ANALYSIS OF FERMI-LEVEL PINNING AND SURFACE-STATE DISTRIBUTION IN INALAS HETEROSTRUCTURES

The band gaps, built-in electric field, and surface Fermi level of a s eries In1-xAlxAs surface-intrinsic-n(+) (SIN+) structures have been st udied by photoreflectance at room temperature. The samples were grown by molecular beam epitaxy with an undoped layer thickness of 1000 Angs trom. Our study indicates that, in contrast to GaAs and AlGaAs, the su rface Fermi level is not pinned at midgap over aluminum concentration of 0.42-0.57. The pinning position is composition dependent. The undop ed layer was subsequently etched to 800, 600, 400, and 200 Angstrom. D ifferent chemical solutions were used in the etching process and the b uilt-in electric field is found independent of the etching process. Al though the surface Fermi level, in general, varies with the undoped la yer thicknesses, there exists, for each Al concentration, a certain ra nge of thicknesses within which the surface Fermi level is weakly pinn ed. From the dependence of electric field and surface Fermi level on t he undoped layer thickness, we conclude that the surface states distri bute over two separate regions within the energy band gap and the dens ities of surface states are as low as 1.36+/-0.05 x 10(11) cm(-2) eV(- 1) for the distribution near the conduction band and 4.38+/-0.05 x 10( 11) cm(-2) eV(-1) for the distribution near valence band. (C) 1998 Ame rican Institute of Physics. [S0021-8979(98)01907-0].