Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.     

Cubic GaN layers grown by metalorganic chemical vapor deposition on GaN templates obtained by nitridation of GaAs

Cubic gallium nitride epitaxial layers were grown by metalorganic chemical vapor deposition on GaN templates obtained by nitridation of GaAs substrates. The GaN structure can be peeled off the GaAs substrate and it can be handled separately. X-ray diffraction, Raman and photoluminescence measurements show that the epitaxial layers are cubic and monocrystalline.     

The magnetic field effect on optical properties of Sm-doped GaN thin films

We report the optical and magnetic properties of Sm doped wide-band-gap Gallium nitride (GaN) prepared by ion implantation. The photoluminescence (PL) intensity obviously increased as the increasing of annealing temperature from 700 to 1,100 °C. Magnetic hysteresis was observed at room temperature. Our study was focusing on the influence of magnetic field on optical property of Sm³⁺ doped GaN. It was discovered that the PL intensity of Sm³⁺ increases remarkably under an external magnetic field. The mechanism of the magnetic ordering-modulated PL peak intensity of Sm³⁺ doped GaN films was discussed. This may open up a suite of potential applications in producing magneto-optical devices.     

The influence of reactor height adjustment on properties in GaN films grown on 6H-SiC by metal organic chemical vapor deposition

The influence of reactor height adjustment on properties in GaN films grown on 6H-SiC by metal organic chemical vapor deposition (MOCVD) was investigated. The property of GaN epilayer was investigated by atomic force microscopy, X-ray diffraction, low-temperature (10 K) photoluminescence and the Raman scattering. It is found that, as the spacing between showerhead and susceptor decreased, the growth rate increased and the tensile stress decreased. This result may be useful to control the stress in GaN thin films grown on silicon carbide substrate by MOCVD.     

Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

High-resistive GaN (>10⁸ Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and Al_xGa_1?xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.     

Basal plane-oriented gallium nitride films on fused silica via acetate dip coating

c-axis-oriented gallium nitride (wurtzite GaN) thin films were fabricated by nitridation of acetate derived precursor films deposited on fused silica substrates without any buffer layer on the top of the substrate. The acetate derived precursors were obtained by (i) preparing a gallium-acetate sol by reacting Ga metal with acetic acid, (ii) coating cleared fused silica substrate with the sol and (iii) after drying the coated films at 100 °C, annealing them in air at 300°, 500° and 900 °C. Only films showing crystallization of α-GaO(OH) (300 °C) and (α + β)-Ga₂O₃ (500 °C) were selected for nitridation. In spite of the amorphous nature of the substrate, the GaN films showed a strong preferred orientation for the basal plane (002) under selected conditions of precursor annealing (300°, 500 °C) and subsequent nitridation (under flowing NH₃) temperature and time. In other cases formation of an additional plane, i.e. (101) was indicated as a weaker peak in X-ray diffraction (XRD) patterns. The precursors and nitride films were characterized by Fourier transform infrared spectroscopy, UV-Visible spectroscopy, XRD, high resolution transmission electron microscopy and atomic force microscopy analyses.     

Employing Al buffer layer with Al droplets-distributed surface to obtain high-quality and stress-free GaN epitaxial films on Si substrates

Al buffer layers with Al droplets-distributed surface have been employed to grow high-quality and stress-free GaN epitaxial films on Si substrates. The Al droplets are proved to efficiently improve the quality of as-grown GaN. On the one hand, they can act as nucleation seeds to facilitate the epitaxial growth, improving the crystalline quality and surface morphology of as-grown GaN epitaxial films. On the other hand, they also can compensate the huge compressive stress produced by Al buffer layer during the cooling process, achieving stress-free film. The density and volume of Al droplets greatly impact the properties of as-grown GaN epitaxial films. The GaN epitaxial film grown on the Al buffer layer with many small Al droplets uniformly distributed on it shows the best crystalline quality with the full-width at half maximum (FWHM) of GaN(0002) and GaN(10–12) as 0.5° and 0.7°, respectively, and flat surface with the smallest surface root-mean-square roughness of 3.8 nm. In addition, it also exhibits relatively better photoelectric properties with an FWHM of near band gap emission peak of 18 nm, carrier concentration of 2.0 × 10¹⁷ cm^?3, and mobility of 137.1 cm²/Vs. This work has revealed the advantages of Al buffer layer and the important effects of buffer layer surface on achieving high-quality GaN by PLD, which is of significance for various applications of GaN-based devices.     

