expressed by

It was found that the composition is close to satisfying the relationship. In In _0.09 Al _0.33 Ga _0.58 N having a composition lattice-matched with GaN close to this [Equation 4] condition, the forbidden band width of the same layer was found to be 3.46 eV from the evaluation results of cathodoluminescence and the like. From this result, c = c ′ = 2.6 eV is obtained in [Equation 3]. Even if the In composition and Al composition are increased, the forbidden band width is not expected to become very large. FIG. 4 shows the dependence of the forbidden band width of the InAlGaN quaternary mixed crystal on the In composition when the In composition and the Al composition are increased while satisfying the lattice matching condition for lattice matching with GaN using this bowing parameter. FIG. 4 also shows the forbidden band width of Al _0.04 Ga _0.96 N as an AlGaN ternary mixed crystal having the same forbidden band width as In _0.07 Al _0.33 Ga _0.6 N used in this embodiment.

For the InAlGaN layer and AlGaN layer used as the semiconductor layer constituting the type II quantum well active layer this time, the barrier height of the Schottky electrode on each semiconductor layer was evaluated by changing the type of metal, and each FIG. 5 summarizes the relationship between the work function of the metal and the barrier height φ _B on each semiconductor layer. FIG. 5 shows the result of measuring the barrier height of the Schottky electrode on each semiconductor layer using four types of metals, Mo, Ti, Cu, and Pd. As the metal work function increases, the barrier heights on the GaN, AlGaN, and InAlGaN layers all increase, but the barrier height on the InAlGaN layer becomes smaller than on the GaN and AlGaN layers. ing. The barrier height φ _B is given by φ _m −χ _s when the metal work function is φ _m and the electron affinity of the semiconductor layer is χ _s . Therefore, the small barrier height means that the electron affinity of the InAlGaN layer is small. It means big.

A nitride compound semiconductor is generally a material that is epitaxially grown such that its surface has a C-plane ((0001) plane), and the polarization is very large on the same plane. There are two types of polarization: spontaneous polarization determined by the elastic constant of the material, and piezo polarization caused by crystal distortion in the material, and the overall polarization is obtained from the above two sums. Only spontaneous polarization occurs in an InAlGaN quaternary mixed crystal formed on GaN so as to lattice match with GaN. On the other hand, since AlGaN formed on GaN has different lattice constants between GaN and AlGaN, crystal distortion occurs in AlGaN, and spontaneous polarization and piezoelectric polarization occur. Here, as shown in O. Ambacher et al. (O. Ambacher et al., J. Appl. Phys 85 (1999) 3222), polarization can be calculated using elastic constants and crystal strain. Based on this calculation, the InAl of the quaternary mixed crystal of InAlGaN when the In composition and the Al composition are increased while satisfying the lattice matching condition for lattice matching with GaN (the Al composition is 4.662 times the In composition as described above). FIG. 6 shows the composition dependency. In FIG. 6, the polarization of Al _0.04 Ga _0.96 N having the same forbidden band width as In _0.07 Al _0.33 Ga _0.6 N is also illustrated.

FIG. 7 shows a band diagram of the In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N multiple quantum well active layer 104 used in this embodiment. In the multiple quantum well active layer 104, as described above, the InAlGaN layer has a higher electron affinity than the AlGaN layer, and the InAlGaN layer and the AlGaN layer have the same forbidden band width, and the InAlGaN layer is more polarized than the AlGaN layer. Can be understood that an internal electric field is generated in the active layer and a so-called type II quantum well structure is taken into consideration. As described above, electrons are accumulated in the InAlGaN layer and holes are accumulated in the AlGaN layer, and light emission from the active layer having the InAlGaN / AlGaN multilayer structure is the band gap of the InAlGaN layer and the AlGaN layer. Light emission having energy smaller than energy, that is, long wavelength light emission. Here, in the above active layer, for example, assuming that the band offset in the conduction band, so-called ΔEc is about 0.3 eV, for the forbidden band width 3.46 eV of the InAlGaN layer and the AlGaN layer, 2.86 eV (corresponding to 434 nm) or less Can emit light.

  Therefore, in this embodiment, the composition of the InAlGaN layer and the AlGaN layer is set so as to form a type II quantum well structure while achieving lattice matching between the GaN layer and the InAlGaN layer. Since the active layer can be formed under a small epitaxial growth condition, longer wavelength light emission can be realized without degrading the crystallinity of the active layer. Therefore, longer wavelength light emission can be realized with high efficiency.

In the present embodiment, the case of using the In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N multiple quantum well active layer 104 has been described, but the present invention is not limited to this. For example, in the case of using an InAlGaN / InGaN multiple quantum well active layer in which InAlGaN quaternary mixed crystal and InGaN are periodically formed on GaN, the type II quantum well is set by setting the composition of InAlGaN quaternary mixed crystal and InGaN. The structure can be realized. In this case, longer wavelength light emission is possible.

