Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700 °C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25 eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5 × 10¹³ and 2 × 10¹⁴ at. cm^− 2 s^− 1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio.     

Growth process from amorphous GaN to polycrystalline GaN on Si (111) substrates

Amorphous GaN (a-GaN) films on Si (111) substrates have been deposited by RF magnetron sputtering with GaN powder target. The growth process from amorphous GaN to polycrystalline GaN is studied by XRD, SEM, PL and Raman. XRD data mean that annealing under flowing ammonia at 850-950 °C for 10 min converts a-GaN into polycrystalline GaN (p-GaN). The growth mechanism can be mostly reaction process through N³⁻ in amorphous GaN replaced by N³⁻ of NH₃. Annealing at 1000 °C, the appearance of GaN nanowires can be understood based on the vapor-liquid-solid (VLS) mechanism. In addition, XRD, PL and Raman measurement results indicate that the quality of GaN films increases with increasing temperature. The tensile stress in the films obtained at 1000 °C is attributable to the expansion mismatch between GaN and Si, with the gallium in the film playing a negligible role.     

Structural, optical and electrical study of undoped GaN layers obtained by metalorganic chemical vapor deposition on sapphire substrates

We investigate optical, structural and electrical properties of undoped GaN grown on sapphire. The layers were prepared in a horizontal reactor by low pressure metal organic chemical vapor deposition at temperatures of 900 °C and 950 °C on a low temperature grown (520 °C) GaN buffer layer on (0001) sapphire substrate. The growth pressure was kept at 10,132 Pa. The photoluminescence study of such layers revealed a band-to-band emission around 366 nm and a yellow band around 550 nm. The yellow band intensity decreases with increasing deposition temperature. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies show the formation of hexagonal GaN layers with a thickness of around 1 μm. The electrical study was performed using temperature dependent Hall measurements between 35 and 373 K. Two activation energies are obtained from the temperature dependent conductivity, one smaller than 1 meV and the other one around 20 meV. For the samples grown at 900 °C the mobilities are constant around 10 and 20 cm² V⁻¹ s^− 1, while for the sample grown at 950 °C the mobility shows a thermally activated behavior with an activation energy of 2.15 meV.     

Structural and optical properties of γ-alumina thin films prepared by pulsed laser deposition

Preparation of γ-alumina thin films by pulsed laser deposition from a sintered α-alumina target is investigated. The films were deposited on (100) silicon substrates at 973 K with varying oxygen partial pressures in the range 2.0 × 10⁻⁵-3.5 × 10^− 1 mbar. X-ray diffraction results indicated that the films were polycrystalline γ-Al₂O₃ with cubic structure. The films prepared in the oxygen partial pressure range 2.0 × 10^− 5-3.5 × 10^− 2 mbar contained nanocrystals of sizes in the range 10-16 nm, and became amorphous at pressures > 3.5 × 10^− 1 mbar. Topography of the films was examined by atomic force microscopy using contact mode and it showed the formation of nanostructures. The root-mean square surface roughness of the film prepared at 2.0 × 10^− 5 mbar and 3.5 × 10^− 1 mbar were 1.4 nm and 3.5 nm, respectively. The thickness and optical properties were studied using ellipsometry in the energy range 1.5-5.5 eV for three different angles of incidence. The refractive index was found to decrease from 1.81 to 1.73 with the increase of oxygen partial pressures from 2.0 × 10^− 5 to 3.5 × 10^− 2 mbar. The variation in the refractive index has been found to be influenced by the microstructure of the films obtained as a function of oxygen partial pressure.     

High quality a-plane GaN films grown on cone-shaped patterned r-plane sapphire substrates

Planar nonpolar (112?0) a-plane GaN films have been grown by metalorganic chemical-vapor deposition directly on cone-shaped patterned r-plane sapphire substrates (PRSS) fabricated by dry etching. High-resolution X-ray diffractometers 2θ-ω scan confirmed that the films grown on PRSS are solely a-plane oriented, and the full width at half maximum values (FWHM) of the X-ray rocking curves for (112?0) GaN along [0001]_GaN and [11?00]_GaN were found to be 684 and 828″, respectively. As compared to the film grown on conventional r-plane sapphire substrate which typically has (112?0) omega FWHM values of 900 and 2124″ along [0001]_GaN and [11?00]_GaN respectively, the film grown on PRSS exhibits overall reduced omega FWHM values, and much smaller anisotropy behavior of crystallinity with respect to the in-plane orientation. The surface morphology is also improved by utilizing the PRSS technique. Cross-sectional transmission electron microscopy analysis shows that the density of threading dislocations has been greatly reduced from ~ 1.0 × 10¹⁰ cm^− 2 above the flat sapphire regions to ~ 1.0 × 10⁷ cm^− 2 above the protruding cone patterns. The improvement of crystal quality and the increase of light extraction efficiency by using cone-shaped PRSS technique lead to a strong enhancement in the light emission of a-plane GaN films. These results indicate that growth of a-plane GaN films on cone-shaped PRSS shows promise for use in high-quality and high-cost-performance nonpolar GaN based devices.     

