Material quality (crystal perfection, electrical resistivity) of GaN buffer layers has great impact on the nitride based HEMT's (high electron mobility transistor) performance. On the one hand, buffer should have at least sheet resistivity in the range of 1E4 - 1E5 ohm/sq, to reduce leakage current, on the other hand, the surface roughness and dislocation density at the AlGaN/GaN heterostructure interface should be low enough to suppress electron scattering mechanism of 2DEG (two dimensional electron gas). So far, the lowest achievable background contretrations of u-GaN on the order of 1E16 cm^-3, are not sufficient to give sheet resistance higher than 1E4 ohm/sq. Due to the difficulties in fabrication of pure low dislocated GaN crystal, the only way to obtain high resistivity GaN layers is intentional compensation with acceptor. In general there are two different methods of deposition resistive GaN layers by MOVPE. First, not desirable for HEMT purposes due to strong impact on material mobility, is the doping with acceptors (Fe, Cr, Mg) [1,2,3], second is the self compensation of the material [4,5] by tuning the growth conditions of GaN buffer [6,7,8].

The self compensation of the GaN material is explained by the introduction of acceptor levels in the crystal structure. The main source of acceptors are probably C atoms comes from TMGa precursor. Experimental results indicate, that compensation effect, is strongly affected by the edge-type TD (threading dislocations) related to the MOVPE growth parameters. It was shown, that laser interferometry is useful method of growth process monitoring, and crystal quality prediction. It is possible to link growth conditions and material properties with reflectance trace.

In this contribution we demonstrate the statistical study of over 50 reflectance traces recorded during MOVPE growth of GaN buffer layers. The correlation between reflectance signal (i.e. signal recovery time) and electrical properties of the buffer (thickness of buried conductive layer, buffer resistivity) obtained by impedance spectroscopy are presented. This results shows, that in-situ laser interferometry can be used as a fast method of prediction of GaN buffer resistivity during AlGaN/GaN HEMT's growth. Based on this observation, the impact of the nucleation layer growth and annealing conditions (thickness/time, pressure, temperature) on the electrical properties of GaN buffer is demonstrated.

Acknowledgements:

This work was co-financed by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08-05), National Centre for Science under the grant no. N N515 495740, by National Centre for Research and Development through Applied Research Program grant no. 178 782, by Wroclaw University of Technology statutory grant S20010, B20011 and Slovak-Polish International Cooperation Program nos. SK-PL-0017-09 & 0005-12,Mateusz Wosko benefited from fellowship co-financed by European Union within European Social Fund.

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