LATTICE MISMATCH AND CONDUCTIVITY IN LTALGAAS LAYERS

The lattice mismatch of as-grown and annealed Low Temperature (LT) Alx Ga1-xAs layers, epitaxially grown by Molecular Beam Epitaxy on (001) S .I. GaAs substrates, has been investigated as a function of growth con ditions. The variation of electrical conduction in the temperature ran ge of 150 K to 400 K has also been examined. The relaxed lattice misma tch due to the LT growth increases as the growth temperature decreases and as the V/III equivalent pressure ratio increases. For the same gr owth conditions the relaxed lattice expansion is higher in the LT GaAs than in the LT AlxGa1-xAs layers. The lattice expansion in LT AlxGa1- xAs decreases by increasing the Al concentration. The conductivity mea surements have revealed the presence of hopping conduction in LT AlxGa 1-xAs layers (T < 300 K) with a characteristic activation energy E-o i ncreasing from 31 meV to 46 meV as the Al concentration rises from 0.1 9 to 0.65. The rise in E-o is caused by the high density of acceptor s tates. These acceptor states can be attributed to gallium vacancies as in the case of LTGaAs layers. The increase in gallium vacancy concent ration is consistent with the lower dilation of the lattice constant i n the LTAlxGa1-xAs layers compared with that of LTGaAs. (C) 1998 Elsev ier Science B.V. All rights reserved.