The Role of Impurities in LP-MOCVD Grown Gallium Nitride
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ABSTRACT We have investigated the relationship of the Hall electron mobility to the background carrier concentration in low pressure MOCVD grown GaN. The highest electron mobility (400 cm 2/Vss) of the unintentionally doped GaN was obtained at a carrier concentration of lxl 017cm-3 and samples with carrier concentrations lower than this exhibited lower mobilities. SIMS analysis shows C and 0 concentrations in the range of 2-3xl 06cm-3 and H in the 2-3x101 cm3 range. Structural defects, stoichiometry and impurities in the GaN films grown under different conditions are investigated to understand their relationship to the electron Hall mobilities. In particular, different growth temperatures and pressures were used to grow undoped GaN and modify the background doping effect of the impurities.
INTRODUCTION Most Ill-V compound semiconductors like GaAs and lnP have high electron or hole mobility when the impurity or carrier concentration is low . In such materials, the mobility decreases gradually as the impurity concentration increases. For GaN films grown by MOCVD, however, the highest mobility tends to occur at an intermediate carrier concentration and the mobility decreases as the background carrier concentration drops below this value. We have investigated this unusual behavior of the electron mobility by several approaches. First, because the defect density in GaN films is much higher than in Si or other Ill-V materials, we must examine the potential role of the defect density. Then, we examined the film stoichiometry and impurity levels to pinpoint the origin of the unusual carrier concentration/mobility behavior. EXPERIMENTAL Samples were grown by MOCVD with TMG and NH 3 as the precursors as described in a previous publication 2 . Several characterization techniques were used to find the correlation between residual stress, stoichiometry, impurities and the electron Hall mobility. Strain was measured by both X-ray 0-20 scans and rocking curves for samples with different mobilities. We also performed RBS and AES analysis on samples with different values of mobility and carrier concentration to determine their Ga to N ratio. Finally, to monitor the impurity levels in the films, we performed concentration depth profiles using SIMS analysis on several samples. Based upon our use of the MOCVD method with a rotating sample holder, the most likely impurities in the film are H, C and 0 from the source gases and Fe, Cr, Ni and Mo from the reactor itself.
521 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
RESULTS A. Dislocation Density of the Films Structural imperfections in the GaN films are one possible explanation for the low electron mobility of the film. To investigate this possibility, several samples with different electron mobilities were chosen for cross-sectional and plane-view TEM. From the crosssectional TEM samples, one can compare the crystal structure of the epilayer/buffer layer/substrate interfaces. When the growth conditions, i.e. growth temperature and thickness of the buffer l
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