38-40 GaN v2 10/9/09 13:32 Page 40
technology nitrides
rhombus4
lead to an increase in nitrogen-deficiency and a reduction
in hydrogen contamination. A closer investigation of the X-
ray diffraction results reveals that the samples that
produce more intense photoluminescence contain several
crystalline phases. Each of these is hexagonal GaN, but
these phases contain slight variations in lattice spacing
that can be attributed to the increase in nitrogen
vacancies. After completing this work, I went in search of
new challenges in Dan Morse’s group at UCSB’s Institute
for Collaborative Biotechnologies. But it wasn’t long
before I found myself hankering for an even better
understanding of the interplay between the structure and
the properties of GaN, and wondering about the answers
to a series of nagging questions: How does the chosen
precursor influence photoluminescence? What extent of
Figure 3. Researchers at UCSB investigated the nitrogen-deficiency is needed to increase the emission
71
Ga NMR spectrum of a GaN sample with good intensity? Is the band edge related luminescence in GaN
photoluminescence properties (a) The sharp peak on powders directly or only indirectly (no oxygen contamination,
the right can be attributed to stoichiometric GaN with no structural protons) linked to the observation of
a gallium-to-nitrogen ratio of 1:1. Increases in nitrogen-deficient phases in the
71
Ga NMR spectrum?
band-edge-related photoluminescence are correlated
with increases in the area underneath a broad peak to Today I’m devoting some of my time to answering these
the left of the sharp signal at 324 ppm. This peak is questions in a team that now includes Brad Chmelka’s
caused by nitrogen deficiencies in the material, the group in the Department of Chemical Engineering. Our
chemical structure being GaN1-x. (b) NMR spectra understanding of the relationship between band edge
were also recorded for
15
N. The similarity in lineshape related photoluminescence and the levels of nitrogen
between the two spectra is quite striking. deficiency in GaN nanostructures is still based on
This data was first presented in Solid State Sciences, experimental evidence, but we now are trying to tie these
8, 1193 (2006). results to underlying physical principles. For example, we
have confirmed our hypothesis that nitrogen-deficiencies
are responsible for the increase in photoluminescence by
and 3 for details). These results indicated that the converting non-luminescing GaN - which was prepared
observed intense photoluminescence could be caused by from gallium oxide under nitrogen gas at 1100 °C - into
nitrogen deficiencies. What’s more, they also contradicted luminescing GaN. Again, the results of NMR
the assumption that stoichiometically balanced GaN is spectroscopy revealed changes at the atomic level.
needed to produce the strongest luminescence.
Since my efforts of ammonolysis began, several groups
The next step, which we reported in 2006, involved the have independently reported increased
NMR study of isotopically labelled GaN nanostructures photoluminescence after re-annealing their GaN LED
that were produced using similar conditions. By labelling epistructures. However, none of these groups appears to
ammonia with the
15
N isotope, we could identify gallium, have established a connection between potential
nitrogen and hydrogen isotopes in our samples. What we decomposition of GaN and an increase in emission, which
Further reading discovered was a striking similarity between the NMR stems from the introduction of nitrogen-deficiencies.
B. Schwenzer, data for the
71
Ga and
15
N isotopes in the samples grown Although it would be fascinating to compare the chemical
J. Hu, at high temperatures (see figure 3). We were also able to composition of the GaN layers in these devices before
R. Seshadri, assign the broad feature in the NMR spectra to crystal and after annealing, one of the drawbacks of NMR is that
S. Keller, structure defects at the nearest-neighbor atomic level. it is that it cannot readily be used to investigate
S. P. DenBaars, This implies that it is very unlikely that chemical shift assembled device structures, due to the relatively large
U. K. Mishra, distributions are due to electronic effects, a cause that dimensions of an LED and the small amount of GaN that
2004 Chem. has been suggested by other researchers in this it contains. However, it might be possible to simulate the
Mater. community. heat treatment techniques used for GaN LEDs, and then
16 5088. study the chemical composition of the active layers in
B. Schwenzer, We continue to believe that these shifts are instead these structures. It should be possible to carry out this
J. Hu, Y. Wu, caused by nitrogen-deficiencies, which is a conclusion analysis using several of the techniques that we employ to
U. K. Mishra that we had drawn in 2004. The addition of determine the characteristics of our bulk samples. These
2006 Solid
15
N NMR data has also enabled us to determine gallium- studies would not only be very interesting from a scientific
State Sci. to-nitrogen ratios, while the
1
H NMR measurements have perspective – they could also help to spur the
8 1193. offered us an insight into hydrogen contamination within performance of LEDs, and enable them to enjoy success
the material. As expected, higher growth temperatures in emerging markets such as solid-state lighting.
40 www.compoundsemiconductor.net September 2009
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