SiC缓冲层用于改善硅基氮化镓薄膜的质量研究

用脉冲激光沉积法在硅衬底上沉积GaN薄膜,为了减小Si衬底与GaN薄膜之间的热失配和晶格失配引入SiC缓冲层.脉冲激光沉积后的GaN薄膜是非晶结构,将样品在氨气氛围中在950℃下退火15min.得到结晶的GaN薄膜.并用X 射线衍射、原子力显微镜、傅立叶红外吸收谱、光致发光谱研究了SiC缓冲层对GaN薄膜的结晶、形貌和光学性质的影响.     

GaN基半导体上BiFeO_3薄膜的生长与性能研究

采用脉冲激光沉积法在(0001)取向的GaN以及AlGaN/GaN调制掺杂结构上制备了(111)取向的BiFeO3(BFO)薄膜。首先在导电氧化物SrRuO3和TiO2缓冲层包覆的GaN上制备了BFO薄膜,分析了在GaN上生长的BFO薄膜的面外取向、外延关系、表面形貌以及电学性能等性质。然后,在AlGaN/GaN调制掺杂结构上采用TiO2缓冲层生长了BFO薄膜,并采用光刻工艺分别在AlGaN表面制备Ti/Al/Ti/Au欧姆电极和BFO表面制备Ni/Au肖特基电极以形成二极管结构。C-V测试表明,由于BFO铁电薄膜极化的作用,BFO/TiO2/AlGaN/GaN结构具有1 V左右的逆时针窗口。     

MgO缓冲层对PZT/AlGaN/GaN异质结构电学性能的影响

采用脉冲激光沉积(PLD)技术,以MgO作为缓冲层,在AlGaN/GaN半导体异质结构上沉积了Pb(Zr0.52T0.48) O3 (PZT)铁电薄膜,从而形成金属-铁电-介质-半导体结构(MFIS).XRD扫描结果表明,通过MgO缓冲层对界面结构的优化,实现了PZT薄膜沿(111)面择优取向生长.电流-电压(I-V ...     

TMGa流量对玻璃衬底上低温沉积GaN的影响

采用电子回旋共振-等离子体辅助增强金属有机物化学气相沉积两步生长法在玻璃衬底上低温沉积GaN薄膜。利用原位反射高能电子衍射、X射线衍射、室温透射光谱和原子力显微镜,研究了不同TMGa流量条件下沉积的GaN薄膜的结晶性、光学性质和表面形貌。结果表明,TMGa流量对GaN薄膜质量影响很大,TMGa流量约为1.4cm3/min(标准状态)条件下沉积的GaN薄膜结晶性较好,呈高度c-轴择优取向,420～1110nm波长光区内透过率超过90%,薄膜表面由大小均匀的亚微米量级表面岛按一致取向堆砌而成。     

金属缓冲层上生长GaN薄膜的结构、发光性能及其生长机理

为了解决硅衬底与GaN之间的晶格失配和热失配问题,实验尝试采用常压化学气相沉积法(APCVD),分别以金属镓(Ga)和氨气(NH3)为镓源和氮源,在加入A1、Au/Al两种金属缓冲层和不加缓冲层的硅衬底上生长氮化镓(GaN)薄膜.采用高分辨X射线衍射仪(HRXRD)、X-ray能谱仪(EDS)、场发射扫描电子显微镜(F...     

AlN缓冲层对提高硅基GaN薄膜质量的作用机理及其研究进展

AlN作为中间层可以有效提高硅基GaN的晶体质量。本文对于AlN作为形核层和插入层提高GaN晶体质量的机理进行分析和总结，并给出AlN的最佳生长条件。     

AlN/Si衬底上ZnO薄膜的择优取向生长

当ZnO薄膜直接沉积在Si衬底上时,由于ZnO与Si的晶格失配度大,不易于获得高质量的ZnO薄膜.因此,选择合适的衬底材料沉积ZnO薄膜,对提高其质量非常重要.本文采用射频磁控溅射法,通过在Si(100)衬底上预沉积AlN作为ZnO薄膜生长的缓冲层,获得了择优取向的ZnO薄膜.我们还讨论了ZnO薄膜在AlN/Si衬底上的取向生长机理.     

预成核对蓝宝石衬底上生长氮化镓低温层的影响北大核心CSCD

金属有机物化学气相沉积(MOCVD)技术以气相源的热分解反应作为基础,其适合规模化生产,是现今生长半导体材料的主要制备方式。在MOCVD生长GaN的过程中,衬底表面初始条件直接影响到材料成核与生长,因此对于外延生长非常关键。本论文研究了GaN外延生长过程中蓝宝石衬底的表面预成核工艺对GaN低温成核的影响。通过对比未处理样品和高温预通TMGa、高温预通TMGa和NH3预成核以及高温预通TMAl和NH3预成核的样品上生长的低温层退火后的形貌,我们发现高温预成核形成的成核点有利于吸引其周围气相源并入,并降低成核岛的密度。结合光学实时反射率监测气相沉积中晶粒的成核过程,进一步横向比较可发现由于高温时AlN更稳定,预成核的效果更好,对退火以后GaN小岛形貌影响更加显著。X射线衍射表征成核层的晶体质量,发现预成核工艺可将退火后成核层的(002)衍射峰半高宽从1636 arcsec降低到最低1088 arcsec。通过对比分析,我们认为高温预成核工艺的优点可能来源于其可以改善成核初期小岛的晶向。这些研究为进一步提高GaN外延质量提供了新的工艺思路。     

