Study on the relationships between Raman shifts and temperature range for a-plane GaN using temperature-dependent Raman scattering

In this paper, Raman shifts of a-plane GaN layers grown on r-plane sapphire substrates by low-pressure metal-organic chemical vapor deposition (LPMOCVD) are investigated. We compare the crystal qualities and study the relationships between Raman shift and temperature for conventional a-plane GaN epilayer and insertion AlN/AlGaN superlattice layers for a-plane GaN epilayer using temperature-dependent Raman scattering in a temperature range from 83 K to 503 K. The temperature-dependences of GaN phonon modes (A₁ (TO), E₂ (high), and E₁ (TO)) and the linewidths of E₂ (high) phonon peak are studied. The results indicate that there exist two mechanisms between phonon peaks in the whole temperature range, and the relationship can be fitted to the pseudo-Voigt function. From analytic results we find a critical temperature existing in the relationship, which can characterize the anharmonic effects of a-plane GaN in different temperature ranges. In the range of higher temperature, the relationship exhibits an approximately linear behavior, which is consistent with the analyzed results theoretically.     

The properties of GaMnN lms grown by metalorganic chemical vapour deposition using Raman spectroscopy

An investigation of room-temperature Raman scattering is carried out on ferromagnetic semiconductor GaMnN films grown by metalorganic chemical vapour deposition with different Mn content values. New bands around 300 and 669 cm^-1, that are not observed in undoped GaN, are found. They are assigned to disorder-activated mode and local vibration mode (LVM), respectively. After annealing, the intensity ratio between the LVM and E₂(high) mode, i.e., I_LVM=I_E2(high), increases. The LO phonon-plasmon coupled (LOPC) mode is found in GaMnN, and the frequency of the LOPC mode of GaMnN shifting toward higher side is observed with the increase in the Mn doping in GaN. The ferromagnetic character and the carrier density of our GaMnN sample are discussed.     

Comparative Analysis of Temperature-dependent Raman Spectra of GaN and GaN/Mg Films

The Raman spectra of unintentionally doped gallium nitride (GaN) and Mg-doped GaN films were investigated and compared at room temperature and low temperature. The differences of E₂ and A₁(LO) mode in two samples are discussed. Stress relaxation is observed in Mg-doped GaN, and it is suggested that Mg-induced misfit dislocation and electron–phonon interaction are the possible origins. A peak at 247 cm^?1 is observed in both the Raman spectra of GaN and Mg-doped GaN. Temperature-dependent Raman scattering experiment of Mg-doped GaN shows the frequency and intensity changes of this peak with temperature. This peak is attributed to the defect-induced vibrational mode.     

Comparative Analysis of Temperature-dependent .Raman Spectra of GaN and GaN/Mg Films

The Raman spectra of unintentionally doped gallium nitride (GaN) and Mg-doped GaN films were investigated and compared at room temperature and low temperature. The differences of E₂ and A₁(LO) mode in two samples are discussed. Stress relaxation is observed in Mg-doped GaN, and it is suggested that Mg-induced misfit dislocation and electron–phonon interaction are the possible origins. A peak at 247 cm⁻¹ is observed in both the Raman spectra of GaN and Mg-doped GaN. Temperature-dependent Raman scattering experiment of Mg-doped GaN shows the frequency and intensity changes of this peak with temperature. This peak is attributed to the defect-induced vibrational mode. Translated from Chinese Journal of Semiconductors, 2005, 26(4) (in Chinese)     

Effect of different electrolytes on porous GaN using photo-electrochemical etching

This article reports the properties and the behavior of GaN during the photoelectrochemical etching process using four different electrolytes. The measurements show that the porosity strongly depends on the electrolyte and highly affects the surface morphology of etched samples, which has been revealed by scanning electron microscopy (SEM) images. Peak intensity of the photoluminescence (PL) spectra of the porous GaN samples was observed to be enhanced and strongly depend on the electrolytes. Among the samples, there is a little difference in the peak position indicating that the change of porosity has little influence on the PL peak shift, while it highly affecting the peak intensity. Raman spectra of porous GaN under four different solution exhibit phonon mode E₂ (high), A₁ (LO), A₁ (TO) and E₂ (low). There was a red shift in E₂ (high) in all samples, indicating a relaxation of stress in the porous GaN surface with respect to the underlying single crystalline epitaxial GaN. Raman and PL intensities were high for samples etched in H₂SO₄:H₂O₂ and KOH followed by the samples etched in HF:HNO₃ and in HF:C₂H₅OH.     

