JPH09116225A - Semiconductor light emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the threshold carrier density of a gallium nitride-based compound semiconductor laser by reducing the state density of a valence band and increasing the transition probability of the band. SOLUTION: A quantum well active layer 4 having a biaxial tensile strain is grown on a substrate crystal 1 having plane orientation of (1-100)-plane, (11-20)-plane, or an equivalent plane, and a resonator is constituted in the direction perpendicular to the (0001)-direction. Therefore, the state density of the upper part of a valence band can be reduced and, at the same time, the transition probability of the band can be increased. In addition, a gallium nitride-based compound semiconductor laser can be obtained, because the threshold current density can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device using a gallium nitride compound semiconductor.

[0002]

2. Description of the Related Art GaN, GaAlN, InGaN, In
A gallium nitride-based compound semiconductor such as GaAlN is a wide-gap semiconductor having a direct transition type, and has been actively studied as a material for forming a light emitting device in the blue to ultraviolet range. Currently, a double-heterostructure high-intensity blue LED having a Zn-doped InGaN layer formed on a sapphire substrate as a light emitting layer is known as a light emitting device using this material (S. Nakamura et al., Appl. Phys. Lett., 64 (199
4) 1687). Further, a gallium nitride-based light emitting device constructed on a ZnO substrate to reduce defects due to lattice strain is disclosed in Japanese Patent Laid-Open No. 5-20.
It is disclosed in Japanese Patent No. 6513. However, no gallium nitride-based compound semiconductor laser by current injection has been realized so far.

[0003]

Laser oscillation by current injection is difficult in gallium nitride-based compound semiconductors because the valence band density of states of this material system is large and the threshold carrier density is high. To do. FIG. 5 shows the band structure in the upper part of the valence band near the Γ point in the case of wurtzite GaN without strain.

Incidentally, the point Γ is the point where the wave number vector k (corresponding to the wave number on the horizontal axis of FIG. 5) of the electrons inside the crystal becomes “0”. In a wurtzite semiconductor, the energy at the Γ point splits into three due to the crystal field and spin-orbit interaction. For convenience, hh (heavy holes) 1 and h are set for these three bands in the state of the wave function at the Γ point, respectively.
h2, lh (light hole). Since the density of states in the upper part of the valence band of GaN is higher than that of general III-V semiconductors such as GaAs, the threshold carrier density that gives laser oscillation increases, and laser oscillation by current injection is difficult. It was Further, in the wurtzite semiconductor, since the properties of the wave functions of hh1 and hh2 are the same, even if strain is applied, the energy split of hh1 and hh2 hardly changes. For this reason, in the wurtzite semiconductor, reduction in the density of states due to compressive strain could not be expected.

The present invention reduces the threshold carrier density required for laser oscillation by reducing the density of states in the upper part of the valence band and increasing the optical transition probability due to the tensile strain of a gallium nitride-based compound semiconductor, and gallium nitride by current injection. The object is to realize a semiconductor laser.

[0006]

The gallium nitride based semiconductor light emitting device of the present invention is a quantum well active layer having biaxial tensile strain on the (1-100) plane of a first crystal having a wurtzite structure. Grows the waveguide to the [0001] of the first crystal.
It is characterized in that it is manufactured in a direction perpendicular to the axis, that is, in the [11-20] direction. In addition, the first crystal (11-2
0) grow an active layer with biaxial tensile strain on the plane,
The waveguide is in a direction perpendicular to the [0001] axis, that is, [1-
The same effect can be obtained by manufacturing in the [100] direction. The same effect can be obtained even when the plane orientation of the first crystal is a plane having a deviation of 10 degrees from (1-100) or (11-20). In other words, in the semiconductor light emitting device according to the present invention, (1) the c-axis of the crystal forming the active layer is substantially parallel to the surface of the substrate on which the device is formed, and (2) the well layer of the active layer. Has a structural feature that tensile strain is applied.

