Comparison between gallium-implanted layers of ZnSe and ZnS xSe 1−x by optical, electrical and electron beam characterization methods

Abstract

Optoelectronic devices in the blue spectral region require doped ZnS x Se 1− x layers for electrical confinement and optical waveguilding. Since ion implantation is often used to realize heavy doping in thin layers, we implanted gallium ions with different doses and energies into ZnSe and ZnS x Se 1− x ( x ≤ 0.4) layers, grown by metalorganic vapor phase epitaxy (MOVPE) on GaAs. Rapid thermal annealing was performed after SiO 2 capping. For characterization, we used electron probe micro-analysis (EPMA), photoluminescence (PL) at 11 K, Raman spectroscopy, far infrared reflectivity (FIR) and Hall measurements. In ZnSe, the Ga depth profiles remain nearly gaussian after annealing, indicating weak diffusion. PL shows that both the overall intensity and the ratio of excitonic to deep centre emissions are maximum for annealing at 850°C (30 s), implying optimum crystalline quality and maximum carrier concentration. In addition, the conductivity is maximum (10 Ω -1 cm -1). However, FIR reveals a heavily doped layer at the GaAs interface, which is assigned to the annealing-induced Zn diffusion into the substrate. Furthermore, the PL spectra show donor-acceptor pair (DAP) transitions which can be attributed to shallow acceptors due to complexes of Ga and intrinsic defects. With increasing ion dose the free carrier concentration saturates at 4 × 10 17 cm -3 probably due to self-compensation caused by zinc vacancies (V Z n ). For ZnS 0.3Se 0.7, EPMA measurements show a diffusion of the Ga towards the surface, while S- and Ga-rich surface defects appear during annealing. After implantation the PL spectra show deep level emissions at 2.08 and 2.35 eV, which were assigned to [Ga Z n − V Z n ] complexes and to Zn S e . Optimum annealing seems to occur at 850°C for 30 s, resulting in a maximum PL intensity with strong DAP and excitonic contributions. Similar behaviour was observed for ZnS 0.4Se 0.6. Up to now, due to the diffusion and compensation, the implanted and annealed ternary layers remain semi-insulating.