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.     

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.     

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.     

Si(111)衬底上HVPE GaN厚膜生长

在Si(111)衬底上,以MOCVD方法高温外延生长的AIN为缓冲层,使用氮化物气相外延(HVPE)方法外延生长了15Km的c面GaN厚膜.并利用X射线衍射(XRD)、光致发光谱(PL)、拉曼光谱(Raman)等技术研究了GaN厚膜的结构和光学性质.分析结果表明,GaN厚膜具有六方纤锌矿结构,外延层中存在的张应力较小,...     

Structural and optical properties of wurtzite InN grown on Si(111)

We report the structural and optical properties of InN films on Si(111) prepared by ion-beam-assisted filtered cathodic vacuum arc technique. X-ray diffraction and Raman spectroscopy measurements indicated that all the InN films were hexagonal crystalline InN. The InN films deposited at substrate temperature of 475 °C exhibited highly (0001) preferred orientation and texturing (cratered) surface morphology. The oxygen incorporated in the InN films was segregated in the form of amorphous indium oxide or oxynitride phases at the grain boundaries. Photoluminescence emission of ∼ 1.15 eV was observed at room temperature from the InN films.     

Improvement of InN layers deposited on Si(111) by RF sputtering using a low-growth-rate InN buffer layer

We investigate the influence of a low-growth-rate InN buffer layer on structural and optical properties of wurtzite nanocrystalline InN films deposited on Si(111) substrates by reactive radio-frequency sputtering. The deposition conditions of the InN buffer layer were optimized in terms of morphological and structural quality, leading to films with surface root-mean-square roughness of ~ 1 nm under low-growth-rate conditions (60 nm/h). The use of the developed InN buffer layer improves the crystalline quality of the subsequent InN thick films deposited at high growth rate (180 nm/h), as confirmed by the narrowing of X-ray diffraction peaks and the increase of the average grain size of the layers. This improvement of the structural quality is further confirmed by Raman scattering spectroscopy measurements. Room temperature PL emission peaking at ~ 1.58 eV is observed for InN samples grown with the developed buffer layer. The crystal and optical quality obtained for InN films grown on Si(111) using the low-growth-rate InN buffer layer become comparable to high-quality InN films deposited directly on GaN templates by RF sputtering.     

Effect of growth temperature on polytype transition of GaN from zincblende to wurtzite

We have investigated effect of growth temperature on the polytype conversion of cubic GaN (c-GaN) grown on GaAs (001) substrates by MOVPE. It was found that the polytype transition of GaN from zincblende (cubic) to wurtzite (hexagonal) structures is much dependent on the growth temperature. Transmission electron microscopy (TEM) observations demonstrate that the GaN grown layers have the cubic structure (c-GaN) and contain bands of stacking faults (SFs) parallels to {111} planes. For low growth temperatures (∼ 900 °C), XRD results demonstrate that the GaN grown layers with the cubic phase purity higher than 85% were obtained. No different types of single diffraction spots, indicating the incorporation of single-crystal h-GaN, on the selected area diffraction (SAD) pattern was observed. It is also found that a density of SFs decreases with the distance from the interface of c-GaN/GaAs. On the other hand, GaN layers exhibited a transition from cubic to mixed cubic/hexagonal phase under conditions of increasing growth temperature (∼ 960 °C) as determined using TEM-SAD technique with complementary XRD and PL observations. In addition, the optical characteristics of c-GaN layers are shown to be very sensitive to the presence of the single-crystal h-GaN.     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

Growth of GaN on SiC/Si substrates using AlN buffer layer by hot-mesh CVD

GaN films were grown on SiC/Si (111) substrates by hot-mesh chemical vapor deposition (CVD) using ammonia (NH₃) and trimetylgallium (TMG) under low V/III source gas ratio (NH₃/TMG = 80). The SiC layer was grown by a carbonization process on the Si substrates using propane (C₃H₈). The AlN layer was deposited as a buffer layer using NH₃ and trimetylaluminum (TMA). GaN films were formed and grown by the reaction between NH_x radicals, generated on a tungsten hot mesh, and the TMG molecules. The GaN films with the AlN buffer layer showed better crystallinity and stronger near-band-edge emission compared to those without the AlN layer.     