For example, when the wurtzite GaN is subjected to a biaxial tensile strain of 2%, the band structure above the valence band near the Γ point is as shown in FIG. As compared with FIG. 5, by applying tensile strain, the lh band consisting of the z orbit shifts to the upper side, and the state density in the upper part of the valence band in the direction parallel to the c axis, that is, the [0001] axis is significantly reduced. Recognize. That is, the energy (vertical axis) changes sharply with respect to the wave number (horizontal axis) in the direction parallel to the c-axis, and the state density is reduced. Therefore, the quantum well active layer is formed in a direction perpendicular to the [0001] axis, that is, in the (1-100) plane or (1
It is possible to reduce the density of states of the valence band by adopting a structure in which the tensile strain is applied by forming the 1-20) plane or a plane equivalent thereto.

Also, the (1-100) plane or the (11-
20) When a quantum well is formed on a plane, the optical transition probability has a polarization direction dependency due to anisotropy in the plane of the quantum well. For example, the polarization dependence of the transition matrix element at the Γ point of the quantum well having the plane orientation of (1-100) is as shown in Table 1 as compared with the case of the undistorted quantum well configured in the (0001) plane. . Table 1 shows the calculation result of the optical matrix element at the band edge in the GaN quantum well.

[0009]

[Table 1]

From Table 1, in the tensile strained quantum well on the (1-100) plane, if the waveguide is formed in the direction perpendicular to [0001], that is, in the [11-20] direction, the transition probability is approximately doubled. It can be seen that the energy value in the table indicates the likelihood of optical transition, and the larger the energy value, the higher the transition probability. As a result, the gain is increased, the threshold carrier density required for oscillation is reduced, and a gallium nitride based semiconductor laser can be realized.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will be described with reference to FIG.

As shown in the figure, this multi-quantum well laser comprises an (1-100) plane n-type ZnO substrate 1, an InGaN buffer layer 2 lattice-matched with the substrate 1, an Si-doped n-InGaAlN layer 3, Active layer 4 consisting of undoped multiple quantum wells, Mg-doped p-InGaAlN
The layers 5 are sequentially laminated and configured. Each of these layers is epitaxially grown by the gas source molecular beam growth method.
Buffer layer 2, n-InGaAlN layer 3, p-InGa
The film thickness of the AlN layer 5 is 2 μm, 0.15 μm,
0.15 μm. Undoped multiple quantum well active layer 4
Is, as shown enlarged, In _0.2 Ga _0.6 Al _0.2 N
It has a double quantum well structure in which barrier layers (film thickness 8 nm) 6 and In _0.1 Ga _0.9 N well layers (film thickness 4 nm) 7 are alternately laminated. Here, the composition ratio of the well layer 7 is a deviation Δa / a of the lattice constant from this when the lattice constant of ZnO is a.
Is set to be -1.8%, and biaxial tensile strain is applied. The n-side In electrode 8 and the p-type InG are formed on the back surface of the substrate 1 of the wafer obtained as described above.
After depositing the Al electrode 9 on the aAlN layer 5, (11-2
Cleavage at the (0) plane [11-20] direction (active layer 4 in FIG.
A resonator having a length of about 800 μm is formed on the side surface of the semiconductor laser to manufacture a semiconductor laser. This semiconductor laser continuously oscillated at room temperature with a threshold current of about 50 mA. Oscillation wavelength is about 42
It was 0 nm.

In the present embodiment, a semiconductor laser having the (11-20) plane as the plane orientation of the ZnO substrate and the resonator formed in the [1-100] direction was similarly prepared. Are almost equivalent. Also,
In the present embodiment, the plane orientation of the ZnO substrate is set to (1-10
0) plane and a plane inclined by 10 degrees in the [0001] direction,
When a semiconductor laser having a resonator formed in the [11-20] direction was manufactured in the same manner, the threshold current and the oscillation wavelength were almost the same.

Next, a second embodiment of the present invention will be described with reference to FIG.

