Optical studies of the phonons and electrons in gallium nitride
TL;DR: In this paper, the phonons and electrons of n-type gallium nitride have been studied using first order Raman spectroscopy and infrared reflectivity and the values of the allowed optical phonon frequencies, the dielectric constants, the optical electron mass and relaxation time, and the Born effective charge are reported.
About: This article is published in Solid State Communications.The article was published on 1970-08-01. It has received 149 citations till now. The article focuses on the topics: Gallium nitride & Raman spectroscopy.
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TL;DR: In this article, the elastic constants for zinc-blende and wurtzite were obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions.
Abstract: Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzite c/a ratio under hydrostatic pressure Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases
952 citations
TL;DR: In the last decade, a rapid and significant development in Raman scattering experiments on GaN and related nitride compounds has been seen as discussed by the authors, where the Γ-point phonon frequencies have been identified for both cubic and hexagonal structures of binary compounds of GaN.
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.
606 citations
TL;DR: In this article, the authors provide a detailed review of the state-of-the-art properties of the group III nitrides (AlN, GaN and InN).
Abstract: The group III nitrides (AlN, GaN and InN) represent an important trio of semiconductors because of their direct band gaps which span the range 1.95-6.2 eV, including the whole of the visible region and extending well out into the ultraviolet (UV) range. They form a complete series of ternary alloys which, in principle, makes available any band gap within this range and the fact that they also generate efficient luminescence has been the main driving force for their recent technological development. High brightness visible light-emitting diodes (LEDs) are now commercially available, a development which has transformed the market for LED-based full colour displays and which has opened the way to many other applications, such as in traffic lights and efficient low voltage, flat panel white light sources. Continuously operating UV laser diodes have also been demonstrated in the laboratory, exciting tremendous interest for high-density optical storage systems, UV lithography and projection displays. In a remarkably short space of time, the nitrides have therefore caught up with and, in some ways, surpassed the wide band gap II-VI compounds (ZnCdSSe) as materials for short wavelength optoelectronic devices. The purpose of this paper is to review these developments and to provide essential background material in the form of the structural, electronic and optical properties of the nitrides, relevant to these applications. We have been guided by the fact that the devices so far available are based on the binary compound GaN (which is relatively well developed at the present time), together with the ternary alloys AlGaN and InGaN, containing modest amounts of Al or In. We therefore concentrate, to a considerable extent, on the properties of GaN, then introduce those of the alloys as appropriate, emphasizing their use in the formation of the heterostructures employed in devices. The nitrides crystallize preferentially in the hexagonal wurtzite structure and devices have so far been based on this material so the majority of our paper is concerned with it, however, the cubic, zinc blende form is known for all three compounds, and cubic GaN has been the subject of sufficient work to merit a brief account in its own right. There is significant interest based on possible technological advantages, such as easier doping, easier cleaving (for laser facets) and easier contacting. It also appears, at present, that the cubic form gives higher electron and hole mobilities than the hexagonal form. The dominant hexagonal structure is similar to that found in a number of II-VI compounds such as CdS and they can therefore be taken as role models. In particular, the lower symmetry gives rise to three separate valence bands at the zone centre and exciton spectra associated with each of these have been reported by many workers for GaN. Interpretation is complicated by the presence of strain in many samples due to the fact that most material consists of epitaxial thin films grown on non-lattice-matched substrates (bulk GaN crystals not being widely available). However, much progress has been made in understanding the physics of these films and we discuss the current position with regard to band gaps, effective masses, exciton binding energies, phonon energies, dielectric constants, etc. Apart from a lack of knowledge of the anticipated valence band anisotropy, it can be said that GaN is now rather well documented. Less detail is available for AlN or InN and we make no attempt to provide similar data for them. The structure of the paper is based on a historical introduction, followed by a brief account of the various crystal growth methods used to produce bulk GaN and epitaxial films of GaN and the ternary alloys. This is then followed by an account of the structural properties of hexagonal GaN as measured by x-ray diffraction and electron microscopy, phonon properties from infrared and Raman spectroscopy, electrical properties, with emphasis on n- and p-type doping, and optical properties, measured mainly by photoluminescence. A brief comparative account of cubic GaN properties follows. Discussion of alloy properties in the context of their use in quantum well and superlattice structures forms an introduction to the device sections which close the paper. These include details of the technology necessary for etching, contacting and forming laser facets, as an introduction to recent results on LEDs and laser diodes. Having described the current position, we speculate briefly on likely future developments.
464 citations
TL;DR: In this paper, the authors review recent progress in the group-III nitride and related materials, and electronic and optical devices based on them, focusing on the current status of wide bandgap gallium nitride, and related semiconductors from both the materials and devices points of view.
