Piezoelectric Effects in GaN/AlGaN Multiple Quantum Wells Probed by Picosecond Time-Resolved Photoluminescence
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j. Y. Lin., H. X. Jiang
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W. W. Chow*, A. Botchkarev**, and H. Morkoq**
Department of Physics, Kansas State University, Manhattan, KS 66506-2601 * Sandia National Laboratories, Albuquerque, NM 85718-0601 Department of Electrical Engineering and Physics, Virginia Commonwealth University, Richmond, Virginia 23284-3072 Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.3 (1999)
ABSTRACT Piezoelectric effects in GaN/AIGaN multiple quantum wells (MQWs) have been directly probed by picosecond time-resolved photoluminescence (PL) spectroscopy. The time-resolved PL spectra of the 40 A well MQWs reveal that the PL transition peak position is in fact blueshifted at early delay times due to the collective effects of quantum confinement of carriers, piezoelectric field, and Coulomb screening. However, the spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. By comparing experimental and calculation results, we have obtained a low limit of the piezoelectric field strength to be about 560 kV/cm in the 40 A well GaN/A10 15 . Ga0 85 . N MQWs. INTRODUCTION The group III-nitride semiconductors have attracted much attention recently due to many important applications, such as blue/UV light emitting diodes (LEDs), laser diodes (LDs), and high-temperature/high-power electronic devices [l]. As demonstrated by GaN LDs, LEDs, and GaN based electronic devices, many GaN based devices must take advantage of multiple quantum well (MQW) structures such as GaN/AIGaN and InGaN/GaN MQWs. In order to optimize the device design, it is necessary to study and understand the physical properties of nitride MQWs as well as the MQW structural effects on the device performance. Recent work on the III-nitride alloy systems and MQWs has shown that localized excitons dominate the optical transitions in these systems at low temperatures[2-4]. And, it has been proposed and shown that piezoelectric fields due to lattice mismatch-induced strain in InGaN/GaN MQWs [5,6] and GaN/A1GaN QWs [6-9] are the primary reason for the large redshift of the photoluminescence (PL) emission peak. Indeed the piezoelectric field in strained layers has been used to modify the operating characteristics of devices fabricated from these materials 1911]. EXPERIMENT Time-resolved PL (TRPL) studies have been carried out for a set of GaN/AIGaN MQW samples grown under identical conditions with well thickness varying from 20 to 50 A . These MQW samples were grown by reactive molecular beam epitaxy (MBE) on sapphire (A120 3) substrates with a 1.5 ptm AlGaixN (x=0.15) buffer layer. The MQWs are composed of ten periods of alternating GaN wells and 200 A AlxGal-xN (x0.15) barriers. All samples were nominally undoped and the GaN epilayers grown under similar conditions were semiinsulating. G 3.3 Mat. Res. Soc. Symp. Proc. Vol. 537 © 1999 Materials Research Society
For PL measurements, samples were attached to copper sample holders and placed inside a closed-cycle He refrigerator with a temper
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