  Further, in this embodiment, for the purpose of relaxing the internal electric field generated in the InAlGaN / AlGaN multiple quantum well active layer 104, an impurity such as Si is added to InAlGaN or AlGaN, and the InAlGaN or AlGaN doped with the impurity is electrically conductive. You may form so that property may be provided.

(Second embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a cross-sectional view showing the structure of a nitride semiconductor light emitting device in the second embodiment of the present invention. FIG. 9 is a configuration diagram showing a band diagram of the active layer portion of the nitride semiconductor light emitting device in the second embodiment of the present invention. In FIG. 8, 201 is an R-plane sapphire substrate, 202 is an n-type GaN layer, 203 is an n-type Al _0.1 Ga _0.9 N cladding layer, and 204 is an In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N multiple quantum well active layer. 205 is a p-type Al _0.1 Ga _0.9 N cladding layer, 206 is a Ti / Al / Ni / Au electrode, 207 is a Ni / Au transparent electrode, and 208 is an Au electrode.

  In this embodiment, a nitride semiconductor layer whose surface is an a-plane ((11-20) plane) is epitaxially grown on a sapphire substrate whose surface is an R-plane ((1-102) plane). The a-plane is a so-called nonpolar plane in which the same number of group III atoms and N atoms are mixed in the plane, and polarization does not occur in the plane direction.

  The nitride semiconductor light emitting device according to the second embodiment is different from the nitride semiconductor light emitting device according to the first embodiment in that an active layer is formed on a nonpolar surface. As shown, no internal electric field is generated in the active layer having an InAlGaN / AlGaN multilayer structure, electrons are uniformly distributed in the InAlGaN layer, and holes are uniformly distributed in the AlGaN layer, thereby enabling more efficient light emission. It becomes.

  Therefore, in the present embodiment, as in the first embodiment, the active layer can be formed under the epitaxial growth conditions with a smaller In composition and less lattice mismatch, so that the emission of longer wavelengths degrades the crystallinity of the active layer. It can be realized without doing. Furthermore, since an internal electric field does not occur in the active layer, the spatial distance between electrons and holes can be shortened, so that emission of a longer wavelength can be realized with high efficiency.

  In the first and second embodiments, a C-plane or R-plane sapphire substrate is used as the substrate. However, the present invention is not limited to this, and any substrate such as SiC, GaN, or Si is used. In addition, a substrate with any plane orientation may be used, and for example, a plane orientation with an off-angle from a representative plane such as the (0001) plane may be used.

  Further, in the first and second embodiments, the MOCVD method is used as the crystal growth method of the epitaxial growth layer, but the present invention is not limited to this. For example, molecular beam epitaxy (MBE) Alternatively, a hydride vapor phase epitaxy (HVPE) method may be used.

  Although the nitride semiconductor light emitting device has been described in the first and second embodiments, the present invention is not limited to the nitride semiconductor light emitting device, and the layer structure of the active layer shown in the first or second embodiment. The same effects as those of the present invention can be obtained for a nitride semiconductor light-receiving element using a light-receiving layer having the same layer structure. That is, with respect to the nitride semiconductor light receiving element to which the present invention is applied, the light receiving efficiency can be improved not only in the wavelength region of the ultraviolet region or the blue-violet region but also in the wavelength region on the longer wavelength side.

  The nitride semiconductor device according to the present invention can be applied to a nitride semiconductor light emitting device such as a light emitting diode and a semiconductor laser in the visible region or a nitride semiconductor light receiving device in the visible region, and is very useful.

1 is a cross-sectional view showing the structure of a nitride semiconductor light emitting device in a first embodiment of the present invention. FIG. 3 is a configuration diagram showing a forbidden band width of an InAlGaN quaternary mixed crystal lattice-matched to GaN in the present invention and a relationship between a lattice constant and a forbidden band width in a nitride semiconductor. The present inventors actually various compositions on _{GaN In x Al y Ga 1-} xy N a quaternary alloy crystal-grown, the _{In x Al y Ga 1-xy} N 4 y in mixed crystal / x classify the values for each crystal distortion amount in each _{in x Al y Ga 1-xy} N 4 -element mixed crystal is a plot. FIG. 3 is a structural diagram showing the In composition dependence of the forbidden band width of an InAlGaN quaternary mixed crystal lattice-matched to GaN in the present invention. FIG. 4 is a configuration diagram showing the relationship between the barrier height of a Schottky electrode and the work function of a Schottky metal on each of GaN, AlGaN, and InAlGaN quaternary mixed crystal lattice-matched to GaN in the present invention. FIG. 3 is a configuration diagram showing the In composition dependence of the polarization of an InAlGaN quaternary mixed crystal lattice-matched to GaN in the present invention. FIG. 3 is a configuration diagram showing a band diagram of an active layer portion of the nitride semiconductor light emitting device in the first embodiment of the present invention. FIG. 5 is a cross-sectional view showing the structure of a nitride semiconductor light emitting device in a second embodiment of the present invention. FIG. 6 is a configuration diagram showing a band diagram of an active layer portion of a nitride semiconductor light emitting device in a second embodiment of the present invention. It is sectional drawing which shows the structure of the light emitting diode in a prior art example. It is a block diagram which shows the band diagram of the active layer part of the light emitting diode in a prior art example.