溅射制备非晶氮化镓薄膜的光学性能

利用直流磁控溅射方法制备了GaN薄膜. X射线衍射及Raman光谱结果表明薄膜样品为非晶结构; 傅立叶红外光谱表明薄膜样品的主要吸收峰为Ga--N键的伸缩振动; 光致发光测试得到了360nm处的紫外发光谱; 测量薄膜样品的紫外-可 见谱, 并利用Tauc公式计算得到样品的光学带隙为3.74eV, 这与光致发光谱得到的结果是一致的.     

Synthesis and optical properties of single-crystalline GaN nanorods

Single-crystalline GaN nanorods were successfully synthesized on Si(1 1 1) substrates through ammoniating Ga₂O₃/Mo films deposited on the Si(1 1 1) substrate by radio frequency magnetron sputtering technique. The as-synthesized nanorods are confirmed as single-crystalline GaN with wurtzite structure by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). Scanning electron microscopy (SEM) displays that the GaN nanorods are straight and smooth with diameters in the range of 100-200 nm and lengths typically up to several micrometers. X-ray photoelectron spectroscopy (XPS) confirms the formation of bonding between Ga and N. The representative photoluminescence spectrum at room temperature exhibits a strong and broad emission band centered at 371.1 nm, attributed to GaN band-edge emission. The growth process of GaN nanorod may be dominated by vapor-solid (VS) mechanism.     

Characterization and properties of GaN films deposited by middle-frequency magnetron sputtering with anode-layer ion source assistance

GaN films were deposited on Si (111) substrates at a high growth rate of 94 nm/min using middle-frequency (MF) magnetron sputtering method with anode-layer ion source assistance. XRD, TEM and PL experiments were used to investigate the structure and optical properties of the resulting films. GaN films produced under optimal conditions have an almost 1:1 N: Ga ratio. The O concentration decreased while the deposition rate increased with the increasing of bias voltages. Hexagonal polycrystal nature of the films was detected by the TEM and XRD measurements. Peaks located at 3.36 eV labeled as free-exciton were detected in the temperature dependence photoluminescence spectra. The binding energies of N 1s and Ga 3d were centered at 397.5 and 19.8 eV, respectively. The results show that the ion beam-assisted MF reactive magnetron sputtering method can be an encouraging method for deposition of polycrystalline GaN films at low temperatures.     

A study of microstructural and optical properties of nanocrystalline ceria thin films prepared by pulsed laser deposition

Thin films of cerium oxide (CeO₂) have been deposited on (100) Si substrates using pulsed laser deposition technique at various substrate temperatures from room temperature (RT) to 973 K at an optimized oxygen partial pressure of 3 Pa. Structural, morphological and optical properties have been carried out using X-ray diffraction (XRD), Raman, ellipsometry and atomic force microscopy techniques. XRD results showed that the deposited films are polycrystalline with cubic structure. At room temperature, the film showed preferred orientation along (111) plane, while at higher temperatures, it exhibited preferred orientation along (200). The crystallite sizes were calculated and were found to be in the range 17-52 nm. The texture coefficient for (200) reflection increased until 573 K, and then decreased in the temperature range 673-973 K. The Raman peak appeared at 463 cm^− 1 due to the F_2g active mode also confirmed the formation of CeO₂ with a cubic structure. There was a systematic variation in the Raman peak intensity, frequency shift and line broadening with the increase of temperature. The ellipsometry studies showed that the refractive index and band gap increased from 2.2 to 2.6 and 3.4 to 3.6 eV, respectively with increasing substrate temperature from RT to 973 K.     

电泳沉积法制备GaN薄膜的结构和组分分析

采用电泳沉积法在Si(111)衬底上制备出了GaN薄膜.用X射线衍射(XRD)、傅立叶红外吸收(FTIR)谱、X射线光电子能谱(XPS)和扫描电镜(SEM)对样品的结构、组分和形貌进行了分析.结果显示所得样品为六方纤锌矿结构的GaN多晶薄膜.     

Synthesis of GaN nanowires through Ga2O3 films' reaction with ammonia

GaN nanowires were synthesized by ammoniating Ga₂O₃ films on Ti layers deposited on Si (111) substrates at 950 °C. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM). The XRD, FTIR and HRTEM studies showed that these nanowires were hexagonal GaN single crystals. SEM observation demonstrated that these GaN nanorods with diameters ranging from 50 nm to 100 nm and lengths up to several micrometers intervene with each other on the substrate.     

