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M2.4.1

Terahertz-Emitting Silicon-Germanium Devices Ralph T. Troeger, Thomas N. Adam, Samit K. Ray, Pengcheng Lv, Ulrike Lehmann, and James Kolodzey Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, U.S.A. ABSTRACT In this paper we report on far-infrared emission in the 1-12 THz frequency range from strained SiGe structures. Pseudomorphic superlattices were grown by Molecular Beam Epitaxy (MBE) at the relatively low substrate temperature of 400°C to prevent germanium segregation. Layer thicknesses, composition, and crystallinity were confirmed by high-resolution X-ray diffraction. Devices were designed to produce confined hole states with various energy separations. Mesa devices were etched in a reactive-ion etching system and tested for edge emission over a wide range of drive currents using an FTIR spectrometer in step-scan mode. THz emission was observed in pulsed mode at current densities as low as 50 A/cm2 and at temperatures as high as 50 K, using a liquid-helium-cooled silicon bolometer detector with a lock-in amplifier. Emission spectral peaks occurred at 7.9 and 9.36 THz for two different samples, in good agreement with k⋅p calculations. INTRODUCTION Recently, emitters and detectors operating in the terahertz gap of the electromagnetic spectrum (0.3-30 THz, 1000-10 µm wavelength) have garnered much interest for possible application in such fields as ranging [1], imaging [2, 3], gas and environmental sensing [4, 5], wireless communications [6], and spectroscopy [7]. Quantum cascade lasers (QCL) fabricated from III-V materials have demonstrated light emission at wavelengths typically below 10 µm [8] over a relatively large temperature range [9]. However, these devices are thought to be severely limited within the THz range due to strong reststrahlen absorption in III-V compound semiconductors [10]. Fabricating THz light emitters based on the SiGe material system would not only circumvent this problem due to the absence of such absorption bands in SiGe, but also enable production of opto-electronic devices compatible with Si CMOS technology. A number of such structures have recently been reported. Stimulated THz emission was demonstrated from boron delta-doped sheets located within strained SiGe quantum wells at a wavelength of approximately 100µm [11]. The optical transition in these structures is between acceptor states in the band gap, which are split by the built-in strain. Electroluminescence has also been reported from structures based on intersubband transistions between confined states in SiGe quantum wells. The observed emission was typically at the upper limit of the THz range, with emission reported at for instance 30 THz [12] and 34-37 THz [13]. A few reports on longer-wavelength emission at 2.9 and 6 THz have been published [14, 15]. In this paper we report on measurements of THz electroluminescence from strained SiGe quantum well structures and its dependence on the pumping conditions.

M2.4.2

THEORY In the SiGe material system, band gap difference ma