Effects of annealing atmosphere on thermoelectric signals from ZnO films

Zinc oxide epilayer films were grown on vicinal cut sapphire substrates by pulsed laser deposition with in situ annealing oxygen pressures varied from 0 to 10 × 10³ Pa. The best crystalline quality was obtained for ZnO layer with annealing oxygen pressure of 6 × 10³ Pa. Laser induced thermoelectric voltage (LITV) were observed along the tilting angle orientation of the substrate when the pulsed KrF excimer laser of 248 nm were irradiated on the films. The largest LITV signal was measured for the film grown at 6 × 10³ Pa annealing oxygen pressure. According to the measured LITV signals, Seebeck anisotropy was evaluated and was found to range from 3 to 12 μV/K for ZnO films annealed at different oxygen pressures from 2 to 10 × 10³ Pa. It is suggested that oxygen ambient plays an important role in the electronic properties of the ZnO films.     

Surface morphology and growth mechanisms for sputtered amorphous silicon nitride thin films

Evolution of surface of sputter-deposited amorphous Si₃N₄ films growth on Si (100) substrates was investigated using atomic force microscopy (AFM). The scaling behaviors of the AFM topographical profiles were analyzed using the one-dimensional power spectral density. The results of root-mean-square surface height variation showed that there is a power law relationship between the surface roughness and deposition time. It is interesting to note that the growth exponent can be divided into one region and two regions, respectively, when Si₃N₄ films are deposited at different working pressures. A very low growth exponent of β = 0.07 ± 0.01 was found when Si₃N₄ films were deposited at a working pressure of 1.6 × 10^− 1 Pa. However, the growth exponent β can be divided into two regions, which is β₁ = 0.09 ± 0.01, β₂ = 0.24 ± 0.03 and β₁ = 0.09 ± 0.01, β₂ = 0.33 ± 0.04, when the films were deposited at a working pressure of 2.1 × 10^− 1 Pa and 2.7 × 10^− 1 Pa, respectively. The mechanisms of anomalous dynamic scaling exponents of Si₃N₄ films deposited at different working pressures were discussed.     

Deposition and characterization of BN/Si(0 0 1) using tris(dimethylamino)borane

Boron nitride thin films could be deposited on Si(0 0 1) by chemical vapor deposition (CVD) at atmospheric pressure using a single source precursor. IR absorption spectra of films deposited between 750 and 1000°C using B[N(CH₃)₂]₃ (tris(dimethylamino)borane, TDMAB) as the boron and nitrogen source showed a peak absorption at ∼1360 cm⁻¹ characteristic of the in-plane vibrational mode seen in h-BN. It was noted that the mode at 800 cm⁻¹ is very weak. The observed growth rate varied exponentially with temperature in the range 850-900°C. Ellipsometry measurements were used to investigate the thickness and optical constant of the films. The refractive index, slightly lower than the bulk material, is close to 1.65-1.7 depending on the surface morphology of the films. The surface morphology of thin layers has been observed by atomic force microscopy with an increase of the surface roughness from 0.3 to 3.5 nm as the growth temperature increases from 800 to 950°C.     

A study of structural transition in nanocrystalline titania thin films by X-ray diffraction Rietveld method

Structural and microstructural analyses of nanocrystalline titania thin films prepared by pulsed laser deposition have been carried out. At lower oxygen partial pressures (≤10⁻⁴ mbar), rutile films were formed, whereas at 1.2 × 10⁻³ mbar of oxygen partial pressure, the thin films contained both rutile and anatase phases. At 0.04 and 0.05 mbar of oxygen partial pressure, the film was purely anatase. Addition of oxygen has also shown a profound influence on the surface morphology of the as deposited titania films. Modified Rietveld method has been used to determine crystallite size, root mean square strain and fractional coordinates of oxygen of the anatase films. The influence of crystallite size and strain on the rutile to anatase phase transition is investigated.     

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.     

