Pressure Dependence of Optical Transitions in InGaN/GaN Multiple Quantum Wells
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dependence of the absorption process probed by PT and that of the emission process measured by PL provides direct insights into the nature of the electronic states involved. EXPERIMENT The Ino. 15Ga 0.85N/GaN MQW sample used in this work is a laser diode structure prepared by metalorganic chemical vapor deposition. It consists of a 10-period In0 .15Gao. 85N/GaN superlattice grown on a 4-jm thick GaN layer deposited on a sapphire substrate, and it is capped by a 0.2-jim GaN:Mg p-type layer. The thicknesses of the well and the barrier are 18 and 62 A, respectively. These values were derived from X-ray diffraction (XRD) measurements of the superlattice period (80 A) and the ratio of the well/barrier growth times (35/120). The averaged The MQW In concentration was determined by Rutherford backscattering spectrometry. 7 structure is pseudomorphically strained to the underlaying GaN layers. Photomodulation measurements were performed in a transmission geometry using a 150W xenon lamp as probing light source and a chopped HeCd laser beam (3250 A) as modulating light. PL signals resulted from excitation by the laser and were dispersed by a 1-M double-grating monochromator. Application of hydrostatic pressure was accomplished by mounting small sample chips with sizes of -200x200 pmr2 into gasketed diamond anvil cells. A small ruby chip was also placed in the DAC for pressure calibration. All the spectra reported in this work were recorded at room temperature (295 K). RESULTS AND DISCUSSION Fig. 1 shows PT spectra taken from the In0 .15Gao. 85N/GaN MQW sample and two thick epilayer samples at (In0 .15Ga 0 .85N and Ino. 0 Ga 0.89N) ambient pressure. The derivative-like U) spectral signatures denoted as E0 in the EGaN spectra are associated with the optical Io l5Ga, 85N/GaN transitions across the band gap of the MOw _ respective samples. Note that the quantum confinement effects on the electron and hole states in the MQW 3.3 eV Ul) have shifted the band gap of In0 .15Ga 0.85N SE=3.045 eV Il0..1Gao.8 5N EGaN to an energy approximately equal to that Epitaxial layer of bulk In0. 1Gao.89N. The second derivative-like spectral feature (denoted as El) in the PT spectrum of the MQW is Eo=2.9 eV due to transitions from ionized Mg 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 acceptor states to the conduction band Photon Energy (eV) edge in the p-type GaN cladding layer. As commonly observed in InGaN alloys and related heterostructure samples, the MQW sample exhibits fairly broad PT Fig. 1. PT spectra of an 1n0.l5Ga 0.85N/GaN MQW sample and PL spectral lineshapes. A Stokes and two (Ino.15Gao.85N and In0.15Gao. 85N) epilayer samples at ambient pressure shift of the PL peak energy (2.99 eV) relative to the transition energy (E0 ) was observed.
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the thick 1n0 .,1 Ga 0 .89 N epilayer, are plotted in Fig.2. The inset of the figure shows a comparison of the
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The pressure induced energy shifts for the E 0 transition and peaktheif the sample, alongPLwith PL MQW emission f
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