JP4493041B2 - Nitride semiconductor light emitting device - Google Patents

Description

  The present invention relates to a nitride semiconductor light emitting device and a method for manufacturing the same, and more particularly to a nitride semiconductor light emitting device applicable to a short wavelength light emitting diode device or a blue-violet semiconductor laser device and a method for manufacturing the same.

General formula _{In x Al y Ga 1-xy} N ( where, x, y are, 0 ≦ x ≦ 1,0 ≦ y ≦ 1,0 a ≦ x + y ≦ 1.) III-V nitride represented by For example, gallium nitride (GaN) has a comparatively large forbidden band width of 3.4 eV, so that the semiconductor is applied to a light emitting device such as a visible light emitting diode element or a short wavelength semiconductor laser element. Can do. In particular, as for the light emitting diode element, a blue light emitting diode element and a green light emitting diode element have already been put into practical use as various display panels, large display devices, traffic signals, or the like. A white light emitting diode element that emits visible light by exciting a fluorescent material is also commercialized as a light source for a liquid crystal backlight. On the other hand, a semiconductor laser element made of a nitride semiconductor is already at a technical level that can be put into practical use as a writing light source for a next-generation high-density optical disk represented by a Blue-Ray Disc. As described above, research and development have been actively promoted toward higher brightness, higher output, or higher efficiency in a semiconductor optical device made of a group III-V nitride semiconductor.

  As a result of research and development on crystal growth technology centered on the conventional metal organic chemical vapor deposition (MOCVD) method, the brightness, output and luminous efficiency of light-emitting devices made of nitride semiconductors have greatly improved. ing. Specifically, a hetero-epitaxial growth technique in which a low-temperature buffer layer is interposed on a substrate made of sapphire, an active layer growth technique having a multiple quantum well structure made of InGaN, and a low resistance p-type by annealing for dopant activation The establishment of major technologies such as GaN growth technology has greatly contributed to the high performance of nitride semiconductor optical devices.

  Furthermore, a substrate made of gallium nitride (GaN) is also commercially available, and the crystallinity of an epitaxial layer grown on the GaN substrate is expected to be further improved. In order to improve performance in the future, it is also important to reduce contact resistance and parasitic resistance in ohmic electrodes.

  Hereinafter, a nitride semiconductor light emitting device using a conventional sapphire substrate will be described.

  FIG. 9 shows a cross-sectional configuration of a light-emitting diode element using a group III-V nitride semiconductor according to a conventional example (see, for example, Patent Document 1).

  With reference to FIG. 9, a configuration of a light emitting diode device according to a conventional example will be described together with a manufacturing method. As shown in FIG. 9, first, an n-type contact layer 802 made of n-type GaN, an n-type clad layer 803 made of n-type AlGaN, a multi-quantum made of InGaN, on a substrate 801 made of sapphire, for example, by MOCVD. A well active layer 804 and a p-type cladding layer 805 made of p-type AlGaN are sequentially formed.

Subsequently, the p-type cladding layer 805, the multiple quantum well active layer 804, and the n-type cladding layer 803 thus formed are selectively subjected to dry etching using, for example, chlorine (Cl ₂ ) gas to form the n-type contact layer 802. Expose part.

  Thereafter, a transparent electrode 806 made of a laminate of nickel (Ni) and gold (Au) thereon is formed on the upper surface of the p-type cladding layer 805. In order to give transparency to the transparent electrode 806, the film thickness of the laminate needs to be 10 nm or less. An n-side electrode 807 made of a laminate of titanium (Ti) and aluminum (Al) thereon is formed on the exposed portion of the n-type contact layer 802.

  Subsequently, a p-side electrode 808 made of gold (Au) and serving as a bonding pad is formed in a partial region on the transparent electrode 806.

In this way, by providing the transparent electrode 806 in addition to the p-side electrode 808, most of the blue light having an emission wavelength of, for example, 470 nm from the multiple quantum well active layer 804 passes through the transparent electrode 806, and the element Can be taken out.
JP-A-6-314822

However, in the conventional nitride semiconductor light emitting device and the manufacturing method thereof, the n-side electrode 807 that is an ohmic electrode is formed on the n-type contact layer 802 made of n-type GaN. In general, GaN has a large forbidden band of 3.4 eV, but its electron affinity is small, so that the potential barrier with the metal forming the ohmic electrode becomes large. For this reason, it is difficult to realize a low-resistance ohmic contact. As a result, the ohmic contact resistance with respect to the n-type contact layer 802 is limited to about 1 × 10 ⁻⁵ Ωcm ² , which is realized by a compound semiconductor device using gallium arsenide (GaAs) or the like of another III-V group compound semiconductor. It is extremely difficult to realize an ohmic contact resistance of 1 × 10 ⁻⁶ Ωcm ² or less. For this reason, there exists a problem that the operating voltage of the light emitting diode element using a III-V group nitride semiconductor cannot be reduced easily.

  In view of the above-described conventional problems, the present invention provides a semiconductor light-emitting device using a group III-V nitride-based compound semiconductor that can achieve low voltage operation by reducing ohmic contact resistance to an n-type semiconductor layer. With the goal.

  In order to achieve the above object, according to the present invention, a nitride semiconductor light emitting device is configured to use a mixed crystal layer made of n-type InAlGaN which is a quaternary mixed crystal in an n-type contact layer on which an ohmic electrode is formed.

The inventors of the present application have obtained the following knowledge as a result of repeated studies on the ohmic contact resistance with the ohmic electrode in the n-type nitride semiconductor layer through various experiments. That is, InAlGaN, which is a quaternary mixed crystal, has a higher electron affinity than GaN, and thus can reduce the difference from the work function of the ohmic electrode. As a result, InAlGaN has a small potential barrier at the contact portion with the ohmic electrode, so that a contact resistance of 1 × 10 ⁻⁶ Ωcm ² or less can be realized.

  With this configuration, a smaller ohmic contact resistance can be realized as compared to the case where an ohmic electrode is formed on the n-type GaN layer, so that a light emitting device capable of a low operating voltage can be obtained.

Specifically, a first nitride semiconductor light emitting device according to the present invention includes an active layer made of a first group III-V nitride semiconductor and one of the opposing surfaces of the active layer facing each other. are formed _{on, in x Al y Ga 1-} xy n ( where, x, y are, 0 <x <1,0 <y <1,0 < a x + y <1.) consists, n-type conductivity And an ohmic electrode formed so as to be in contact with the mixed crystal layer.

According to a first nitride semiconductor light emitting device, made of _{In x Al y Ga 1-xy} N, and a mixed crystal layer is a contact layer having n-type conductivity, which is formed in contact with該混crystal layer Since an ohmic electrode is provided, the contact resistance between the n-type mixed crystal layer and the ohmic electrode can be reduced without increasing the n-type doping concentration from the above knowledge. As a result, a nitride semiconductor light emitting device having a smaller series resistance and a lower operating voltage can be realized.

  The first nitride semiconductor light emitting device further includes a substrate and a base layer formed on the substrate and made of a second group III-V nitride semiconductor, and the mixed crystal layer is lattice-matched with the base layer. It is preferable.

  In this case, the mixed crystal layer lattice-matched with the base layer can be thickened without generating cracks. For example, the p-side ohmic electrode and the n-side ohmic electrode are formed on one surface side. Since the series resistance in the peripheral region of the n-side ohmic electrode can be further reduced, the operating voltage can be further reduced.

In the first nitride semiconductor light emitting device, the mixed crystal _{layer, In x Al y Ga 1-} xy N ( where, x, y are 0 <x <1,0 <y <1,0 <x + y <1 The ratio of the composition y to the composition x (y / x) is preferably 3.5 or more and 3.7 or less.

  In this way, the mixed crystal layer is lattice-matched with the nitride semiconductor layer serving as the underlying layer, and crystallinity is improved.

In the first nitride semiconductor light emitting device, the ohmic electrode preferably has a contact resistance of 1 × 10 ⁻⁶ Ωcm ² or less.

  In this way, the contact resistance of the n-side ohmic electrode is sufficiently reduced.

The first nitride semiconductor light emitting device is formed so as to be in contact with the mixed crystal layer, and is made of Al _z Ga _{1 -zN} (where z is 0 <z ≦ 1), and has n-type conductivity. A mixed crystal layer, the composition of Al, Ga or In being inclined so that the lower end of the conduction band at the interface between the mixed crystal layer and the first cladding layer is continuous. Preferably it is.

As described above, when the first clad layer made of Al _z Ga _1-z N and having n-type conductivity is formed so as to be in contact with the mixed crystal layer, the first clad layer contains Al in the composition. In general, the refractive index is smaller and the forbidden band width is larger than that of an active layer not containing Al. For this reason, the light confinement function in the direction perpendicular to the active layer is improved and the current confinement function is also improved. Further, since the composition of the mixed crystal layer made of InAlGaN is inclined, the heteropotential barrier between the mixed crystal layer and the first cladding layer made of AlGaN can be reduced, so that high-efficiency light emission is possible and the series resistance is reduced. A small nitride semiconductor light emitting device can be realized.

