Synthesis and characterization of heteroepitaxial GaN films on Si(111)

We report crack-free and single-crystalline wurtzite GaN heteroepitaxy layers have been grown on Si (111) substrate by metal-organic chemical vapor deposition(MOCVD). Synthesized GaN epilayer was characterized by X-ray diffraction(XRD), atomic force microscope (AFM) and Raman spectrum. The test results show that the GaN crystal reveals a wurtzite structure with the <0001> crystal orientation and XRD ω-scans showed a full width at half maximum (FWHM) of around 583 arcsec for GaN grown on Si substrate with an HT-AlN buffer layer. In addition, the Raman peaks of E₂^high and A₁(LO) phonon mode in GaN films have an obvious redshit comparing to bulk GaN eigen-frequency, which most likely due to tensile strain in GaN layers. But the AO phonon mode of Si has a blueshit which shows that the Si substrate suffered a compressive strain. And we report that the AlN buffer layer plays a role for releasing the residual stress in GaN films.     

Thin film of aluminum oxide through pulsed laser deposition: a micro-Raman study

Successful use of micro-Raman spectroscopy to characterize the surface of the thin Al₂O₃ films deposited on Si(010) substrate through pulsed laser ablation is presented. The micro-Raman spectra have been recorded on several spots (sized ≈1 μm) over the length and the breadth of the films in a systematic fashion. The as-deposited films are inhomogeneous from spot-to-spot over the surface. The variation in characteristic properties of 521 cm⁻¹ line of crystalline Si (c-Si) have been used to estimate the thickness of the Al₂O₃ films. The thickness of the films has been found to depend on the position of the substrate with respect to the plume boundaries produced by the laser pulse after hitting the Al target. The thickness also depends on the position of the spot on a film placed at a fixed distance from the target surface. The thickness profiling of the deposited films on the basis of micro-Raman spectroscopic investigations corroborates the theoretical prediction of the model of the plume formation and the formation of the shock-front region in the pulsed laser deposition technique. All the seven Raman active modes (2A_1g+5E_g) observable in single crystals of sapphire (c-Al₂O₃) powder have been identified in the complex spectra obtained from deposited film surfaces under varying experimental conditions. The intensity variation of an isolated line (750 cm⁻¹) of Al₂O₃ corroborates the deposition profiling obtained from c-Si line parameters. The most intense Raman lines in the micro-Raman spectra of the deposited films are two infrared active modes around 450 and 600 cm⁻¹. The strong presence of these infrared active modes in the micro-Raman spectra suggests a highly distorted micro-crystalline structure of the films. The local micro-stress created by the micro-crystals on the Si-substrate is maximum near the stopping distance of the plume. Largest micro-crystal of Al₂O₃ are also seen over the spot near the stopping distance of the plume.     

Study of the growth of CuAlS2 thin films on oriented silicon (111)

Within the chalcopyrite family the sulphur based compounds CuMS₂ (M = In, Ga, Al) have attracted much interest in recent years because they show a direct wide band-gap covering from E_gap = 1.53 eV (CuInS₂) over E_gap = 2.43 eV (CuGaS₂) to E_gap = 3.49 eV (CuAlS₂). Therefore they are particularly suitable for optoelectronic as well as photovoltaic applications. The CuAlS₂ semiconductor is one of these compounds and has good luminescent properties and a wide direct gap of 3.5 eV making it suitable for the use as material for light-emitting devices in the blue region of the spectrum. To dig up fully its potential a better understanding of the fundamental properties of the CuAlS₂ film itself is essential, which could be achieved from high-quality single-crystalline materials. So, the aim of this work has been to study the growth of multilayer CuAlS₂ thin films on Si(111) substrates at a substrate temperature of 723 K. One, two and three layers with 60, 120 and 180 nm thicknesses, respectively, were deposited on Si(111) substrate. The effect of the CuAlS₂ layer numbers on the structure, morphology and optical properties of the samples was investigated. The X-ray diffraction studies revealed that all the samples are polycrystalline in nature, single CuAlS₂ phase and exhibiting chalcopyrite structure with a preferred orientation along the (112) direction. However, the sample with three CuAlS₂ layers exhibit the highly oriented (112) plane with grain sizes of 80 nm. So we show that this experimental process affects significantly the structural properties of the CuAlS₂ films. Raman spectroscopic measurements indicated five prominent peaks at 193, 205, 325, 335 and 370 cm^− 1. The possible origin of the 370 cm^− 1 peak was investigated and was found to be some local vibration in the structure. The peaks at 193-205 and 335 cm^− 1 were ascribed to A₁ and B₂ modes, respectively.     

