Development of A II B VI Semiconductors Doped with Cr for IR Laser Application
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Development of AIIBVI Semiconductors Doped with Cr for IR Laser Application V.A. Kasiyan, R.Z. Shneck, Z.M. Dashevsky and S.R. Rotman1 Department of Materials Engineering, 1Department of Electrical Engineering Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel ABSTRACT Electrical and optical measurements obtained with CdSe single crystals doped with chromium from a gas source CrSe over a wide temperature range (500-1050 oC) are compared with ZnSe annealed in liquid metal (Zn). These processes are intended to control the concentrations of the impurity and intrinsic defects. The low temperature annealing of CdSe crystals in CrSe atmosphere allows obtaining high electron mobility up to 9000 cm2/Vs at 80 K and demonstrates the low native defect concentration. A high temperature annealing gives rise to increased electron concentration with decreased mobility. Optical absorption measurements show that at the high annealing temperature effective doping with Cr takes place. The impurity absorption beyond the absorption edge is interpreted by the excitation of Cr ++ and Cr+ deep levels. INTRODUCTION Compact, tunable, room-temperature, solid-state laser sources operating in the mid-IR (1.5 – 3 µm) spectral region are of interest for a number of applications such as eye safe laser radar, chemical sensing, IR counter-measures and spectroscopy. Efficient room temperature lasing has been demonstrated with Cr++ active ions doped into II-VI chalcogenides such as ZnSe and CdSe [1, 2]. Transition metals incorporated into Zn(Cd)Se substitute metallic atoms and create deep levels in the band gap [3]. Only one strong emission at 2.67 eV is observed in the spectrum of the undoped ZnSe, which corresponds to the room temperature band gap energy in ZnSe. The luminescence of doped compounds exhibits two new bands near 2.2 and 1.4 eV corresponding to the photogeneration (Cr++ + eVB → Cr+) and photoionization (Cr+ → Cr++ + eCB) processes in ZnSe. Hence the positions of Cr+ and Cr++ states were found 1.24 and 2.26 eV beneath the conduction band edge [4]. From the intensities of Cr+/Cr++ related processes a relatively large capture cross section of the Cr++ ion has been deduced. In addition to these high energy transitions, a typical room temperature absorption spectrum of CdSe:Cr++ reveals a peak at 1.9 µm [5] corresponding to the inter-center transition 5T2→5E of Cr++ in CdSe [6]. Mid-infrared tunable laser media based on II-VI semiconductors doped by transition metals were demonstrated in [7] based on this transition. Tetrahedrally-coordinated Cr++ ions are especially attractive as laser centers on account of high luminescence quantum yields for emission in the 2-3 µm range. The strength of the emission depends on the concentration of the Cr++ ions in the material. In developing of a II-VI:Cr laser material we consider two questions: (a) The total amount of the Cr dopant atoms depends on the available metallic vacancies for replacement by Cr. To investigate the vacancy replacement process we studied the annealing of ZnSe
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