SiNx介质薄膜制备及其对GaN表面金刚石形核生长的影响

SiNx作为GaN和金刚石异质结构的中间层，不仅是下层GaN材料的保护层，也是上层金刚石的形核生长层，因此SiNx介质薄膜对于GaN表面合成高质量金刚石具有重要的意义。研究分别采用低压化学气相沉积（LPCVD）和磁控溅射（MS）方法在GaN-Si衬底上制备SiNx介质薄膜。利用扫描电镜、傅立叶红光光谱、X射线衍射、激光拉曼等技术对SiNx薄膜的表面形貌、晶体结构和表面官能团等进行分析。结果表明，采用LPCVD镀制的非晶态SiNx介质薄膜经籽晶播种、形核生长金刚石后，金刚石/SiNx/GaN界面完整致密；采用MS制备的SiNx介质薄膜呈晶态特征，对应的界面出现明显的刻蚀坑。沉积方式会影响SiNx薄膜的晶体结构和微观形貌，高致密度的非晶态结构有利于金刚石层快速形核生长，对于构建金刚石基GaN结构更为有利。     

以ZnO为缓冲层制备硅基氮化镓薄膜

用脉冲激光沉积法(PLD)先在600℃的Si(111)衬底上沉积ZnO薄膜,然后用磁控溅射法再沉积GaN薄膜。直接沉积得到的GaN薄膜是非晶结构,将样品在氨气氛围中在850、900、950℃下退火15min得到结晶的GaN薄膜。用X射线衍射(XRD)、傅立叶红外吸收谱(FTIR)、光致发光谱(PL)和扫描电子显微镜(SEM)研究了ZnO缓冲层对GaN薄膜的结晶和形貌的影响。     

The growth of GaN films by alternate source gas supply hot-mesh CVD method

Gallium nitride (GaN) films and Aluminium nitride (AlN) layers were deposited on SiC/Si (111) substrates by an alternating source gas supply or an intermittent supply of a source gas such as ammonia (NH₃), trimethylgallium (TMG) or trimethylaluminum (TMA) in a hot-mesh chemical vapor deposition (CVD) apparatus. The AlN layer was deposited as a buffer layer using NH₃ and TMA on a SiC layer grown by carbonization on Si substrates using propane (C₃H₈). GaN films were grown on an AlN layer by a reaction between NH_x radicals generated on a ruthenium (Ru) coated tungsten (W)-mesh and TMG molecules. An alternating source gas supply or an intermittent supply of one of the source gases during the film growth are expected to be effective for the suppression of gas phase reactions and for the enhancement of precursor migration on the substrate surface. By the intermittent supply of alkylmetal gas only during the growth of the AlN layer, the defect generation in the GaN films was reduced. GaN film growth by intermittent supply on an AlN buffer layer, however, did not lead to the improvement of the film quality.     

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.     

Growth of highly c-axis oriented aluminum nitride thin films on β-tantalum bottom electrodes

We have investigated the influence of tantalum (Ta) bottom electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films. AlN thin films and Ta electrodes were prepared by using rf magnetron sputtering method. The crystal structure of the Ta electrodes was tetragonal (β-Ta, a metastable phase) at room temperature. The crystallinity and orientation of the AlN thin films and Ta electrodes strongly depended on sputtering conditions. Especially, the crystallinity and crystal orientation of the Ta electrodes were influenced by their film thickness and the substrate temperature. When the thickness of the Ta bottom electrodes was 200 nm and the substrate temperature was 100 °C, the AlN thin films indicated high c-axis orientation (the full width at half maximum of rocking curve of 3.9°). The crystal orientation of the AlN film was comparable to that of AlN thin films deposited on face centered cubic (fcc) lattice structure metal, such as Au, Pt and Al, bottom electrodes.     

Characterization of reactively sputtered c-axis orientation (Al, B)N films on diamond

In this research, we demonstrated the viability of oriented AlN layer that incorporated BN to enhance the texturing. Wurtzite (Al, B)N films were deposited on a diamond wafer (diamond film on Si wafer) by a co-sputtering technique. The preferred orientation structure is sensitive to sputtering control factors. The relationship between the microstructures and process conditions were examined with XRD, TEM and AFM analysis. The cross-section TEM images showed that amorphous and randomly aligned structures were produced in the initial sputtering period, but the higher c-axis orientation structure formed as the sputtering time increased. The thickness of the amorphous and randomly aligned layer decreased with increasing sputtering power, nitrogen concentration, substrate temperature and bias voltage. As the thickness of the amorphous and the randomly aligned layer decreased, an (Al,B)N film with higher film quality than AlN was observed.     

