Production of epitactic GaN layers on substrates

Details Production of epitactic GaN layers on substrates Production of epitactic GaN layers on substrates

2000-06-27

2001-07-03

Kunemund, Robert (Department: 1765)

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

Forming from vapor or gaseous state

With decomposition of a precursor

C117S104000, C117S952000

Reexamination Certificate

active

06254675

ABSTRACT:

The present invention relates to a process for the application of an epitactic GaN layer to a substrate by pyrolysis of precursor compounds.
Owing to the development of blue light emitting diodes (LEDs) and laser diodes, gallium nitride (GaN) has recently increased in importance. Hexagonal GaN has a direct band gap of about 3.4 eV (Lacklison et al., J. Appl. Phys. 78 (1995), 1838), enabling it to be employed very well in short-wave optical devices (Nakamura and Fasol, The Blue Laser Diode: GaN-based Light Emitters and Lasers, Berlin, Springer Verlag, 1997; Pearton and Kuo, MRS Bulletin 22 (1997), 17).
At present, single-crystal GaN layers for opto-electronic components are produced by deposition of atoms or molecules from the gas phase (chemical vapor deposition, CVD) or by molecular beam epitaxy (MBE), as described in Lin et al. (J. Appl. Phys. 74 (1993), 5038) and Lee et al. (J. Crystal Growth 182 (1997), 11). In the MBE process, metallic gallium and molecular nitrogen are generally used as starting materials. In most CVD processes, by contrast, volatile organo-metallic precursor compounds, such as, for example, trimethylgallium or triethylgallium, and ammonia are employed. The disadvantages of these processes are that they are complex and expensive.
The object on which the invention is based was thus to provide a simpler and economically advantageous process for the production of epitactic GaN layers on substrates.
Surprisingly, it has been found that epitactic GaN layers can be produced on substrates by applying carbodiimide-containing precursor compounds followed by pyrolysis. After the application, the precursor compounds are preferably distributed uniformly, for example by spin coating, dip coating or by spray coating. The thickness of the resultant layer can be varied depending on the rotational speed during spin coating and the viscosity of the precursor solution. Under ammonia as reactive gas, the precursor film can then be converted into crystalline GaN at temperatures of preferably about 900° C. By introducing doping elements into the precursor solution, the composition of the GaN layer can likewise be influenced.
The present invention thus relates to a process for the application of an epitactic GaN layer to a substrate, in which a gallium carbodiimide-containing precursor compound is applied to the substrate and converted by pyrolysis into crystalline GaN, in particular into GaN having a hexagonal or cubic crystal structure.
The precursor compound is obtainable by reaction of a gallium salt with a carbodiimide-containing organic compound under suitable reaction conditions under which gallium atoms can form a bond to carbodiimide. The gallium salt employed is preferably a gallium halide, particularly preferably GaCl
3
. The resultant precursor compound contains gallium and carbodiimide advantageously in a molar ratio of from 1:0.5 to 1:3, in particular in a molar ratio of from 1:0.8-1.2, for example of about 1:1.
The precursor compound can have a structure of the general formula (I):
Ga
w
(NCN)
x
(R)
y
A
z
  (I)
where
R is an organic or inorganic radical, preferably a silyl-containing radical, which may furthermore contain alkyl, alkenyl or aryl groups, halogen and/or hydrogen,
A is an anion, preferably a halide ion, and the ratio w:x:y:z is in the range from 1:0.5-3:0.01-1:0.5-1.5 and preferably from 1:0.8-1.2:0.01-0.3:0.8-1.2.
Particular preference is given to the gallium precursor compounds prepared by reaction of gallium halides with a compound of the formula (II)
R′
3
Si—N═C═N—SiR′
3
  (II)
in which
R′ are each, independently of one another, an organic or inorganic radical, for example a C
1
-C
3
alkyl or alkenyl group, preferably a methyl group, an aryl group, a halogen or H.
On reaction of the compound (II) with the gallium halide, the reaction products formed are a volatile trialkylhalosilane and the polymeric crosslinked gallium carbodiimide-containing precursor compound. After the preparation, the crosslinked polymer remains essentially dissolved in the compound (II) and, if necessary, further solvent. The precursor compound can be isolated as a solid by removal of volatile substances, for example under reduced pressure.
Suitable substrates for process according to the invention are basically any desired substrates which withstand the temperatures prevailing during the pyrolysis treatment. It is advantageous to use substrates which have similar coefficients of thermal expansion to cubic or hexagonal GaN. Preference is given to substrates comprising elements from main groups III to VI and sub-groups IV to VI of the Periodic Table of the Elements or compounds containing such elements. Preferred examples of substrates are elements or compounds having metallic or semiconducting properties, for example Si and GaAs. Further preferred examples are substrates having graphite surfaces or ceramic surfaces, for example SiC, Al
2
O
3
, Si
3
N
4
, TiC or TiN. The particularly preferred substrate is &agr;-Al
2
O
3
, in particular &agr;-Al
2
O
3
—R.
Particularly suitable for the deposition of heteroepitactic &agr;-GaN are &agr;-Al
2
O
3
(0001), &agr;-Al
2
O
3
(01-12), &agr;-Al
2
O
3
(11-20), &agr;-Al
2
O
3
(10-10), and 6H—SiC (0001). For cubic GaN (&bgr;-GaN), GaAs (001) as well as Si (001) and Si (111) substrates are of particular importance.
The substrate to be coated is brought into contact with a solution or suspension of the precursor compound or with the precursor compound itself if this is liquid, a layer of the precursor compound being deposited on the surface of the substrate. This layer of the precursor compound is then preferably distributed as uniformly as possible over the substrate surface to be coated. This distribution can be carried out by spin coating, dip coating or spraying. Application by spin coating is described in detail in the examples. In the case of dip coating, the substrate is preferably immersed in the precursor and then removed at a uniform, defined rate, the substrate being uniformly coated with the exception of the lower edge. In the case of spray coating, the precursor is atomized, and the substrate is held in the spray cone or spray mist. It is also conceivable to apply the precursor by spreading, for example with a brush.
The preparation of the precursor compounds and in particular the application of the precursor compound to the substrate preferably take place under an inert atmosphere, for example a noble-gas atmosphere, such as, for example, argon.
The pyrolysis treatment of the precursor compound applied to the substrate surface as a coating is carried out, in particular, by heating to a temperature of at least 600° C., preferably of about 900° C., under a reactive atmosphere containing ammonia. The ammonia partial pressure is preferably from 10
4
to 10
6
Pa, particularly preferably about 10
5
Pa.
An essential advantage of the process according to the invention consists in that the precursor compound can comprise further doping elements, such as Si, Ge and/or Mg. If the precursor compound is prepared by reaction of gallium halides with compounds of the formula (II), silicon is always present in the precursor and is also incorporated into the resultant GaN layer in small amounts (less than 1 atom-%). Further silicon and other doping elements, such as Mg or Ge, can be introduced by additions to the precursor, for example via elemental halides or auxiliary polymers. There are basically no restrictions with respect to the elements which are suitable. The doping elements are preferably employed in a proportion of up to 10 atom-%, based on the amount of gallium present in the precursor compound.
The thickness of the epitactic GaN layer can be varied within broad limits, for example by selecting precursor compounds of different viscosity and/or by repeating the coating operation one or more times until a layer having the respective desired thickness is built up. The thickness preferably extends from individual atom layers up to 10 &mgr;m. Thicknesses of up to 1 &mgr;m are particul

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