溅射后退火簇状GaN纳米棒的生长及其镁的作用

采用磁控溅射技术先在硅衬底上制备Ga2O3/Mg薄膜,然后在1000℃时于流动的氨气中进行氨化反应制备GaN薄膜.X射线衍射(XRD)、X射线光电子能谱(XPS)、选区电子衍射(SAED)和高分辨透射电子显微镜(HRTEM)的结果表明采用此方法得到了六方纤锌矿结构的GaN单晶纳米棒.通过扫描电镜(SEM)观察发现纳米棒的形貌,纳米棒的直径在200～600nm之间.我们对镁层的作用进行了简单探讨.     

Effects of SiC buffer layer growth temperature on the residual strain of GaN/SiC/Si thin films

Effects of SiC buffer layers were studied on the residual strain of GaN films grown on 3C-SiC/Si (111) substrates. It was clearly observed by Raman scattering measurement that the residual strain of the GaN/Si is reduced by inserting the SiC intermediate layer. Furthermore, residual strain within the GaN/SiC/Si films decreased when the growth temperature of the SiC buffer layer decreased. It was proposed that the irreversible creep phenomenon occurs during the high temperature growth of SiC, affecting nature of the residual strain within the SiC and the GaN layers.     

Synthesis of thermally evaporated ZnSe thin film at room temperature

Zinc selenide (ZnSe) thin film on glass substrates were prepared by thermal evaporation under high vacuum using the quasi-closed volume technique at room temperature (300 ± 2 K). The deposited ZnSe properties were assessed via X-ray diffraction, atomic force microscope (AFM), UV-Vis specrophotometry, Raman spectroscopy, photo-luminescence, Fourier transform infrared spectroscopy (FT-IR) and spectroscopic ellipsometry. The X-ray diffraction patterns of the film exhibited reflection corresponding to the cubic (111) phase (2θ = 27.20°). This analysis indicated that the sample is polycrystalline and have cubic (Zinc blende) structure. The crystallites were preferentially oriented with the (111) planes parallel to the substrates. The AFM images showed that the ZnSe films have smooth morphology with roughness 6.74 nm. The transmittance spectrum revealed a high transmission of 89% in the infrared region (≥ 600 nm) and a low transmission of 40% at 450 nm. The maximum transmission of 89.6% was observed at 640 nm. Optical band-gap was calculated from the transmission data of specrophotometry, photo-luminescence and ellipsometry and was 2.76, 2.74 and 2.82 eV respectively. Raman spectroscopic studies revealed two longitudinal optical phonon modes at 252 cm ^-1 and 500 cm ^-1. In photoluminescence study, the luminescence peaks was observed at 452 nm corresponding to band to band emission. FT-IR study illustrated the existence of Zn-Se bonding in ZnSe thin film. The optical constants were calculated using spectroscopic ellipsometry and were determined from the best fit ellipsometric data in the wavelength regime of interest from 370-1000 nm. These results manifested excellent room temperature ZnSe synthesis and characteristics for opto-electronics technologies.     

氨化Si基Ga2O3/In制备GaN薄膜

研究了Ga2O3/In 膜反应自组装制备GaN薄膜,再将Ga2O3/In膜在高纯氨气气氛中氨化反应得到GaN薄膜,用X射线衍射(XRD),傅里叶红外吸收(FTIR),扫描电镜(SEM),原子力显微镜(AFM),透射电镜(TEM)对样品进行结构,形貌的分析.测试结果表明:用此方法得到了六方纤锌矿结构的GaN多晶膜,且900℃时成膜的质量最好.     

Electrical and optical properties of silicon-doped gallium nitride polycrystalline films

Si-doped GaN films in polycrystalline form were deposited on quartz substrates at deposition temperatures ranging from 300–623 K using r.f. sputtering technique. Electrical, optical and microstructural properties were studied for these films. It was observed that films deposited at room temperature contained mainly hexagonal gallium nitride (h-GaN) while films deposited at 623 K were predominantly cubic (c-GaN) in nature. The films deposited at intermediate temperatures were found to contain both the hexagonal and cubic phases of GaN. Studies on the variation of conductivity with temperature indicated Mott’s hopping for films containing c-GaN while Efros and Shklovskii (E-S) hopping within the Coulomb gap was found to dominate the carrier transport mechanism in the films containing h-GaN. A crossover from Mott’s hopping to E-S hopping in the ‘soft’ Coulomb gap was noticed with lowering of temperature for films containing mixed phases of GaN. The relative intensity of the PL peak at ∼2·73 eV to that for peak at ∼3·11 eV appearing due to transitions from deep donor to valence band or shallow acceptors decreased significantly at higher temperature. Variation of band gap showed a bowing behaviour with the amount of cubic phase present in the films.