UV Raman studies on carbon nitride structures

Visible (514 nm) and deep UV (257 nm) Raman spectra of monoclinic tetracyanoethylene (tcne) are recorded at ambient conditions and also after laser heating at ambient pressure and at 40 GPa. Tetracyanoethylene (C₂(CN)₄) is a convenient precursor to synthesize hard C₃N₄ materials. At low incident laser powers the UV Raman spectra of virgin tcne resemble visible Raman spectra, and at higher powers there appear new, broad modes that increase in intensity as a function of laser power. When tcne is laser-heated at ambient pressure, there are two broad UV Raman peaks about 1,405 cm⁻¹ and 1,604 cm⁻¹ whereas visible laser Raman excitation results in too high a fluorescent background to show up any Raman modes. Raman spectrum of tcne laser heated at 40 GPa show broad peaks indicative of multiphase formation. The spectrum has additional modes at lower frequencies, and comparison with calculated Raman frequencies points to possible formation of α-C₃N₄.     

Raman scattering studies of Mn-doped ZnO thin films deposited under pure Ar or Ar + N2 sputtering atmosphere

This paper investigates the anomalous and specific Raman modes present in Mn-doped ZnO thin films deposited using the magnetron co-sputtering method. To trace these peaks, we prepared Mn-doped ZnO films with different Mn concentrations by altering the sputtering power of the Mn target in a pure Ar or Ar + N₂ sputtering atmosphere. A broad band observed in the Raman spectra of heavily Mn-doped ZnO films ranges from 500 to 590 cm^− 1. This band involves the enhanced A₁ longitudinal mode and activated silent modes of ZnO, as well as a characteristic mode of Mn₂O₃. Four anomalous Raman peaks at approximately 276, 510, 645 and 585 cm^− 1 are present in pure and Mn-doped ZnO films deposited under the Ar + N₂ sputtering atmosphere. The peaks at 276 cm^− 1 and 510 cm^− 1 may originate from the complex defects of Zn_i-N_O and Zn_i-O_i, respectively, while the peak at approximately 645 cm^− 1 could be due to a complex defect of Zn_i coupled with both the N and Mn dopants. The results of this study suggest classifying the origins of anomalous and specific Raman peaks in Mn-doped ZnO films into three major types: structural disorder and morphological changes caused by the Mn dopant, Mn-related oxides and intrinsic host-lattice defects coupled with/without the N dopant.     

Influence of double buffer layers on properties of Ga-polarity GaN films grown by rf-plasma assisted molecular-beam epitaxy

Ga-polarity GaN thin films were grown on sapphire (0001) substrates by rf-plasma assisted molecular beam epitaxy (MBE) using a double buffer layer, which consisted of an intermediate-temperature GaN buffer layer (ITBL) grown at 690 °C and a conventional AlN buffer layer deposited at 740 °C. Raman scattering spectra showed that the E₂ (high) mode of GaN film grown on conventional AlN buffer layer is at about 570 cm⁻¹, and shifts to 568 cm⁻¹ when an ITBL was used. This indicates that the ITBL leads to the relaxation of residual strain in GaN film caused by mismatches in the lattice constants and coefficients of thermal expansion between the GaN epilayer and the sapphire substrate. Compared to the GaN film grown on the conventional AlN buffer layer, the GaN film grown on an ITBL shows higher Hall mobility and substantial reduction in the flicker noise levels with a Hooge parameter of 3.87×10⁻⁴, which is believed to be, to date, the lowest reported for GaN material. These results imply that the quality of Ga-polarity GaN films grown by MBE can be significantly improved by using an ITBL in addition to the conventional low-temperature AlN buffer layer.     

Nanostructured GaN on silicon fabricated by electrochemical and laser-induced etching

Nanostructured GaN layers have been fabricated by electrochemical and laser-induced etching (LIE) processes based on n-type GaN thin films grown on the Si (111) substrate with AlN buffer layers. The effect of varying current and laser power density on the morphology of the GaN layers is investigated. The etched samples exhibited a dramatic increase in photoluminescence intensity as compared to the as grown samples. The average diameter of the GaN crystallites was about 7-10 nm, as determined from the PL data The Raman spectra also displayed stronger intensity peaks, which were shifted and broadened as a function of etching parameters. A strong band at 522 cm^− 1 is from the Si (111) substrate, and a small band at 301 cm^− 1, due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm^− 1 and 568 cm^− 1due to E2 (low) and E2 (high) respectively.     

