Defect reduction in GaN and related materials

Details Defect reduction in GaN and related materials Defect reduction in GaN and related materials

2001-04-16

2003-12-02

Coleman, William David (Department: 2823)

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

Specified wide band gap semiconductor material other than...

C257S183000, C257S615000

Reexamination Certificate

active

06657232

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to materials with reduced bulk and surface defects and more particularly to GaN layers for optoelectronic materials, electronic materials, and other materials with reduced defects.
2. Background Description
Gallium nitride and its alloys with InN and AlN have recently emerged as important semiconductor materials with applications to yellow, green, blue and ultraviolet portions of the spectrum as emitters and detectors, and as high power/temperature electronics. Estimates are in the billions of dollars per annum for business activity surrounding nitride semiconductor based light emitters and to some extent power devices.
GaN and related heterostructures, however, suffer from a large concentration of structural and point defects. This is due to lack of native substrates being available. The most commonly used substrate is sapphire. There is a large lattice and thermal mismatch between GaN and sapphire. To circumvent this, a process of called “Lateral Epitaxial Overgrowth” is utilized in many instances. This process is imperative in lasers with long longevity. This process requires a growth sequence to be completed. Then the wafer is removed from the reactor, patterned with SiO
2
or Si
3
N
4
and put back in the growth vessel for the continuation of the growth. The post pattern growth process is tailored to promote lateral growth followed by vertical growth after complete coalescence. During lateral growth, the area above the dielectric mask grows out and merges with the one from the other side. Fundamentally, that region of the material will have structural defects unless the GaN below the dielectric mask is coherent. A schematic representation of this process is shown in FIG.
1
. As can be seen in
FIG. 1
, there is shown a cross-sectional view of a portion of a wafer. A GaN layer
1
over a substrate
2
has a SiO
2
dielectric mask
3
. GaN epitaxy
4
is grown over dielectric mask
3
. Defects form in lateral growth wings
5
over the dielectric mask
3
. There are still many technological and fundamental problems with this approach and the method is viewed as temporary.
Another approach that has been explored is to grow very thick, 200-300 &mgr;m GaN layers on sapphire by hydride Vapor Phase Epitaxy, remove the GaN layer from the substrate, polish both sides, and fine polish to render the top face epitaxy worthy. With process conditions that have not been reported, one laboratory was able to reduce the extended defect concentration near the top surface. However, the seemingly same approach has not yielded the same success in other laboratories. In any case, this too is a rather involved process.
Nitride semiconductors have been deposited by vapor phase epitaxy (i.e., both hydride VPE (HVPE) which has been developed for thick GaN layers and organometallic VPE (MOVPE) which has been developed for heterostructures), and in a vacuum by a slew of variants of molecular beam epitaxy (MBE).
Nitride-based light emitting diodes (LEDs) with lifetimes approaching 100,000 hours (extrapolated) and brightness near 70 lm/W in the green have been obtained. These LEDs are already being used in full color displays, moving signs, traffic lights, instrumentation panels in automobiles and aircraft, airport runways, railway signals, flashlights, underwater lights. The technology is in the process of being extended to standard illumination under the name “Solid State Lighting” (SSL). SSL is expected to result in substantial energy savings by as much as a factor of six compared to standard tungsten bulbs. Along similar lines, blue lasers are being explored as the read and write light source for increased data storage density for the next generation of digital video disks (DVDs). Already, the room temperature CW operation in excess of 10,000 hours has been reported. To be versatile, this level of lifetime with a power level of about 20 mW at 60 C. is required. The present device lifetimes under these more stringent operating conditions are near 400 hours which is a long way from the needed 10,000 hours.
The large bandgap of GaN with its large dielectric breakdown field, coupled with excellent transport properties of electrons and good thermal conductivity, are well suited for high power electronic devices. Already, high power modulation doped field effect transistors (MODFETs) with a record power density of about 10 W/mm in small devices, and a total power of 8 W in large devices have been achieved. In addition to high power, and high frequency operation, applications include amplifiers that operate at high temperature and other unfriendly environments, and low cost compact amplifiers for earthbound and space applications.
When used as UV sensors in jet engines, automobiles, and coal burning furnaces (boilers), GaN-based devices will allow optimal fuel efficiency and control of effluents for a cleaner environment. Again, this is a direct result of the large bandgaps accessible by nitrides, as well as their robust character. GaN/Al
x
Ga
1−x
N (from now on denoted GaN/AlGaN) UV pin detectors have demonstrated sensitivities of about 0.20 A/W or higher, and speed of response below a nanosecond.
Despite this progress, the defect concentrations of both structural and point defects are still high. This is mainly attributed to the lack of native substrates. To circumvent this somewhat, a flurry of activity has been expended on lateral overgrowth methods to block dislocations. However, if and when the base layer lacks long range coherence, the overgrown layer will naturally lack that coherence making it rather doubtful that a defect free material will emerge where the lateral growth fronts meet. Nevertheless, lasers with long longevity could be obtained only by this process as the overall structural defect density is reduced, primarily above the masked regions, by several orders of magnitude to around 10
7
-10
8
cm
−2
from about 10
10
cm
−2
.
For electronic devices to hold promise in a given semiconductor, carrier mobility is generally used as a figure of merit. In addition, the carrier mobility is also used to deduce information regarding scattering centers and processes involved. GaN is no exception and consequently electron mobility in samples prepared by various methods has been a subject of discussion. In this vein, room temperature electron mobilities for MOVPE grown silicon doped GaN layers are typically reported to be in the range of 350-600 cm
2
V
−1
S
−1
, whereas that reported for hydride vapor phase epitaxy (in several tens of microns thick layers) is about 800 cm
2
V
−1
s
−1
. The highest room temperature mobility ever reported for GaN was 900 cm
2
V
−1
s
−1
deposited by MOVPE, which has not been confirmed, for a 4 &mgr;m thick layer. In contrast, the highest room temperature mobility for plasma-MBE grown GaN is around 300 cm
2
V
−1
s
−1
on sapphire substrates and 560 cm
2
V
−1
s
−1
on SiC for ammonia-MBE on sapphire is about 550 cm
2
V
−1
s
−1
in 2 &mgr;m thick layers. More recently, a combination of lateral epitaxial overgrowth by MOCVD and subsequent growth by RF MBE method has resulted in relatively high electron mobilities in GaN, approaching 800 cm
2
V
−1
s
−1
. The MOCVD grown layers are several microns thick whereas the MBE grown layers are thin and grown at growth rates in the low tenth of a micron per hour range. These figures compare with earlier predictions, which seemed to have converged around 900 cm
2
V
−1
s
−1
. Recently, these predictions had to be revisited as the room temperature mobility in modulation doped AlGaN/GaN structures began to approach about 2,000 cm
2
V
−1
s
−1
. Electron mobilities limited by polar optic phonon scattering have been predicted by Ridley to be 2200 cm
2
V
−1
s
−1
for an electron effective mass of m*=0.22 m
0
.
SUMMARY OF THE INVENTION
The present invention is directed to reducing defects emanatin

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