Electrical and Photoelectrical Characterization of Deep Defects in Cubic GaN on GaAs
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Introduction Cubic GaN layers grown on GaAs are of high interest for production of devices such as cleaved fabry perot type blue cavities using the substrate facets /1/. Furthermore, reaching very high p-type doping levels is feasible for cubic GaN /2/. However, for all applications based on cubic GaN the exact knowledge of defects in the layers is essential for optimization of devices. Therefore, intensive investigations on defects in GaN grown with MBE on semiinsulating (SI)GaAs substrates were made with photoluminescence (PL), cathodoluminescence (CL) and temperature dependent Halleffect measurements (TDH). Some donor-acceptor-pair transitions were found in PL at 3.15eV /3/ involving a donor at 25meV and an acceptor at 130meV, as well as at 3.178eV and 3.056eV /4/. An acceptor bound exciton transition at 3.088 /4/involves an acceptor at an activation energy of 212meV. In p-type undoped c-GaN grown under N-rich conditions an acceptor at an activation energy of 0.445eV was observed with TDH /5/, whereas in n-type undoped c-GaN-layers grown under Ga-rich conditions a shallow donor at an activation energy of 26meV and an deep donor at an activation energy of 0.6eV were found /5/. A further deep CL peak was observed at 2.4eV in undoped c-GaN /1/. G 3.14 Mat. Res. Soc. Symp. Proc. Vol. 537 01999 Materials Research Society
However, the electrical and photoelectric characterization of cubic GaN /SI-GaAs involves some principle problems. SI-GaAs itself contains many deep levels which must be isolated in the GaN/GaAs heterostructure spectra, complicating the analysis of the GaN layers. The aim of our works is the detection and isolation of deep levels in n- and p-type GaN layers and to separate them from the defects in the SI-GaAs substrate using thermal and photoelectric techniques by using different contact arrangements and different excitation energies. Experimental Cubic GaN (c-GaN) films with a phase purity better than 99.9% (estimated by both X-ray diffraction and Raman measurements) were grown on SI-GaAs substrates orientated in (001) direction by rf-plasma-assisted molecular beam epitaxy (MBE) at a substrate temperature of 720 0C. Details of the nucleation process and growth parameters were described in /6/. All samples were nominally undoped. However, in c-GaN the type of conductivity can be influenced by the growth conditions and both p- and n-type GaN layers with low carrier concentrations were obtained /5/. Three different kinds of GaN- layers were investigated. 1.) The first group were nominally undoped p-type GaN-layers with carrier concentrations of (l-5)x10 16cm-3 and Hall mobilities between 220 and 300 cm2 /Vs (samples signed as P1 and P2), 2.) an nominally undoped, p-type GaN- layer on top at a p-type GaAs buffer layer (signed B1), and 3.) n-type 4 GaN layers, nominally undoped or slightly Si-doped with carrier concentrations below 101 cm-3 2 and mobilities of a few 100cm /Vs (signed as NI and N2). The SI-GaAs substrate is identical for all heterostructures investigated. Ohmic contacts were prepared on
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