High-pressure Raman scattering in wurtzite indium nitride

Abstract

We perform Raman-scattering measurements at high hydrostatic pressures on c-face and a-face InN layers to investigate the high-pressure behavior of the zone-center optical phonons of wurtzite InN. Linear pressure coefficients and mode Grüneisen parameters are obtained, and the experimental results are compared with theoretical values obtained from ab initio lattice-dynamical calculations. Good agreement is found between the experimental and calculated results.Over the last few years, indium nitride (InN) has received intensive research interest because of its great potential to develop a wide variety of applications such as high-frequency electronic devices, high-efficiency tandem solar cells, or infrared light-emitting devices. 1 Although the growth of high quality InN is still challenging, bulk material with residual electron densities lower than 5 Â 10 17 cm À3 and electron mobilities higher than 2000 cm 2 V À1 s À1 has been achieved by molecular beam epitaxy (MBE). 2 In spite of the great interest of InN from both fundamental and applied points of view, many material properties of this compound remain to be investigated. The production of highquality InN layers and the discovery of the 0.7 eV fundamental band gap of wurtzite InN (w-InN) made necessary to revise many material parameters of InN and InN-based alloys, including their high-pressure behavior. 3,4 While the available experimental data on the vibrational properties of InN stem mainly from Raman spectroscopy (see for instance Ref. 5 and references therein), a deeper understanding of the lattice dynamics in InN has only been accomplished very recently by means of grazing incidence inelastic x-ray scattering (IXS) measurements on high-quality InN epilayers. 6 Regarding the high-pressure vibrational properties of InN, Pinquier et al. 7,8 used Raman scattering to study the pressure dependence of the E 2h , A 1 (TO), and A 1 (LO) phonon modes of w-InN in a layer with a high background electron concentration (2.3 Â 10 19 cm À3 ). These authors observed the wurtzite-to-rocksalt transition and studied the pressure dependence, up to 50 GPa, of broad bands arising from the rocksalt phase. 8 However, the pressure dependence of the long-lived E 2l mode and of the E 1 modes of w-InN was not reported in those works. Recently, Yao and co-workers employed Raman scattering to investigate the structural stability of poorly crystalline w-InN nanowires under high pressure. 9 The pressure coefficients and mode Grüneisen parameters obtained by these authors for the A 1 (TO), E 2h , and A 1 (LO) modes were sizably lower than those reported in Refs. 7 and 8.In this letter, we present Raman-scattering measurements under high hydrostatic pressure on high-quality c-face and a-face InN epilayers grown by MBE. Experiments on both types of samples have allowed us to study the pressure behavior of the non-polar E 2l and E 2h optical phonons and of the polar A 1 (TO), E 1 (TO), and LO phonons of w-InN. The linear pressure coefficients and mode Grüneisen parameters of these modes hav...

High-pressure lattice dynamics in wurtzite and rocksalt indium nitride investigated by means of Raman spectroscopy

“…[4][5][6][7] Pinquier and coworkers 4,5 investigated the pressure dependence of the E 2h , A 1 (TO), and LO phonons of w-InN. Pressure coefficients and mode Grüneisen parameters were obtained for these phonon modes, and the wurtzite-to-rocksalt transition was found to lie in the 12-14 GPa range, in agreement with x-ray diffraction measurements 8 and with the predictions of ab initio calculations.…”

“…In turn, Yao and coworkers 6 carried out high-pressure Ramanscattering measurements to study the structural stability of poorly crystalline w-InN nanowires. Recently the softening of the E 2l mode has been observed, and the pressure behavior of the E 1 (TO) mode has been investigated on a-face layers, 7 with good agreement between the experimental data and ab initio calculations relying on density functional theory.…”

“…As discussed in Ref. 7, the Raman spectra of the InN flakes loaded into the DAC contain contributions of both A 1 and E 1 symmetries, which is attributed to symmetryforbidden enhancement of the E 1 (LO) signal due to the impurity-induced Fröhlich interaction mechanism. 11 In the case of a-face material (sample D), the Raman spectra also exhibit weak features corresponding to the zone-center A 1 (TO) and E 1 (TO) modes (not shown here; see Ref.…”

“…11 In the case of a-face material (sample D), the Raman spectra also exhibit weak features corresponding to the zone-center A 1 (TO) and E 1 (TO) modes (not shown here; see Ref. 7).…”

Raman scattering by the E2h and A1(LO) phonons of InxGa1−xN epilayers (0.25 < x < 0.75) grown by molecular beam epitaxy

“…6 can be solely attributed to experimental errors or to an additional effect. For instance, the observed deviations could be originated by a contribution of the forbidden E 1 (LO) mode due to disorder or to sample mosaicity, as occurs in disordered III-nitrides 36,37 or in nanostructured material. 38,39 A similar analysis using the appropriate phonon deformation potentials can be applied to obtain the compositional dependence of the frequency of the E 2h mode in strain-free In x Ga 1Àx N. Figure 7 shows the calculated frequency values for the E 2h mode of strain-free In x Ga 1Àx N for the samples studied in this work.…”