Deposition and characteristics of GaN films on Ni metal substrate by ECR-PEMOCVD

Gallium nitride (GaN) films were deposited on Ni metal substrate using electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system. With this approach, highly c-oriented GaN films with smooth surface were obtained at an extremely low temperature of ~480 °C. The trimethyl gallium (TMGa) flux dependent structural, morphological, and optical characteristics of GaN films were investigated by X-ray diffraction analysis, reflection high energy electron diffraction, atomic force microscopy and photoluminescence analysis. The results indicate that it is feasible to deposit GaN films on Ni metal substrate under the proper deposition procedures. The high quality GaN films with high c-axis orientation and strong ultraviolet emission peak are successfully achieved under the optimized TMGa flux of 1.2 sccm. The GaN/Ni structure has great potential for the development of high power devices with excellent heat dissipation.     

Towards the fabrication of a UV light source based on CuCl thin films

CuCl is an ionic wide band gap I–VII semiconductor which has a band gap of 3.4 eV (at 300 K) and a large excitonic binding energy of 190 meV compared to other wide band gap semiconductors, e.g. ZnO (60 meV) and GaN (25 meV). It can be useful as a UV source which can emit light in the blue-UV range. The large excitonic binding energy of CuCl should improve the quantum efficiency compared to ZnO or GaN. In room temperature UV–VIS spectroscopy measurements we have observed strong free Z₃ and Z_1,2 excitonic peaks in vacuum-deposited CuCl films incorporated within structures for electroluminescent devices. Using room temperature photoluminescence (PL) we have also observed a strong free excitonic peak which is attributed to Z₃. We also report on the design of a UV source using electrically pumped CuCl thin films on Si and ITO coated glass substrates. This could open up the possibility of fabricating UV/blue light emitters utilizing CuCl.     

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.     

Light emitting mechanisms dependent on stoichiometry of Si-rich-SiN<Subscript>x</Subscript> films grown by PECVD

Light emission and morphology of silicon-rich silicon nitride films grown by plasma-enhanced chemical vapor deposition were investigated versus film’s stoichiometry. The excess silicon content in the films was controlled varying the NH₃/SiH₄ gas flow ratio from 0.45 up to 1.0. High-temperature annealing was employed to form the silicon quantum dots (QDs) and to enhance the photoluminescence (PL) in visible spectral range. The PL spectrum was found to be complex. The competition of five PL bands leads to the non-monotonous variation of total PL peak position in the range of 1.55–2.95 eV when the Si excess content increases. The shape of PL spectra depends also on an excitation light wavelength. It is shown that for the films fabricated with R ≤ 0.56 and R ≥ 0.67 the dominant contribution into PL spectra is given by native SiN_x defects, whereas in the films obtained with R = 0.59–0.67 the Si-QDs form the main radiative channel. The highest PL intensity is detected in Si-rich SiN_x films grown at R = 0.59–0.67 as well. PL mechanisms are discussed in terms of the contribution of different radiative channels in the light emission process that can show the ways for the optimization of SiN_x light-emitting properties.     

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,...     

Optical Properties of Mn-Implanted GaN Nanorods

We have investigated the optical properties of vertical GaN nanorods with diameters of 150 nm grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy followed by Mn ion implantation and annealing. The GaN nanorods are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. For GaMnN nanorods, Arrhenius plots of the intensities of the Mn acceptor give a thermal activation energy of Δ=350 meV, indicating that the thermal quenching of the Mn-related PL peak is due to the dissociation of an acceptor-bound hole from the temperature-dependent PL spectra. This suggests that the Mn-bound holes in GaN nanorods exhibit the impurity states predicted by the hydrogen model.     

