Effects of Mg-doping temperature on the structural and electrical properties of nonpolar a-plane p-type GaN films

Nonpolar(11–20) a-plane p-type GaN films were successfully grown on r-plane sapphire substrate with the metal–organic chemical vapor deposition(MOCVD) system. The effects of Mg-doping temperature on the structural and electrical properties of nonpolar p-type GaN films were investigated in detail. It is found that all the surface morphology, crystalline quality, strains, and electrical properties of nonpolar a-plane p-type GaN films are interconnected, and are closely related to the Mg-doping temperature. This means that a proper performance of nonpolar p-type GaN can be expected by optimizing the Mg-doping temperature. In fact, a hole concentration of 1.3×10^(18)cm^(-3), a high Mg activation efficiency of 6.5%,an activation energy of 114 me V for Mg acceptor, and a low anisotropy of 8.3% in crystalline quality were achieved with a growth temperature of 990℃. This approach to optimizing the Mg-doping temperature of the nonpolar a-plane p-type GaN film provides an effective way to fabricate high-efficiency optoelectronic devices in the future.

Near-field radiative heat transfer between nanoporous GaN films

Photon tunneling effects give rise to surface waves,amplifying radiative heat transfer in the near-field regime.Recent research has highlighted that the introduction of nanopores into materials creates additional pathways for heat transfer,leading to a substantial enhancement of near-field radiative heat transfer(NFRHT).Being a direct bandgap semiconductor,GaN has high thermal conductivity and stable resistance at high temperatures,and holds significant potential for applications in optoelectronic devices.Indeed,study of NFRHT between nanoporous GaN films is currently lacking,hence the physical mechanism for adding nanopores to GaN films remains to be discussed in the field of NFRHT.In this work,we delve into the NFRHT of GaN nanoporous films in terms of gap distance,GaN film thickness and the vacuum filling ratio.The results demonstrate a 27.2%increase in heat flux for a 10 nm gap when the nanoporous filling ratio is 0.5.Moreover,the spectral heat flux exhibits redshift with increase in the vacuum filling ratio.To be more precise,the peak of spectral heat flux moves fromω=1.31×10^(14)rad·s^(-1)toω=1.23×10^(14)rad·s^(-1)when the vacuum filling ratio changes from f=0.1 to f=0.5;this can be attributed to the excitation of surface phonon polaritons.The introduction of graphene into these configurations can highly enhance the NFRHT,and the spectral heat flux exhibits a blueshift with increase in the vacuum filling ratio,which can be explained by the excitation of surface plasmon polaritons.These findings offer theoretical insights that can guide the extensive utilization of porous structures in thermal control,management and thermal modulation.

Synthesis of GaN films on porous silicon substrates

A novel and simple method was employed to synthesize GaN films on porous silicon （PS） substrates, GaN films were obtained through the reaction between NH3 and Ga2O3 films deposited on the substrates with magnetron sputtering. Since GaN and PS are all good materials for luminescence, it is expected to obtain some new properties from GaN on PS. The samples were analyzed with X-ray diffraction （XRD） to identify crystalline structure. Fourier transmit infrared （FFIR） spectrum was used to analyze the chemical state of the samples. The films were observed with scanning electron microscopy （SEM） and were found to consist of many big crystal grains. Photoluminescence （PL） spectrum was used to illuminate the optical property of the GaN films.

Preparation and characterization of GaN films grown on Ga-diffused Si (111) substrates

Hexagonal GaN films were prepared by nitriding Ga_2O_3 films with flowingammonia. Ga_2O_3 films were deposited on Ga-diffused Si (111) substrates by radio frequency (r.f.)magnetron sputtering. This paper have investigated the change of structural properties of GaN filmsnitrided in NH_3 atmosphere at the temperatures of 850, 900, and 950 deg C for 15 min and nitridedat the temperature of 900 deg C for 10, 15, and 20 min, respectively. X-ray diffraction (XRD),scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectronspectroscopy (XPS) were used to analyze the structure, surface morphology and composition ofsynthesized samples. The results reveal that the as-grown films are polycrystalline GaN withhexagonal wurtzite structure and GaN films with the highest crystal quality can be obtained whennitrided at 900 deg C for 15 min.

