Raman scattering study of wurtzite and rocksalt InN under high pressure C. Pinquier, F. Demangeot, and J. Frandon Laboratoire de Physique des Solides, CNRS-UMR 5477, IRSAMC, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 4, France J.-C. Chervin, A. Polian, B. Couzinet, and P. Munsch Physique des Milieux Denses, IMPMC, CNRS-UMR 7590, Université Pierre et Marie Curie, 140 rue de Lourmel, 75015 Paris, France O. Briot, S. Ruffenach, B. Gil, and B. Maleyre Groupe d’Etude des Semiconducteurs, Université Montpellier II, Case Courrier 074, 34095 Montpellier Cedex 5, France Received 9 January 2006; revised manuscript received 15 February 2006; published 30 March 2006 Indium nitride under high pressure up to 50 GPa was analyzed by means of Raman spectroscopy. The wurtzite to rocksalt phase transition was evidenced at the pressure of 13.5± 0.5 GPa and the pressure depen- dence of vibration modes of both structures was investigated, leading to the determination of linear pressure coefficients and mode Grüneisen parameters. Influence of the pressure dependence of the energy gap on the spectra intensity is also discussed. DOI: 10.1103/PhysRevB.73.115211 PACS numbers: 61.50.Ks, 63.20.-e I. INTRODUCTION Group-III nitrides have been widely investigated because of their promising applications for optoelectronic devices. Indium nitride was the least studied as it is rather difficult to grow high quality material, and particularly to elaborate thick undoped layers. This material has known increased in- terest thanks to recent progress in growth techniques; never- theless, to date, the knowledge of InN properties remains quite poor. InN films are usually grown on various buffer layers and substrates, and so, are generally strained due to the significant lattice mismatch and the difference in thermal expansion between the film and the underlying layer. Raman spectroscopy can be employed to probe the strain fields: for this purpose, deformation potentials are key parameters. While they have been extensively studied in the case of GaN Refs. 1–3 and AlN, 4,5 only one report can be found for InN. 6 However, in the last reference, owing to the lack of experimental data, calculated mode Grüneisen parameters 7 were used. The experimental evaluation of the latter requires high-pressure measurements. In a previous paper on Raman scattering in wurtzite InN under hydrostatic pressure, 8 up to 13.2 GPa, we evidenced the beginning of the wurtzite to rocksalt phase transition and analyzed the long-wavelength E 2 and A 1 LO phonons rela- tive to the wurtzite structure. Here, higher pressure experi- ments up to 50 GPa are reported: the phase transition was achieved and the Raman signal of rocksalt InN is discussed. Linear pressure coefficients and mode Grüneisen parameters are determined, not only for the wurtzite E 2 and A 1 LO phonons, but also for the A 1 TO phonon, as well as the rocksalt modes. Moreover, the study is completed by quan- titative results on Raman signal intensities. II. EXPERIMENTAL DETAILS The sample under study is a 1.4-m-thick wurtzite InN film, directly deposited on a sapphire substrate by metalor- ganic vapor phase epitaxy MOVPE. 9 The doping level de- termined by Hall measurements is 2.3  10 19 cm -3 , which is typical for such layers. 10 Due to the large lattice mismatch of 26% between the two materials, InN could be easily de- tached from the substrate, thus producing free-standing flakes of 20 m in size. Pressure experiments were conducted using a membrane diamond anvil cell DAC. 11 The culets of the anvils were 400 m in diameter. The pressure transmitting medium was argon, loaded at high pressure by an optically monitored method. 12 The stainless steel gasket was preindented to a thickness of approximatively 40 m, and the hole forming the experimental volume had a diameter of 160 m, just after loading, as shown in the inset of Fig. 1. The pressure values up to 50 GPa were determined by the ruby fluores- cence method. 13,14 The accuracy of this measurement ±0.1 GPa is limited by pressure heterogeneity inside the DAC, which cannot be neglected for the highest pressures. 15 Spectra of InN Raman scattering and ruby luminescence were recorded in backscattering geometry by means of a HR 460 Raman spectrometer with a holographic notch filter and a charge coupled device CCD camera, and equipped with a microscope. The 2.41 eV 514.5 nm Ar + line was used for excitation. Acquisition time ranged between 300 and 900 s. Then, all spectra were normalized in time or to the integrated area, as explained later. III. RESULTS AND DISCUSSION Raman spectra normalized in time under increasing pressure up to 14.1 GPa are displayed in Fig. 1. The lumi- nescence at high frequency is due to the diamond anvils, as well as the peak at 760 cm -1 , marked with an asterisk, which corresponds to the diamond 2TA phonon. The modes respec- tively observed at 440, 491, and 592 cm -1 at atmospheric pressure are attributed to the phonons of InN, namely A 1 TO, long-wavelength E 2 , and A 1 LO, in agreement with PHYSICAL REVIEW B 73, 115211 2006 1098-0121/2006/7311/1152115/$23.00 ©2006 The American Physical Society 115211-1