Structural and optical properties of GaN-based nanocrystalline thin films

We study the effect of annealing on structure, morphology and optical properties of nanocrystalline films of GaN, GaN:O and GaN:Mn prepared by ion assisted deposition on silicon, quartz and glassy carbon substrates. Blisters and holes having diameters proportional with the thickness of the film were observed in GaN:O deposited on silicon and glassy carbon. The Mn excess in GaN:Mn turns through annealing into Mn_xN_y islands. The degree of short- and intermediate-range order in the films was investigated by micro-Raman spectroscopy, extending to about 3 nm for gallium oxynitride films annealed to 973 K and to more than 10 nm in GaN samples. A diminished oxygen content following the annealing procedures on GaN:O samples is noticed from the reduced intensity of the oxygen mode at 1000 cm^− 1 in the Raman spectra. This observation is supported with X-ray photoemission spectroscopy measurements. The presence of oxygen at concentrations above 15at.% in the films leads to an abrupt nanocrystalline-amorphous transition.     

AlGaN/GaN-heterostructures on (111) 3C-SiC/Si pseudo substrates for high frequency applications

We present the realization of high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures, which were grown on silicon substrates using an ultrathin SiC transition layer. The growth of AlGaN/GaN heterostructures on 3C-SiC(111)/Si(111) was performed using metalorganic chemical vapour deposition (MOCVD). The 3C-SiC(111) transition layer was realized by low pressure CVD and prevented Ga-induced meltback etching and Si-outdiffusion in the subsequent MOCVD growth. The two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface showed an electron sheet density of 1.5 × 10¹³ cm^− 3 and a mobility of 870 cm²/Vs. The HEMTs DC and RF characteristics were analysed and showed a peak cut-off frequency as high as 29 GHz for a 250 nm gate length.     

Structural and optical properties of tellurite thin film glasses deposited by pulsed laser deposition

Tellurite (TeO₂-TiO₂-Nb₂O₅) thin film glasses have been produced by pulsed laser deposition at room temperature at laser energy densities in the range of 0.8-1.5 J/cm² and oxygen pressures in the range of 3-11 Pa. The oxygen concentration in the films increases with laser energy density to reach values very close to that of the bulk glass at 1.5 J/cm², while films prepared at 1.5 J/cm² and pressures above 5 Pa show oxygen concentration in excess of 10% comparing to the glass. X-ray photoelectron spectroscopy shows the presence of elementary Te in films deposited at O₂ pressures ≤ 5 Pa that is not detected at higher pressures, while analysis of Raman spectra of the samples suggests a progressive substitution of TeO₃ trigonal pyramids by TeO₄ trigonal bipyramids in the films when increasing their oxygen content. Spectroscopic ellipsometry analysis combined with Cauchy and effective medium modeling demonstrates the influence of these compositional and structural modifications on the optical response of the films. Since the oxygen content determines their optical response through the structural modifications induced in the films, those can be effectively controlled by tuning the deposition conditions, and films having large n (2.08) and reduced k (< 10^− 4) at 1.5 μm have been produced using the optimum deposition conditions.     

Optical and structural properties of Mg-ion implanted GaN nanowires

Mg⁺ ions (60 keV) were implanted into GaN nanowires (NWs) with total fluxes of 5 × 10¹²-5 × 10¹⁴ cm⁻² followed by thermal annealing at 700 °C in N₂ ambient. Transmission electron microscopic images showed amorphous layer formation and defect accumulation in the higher dose Mg-implanted GaN NWs after annealing. Photoluminescence spectra (300 K) of the annealed Mg-implanted GaN NWs exhibited near-band-edge (NBE) emission, donor-acceptor pair (DAP) emission, and defect-related yellow luminescence. With increasing dose, the NBE and DAP emissions are red shifted. Similar phenomena were observed in samples implanted with Ar to produce similar amounts of lattice disorder. The NWs show a much higher sensitivity to defect accumulation than GaN thin films.     

Dependence of magnetic properties on the Fe ion in Ba-doped BiFeO3 multiferroic films

Ba-doped BiFeO₃ thin films were prepared on Pt/TiO₂/SiO₂/Si (100) substrate at various oxygen pressures by pulsed laser deposition. The influences of oxygen pressure on the crystalline structures, magnetic and electrical properties of these films were studied. Studies showed that the oxygen vacancy content and accordingly the leakage current density of the films decreased with the increase of the oxygen pressure, and the concentration of the Fe²⁺ increased with the decrease of the oxygen pressure. Strong ferromagnetism was observed in these films. The magnetization of the films increased with the decrease of the oxygen pressure and the largest remnant magnetization of 2.46 × 10⁴ A/m was obtained at an oxygen pressure of 0.7 Pa. This enhancement of the magnetization can be attributed to the increase of the Fe²⁺ concentration which may be related closely to the increase of the oxygen vacancy in the films.     

