Ultrathin AlN/GaN Heterojunctions by MBE for THz Applications
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0955-I13-05
Ultrathin AlN/GaN Heterojunctions by MBE for THz Applications Yu Cao and Debdeep Jena Electrical Engineering, University of Notre Dame, Notre Dame, IN, 46556
ABSTRACT High electron mobility transistors (HEMTs) based on all-binary AlN/GaN heterostructures are attractive candidates for the generation and detection of terahertz (THz) radiation. The basic requirements are that a) the electron density in the 2DEG should be large enough for ensuring that the electron-electron scattering mean-free path is much shorter than the distance between the contacts to ensure a viscous, fluid-like flow, and b) the mobility should be high enough such that the possible plasma modes are not strongly damped. In comparison to other semiconductors, the large spontaneous and piezoelectric polarization difference between coherently strained ultrathin AlN layers grown on GaN can achieve 2DEG densities exceeding 3*1013 cm -2 , and possibly approaching values as high as 6*1013 cm -2 . The plasma frequencies achievable in such extremely high density 2DEGs are higher than HEMTs based on other material systems. The absence of alloy scattering in these all-binary heterojunctions allows for high mobilities as well, as is demonstrated in this work by RF MBE growth; this is critical for sustaining plasma oscillations. INTRODUCTION In 1970, terahertz electromagnetic radiation was observed from a LiNbO3 crystal under picosecond-pulsed optical excitation [1]. Since then, a lot of research has been done to generate THz radiation using either picosecond or femtosecond optical pulses incident upon various materials such as LiNbO3, ZnTe, GaAs, InAs etc [2-6]. In 2002, Köhler et al demonstrated a GaAs-based electrically pumped quantum cascade laser operating at low temperatures for the generation of monochromatic 4.4 THz radiation [7]. This structure was further developed and more frequencies have been achieved [8, 9]. For all-electrical devices, Dyakanov and Shur proposed that both gated and ungated 2D electron gases satisfying certain boundary criteria can be used for generating terahertz radiation [10, 11]. Knap et al studied HEMTs based on InGaAs and AlGaN/GaN heterostructures, and demonstrated voltage-tunable emission from 0.4 to 5 THz [12-14]. Dyakanov and Shur have predicted [11] that the electromagnetic radiation frequency f 02 D is dependent on the square root of sheet carrier density ns. Radiation from 2DEGs with low ns has been studied in GaAs and InGaAs systems, which give f 02 D less than 10 THz. A high 2DEG density with high electron mobility is necessary for generating higher frequencies. The widest range for ns among semiconductors is in the III-V nitride semiconductors. Since 2DEGs in nitride HEMTs are generated by spontaneous and piezoelectric polarization, the highest 2DEG density possible is in an AlN/GaN heterostructure. In this report, the MBE growth, the structural and the transport properties of ultrathin AlN/GaN heterojunctions with very high ns (~3*1013cm-2) are described and shown to be attractive for THz devices base
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