Properties of GaN and related compounds studied by means of Raman scattering

Abstract

In the last decade, we have seen very rapid and significant developments in Raman scattering experiments on GaN and related nitride compounds: the Γ-point phonon frequencies have been identified for both cubic and hexagonal structures of binary compounds of GaN, AlN, and InN. The phonon spectra of their ternary alloys, InGaN and AlGaN, were also intensively studied. On the basis of these studies, characterizations of strain, compositional fluctuation, defects, impurities, etc, are now being intensively conducted. Besides such pure lattice properties, coupled modes between a lattice vibration (LO phonon) and a collective excitation of free carriers (plasmon) in GaN have been thoroughly studied, and the results are now widely applied to characterize carrier-transport properties. Low-dimensional structures of nitrides such as quantum dots and superlattices will soon enter the most active field of Raman scattering characterization. This article briefly reviews the present status of Raman scattering experiments on GaN and related nitride compounds and presents some future prospects.

Realising epitaxial growth of GaN on (001) diamond

“…The A 1 (LO) phonon mode is here denoted as GaN (quasi-LO) because of its potential mixing with E 1 (LO) to produce a carrier concentration dependent coupled longitudinal optical mode. 43,44 For all the samples in this category, the above-mentioned E 2 (high) and the quasi-LO are clearly visible, respectively at 565.5 cm À1 and 734.4 cm À1 , indicating that some stress is present in the samples. The peak wavenumber for the E 2 (high) is slightly lower than the value obtained for bulk GaN, thus demonstrating a small tensile stress in the layer, which is estimated at 0.4 GPa from the relation 45…”

“…The cause for this difference could not be determined unambiguously, but it most likely originates from a larger carrier concentration (about a ¼ 1:5 Â 10 17 cm À3 ), resulting in LO phonon-plasmon coupling of which the L þ branch is at 738 cm À1 and the L À branch is just visible at 244 cm À1 . 44 …”

Raman spectroscopy of wurtzite and zinc-blende GaAs nanowires: Polarization dependence, selection rules, and strain effects

“…As it has been shown in the case of GaN and SiC, the main characteristics of the phonon dispersion of wurtzite GaAs can be deduced in an approximated way thanks to the straight forward relation between the two structures. 43,44 From the crystallographic point of view, zinc-blende and wurtzite structures differ only in the stacking periodicity of the Ga-As bilayers parallel to the ͑111͒ ZB / ͑0001͒ WZ oriented planes. For illustration, a schematic representation of the crystalline structure of zincblende and wurtzite structures is given in Figs.…”

Piezoelectric domains in the AlGaN hexagonal microrods: Effect of crystal orientations

“…14,[39][40][41] The strain coefficient, ε 33 is calculated by analyzing the deformation of crystal from line scan data of the PFM image. By using the above expression, we calculated the approximate value of d 33 along the c-axis as 4.14 pm/V. The calculated value of d 33 is slightly different from the reported values 5.1 and 3.1 pm/V for unclamped AlN and GaN, respectively.…”

“…Here, d 31 , d 33 and d 51 are known as the piezoelectric coefficients found in the wurtzite crystal structure. 7 In any 3D coordinate system, each mutually independent element in the piezoelectric third rank tensor depends only on the crystallographic structure and orientations of the material.…”