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.

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.

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.

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.

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.

Mosaic Structure Evolution in GaN Films with Annealing Time Grown by Metalorganic Chemical Vapour Deposition

We investigate mosaic structure evolution of GaN films annealed for a long time at 800℃ grown on sapphire substrates by metalorganic chemical vapour deposition by high-resolution x-ray diffraction. The result show that residual stress in GaN films is relaxed by generating edge-type threading dislocations （TDs） instead of screw-type TDs. Compared to as-grown GaN films, the annealed ones have larger mean twist angles corresponding to higher density of edge-type TDs but smaller mean tilt angles corresponding to lower density of screw-type TDs films. Due to the increased edge-type TD density, the lateral coherence lengths of the annealed GaN films also decrease. The results obtained from chemical etching experiment and grazing-incidence x-ray diffraction （GIXRD） also support the proposed structure evolution.

Elimination of Crystallographic Wing Tilt of Canti-Bridged Epitaxial Laterally Overgrown GaN Films by Optimizing Growth Procedure

Canti-bridged epitaxial lateral overgrowth （CBELO） of GaN is performed by metalorganic chemical vapour deposition （MOCVD） on maskless V-grooved sapphire substrates prepared by wet chemical etching with different mesa widths. The wing tilt usually observed in ELO is not found in the CBELO GaN with wide mesa widths, while it can be detected obviously in the GaN with narrow mesa widths. The wing tilt of CBELO GaN grown on a grooved sapphire substrate with narrow mesa can be controlled by adjusting the thickness of the nucleation layer. The dependence of the wing tilt on the nucleation layer thickness is studied. Cross-sectional scanning electron microscopy is used to characterize the geometry of the wing regions, and double crystal x-ray diffraction is used to analyse the structural characteristics and to measure the magnitude of the crystalline wing tilt. It is found that the crystalline wing tilt can be eliminated completely by first growth of a thin nucleation GaN layer then the CBELO GaN. Possible reason and the origin of the wing tilt in CBELO GaN films are also discussed.

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.

Effect of thickness on the microstructure of GaN films on Al_2 O_3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire （0001） substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional （3D） island mode to two-dimensional （2D） layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The ll0-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.

低成本高结晶GaN纳米线柔性薄膜制备及其场发射性能

旨在探究非贵金属Cu替代贵金属Au作为催化剂在柔性碳膜上制备高结晶的GaN纳米线的可行性,并研究其场发射特性及机理。采用非贵金属Cu替代贵金属Au作为催化剂,在柔性碳膜上制备了直径为20~100 nm、长度为3~15μm的高结晶的GaN纳米线,并通过工艺参数对其结构与尺寸进行调控,得到GaN纳米线薄膜的催化生长机制。通过对其场发射特性进行研究,发现其场发射性能与其纳米结构紧密相关,催化剂厚度以及薄膜弯曲状态可显著影响其场发射性能。结果表明,采用Cu作为催化剂所制备的GaN纳米线柔性薄膜的场发射电流具有较好的稳定性。该研究为GaN纳米线的低成本制备方法提供了可借鉴思路,同时也为场发射柔性器件的制作提供了可行的技术手段。

SiC基GaN上多晶金刚石散热膜生长及其影响

通过微波等离子体化学气相沉积(MPCVD)法,在SiC基GaN高电子迁移率晶体管(HEMT)异质结构材料上生长多晶金刚石散热膜,采用光学显微镜(OM)、拉曼光谱、非接触霍尔测试系统、X射线衍射(XRD)和扫描电子显微镜(SEM)对生长样品进行表征,研究了生长温度、多晶金刚石散热膜厚度对GaN HEMT异质结构材料性能的影响。测试结果表明,当多晶金刚石生长温度为625℃,散热膜厚度为20μm时,GaN材料载流子迁移率降低9.8%,载流子浓度上升5.3%,(002)衍射峰半高宽增加40%。生长温度越高,金刚石散热膜的生长速率越快。当金刚石散热膜厚度相差不大时,生长温度越高,GaN所受拉应力越大,材料电特性衰退越明显。多晶金刚石高温生长过程中,金刚石引入的应力未对GaN结构产生破坏作用,GaN材料中没有出现孔洞等缺陷。