The first nitride semiconductor light emitting device is formed so as to be in contact with the mixed crystal layer, and is made of Al _z Ga _{1 -z} N (where z is 0 <z ≦ 1), and is of n-type. A first cladding layer having conductivity is further provided, and the first cladding layer has an Al composition gradient such that the lower end of the conduction band at the interface between the first cladding layer and the mixed crystal layer is continuous. It is preferable.

  As described above, since the heteropotential barrier between the mixed crystal layer and the first cladding layer can be reduced also by inclining the composition of the first cladding layer, the nitride semiconductor enables high-efficiency light emission and low series resistance. A light emitting element can be realized.

  The first nitride semiconductor light emitting device is formed on the other surface of the active layer, is made of a third group III-V nitride semiconductor, has a p-type conductivity second cladding layer, It is preferable to further include a metal electrode formed so as to be in contact with the second cladding layer and having a reflectance higher than 70% at a wavelength of emitted light emitted from the active layer, and the emitted light is preferably extracted through the mixed crystal layer.

  In this way, a light emitting diode element with improved emission light extraction efficiency can be realized.

  In this case, the metal electrode preferably contains platinum (Pt), silver (Ag), or rhodium (Rh) as a main component. These metals are preferable because they have a reflectance higher than 70% at the wavelength of emitted light.

  In this case, it is preferable that the first nitride semiconductor light emitting device further includes a metal film formed so as to be in contact with the metal electrode and having a thickness of 10 μm or more.

  In this case, the metal film having a thickness of 10 μm or more has a low thermal resistance and excellent heat dissipation, so that a large output operation is possible.

  In this case, the metal film preferably contains gold (Au) as a main component.

  If it does in this way, a metal film with small heat resistance can be easily formed by plating.

  The first nitride semiconductor light emitting device further includes a second cladding layer formed on the other surface of the active layer, made of a third group III-V nitride semiconductor and having p-type conductivity, The two cladding layers preferably have a stripe structure that becomes a waveguide, and the stripe structure preferably causes laser oscillation in the active layer.

  In this way, it is possible to realize a semiconductor laser element having a smaller series resistance and capable of operating at a low voltage.

A second nitride semiconductor light emitting device according to the present invention is made of GaN and has an n-type conductivity, a pn junction structure formed on one surface of the substrate and including an active layer, formed in contact with the other _{surface, in x Al y Ga 1-} xy N ( where, x, y are 0 <x <1,0 <y < 1,0 <x + y <1.) from And a mixed crystal layer having n-type conductivity and an ohmic electrode formed so as to be in contact with the mixed crystal layer.

According to the second nitride semiconductor light emitting device, the pn junction structure including the active layer formed on one surface of the GaN substrate having n-type conductivity has excellent crystallinity, and thus emits high efficiency light. realizable. Moreover, since the mixed crystal layer having a _{In x Al y Ga 1-xy} N consists of n-type conductivity is formed in contact with the other surface of the substrate, the findings described above,該混crystal layer Since the contact resistance with the ohmic electrode in contact therewith can be reduced, a nitride semiconductor light emitting device having a low series resistance and capable of low voltage operation can be realized.

Third nitride semiconductor light emitting device according to the present _{invention, In x Al y Ga 1-} xy N ( where, x, y is the 0 <x <1,0 <y < 1,0 <x + y <1 And an n-type conductive substrate, a pn junction structure formed on the substrate and including an active layer, and an ohmic electrode formed in contact with the substrate. To do.

According to a third nitride semiconductor light emitting device, due to the use of quaternary alloy consisting of _{In x Al y Ga 1-xy} N on the substrate itself, the contact resistance of the ohmic electrode formed in contact with the substrate Since it can be made small, a nitride semiconductor light emitting device with low series resistance and capable of low voltage operation can be realized.

Fabrication of a nitride semiconductor light emitting device according to the present invention, on the _{substrate, In x Al y Ga 1-} xy N ( where, x, y are, 0 <x <1,0 <y <1,0 <x + y <1.) And a step (a) of forming an n-type conductive mixed crystal layer by epitaxial growth, and an active layer, p-type so as to be in contact with the mixed crystal layer on the mixed crystal layer. And a step (b) of forming a pn junction structure including a semiconductor layer and an n-type semiconductor layer by epitaxial growth, and a step (c) of forming an ohmic electrode so as to be in contact with the mixed crystal layer. .

According to the manufacturing method of the nitride semiconductor light-emitting device of the present invention, on a substrate, made of _{In x Al y Ga 1-xy} N, since the mixed crystal layer having n-type conductivity is formed by epitaxial growth,該混crystals Since the contact resistance with the ohmic electrode so as to be in contact with the layer is reduced by the above-described knowledge, it is possible to realize a nitride semiconductor light emitting device with lower series resistance and lower operating voltage.

  In the method for manufacturing a nitride semiconductor light emitting device of the present invention, in the step (a), a base layer made of a first group III-V nitride semiconductor is formed on the substrate before the mixed crystal layer is formed. The mixed crystal layer is preferably epitaxially grown so as to lattice match with the underlayer.

  In this way, the mixed crystal layer lattice-matched with the base layer can be thickened without generating cracks. For example, the p-side ohmic electrode and the n-side ohmic electrode are formed on one surface side. In this case, since the series resistance in the peripheral region of the n-side ohmic electrode can be further reduced, the operating voltage can be further reduced.

  The method for manufacturing a nitride semiconductor light emitting device of the present invention includes a step (d) of separating the mixed crystal layer and the pn junction structure from the substrate, and the active layer is emitted on the p-type semiconductor layer in the pn junction structure. A step (e) of forming a metal electrode having a reflectance greater than 70% at the emission wavelength of the emitted light, and a step (f) of forming a metal film having a thickness of 10 μm or more so as to be in contact with the metal electrode It is preferable to provide.

  In this case, the light extraction efficiency is improved by adopting a structure in which light is extracted through the metal electrode and the n-type layer having a reflectance higher than 70% formed so as to be in contact with the p-type semiconductor layer. Further, since the heat dissipation is improved by forming a metal film having a thickness of 10 μm or more so as to be in contact with the metal electrode, it is possible to realize a light-emitting diode element capable of high-efficiency light emission and high-power operation. it can.

  In this case, the step (a) includes a step of forming a semiconductor layer made of the second group III-V nitride semiconductor on the substrate so as to be in contact with the substrate before forming the mixed crystal layer. In the step (d), the substrate is separated by irradiating light having a wavelength absorbed by the semiconductor layer from the surface opposite to the semiconductor layer in the substrate and decomposing the semiconductor layer by the irradiated light. preferable.

  As described above, by irradiating the semiconductor layer with light from the surface of the substrate opposite to the semiconductor layer to decompose the semiconductor layer, the stress in the semiconductor layer is alleviated. Due to the relaxation of the stress, the generation of cracks generated in the pn junction structure is suppressed. Therefore, even if the pn junction structure (epitaxial growth layer) is a substrate (wafer) having a relatively large area, the crack is generated. And can be separated with good reproducibility.

In this case, the substrate is made of sapphire, MgO, or LiGa _u Al _1-u O ₂ (where u is 0 ≦ u ≦ 1), and the semiconductor layer is made of GaN, In _x Al _y Ga _1−. It is preferably made of _xy N (where x and y are 0 <x <1, 0 <y <1, 0 <x + y <1) or ZnO.

In this way, a nitride semiconductor layer such as GaN, InGaN, or AlGaN having excellent crystallinity can be formed on the main surface of the substrate by epitaxial growth. In addition, for example, when the substrate is separated by irradiating the third harmonic light of a Nd: YAG laser having a wavelength of 355 nm, GaN, In _x Ga _1-x N (where x is 0 <x ≦ 1) Alternatively, since ZnO is easily decomposed by absorbing laser light having a wavelength of 355 nm, the nitride semiconductor layer having excellent crystallinity can be easily separated from the substrate.

  When light is used for separating the substrate, the light source is preferably laser light that oscillates in a pulsed manner or a bright line of a mercury lamp.

  In this way, when laser light that oscillates in a pulsed manner is used, the output power of the light can be significantly increased, so that the nitride semiconductor layer grown on the substrate can be easily separated. Further, when the emission line of a mercury lamp is used, although the optical power is inferior to the laser light, the spot size can be made larger than that of the laser light, so that the throughput in the light irradiation process is improved.

  When the substrate is separated, the method for manufacturing a nitride semiconductor light emitting device according to the present invention is different from the group III-V nitride semiconductor in the pn junction structure between step (b) and step (d). It is preferable that the method further includes a step (g) of attaching a holding material.

  This facilitates the handling of the pn junction structure from which the substrate is removed.

  In this case, a step (h) of separating the holding material from the pn junction structure may be provided after the step (d). That is, when, for example, a polymer film having no conductivity is used for the holding material, it is preferable to separate and remove the holding material from the pn junction structure.