Surface phonon polariton of wurtzite AlN thin film grown on sapphire

Polarized infrared attenuated total reflection (ATR) with Otto configuration was employed to access the surface phonon polariton (SPP) characteristics of wurtzite aluminium nitride (AlN) thin film grown on sapphire (Al₂O₃) substrate. The surface and guided wave polariton dispersion curves for the studied structure were derived by taking into account thin film and substrate anisotropy. From the p-polarized ATR spectrum, one prominent peak corresponds to the SPP mode of AlN thin film was clearly observed at 824 cm⁻¹. In addition, four guided wave modes were also detected at 467, 593.5, 633.5 and 668 cm⁻¹. For the s-polarized ATR spectrum, five pronounced dips associated to the guided wave modes were detected. The obtained results were in good agreement with the ATR spectra calculated based on the standard transfer matrix formulation. The origin of the observed ATR dips were verified from the dispersion curves simulated based on air/AlN epilayer/AlN buffer layer/Al₂O₃ model.     

Growth and surface characterization of magnetron sputtered zinc nitride thin films

Zinc nitride films were deposited on Si(100) substrates at room temperature using RF-magnetron sputtering in pure N₂ and in Ar + N₂ atmospheres. Two active phonon modes (270.81 and 569.80 cm^− 1) are observed in Raman spectra for films deposited in Ar + N₂ atmosphere. Atomic force microscopy showed that the average surface roughness of the films deposited in pure N₂ atmosphere (1.3-3.33 nm) was less than for those deposited in a mixed Ar + N₂ atmosphere (10.3-12.8 nm). Low temperature cathodoluminescence showed two emission bands centered at 2.05 eV and 3.32 eV for both types of films.     

Deposition and characterization of c-axis oriented aluminum nitride films by radio frequency magnetron sputtering without external substrate heating

Aluminum nitride (AlN) films were deposited on a variety of substrates (glass, Si, oxidized Si, Al-SiO₂-Si, Cr-SiO₂-Si, and Au-Cr-SiO₂-Si) by radio frequency (RF) magnetron sputtering using an AlN target. The films were deposited without external substrate heating. The effect of RF power, ambient gas (Ar and Ar-N₂) and sputtering pressure on deposition rate and crystallinity were investigated. The structure and morphology of the films were studied by X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. These investigations revealed that the AlN films prepared in mixed gas ambient (Ar-N₂) were highly c-axis oriented with moderate surface roughness on all the substrate. A strong IR absorption band was observed around 670 cm^− 1 which confirms the presence of Al-N bond in the film. The dc resistivity of the films was measured to be in the range of 10¹¹ to 10¹² Ω-cm at moderate electric fields. The application of these films in piezoelectric based micro-electro-mechanical systems is discussed.     

Effects of hydrostatic pressure on Raman scattering in Ge quantum dot superlattices

Self-organized Ge/Si quantum dots (QD's) in strained Si/Ge short-period superlattices are studied by Raman scattering under hydrostatic pressure excited in-resonance and off-resonance with the confined Ge-like E₁ transition of Ge using 488 and 514.5 nm lines from an argon-ion laser and 632.8 nm line from a He-Ne laser. The Raman spectra of Ge-Ge, Si-Ge and Si-2TA modes of the QD's were obtained as a function of pressure in the range of 1-70 kbar. Our results show that the mode Grüneisen parameter of the Ge-Ge phonon mode in QD's is found to be γ=0.81±0.01, which is smaller than that of the bulk Ge. We observe resonance effects with the confined Ge-like E₁ transition and the pressure coefficient of this resonating electronic transition obtained is ∼5±1 meV kbar⁻¹.     

Hot-wire chemical vapor deposition and characterization of p-type nanocrystalline SiC films and their use in Si heterojunction solar cells

Wide optical bandgap p-type nanocrystalline silicon carbide (p-nc-SiC) films deposited by hot-wire chemical vapor deposition were used as window layers in n-type crystalline Si heterojunction (HJ) solar cells. The effect of H₂ flow rates on the material properties of p-nc-SiC films was investigated by X-ray diffractometer and Raman spectroscopy. Moreover, the optical and electrical properties, such as optical bandgap (E_g), dark conductivity, and activation energy (E_a), of p-nc-SiC films were also measured. It was found that H₂ flow rates played an important role in forming of p-nc-SiC films and increasing the E_g and decreasing the E_a of p-nc-SiC films. Moreover, the effect of hydrogenation process of the amorphous Si buffer layer on solar cell characteristics was investigated. After the deposition and hydrogenation parameters were optimized, the Si HJ solar cells with the open-circuit voltage of 0.59 V, short-circuit current density of 38.06 mA/cm², fill factor of 62.03%, and the conversion efficiency of 14.09% could be obtained.     

