Growth and Optical Properties of GaN Grown by MBE on Novel Lattice-Matched Oxide Substrates
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Among the problems which stand in the way of the realisation of a laser diode we may identify: (1) The very high density of structural imperfections, that may be described either as dislocations [2] or grain boundaries [3], (2) The relative difficulty of p-doping, ascribed to acceptor passivation, (3) The presence of an uncontrolled deep centre, that contributes to a strong yellow luminescence band in competition with the bandedge emission [4]. All of these problems relate, to a greater or lesser extent, to the lattice mismatch that exists in all cases between the I11-N epilayer or heterostructure and the underlying substrate. The purpose of this contribution is to advocate the use of some novel oxide substrates for the epitaxial growth of GaN and its II1-N alloys. Table 1 compares the structural properties of several III-N compounds with those of the substrates that may be used for III-N epitaxy. It is clear that the lithium salts offer much better lattice matching than any of the conventional substrates. In addition, they have comparable coefficients of
535 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
thermal expansion. This is necessary to preserve lattice matching at the high growth temperatures used for widegap II-V materials.
Crystal
Band Gap (eV)
a-axis (nm)
Mismatch %
Thermal (10-6 /K)
AIN
6.20
0.3112
-2.10
4.2
GaN
4.20
0.3180
-
5.6
InN
1.95
LiAIO 2
6.20
0.3134
-1.45
7.5
LiGaO 2
4.20
0.3186
+0.19
7
Li 2 GeO 3
5.35
0.3203
-0.16
---
A12 0 3
8.70
0.3476
9.30
7.5
ZnO
5.60
0.3252
2.26
2.9
6H-SiC
3.70
0.3080
3.14
---
2.30
0.32530
1
---
I
Table 1: structuralpropertiesof ll-N compounds and substrates. In terms of crystal structure, lithium aluminate (LiA10 2 ) is tetragonal, while the gallate and germanate are orthorhombic. These compounds all feature purely tetrahedral bonding, so that they may be said to be "structure-matched" as well as lattice-matched to both the wurtzite and cubic forms of the III-N semiconductors. . In addition, they are all widegap materials with high transparency in the ultraviolet spectral region. Finally, the germanate is particu;arly interesting dfor laser applications because of its strong cleavage normal to the direction of growth (c-axis). SAMPLE GROWTH The melting points of the aluminate, gallate and germanate are 1275 °C, 1600 oC and 1700 OC.respectively. At the Optical Materials Centre in the University of Strathclyde, Li2GeO3 and LiGaO2 were grown by the Czochrlski technique using an induction heated furnace. The crystals were grown in a nitrogen atmosphere from a melt contained in an iridium crucible. Seed material was obtained by spontaneously nucleating a stoichiometric melt. The crystal growth furnace employs power/crystal weight feedback to control the
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growth rate. The optimum growth conditions for both the germanate and gallate was found to be a translation rate of 1-2 mm per hour and a rotation rate of 20-30 rpm. GaN layers were deposited on the gallate substrate at the University of Nott
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