284 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 2, MARCH/APRIL 2008 III–VI Chalcogenide Semiconductor Crystals for Broadband Tunable THz Sources and Sensors Krishna C. Mandal, Sung Hoon Kang, Michael Choi, Jian Chen, Xi-Cheng Zhang, Fellow, IEEE, James M. Schleicher, Charles A. Schmuttenmaer, and Nils C. Fernelius, Senior Member, IEEE Abstract—The layered chalcogenide semiconductor GaSe has been grown under various crystal growth conditions for optimum performance for tunable terahertz (THz) wave generation and broadband THz detection. Low-temperature photoluminescence (PL), Raman spectroscopy, optical absorption/transmission, electrical charge transport property measurements, and THz time-domain spectroscopy (TDS) have been used to characterize the grown crystals. It is observed that indium doping enhances hardness of the grown GaSe crystals, which is very useful for processing and fabricating large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient ≤0.1 cm −1 in the spectral range of 0.62–18 μm), high dark resistivity (≥10 9 Ω cm), wide bandgap (2.01 eV at 300 K), good anisotropic ( ||and ⊥) electrical transport properties (μ e/h ,τ e/h , and μτ e/h ) and long-term stability. The THz emission measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz), and sensors (up to 100 THz). Additionally, new THz frequencies (0.1–3 THz) have been observed for the first time from an anisotropic binary and a ternary semiconductor crystal. Details of characterizations as well as optimum crystal growth conditions including simulation and computer modeling are described in this paper. Index Terms—Crystal growth, optical characterization, tera- hertz (THz), time-domain measurements. I. INTRODUCTION L AYERED chalcogenide semiconductors have been stud- ied for a long time due to their unique properties coming from their layered structures. Their anisotropic properties result from strong covalent bonding within the layer planes and weak van der Waals type bonding between them [1]. Among various chalcogenides, the III–VI semiconductor GaSe has been stud- ied for a long time due to its large nonlinear optical coefficient (d 22 = 75 pm/V) and its high structural anisotropy [1]–[4]. The nonlinear optical effects of GaSe can be utilized for generation and detection of broadband tunable terahertz (THz) Manuscript received September 9, 2007; revised October 15, 2007. This research was supported in part by the Air Force under Contract FA86540-06- M-5411 and Contract FA8650-07-C-5306. K. C. Mandal, S. H. Kang, and M. Choi are with the EIC Labo- ratories, Inc., Norwood, MA 02062 USA (e-mail: kmandal@eiclabs.com; skang@eiclabs.com; mchoi@eiclabs.com). J. Chen and X.-C. Zhang are with the Rensselaer Polytechnic Institute, Troy, NY 12180 USA (e-mail: zhangxc@rpi.edu). J. M. Schleicher and C. A. Schmuttenmaer are with the Yale Uni- versity, New Haven, CT 06520 USA (e-mail: james.schleicher@yale.edu; charles.schmuttenmaer@yale.edu). N. C. Fernelius is with the Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433 USA (e-mail: nils.fernelius@wpafb.af.mil). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2007.912767 radiation [5], [6]. In spite of its interesting characteristics, GaSe has not been widely studied due to the difficulty of growing and processing of large crystals resulting from its low mechanical hardness [7]. In order to address this issue, we have studied In- doped GaSe and Ge-doped GaTe crystals for THz applications. The crystals have been grown based on numerical simulation for optimizing THz wave generation and detection properties. One of the primary difficulties in GaSe single crystal growth is its extremely low thermal conductivity (0.37 W/mK) along the c-axis near the melting temperature. Also, the thermal conduc- tivity is anisotropic. The radial conduction in the solid is more efficient for heat removal from the growth interface compared to axial conduction in GaSe growth. Anisotropy and liquid/solid conductivity ratios are expected to strongly influence the inter- face shape, which, in turn, affects twinning and other defects. Furthermore, the Prandtl number of GaSe is about 2.8, which results in strongly coupled melt flow and heat transfer. It is ex- pected that any disturbance of melt flow from the pulling rate and/or rotation rate will significantly affect the temperature dis- tribution, and consequently, the interface shape. It is, therefore, extremely important to properly control the melt flow, growth interface, and solute transport. The crystal growth parameters were determined based on simulation and modeling studies us- ing a numerical model, multizone adaptive scheme for transport and phase-change processes (MASTRAPP). We have also in- vestigated two other chalcogenide semiconductors, GaTe and GaSe x Te 1−x , as new THz sources for the first time. In this ar- ticle, we report details of crystal growth based on numerical modeling and simulation, characterizations of the grown crys- tals, and the THz results for broadband tunable THz sources and sensors. II. EXPERIMENTAL Opto-electronic properties of THz crystals are strongly and negatively influenced by the presence of trace levels of residual impurities, since they substantially reduce charge carrier trans- port and optical absorption/transmission properties of the grown crystal. We have grown GaSe, GaTe, and GaSe 0. 5 Te 0. 5 crystals from stoichiometric amounts of high-purity (7 N, Alfa Aesar) Ga, and vacuum-distilled and zone-refined (≥7 N) Se or Te. For GaSe and GaTe crystal growth, 1000 ppm indium (In) and 0.8 wt% germanium (Ge) were added as dopants, respectively. For growing high-quality large single crystals, it is important to control heat transfer and the fluid flow pattern in the furnace. In order to predict heat transfer in the furnace, numerical mod- eling and simulation were conducted by MASTRAPP [8]–[11]. Based on simulation results, the temperature distributions in 1077-260X/$25.00 © 2008 IEEE Authorized licensed use limited to: Rensselaer Polytechnic Institute. Downloaded on February 11, 2010 at 13:00 from IEEE Xplore. Restrictions apply.