Nitride semiconductor device

Details Nitride semiconductor device Nitride semiconductor device

2001-04-10

2004-02-10

Leung, Quyen (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010

Reexamination Certificate

active

06690700

ABSTRACT:

BACKGROUND OF THE INVENTION
Short-wavelength lasers fabricated from Group III-nitride semiconductor materials, whose general formula is Al
x
Ga
1−x−y
In
y
N, where Al is aluminum, Ga is gallium, In is indium, N is nitrogen, and x and y are compositional ratios, have been widely reported. However, such lasers have a far-field pattern, which is the Fourier transformation of the near-field pattern, that exhibits more than one peak. See, for example, D. Hofstetter et al., 70 APPL. PHYS. LETT., 1650 (1997). A laser that generates coherent light having a far-field pattern that exhibits multiple peaks can be used in substantially fewer practical applications than a laser that generates coherent light having a far-field pattern that exhibits a single peak.
The far-field pattern of the light generated by such conventional lasers exhibits multiple peaks, rather than the desired single peak, because the optical waveguide layer provides insufficient optical confinement and allows light to leak from the optical waveguide layer into the contact layer underlying the cladding layer. The contact layer then acts as a parasitic optical waveguide, resulting in spurious laser oscillation in a high-order mode. The contact layers are included in the laser to inject current into the active layer.
Attempts to achieve sufficient optical confinement have included increasing the thickness of the cladding layers compared with the conventional thickness value, and increasing the refractive index difference between the cladding layers and the optical waveguide layers. However, when implemented conventionally, these measures cause cracks in the cladding layers. This seriously impairs the production yield of lasers that incorporate such measures.
FIG. 1
illustrates the structure of the conventional GaN-based laser diode
10
. The electrodes have been omitted to simplify the drawing. The laser diode is composed of the GaN low-temperature-deposited buffer layer
12
, the GaN n-contact layer
13
, the n-type AlGaN cladding layer
14
, the n-type GaN optical waveguide layer
15
, the active layer
16
, the p-type GaN optical waveguide layer
17
, the p-type AlGaN cladding layer
18
, and the GaN p-contact layer
19
. These layers are successively grown on the substrate
11
. The material of the substrate is sapphire, SiC, spinel, MgO, GaAs, silicon, or some other suitable material.
The growth temperatures and growth thicknesses of the layers of a conventional laser diode having a structure similar to that shown in
FIG. 1
are disclosed by Okumura in Japanese Laid-Open Patent Application No. H 10-261838. The low-temperature-deposited buffer layer
12
is a 35 nm-thick layer of GaN deposited at a temperature of 550° C. The GaN n-contact layer
13
is a 3 &mgr;m-thick layer of silicon-doped n-type GaN deposited at a temperature of 1050° C. The n-type cladding layer
14
is a 700 nm-thick layer of silicon-doped Al
0.1
Ga
0.9
N deposited at a temperature of 1050° C. The n-type optical waveguide layer
15
is a 50 nm-thick layer of silicon-doped GaN deposited at a temperature of 1050° C. The active layer
16
is an 18 nm-thick composite layer deposited at a temperature of 750° C. The composite layer is composed of three 2 nm-thick layers of In
0.05
Ga
0.95
N interleaved with four 3 nm-thick layers of In
0.2
Ga
0.8
N. The laser diode additionally includes an anti-evaporation layer (not shown), which is a 10 nm-thick layer of Al
0.2
Ga
0.8
N deposited at a temperature of 750° C. The p-type optical waveguide layer
17
is a 50 nm-thick layer of magnesium-doped GaN deposited at a temperature of 1050° C. The p-type cladding layer
18
is a 700 nm-thick layer of magnesium-doped Al
0.1
Ga
0.9
N deposited at a temperature of 1050° C. The p-contact layer
19
is a 200 nm-thick layer of magnesium-doped p-type GaN deposited at a temperature of 1050° C.
The above-mentioned layers are successively grown on the C plane of the sapphire substrate
11