Structure and Strain Properties of GaN Films Grown on Si（111） Substrates with AlxGa1-xN/AlyGa1-yN Superlattices

CaN films with an AlxGa1-xN/AlyGa1-xN superlattice （SL） buffer layer are grown on Si（111） substrates by metal-organic chemical vapor deposition （MOCVD）. The structure and strain properties of the samples are studied by optical microscopy, Raman spectroscopy, x-ray diffractometry and atomic force microscopy. The results show that the strain status and crystalline quality of the CaN layers are strongly dependent on the difference of the Al composition between AlxCa1-xN barriers and AlyCa1-yN wells in the SLs. With a large Al composition difference, the CaN film tends to generate cracks on the surface due to the severe relaxation of the SLs. Otherwise, when using a small Al composition difference, the crystalline quality of the CaN layer degrades due to the poor function of the SLs in filtering dislocations. Under an optimized condition that the Al composition difference equals 0.1, the crack-free and compressive strained CaN film with an improved crystalline quality is achieved. Therefore, the AlxGa1-xN/AlyGal-yN SL buffer layer is a promising buffer structure for growing thick CaN films on Si substrates without crack generation.

Growth of a-Plane GaN Films on r-Plane Sapphire by Combining Metal Organic Vapor Phase Epitaxy with the Hydride Vapor Phase Epitaxy

Hydride vapor phase epitaxy （HVPE） is utilized to grow nonpolar a-plane GaN layers on r-plane sapphire templates prepared by metal organic vapor phase epitaxy （MOVPE）. The surface morphology and microstructures of the samples are characterized by atomic force microscopy. The full width at half maximum （FWHM） of the HVPE sample shows a W-shape and that of the MOVPE sample shows an M-shape plane with the degree of 0 in the high-resolution x-ray diffraction （HRXRD） results. The surface morphology attributes to this significant anisotropic. HRXRD reveals that there is a significant reduction in the FWHM, both on-axis and off-axis for HVPE GaN are compared with the MOVPE template. The decrease of the FWHM of E2 （high） Raman scat tering spectra further indicates the improvement of crystal quality after HVPE. By comparing the results of secondary- ion-mass spectroscope and photoluminescence spectrum of the samples grown by HVPE and MOVPE, we propose that C-involved defects are originally responsible for the yellow luminescence.

Structural Characterization of Thin Epitaxial GaN Films on Polymer Polyimides Substrates by Ion Beam Assisted Deposition

The Epitaxial GaN thin films have been fabricated by Ion Beam Assisted Deposition (IBAD) process using nitrogen ions with hyperthermal energies on the polyimides polymer substrates. By applying with the Reflection of High-Energy Electron Diffraction (RHEED), Scanning Electron Microscopy (SEM) and Quantum Design Physical Properties Measurement System, the behaviour of hexagonal GaN thin films is investigated. The result showed that the high quality of the deposited GaN layers kept appearing for many parameters depending on the temperature greatly. The behaviour of high quality of epitaxial GaN coating on the polyimide polymer substrates is a promising material for optoelectronic devices and semiconductor devices application.

The Characteristic of Threading Dislocation in Different Structures GaN Films Grown by MOCVD

Three mechanisms to reduce threading dislocations(TDs) in GaN films as the epitaxial films grow thicker are suggested by SEM and

Fabrication of hexagonal gallium nitride films on silicon (111) substrates

Hexagonal gallium nitride films were successfully fabricated throughammoniating Ga_2O_3 films deposited on silicon (111) substrates by electrophoresis. The structure,composition, and surface morphology of the formed films were characterized by X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmissionelectron microscopy (TEM). The measurement results reveal that the polycrystalline GaN films withhexagonal wurtzite structure were successfully grown on the silicon (111) substrates. Preliminaryresults suggest that varying the ammoniating temperature has obvious effect on the quality of theGaN films formed with this method.