Large-scale electrochemical synthesis of SnO2 nanoparticles

Tin oxide nanoparticles were synthesized by electrochemical oxidation of a tin metal sheet in a non-aqueous electrolyte containing NH₄F. The as-prepared nanoparticles were then thermally annealed at 700 °C for 6 h. The resulting particles were characterized by a variety of experimental techniques, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman, UV-visible, and photoluminescence (PL) spectroscopy. The XRD patterns clearly showed that the amorphous phase of the as-synthesized SnO₂ particles was transformed into a rutile-type crystalline structure after thermal treatment; and from the line broadening of the XRD peaks, the average size of the annealed particles was found to be 15.4, 12.5, 11.8 nm for the particles initially synthesized at 20, 30, and 40 V, respectively. Consistent results were also observed in HRTEM measurements which showed clear crystalline lattice fringes of the calcined nanoparticles, as compared to the featureless profiles of the as-produced counterparts. In Raman spectroscopic studies, three dominant peaks were observed at 480, 640, and 780 cm⁻¹ which were ascribed to the E_1g, A_1g, and B_2g Raman active vibration modes, respectively, and the wavenumbers of these peaks blue-shifted with decreasing particle size. Additionally, a broad strong emission band was observed in room-temperature photoluminescence measurements.     

Combined XPS and first principles study of double-perovskite Ca2GdTaO6

The electronic structure and the vibrational property of the double perovskite oxide, Ca₂GdTaO₆ (CGT), synthesized by solid-state reaction technique are investigated. Density functional theory calculations performed by the VASP show a direct band gap energy of 3.2 eV. The calculated density of states (DOS) has been compared to the valence band spectrum measured by X-ray photoelectron spectroscopy (XPS). The calculated electronic structures of CGT are qualitatively similar to those of the XPS spectra in terms of spectral features and relative intensities. The DOS of CGT shows that the Gd f, Ta d and O 2p states are hybridized. The band-structure calculation is used to obtain the optical dielectric constant of the sample. The inter-band contributions to the optical properties of CGT have been analysed. Raman spectrum of the sample taken at 488 nm excitation wavelength shows five primary strong peaks at 112, 215, 320, 458 and 767 cm^?1. Lorentzian lines with 17 bands have been used to fit the Raman spectrum. The eigen frequencies of different phonon modes have been theoretically calculated. The calculated results are compared with the experimental data.     

Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architectures

We describe a simple route to flower like ZnO architectures, based on the decomposition of zinc acetate precursor in water-ethylene glycol solution at 140-160 °C for 1d through hydrothermal method. The PXRD pattern reveals that the ZnO crystals are of hexagonal wurtzite structure. Ethylene glycol plays a key role on the morphology control of ZnO crystals. The SEM images of ZnO products prepared at 140 °C and 160 °C mainly exhibit flower like architecture composed of many rods. Whereas, the product prepared at 180 °C shows bunches accompanying a few number of free rods. TEM results reveal that the rods resemble swords with decrease in size from one end to another. From Raman spectrum, the peaks at 437 cm^− 1, 382 cm^− 1 and 411 cm^− 1 correspond to E₂ (high), A₁ (TO) and E₁ (TO) of ZnO crystals respectively. The photoluminescence spectrum exhibits strong UV emission at ~ 397 nm, which comes from recombination of exciton. The possible mechanism for the formation of flower like ZnO architecture is proposed.     