  According to the nitride semiconductor light emitting device and the manufacturing method thereof according to the present invention, the ohmic contact resistance in the n-type semiconductor layer can be reduced and low voltage operation is possible. For example, when applied to a semiconductor laser device, power consumption Can be reduced and a longer life can be realized.

(First embodiment)
A first embodiment of the present invention will be described with reference to the drawings.

  FIG. 1A shows a nitride semiconductor light emitting device according to the first embodiment of the present invention, which shows a cross-sectional configuration of the light emitting diode device, and FIG. 1B shows an active layer of the light emitting device and its lower portion. The band diagram of electronic energy is shown.

  The structure of the light-emitting diode device according to the first embodiment will be described together with the manufacturing method with reference to FIG.

First, for example, an underlayer 102 made of n-type GaN having a thickness of 2 μm is formed on the main surface of a substrate 101 made of sapphire (single crystal Al ₂ O ₃ ) by metal organic chemical vapor deposition (MOCVD). Is an n-type contact layer 103 made of n-type InAlGaN that is a quaternary mixed crystal of 100 nm, a multiple quantum well (MQW) active layer 104 made of InGaN, and a p-type made of p-type Al _0.1 Ga _0.9 N having a thickness of 0.1 μm. The mold cladding layer 105 is formed sequentially by epitaxial growth. Here, the n-type contact layer 103 also functions as an n-type cladding layer that facilitates confinement of injected electrons and emitted light in the MQW active layer 104. Further, the MQW active layer 104 has a well layer made of In _0.35 Ga _0.65 N with a thickness of 3 nm, a barrier layer made of GaN with a thickness of 10 nm, and has three well layers.

Of the organometallic raw materials, the gallium (Ga) source is, for example, trimethylgallium (TMG), the aluminum (Al) source is, for example, trimethylaluminum (TMA), and the indium (In) source is, for example, trimethylindium (TMI). . The nitrogen (N) source is ammonia (NH ₃ ). Further, for example, silane (SiH ₄ ) containing silicon (Si) is used for the n-type dopant source, and for example, cyclopentadienyl magnesium (Cp ₂ Mg) containing magnesium (Mg) is used for the p-type dopant source. .

Next, dry etching using, for example, chlorine (Cl ₂ ) gas is selectively performed on the formed p-type cladding layer 105, MQW active layer 104 and n-type contact layer 103, so that a part of the n-type contact layer 103 is formed. To expose.

  Next, the upper surface of the exposed n-type contact layer 103 is made of titanium (Ti), aluminum (Al), nickel (Ni), and gold (Au) on the exposed upper surface of the n-type contact layer 103 by sputtering or vacuum deposition, and has ohmic characteristics. The n-side electrode 106 having the above is selectively formed.

  Next, a transparent electrode 107 made of a laminate of Ni and Au having a thickness of about 10 nm is sequentially formed on the upper surface of the p-type cladding layer 105 from the substrate side. Thereafter, a p-side electrode 108 made of Au and serving as a bonding pad is formed in a partial region on the transparent electrode 107. The order of forming the n-side electrode 106, the transparent electrode 107, and the p-side electrode 108 is not particularly limited.

  With this configuration, blue light having a wavelength of, for example, 470 nm generated by recombination of electrons and holes in the MQW active layer 104 passes through the transparent electrode 107 and is extracted outside.

  The MQW active layer 104 can emit emitted light of, for example, about 340 nm to 550 nm by increasing or decreasing the In composition in the well layer made of InGaN or using a quaternary mixed crystal of InAlGaN.

  In the first embodiment, not only a quaternary mixed crystal made of InAlGaN is used for the n-type contact layer 103 but also the n-type contact layer 103 has a composition that lattice-matches with the underlying layer 102 made of GaN.

Here constitutes the n-type contact layer _{103 In x Al y Ga 1-} xy N ( where, x, y are 0 <x <1,0 <y < 1,0 <x + y <1.) Of Assuming that the lattice constant is given by linear interpolation, it is expressed by the following [Equation 1]. Here, a _InN , a _AlN and a _GaN are lattice constants of InN, AlN and GaN, respectively, that is, a _InN = 3.548Å, a _AlN = 3.112Å and a _GaN = 3.189３.

a = x · a _InN + y · a _AlN + (1−xy) · a _GaN (1)
Therefore, the lattice constant of In _x Al _y Ga _1-xy N is equal to 3.189Å is the lattice constant of GaN, the relationship between y of x and Al composition of In composition, the following [Expression 2] Represented.

y = 4.662x .... [Formula 2]
FIG. 2 shows the results obtained by the inventors of the present invention by actually forming a mixed crystal layer of InAlGaN having various composition ratios on the GaN layer by epitaxial growth and measuring the strain amount of the crystal for each formed mixed crystal layer. ing. The composition of the grown mixed crystal layer is measured by an electron probe micro-analysis (PMA) method, and the amount of strain in the mixed crystal layer is determined by an X-ray diffraction pattern and reciprocal lattice mapping. It was calculated by evaluating the deviation. As can be seen from FIG. 2, the composition of the mixed crystal layer in which the strain amount is close to 0 is closer to the composition having the relationship represented by the following [Formula 3] rather than the composition represented by [Formula 2]. Became clear.

y = 3.6x .... [Formula 3]
Note that the n-type InAlGaN layer determined by [Equation 3] is not limited to a semiconductor light-emitting element, but also has a low contact resistance regardless of its crystallinity as an n-type contact layer in other electronic devices such as field effect transistors. It is useful because it can be realized.

The n-type contact layer 103 shown in FIG. 1A is composed of In _0.09 Al _0.33 Ga _0.58 N as a composition ratio that substantially lattice matches under conditions close to those of [Formula 3]. Here, it is confirmed that the forbidden band width of the n-type contact layer 103 is 3.46 eV based on an evaluation result by a cathodoluminescence method or the like.

The forbidden band width E _g of the quaternary mixed crystal made of InAlGaN is given by the following [Equation 4]. _Here, E g_InN is a forbidden band width of InN, E _g _ _AlN is a forbidden band width of AlN, E _g _ _GaN is a forbidden band width of GaN. C and c ′ are so-called bowing parameters.

_{E g = x · E g _} InN + y · E g _ AlN + (1-x-y) · E g _ GaN
-C. (1-xy) -c'.y. (1-xy) ..... [Formula 4]
Since the forbidden band width of In _0.09 Al _0.33 Ga _0.58 N is 3.46 eV, it is calculated that c = c ′ = 2.6 eV. Even if the In composition and the Al composition are increased, the forbidden band of the quaternary mixed crystal is obtained. It can be seen that the width E _g is not so large.

Furthermore, the n-type contact layer 103 is doped with Si at a high concentration of, for example, 1 × 10 ¹⁸ cm ⁻³ or more, which is an n-type dopant. With these configurations, the contact resistance between the n-side electrode 106 formed in contact with the n-type contact layer 103 and the n-type contact layer 103 can be an extremely small value of 1 × 10 ⁻⁶ Ωcm ² or less. .

On the other hand, as a comparative example, when the n-side electrode 106 is formed on an n-type GaN layer doped with Si as much as the n-type contact layer 103, the contact resistance is about 5 × 10 ⁻⁵ Ωcm ² . Therefore, the contact resistance can be greatly reduced by forming the n-side electrode 106 that is an ohmic electrode on the n-type contact layer 103 made of In _0.09 Al _0.33 Ga _0.58 N as in the first embodiment. I understand.

  The n-type contact layer 103 according to this embodiment has a forbidden band width larger than 3.46 eV and 3.4 eV of gallium nitride (GaN). Nevertheless, considering that the ohmic contact resistance is greatly reduced, the n-type contact layer 103 made of InAlGaN has a high electron affinity, and the work function (= electron affinity) of the n-side electrode 106 and n It is considered that the potential barrier generated by the difference from the electron affinity of the type contact layer 103 is reduced.

  FIG. 1B is a band diagram of electron energy considering the above. Here, the MQW active layer 104 of the light emitting diode device according to the present embodiment and a band diagram below the MQW active layer 104 are schematically shown. As shown in FIG. 1B, it is considered that the electron energy at the lower end of the conduction band Ec in the n-type contact layer 103 made of InAlGaN is lowered, and as a result, the ohmic contact resistance is reduced.

  As described above, according to the first embodiment, in the nitride semiconductor light emitting device, that is, the light emitting diode device, the composition of the n-type contact layer 103 in contact with the n-side electrode 106 that is an ohmic electrode is changed to a quaternary mixed material composed of InAlGaN. By using a crystal, since the electron affinity in InAlGaN is larger than that in GaN, the position of the lower end of the conduction band in InAlGaN is lower than that in GaN. For this reason, since the ohmic contact resistance can be made extremely small, the series resistance and the operating voltage in the light emitting diode element can be reduced.

(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to the drawings.

