Electrical Characterization of GaN Based Ultraviolet and Blue Light Emitting Diodes
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Electrical Characterization of GaN Based Ultraviolet and Blue Light Emitting Diodes Alphonse-Marie Kamto Tegueu1, Okechukwu Akpa2, Arindra Guha2, and Kalyankumar Das2 1 Electrical Engineering, University of Arkansas, Fayetteville, AR, 72701 2 Electrical Engineering, Tuskegee University, Tuskegee, AL, 36088 ABSTRACT Gallium nitride based ultraviolet (UV) and blue AlGaN/GaN/AlGaN double heterojunction structure light emitting diodes (LEDs) were electrically characterized using current-voltage (I-V) and capacitance-voltage (C-V) measurements as a function of frequency. An analysis of logarithmic plots of the forward I-V characteristics indicated that x qV / nkT , where x was observed current in these diodes was proportional to V , as opposed to e to be either 1 or 2 at low biases increasing to as high as 40 at higher biases. The dependence x of diode forward current on V is likely to be due to space charge limited current in the presence of a high concentration of deep level states in the bandgap. The concentration of deep states and their position in the band gap were extracted from these logarithmic plots. For both the blue and the UV LEDs, several closely spaced levels were obtained, located most likely in the range between EV and EV + 0.5 eV with concentrations of the order of 1016/cm3 to 1017/cm3. Capacitance-voltage measurements as a function of frequency (200 Hz - 1 MHz) at room temperature yielded a density of approximately 1 x 1015 cm-3 located at 0.46 eV above the valence band-edge for both the UV and blue LED. Even though the location of these deep states from the I-V and C-V measurements are within the same range, the two orders magnitude difference in the concentration of deep states is not well understood at this point. INTRODUCTION Ultraviolet and blue LEDs, and laser diodes (LDs) based on group III-nitride semiconductor compounds represent one of the most important breakthroughs, in electronics and optoelectronics, of recent years. Gallium nitride based LEDs have started to replace traditional light bulbs in traffic lights and are likely to gain an ever increasing market share for many other lighting applications including general purpose illumination. However, I-V characteristics of these LEDs appear to be highly non-ideal, as evidenced by high values of the ideality factor (n > 2) obtained from semilogarithmic plots of the forward characteristics. High ideality factors imply that the recombination process in these diodes cannot be conveniently described by the Shockley-Read-Hall recombination statistics [1, 2]. As reported previously, logarithmic plots of the forward characteristics of blue LEDs indicated a space-charge-limited current (SCLC) conduction through the active region of the diodes [3]. Several different slopes observed in these logarithmic plots were considered to be representative of sets of high-density deep-level states [4]. Deep-level state densities of 5.60 x 1016 cm-3, 8.12 x 1016 cm-3 and 1.21 x 1017 cm-3 most likely located at 0.27 eV, 0.30 eV and 0.37 eV, respectively
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