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.     

Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature to achieve c-oriented α-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and β-GaN inclusions     

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.     

Growth of high c-orientated crystalline GaN films on amorphous Cu/glass substrates with low-temperature ECR-PEMOCVD

A low temperature growth method based on electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system (ECR-PEMOCVD) was proposed for the growth of gallium nitride (GaN) films on ordinary soda-lime glass substrates with sputtered Cu as intermediate layer (Cu/glass substrates). The influence of deposition temperature on the properties of the GaN films on Cu/glass substrates was systematically investigated by means of In-situ reflection high energy electron diffraction, X-ray diffraction, atomic force microscopy and photoluminescence spectra. With this method, high c-orientated crystalline GaN films with relatively smooth surface were achieved on amorphous Cu/glass substrate at an extremely low temperature of ~400 °C. The successfully growth of crystalline GaN films on amorphous Cu/glass substrates show great potential for significant improvements in the scalability and cost of GaN based devices, since the adverse effects with high temperature process for glass substrates can be effectively suppressed by this technique.     

Microstructure and optical properties of GaN films on sapphire substrates

Transmission electron microscopy (TEM), double crystal X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurement were applied to study the correlation between the microstructure and material properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. Corresponding to the density of the threading dislocation (TD) increasing approximately one order, the yellow luminescence (YL) intensity was strengthened from negligible to two orders higher than the band-edge emission intensity. The full width of half maximum (FWHM) of GaN (0002) peak of the XRD rocking curve was widened from 11 to 15 min, and in Raman spectra, the width of E₂ mode is broadened from 5 cm⁻¹ to 7 cm⁻¹. A ‘zippers’ structure of GaN buffer layer was discovered by high-resolution electron microscope (HREM).     

Structural and functional properties of ZnO thin films grown on Si substrates by air assisted USP method from non-aqueous solutions at low-temperature

In this work, we deals with the processing and characterization of transparent conducting ZnO thin films on p-type Silicon substrates (1 0 0) by air assisted Ultrasonic Spray Pyrolysis (USP) method. The thin films from different Zn acetate precursor solution concentrations (0.1, 0.2, 0.3 and 0.4 M) were deposited at several temperatures (400, 450 and 500 °C) with thickness from ～100 to ～500 nm. The effects of precursor solution concentration, deposition time and temperature on the structural, morphological, optical, and electrical properties of ZnO films were studied by X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), UV–Vis-NIR spectroscopy, and Hall Effect techniques, respectively. It has been shown that on the ZnO film surface, the preferred orientation, the average crystallite size, the electrical resistivity and the RMS surface roughness depend on the substrate temperature. The grown films have showed a good adhesion and an excellent optical transmission of about 80–95% within the visible range (400–800 nm) and a direct band gap from 3.35 to 3.23 eV with the increase of the substrate temperature and the deposition time. All the PL spectra have exhibited a typical green-yellow emission band. Additionally photovoltaic (PV) activities of n-ZnO/p-Si heterostructures fabricated are investigated.     

Structural, Optical and Magnetic Properties of Ce�CGaN Based Diluted Magnetic Semiconductor

First ever Ce based GaN diluted magnetic semiconductor is reported. MOCVD grown GaN thin films were implanted with 3×10¹⁴ cm^?2 dose of cerium ions. Photoluminescence (PL), optical transmission, Raman, high-resolution X-ray diffraction (HRXRD) measurements were performed on samples to study the optical and structural properties of the materials. Band gap narrowing is observed in optical transmission measurements, which points to incorporation of cerium ions into GaN host lattice. Superconducting Quantum Interference device (SQUID) was used in order to investigate the magnetic properties of implanted samples as a function of temperature and applied field. Hysteresis loops were recorded at 100 K and 300 K for implanted and as-grown samples. Hysteresis behavior and temperature-dependent magnetization measurements revealed the presence of ferromagnetic ordering in Ce implanted GaN samples, which points to the realization of Ce:GaN diluted magnetic semiconductor.