Much of the best GaN material grown today is produced in cold-wall MOCVD reactors by running TMGa and NH3 in a very high V-to-III gas-flow ratio,~103 and up to 40000, since a high NH3 partial pressure is necessary to alleviate nitrogen loss during the growth.
The aim of this work is to explore the growth of AlN, AlGaN and GaN on SiC substrates in a lower V-to-III gas-flow ratio range, which may have advantages in avoiding the excess ammonia supply, reduced adduct formation and less toxic exhaust while achieving layers of device quality material. The growth runs are performed in a hot-wall MOCVD system, which is a new approach to the MOCVD growth of III-nitrides, though for the growth of device quality SiC epitaxial layers the situation is different. Today, the hot-wall reactor design dominates the SiC CVD world market and it is proven to supply very good heating efficiency, temperature homogeneity and uniform characteristics of the layers.
In the present experiments, with nitrogen as a carrier, TMAl and TMGa are added to NH3 at N-to-Al and N-to-Ga gas-flow ratio of ~50 and 250, respectively. The NH3 flow is 50 ml/min and the precursors are delivered jointly.
High speed growth of epitaxial AlN and Al0.2Ga0.8N layers are achieved, 1.5 and 3 μm/h, respectively, at a growth temperature of 1200oC. The data analysis of the IR spectroscopic ellipsometry response of a 380 nm thick AlN, 55 nm thick Al0.2Ga0.8N, and 580 nm thick GaN layer provides the following results: E1(TO) phonon mode broadening values of 5cm-1, 4.2cm-1 and 4.6cm-1, respectively. Small E1(TO) broadening value indicates high compositional homogeneity and high crystal quality.
As a next step, we will show results from epitaxial growth under same growth conditions but using H2 as a carrier gas. For example, relatively higher E1(TO) broadening value, 5.7cm-1, is measured for an AlN layer with similar thickness,~340 nm, grown however in 60 min when in H2 ambient.
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