Quaternary AlInGaN MQWs for Ultraviolet LEDs
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Quaternary AlInGaN MQWs for Ultraviolet LEDs J. P. Zhang, J. W. Yang, V. Adivarahan, H. M. Wang, Q. Fareed, E. Kuokstis, A. Chitnis, M. Shatalov, G. Simin, M. Asif Khan, R. Gaska1, M. S. Shur1 Dept. of Electrical Engineering, University of South Carolina, Columbia, SC 29208, U.S.A 1 Sensor Electronic Technology, Inc., Latham, NY 12110, U.S.A ABSTRACT We report a pulsed atomic layer epitaxy (PALE) growth technique for quaternary AlInGaN films for ultraviolet optoelectronic applications. Using the PALE approach high quality quaternary AlInGaN/AlInGaN multiple quantum wells (MQWs) were successfully grown over sapphire substrates. From X-ray diffraction, atomic force microscopy, and photoluminescence study, a high structural and optical quality was established for the AlInGaN MQWs. Incorporating the PALE grown quaternary MQWs as the active layer of light emitting diode (LED) on sapphire or SiC substrates we also demonstrated room temperature deep ultraviolet electroluminescence under dc and pulsed electrical pumping. The peak emission wavelength can be tuned from 305 nm to 340 nm with spectrum FWHM of about 20 nm by varying the alloy compositions of the quaternary AlInGaN active layers using PALE. Comparative study of LEDs over sapphire and SiC substrates was also done in order to determine the influence of epilayer design on the performance parameters and the role of the substrate absorption.
INTRODUCTION There have been several reports [1]-[4] of using blue GaN/InGaN multiple quantum well (MQW) light emitting diodes (LEDs) as pumps for YAG:Ce3+ and other inorganic and polymer phosphors for solid state white lighting applications. This approach however has been shown to have severe color rendering and low power conversion efficiency problems [1]. These can be greatly minimized by using down conversion phosphors and ultraviolet (UV) light emitting diodes with emission wavelengths below 350 nm. This also increases the number of available phosphor choices. Deep UV laser/LED pulsed light sources with emission below 350nm are also required for spectroscopic systems used in chemical identification and short distance communication links. Recently, using AlGaN/AlGaN MQWs in the active region several groups [5]-[8] have demonstrated UV LEDs with peak emission wavelength around 340-350 nm. Nishida et al. [5] grew their device structures over n+-SiC and n+-GaN substrates, which allows a vertically conducting geometry and an easier thermal management. However, for wavelengths well below 360 nm, these conducting substrates are highly absorbing. In past, we have demonstrated a unique strain energy band engineering (SEBE) approach to tailor the built-in strain [8], [9] and significantly improve the quantum well emission properties by using the quaternary AlInGaN material system [10], [11]. These improvements result not only from the strain management but also from an overall material quality improvement by the In- incorporation in the ternary AlGaN layers [12]. Using quaternary AlInGaN/AlInGaN MQWs in the active region UV LEDs on sa
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