  FIG. 3A shows a nitride semiconductor light emitting device according to the second embodiment of the present invention, which shows a cross-sectional configuration of the light emitting diode device, and FIG. 3B shows an active layer of the light emitting device and its lower portion. The band diagram of electronic energy is shown.

  The structure of the light-emitting diode device according to the second embodiment will be described together with a manufacturing method with reference to FIG.

First, for example, by an MOCVD method, a base layer 202 made of n-type GaN having a thickness of 2 μm and an n-type InAlGaN made of n-type InAlGaN having a thickness of 100 nm are formed on the main surface of the substrate 201 made of sapphire. An n-type cladding layer 204 made of n-type Al _0.1 Ga _0.9 N having a thickness of 0.5 μm, an MQW active layer 205 made of InGaN, and a p-type Al _0.1 Ga _0.9 N having a thickness of 0.1 μm. The p-type cladding layer 206 to be formed is sequentially formed by epitaxial growth. Here, in the MQW active layer 205, the well layer is made of In _0.35 Ga _0.65 N having a thickness of 3 nm, the barrier layer is made of GaN having a thickness of 10 nm, and has three well layers.

Next, the p-type cladding layer 206, the MQW active layer 205, the n-type cladding layer 204, and the n-type contact layer 203 thus formed are selectively etched by, for example, chlorine (Cl ₂ ) gas to form an n-type. A part of the contact layer 203 is exposed.

  Next, an n-side electrode 207 made of Ti, Al, Ni and Au from the substrate side and having ohmic characteristics is formed on the upper surface of the exposed n-type contact layer 203 by sputtering, vacuum vapor deposition, electron beam vapor deposition, or the like. Selectively form.

  Next, a transparent electrode 208 made of a laminate of Ni and Au having a thickness of about 10 nm is sequentially formed on the upper surface of the p-type cladding layer 206 from the substrate side. Thereafter, a p-side electrode 209 made of Au and serving as a bonding pad is formed in a partial region on the transparent electrode 208. The formation order of the n-side electrode 207, the transparent electrode 208, and the p-side electrode 209 is not particularly limited.

  With this configuration, blue light having a wavelength of, for example, 470 nm generated by recombination of electrons and holes in the MQW active layer 205 passes through the transparent electrode 208 and is extracted outside.

Similar to the n-type contact layer 103 according to the first embodiment, the composition of the n-type contact layer 203 according to the second embodiment is In _0.09 Al _0.33 Ga _0.58 N.

In the second embodiment, since the forbidden bandwidth of the n-type cladding layer 204 made of n-type Al _0.1 Ga _0.9 N is 3.59 eV, the n-type contact layer made of n-type In _0.09 Al _0.33 Ga _0.58 N 203 adjusts the forbidden band width of 0.13 eV to the n-type cladding layer 204 stepwise or continuously so as to be 3.59 eV at each interface, thereby forming a hetero barrier (potential barrier due to a heterojunction). Has eased. That is, the n-type contact layer 203 is provided with a composition gradient region at the interface with the n-type cladding layer 204 and in the vicinity thereof. Thereby, there is no potential barrier at the interface between the n-type cladding layer 204 and the n-type contact layer 203, and the series resistance is further reduced.

In the second embodiment, the composition gradient region is provided on the interface side of the n-type contact layer 203 with the n-type cladding layer 204. Conversely, the n-type contact layer in the n-type cladding layer 204 is provided. By providing a composition gradient region on the interface side with 203, the potential barrier at the interface between the n-type cladding layer 204 and the n-type contact layer 203 may be eliminated. That is, when the composition gradient region is provided in the n-type cladding layer 204, the n-type cladding layer 204 has a forbidden band width of 3.46 eV at the interface between the n-type cladding layer 204 and the n-type contact layer 203. The composition may be Al _0.04 Ga _0.96 N.

Furthermore, the n-type contact layer 203 is doped with Si as an n-type dopant at a high concentration of, for example, 1 × 10 ¹⁸ cm ⁻³ or more. Therefore, the contact resistance between the n-side electrode 207 formed in contact with the n-type contact layer 203 and the n-type contact layer 203 can be an extremely small value of 1 × 10 ⁻⁶ Ωcm ² or less.

FIG. 3B is a band diagram of electron energy considering the high electron affinity and composition gradient region of the n-type contact layer 203 made of In _0.09 Al _0.33 Ga _0.58 N in the light-emitting diode device according to the second embodiment. is there. Here, an MQW active layer 205 and a band diagram below the MQW active layer 205 of the light-emitting diode element according to the present embodiment are schematically shown. As shown in FIG. 3B, the electron energy at the lower end of the conduction band Ec on the n-type GaN layer 202 side in the n-type contact layer 203 made of InAlGaN is lowered.

  As described above, according to the second embodiment, in the nitride semiconductor light emitting device, that is, the light emitting diode device, the composition of the n-type contact layer 203 in contact with the n-side electrode 207 that is an ohmic electrode is changed to a quaternary mixed material composed of InAlGaN. Since the electron affinity in InAlGaN is larger than that in GaN, the ohmic contact resistance can be extremely reduced because the position of the lower end of the conduction band in InAlGaN is lower than that in GaN. It is possible to reduce the series resistance and operating voltage in the diode element.

(Third embodiment)
Hereinafter, a third embodiment of the present invention will be described with reference to the drawings.

  FIG. 4 shows a nitride semiconductor light emitting device according to the third embodiment of the present invention, and shows a cross-sectional structure of the light emitting diode device.

As shown in FIG. 4, the light-emitting diode device according to the third embodiment includes a p-type cladding layer 305 made of p-type Al _0.1 Ga _0.9 N having a thickness of 0.1 μm and stacked sequentially from the bottom. An MQW active layer 306, an n-type cladding layer 307 made of n-type Al _0.1 Ga _0.9 N having a thickness of 0.5 μm, and an n-type contact layer 308 made of n-type InAlGaN that is a quaternary mixed crystal having a thickness of 100 nm. Have. Here, in the MQW active layer 306, the well layer is made of In _0.35 Ga _0.65 N having a thickness of 3 nm, the barrier layer is made of GaN having a thickness of 10 nm, and has three well layers. Here, the p-type cladding layer 305, the MQW active layer 306, the n-type cladding layer 307 and the n-type contact layer 308 are the pn junction structure 300 including the MQW active layer 306. The pn junction structure 300 is formed by epitaxial growth from the n-type contact layer 308 side, as will be described later.

  An n-side electrode 309 made of Ti, Al, Ni, and Au stacked from the n-type contact layer 308 side and having ohmic characteristics is selectively formed in a partial region on the n-type contact layer 308. Yes.

On the opposite surface of the p-type cladding layer 305 to the MQW active layer 306, an insulating film 304 made of silicon oxide (SiO ₂ ) having an opening and a side end face exposed with a thickness of about 400 nm is formed. A high reflection electrode 303 made of platinum (Pt) is formed so as to fill the opening.

  On the surface opposite to the p-type cladding layer 305 in the insulating film 304 and the highly reflective electrode 303, plating is made of Au having a thickness of about 50 μm, using the base metal film 302 in which Ti and Au are laminated as the base layer. A layer 301 is formed.

  With this configuration, for example, blue light having a wavelength of 470 nm generated by recombination of electrons and holes in the MQW active layer 306 is reflected by the highly reflective electrode 303, and the n-side electrode 309 in the n-type contact layer 308. Is taken out through the region where no is provided.

Similar to the second embodiment, the n-type contact layer 308 is made of In _0.09 Al _0.33 Ga _0.58 N, and the n-type contact layer 308 and the n-type cladding layer 307 made of Al _0.1 Ga _0.9 N In order to relax the hetero barrier by adjusting the forbidden band width of .13 eV stepwise or continuously, a composition gradient region is provided at the interface with the n-type cladding layer 307 in the n-type contact layer 308 and in the vicinity thereof. . As a result, there is no potential barrier between the n-type cladding layer 307 and the n-type contact layer 308, and a light-emitting diode element with further reduced series resistance can be realized.

In the third embodiment, the composition gradient region is provided on the interface side of the n-type contact layer 308 with the n-type cladding layer 307. On the contrary, the n-type contact layer in the n-type cladding layer 307 is provided. By providing a composition gradient region on the interface side with 308, the potential barrier at the interface between the n-type cladding layer 307 and the n-type contact layer 308 may be eliminated. That is, when the composition gradient region is provided in the n-type cladding layer 307, the n-type cladding layer 307 has a forbidden band width of 3.46 eV at the interface between the n-type cladding layer 307 and the n-type contact layer 308. The composition may be Al _0.04 Ga _0.96 N.

  The highly reflective electrode 303 made of Pt has a high reflectance of about 73% in the visible region and the ultraviolet region. Therefore, as long as the work function is large and a relatively good ohmic characteristic can be realized with respect to the p-type cladding layer 305, another metal such as silver (Ag) having a reflectivity of about 97% is substituted for platinum (Pt). ) Or a metal such as rhodium (Rh) having a reflectivity of about 84%.