Effect of Al Doping in the InGaN/GaN Multiple Quantum Well Light Emitting Diodes Grown by Metalorganic Chemical Vapour Deposition

The effect of Al doping in the GaN layer of InGaN/GaN multiple quantum-well light emitting diodes （LEDs） grown by metalorganic chemical vapour deposition is investigated by using photoluminescence （PL） and highresolution x-ray diffraction. The full width at half maximum of PL of A1 doped LEDs is measured to be about 12nm. The band edge photoluminescence emission intensity is enhanced significantly. In addition, the in-plane compressive strain in the Al-doped LEDs is improved significantly and measured by reciprocal space map. The output power of Al-doped LEDs is 130mW in the case of the induced current of 200mA.

Luminescent Characteristics of Near Ultraviolet InGaN/GaN MQWs Grown on Grooved Sapphire Substrates Fabricated by Wet Chemical Etching

InGaN/GaN multiple quantum wells （MQWs） are grown on planar and maskless periodically grooved sapphires by metal organic vapour phase epitaxy （MOCVD）. High-resolution x-ray rocking curves and transmission electron microscopy （TEM） are adopted to characterize the film quality. Compared with the MQWs grown on planar sapphire, the sample grown on grooved sapphire shows better crystalline quality： a remarkable reduction of dislocation densities is achieved. Meanwhile, the MQWs grown on grooved sapphire show two times larger PL intensity at room temperature. Temperature-dependent PL measurements are adopted to investigate the luminescence properties. The luminescence thermal quenching based on a fit to the Arrhenius plot of the normalized integrated PL intensity over the measured temperature range suggests that the nonradiative recombination centres （NRCs） are greatly reduced for the sample grown on grooved sapphire. We assume that the reduction of dislocations which act as NRCs is the main reason for the sample grown on pattern sapphire having higher PL intensity.

Optical Emission Spectroscopy of Electron Cyclotron Resonance-Plasma Enchanced Metalorganic Chemical Vapor Deposition Process for Deposition of GaN Film

An investigation was made into the nitrogen-trimethylgallium mixed electron cyclotron resonance （ECR） plasma by optical emission spectroscopy （OES）. The ECR plasma enhanced metalorganic chemical vapour deposition technology was adopted to grow GaN film on an α-Al2O3 substrate. X-ray diffraction （XRD） analyses showed that the peak of GaN （0002） was at 20 = 34.48°, being sharper and more intense with the increase in the Ne： trimethylgallium（TMG） flow ratio. The results demonstrate that the electron cyclotron resonance-plasma enchanced met- alorganic chemical vapor deposition （ECR-MOPECVD） technology is evidently advantageous for the deposition of GaN film at a low growth temperature.

The influence of an Si_xN_y interlayer on a GaN film grown on an Si(111) substrate

A method to drastically reduce dislocation density in a GaN film grown on an Si（111） substrate is newly developed. In this method, the SixNy interlayer which is deposited on an A1N buffer layer in situ is introduced to grow the GaN film laterally. The crack-free GaN film with thickness over 1.7 micron is successfully grown on an Si（lll） substrate. A synthesized GaN epilayer is 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 the full width at half maximum of the X-ray diffraction curve in the （0002） plane is as low as 403 arcsec for the GaN film grown on the Si substrate with an SixNy interlayer. In addition, Raman scattering is used to study the stress in the sample. The results indicate that the SizNy interlayer can more effectively accommodate the strain energy. So the dislocation density can be reduced drastically, and the crystal quality of GaN film can be greatly improved by introducing an SixNy interlayer.