Raman scattering studies on manganese ion-implanted GaN

This paper reports that the Raman spectra have been recorded on the metal-organic chemical vapour deposition epitaxially grown GaN before and after the Mn ions implanted. Several Raman defect modes have emerged from the implanted samples. The structures around 182 cm^-1 modes are attributed to the disorder-activated Raman scattering, whereas the 361 cm^-1 and 660 cm^-1 peaks are assigned to nitrogen vacancy-related defect scattering. One additional peak at 280 cm^-1 is attributed to the vibrational mode of gallium vacancy-related defects and/or to disorder activated Raman scattering. A Raman-scattering study of lattice recovery is also presented by rapid thermal annealing at different temperatures between 700 °C and 1050 °C on Mn implanted GaN epilayers. The behaviour of peak-shape change and full width at half maximum (FWHM) of the A₁(LO) (733 cm^-1) and E^H₂ (566 cm^-1) Raman modes are explained on the basis of implantation-induced lattice damage in GaN epilayers.     

Synthesis and characterization of GaN nanowires by a catalyst assisted chemical vapor deposition

GaN nanowires have been fabricated on Si(1 1 1) substrates by chemical vapor deposition (CVD) method with NiCl₂ as catalyst and their compositions, microstructures, morphologies and light emitting properties were characterized by X-ray diffraction (XRD), FT-IR spectrophotometer (FTIR), scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), Raman spectroscopy and photoluminescence (PL). The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure and high crystalline quality, having the size of 20-50 nm in diameter and several tens of microns in length with some nano-droplets on their tips, which reveals that the growth mechanism of GaN nanowires agrees with vapor-liquid-solid (VLS) process. Five first-order Raman active phonon bands move to low shift and A₁(TO), E₁(TO), and E₂ (high) bands are overlapped and broaden, which is caused by uncertainty in the phonon wave vector. Five non-first-order active Raman phonons also appear, which is caused by the small dimension and high surface disorder degree. A blue-shift of the band-gap emission occurs due to quantum confinement effect.     

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E _2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.     

Synthesis and optical properties of single crystalline GaN nanorods with a rectangular cross-section

GaN nanorods were synthesized from the reaction of a Ga/Ga₂O₃ mixture with NH₃ on Si substrates by chemical vapor deposition. The synthesized products were characterized by scanning and transmission electron microscopy, X-ray diffraction, photoluminescence and Raman spectroscopy. The nanorods are highly single crystalline and possess uniform smooth surfaces. PL revealed only a strong emission at 3.268 eV, ascribed to free exciton (FX) transitions, at room temperature; while the well-known yellow luminescence band centered at 2.2-2.3 eV was not detected. Four first-order phonon modes, corresponding to the A₁(TO), E₁(TO), E₂(high), and A₁(LO) at ∼531, 554, 564, and 721 cm⁻¹, respectively, were observed by Raman backscattering. The red-shift of the FX emission peak and the down-shifts of the Raman modes by a few wave numbers are attributed to the presence of tensile strain inside GaN nanorods.     

Effect of Si doping on epitaxial lateral overgrowth of GaN: A spatially resolved micro-Raman scattering study

We report spatially resolved Raman scattering from Si-doped epitaxial laterally overgrown GaN structures to investigate spatial variations in stress and free electron concentration. The doping-induced increase in the free electron concentration is relatively higher in the laterally overgrown regions than in the coherently grown regions due to the increased contribution of the high-energy A₁ longitudinal optical phonon–plasmon coupled mode. In addition, the E₂(high) [E₂(low)] phonon energy shifts downward (upward) more significantly in the laterally overgrown regions than in the coherently grown regions. The doping-induced Raman shifts of the E₂(high) and E₂(low) phonons in the laterally overgrown regions are approximately ?0.6 and 0.11 cm^?1, respectively, corresponding to the in-plane stress of ～0.22 GPa.     

