|About this Abstract
||2010 Electronic Materials Conference
||TMS 2010 Electronic Materials Conference
||LATE NEWS: L7, Molecular Beam Epitaxy of N-Polar InGaN
||Digbijoy Neelim Nath, Fatih Akyol, Emre Gur, Steven A Ringel, Siddharth Rajan
|On-Site Speaker (Planned)
||Digbijoy Neelim Nath
The N-orientation of GaN is advantageous for growth of In-containing compounds since it enables higher temperature growth of InGaN<SUP>1</SUP>. In addition, the inverted direction of polarization may provide advantages in LED performance. In this work we describe a comprehensive model for the growth of N-polar InGaN, and demonstrate the quality of the InGaN films through PL and LED measurements. This is the first demonstration of a N-polar green LED by molecular beam epitaxy. To develop a better understanding of growth kinetics, Ga-polar and N-polar GaN templates were co-loaded and growth of InGaN was done while keeping Ga-flux constant at 9.5 x 10<SUP>-8</SUP> Torr and varying temperature. Composition was determined by room temperature photoluminescence (PL) measurements. N-polar InGaN was found to incorporate significantly higher indium than co-loaded Ga-polar samples under identical growth conditions. N-polar samples show a 100 times increase in intensity for 100°C increase in growth temperature. Peak PL intensity and the full width half maximum (FWHM) values for the PL improved significantly as the temperature was increased. Based on InN decomposition, a growth model was developed for N-polar InGaN growth to explain the temperature dependence of indium composition. The experimentally obtained indium mole-fractions were used to establish the constants in the model, and excellent agreement was observed. Comparison with this and other Ga-polar growth work<SUP>2,3,4,5</SUP> shows the higher temperature range available for InGaN growth on the N-polar orientation. have been included to emphasize the overall higher achievability of indium composition for N-face polarity. Based on our growth model, Ga-flux required for obtaining 28% indium mole-fraction for InGaN grown at ~ 625°C-630°C was calculated and an N-polar InGaN/In<SUB>0.05</SUB>Ga<SUB>0.95</SUB>N MQW was grown to obtain green (520 nm) emission. The epitaxial structure was verified using XRD measurements and simulation. The In-composition extracted from XRD simulation (31.5%) was close to what our growth model predicted. Smooth morphology was observed by AFM (rms ~ 0.78 nm, 2 μm x 2 μm). The room temperature PL measurements in unbiased condition gave a sharp peak at 600 nm. We expect a red-shift caused by the polarization-induced field in the quantum wells, as predicted by the energy band diagram (inset). LEDs were fabricated using e-beam evaporated p-contact (Ni/Au/Ni) on top and an n-contact (Ti/Au) on n-GaN layer. The forward-biased I-V characteristics measured on a diode of area 103460 μm2 indicated low leakage up to -7 V reverse bias. Bright green emission was observed from the LED structure. Energy band diagram analysis using the measured composition and thickness values matches the green emission characteristics observed in the LED. Detailed electroluminescence spectra of the diode will be presented.