by metal-organic vapor phase epitaxy (MOVPE). Alternatively, molecular beam epitaxy (MBE) or halide vapor phase epitaxy (HVPE) may be used.
After the above-described stack of layers has been deposited on the substrate, the stack of layers is annealed at 800° C. in a nitrogen atmosphere to activate the dopants in the magnesium-doped p-type layers, and hence reduce the resistance of these layers. It has also been disclosed that the Al
0.2
Ga
0.8
N anti-evaporation layer (not shown) can be doped with magnesium, which facilitates hole injection from the p-type GaN optical waveguide layer
17
.
Okumura reported that providing a ridge structure in the stack, depositing electrodes on the p-contact layer
19
and the n-contact layer
13
and cleaving the stack produced a laser diode that generated coherent light with a wavelength of 430 nm and had a threshold current of 40 mA.
Okumura's disclosure does not indicate the far-field pattern of the light emitted by the laser just described.
In 37 JPN. APPL. PHYS., L905-L906 (1998), Yasuo Ohba et al. proposed a structure that uses a GaN active layer for generating light at short wavelengths. The GaN active layer required that the molar fraction of AlN in the AlGaN cladding layers be increased to maintain the band-gap difference between the active layer and the cladding layers. However, the increased aluminum molar fraction gave rise to a lattice mismatch between the materials of the AlGaN cladding layer and the GaN buffer layer on which it was deposited. The lattice mismatch was sufficiently large to cause cracks in the cladding layer.
Ohba et al. disclosed using a single-crystal AlN buffer layer interposed between the substrate and the n-type cladding layer to solve the cracking problem. First, a 600 nm-thick single-crystal AlN buffer layer was grown at a temperature of 1300° C. on a sapphire substrate. Successively grown on the buffer layer were an n-type cladding layer, which was a 1.2 &mgr;m-thick layer of silicon-doped n-type Al
0.25
Ga
0.75
N deposited at a temperature of 1150° C.; an n-type optical waveguide layer, which was a 100 nm-thick layer of silicon-doped GaN deposited at a temperature of 1150° C.; an active layer, which was a 50 nm-thick composite layer composed of one layer of Al
0.2
Ga
0.8
N, five layers of GaN interleaved with four layers of Al
0.1
Ga
0.9
N and one layer of Al
0.2
Ga
0.8
N; a p-type optical waveguide layer, which was a 100 nm-thick layer of magnesium-doped GaN grown at a temperature of 1150° C.; a p-type cladding layer, which was a 700 nm-thick layer of magnesium-doped Al
0.25
Ga
0.75
N grown at a temperature of 1150° C.; and p-contact layer, which was a 600 nm-thick layer of magnesium-doped GaN contact layer grown at a temperature of 1150° C.
After the above-stack of layers was deposited, the stack was annealed in a nitrogen atmosphere at 800° C. to activate the dopants in the magnesium doped p-type layers, and hence reduce the resistance of these layers. Ohba et al. reported that no cracking was observed even when the n-type cladding layer was grown on the single-crystal AlN buffer layer to an overall thickness of 1.8 &mgr;m. According to Ohba et al., p-type AlGaN has a reasonably low resistivity at AlN molar fractions up to 25%. Ohba et al. further reported that, when electrodes were added to the device just described, the device emitted high-intensity light, but no coherent light was generated, even near the breakdown voltage of the device.
In Japanese Laid-Open Patent Application No. H 10-242587, Nagahama et al. stated: “In the case of an LD (laser diode), a cladding layer that provides optical confinement must be grown preferably with a thickness of at least 0.1 &mgr;m, but if a thick AlGaN layer is grown directly on GaN and AlGaN layers, cracks will develop in the AlGaN layer that is subsequently grown. This has made device production difficult in the past.” Nagahama et al. went on to disclose a technique that introduced a 10 to 500 nm-thick anti-cracking layer on which a layer can be grown thick enough for the subsequently-grown aluminum-containing layer to function as a cladding layer. Na

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