Raman-scattering spectrum of GaN straight nanowires

The Raman spectrum of GaN straight nanowires deposited on a LaAlO₃ crystal substrate was studied. The E₂ (high) phonon frequency at 560 cm^-1 shows a 9 cm^-1 shift compared with the calculated value. The low-energy shift and band broadening of the Raman modes result from the nanosize effect. The unique property of the low intensity ratio of I_E2/I_A1(LO) on the Raman spectrum from the GaN straight nanowires was observed. Received: 5 June 2000 / Accepted: 7 June 2000 / Published online: 2 August 2000     

Temperature dependence of biaxial strain and its influence on phonon and band gap of GaN thin film

Hexagonal GaN epilayer grown on sapphire substrate by metal organic chemical vapour deposition (MOCVD) is studied using Raman scattering and photoluminescence in a temperature range from 100\,K to 873\,K. The model of strain (stress) induced by the different lattice parameters and thermal coefficients of epilayer and substrate as a function of temperature is set up. The frequency and the linewidth of $E_2^{\rm high}$ mode in a GaN layer are modelled by a theory with considering the thermal expansion of the lattice, a symmetric decay of the optical phonons, and the strain (stress) in the layer. The temperature-dependent energy shift of free exciton A is determined by using Varshni empirical relation, and the effect of strain (stress) is also investigated. We find that the strain in the film leads to a decreasing shift of the phonon frequency and an about 10meV-increasing shift of the energy in a temperature range from 100\,K to 823\,K.     

Effects of donor density and temperature on electron systems in AlGaN/AlN/GaN and AlGaN/GaN structures

It was reported by Shen et al that the two-dimensional electron gas (2DEG) in an AlGaN/AlN/GaN structure showed high density and improved mobility compared with an AlGaN/GaN structure, but the potential of the AlGaN/AlN/GaN structure needs further exploration. By the self-consistent solving of one-dimensional Schr\"{o}dinger--Poisson equations, theoretical investigation is carried out about the effects of donor density (0--1\times 10¹⁹cm^-3 and temperature (50--500K) on the electron systems in the AlGaN/AlN/GaN and AlGaN/GaN structures. It is found that in the former structure, since the effective \Delta E_c is larger, the efficiency with which the 2DEG absorbs the electrons originating from donor ionization is higher, the resistance to parallel conduction is stronger, and the deterioration of 2DEG mobility is slower as the donor density rises. When temperature rises, the three-dimensional properties of the whole electron system become prominent for both of the structures, but the stability of 2DEG is higher in the former structure, which is also ascribed to the larger effective \Delta E_c. The Capacitance--Voltage (C-V) carrier density profiles at different temperatures are measured for two Schottky diodes on the considered heterostructure samples separately, showing obviously different 2DEG densities. And the temperature-dependent tendency of the experimental curves agrees well with our calculations.     

Temperature characterization of Raman scattering in an AlGaN/GaN heterostructure

Raman scattering from an AlGaN/GaN heterostructure was performed in the temperature range from 77 to 773 K. The first- and second-order Raman scattering of the A₁ longitudinal-optical phonon–plasmon coupled mode from an AlGaN/GaN interface as well as the Raman scattering from the GaN layer were observed. All the modes downshift, and their intensities weaken with increasing temperature. The free-carrier concentration estimated by the frequency of the coupled mode from an AlGaN/GaN interface is 7.5 times as high as that of n-AlGaN, indicating mass free-carrier transfer from the AlGaN barrier to the GaN well. Moreover, the temperature dependence of the phonon frequency is well described by an empirical formula. PACS 78.30.Fs; 63.20.Ls; 61.82.Fk; 68.60.Dv; 81.15.Gh     

Linear electro-optical properties of zincite

Orientational, dispersion, and temperature dependences of electro-optical coefficients of Li-doped ZnO single crystals are investigated. According to the data on the orientational dependences obtained far from the electronic absorption band, the values of all linearly independent components of the Pockels tensor are specified. In the range from 2 eV to E _g , at both 100 and 300 K, the dispersion obeys a power law with m=2, which indicates a two-dimensional character of the van Hove singularity. The anomalies in the temperature dependences in the range from 15 to 700 K are explained by the competing contributions of the anharmonicities of the electron and lattice subsystems, which lead to anomalous behavior of the coefficients at both low (T < 100 K) and high (T > 100 K) temperatures. In the low-temperature range, the identified anomalies correlate with data of independent measurements of birefringence, spontaneous polarization, and dilatometry.     

