Epitaxial Growth of GaN on Lattice-Matched Hafnium Substrates
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ABSTRACT A method of producing epitaxial GaN on single-crystal Hf has been developed. The metal substrate is formed by a strain-anneal process yielding macroscopic (5-mm) grain sizes, followed by polishing, chemical etching, and Ar ion sputtering at elevated temperature in ultrahigh vacuum. The growth is conducted by plasma-assisted molecular beam epitaxy using an initial passivation layer deposited at low temperature and subsequent growth at 700 'C. The resulting films are in registry with the hcp substrate lattice as observed by reflection high-energy electron diffraction during growth and verified by plan-view transmission electron microscopy. High-resolution x-ray rocking curve linewidths of the GaN and Hf [ 1012] peaks are as narrow as 900 and 180 arc seconds, respectively. The [0002] peak separation confirms the approximately 2.7% mismatch in the c axis spacing. Initial photoluminescence observations at 20 K of a (donor-bound exciton) peak at 3.467 eV are consistent with the assumption of a nearly strain-free film resulting from the exact basal-plane lattice match and close thermal coefficient match between GaN and Hf.
INTRODUCTION The goals of optoelectronic device development in the III-N materials system require highcrystal-quality strain-free films, first for the advancement of fundamental understanding of the
materials and second for the fabrication of high-performance devices such as laser diodes. It is by now widely appreciated that the "device-quality" CVD-grown GaN on sapphire has a very high dislocation density [1] and in general will be under compressive stress due to the thermal coefficient mismatch, although special processing techniques apparently allow substantial relaxation of the thermal strain. In contrast, films grown on SiC or Si are put in tension during the cool-down from growth temperatures. In the absence of any viable bulk GaN growth technology, there is a compelling incentive to develop lattice-matched heteroepitaxial substrates. The group-IVB refractory metals Ti, Zr, and especially Hf comprise an intriguing alternative that has been under development by the present authors. The characteristics of these metals that suit them particularly well for the present applications can be summarized as follows: • • • * • *
Hexagonal close-packed lattice (induces wurtzite-structure nitride films). Exact basal-plane lattice match between Hf and GaN. Small mismatch along the c axis (2.7%.) Solid solutions can match a range of AIGaN and InGaN lattice parameters. Close thermal expansion coefficient match between Hf and GaN (about 4.6% mismatch). Low vapor pressure at nitride growth temperatures.
* High thermal and electrical conductivity (built-in ohmic contact and heat sink). 55 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
Along with these notable advantages come several challenges, principally the facts that singlecrystal Hf is not readily available and GaN is not thermodynamically stable on Hf. The former problem is addressed in the present work with a strain-anneal
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