Pressure Dependence of Optical Transitions in InGaN/GaN Multiple Quantum Wells
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Downloaded from https://www.cambridge.org/core. IP address: 91.243.91.127, on 29 Oct 2018 at 20:02:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002441
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 In0.15Ga0.85N/GaN MQW sample used in this work is a laser diode structure prepared by metalorganic chemical vapor deposition. It consists of a 10-period In0.15Ga0.85N/GaN superlattice grown on a 4-µm thick GaN layer deposited on a sapphire substrate, and it is capped by a 0.2-µm 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 underlaying GaN layers.7 Photomodulation measurements were performed in a transmission geometry using a 150W xenon lamp as probing light source and a chopped HeCd laser beam (3250 Å) 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 ~200×200 µm 2 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
PT Signal (arb.units)
Fig.1 shows PT spectra taken E0=3.025 eV 295 K from the In0.15Ga0.85N/GaN MQW In0.11Ga0.89N sample and two thick epilayer samples Epitaxial layer (In0.15Ga0.85N and In0.11Ga0.89N) at ambient pressure. The derivative-like spectral signatures denoted as E0 in the EGaN spectra are associated with the optical transitions across the band gap of the In0.15Ga0.85N/GaN MQW respective samples. Note that the quantum confinement effects on the electron and hole states in the MQW E1=3.3 eV have shifted the band gap of In0.15Ga0.85N E0=3.045 eV In0.15Ga0.85N EGaN to an energy approximately equal to that Epitaxial layer of bulk In0.11Ga0.89N. The second derivative-like spectral feature (denoted as E1) in the PT spectrum of the MQW is E0=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 edge in the p-type GaN cladding layer. Photon Energy (eV) As commonly observed in InGaN alloys and related heterostructure samples, the MQW sample exhibits fairly broad PT Fig. 1. PT spectra of an In0.15Ga0.85N/GaN MQW sample and PL spectral lineshapes. A Stokes and two (In0.15Ga0.85N and In0.15Ga0.85N) epilayer samples shift of the PL peak energy (2.99 eV) at ambient pressure relative to the transition energy (E0) was observed.
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