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.     

Growth of GaN and AlN thin films by laser induced molecular beam epitaxy

Hexagonal GaN and AlN thin films were grown by laser induced molecular beam epitaxy using Al or Ga metal as target material and N₂as nitrogen source. The films were deposited on sapphire (0001) and SiC (0001) substrates. Epitaxial growth of GaN has been achieved at 730°C and 10⁻³ mbar N₂ pressure. The AlN films were polycrystalline with predominant (0001) orientation.     

The effects of LT AlN buffer thickness on the properties of high Al composition AlGaN epilayers

To fabricate nitride-based ultraviolet optoelectronic devices, a deposition process for high-Al-composition AlGaN (Al content > 50%) films with reduced dislocation densities must be developed. This paper describes the growth of high-Al-composition AlGaN film on (0001) sapphire via a LT AlN nucleation layer by low pressure metalorganic chemical vapor deposition (LPMOCVD). The influence of the low temperature AlN buffer layer thickness on the high-Al-content AlGaN epilayer is investigated by triple-axis X-ray diffraction (TAXRD), scanning electron microscopy (SEM), and optical transmittance. The results show that the buffer thickness is a key parameter that affects the quality of the AlGaN epilayer. An appropriate thickness results in the best structural properties and surface morphology.     

Deposition of c-axis orientation aluminum nitride films on flexible polymer substrates by reactive direct-current magnetron sputtering

Aluminum nitride (AlN) piezoelectric thin films with c-axis crystal orientation on polymer substrates can potentially be used for development of flexible electronics and lab-on-chip systems. In this study, we investigated the effects of deposition parameters on the crystal structure of AlN thin films on polymer substrates deposited by reactive direct-current magnetron sputtering. The results show that low sputtering pressure as well as optimized N₂/Ar flow ratio and sputtering power is beneficial for AlN (002) orientation and can produce a highly (002) oriented columnar structure on polymer substrates. High sputtering power and low N₂/Ar flow ratio increase the deposition rate. In addition, the thickness of Al underlayer also has a strong influence on the film crystallography. The optimal deposition parameters in our experiments are: deposition pressure 0.38 Pa, N₂/Ar flow ratio 2:3, sputtering power 414 W, and thickness of Al underlayer less than 100 nm.     

Epitaxial growth of gallium nitride by ion-beam-assisted evaporation

The epitaxial growth of gallium nitride thin film was obtained on the inclined Si(111) substrates by the process of ion-beam-assisted evaporation (IBAE) at the low temperature of 500 °C. The film composition determined by Rutherford backscattering spectrometry shows that the synthesized film is a stoichiometric nitride. The epitaxial quality of GaN film is enhanced by minimizing the bombardment-induced film damage by decreasing the ion flux. However, the crystallinity of the GaN film becomes very poor when the ion flux is not sufficient to densify the film. The optimum flux ratio of N⁺₂ to Ga and the energy of incident N⁺₂ ions for the epitaxial growth were found to be 3.4 and 50 eV, respectively. The GaN film deposited on the 4 °-inclined Si (111) with respect to substrate surface shows much better crystalline quality compared with that on the 0 ° inclined Si(111) due to many stable nucleation sites. A thin amorphous layer exists at the interface between GaN and Si(111) substrate and acts as a buffer zone enabling the subsequent epitaxial growth of GaN by relaxing the large misfit strain (23%) in the early stage of film growth. The epitaxial GaN film shows an orientational relation with the Si(111) substrate.     

Dielectric properties and thickness metrology of strain engineered GaN/AlN/Si (111) thin films grown by MOCVD

Maturity of silicon nanoelectronics and the high quality of 300 mm epi-Si wafers make these substrates an ideal choice for the growth of high quality III-Nitride devices. The results of our substrate engineering technique, which involves implantation of nitrogen into Si through an AlN thin film, have shown to simultaneously and significantly reduce the dislocation density and macro-cracks in epitaxially grown 2 μm GaN films. In this study, high quality strain engineered GaN films were grown by metalorganic chemical vapor deposition (MOCVD) and spectroscopic ellipsometry was used to characterize the dielectric properties, thickness, and stress of the complex structure. The uniaxial, anisotropic dielectric functions of wurtzite GaN and AlN were determined for the processes used in this study, and using this information, the thickness of each layer was determined in the completed film stack. IR spectroscopic ellipsometry (IRSE) was used as the non-destructive characterization technique to identify the IR sensitive phonon modes in AlN. The stress evolution in the films was investigated as a function of the phonon frequency shift and the broadening of the phonon modes. The results obtained by IRSE were further complemented by high resolution X-ray diffraction (HRXRD) and Raman scattering measurements.