  As described above, according to the second embodiment, as in the first embodiment, the composition of the n-type contact layer 308 in contact with the n-side electrode 309 that is an ohmic electrode in the light-emitting diode element is 4 made of InAlGaN. By using the original mixed crystal, since the electron affinity in InAlGaN is larger than that in GaN, the position of the lower end of the conduction band in InAlGaN is lower than that in GaN. For this reason, since the ohmic contact resistance can be made extremely small, the series resistance and the operating voltage in the light emitting diode element can be reduced.

  Further, on the surface of the p-type cladding layer 305 opposite to the MQW active layer 306, a highly reflective electrode 303 having a reflectance of 70% or more is provided, so that the emitted light generated in the MQW active layer 306 is highly reflected. After reflecting the electrode 303, it is transmitted through the n-type contact layer 308 side and taken out. For this reason, the light extraction efficiency is greatly improved.

  Further, since the plating layer 301 made of Au with the metal base film 302 interposed is provided on the surface opposite to the p-type cladding layer 305 in the highly reflective electrode 303, the heat generated from the MQW active layer 306 is plated. Diffused through layer 301. As a result, heat dissipation is improved, and high output operation is possible. Further, since the structure does not include an insulating substrate such as sapphire, the electrostatic withstand voltage is improved.

  Hereinafter, a method of manufacturing the light-emitting diode element configured as described above will be described with reference to the drawings.

  FIG. 5A to FIG. 5F show cross-sectional structures in the order of steps of a method for manufacturing a light-emitting diode element according to the third embodiment of the present invention.

First, as shown in FIG. 5A, for example, an MOCVD method is used to form an underlayer 402 made of n-type GaN on a main surface of a substrate 401 made of sapphire, and n made of n-type InAlGaN that is a quaternary mixed crystal. A type contact layer 308, an n-type cladding layer 307 made of n-type Al _0.1 Ga _0.9 N, an MQW active layer 306 made of InGaN, and a p-type cladding layer 305 made of p-type Al _0.1 Ga _0.9 N are sequentially formed by epitaxial growth.

Subsequently, an insulating film 304 made of SiO ₂ having a thickness of 400 nm is deposited on the entire surface of the p-type cladding layer 305 by, for example, chemical vapor deposition (CVD). Thereafter, a resist pattern (not shown) having a plurality of openings is formed in a region of the insulating film 304 where the highly reflective electrode is to be formed by lithography, and the insulating film 304 is etched using the formed resist pattern as a mask. The p-type cladding layer 305 is partially exposed by forming a plurality of openings in the insulating film 304. Here, each opening corresponds to one electrode formation region. Subsequently, a Pt film is deposited on the resist pattern and the p-type cladding layer 305 exposed from each opening by, for example, electron beam evaporation. Thereafter, a highly reflective electrode 303 made of Pt is formed on the p-type cladding layer 305 exposed from each opening of the insulating film 304 by a so-called lift-off method for removing the resist pattern and the Pt film thereon.

Here, in order to further reduce the contact resistance between the p-type cladding layer 305 and the highly reflective electrode 303, for example, n-type made of p ⁺ -type GaN doped with Mg at about 1 × 10 ²¹ cm ^−3. A contact layer may be formed on the p-type cladding layer 305.

  Next, as shown in FIG. 5B, a base metal film 302, which is a laminated film of Ti and Au and has a thickness of about 200 nm, is formed on the highly reflective electrode 303 and the insulating film 304 by electron beam evaporation. Form. Thereafter, a plating layer 301 made of Au having a thickness of about 50 μm is formed by plating. Here, the thickness of the plating layer 301 should just be 10 micrometers or more.

Next, as shown in FIG. 5C, a holding material 403 made of a polymer film having a thickness of about 100 μm is bonded onto the surface of the plating layer 301 on the side opposite to the base metal film 302. Here, the polymer film to be the holding material 403 is made of, for example, polyester, and adheres to the plating layer 301 through an adhesive layer that is foamed by heating and whose adhesive strength is reduced or disappears. Subsequently, the substrate 401 is irradiated with the third harmonic light of an Nd (neodymium): YAG (yttrium aluminum garnet) laser having a wavelength of 355 nm while scanning in the substrate surface from the surface opposite to the base layer 402. The irradiated laser light is not absorbed by the substrate 401 made of sapphire from the wavelength, but is absorbed only by the base layer 402 made of n-type GaN. For this reason, local heat generation occurs only in the portion of the base layer 402 irradiated with laser light, and the generated heat causes thermal decomposition of the vicinity of the interface of the base layer 402 with the substrate 401. That is, GaN is thermally decomposed into nitrogen (N ₂ ) gas and metal gallium (Ga). Subsequently, the substrate 401 is easily separated from the pn junction structure 300 by removing the metal Ga with an acidic solution such as hydrochloric acid (HCl). As a result, a pn junction structure 300 to be a light emitting diode is formed on the plating layer 301, and a device structure in which the n-type contact layer 308 is exposed on the upper surface is obtained.

  Note that as a light source for irradiating the base layer 402, a bright line of a KrF excimer laser having a wavelength of 248 nm or a mercury lamp having a wavelength of 365 nm can be used instead of the third harmonic light of the Nd: YAG laser.

  Further, as a method of separating the substrate 401 from the pn junction structure 300, a method of polishing and removing the substrate 401 may be used instead of the light irradiation method.

  Next, as shown in FIG. 5D, an n-side electrode 309 that is an ohmic electrode is formed on the n-type contact layer 308 exposed by the separation of the substrate 401 by, for example, an electron beam evaporation method and a lift-off method. .

Next, as shown in FIG. 5E, the pn junction structure 300, which is an epitaxial growth layer including a plurality of devices, is divided into 350 μm square chips individually. Specifically, for example, chlorine (Cl ₂ ) gas is used to selectively etch the insulating film 304 in a region where the insulating film 304 can be divided so as to include the highly reflective electrodes 303 inside the pn junction structure 300. Subsequently, the base metal film 302 and the plating layer 301 are selectively removed by aqua regia (an acidic aqueous solution in which concentrated hydrochloric acid and concentrated nitric acid are mixed at about 3: 1), for example. Thereby, a plurality of light emitting diode elements (chips) are arranged in a state of being bonded onto the holding material 403. When the holding material 403 is heated to 200 ° C., for example, as described above, the adhesive layer applied to the holding material 403 made of a polymer film is foamed by heating and the adhesive force is almost lost. It can be easily peeled off from the holding material 403. Thus, one light emitting diode element shown in FIG.

  The underlying layer 402 made of n-type GaN formed on the substrate 401 is not limited to GaN. That is, as long as the portion that absorbs the irradiated laser beam is present in the underlayer 402, for example, a group III-V nitride compound semiconductor such as AlGaN or InGaN, or a group II-VI oxide may be used. Some zinc oxide (ZnO) may be used.

In addition to sapphire, magnesium oxide (MgO) or LiGa _u Al _1-u O ₂ (where u is 0 ≦ u ≦ 1) is used for the substrate 401 that does not absorb the laser light. be able to.

  The holding material 403 is not limited to a polymer film. For example, a so-called heterogeneous substrate different from a III-V group nitride semiconductor such as silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), gallium phosphide (GaP) or copper tungsten (CuW) is irradiated with light. You may affix on the plating layer 301 before or after performing. In this case, the dissimilar substrate which is the holding material 403 is not necessarily peeled off from each light emitting diode chip in the step shown in FIG.

(Fourth embodiment)
Hereinafter, a fourth embodiment of the present invention will be described with reference to the drawings.

  FIG. 6 shows a nitride semiconductor light emitting device according to the fourth embodiment of the present invention, and shows a cross-sectional structure of the semiconductor laser device.

As shown in FIG. 6, for example, a lower base layer 502A made of n-type or undoped GaN having a thickness of 2 μm is formed on a substrate 501 made of sapphire, and on the lower base layer 502A, A mask layer 503 made of SiO ₂ having a stripe-shaped opening pattern with an opening width of about 5 μm and an opening interval of about 10 μm and a thickness of 100 nm is formed. On the mask layer 503, an upper base layer made of n-type or undoped GaN having a thickness of 2 μm and grown by selective growth (lateral growth) from the lower base layer 502A exposed from the opening pattern on the mask layer 503. 502B is formed. By this selective lateral growth, the dislocation density of 10 ¹⁹ cm ^{−2 in} the lower base layer 502A is reduced to 10 ⁶ cm ^{−2 in} the upper portion of the mask layer 503 in the upper base layer 502B.

