Pressure dependence of optical transitions in In 0.15 Ga 0.85 N/GaN multiple quantum wells W. Shan, J. W. Ager III, and W. Walukiewicz Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 E. E. Haller Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 and Department of Materials Sciences and Mineral Engineering, University of California, Berkeley, California 94720 M. D. McCluskey, N. M. Johnson, and D. P. Bour Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304 Received 26 June 1998; revised manuscript received 20 August 1998 The effects of hydrostatic pressure on optical transitions in In 0.15 Ga 0.85 N/GaN multiple quantum wells MQW’s have been studied. The optical transition associated with confined electron and hole states in the MQW’s was found to shift linearly to higher energy with pressure but exhibit a significantly weaker pressure dependence compared to bulklike thick epitaxial-layer samples. Similar pressure coefficients obtained by both photomodulation and photoluminescence measurements rule out the possibility of the transition involving localized states deep in the band gap. We found that the difference in the compressibility of In x Ga 1-x N and GaN induces a tensile strain in the compressively strained In x Ga 1-x N well layers, partially compensating the externally applied hydrostatic pressure. This mechanical effect is primarily responsible for the smaller pressure dependence of the optical transitions in the In x Ga 1-x N/GaN MQW’s. In addition, the pressure-dependent measurements allow us to identify a spectral feature observed at an energy below the GaN band gap. We conclude that this feature is due to transitions from ionized Mg acceptor states to the conduction band in the p-type GaN cladding layer rather than a confined transition in the MQW’s. S0163-18299851940-6 The In x Ga 1 -x N alloy system and related heterostructures such as quantum wells QW’s have been attracting much attention because of their importance in both scientific and technological aspects. 1–3 One of the fundamental issues is the evolution of the pressure dependence of the energy band gap for the alloy system. The most-recent experimentally determined values of pressure coefficient for the direct band gap of GaN are (3.9– 4.2) 10 -3 eV/kbar. 4–6 Christensen and Gorczyca predicted a pressure coefficient of 3.310 -3 eV/kbar for the band gap of InN based on their self- consistent band-structure calculations. 7 Pressure-dependent photoluminescence studies on bulklike In x Ga 1 -x N epitaxial layers, which have yielded the pressure coefficients of near- band-edge luminescence emission from layers with small InN mole fraction (0 x 0.15), do not substantially differ from that of GaN. 8,9 However, recent pressure-dependent studies of the optical properties of In x Ga 1 -x N/GaN QW’s have found that the pressure coefficients of luminescence emission depend on QW sample structure and the In concentration. 10,11 Perlin and co-workers reported that the pressure coefficients of PL and EL emissions from In x Ga 1 -x N/GaN/Al x Ga 1 -x N QW samples are much smaller than those of the GaN band gap. 10 The results were ex- plained by assuming that highly localized states, with small pressure coefficients, are involved in the emission processes in their QW’s. In this paper we present the results of a high-pressure study of optical transitions in an In 0.15 Ga 0.85 N/GaN multiple- quantum-well MQW sample by performing photomodu- lated transmission PT and photoluminescence PL mea- surements. Since it is well known that in most instances, deep localized states have a pressure dependence smaller than the band-edge states, a comparison between the pressure dependence of the absorption process probed by PT and that of the emission process measured by PL can provide direct insights into the nature of the states involved. The In 0.15 Ga 0.85 N/GaN MQW sample used in this work is a laser diode structure prepared by metalorganic chemical vapor deposition. It consists of a 10-period In 0.15 Ga 0.85 N/GaN superlattice grown on a 4-m-thick GaN layer deposited on a sapphire substrate, and capped by a 0.2-m GaN:Mg p-type layer. The thicknesses of the well and the barrier are 18 and 62 Å, respectively. These values were derived from x-ray-diffraction XRD measurements of the superlattice period 80 Å and the ratio of the well/barrier growth times 35/120. The averaged In concentration was determined by Rutherford backscattering spectrometry. The MQW structure is pseudomorphically strained to the under- lying GaN layers. 12 Photomodulation spectroscopic measurements were per- formed in a transmission geometry suing a 150 W xenon lamp as probing light source and a chopped HeCd laser beam 3250 Å as modulating light. PL signals resulted from exci- tation 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 200200 m 2 into gasketed diamond anvil cells. A small ruby chip was also placed in the diamond anvil cell DAC for pressure calibration. All the spectra reported in this work were recorded at room temperature 295 K. Figure 1 shows PT spectra taken from the In 0.15 Ga 0.85 N/GaN MQW sample and a thick In 0.15 Ga 0.85 N RAPID COMMUNICATIONS PHYSICAL REVIEW B 15 OCTOBER 1998-II VOLUME 58, NUMBER 16 PRB 58 0163-1829/98/5816/101914/$15.00 R10 191 © 1998 The American Physical Society