The junction temperatures of ultraviolet (UV) light-emitting diodes (LEDs) were determined in situ using noncontact micro-Raman spectroscopy. This method is based on the systematic downshift of the Raman peak as junction temperature rises. A calibration measurement was carried out first to establish the relation between junction temperature and the Raman peak, followed by temperature measurements as the forward current increased from 50 mA to 110 mA. A temperature rise from 27°C to 107°C was observed. We have demonstrated that micro-Raman spectroscopy is a viable technique for UV LED junction temperature determination.Due to their high power-conversion efficiency, long lifetime, and low operating cost, light-emitting diodes (LEDs) have been widely used in solid-state lighting applications. It is expected that white LEDs will gradually replace traditional lighting such as incandescent and fluorescent bulbs. 1 However, to achieve this goal, LED chips need to be designed so as to minimize heat generation and improve heat sinking, since their lifetime and luminous efficacy decrease as the junction temperature increases. 2 Therefore, it is very important to accurately determine the junction temperature of the LED during its operation. Temperature measurement methods of semiconductor devices fall into three categories: physical contact, electrical, and optical. 3 The electrical measurement method of LED temperature is based on the temperaturedependent forward voltage, derived from the Shockley diode equation and Varshni relation. 4 This method, although it provides the average junction temperature with good accuracy, lacks spatial resolution information. The existing optical methods determine the junction temperature from the slope of the high-energy portion of the electroluminescence (EL) peak 5,6 and/or the shift of the EL peak. 4 However, even these methods offer no spatial resolution and are applicable to only a single emission peak in the EL spectrum. The micro-Raman spectroscopy method, based on the measurement of the Raman peak shift upon temperature change, can be used to determine device temperature because of its high accuracy and high spatial resolution. For example, 1 lm spatial resolution and 10°C temperature accuracy were achieved by micro-Raman spectroscopy for GaN-based heterostructure fieldeffect transistors (HFETs). 7,8 The advantage of micro-Raman spectroscopy over EL-based methods is that it is also applicable to LEDs which exhibit multiple EL peaks. Temperature determination of ultraviolet (UV) LED chips by micro-Raman spectroscopy with good agreement with theory was reported previously. Schwegler et al. 9 determined the temperature of InGaN multiple-quantum-well

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 current in these diodes was proportional to x V , as opposed to nkT qV e / , where x was observed to be either 1 or 2 at low biases increasing to as high as 40 at higher biases. The dependence of diode forward current on x 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 E V and E V + 0.5 eV with concentrations of the order of 10 16 /cm 3 to 10 17 /cm 3 . Capacitance-voltage measurements as a function of frequency (200 Hz -1 MHz) at room temperature yielded a density of approximately 1 x 10 15 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. Mater. Res. Soc. Symp. Proc. Vol. 955

Light emitting diodes (LEDs) are normally operated at high current levels resulting in substantial junction heating. However, the junction temperature cannot be measured directly. In the study reported here, it was estimated junction temperature in LEDs with peak emission 400 nm. Temperature was estimated from current-voltage (I-V) measurements as a function temperature and peak-shift of optical emission spectra with increasing temperature. For a diode operated at a current-level of 100 mA, a temperature of 100 o C was estimate from current voltage measurement and 160 o C from the peak-shift in the optical emission spectra. The difference in temperature estimated using the two different technique is not understood at this point. Mater. Res. Soc. Symp. Proc. Vol. 955