Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-04-09
2002-08-06
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06430202
ABSTRACT:
BACKGROUND
The present invention relates generally to the field of laser diodes, and more particularly to short-wavelength nitride based laser diodes. Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as high density-optical storage.
Laser diodes based on higher bandgap semiconductor alloys such as AlGaInN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, S. Nakamura et al., “CW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates”, Applied Physics Letters, Vol. 72(6), 2014 (1998), S. Nakamura and G. Fasol, “The Blue Laser Diode-GaN based Light Emitters and Lasers”, (Springer-Verlag, 1997) and also A. Kuramata et al., “Room-temperature CW operation of InGaN Laser Diodes with a Vertical Conducting Structure on SiC Substrate”, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998), all of which are incorporated by reference in their entirety.
For laser diodes and arrays incorporated into printing systems, reliable, low threshold operation is a basic requirement. Among the difficulties associated with achieving low threshold operation is the confinement of injected electrons in the quantum well active region. If the injected electrons are not appropriately confined, the electrons leak away from the quantum well active region and recombine with the holes injected from the p-layers of the structure. For example, in the nitride laser structure pioneered by S. Nakamura of Nichia Chemical Company, a thin, high bandgap layer is placed immediately adjacent to the active region to confine the injected electrons. In the Nakamura structure, a 200 Å layer of Al
0.2
Ga
0.8
N:Mg acts as a tunnel barrier layer to prevent the energetic electrons (electrons having sufficient energy to escape from the quantum wells) from diffusing into the p-type material, where recombination with the available holes would occur. Reducing electron leakage lowers the laser threshold current and its temperature sensitivity while raising the quantum efficiency of the laser.
FIG. 1
shows conventional nitride laser structure
100
. Conventional nitride laser structure
100
uses both GaN:Mg p-waveguide layer
115
and GaN:Si n-waveguide layer
116
with Al
0.2
Ga
0.8
N:Mg tunnel barrier layer
110
positioned over In
0.12
Ga
0.88
N/In
0.02
Ga
0.98
N:Si multiple quantum well active region
120
. Al
0.07
Ga
0.93
N:Mg p-cladding layer
130
is positioned over p-waveguide layer
115
while Al
0.07
Ga
0.93
N:Si n-cladding layer
131
is positioned below n-waveguide layer
116
. GaN:Mg layer
140
serves as a capping layer to facilitate ohmic contact while Al
2
O
3
layer
150
serves as the growth substrate. An Ni/Au p-contact (not shown) on top of GaN:Mg layer
140
, a Ti/Al contact (not shown) on exposed surface of GaN:Si layer
155
. GaN:Si layer
155
is a lateral contact layer while In
0.03
Ga
0.97
N:Si layer
156
is the defect reduction layer to prevent defect propagation. GaN layer
160
functions as a buffer layer.
FIG. 2
illustrates the function of tunnel barrier layer
110
using a simplified band diagram. Tunnel barrier layer
110
is a p-type AlGaN layer which acts as a strong confinement barrier for injected electrons. Quantum wells
220
,
221
,
222
,
223
and
224
comprising active region
120
are InGaN while tunnel barrier layer
110
is AlGaN. The potential energy level
250
for the conduction band electrons and quasi-fermi level
255
are shown for AlGaN tunnel barrier layer
110
with low p-doping energy level
230
and high p-doping energy level
240
are shown with respect to potential energy level
250
for electrons and fermi level
255
for the conduction band. Quasi-fermi level
260
for the holes is shown along with potential energy level
265
for holes. Successful operation of Nakamura type laser structures requires successful p-type doping of high-bandgap AlGaN tunnel barrier layer
110
. However, the growth of tunnel barrier layer
110
presents many practical difficulties, including the difficulty of p-doping high aluminum content alloys and the difficulty of reliably growing high aluminum content alloys because of parasitic pre-reactions between trimethylaluminum and ammonia during metalorganic chemical vapor deposition (MOCVD). If the hole concentration or aluminum content in tunnel barrier layer
110
is insufficient, the ability of layer
110
to contain electrons is reduced since electron confinement increases with the p-type doping level.
P-cladding layer
130
can be used to confine injected electrons in a nitride laser diodes if it is placed in close proximity, typically within 1 minority carrier diffusion length, to the multiple-quantum well active region. A difficulty with this approach is that multiple-quantum well active region
120
is typically located at the core of a waveguide region to maximize the spatial overlap with the optical mode as shown in
FIG. 3
for conventional nitride laser diode structure
100
. However, this places p-cladding layer more than 1 minority carrier diffusion length from multiple-quantum well region
120
. Refractive index profile
310
and corresponding fundamental transverse optical mode
320
are shown as a function of distance relative to the interface between n-cladding layer
131
and n-waveguide layer
116
. The waveguide thickness is adjusted independently to maximize the optical confinement factor, &Ggr;. Optical confinement factor, &Ggr; is the fraction of the power that overlaps multiple-quantum well active region
120
where the optical gain is generated. For nitride laser diodes, the typical thickness for the waveguide above and below multiple-quantum well active region
120
is about 100 nm which is greater than 1 electron diffusion length. This places p-cladding layer
130
in conventional nitride laser diode structure
100
to far away from multiple-quantum well active region
120
to confine the injected electrons.
SUMMARY OF INVENTION
In accordance with the present invention, a p-type cladding layer is used to eliminate the p-type waveguide and eliminate the need for a p-type, very high bandgap, high-aluminum content AlGaN tunnel barrier layer in nitride laser diodes. The p-type cladding layer is used to suppress electron leakage. In addition to the p-type cladding layer, a high-Al-content tunnel barrier, a superlattice structure or a distributed electron reflector may be placed at the multiple quantum well region/p-cladding layer interface. Although a p-type cladding layer is used for suppressing electron leakage in laser diodes fabricated from other materials such as arsenides and phosphides, the use of p-cladding layer in nitride laser diodes is not straightforward. The minority carrier diffusion lengths (average distance minority carrier travels before recombination occurs) in nitrides are many times shorter than in other laser diode materials. Hence, the p-cladding layer typically lies several diffusion lengths away from the multiple-quantum well active region. Consequently, injected electrons are not appreciably confined by the p-cladding layer which leads to the requirement for the high-aluminum content tunnel barrier layer. In red and infrared laser diodes, the waveguide thickness is a mere fraction of the diffusion length, so that the cladding layer can effectively suppress leakage, see for example, “Drift leakage current in AlGaInP quantum well laser diodes, “D. P. Bour, D. W. Treat, R. L. Thomton, R. S. Geels, and D. F. Welch, IEEE Journal of Quantum Electronics, vol. 29, pp. 1337-1343 (1993).
A high optical confinement factor can still be achieved for nitride laser diode structures if a p-cladding layer is positioned adjacent to the multiple-quantum well active region instead of the typical 100 nm distance away which maximizes the optical conf
Bour David P.
Kneissl Michael A.
Romano Linda T.
Van de Walle Christian G.
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