GaN薄膜的太赫兹光谱响应研究

目前,太赫兹时域光谱(THz-TDS)已经成为研究固体光学参数及色散关系的有效工具。采用太赫兹时域光谱仪对纤锌矿结构GaN薄膜在0~8.0 THz范围内的吸收光谱、介电常数、折射率以及介电损耗等进行了研究。研究结果表明,频率为4.65 THz的太赫兹响应是由GaN的E2(low)声子振动模式主导的,获得的低频介电常数8.9和高频介电常数6.0与理论值接近;进一步研究了频率在4.24~4.40 THz之间的太赫兹介电常数响应谱,获得的GaN薄膜的中心振动频率与太赫兹吸收光谱一致,介电损耗值很小且逐渐趋近于0,表明GaN有良好的介电特性。得出的结论拓展了GaN基电子元器件在THz波段中的应用,对进一步研究GaN基电子元器件在THz波段的质量与可靠性具有借鉴意义。

GaN衬底上ZnS纳米薄膜的结构、光学和电学特性

采用脉冲激光沉积技术在GaN衬底上制备了ZnS纳米薄膜。通过XRD和SEM对薄膜结晶情况和截面结构进行了表征,并测量了ZnS纳米薄膜的透射光谱和ZnS/GaN异质结的Ⅰ-Ⅴ特性曲线。ZnS纳米薄膜在可见光区的透过率较高,平均透过率在80%以上,经过退火处理,透过率增大。Ⅰ-Ⅴ特性曲线表明,ZnS/GaN形成了异质结,具有和普通二极管相似的整流特性。在正向偏压下电流随着电压的增加而增大。退火处理后异质结的导通电压减小。这些特性表明,ZnS纳米薄膜在无人机载传感器和航空电子系统中的光电二极管、光电探测器、光伏电池等领域有着潜在的应用价值。

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

多孔GaN薄膜的制备与光学性能研究

将表面沉积有金纳米颗粒的GaN薄膜在H 2与N 2的混合气氛下进行高温退火,成功制备了多孔GaN薄膜。多孔GaN薄膜的表面形貌可通过退火温度、退火时间及金沉积时间等参数进行调控。利用高分辨X射线衍射(HRXRD)和拉曼光谱表征了不同GaN结构的晶体质量,与平面GaN薄膜相比,多孔GaN薄膜的位错密度和残余应力均有所降低,在退火温度为1000℃时其位错密度最小,应力的释放程度较大。采用光致发光(PL)光谱表征了其光学性质,与平面GaN薄膜相比,多孔GaN薄膜的发光强度显著提高,这可归因于多孔结构的孔隙率增大,有效增加了光的散射能力。此外,通过电化学工作站测试了不同GaN结构的光电流密度,结果表明,具有更大比表面积的多孔GaN薄膜在作为工作电极时,光电流密度是平面GaN薄膜的2.67倍。本文通过高温刻蚀手段成功制备了多孔GaN薄膜,为GaN外延层晶体质量与光学性能的提升及在光电催化等领域中的应用提供了一定的理论指导。

Effect of TMGa flux on GaN films deposited on Ti coated on glass substrates at low temperature

Highly c-axis-oriented GaN films were deposited on Ti coated glass substrates using low temperature electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition system(ECR-PEMOCVD)with trimethyl gallium(TMGa)as gallium source.The influence of TMGa flux on the properties of GaN films were systematically investigated by reflection high energy electron diffraction(RHEED),X-ray diffraction analysis(XRD),atomic force microscopy(AFM)and Raman scattering.The GaN film with small surface roughness and high c-axis preferred orientation was successfully achieved at the optimized TMGa flux of 1.0 sccm.The ohmic contact characteristic between GaN and Ti layer was clearly demonstrated by the near-linear current-voltage(I-V)curve.The GaN/Ti/glass structure has great potential to dramatically improve the scalability and reduce the cost of solid-state lighting light emitting diodes.

生长在图形化蓝宝石衬底上的GaN薄膜光学性质研究

图形化蓝宝石衬底(PSS)技术作为提高LED发光效率的方法之一,一直备受关注。研究了PSS上生长的GaN薄膜的表面形貌、吸收光谱、红外光谱、拉曼散射(Raman)和太赫兹光谱等,并与常规蓝宝石衬底上的GaN外延薄膜进行对比。结果表明,PSS上生长的GaN外延薄膜的表面形貌、光提取效率(LEE)得到明显改善。此研究成果对进一步提高GaN基短波(蓝/紫光)发光二极管(LED)的发光效率具有一定的借鉴意义,为进一步拓展PSS器件的太赫兹响应提供了依据。