Hexagonal GaN is grown on (0001) sapphire substrates using an atmospheric pressure organometallic vapor phase epitaxy method. We investigate the influences of the initial treatment of sapphire substrate, such as initial nitridation and low-temperature GaN buffer layer deposition, upon the surface morphology and crystallinity. The thermal stability of grown GaN layers is also investigated using a thermal etching process in a H2 atmosphere in order to obtain the information concerning the surface polarity of GaN(0001) layers. When sapphire substrates are initially nitrided, highly crystalline GaN layers with large hexagonal facets are obtained and its surface appears to be (0001)N. On the other hand, the deposition of a thicker buffer layer on the nitrided sapphire substrates improves the surface morphology, and the surface polarity of the mirror surface appears to be (0001)Ga. The initial nitridation of sapphire substrates and the GaN buffer layer deposition are considered to be important processes from viewpoints of the (0001) surface polarity.
High-quality GaAs epilayers with dislocation densities of 1.2×106 cm−2 on (100)Si substrates have been obtained by insertion of an InGaAs strained interlayer combined with thermal cycle annealing instead of strained layer superlattices. All the layers were grown by low-pressure metalorganic vapor phase epitaxy. The threading dislocation density near the surface of 4 μm thick GaAs was measured by plan-view transmission electron microscopy. The threading dislocation density was found to be very sensitive to the In composition of the interlayer and the specifics of thermal cycle annealing.
We have investigated the influence of low-temperature buffer
layer deposition conditions, such as thickness and thermal annealing
time, on the properties of the high-temperature GaN growth layer. The
surface morphology of the buffer layer after thermal annealing at
1040°C depends on both the thickness of the buffer layer and
the annealing time. When a thick buffer layer was used, large
trapezoidal growth nuclei were formed after annealing, which led to
the poor crystallinity of the GaN growth layer. On the other hand,
when a thin buffer layer or a fully annealed thick buffer layer was
used, small growth nuclei having a relatively small misorientation
were formed, which led to good crystallinity of GaN growth
layer. Thus, the thermal annealing time must be optimized, taking the
thickness of the buffer layer into the consideration.
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