Spectra Analysis of a Novel Ti-Doped LiAlO2 Single Crystal

LiAlO2 single crvstals doped with Ti at concentration 0.2at.% are grown by the Czochralskl technique with dimensions φ42×55mm. Ti ions in the crystal are quadrivalence proven by comparing the absorption and fluorescence spectra of pure LiAlO2 and Ti： LiAlO2. After air and Li-rich atmosphere annealing, the absorption peaks in the range of 600-800nm disappear. We conclude that 682 and 756nm absorption peaks are attributed to the VLi and Vo absorptions, respectively： The peaks at 716nm and 798nm may stem from the VLi^＋ and absorptions. The colour-centre model can be applied to explain the experimental phenomena. Ti^4＋-doping produces more lithium vacancies in the LiAlO2 crystal. The intensities of [LiO4] and the associated bonds remain unchanged, which improves the anti-hydrolyzation and thermal stability of LiAlO2 crystals.

Three-Step Growth Optimization of AlN Epilayers by MOCVD

A three-step growth process is developed for depositing high-quality aluminium-nitride （AlN） epilayers on （001） sapphire by low pressure metalorganic chemical vapour deposition （LP-MOCVD）. We adopt a low temperature （LT） A1N nucleation layer （NL）, and two high temperature （HT） A1N layers with different V/Ⅲ ratios. Our results reveal that the optimal NL temperature is 840-880℃, and there exists a proper growth switching from low to high V/Ⅲ ratio for further reducing threading dislocations （TDs）. The screw-type TD density of the optimized AIN film is just 7.86×10^6 cm^-2, about three orders lower than its edge-type one of 2×10^9 cm^-2 estimated by high-resolution x-ray diffraction （HRXRD） and cross-sectional transmission electron microscopy （TEM）.

Temperature dependence of the point defect properties of GaN thin films studied by terahertz time-domain spectroscopy

The dielectric functions of GaN for the temperature and frequency ranges of 10–300 K and 0.3–1 THz are obtained using terahertz time-domain spectroscopy.It is found that there are oscillations of the dielectric functions at various temperatures.Physically,the oscillation behavior is attributed to the resonance states of the point defects in the material.Furthermore,the dielectric functions are well fitted by the combination of the simple Drude model together with the classical damped oscillator model.According to the values of the fitting parameters,the concentration and electron lifetime of the point defects for various temperatures are determined,and the temperature dependences of them are in accordance with the previously reported result.Therefore,terahertz time-domain spectroscopy can be considered as a promising technique for investigating the relevant characteristics of the point defects in semiconductor materials.

Fabrication of GaN epitaxial films on Al_2O_3/Si (001) substrates

Single crystal GaN films of hexagonal modification have been fabricated on Al2O3/Si （001） substrates via a low pressure metalorganic chemical deposition （LP-MOCVD） method. The full width at half-maximum of （0002） X-ray diffraction peak for the GaN film 1.1 μm thick was 72 arcmin, and the mosaic structure of the film was the main cause of broadening to the X-ray diffraction peak. At room temperature, the photoluminescence （PL） spectrum of GaN exhibited near band edge emission peaking at 365 nm.

GaN/metal/Si heterostructure fabricated by metal bonding and laser lift-off

A process methodology has been adopted to transfer GaN thin films grown on sapphire substrates to Si substrates using metal bonding and laser lift-off techniques. After bonding, a single KrF （248 nm） excimer laser pulse was directed through the transparent sapphire substrates followed by low-temperature heat treatment to remove the substrates. The influence of bonding temperature and energy density of the excimer laser on the structure and optical properties of GaN films were investigated systemically. Atomic force microscopy, X-ray diffraction and photoluminescence measurements showed that （1） the quality of the GaN film was higher at a lower bonding temperature and lower energy density; （2） the threshold of the energy density of the excimer laser lift-off GaN was 300 mJ/cm^2. The root-mean-square roughness of the transferred GaN surface was about 50 nm at a bonding temperature of 400 ℃.