460 citations
TL;DR: In this article, the authors measured the Ramaman spectra of n-type gallium nitride with different carrier concentrations and found that the phonon band shifted towards the high frequency side and broadened with an increase in carrier concentration.
Abstract: Raman spectra of n‐type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high‐frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line‐shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation‐potential and electro‐optic mechanisms.
344 citations
References
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TL;DR: A review of progress in the theoretical and experimental study of the Raman effect in crystals during the past ten years is given in this article, where the theory of those properties of long-wavelength lattice vibrations in both cubic and uniaxial crystals which can be studied by Raman scattering.
Abstract: A review is given of progress in the theoretical and experimental study of the Raman effect in crystals during the past ten years. Attention is given to the theory of those properties of long-wavelength lattice vibrations in both cubic and uniaxial crystals which can be studied by Raman scattering. In particular the phenomena observed in the Raman scattering from crystals which lack a centre of inversion are related to the theory. The angular variations of the scattering by any type of lattice vibration in a crystal having any symmetry can be easily calculated using a complete tabulation of the Raman tensor. Recent measurements of first-order lattice vibration spectra are listed. A discussion of Brillouin scattering is included. The relation of second-order Raman spectra to critical points in the lattice vibration density of states is discussed, and measurements of the second-order spectra of diamond and the alkali halides are reviewed. The theory and experimental results for Raman scattering by electroni...
1,831 citations
TL;DR: In this paper, the frequency and symmetry character of the fundamental modes of the Raman effect in zinc oxide has been determined using the continuous helium-neon and ionized argon lasers as sources.
Abstract: The Raman effect in zinc oxide has been measured using the continuous helium-neon and ionized argon lasers as sources. The frequency and symmetry character of the fundamental modes have been determined. The results are: two ${E}_{2}$ vibrations at 101 and 437 ${\mathrm{cm}}^{\ensuremath{-}1}$; one transverse ${A}_{1}$ at 381 ${\mathrm{cm}}^{\ensuremath{-}1}$ and one transverse ${E}_{1}$ at 407 ${\mathrm{cm}}^{\ensuremath{-}1}$; one longitudinal ${A}_{1}$ at 574 ${\mathrm{cm}}^{\ensuremath{-}1}$ and one longitudinal ${E}_{1}$ at 583 ${\mathrm{cm}}^{\ensuremath{-}1}$.
1,757 citations
TL;DR: In this article, the effects of the competition between the long-range electrostatic forces and the short-range forces due to anisotropy in the interatomic force constants on the vibrational spectrum has been included.
Abstract: First-order Raman scattering from BeO, ZnO, ZnS, and CdS, all having the wurtzite structure (${C}_{6v}$), has been investigated. A discussion of the effects of the competition between the long-range electrostatic forces and the short-range forces due to anisotropy in the interatomic force constants on the vibrational spectrum has been included. A series of scattering diagrams are presented showing the geometrical arrangements necessary to observe all the $k=0$ phonons for this type of crystal structure. In BeO the ${E}_{2}$ mode was resolved from the transverse modes for the first time. The assignments of the ${E}_{2}$ modes in ZnS differ from previous investigations. From absolute intensity measurements, electro-optic coefficients for BeO, ZnO, and CdS were determined.
886 citations
TL;DR: In this paper, the Clausius-Mossotti theory of the electronic dielectric constant of diatomic crystals is extended to the case of the diamond, zincblende, wurtzite, and rock-salt types.
Abstract: Electronegativity difference is redefined as a scaling parameter, generalizing the concept of valence difference. A procedure for its evaluation is developed in terms of the dielectric constants of diatomic crystals. A simple alternative to the Clausius-Mossotti theory of the electronic dielectric constant is developed in terms of this concept. The effect of $d$-electron states and of hydrostatic pressure are discussed, and procedures for their approximate evaluation are developed. The treatment is extended to 68 crystals of the diamond, zincblende, wurtzite, and rock-salt types; values of the electronegativity parameter are tabulated for these crystals.
829 citations
TL;DR: In this paper, a dielectric two-band model was proposed to predict ionization potentials and electronic band structures of binary compounds, where only the density and dielectrically defined electronegativity difference are required as input data.
Abstract: We construct a dielectric two-band model which describes and predicts ionization potentials and electronic band structures of binary compounds ${A}_{N}{B}_{8\ensuremath{-}N}$. Only the density and dielectrically defined electronegativity difference are required as input data. The method is applied to 19 tetrahedrally coordinated crystals with excellent results. Predictions are also made for 18 other tetrahedrally coordinated crystals. The extension to octahedral coordination is discussed, and direct band gaps are calculated for 28 such crystals. It is also proposed that crystal structure and phase transitions can be predicted using this method.
430 citations