On the upper base layer 502B, an n-type contact layer 504 made of n-type InAlGaN having a quaternary mixed crystal having a thickness of 100 nm and having a composition gradient region on the upper portion, an n-type Al _0.07 having a thickness of 1.2 μm. N-type cladding layer 505 made of Ga _0.93 N, MQW active layer 506 made of InGaN, p-type cladding layer 507 made of p-type Al _0.07 Ga _0.93 N having a thickness of 0.5 μm, and p-type GaN having a thickness of 200 nm. The p-type contact layer 508 is sequentially formed by epitaxial growth. Here, Si is introduced as a dopant into each n-type nitride semiconductor layer of conductivity type, and Mg is introduced as a dopant into each p-type nitride semiconductor layer of conductivity type. Further, the MQW active layer 506 has a well layer made of In _0.1 Ga _0.9 N with a thickness of 3 nm, a barrier layer made of GaN with a thickness of 7 nm, and has three well layers. Further, the crystal growth conditions are optimized so that blue-violet emission light having an oscillation wavelength of 405 nm is generated from the MQW active layer 506.

  The epitaxial growth layer is selectively removed so as to expose a part of the n-type contact layer 504, and Ti, Al, and Ni are sequentially stacked on the exposed n-type contact layer 504 from the substrate 501 side. And an n-side electrode 509 made of Au and having ohmic characteristics.

  The upper portions of the p-type contact layer 508 and the p-type cladding layer 507 are formed in a stripe shape (ridge shape) having a width of about 2 μm, for example. Here, the stripe portion is formed in a region above the opening pattern in the mask layer 503 and having a low dislocation density.

  On the p-type contact layer 508 formed in a stripe shape, a p-side electrode 511 made of Ni, Pt, and Au sequentially stacked from the substrate 501 side and having ohmic characteristics is formed.

On the exposed surface of the epitaxial growth layer, a protective insulating film 510 made of SiO ₂ having a thickness of 200 nm is formed except for the n-side electrode 509 and the p-side electrode 511. On the n-side electrode 509 and the p-side electrode 511, pad electrodes 512 made of Ti and Au, which are sequentially stacked from the substrate 501 side, are formed. Here, the pad electrode 512 on the p-side electrode 511 is formed so as to cover the striped portion.

  The stripe-shaped portion functions as a waveguide for blue-violet laser light because light emitted from the MQW active layer 506 is confined by the refractive index difference between the protective insulating film 510 and the p-type cladding layer 507. The waveguides formed in a stripe shape are perpendicular to the longitudinal direction of the waveguide (resonance direction of the laser beam), for example, so that the resonator length is 700 μm and the cleavage planes face each other. Is formed. A dielectric film is coated on one end face of the both cleaved faces so as to obtain a high reflectance, thereby forming a blue-violet semiconductor laser structure.

  With the above configuration, it is possible to suppress the occurrence of the kink phenomenon in the current-light output characteristics due to spatial hole burning during high output operation, and to realize a high output laser device having a stable single transverse mode.

As a feature of the fourth embodiment, as in the first to third embodiments, the composition of the contact portion of the n-type contact layer 504 with the n-side electrode 509 which is an ohmic electrode is In _0.09 Al _0.33 Ga _0.58 N It is. Further, in order to relax the hetero barrier by adjusting the forbidden band width of 0.07 eV between the n-type contact layer 504 and the n-type cladding layer 505 made of Al _0.07 Ga _0.93 N stepwise or continuously, A composition gradient region is provided at the interface of the layer 504 with the n-type cladding layer 505 and in the vicinity thereof. Thereby, there is no potential barrier between the n-type cladding layer 505 and the n-type contact layer 504, and a semiconductor laser device with a further reduced series resistance can be realized.

In the fourth embodiment, the composition gradient region is provided on the interface side of the n-type contact layer 504 with the n-type cladding layer 505. Conversely, the n-type contact layer in the n-type cladding layer 505 is provided. The potential barrier at the interface between the n-type cladding layer 505 and the n-type contact layer 504 may be eliminated by providing a composition gradient region on the interface side with the 504. That is, when the composition gradient region is provided in the n-type cladding layer 505, the n-type cladding layer 505 has a forbidden band width of 3.46 eV at the interface between the n-type cladding layer 505 and the n-type contact layer 504. The composition may be Al _0.04 Ga _0.96 N.

Further, the n-type contact layer 504 is doped with Si, which is an n-type dopant, at a high concentration of, for example, 1 × 10 ¹⁸ cm ⁻³ or more. Therefore, the contact resistance between the n-side electrode 509 formed in contact with the n-type contact layer 504 and the n-type contact layer 504 can be an extremely small value of 1 × 10 ⁻⁶ Ωcm ² or less.

  As described above, according to the fourth embodiment, in the nitride semiconductor light emitting device, that is, the semiconductor laser device, the composition of the n-type contact layer 504 in contact with the n-side electrode 509 that is an ohmic electrode is changed to a quaternary mixed material composed of InAlGaN. By using a crystal, since the electron affinity in InAlGaN is larger than that in GaN, the position of the lower end of the conduction band in InAlGaN is lower than that in GaN. For this reason, since the ohmic contact resistance can be extremely reduced, the series resistance and the operating voltage in the semiconductor laser element can be reduced. Therefore, it is possible to realize a blue-violet semiconductor laser element that can reduce power consumption and has a long lifetime.

In the fourth embodiment, a laser element has been described in which a striped waveguide is formed on a substrate 501 made of sapphire on a region having a low dislocation density formed by a selective lateral growth method. The lower base layer 502A, the mask layer 503, and the upper base layer 502B are not provided as long as the order of the dislocation density in the epitaxial growth layer including the n-type contact layer 504 can be, for example, 10 ⁶ cm ^−2. Gallium nitride (GaN) can be used as the substrate 501. Even when sapphire is used for the substrate 501, dislocation density generated in the epitaxial growth layer can be reduced by providing a single-layer base layer made of aluminum nitride (AlN) which is relatively thick, for example, about 1 μm. Good.

  Further, the blue-violet semiconductor laser device according to the fourth embodiment includes, for example, n-type GaN and n-type GaN between the MQW active layer 506 and the n-type cladding layer 505 and between the MQW active layer 506 and the p-type cladding layer 507. Each of the light guide layers made of p-type GaN and improving the confinement efficiency of the emitted light may be formed.

Further, a so-called electron barrier layer made of, for example, p-type Al _0.15 Ga _0.85 N is provided on the MQW active layer 506 to suppress overflow of electrons from the MQW active layer 506 to the p-type semiconductor layer side. Good.

(Fifth embodiment)
Hereinafter, a fifth embodiment of the present invention will be described with reference to the drawings.

  FIG. 7 shows a nitride semiconductor light emitting device according to the fifth embodiment of the present invention, and shows a cross-sectional configuration of the semiconductor laser device.

  The structure of the semiconductor laser device according to the fifth embodiment will be described together with the manufacturing method with reference to FIG.

First, as shown in FIG. 7, an n-type cladding layer 602 made of n-type Al _0.07 Ga _0.93 N having a thickness of 1.2 μm is formed on the main surface of an n-type substrate 601 made of n-type GaN, for example, by MOCVD. Then, an MQW active layer 603 made of InGaN, a p-type cladding layer 604 made of p-type Al _0.07 Ga _0.93 N having a thickness of 0.5 μm, and a p-type contact layer 605 made of p-type GaN having a thickness of 200 nm are successively grown by epitaxial growth. Form. Here, in the MQW active layer 603, the well layer is made of In _0.1 Ga _0.9 N having a thickness of 3 nm, the barrier layer is made of GaN having a thickness of 7 nm, and has three well layers. Further, the crystal growth conditions are optimized so that blue-violet emission light having an oscillation wavelength of 405 nm is generated from the MQW active layer 603.

Next, the surface of the n-type substrate 601 opposite to the n-type cladding layer 602 is polished to thin the n-type substrate 601 to 150 μm or less (back surface polishing). Thereafter, the n-type In _0.09 Al _0.33 Ga _0.58 N, which is a quaternary mixed crystal having a thickness of 100 nm, is formed on the back surface of the polished n-type substrate 601, that is, the surface opposite to the n-type cladding layer 602 by MOCVD. An n-type contact layer 606 is formed.

Next, as in the fourth embodiment, a striped (ridged) portion having a width of about 2 μm, for example, is selectively formed on the p-type contact layer 605 and the p-type cladding layer 604 by etching. Subsequently, a protective insulating film 607 made of SiO ₂ having a thickness of 200 nm is selectively formed on both sides of the striped portion on the p-type cladding layer 604 by CVD.

  Next, a p-side electrode 608 having an ohmic characteristic made of Ni, Pt, and Au is formed on the p-type contact layer 605 from the substrate side by an electron beam evaporation method or the like, and subsequently, on the p-side electrode 608. A pad electrode 610 made of Ti and Au is formed.

  Next, an n-side electrode 609 made of Ti, Al, Ni, and Au and having ohmic characteristics is formed on the surface of the n-type contact layer 606 opposite to the n-type substrate 601 from the substrate side. Note that the order of forming the p-side electrode 608 and the n-side electrode 609 is not particularly limited.

Also in the fifth embodiment, the contact resistance between the n-side electrode 609 formed in contact with the n-type contact layer 606 and the n-type contact layer 606 is an extremely small value of 1 × 10 ⁻⁶ Ωcm ² or less. be able to.