As shown, (1-100) plane n-type Zn
The composition x of In1-xGaxN grown on the O substrate 1 is 0.8.
On the InGaN composition gradient layer 11 that continuously changes from 0.5 to 0.5, In _0.5 Ga lattice-matched to the composition gradient layer 11
_0.5 N buffer layer 12, Si-doped n-InGa
An AlN layer 13, an active layer 14 composed of undoped multiple quantum wells, and a Mg-doped p-InGaAlN layer 15 are sequentially stacked. Each of these layers is epitaxially grown by the gas source molecular beam growth method. Buffer layer 12, n-InGaAlN layer 13, p-InGaAl
The thickness of the N layer 15 is 2 μm, 0.15 μm,
0.15 μm. Undoped multiple quantum well active layer 1
4 is In _0.35 Ga _0.5 Al, as shown enlarged.
It has a double quantum well structure in which _0.15 N barrier layers (film thickness 5 nm) 16 and In _0.2 Ga _0.8 N well layers (film thickness 3 nm) 17 are alternately laminated. Here, the composition ratio of the well layer 17 is I
When the lattice constant of the n _0.5 Ga _0.5 N buffer layer is a,
The deviation Δa / a of the lattice constant from this is set to −2.0%, and biaxial tensile strain is applied. The n-side In electrode 8 is formed on the back surface of the substrate 1 of the wafer obtained as described above, and the Al is formed on the p-type InGaAlN layer 5.
After vapor deposition of the electrode 9, the (11-20) plane is cleaved to form a resonator having a length of about 800 μm in the [11-20] direction, and a semiconductor laser is manufactured. This semiconductor laser continuously oscillated at room temperature with a threshold current of about 60 mA. The oscillation wavelength was about 450 nm.

In the above embodiment, InG is used as the quantum well layer.
Although aN and ZnO are used as the substrate, the structure used in the light emitting device of the present invention is not limited to this, and the structures shown in FIGS.

The semiconductor laser shown in FIG. 3 is an n-type ZnO.
On the (1-100) plane of the substrate 1, an InGaN buffer layer 2 lattice-matched with the substrate 1 is grown.
An n-InGaAlN layer 3, an undoped single quantum well active layer 21, and a p-InGaAlN clad layer 5 are sequentially stacked on top of this. Each of these layers is epitaxially grown by the gas source molecular beam growth method. Here, the quantum well active layer 21 is formed of GaN as shown in an enlarged manner.
_0.95 As _0.05 Well layer (film thickness 5 nm) 22 is In _0.2 Ga
It has a single quantum well structure sandwiched by _0.6 Al _0.2 N barrier layers (film thickness 10 nm) 23. Here, the composition ratio of the well layer 22 is set so that the deviation Δa / a of the lattice constant from ZnO is −1.8% when the lattice constant of ZnO is a, and the biaxial tensile strain. Is being applied. On the back surface of the substrate 1 of the wafer obtained as described above, n-side In
An Al electrode 9 is vapor-deposited on the electrode 8 and the p-type InGaAlN layer 5, and then cleaved at the (11-20) plane to form a resonator having a length of about 800 μm in the [11-20] direction, and a semiconductor laser is manufactured. This semiconductor laser continuously oscillated at room temperature with a threshold current of about 50 mA. The oscillation wavelength was about 450 nm.

In the semiconductor laser shown in FIG. 4, the InGaN buffer layer 2 is grown on the (1-100) plane of the sapphire substrate 31, and the n-InGaA layer is grown on the buffer layer 2.
1N layer 3, undoped multiple quantum well active layer 4, p-In
The GaAlN clad layer 5 is sequentially laminated. Each of these layers is epitaxially grown by a metal organic chemical vapor deposition method. Here, the quantum well active layer 4 has In _0.2 Ga _0.6 Al _0.2 N barrier layer 6 (thickness 8 nm) 6 and In _0.1 Ga _0.9 N well layer (thickness 4 nm), as shown enlarged.
7 has a multiple quantum well structure in which two periods are alternately laminated. Here, the composition ratio of the well layer 7 is set so that the deviation Δa / a of the lattice constant from this is −1.8% when the lattice constant of the InGaN buffer layer is a.
Biaxial tensile strain is applied. The p-InGaAlN cladding layer 5 of the wafer obtained as described above
And part of the quantum well active layer 4 are removed by etching to expose the n-InGaAlN cladding layer 3 and p-
After the Al electrode 9 is vapor-deposited on the clad layer and the n-clad layer, it is cleaved at the (11-20) plane to form a resonator having a length of about 800 μm in the [11-20] direction to manufacture a semiconductor laser. This semiconductor laser has a threshold current of about 7 at room temperature.
It oscillated continuously at 0 mA. The oscillation wavelength was about 420 nm.

The present invention is not limited to the laser structures shown in the embodiments, but can be applied to various semiconductor lasers such as distributed feedback lasers, Bragg reflection lasers, wavelength tunable lasers, and lasers with an external resonator.

