Method for producing a gallium nitride epitaxial layer

Details Method for producing a gallium nitride epitaxial layer Method for producing a gallium nitride epitaxial layer

2000-07-07

2001-12-04

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

With decomposition of a precursor

C117S096000, C117S097000, C117S952000, C257S103000

Reexamination Certificate

active

06325850

ABSTRACT:

The present invention relates to a process for producing an epitaxial layer of gallium nitride (GaN) as well as to the epitaxial layers of gallium nitride (GaN) which can be obtained by said process. Such a process makes it possible to obtain gallium nitride layers of excellent quality.
It also relates to the short-wavelength optical devices or the high-power high-frequency electronic devices provided with such an epitaxial gallium nitride layer.
It relates in particular to optoelectronic components formed on such gallium nitride layers.
Processes are known for obtaining relatively thick GaN layers, for example from 100 to 200 micrometers. The method commonly used is chloride and hydride vapor phase epitaxy (HVPE). Either sapphire substrates or GaN layers on sapphire 200 micrometers in thickness are used, these layers being fabricated by OrGaNoMetallic Pyrolisis Vapor Phase Epitaxy (OMVPE). However the crystal lattice parameter mismatch between sapphire and GaN is such that the build-up of stresses in the layers results in cracks and prevents the sapphire substrate from being removed. All the experimental innovations (treatment of the surface of the sapphire at the start of growth with GaCl, deposition of a ZnO interlayer) have not made it possible to solve this problem. At the present time, the relatively thick GaN layers have a double X-ray diffraction (DXD) line width of the order of at best 300 arcsec, which means that the crystallographic quality does not exceed that of the layers formed by OMVPE or by molecular beam epitaxy (MBE).
In other words, no potential sapphire, ZnO, 6H—SiC or LiAlO
2
substrate is ideal for nitride epitaxy (excessively high lattice mismatch and thermal expansion coefficient mismatch, thermal instability).
Moreover, the lasing effect (by optical pumping) on GaN has been known for a long time. Although diode lasers based on III-V nitride have been produced, the crystal quality of the nitride layers constituting the structure of these lasers is very average. Dislocation densities ranging from 10
9
to 10
10
cm
−2
have been measured.
In fact, the defects associated with the formation of relatively thick epitaxially grown GaN layers indicated above have considerably slowed down the development of diode lasers provided with such layers: high residual n, absence of single crystals and of suitable substrates, impossibility of producing p-doping.
The publication by D. Kalponek et al., Journal of Crystal Growth, 170 (1997) 340-343 mentions the localized nitride growth in apertures formed in a mask so as to form pyramidal structures. However, this document neither describes nor suggests the formation, by coalescence, of features or islands of smooth gallium nitride layers.
The publication Y. Kato, S. Kitamura, K. Hiramatsu and N. Sawaki, J. Cryst. Growth, 144, 133 (1994) describes the selective growth of gallium nitride by OMVPE on sapphire substrates on which has been deposited a thin gallium nitride layer masked by an SiO
2
layer etched so as to reveal continuous bands of gallium nitride.
However, the localized epitaxy thus carried out involves neither the lateral growth nor the growth anisotropy as will be described below.
The document EP 0,506,146 describes a process for local and lateral growth using a mask, shaped by lithography, to localize the growth. The examples of smooth layers relate in no case to gallium nitride. These examples mention GaAs homoepitaxy on a GaAs substrate and InP homoepitaxy on an InP substrate.
The object of the process according to the invention is to obtain crystalline layers allowing the production of optoelectronic devices (especially diode lasers) having life times and performance characteristics which are superior to those obtained previously.
SUMMARY OF INVENTION
The inventors have found that the treatment of a substrate by deposition of a suitable dielectric followed by deposition of gallium nitride, which is itself followed by thermal annealing, causes the formation of gallium nitride islands which are virtually defect-free.
The coalescence of such islands caused by the heat treatment results in a gallium nitride layer of excellent quality.
The invention relates firstly to a process for producing a layer of gallium nitride (GaN), characterized in that it comprises the deposition on a substrate of a dielectric layer functioning as a mask and the regrowth of gallium nitride on the masked substrate under epitaxial deposition conditions so as to induce the deposition of gallium nitride features and the anisotropic and lateral growth of said features, the lateral growth being continued until coalescence of the various features. The term “islands” instead of “features” may also be employed.
The substrate generally has a thickness of a few hundred micrometers (in particular, approximately 200 micrometers) and may be chosen from the group consisting of sapphire, ZnO, 6H—SiC, LiAlO
2
, LiGaO
2
and MgAl
2
O
4
. The substrate is preferably treated beforehand by nitriding.
Preferably, the dielectric is of the Si
x
N
y
type, especially Si
3
N
4
. SiO
2
may also be mentioned, but other well-known dielectrics could be used. The deposition of the dielectric is carried out in the gallium nitride growth chamber from silane and ammonia.
Preferably, the carrier gas is an N
2
/H
2
mixture.
According to a first embodiment, the dielectric layer is an atomic monolayer, or a cover of the order of the atomic plane.
Next, epitaxial regrowth on the substrate is carried out using OMVPE. Regular features or islands develop. Examination in a high-resolution electron microscope shows that the GaN dislocation density in the regular features or islands, which has therefore grown without heteroepitaxial strains, is very much less than that produced by the direct deposition of gallium nitride on the substrate. Thus, the GaN growth, which takes place laterally in the [10{overscore (1)}0] directions on a dielectric surface, and therefore without being in epitaxial relationship with the sapphire substrate, results in much better GaN crystal quality than the usual processes. After said features have been obtained, the growth may be continued, either using OMVPE or HVPE. Growth takes place laterally, until coalescence of the islands. These surfaces resulting from the coalescence of islands exhibit crystal quality superior to the layers grown heteroepitaxially on sapphire.
The gallium nitride deposition is generally carried out in two steps. A first step, at a temperature of approximately 600° C. for the deposition of a buffer layer, from which the GaN features will emerge, then at a higher temperature (approximately 1000-1100° C.) for the growth of an epilayer from said features.
According to a second embodiment, the invention relates to a process characterized in that the dielectric layer is etched, so as to define apertures and to expose the facing regions of the substrate, and gallium nitride is regrown under epitaxial deposition conditions on the masked and etched substrate so as to induce the deposition of gallium nitride features on the facing regions and the anisotropic and lateral growth of said features, the lateral growth being continued until coalescence of the various features.
According to a third embodiment, the invention relates to a process for producing an epitaxial layer of gallium nitride (GaN), comprising the deposition of a thin gallium nitride layer on a substrate characterized in that:
a dielectric layer is deposited on said thin gallium nitride layer;
the dielectric layer is etched so as to define apertures and to expose those regions of said thin gallium nitride layer which face them;
gallium nitride is regrown under epitaxial deposition conditions on the expitaxially grown, masked and etched substrate so as to induce the deposition of gallium nitride features on the facing regions and the anisotropic and lateral growth of said features, the lateral growth being continued until coalescence of the various features.
The process according to the invention is noteworthy in that it limits the density of defects gen

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