  Next, the waveguide is made a resonator by forming cleavage planes facing each other in a direction perpendicular to the longitudinal direction of the striped waveguide. Thereafter, the reflective end face of the cleavage plane is coated with a dielectric to increase the reflectivity of the reflective end face to obtain a semiconductor laser structure.

  With the above configuration, light emission from the MQW active layer 603 is confined in the waveguide by the refractive index difference between the protective insulating film 607 and the p-type cladding layer 604, and as a result, the occurrence of a kink phenomenon in the current-light output characteristics is generated. A high-power laser device that can be suppressed and has a stable single transverse mode can be realized.

  According to the fifth embodiment, in the nitride semiconductor light emitting device, that is, the semiconductor laser device, the composition of the n-type contact layer 606 in contact with the n-side electrode 609 that is an ohmic electrode is made of InAlGaN, as in the fourth embodiment. By using a quaternary mixed crystal, since the electron affinity in InAlGaN is larger than that in GaN, the position of the lower end of the conduction band in InAlGaN is lower than that in GaN. For this reason, since the ohmic contact resistance can be extremely reduced, the series resistance and the operating voltage in the semiconductor laser element can be reduced. Therefore, it is possible to realize a blue-violet semiconductor laser element that can reduce power consumption and has a long lifetime.

Furthermore, in the fifth embodiment, since n-type GaN is used for the n-type substrate 601, unlike the case where, for example, insulating sapphire is used, a part of the epitaxial growth layer is formed when the n-side electrode 609 is formed. There is no need to etch. In addition, unlike sapphire, GaN is the same nitride semiconductor as the epitaxial growth layer, and therefore can be easily cleaved, so that the flatness and reflectance of the end face of the resonator are improved. As a result, a semiconductor laser element that operates at a lower threshold current can be realized. For example, when GaN is used for the n-type substrate 601, the order of dislocation density in the epitaxial growth layer can be reduced to 10 ⁶ cm ⁻² or less, so that the lifetime can be further increased.

  The blue-violet semiconductor laser device according to the fifth embodiment includes, for example, n-type GaN and n-type GaN between the MQW active layer 603 and the n-type cladding layer 602 and between the MQW active layer 603 and the p-type cladding layer 604. Each of the light guide layers made of p-type GaN and improving the confinement efficiency of the emitted light may be formed.

Further, a so-called electron barrier layer made of, for example, p-type Al _0.15 Ga _0.85 N may be provided on the MQW active layer 603 to suppress overflow of electrons from the MQW active layer 603 to the p-type semiconductor layer side. Good.

(Sixth embodiment)
Hereinafter, a sixth embodiment of the present invention will be described with reference to the drawings.

  FIG. 8 shows a cross-sectional configuration of a semiconductor laser device, which is a nitride semiconductor light emitting device according to the sixth embodiment of the present invention.

  The difference of the sixth embodiment from the fifth embodiment is that n-type InAlGaN that is a quaternary mixed crystal is used for the n-type substrate 601 itself made of n-type GaN, and that the n-type InAlGaN substrate and the n-type InAlGaN are used. A composition gradient layer made of n-type InAlGaN is provided between the n-type cladding layer made of AlGaN.

  The structure of the semiconductor laser device according to the sixth embodiment will be described together with the manufacturing method with reference to FIG.

  First, an n-type substrate 701 made of n-type InAlGaN shown in FIG. 8 is produced.

For example, a buffer layer (not shown) made of GaN having a thickness of about 200 nm is formed on a sapphire substrate (not shown) by a hydride vapor phase epitaxy (HVPE) method. Similarly to the fifth embodiment, an n-type In _0.09 Al _0.33 Ga _0.58 N layer having a thickness of about 300 μm lattice-matched with gallium nitride (GaN) is formed. Specifically, In, Al, and Ga are introduced into the boats respectively, and then hydrogen chloride (HCl) gas is supplied onto each boat, so that indium chloride (InCl), aluminum chloride (AlCl), and chloride are supplied. Each group III source gas of gallium (GaCl) is manufactured. Epitaxial growth is performed using the manufactured group III source gas and ammonia (NH ₃ ) gas containing a nitrogen source at a substrate temperature of about 1000 ° C., for example. By increasing the flow rate of the group III source gas, for example, high-speed growth with a growth rate of 50 μm / h or more is possible. Here, during the growth of the buffer layer, only GaCl out of the group III source gas is used. Silane (SiH ₄ ) gas is used for doping silicon (Si) which is an n-type dopant.

Subsequently, the third harmonic light of an Nd: YAG laser having a wavelength of 355 nm is irradiated from the surface of the sapphire substrate opposite to the n-type substrate 701. By irradiation with this laser light, GaN constituting the buffer layer is thermally decomposed at the interface of the buffer layer with the sapphire substrate and in the vicinity thereof, so that the n-type In _0.09 Al _0.33 Ga _0.58 N layer is separated from the sapphire substrate. Thus, an n-type substrate 701 made of n-type In _0.09 Al _0.33 Ga _0.58 N can be obtained.

Next, after the decomposition product of the buffer layer on the n-type substrate 701 is removed, the composition gradient layer 702 made of n-type InAlGaN having a thickness of 100 nm is formed on the n-type substrate 701 by MOCVD, for example. An n-type cladding layer 703 made of 1.2 μm n-type Al _0.07 Ga _0.93 N, an MQW active layer 704 made of InGaN, a p-type cladding layer 705 made of p-type Al _0.07 Ga _0.93 N having a thickness of 0.5 μm, and a thickness A p-type contact layer 706 made of p-type GaN having a thickness of 200 nm is sequentially formed by epitaxial growth. Here, in the MQW active layer 704, the well layer is made of In _0.1 Ga _0.9 N having a thickness of 3 nm, the barrier layer is made of GaN having a thickness of 7 nm, and has three well layers. Further, crystal growth conditions are optimized so that blue-violet emission light having an oscillation wavelength of 405 nm is generated from the MQW active layer 704.

Next, as in the fourth or fifth embodiment, a stripe-shaped (ridge-shaped) portion having a width of, for example, about 2 μm is selectively formed on the p-type contact layer 706 and the p-type cladding layer 705 by etching. To do. Subsequently, a protective insulating film 707 made of SiO ₂ having a thickness of 200 nm is selectively formed on both sides of the stripe portion on the p-type cladding layer 705 by CVD.

  Next, a p-side electrode 708 having ohmic characteristics made of Ni, Pt, and Au is formed on the p-type contact layer 706 from the n-type substrate 701 side by an electron beam evaporation method or the like, and then the p-side electrode 708 is formed. A pad electrode 710 made of Ti and Au is formed thereon.

Next, thinning (backside polishing) is performed until the thickness of the n-type substrate 701 on the opposite side of the composition gradient layer 702 is about 150 μm. Thereafter, an n-side electrode 709 made of Ti, Al, Ni and Au and having ohmic characteristics is formed on the back surface of the thinned n-type substrate 701 from the substrate side. As a result, the n-side electrode 709 can obtain an extremely small value of 1 × 10 ⁻⁶ Ωcm ² or less as the contact resistance with the n-type substrate 701.

  Next, the waveguide is made a resonator by forming cleavage planes facing each other in a direction perpendicular to the longitudinal direction of the striped waveguide. Thereafter, one end face of the cleavage plane is coated with a dielectric, and the reflectance of the end face is increased to obtain a semiconductor laser structure.

  With the above configuration, light emission from the MQW active layer 704 is confined in the waveguide by the refractive index difference between the protective insulating film 707 and the p-type cladding layer 705, and as a result, the occurrence of a kink phenomenon in the current-light output characteristics is generated. A high-power laser device that can be suppressed and has a stable single transverse mode can be realized.

Further, since the forbidden band width of Al _0.07 Ga _0.93 N constituting the n-type clad layer 703 is 3.53 eV, it is 0. 0 between the n-type In _0.09 Al _0.33 Ga _0.58 N constituting the n-type substrate 701. In order to relax the hetero barrier by adjusting the forbidden band width of 07 eV stepwise or continuously, the composition gradient layer 702 made of n-type InAlGaN is provided. Thereby, there is no potential barrier between the n-type cladding layer 703 and the n-type substrate 701, and the series resistance is further reduced.

In the sixth embodiment, the composition gradient layer 702 is provided between the n-type substrate 701 and the n-type cladding layer 703. Instead of the composition gradient layer 702, the n-type substrate in the n-type cladding layer 703 is used. A potential barrier at the interface between the n-type cladding layer 703 and the n-type substrate 701 may be eliminated by providing a composition gradient region on the interface side with the 701. That is, when the composition gradient region is provided in the n-type cladding layer 703, the composition of the n-type cladding layer 703 is set so that the forbidden band width is 3.46 eV at the interface between the n-type cladding layer 703 and the n-type substrate 701. _Is preferably Al _0.04 Ga _0.96 N.

