Characterization of Minority-Carrier Hole Transport in Nitride-Based Light-Emitting Diodes with Optical and Electrical T
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E10.9.1
Characterization of Minority-Carrier Hole Transport in Nitride-Based LightEmitting Diodes with Optical and Electrical Time-Resolved Techniques R. J. Kaplar, S. R. Kurtz, D. D. Koleske, A. A. Allerman, A. J. Fischer, and M. H. Crawford Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT Forward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters τp = 758 ± 44 ns, Lp = 588 ± 45 nm, and µp = 0.18 ± 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of subbandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of τp = 3.0 µs was observed, and the diffusion length was shorter, Lp ≈ 280 nm. Mobility was an order of magnitude smaller than in GaN, µp ≈ 10-2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects. INTRODUCTION Transport of massive, minority-carrier holes in nitride-based devices such as photodetectors and light-emitting diodes (LEDs) is a critical factor influencing performance. However, minority-carrier transport needs to be investigated directly, since majority-carrier Hall mobility measurements overlook the influence of a large density of defects. Reported response times of nitride-based minority-carrier devices show great disparity depending on growth conditions and substrate [1-3], suggesting that diffusive hole transport is very sensitive to material quality. Sub-nanosecond lifetimes are measured using time-resolved photoluminescence (TRPL), but these measurements are often insensitive to (or ignore) carrier diffusion due to temperature, electric field, or surface effects [4]. Electron-beam induced current (EBIC) measurements provide an indication of minority-carrier diffusion length [5,6], but lifetimes determined using this technique rely on values of majority-carrier mobility obtained from Hall measurements. To investigate minority-hole transport directly, we have implemented a new technique that combines measurements of the electrical and optical transient responses of an LED to a forward-to-reverse bias voltage pulse. Minority-carrier lifetime, diffusion length, and mobility may be determined. We have observed hole transport in p+n quantum-well LEDs where the QW is positioned in the n-region. Two classes of LEDs were investigated, InGaN/GaN (QW emission ≈ 400 nm) and AlGaN/AlGaN (QW emission ≈ 298 nm). All devices were grown on sapphire using metal-organic chemical vapor deposition (MOCVD).
E10.9.2
EXPERIMENTAL Forward-to-reverse bias current recovery was observed using a transmission-line circuit with
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