Effect of GaAs surface pretreatment on electrical properties of MBE-ZnSe/GaAs substrate interfaces

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H₂SO₄:1H₂O₂: 1H₂O), (NH₄)₂S_x-, NH₄I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. A HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, N_ss, with N_ss,_min below 4 x 10¹¹cm^-2 eV^-1 near E_c- E_FS= 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH₄)₂S_x- pretreated interface. Nevertheless, C-V characteristics of ZnSe/nGaAs samples, which were measured for the first time, indicate that interface Fermi level, E_FS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining E_FS position. The influence of N_ss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with N_ss(E) > 1 x 10¹³ cm^-2 eV^-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.