  As described above, according to the sixth embodiment, in the nitride semiconductor light emitting device, that is, the semiconductor laser device, the composition of the n-type substrate 701 in contact with the n-side electrode 709 that is an ohmic electrode is changed to a quaternary mixed crystal made of InAlGaN. Thus, since the electron affinity in InAlGaN is larger than that in GaN, the position of the lower end of the conduction band in InAlGaN is lower than that in GaN. For this reason, since the ohmic contact resistance can be extremely reduced, the series resistance and the operating voltage in the semiconductor laser element can be reduced. Therefore, it is possible to realize a blue-violet semiconductor laser element that can reduce power consumption and has a long lifetime.

  Furthermore, in the sixth embodiment, since n-type InAlGaN is used for the n-type substrate 701, a part of the epitaxial growth layer is formed when the n-side electrode 709 is formed, unlike the case of using insulating sapphire, for example. There is no need to etch. Further, since InAlGaN is the same nitride semiconductor as the epitaxial growth layer unlike sapphire, it can be easily cleaved, so that the flatness and reflectivity of the end face of the resonator are improved. As a result, a semiconductor laser element that operates at a lower threshold current can be realized.

  The blue-violet semiconductor laser device according to the sixth embodiment includes, for example, n-type GaN and n-type GaN between the MQW active layer 704 and the n-type cladding layer 703 and between the MQW active layer 704 and the p-type cladding layer 705. Each of the light guide layers made of p-type GaN and improving the confinement efficiency of the emitted light may be formed.

Further, a so-called electron barrier layer made of, for example, p-type Al _0.15 Ga _0.85 N is provided on the MQW active layer 704 to suppress overflow of electrons from the MQW active layer 704 to the p-type semiconductor layer side. Good.

  In each of the first to sixth embodiments, the plane orientation of the main surface of sapphire, GaN or InAlGaN used as the substrate for epitaxial growth may be any plane orientation as long as a light emitting element can be formed. For example, a (0001) plane that is a typical plane orientation, or a plane orientation in which an off-angle is provided from the (0001) plane may be used. Furthermore, when the substrate is applied to a semiconductor laser element, it may have any plane orientation as long as it can be cleaved in a direction perpendicular to the longitudinal direction of the striped waveguide.

In addition to III-V nitrides such as sapphire or GaN, MgO, LiGaO ₂ or LiGa _u Al _1-u O ₂ (where u is 0 ≦ u ≦ 1) is used as the substrate material. be able to. Any of these substrate materials can form a nitride semiconductor such as GaN having excellent crystallinity. In addition, for example, when the substrate is separated from the epitaxial growth layer by irradiation with the third harmonic light of an Nd: YAG laser having a wavelength of 355 nm, GaN, In _x Ga _1-x N (where x is 0 <X ≦ 1) or ZnO is easily decomposed by absorbing the third harmonic light of the Nd: YAG laser, so that the epitaxial growth layer made of a nitride semiconductor having excellent crystallinity can be easily separated from the substrate. Can do.

  In addition, the epitaxial growth method of the nitride semiconductor is not limited to the MOCVD method, and for example, a molecular beam epitaxy (MBE) method or an HVPE method may be appropriately used depending on the thickness or composition of each epitaxial growth layer.

  The nitride semiconductor light emitting device or the manufacturing method thereof according to the present invention can operate at a low voltage, and is useful for a light emitting diode device in a visible region and an ultraviolet region, a semiconductor laser device for a high density optical disk, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS (a) is a structure sectional drawing which shows the light emitting diode element which concerns on the 1st Embodiment of this invention. FIG. 4B is a band diagram of the main part of the light-emitting diode element according to the first embodiment of the present invention. 4 is a graph showing the relationship between the value of the ratio of the Al composition y to the In composition x in the quaternary mixed crystal layer (contact layer) made of InAlGaN according to the first embodiment of the present invention and the amount of crystal strain. (A) is a structure sectional drawing which shows the light emitting diode element which concerns on the 2nd Embodiment of this invention. (B) is a band diagram of the principal part of the light-emitting diode device according to the second embodiment of the present invention. It is a structure sectional view showing a light emitting diode device concerning a 3rd embodiment of the present invention. (A)-(f) is the structure sectional drawing of the order of a process which shows the manufacturing method of the light emitting diode element which concerns on the 3rd Embodiment of this invention. It is a structure sectional view showing a semiconductor laser device concerning a 4th embodiment of the present invention. It is a structure sectional view showing a semiconductor laser device concerning a 5th embodiment of the present invention. It is a structure sectional view showing a semiconductor laser device concerning a 6th embodiment of the present invention. It is a structure sectional view showing a conventional light emitting diode element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Substrate 102 Underlayer 103 n-type contact layer 104 Multiple quantum well (MQW) active layer 105 p-type cladding layer 106 n-side electrode (ohmic electrode)
107 transparent electrode 108 p-side electrode 201 substrate 202 base layer 203 n-type contact layer 204 n-type cladding layer 205 multiple quantum well (MQW) active layer 206 p-type cladding layer 207 n-side electrode (ohmic electrode)
208 Transparent electrode 209 P-side electrode 300 Pn junction structure 301 Plating layer 302 Base metal film 303 High reflective electrode 304 Insulating film 305 P-type cladding layer 306 Multiple quantum well (MQW) active layer 307 n-type cladding layer 308 n-type contact layer 309 n-side electrode (ohmic electrode)
401 Substrate 402 Underlayer 403 Holding material 501 Substrate 502A Lower underlayer 502B Upper underlayer 503 Mask layer 504 n-type contact layer 505 n-type cladding layer 506 Multiple quantum well (MQW) active layer 507 p-type cladding layer 508 p-type contact layer 509 n-side electrode (ohmic electrode)
510 Protective insulating film 511 p-side electrode (ohmic electrode)
512 pad electrode 601 n-type substrate 602 n-type cladding layer 603 multiple quantum well (MQW) active layer 604 p-type cladding layer 605 p-type contact layer 606 n-type contact layer 607 protective insulating film 608 p-side electrode (ohmic electrode)
609 n-side electrode (ohmic electrode)
610 pad electrode 701 n-type substrate 702 graded composition layer 703 n-type cladding layer 704 multiple quantum well (MQW) active layer 705 p-type cladding layer 706 p-type contact layer 707 protective insulating film 708 p-side electrode (ohmic electrode)
709 n-side electrode (ohmic electrode)
710 Pad electrode

Claims (10)

An active layer made of a first group III-V nitride semiconductor;
A first clad layer formed on one of opposing surfaces of the active layer and having n-type conductivity;
An underlayer made of GaN formed on the other surface of the first cladding layer opposite to the active layer;
An ohmic electrode formed in contact with the first cladding layer,
Said first cladding _{layer, In x Al y Ga 1-} xy N ( provided that has a large electron affinity than GaN, x, y is the 0 <x <1,0 <y < 1,0 <x + y <1 And a band gap larger than that of the underlayer.   2. The nitride semiconductor light emitting device according to claim 1, wherein a laminated film of Ti and Al is formed on the first cladding layer in contact with the first cladding layer. 3. The nitride semiconductor light emitting device according to claim 1, wherein the ohmic electrode has a contact resistance of 1 × 10 ⁻⁶ Ωcm ² or less. The first cladding layer is made of Al _z Ga _1-z N (where z is 0 <z ≦ 1), and has a composition gradient region having n-type conductivity,
The composition gradient region is characterized in that the composition of Al, Ga or In is inclined such that the lower end of the conduction band at the interface with the region excluding the composition gradient region of the first cladding layer is continuous. The nitride semiconductor light-emitting device according to claim 1. An n-type semiconductor layer formed of Al _z Ga _1-z N (where z is 0 <z ≦ 1) and having n-type conductivity is formed to be in contact with the first cladding layer. Prepared,
2. The Al composition of the n-type semiconductor layer is inclined so that a lower end of a conduction band at an interface between the n-type semiconductor layer and the first cladding layer is continuous. The nitride semiconductor light emitting device described. A second cladding layer formed on the other surface of the active layer, made of a second group III-V nitride semiconductor and having p-type conductivity;
A metal electrode formed so as to be in contact with the second cladding layer and having a reflectance of greater than 70% at a wavelength of emitted light emitted from the active layer;
The nitride semiconductor light emitting device according to claim 1, wherein the emitted light is extracted through the first cladding layer.   The nitride semiconductor light emitting device according to claim 6, wherein the metal electrode contains platinum (Pt), silver (Ag), or rhodium (Rh) as a main component.   The nitride semiconductor light-emitting element according to claim 6, further comprising a metal film formed to be in contact with the metal electrode and having a thickness of 10 μm or more.   The nitride semiconductor light emitting device according to claim 8, wherein the metal film contains gold (Au) as a main component. A second cladding layer formed on the other surface of the active layer, made of a second group III-V nitride semiconductor and having p-type conductivity;
6. The nitride semiconductor light emitting device according to claim 1, wherein the second cladding layer has a stripe structure serving as a waveguide, and the stripe structure causes laser oscillation in the active layer. 7. element.

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