Photoreflectance Probing of Below Gap States in Gan/Algan High Electron Mobility Transitor Structures
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PHOTOREFLECTANCE PROBING OF BELOW GAP STATES IN GaN/AlGaN HIGH ELECTRON MOBILITY TRANSITOR STRUCTURES D.K. Gaskill, O.J. Glembocki, B. Peres*, R. Henry, D. Koleske**, and A. Wickenden*** Naval Research Laboratory, Washington, D.C. 20375 * EMCORE, Somerset, NJ 08873 ** Sandia National Laboratories, Albuquerque, NM 87185-0601 *** Army Research Laboratory, Adelphi, MD 20783-1197 ABSTRACT Optical Impedence Spectroscopy of GaN/AlGaN high electron mobility transistor structures (HEMTs) using photoreflectance exhibit a photoreflectance lags – the component of the modulated reflectance out of phase with the chopper – ranging from 0.1 to 0.5. Photoreflectance was performed using below gap pumping on various samples. Samples that do exhibit appreciable photoreflectance lag for above gap pumping show significantly enhanced photoreflectance signals for below gap pumping. Yet, samples that do not exhibit appreciable photoreflectance lag for above gap pumping do not exhibit a signal for below gap pumping. This implies that the photoreflectance phase lag is due to mid-gap trap states. At least 2 types of traps are found, above and below about 2.5 eV. This result means that that photoreflectance can be used as a probe of HEMT device quality.
INTRODUCTION GaN and its alloys are of great interest for RF-based devices because of desirable electronic transport and thermal dissipation properties inherent to these material systems. [1] Currently, investigations have focused on devices made from high electron mobility transitors (HEMT). It has been found in this laboratory, and others that HEMT device performance is often associated with defects in the GaN and AlGaN layers. These defects may be due to a combination of factors such as substrate lattice mismatch, nucleation layer formation and growth parameters. For example, the commonly referred-to yellow band photoluminescence in GaN may be due to more than one type of defect and it has been demonstrated that the intensity of the yellow band can be controlled during the epitaxial layer synthesis. In this paper, we use photoreflectance (PR) as an optical impedence spectroscopy to probe the material properties of GaN/AlGaN HEMT structures. The photoreflectance signal consists of two portions, that portion of the change in reflectance which is in-phase with the chopped pump light and the portion that is out-of-phase with the chopped pump. The out-ofphase portion contains information about how the sample responds to the pump light. [2] Sample doping geometry and electron barriers and traps all affect the out-of-phase portion of the PR signal.
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THEORETICAL CONSIDERATIONS The photoreflectance technique has traditionally been used to study interband transitions in various semiconductors. The technique produces derivative like line shapes for transitions at various critical points in the Brilloun zone. The derivative nature of PR comes from the FranzKeldysh effect, which results in the reflectivity and absorption of a material being dependent on an applied or built-in electric
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