Metal‐Insulator Transition in GaN Crystals

Abstract

By means of high-pressure Raman spectroscopy of coupled LO phonon-plasmon modes, a qualitatively new model of the metal-insulator transition observed in bulk GaN crystals at pressure around 20 GPa is proposed. This transition was interpreted recently as caused by the emergence of the localized donor state into the energy gap of GaN. Our new experimental evidence suggests that two types of donors can supply electrons to the bulk crystal: the first of a localized character and the second of a shallow character with contributions to the electron concentration of 5 x 10'' and 3 x 1Ol8 ~m -~, respectively. The localized state can be associated with the nitrogen vacancy, while the shallow state can be tentatively attributed to impurities such as oxygen. A number of interesting phenomena take place beyond the metal-insulator transition in bulk GaN and an attempt to explain them will be made in the present work.

Raman and photoluminescence mapping of InxGa1−xN (x ∼ 0.4) at high pressure: Optical determination of composition and stress

“…This is in line with the aforementioned significant relaxation of the epilayer after sample breaking. Moreover, the obtained pressure coefficients for the two modes lie between those reported in the literature for GaN {3.6-4.2 cm À1 ÁGPa À1 for E 2 2 and 3.8-4.4 cm À1 ÁGPa À1 for A 1 (LO)} [16][17][18] and InN {5.1-5.6 cm À1 ÁGPa À1 for E 2 2 and 4.8-6.0 cm À1 ÁGPa À1 for A 1 (LO)}, [19][20][21][22] suggesting the intermediate stiffness of the alloy. In fact, linear interpolation for x ¼ 0.37 between the mean values of the pressure slopes for the end members of the In x Ga 1Àx N system yields 4.5 and 4.6 cm À1 ÁGPa À1 for the E 2 2 and the A 1 (LO) mode, respectively, in excellent agreement with our results.…”

Electronic and structural study of β‐FeSi2 under high pressure

“…The direct bandgap energy and pressure coefficient were determined to be 0.873 eV at ambient pressure and +15.9 meV/GPa, respectively. The pressure coefficient is smaller than those of common semiconductors such as GaAs and GaN, 108 meV/GPa for GaAs and about 40 meV/GPa for GaN, respectively [14,15].…”

Transverse effective charge and its pressure dependence in GaN single crystals

“…17 The common pressure scale for all experiments was based on the position of the E 2 mode and then transformed to the pressure using published data. 18 While we were able to measure the E 1 (TO) mode in this experiment, we were not able to measure the E 1 (LO) mode because it is not allowed in the presently used backscattering geometry. Figures 1 and 2 show the measured spectra obtained in different Raman geometries.…”