Vibrational modes in electrodeposited amorphous silicon: FT-IR analysis

Infrared spectra of 13 samples of amorphous silicon bonded with hydrogen, fluorine and carbon, prepared by electrodeposition using a mixture of ethylene glycol and fluosilicic acid were analysed in the wave number region 4000-400 cm^–1 with a Fourier transform infrared spectrometer. Strong absorption peaks were observed at 1000 cm^–1 due to the SiF_x stretching mode. Small peaks were seen around 2300 and 640 cm^–1 due to SiH stretching and wagging modes of absorption. The number of bonded hydrogen atoms in the film deposited at 0.05 M, 50 mA cm^–2 was calculated to be 6.2579×10²¹ and 1.2302×10²⁰ atm cm^–3 using integrated absorption of the CH and SiH stretching modes, respectively. The absorption coefficient around the SiF_x stretch region was found to vary from 1300–2500 cm^–1 as the molarity of the electrolyte was increased. Binding energy shifts in X-ray photoelectron spectrum were used as a cross check to confirm the silicon bonding with carbon, hydrogen, oxygen and fluorine atoms. The absence of columnar growth in SEM photographs indicates no polysilane formation in the films.     

Electrical and materials properties of AlN/ HfO2 high-k stack with a metal gate

In this study, aluminum nitride (AlN) was grown by molecular layer deposition on HfO₂ that had been deposited on 200 mm Si (100) substrates. The AlN was grown on HfO₂ using sequential exposures of trimethyl-aluminum and ammonia (NH₃) in a batch vertical furnace. Excellent thickness uniformity on test wafers from the top of the furnace to the bottom of the furnace (across the furnace load) was obtained. The equivalent oxide thickness was 16.5-18.8 Å for the AlN/HfO₂ stack on patterned device wafers with a molybdenum oxynitride metal gate with leakage current densities from low 10^− 5 to mid 10^− 6 A/cm² at threshold voltage minus one volt. There was no change in the work function with the AlN cap on HfO₂ with the MoN metal gate, even with a 1000 °C anneal.     

Preparation and optical properties of GaN nanocrystalline powders

Abstract

GaN nanocrystalline powders were synthesised by decomposition of gallium nitrate, followed by nitrogenising with ammonia under different temperature. X-ray diffraction (XRD) and the transmission electron microscopy (TEM) indicated that the crystallinity of the powder is improved and the average size of the GaN nanocrystallites increases from 4·8 to 23·9 nm as the temperature increases from 850 to 1050°C. The Raman spectra displayed four broadened peaks corresponding to A₁ (LO), A₁ (TO), E₁ (TO) and E₂ (high) modes of würtzite GaN respectively. Two additional modes at 252 and 421 cm^–1 attributed to boundary phonons activated by the finite size effects and octahedral Ga–N₆ bonds were observed respectively. A strong blue photoluminescence (～353 nm) was detected for room temperature measurement, indicating that the GaN nanocrystalline powders have few defects and high quality.     

Characterization of fluorine-doped silicon dioxide films by Raman spectroscopy and Electron-spin resonance

We have measured Raman and Electron-spin resonance (ESR) spectra of fluorine-doped SiO₂ films deposited by two different methods. In high-density plasma (HDP) films, the Raman band at about 490 cm^− 1 becomes drastically stronger as the F/Si ratio increases, whereas the Raman band from threefold ring defect is independent of the F/Si ratio. The unusual increase of the intensity of the 490 cm^− 1 band in HDP films has been interpreted in terms of the existence of Si-Si clusters. From a comparison between Raman spectra of HDP film and plasma chemical vapor deposition using tetraethoxysilane (p-TEOS) film with the same F/Si ratios it has been found that HDP film has more Si-Si bonds and threefold ring defects than p-TEOS film. Furthermore, the polarized Raman spectra in the 810 cm^− 1 bands indicate that inhomogeneous SiO₂ clusters of various sizes should exist in the network structure of HDP film. The result of the ESR measurement shows that HDP films have fewer dangling bonds than p-TEOS films. It is considered that many Si-Si clusters, threefold ring defects, and inhomogeneous SiO₂ cluster sizes, and the few dangling bonds in HDP films give rise to the film properties of low stress, good adhesion with Si substrate, and low water permeation.     

Structural and optical properties of single-crystalline ultraviolet-emitting needle-shaped ZnO nanowires

Well-crystallized with excellent optical properties, needle-shaped ZnO nanowires have been synthesized on silicon substrate in a high density via the thermal evaporation of metallic zinc powder without the use of catalysts or additives. Extensive structural analysis showed that the grown nanowires are highly crystalline with the wurtzite hexagonal phase, grown along the [0001] in the c-axis direction. The presence of an optical-phonon E₂ mode in Raman spectrum at 437 cm^{− 1} and sharp and strong UV emission at 379 nm with no green emission in the room-temperature photoluminescence (PL) spectrum confirms good crystallinity with the excellent optical properties for the deposited nanowires.