Semiconductor structures using a group III-nitride...

Details Semiconductor structures using a group III-nitride... Semiconductor structures using a group III-nitride...

1999-03-26

2003-02-18

Tran, Minh Loan (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Incoherent light emitter structure

With particular semiconductor material

C257S012000, C257S013000, C257S022000, C257S096000, C257S184000, C257S192000, C257S197000

Reexamination Certificate

active

06521917

ABSTRACT:

FIELD OF THE INVENTION
This application relates to semiconductor structures and processes, and particularly relates to group Ill-nitride materials systems and methods such as might be used in blue laser diodes.
BACKGROUND OF THE INVENTION
The development of the blue laser light source has heralded the next generation of high density optical devices, including disc memories, DVDs, and so on.
FIG. 1
shows a cross sectional illustration of a prior art semiconductor laser devices. (S. Nakamura, MRS BULLETIN, Vol. 23, No. 5, pp. 37-43, 1998.) On a sapphire substrate
5
, a gallium nitride (GaN) buffer layer
10
is formed, followed by an n-type GaN layer
15
, and a 0.1 &mgr;m thick silicon dioxide (SiO
2
) layer
20
which is patterned to form 4 &mgr;m wide stripe windows
25
with a periodicity of 12 &mgr;m in the GaN<1-100>direction. Thereafter, an n-type GaN layer
30
, an n-type indium gallium nitride (In
0.1
Ga
0.9
N) layer
35
, an n-type aluminum gallium nitride (Al
0.14
Ga
0.86
N)/GaN MD-SLS (
M
odulation
D
oped
S
trained-
L
ayer
S
uperlattices) cladding layer
40
, and an n-type GaN cladding layer
45
are formed. Next, an In
0.02
Ga
0.98
N/In
0.15
Ga
0.85
N MQW (
M
ultiple
Q
uantum
W
ell) active layer
50
is formed followed by a p-type Al
0.2
Ga
0.8
N cladding layer
55
, a p-type GaN cladding layer
60
, a p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
65
, and a p-type GaN cladding layer
70
. A ridge stripe structure is formed in the p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
55
to confine the optical field which propagates in the ridge waveguide structure in the lateral direction. Electrodes are formed on the p-type GaN cladding layer
70
and n-type GaN cladding layer
30
to provide current injection.
In the structure shown in
FIG. 1
, the n-type GaN cladding layer
45
and the p-type GaN
60
cladding layer are light-guiding layers. The n-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
40
and the p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
65
act as cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW layer
50
. The n-type In
0.1
Ga
0.9
N layer
35
serves as a buffer layer for the thick AlGaN film growth to prevent cracking.
By using the structure shown in
FIG. 1
, carriers are injected into the InGaN MQW active layer
50
through the electrodes, leading to emission of light in the wavelength region of 400 nm. The optical field is confined in the active layer in the lateral direction due to the ridge waveguide structure formed in the p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
65
because the effective refractive index under the ridge stripe region is larger than that outside the ridge stripe region. On the other hand, the optical field is confined in the active layer in the transverse direction by the n-type GaN cladding layer
45
, the n-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layers
40
, the p-type GaN cladding layer
60
, and the p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
55
because the refractive index of the of the active layer is larger than that of the n-type GaN cladding layer
45
and the p-type GaN cladding layer
60
, the n-type Al
0.14
Ga
0.86
N/GaN M D-SLS layer
40
, and the p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
60
. Therefore, fundamental transverse mode operation is obtained.
However, for the structure shown in
FIG. 1
, it is difficult to reduce the defect density to the order of less than 10
8
cm−
2
, because the lattice constants of AlGaN, InGaN, and GaN differ sufficiently different from each other that defects are generated in the structure as a way to release the strain energy whenever the total thickness of the n-type In
0.1
Ga
0.9
N layer
35
, the In
0.02
Ga
0.98
N/In
0.15
Ga
0.85
N MQW active layer
50
, the n-type AL
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
40
, the p-type Al
0.14
Ga
0.86
N/GaN MD-SLS cladding layer
65
, and the p-type Al
0.2
Ga
0.8
N cladding layer
55
exceeds the critical thickness. The defects result from phase separation and act as absorption centers for the lasing light, causing decreased light emission efficiency and increased threshold current. The result is that the operating current becomes large, which in turn causes reliability to suffer.
Moreover, the ternary alloy system of InGaN is used as an active layer in the structure shown in FIG.
1
. In this case, the band gap energy changes from 1.9 eV for InN to 3.5 eV for GaN. Therefore, ultraviolet light which has an energy level higher than 3.5 eV cannot be obtained by using an InGaN active layer. This presents difficulties, since ultraviolet light is attractive as a light source for the optical pick up device in, for example, higher density optical disc memory systems and other devices.
To better understand the defects which result from phase separation in conventional ternary materials systems, the mismatch of lattice constants between InN, GaN, and AlN must be understood. The lattice mismatch between InN and GaN, between InN and AlN, and between GaN and AlN, are 11.3%, 13.9%, and 2.3%, respectively. Therefore, an internal strain energy accumulates in an InGaAlN layer, even if the equivalent lattice constant is the same as that of the substrate due to the fact that equivalent bond lengths are different from each other between InN, GaN, and AlN. In order to reduce the internal strain energy, there is a compositional range which phase separates in the InGaAlN lattice mismatched material system, where In atoms, Ga atoms, and Al atoms are inhomogeneously distributed in the layer. The result of phase separation is that In atoms, Ga atoms, and Al atoms in the InGaAlN layers are not distributed uniformly according to the atomic mole fraction in each constituent layer. In turn, this means the band gap energy distribution of any layer which includes phase separation also becomes inhomogeneous. The band gap region of the phase separated portion acts disproportionately as an optical absorption center or causes optical scattering for the waveguided light. As noted above, a typical prior art solution to these problems has been to increase drive current, thus reducing the life of the semiconductor device.
As a result, there has been a long felt need for a semiconductor structure which minimizes lattice defects due to phase separation and can be used, for example, as a laser diode which emits blue or UV light at high efficiency, and for other semiconductor structures such as transistors.
SUMMARY OF THE INVENTION
The present invention substantially overcomes the limitations of the prior art by providing a semiconductor structure which substantially reduces defect densities by materially reducing phase separation between the layers of the structure. This in turn permits substantially improved emission efficiency.
To reduce phase separation, it has been found possible to provide a semiconductor device with InGaAlN layers having homogeneous In content, Al content, Ga content distribution in each layer. In a light emitting device, this permits optical absorption loss and waveguide scattering loss to be reduced, resulting in a high efficiency light emitting device.
A quaternary material system such as InGaAlN has been found to provide, reproducibly, sufficient homogeneity to substantially reduce phase separation where the GaN mole fraction, x, and the AlN mole fraction, y, of all the constituent layers in the semiconductor structure satisfy the condition that x+1.2y nearly equals a constant value.
A device according to the present invention typically includes a first layer of InGaAlN material of a first conductivity, an InGaAlN active layer, and a layer of InGaAlN material of an opposite conductivity successively formed on one another. By maintaining the mole fractions essentially in accordance with the formula x+1.2y equals a constant, for example on the order of or nearly equal to one, the lattice constants of the constituent layers remain substantially equal to each other, leading to decreased generation of defects.
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