Structural, optical and electrical properties of In doped CdO thin films for optoelectronic applications

Thin films of indium doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature and partial oxygen pressure on structural, optical and electrical properties was studied. We find that the optical transparency of the films largely depends on the growth temperature, while partial oxygen pressure has virtually no effect on the transparency of the films. Electrical properties are found to be sensitive to both the growth temperature and oxygen pressure. It is observed that conductivity and carrier concentration decreases with temperature. The film grown at 200 °C under an oxygen pressure of 5.0 × 10^− 4 mbar shows high mobility (155 cm²/V s), high carrier concentration (1.41 × 10²¹ cm³), and low resistivity (2.86 × 10^− 5 Ω cm).     

The effect of deposition parameters on the properties of SrCu2O2 films fabricated by pulsed laser deposition

SrCu₂O₂ (SCO) thin films have been fabricated by pulsed laser deposition at oxygen partial pressures between 5 × 10^− 5-5 × 10^− 2 mbar and substrate temperatures from 300 °C to 500 °C. All films were single-phase SrCu₂O₂, p-type materials. Films deposited at a substrate temperature of 300 °C and oxygen pressure 5 × 10^− 4 mbar exhibited the highest transparency (∼ 80%), having conductivity 10^− 3 S/cm and carrier concentration around 10¹³ cm^− 3. Films deposited at oxygen partial pressure higher than 10^− 3 mbar exhibited higher conductivity and carrier concentration but lower transmittance. Depositions at substrate temperatures higher than 300 °C gave films of high crystallinity and transmittance even for films as thick as 800 nm. The energy gap of SrCu₂O₂ thin films was found to be around 3.3 eV.     

Structural characterization of group III nitrides grown by pulsed laser deposition

We have grown thin GaN epitaxial films on Si(1 1 1) and sapphire (0 0 0 1) with AlN buffer layers using pulsed laser deposition and investigated the distortion of the films using synchrotron radiation grazing incidence-angle X-ray diffraction for the first time. We have found that the strains in the AlN films were almost fully relaxed at the growth temperature (800 °C) by the introduction of the misfit dislocations. However, we have also found that the introduction of misfit dislocations at the interfaces between the AlN buffer layers and the GaN films is not rigorous enough to release the stain fully at the growth temperature.     

Effect of process parameters on the growth and properties of impurity-doped zinc oxide transparent conducting thin films by RF magnetron sputtering

Zinc oxide transparent conducting thin films co-doped with aluminum and ruthenium were grown on polyethylene terephthalate substrates at room temperature using RF magnetron sputtering. The crystal growth and physical properties of the films were investigated with respect to the variation of discharge power density from 1.5 to 6.1 W/cm² and sputtering pressure from 0.13 to 2.0 Pa. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) showed that the films grown with 3.6 W/cm² power density and sputtering pressure of 0.4 Pa had the best crystallinity and larger pyramid-like grains. The optimized electrical resistivity had a lowest measured value of about 9 × 10⁻⁴ Ω cm. The low carrier mobilities of the films (3-8.9 cm² V⁻¹ s⁻¹) have been discussed in terms of what is believed to be the dominant effect of ionized impurity scattering, but in addition chemisorption of oxygen on the film surface and effect of grain boundaries are also thought to be significant. The transmittances of the films in the visible range are greater than 80%, while the optical band gaps are in the order of 3.337-3.382 eV.     

Room-temperature red emission from light-emitting diodes with Eu-doped GaN grown by organometallic vapor phase epitaxy

We obtained room-temperature red emission from GaN-based light-emitting diodes (LEDs) using a Eu-doped GaN (GaN:Eu) as an active layer. The bright emission was observed under normal lighting condition, which is associated with the intra-4f shell transition of Eu³⁺ ions. The LED properties depends on the growth condition of GaN:Eu layer. Since the high-quality GaN can be grown at higher growth pressure, the intense electroluminescence (EL) was observed in the LED with a GaN:Eu active layer grown at atmospheric pressure, which is due to the enhancement of the energy transfer efficiency from the GaN host material to the Eu ions. At a d.c. current of 20 mA, the light output power and external quantum efficiency were 17 μW and 0.04%, respectively. These results indicate the feasibility of GaN:Eu to realize a GaN-based red emitter for fabrication of nitride-based monolithic optical devices.