[0020]

As described above, the gallium nitride-based compound semiconductor light emitting device of the present invention has a plane orientation of (1-100) plane,
Alternatively, since a quantum well active layer having biaxial tensile strain is grown on a (11-20) plane substrate crystal and a waveguide is formed in a direction perpendicular to the [0001] direction, the valence band It is possible to reduce the density of states in the upper portion and increase the transition probability. As a result, the gain is increased and the threshold current density can be reduced, so that a gallium nitride-based compound semiconductor laser can be realized.

[0021]

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a semiconductor laser according to an embodiment of the present invention.

FIG. 3 is a configuration diagram of a semiconductor laser according to an embodiment of the present invention.

FIG. 4 is a configuration diagram of a semiconductor laser according to an embodiment of the present invention.

FIG. 5 is a diagram showing energy dispersion in the upper part of the valence band of wurtzite GaN in the absence of strain. .

FIG. 6 is a diagram showing energy dispersion in the upper part of the valence band of wurtzite GaN when a 2% biaxial tensile strain is applied.

[Explanation of symbols]

1 ... (1-100) plane n-type ZnO substrate, 2 ... InGaN
Buffer layer, 3 ... n-InGaAlN layer, 4 ... Undoped multiple quantum well active layer, 5 ... p-InGaAlN layer, 6
... In _0.2 Ga _0.6 Al _0.2 N barrier layer, 7 ... In _0.1 Ga
_0.9 N well layer, 8 ... In electrode, 9 ... Al electrode, 11 ... I
nGaN composition gradient layer, 12 ... In _0.5 Ga _0.5 N buffer layer, 13 ... n-InGaAlN layer, 14 ... Undoped multiple quantum well active layer, 15 ... p-InGaAlN layer, 16
... In _0.35 Ga _0.5 Al _0.15 N barrier layer, 17 ... In _0.2 G
a _0.8 N well layer, 21 ... Undoped single quantum well active layer, 22 ... GaN _0.95 As _0.05 well layer, 23 ... In _0.2
Ga _0.6 Al _0.2 N barrier layer, 31 ... Sapphire substrate.

Front page continuation (72) Inventor Toshiaki Tanaka 1-280 Higashi Koikekubo, Kokubunji, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor Akinada Watanabe 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi Research Center Co., Ltd. In-house

Claims (6)

[Claims] 1. A first crystal composed of at least a compound semiconductor and having a wurtzite structure, a two-layer clad layer of a first conductivity type and a second conductivity type, and a quantum well sandwiched between the clad layers. A semiconductor light emitting device having an active layer epitaxially grown, wherein the quantum well active layer is (1-100)
The quantum well active layer is formed on a surface having a deviation of 10 degrees or less from the surface, or a surface equivalent thereto, and the well layer of the quantum well active layer has a lattice constant in the state without biaxial stress as described above. A semiconductor light emitting device, characterized in that it is made of a material smaller than the lattice constant of the crystal. 2. A first crystal having at least a compound semiconductor and having a wurtzite structure, two clad layers of a first conductivity type and a second conductivity type, and a quantum well sandwiched between the clad layers. A semiconductor light emitting device having an active layer epitaxially grown, wherein the quantum well active layer is (11-20)
The quantum well active layer is formed on a surface having a deviation of 10 degrees or less from the surface, or a surface equivalent thereto, and the well layer of the quantum well active layer has a lattice constant in the state without biaxial stress as described above. A semiconductor light emitting device, characterized in that it is made of a material smaller than the lattice constant of the crystal. 3. The semiconductor light emitting device according to claim 1 or 2, wherein a waveguide is formed in a direction perpendicular to the [0001] direction. 4. The semiconductor light emitting device according to claim 1, wherein the quantum well active layer is InxGayA.
l1-x-yNzAs1-z (0 <x ≦ 1, 0 <y ≦ 1, 0 <z
<1) A semiconductor light emitting device characterized in that 5. A semiconductor light emitting device according to any one of claims 1 to 4, wherein the first crystal is epitaxially grown on a ZnO substrate. 6. The semiconductor light emitting device according to claim 1, wherein the oscillation wavelength is 350 nm to 550 nm.