Explanation of symbols

101 C-plane sapphire substrate
102 n-type GaN layer
103 n-type Al _0.1 Ga _0.9 N cladding layer
104 In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N Multiple quantum well active layer
105 p-type Al _0.1 Ga _0.9 N cladding layer
106 Ti / Al / Ni / Au electrode
107 Ni / Au transparent electrode
108 Au electrode
201 R surface sapphire substrate
202 n-type GaN layer
203 n-type Al _0.1 Ga _0.9 N cladding layer
204 In _0.07 Al _0.33 Ga _0.6 N / Al _0.04 Ga _0.96 N Multiple quantum well active layer
205 p-type Al _0.1 Ga _0.9 N cladding layer
206 Ti / Al / Ni / Au electrode
207 Ni / Au transparent electrode
208 Au electrode
301 C-plane sapphire substrate
302 n-type GaN layer
303 n-type AlGaN cladding layer
304 InGaN / GaN multiple quantum well active layer
305 p-type AlGaN cladding layer
306 Ti / Al electrode
307 Ni / Au transparent electrode
308 Au electrode

Claims (14)

An active layer formed by periodically laminating a first nitride semiconductor layer and a second nitride semiconductor layer having a composition different from the composition of the first nitride semiconductor layer;
The conduction band bottom energy of the first nitride semiconductor layer is lower than the conduction band bottom energy of the second nitride semiconductor layer, and the valence band top energy of the first nitride semiconductor layer is rather lower than the valence band upper edge energy of the second nitride semiconductor layer,
The first nitride semiconductor layer has the general formula are formed by _{In x Al y Ga 1-xy} N (0 <x <1, 0 <y <1) 4 mixed crystal layer consisting of a compound represented by And
The second nitride semiconductor layer is a binary mixed crystal layer or a ternary mixed crystal layer made of a compound represented by a general formula of Al _x Ga _1-x N (0 ≦ x ≦ 1), or a general formula of A nitride semiconductor light emitting / receiving element, which is formed of a binary mixed crystal layer or a ternary mixed crystal layer made of a compound represented by In _x Ga _1-x N (0 <x ≦ 1) . 2. The nitride semiconductor light emitting / receiving device according to claim 1, wherein the forbidden band width of the first nitride semiconductor layer is equal to the forbidden band width of the second nitride semiconductor layer. The active layer is formed above a base layer formed on a substrate,
2. The nitride semiconductor light emitting / receiving device according to claim 1, wherein the first nitride semiconductor layer or the second nitride semiconductor layer is formed so as to lattice match with the base layer. . 2. The nitride semiconductor light emitting / receiving device according to claim 1, wherein the polarization of the first nitride semiconductor layer is different from the polarization of the second nitride semiconductor layer. The composition of the quaternary mixed crystal layer, wherein the quaternary alloy layer having a composition lattice-matched to GaN layer, according to claim 1 nitride semiconductor light emitting and receiving element according. 6. The nitride semiconductor light emitting / receiving device according to claim 5 , wherein the y / x value is 3.5 or more and 3.7 or less. 2. The nitride semiconductor light emitting / receiving device according to claim 1, wherein a lattice constant of the first nitride semiconductor layer is different from a lattice constant of the second nitride semiconductor layer. 3. 2. The nitriding according to claim 1, wherein the first nitride semiconductor layer and the second nitride semiconductor layer are stacked on a nonpolar plane containing the same number of nitrogen atoms and group III atoms. Semiconductor light emitting / receiving element. The substrate, sapphire, SiC, or the general formula consists of _{In x Al y Ga 1-xy} N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) compound represented by, and the underlying layer is made of GaN The nitride semiconductor light-emitting / light-receiving element according to claim 3. 2. The nitride semiconductor light emitting / receiving device according to claim 1, wherein surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer are (0001) planes. 9. The nitride semiconductor according to claim 8 , wherein surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer are (11-20) planes or (1-100) planes. Light emitting / receiving element. The surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer are (11-20) planes, and the first nitride semiconductor layer and the second nitride semiconductor layer are surfaces 9. The nitride semiconductor light emitting / receiving device according to claim 8 , wherein is formed above a sapphire substrate having a (1-102) plane. The clad layer having a refractive index smaller than the refractive index of the portion having the maximum refractive index in the active layer that provides light emission is formed above and below the active layer. Nitride semiconductor light emitting / receiving element. The clad layer is formed of a binary mixed crystal layer or a ternary mixed crystal layer made of a compound represented by a general formula of Al _x Ga _1-x N (0 <x ≦ 1). The nitride semiconductor light emitting / receiving element according to claim 13 .

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