J. Frandon

Raman determination of phonon deformation potentials in α-GaN

Abstract The effect of the built-in biaxial stress on the E2 and A1 (LO) q = 0 phonon modes of wurtzite GaN layers deposited by Metal Organic Vapor Phase Epitaxy on (0 0 0 1) direction on sapphire substrates is studied by Raman spectroscopy. Shifts in phonon frequencies are measured, which we correlate to the residual strain fields in the epilayers. Using stress calibration measurements taken from reflectance data, the biaxial pressure coefficients of mode frequencies are determined and used to calculate the corresponding deformation potentials.

Interplay of electrons and phonons in heavily doped GaN epilayers

Raman spectroscopy is used to analyze the effect of electrons on the lattice dynamics of unintentionally heavily doped GaN. The deposition temperature of GaN buffer layers on sapphire substrates is found to have an important influence on the presence of free carriers in GaN layers, evidenced by plasmon coupling to the A1(LO) phonon. Data from infrared measurements are used to calculate the Raman line shape of q=0 coupled A1(LO)-plasmon modes in a dielectric approach and give a good fit of the L−(q=0) component observed in Raman spectra. In particular, the fitting procedure applied to spatially resolved micro-Raman measurements reveals an inhomogeneous concentration of electrons on the scale of hexagonal microcrystallites. Partial screening of phonons with wave vectors differing from the q=0 transfer of incident and scattered photons is invoked to explain LO-like scattering over the whole spectral range of optical phonons, attributed to charge density fluctuations on account of its polarization properties.

Molecular-beam epitaxy of gallium nitride on (0001) sapphire substrates using ammonia

Ammonia is used for growing undoped GaN layers by gas source molecular-beam epitaxy on c-plane sapphire substrates. The growth mode is layer by layer as shown by the observation of reflection high-energy electron diffraction intensity oscillations. The structural quality is studied by x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Low-temperature photoluminescence (PL) and reflectivity demonstrate intrinsic excitonic emission. Room-temperature PL exhibits a strong band-edge intensity and a weak deep-level emission, the so-called yellow band. Finally, secondary ion mass spectroscopy is carried out to check the residual impurity levels of Si, C, and O.

Phonon deformation potentials of wurtzite AlN

A strained AlN buffer layer used for the growth of a nitride-based superlattice on silicon carbide was studied by combining x-ray diffraction measurements and Raman spectroscopy. The deformation potentials have been derived from strains and frequency shifts for most long-wavelength optical phonons. The obtained values are compared with recent theoretical calculations and experimental determinations, restricted for the latter to a few accessible modes on account of constraints imposed by the methods of investigation.

GaN nanoindentation: A micro-Raman spectroscopy study of local strain fields

We have investigated strain fields around GaN nanoindentations. Stress relaxation around the edges of the nanoindentation was evident in atomic force microscopy images. More detailed information on the strain fields was obtained from Raman scattering, which has been used to analyze the shape of the strain field around the indentation. We find that the Berkovich tip giving a triangular imprint on the sample generates a strain field, which represents a hexagonal pattern. Negative values of the strain indicate that the residual stress is compressive. Strain is larger in the center of the indentation than outside. Analysis of the ratio of the frequency shift of the E2 and A1(LO) modes suggests that the residual strains are close to biaxial state outside the indentation contact zone, and mostly hydrostatic within the indentation center.

Phonons in a strained hexagonal GaN–AlN superlattice

A GaN (6.3 nm)-AlN (5.1 nm) superlattice, grown by molecular beam epitaxy on a sapphire substrate and an AlN buffer layer, has been studied by means of micro-Raman spectroscopy. Most of the observed features have been identified and assigned to optical phonons of the superlattice layers. The average biaxial strain in GaN layers has been deduced from the detailed analysis of the frequency shift observed on the phonon lines. Additional measurements on the bevelled sample clearly suggest the significant increase of this strain for decreasing distances from the interface with the buffer layer.

Coupling of GaN- and AlN-like longitudinal optic phonons in Ga1−xAlxN solid solutions

Long-wavelength optical phonons of Ga1−xAlxN solid solutions have been identified in a wide compositional range by Raman spectroscopy. The A1 and E1 polar phonon frequencies evolve continuously with x from one-member crystal to the other. The same behavior seems to hold true for the silent B1 mode, which manifests itself by an interference with an unidentified continuum. Coupling of the longitudinal-optic (LO) modes associated with the two types of bonds, via the macroscopic electric field, is treated by a generalization of the dielectric model [D. T. Hon and W. L. Faust, J. Appl. Phys. 1, 241 (1973)]. This approach accounts for the observed frequencies and supports the apparent one-mode behavior of the polar LO phonons.

Angular dispersion of polar phonons in a hexagonal GaN-AlN superlattice

Abstract Raman spectra of a wurtzite GaN (6.3 nm)–AlN (5.1 nm) superlattice have been recorded under visible excitation. When the orientation of the phonon wavevector is varied, the angular dispersion of polar phonons from the superlattice is clearly evidenced. These experimental data are found to be in good agreement with the results of a previous calculation based on a dielectric continuum model, taking into account the strain of the two types of layers, which predicts dispersive interface and quasi-confined modes.

Raman study of the A1(LO) phonon in relaxed and pseudomorphic InGaN epilayers

The behavior of the A1(LO) phonon mode of relaxed and pseudomorphic InxGa1−xN epilayers, at the surface, is investigated by Raman spectroscopy. This study involves relaxed and pseudomorphic samples, with a compositional range of 0.12⩽x<0.30 and 0.04<x⩽0.20, respectively. Raman measurements were performed under excitation at 3.71 eV. Due to the low depth penetration of the incident light (40 nm), the major contribution to Raman scattering comes from the surface, where strain and composition have been independently determined. For relaxed samples, a linear dependence of the A1(LO) phonon frequency is obtained, as theoretically expected for an one-mode behavior alloy: Ω0(x)=(736±1)-(149±2)x. In the case of pseudomorphic samples, the phonon frequency is almost composition independent up to x=0.11, probably due to the opposite effects of strain and alloying.

Inelastic light scattering by phonons in hexagonal GaN-AlN nanostructures

Two selected examples have been chosen to illustrate the ability of non-resonant Raman scattering to probe phonons in hexagonal GaN-AlN artificial structures. The angular dispersion of polar phonons is investigated in a long period GaN-AlN superlattice and compared with the results of calculations based on a dielectric continuum model. On the other hand, the Raman signature of the self-assembled GaN quantum dots and of the ALN spacers of a multi-layered struct;re is used to determine the strain field in the stucture. The dots are shown to be fully strained on the ALN lattice parameter while the spacers exhibit on the average a slight tensile strain.