Enhanced terahertz emission from impurity compensated GaSb Ricardo Ascázubi, 1 Carl Shneider, 1 Ingrid Wilke, 1 Robinson Pino, 2 and Partha S. Dutta 2 1 Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA 2 Department of Electrical, Computer and Systems Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA Received 11 November 2004; revised manuscript received 3 March 2005; published 14 July 2005 We report femtosecond optically excited terahertz THz emission from tellurium doped GaSb at room temperature. The influence of the majority and minority carrier type and concentrations on the strength of the THz emission is investigated. Strong enhancement of THz emission in GaSb is observed as a result of compensation of native acceptors by tellurium donors. Surface field acceleration and the photo-Dember effect are identified as THz emission mechanisms in GaSb and modeled in dependence of the majority and minority carrier type and concentrations in our GaSb samples. THz emission from p-type GaSb is dominated by the photo-Dember effect whereas THz emission from n-type GaSb is dominated by surface field acceleration. The doping conditions under which THz emission is maximized are identified. DOI: 10.1103/PhysRevB.72.045328 PACS numbers: 78.47.+p I. INTRODUCTION Optical excitation of semiconductor surfaces with femto- second fs titanium-sapphire Ti:S laser pulses is an impor- tant method to generate ultrashort terahertz THz radiation pulses. This type of THz radiation source has enabled the development of time domain ultrafast THz spectroscopy and THz imaging over the last decade. 1 However, important applications of these techniques in basic and applied science such as nonlinear THz-spectroscopy, nondestructive testing or biomedical imaging are still limited by the power of the available sources. 2 The development of bright, high bandwidth THz radiation sources is important in order to expand the applications of these techniques. For this purpose it is necessary to understand the THz emission process as determined by semiconductor properties. Small band gap semiconductors are promising sources of optically excited THz radiation. One of the strongest THz surface emitters is InAs with a band gap of 0.35 eV. 3 Another recently discovered strong THz emitter is InN with a band gap of 0.7–0.8 eV. 4 Furthermore, small band gap semiconductors are attractive candidates for compact and lightweight time-domain THz spectroscopy and imaging systems powered by femtosecond fiber lasers with emission wavelengths at   1.55 m E  0.8 eV. In previous experiments, optically excited THz emission from semiconductors was primarily investigated in depen- dence of experimental parameters extrinsic to the semicon- ductor such as excitation wavelength, excitation fluence, temperature, electric or magnetic fields. 3,5–10 Experiments which focused on the dependence of the THz emission on the doping of the semiconductor were limited to the influence of the majority carrier type. 11–13 It was observed that in small band gap semiconductors such as InAs and InSb the origin of THz emission is the photo-Dember effect 5,6,9,13,14 whereas in wide band gap semiconductors GaAs, InP surface field acceleration is the responsible physical mechanism. 7,8,15 Hitherto, the discrimination between the photo-Dember effect and surface-field-acceleration based THz emission has been made by analysis of the temporal wave form of the THz transient. The surface field acceleration model predicts a polarity reversal of the THz wave form if the majority carrier type changes from p type to n type whereas the photo- Dember model does not predict polarity reversal. However, the exact interplay of surface field and photo-Dember field mechanisms in THz emission from optically excited semi- conductors is still under investigation. 3,4,7,8 An examination of the photo-Dember model and the surface field acceleration model reveals that in both cases the strength of the radiated THz electric field characteristically depends on the majority and minority carrier concentrations. In this paper, optically excited THz emission from 17 high purity, selectively doped GaSb crystals is reported. GaSb is a small band gap semiconductor with a bandgap of 0.725 eV at room temperature. 16 In contrast to previous work, 11–13 we investigate the influence of the majority and minority carrier concentrations on the strength of the THz emission. We demonstrate that by systematically varying the majority and minority carrier type and carrier concentrations over three orders of magnitude the THz emission mechanism in GaSb can be tuned from being dominated by the photo-Dember effect to being dominated by surface field acceleration. We demonstrate further that within each regime photo-Dember based THz emission and surface field accel- eration based THz emission are maximized under specific majority and minority carrier concentrations. II. EXPERIMENTAL DESCRIPTION As-grown undoped GaSb is invariably p type in nature due to the presence of native defects such as gallium vacancies V Ga  and gallium antisites Ga Sb . 16 In bulk GaSb crystals grown from stoichiometric melt, the net acceptor concentration is in the range of 1 – 2  10 17 cm -3 . 16 Dopants such as Te, Se, and S are commonly used to grow n-type GaSb crystals. The GaSb wafers used in this study were extracted from a tellurium Te compensated GaSb bulk crys- tal that was grown via the vertical Bridgman method. Details PHYSICAL REVIEW B 72, 045328 2005 1098-0121/2005/724/0453285/$23.00 ©2005 The American Physical Society 045328-1