J. Am. Chem. Soc., ASAP Article 10.1021/ja0604865 S0002-7863(06)00486-0
Web Release Date: March 28, 2006
Distributions of Conduction Electrons as Manifested in MAS NMR of Gallium Nitride
James P. Yesinowski,* Andrew P. Purdy, Huaqiang Wu, Michael G. Spencer, Janet Hunting, and Francis J. DiSalvo
Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375, and School of Electrical & Computer Engineering and Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853
A strategy is demonstrated for identifying unambiguously and characterizing quantitatively the effects of distributions of conduction electron concentrations arising from intentional or unintentional dopants in semiconductors by magic-angle spinning (MAS) NMR. The 71Ga MAS NMR spectra of a number of chemically synthesized GaN samples with no intentional doping show inhomogeneously broadened absorptions to high frequency of the main peak. These broad signals are shown, from spin-lattice relaxation time measurements as a function of shift position in a single sample, to be due to Knight shifts arising from degenerate conduction electrons. For a GaN sample with Ge as an intentional dopant at the 0.13% (wt) level, the spectrum is dramatically broadened and shifted to high frequency by up to several hundred parts per million. Analysis of the inhomogeneously broadened line shape yields a quantitative probability density function for electron carrier concentration in the bulk sample that reflects significant compositional heterogeneity due to a variety of possible sources.