William Angerer - Department of Physics and Astronomy - University ...

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a photoluminescence measurement.
The temperature of the GaN film was measured by a Lake Shore Cryotronic,
Inc. carbon-glass resistor model CGR-1-2000. This device was developed for use
with a Lake Shore temperature monitor device as an extremely accurate temperature
sensor over a wide temperature range. We did not have access to a commercial
temperature system.
We built the system displayed in Fig. 4.11 to monitor the
temperature. The resistance of the carbon-glass resistor is a function of temperature.
By measuring the voltage drop across the carbon-glass resistor for a fixed current
input, the resistance and, hence, temperature are measured. While this system is
much less accurate than the commercial systems, the photoluminescence spectra of
GaN at ",,80 K is insensitive to temperature change on the order of a few K. Our
system has the necessary temperature resolution to this level.
Figs. 4.12 and 4.13 display the photoluminescence for an undoped GaN /Ah03
sample. The data shows weak structure from ",,390-410 nm (3.02-3.18 eV) at 293 K
and a much stronger signal at 378.5 nm (3.276 e V). The difference in scale of the photoluminescence
data indicates that the photoluminescence intensity is much stronger
at lower temperatures. This data is consistent with shallow donor acceptor recombination.
Lowering the temperature of the GaN/Ah03 sample increases the population
in these states and, thus, facilitates photoluminescence.
Figs. 4.14 and 4.15 display photoluminescence spectra for an n-doped Ga.\" sam-
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