High temperature electron transport properties of AlGaN/GaN heterostructures with different Al-contents

Electron transport properties in AlGaN/GaN heterostructures with different Al-contents have been investigated from room temperature up to 680 K. The temperature dependencies of electron mobility have been systematically measured for the samples. The electron mobility at 680 K were measured as 154 and 182 cm²/V·s for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively. It was found that the electron mobility of low Al-content Al_0.15Ga_0.85N/GaN heterostructure was less than that of high Al-content Al_0.40Ga_0.60N/GaN heterostructure at high temperature of 680 K, which is different from that at room temperature. Detailed analysis showed that electron occupations in the first subband were 75% and 82% at 700 K for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively, and the two dimensional gas (2DEG) ratios in the whole electron system were 30% and near 60%, respectively. That indicated the 2DEG was better confined in the well, and was still dominant in the whole electron system for higher Al-content AlGaN/GaN heterostructure at 700 K, while lower one was not. Thus it had a higher electron mobility. So a higher Al-content AlGaN/GaN heterostructure is more suitable for high-temperature applications.     

Raman spectroscopy study of lattice dynamics of macro-, micro-, and nanostructured barium titanates

The temperature dependences of the components A ₁(2TO) and E(1TO) of the soft ferroelectric mode during phase transitions in single crystals, ceramics, polycrystalline and epitaxial thin films of barium titanate, as well as a superlattice consisting of alternating layers of barium and strontium titanates, have been studied using the Raman spectroscopy method. Abrupt changes in soft mode frequencies have been observed in the single crystal during phase transitions between tetragonal, orthorhombic, and rhombohedral phases. Smoothing of the temperature dependences of soft modes and the coexistence of phases have been observed in ceramics and polycrystalline films. In the epitaxial film, the sequence of structural transformations fundamentally differs from that observed in the single crystal; in the superlattice, the ferroelectric phase is stable to 550 K.     

Pressure dependence of elastic constants in zinc-blende GaN and InN and their influence on the pressure coefficients of the light emission in cubic InGaN/GaN quantum wells

We have studied the influence of nonlinear elastic effects on the pressure coefficients of light emission, dE_E/dP, in cubic InGaN/GaN quantum wells. By means of ab-initio calculations, we have determined the pressure dependences of the elastic constants, C₁₁, C₁₂ and C₄₄ in zinc-blende InN and GaN. Further, we show that the pressure dependence of the elastic constants results in significant reduction of dE_E/dP in cubic InGaN/GaN quantum wells and essentially improves the agreement between experimental and theoretical values.     

Oscillator parameters of transverse phonons in <Emphasis Type="Italic">a</Emphasis>-domain lead titanate

The temperature behavior of polarized Raman spectra of a-domain PbTiO₃ single crystals has been investigated. The spectra obtained are fit by a damping harmonic oscillator model with allowance for a complex background structure. A full set of oscillator parameters of all phonon modes in the frequency range 12 < ν < 1200 cm^?1 is obtained and their temperature dependences in the range from 300 to 800 K are presented.     

Study on the relationships between Raman shifts and temperature range for α-plane GaN using temperature-dependent Raman scattering

In this paper,Raman shifts of a-plane GaN layers grown on r-plane sapphire substrates by low-pressure metal-organic chemical vapor deposition(LPMOCVD) are investigated.We compare the crystal qualities and study the relationships between Raman shift and temperature for conventional a-plane GaN epilayer and insertion AlN/AlGaN superlattice layers for a-plane GaN epilayer using temperature-dependent Raman scattering in a temperature range from 83 K to 503 K.The temperature-dependences of GaN phonon modes(A1(TO),E2(high),and E1(TO)) and the linewidths of E2(high) phonon peak are studied.The results indicate that there exist two mechanisms between phonon peaks in the whole temperature range,and the relationship can be fitted to the pseudo-Voigt function.From analytic results we find a critical temperature existing in the relationship,which can characterize the anharmonic effects of a-plane GaN in different temperature ranges.In the range of higher temperature,the relationship exhibits an approximately linear behavior,which is consistent with the analyzed results theoretically.