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The low-frequency noise sources are investigated in as-prepared and aged GaN light-emitting diodes ͑LEDs͒. Accelerated aging is performed by thermal ͑300 h at 240°C͒ and electrical forward-bias stressing ͑20 and 50 mA for 2500 h͒. At low currents I Ͻ I RTS , where I RTS is a critical current, the low-frequency noise is dominated by random telegraph signal ͑RTS͒ noise on top of the 1/ f noise. An explanation is given for the giant relative current jumps ⌬I / I Ϸ 50% and an expression for I RTS is derived. The RTS noise in our devices is a less-sensitive diagnostic tool for studying the results of accelerated aging. Two components of the 1 / f noise were observed: one is related to the quantum-well junction and the other is due to series resistance noise. The two 1 / f spectra have different current dependences. It was found that the junction 1 / f noise is not significantly affected by aging. However, a strong increase in series resistance noise, by a factor of 60-800 compared to unstressed devices, is observed after strong electrical and thermal aging. This high increase goes hand in hand with a relatively small increase in the value of the series resistance ͑13%-90%͒. This makes 1 / f noise a very sensitive reliability indicator for GaN LEDs after accelerated aging. We discuss the physical origin of LED degradation.

Heat removal from III-Nitride-based devices into a substrate depends also on an acoustic coupling at III-Nitride/substrate interface. We investigate thermal boundary resistance (TBR) and its effects on temperature distribution for GaN layers on Si, SiC, or sapphire substrates. Micro-Raman method is used for the investigation of TBR at the GaN/Si interface while the transient interferometric mapping (TIM) method is used for investigation of GaN/SiC and GaN/sapphire systems. Thermal modeling is used to analyze the experimental data. We found TBR to be ~7×10−8 m2 K/W for GaN/Si and ~1.2×10−7 m2 K/W for GaN/SiC interfaces. The role of TBR at the GaN/sapphire interface in the poor heat transfer from GaN to substrate is found to be less important. It is suggested that the substrate cooling efficiency may be improved if fewer defects are present at the interface to the GaN epistructure. ©2007 American Institute of Physic

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A method for the extraction of power dissipation sources inside semiconductor devices on a nanosecond-time scale is proposed using a backside transient interferometric mapping technique. The two-dimensional power dissipation density is extracted from the time and space derivative of the measured optical phase shift. The method is applied to the analysis of moving current filaments in an electrostatic discharge protection device operating in the avalanche regime. It is found that the filament dynamics is governed by the negative temperature dependence of the impact ionization coefficient. The total power calculated from the optical measurements is in excellent agreement with the electrical input power.