38-40 GaN v2 10/9/09 13:32 Page 39
nitrides technology
rhombus4
Figure 2. This
GaN powder
was grown at a
temperature
above 1100
o
C.
Its grey color
stems from the
decomposition
of this material
into metallic
gallium and
nitrogen gas
(a). However,
under excitation
from a ultra-
violet lamp, this
sample is still
capable of
Credit: Charles Boch, UCSB Credit: Charles Boch, UCSB intense PL (b)
would therefore hamper the efficiency of any form of stronger photoluminescence than those grown at lower
optoelectronic device. One of the strengths of temperatures, which were powders that displayed a light
ammonolysis is its versatility for preparing GaN yellow color. This surprised me because I expected the
nanostructures - it is possible to use a wide range of lightly colored materials to have similar properties to the
gallium-containing compounds as a starting material. transparent/yellow, high-quality, epitaxially grown films.
However, I avoided gallium alkoxides and other organo-
gallium compounds because of the threat of carbon My next goal was to try and make sense of these
contamination, and selected metallic gallium and gallium seemingly contradictory results. Any additional phases of
oxide-based precursors instead. gallium oxide, or any other possible contamination source
for that matter, could be eliminated in all the samples by
The equipment that I used for producing GaN by X-ray diffraction results. But what about forms of
ammonolysis is incredibly simple. A single-zone tube contamination that don’t have a crystalline phase? They
furnace that features a quartz tube that can be connected would not be exposed by diffraction, because this
to tubing at either end fulfils the requirement for a technique only probes the long-range order in materials.
continuous gas flow (see figure 1). Gas supplies feed Hydrogen, for example, which has been claimed to
ammonia and nitrogen into this reactor, and precursor quench band edge related photoluminescence, would be
materials placed in an alumina crucible provide the source difficult to pick up by X-ray diffraction and most other
for group III metals, such as gallium and indium. As the analysis methods. Fortunately there is a technique that
minimum temperature for the conversion of gallium oxides doesn’t just detect hydrogen, but also determines how it
to GaN under ammonia flow is at least 800
o
C, I decided is bound to its nearest neighboring atoms – nuclear
to prepare a series of GaN powders using various magnetic resonance (NMR). I then started an ongoing
different reaction temperatures within the 900
o
-1100
o
C study of my samples using this technique in collaboration
range. These nanostructured samples,which were with Jerry Hu, the Spectroscopy Facility manager and
produced with a range of precursors, are composed of NMR expert at UCSB’s Materials Research Laboratory.
the hexagonal form of GaN, according to X-ray diffraction This was the first ever study of hydrogen contamination in
measurements. GaN by
1
H NMR.
The photoluminescence results produced by these During these studies I discovered that NMR is, in general,
samples gave me a tremendous shock. They directly incredibly well suited to studying atomic level binding in
opposed the widely held view in the nitride community GaN powders. In addition to revealing hydrogen
that better quality material produces brighter emission. impurities, it is possible to investigate the type of
In my case I found that the very opposite was true - better coordination of gallium within the material, thanks to the
quality material produced far weaker emission than its
71
Ga isotope being NMR active.
poorer quality counterpart.
Hu and I recorded
71
Ga NMR spectra of different GaN
The more defective GaN powders were produced at samples, and discovered striking differences between
1100
o
C, a growth temperature that drives the different materials on the atomic level. To our big surprise,
decomposition of GaN into metallic gallium and nitrogen we found a strong correlation between the
71
Ga NMR
gas. These samples were grey in color and produced far spectral features and photoluminescence (see figures 2
September 2009 www.compoundsemiconductor.net 39
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44