Characterization of GaN/Si Using Capacitance Spectroscopies
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Characterization of GaN/Si Using Capacitance Spectroscopies Steven R. Smith1,2, John C. Roberts3, P. Rajagopal3, J. W. Cook3, E. L. Piner3, K. J. Linthicum3, and Said Elhamri4 1 Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLPS, Wright-Patterson-Air Force Base, OH, 45433-7707 2 University of Dayton Research Institute, 300 College Park, Dayton, OH, 45469-0178 3 Nitronex, Inc., Raleigh, NC, 27606 4 Physics, University of Dayton, Dayton, OH, 45469-2314
EXTENDED ABSTRACT Layers of GaN deposited on Si substrates have been studied using Thermal Admittance Spectroscopy (TAS) and Optical Admittance Spectroscopy (OAS). Transparent front-side contacts were used to facilitate the optical measurements. Six specimens were cut from the same location in two different wafers, and three samples were randomly chosen from other growths. A shallow level at EC – 0.051 eV was found in all the specimens using TAS. In some specimens this peak was asymmetric, indicating more than one level near this energy. Deeper levels were also seen in the high-temperature portion of the spectra, but were poorly resolved in most specimens. Illuminating the specimen with UV light at 25 K resulted in the thermal position of the peak shifting to a lower temperature, and hence, the calculated energy, of the peak shifting lower. The amplitude of the peak also decreased. Transient OAS measurements revealed the interesting phenomenon of negative persistent photo conductance at room temperature in some of the specimens when the illumination photon energy was less than the bandgap. The negative response time was very short. At lower temperatures, below 100 K, the negative response diminished, but the response time was still short. At wavelengths above the bandgap energy, normal transient response was seen, with a longer time constant.
INTRODUCTION GaN has important applications for high-temperature and high-power devices; and, for blue and UV light-emitting diodes. Also of importance is the substrate on which the layers are grown. 1, 2 Shallow levels in the material may be examined by Thermal Admittance Spectroscopy,3, 4 and the interband transitions and near midgap levels can be detected by Optical Admittance Spectroscopy.5, 6 OAS is also useful for examining the transient optical behavior.7
DISCUSSION TAS of the GaN/Si films revealed a peak at low temperature (T~ 50K) characteristic of a shallow level. Furthermore, this peak was not always symmetric, which implies the presence of more than one level in close proximity. The energy calculated from the Arrhenius plot for this peak was EC – 0.051 eV. Every specimen exhibited this peak in the low
temperature TAS spectra. In the asymmetric case, a ‘shoulder’ on the left side of the peak could not be resolved sufficiently to calculate an energy value. However, its thermal position implies an energy lower than 0.051 eV. Thermal Admittance spectra obtained for specimens before and after exposure to UV light at 25 K, shifted the thermal position of the TAS peak to a lower
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