Layered Compound Semiconductor GaSe and GaTe Crystals for THz Applications
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Layered Compound Semiconductor GaSe and GaTe Crystals for THz Applications Krishna C. Mandal, Sung H. Kang, and Michael K. Choi Advanced Materials Division, EIC Laboratories, Inc., 111 Downey Street, Norwood, MA, 02062 ABSTRACT The single crystal growth of layered semiconductors GaSe and GaTe by the vertical Bridgman technique using zone refined selenium (Se), tellurium (Te) and high purity gallium (Ga) have been described. The grown crystals (2.5 cm diameter and ~10 cm long) have demonstrated efficient broadband tunable THz emission and operation as sensitive THz detectors. The crystals have shown promising characteristics with good optical quality, high dark resistivity, wide band gap (GaSe-2.01 eV and GaTe-1.66 eV at 300 K), good anisotropic electrical properties (σ║ vs σ⊥ and µ║ vs µ⊥) and long term stability. Different steps involved in processing GaSe and GaTe crystals as THz sources and sensors are described. INTRODUCTION Layered compound semiconductors have been studied for a long time due to their structures giving rise to unique properties. Strong covalent bonding within the layer planes and weak van der Waals bonding between the planes make layered semiconductors highly anisotropic. Because of its large nonlinear optical coefficient (d22=75 pm/V), high anisotropy, and ideal bandgap with high carrier mobilities, the layered semiconductor GaSe, has been of interest for efficient non-linear optical and radiation detector applications [1-3]. Nonlinear optical effects are also utilized for generation and detection of broadband tunable THz radiation [4-5]. However, GaSe has not been widely studied due to difficulty of growing and processing of large GaSe crystals due to low mechanical hardness [6]. GaSe has many advantages including easy cleaving, high resistance for radiation damage and above room temperature operation in harsh environments. In this paper, we report crystal growth and evaluation of GaSe, and for the first time, GaTe as THz sources. The GaSe and GaTe crystals were grown at EIC by vertical Bridgman methods based on thorough simulation and modeling studies and using a numerical model, Multizone Adaptive Scheme for TRAnsport and Phase-change Processes (MASTRAPP). We report GaSe and GaTe crystal growth procedures based on numerical modeling and present characterization results for broadband tunable THz sources and detectors. EXPERIMENTAL The growth of GaSe and GaTe single crystals has been carried out using a modified vertical Bridgman growth method described earlier [3, 7]. We have grown GaSe and GaTe crystals from stoichiometric amounts of high purity (7N, Alfa Aesar) Ga and zone-refined (≥7N) Se or Te. For GaSe crystal growth, 2.5 mol% Cr or 1000 ppm In was added as a dopant source. The precursor materials were zone-refined at EIC in a zone refiner by passing 30-40 cycles before synthesis. Synthesis of GaSe and GaTe were carried out at 1030°C, and 965°C respectively in a sealed quartz ampoule evacuated to 10-6 Torr using a single zone horizontal furnace. The sealed ampoule was then
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