Plasma assisted molecular beam epitaxial growth of GaN with low growth rates and their properties

A systematic investigation on PA-MBE grown GaN with low growth rates(less than 0.2μm/h)has been conducted in a wide growth temperature range,in order to guide future growth of sophisticated fine structures for quantum device applications.Similar to usual growths with higher growth rates,three growth regions have been revealed,namely,Ga droplets,slightly Ga-rich and N-rich 3D growth regions.The slightly Ga-rich region is preferred,in which GaN epilayers demonstrate optimal crystalline quality,which has been demonstrated by streaky RHEED patterns,atomic smooth surface morphology,and very low defect related yellow and blue luminescence bands.The growth temperature is a critical parameter to obtain high quality materials and the optimal growth temperature window(~700-760℃)has been identified.The growth rate shows a strong dependence on growth temperatures in the optimal temperature window,and attention must be paid when growing fine structures at a low growth rate.Mg and Si doped GaN were also studied,and both p-and n-type materials were obtained.     

射频等离子体辅助MBE生长GaN及Mg掺杂的光致发光

采用射频等离子体辅助分子束外延(RF plasma-assisted MBE)系统生长非故意掺杂GaN和p型GaN,并且通过室温和低温光致发光(PL)谱测试研究了材料的发光特性及与杂质态的关系,对于GaN外延层出现的黄带发光进行分析。结果表明,富Ga条件下生长的GaN材料特性要优于富N生长的材料;非故意掺杂的富Ga样品中出现的黄带发光(YL)与GaN中生成能最低的氮空位(V_N)缺陷有关;不同的Mg掺杂浓度对样品的PL特性有较大的影响;结合Hall效应测量结果,认为在Mg重掺杂的样品中出现的黄带发光,与GaN的自补偿效应以及重掺杂导致的晶体质量下降有关。     

Influence of AlN Buffer Thickness on GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

Hexagonal GaN is grown on a Si(111) substrate with AlN as a buffer layer by gas source molecular beam epitaxy (GSMBE) with ammonia. The thickness of AlN buffer is changed from 9 to 72nm. When the thickness of AlN buffer is 36nm, the surface morphology and crystal quality of GaN is optimal. The in-situ reflection high energy electron diffraction (RHEED) reveals that the transition to a two-dimensional growth mode of AlN is the key to the quality of GaN. However, the thickness of AlN buffer is not so critical to the residual in-plane tensile stress in GaN grown on Si(111) by GSMBE for AlN thickness between 9 to 72nm.     

Control of polarity of heteroepitaxial ZnO films by interface engineering

Control of polarity of heteroepitaxial ZnO films has been examined by interface engineering. ZnO films were grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN template and c-plane sapphire substrates. Polarity of all the samples is determined by coaxial impact collision ion scattering spectroscopy. Zn- and O-polar ZnO films have successfully grown by Zn- and O-plasma pre-exposures on Ga-polar GaN templates prior to ZnO growth. High-resolution transmission electron microscopy revealed formation of a single-crystalline monoclinic Ga₂O₃ interface layer by O-plasma pre-exposure on Ga-polar GaN templates, while no interface layer was observed for Zn pre-exposed ZnO films. The polarity of ZnO films grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. Fabrication of polarity inverted ZnO heterostructure has been studied: polarity of ZnO films on Ga-polar GaN templates was changed from Zn-polar to O-polar by inserting a MgO layer. High-resolution transmission electron microscopy revealed atomically flat interfaces at both lower and upper ZnO/MgO interfaces and no inversion domain boundaries were detected in the upper ZnO layer.     

Microstructural and compositional characteristics of GaN films grown on a ZnO-buffered Si (111) wafer

Polycrystalline GaN thin films have been deposited epitaxially on a ZnO-buffered (111)-oriented Si substrate by molecular beam epitaxy. The microstructural and compositional characteristics of the films were studied by analytical transmission electron microscopy (TEM). A SiO(2) amorphous layer about 3.5 nm in thickness between the Si/ZnO interface has been identified by means of spatially resolved electron energy loss spectroscopy. Cross-sectional and plan-view TEM investigations reveal (GaN/ZnO/SiO(2)/Si) layers exhibiting definite a crystallographic relationship: [111](Si)//[111](ZnO)//[0001](GaN) along the epitaxy direction. GaN films are polycrystalline with nanoscale grains ( approximately 100 nm in size) grown along [0001] direction with about 20 degrees between the (1l00) planes of adjacent grains. A three-dimensional growth mode for the buffer layer and the film is proposed to explain the formation of the as-grown polycrystalline GaN films and the functionality of the buffer layer.     

“Anomalous” pseudodielectric function of GaN: Experiment, modelling and application to the study of surface properties

Studying GaN films exposed to Ar plasma by spectroscopic ellipsometry and reflectance, we found an “anomalous” pseudodielectric function (PDF) for which the imaginary part is significantly higher as compared to GaN, while the real part of the PDF remains close to the value for GaN. In addition, a higher reflectance at low angles of incidence was observed. The data are explained in terms of a thin highly absorbing surface layer arising due to non-stoichiometry in the near-surface region. Comparison to samples grown by molecular beam epitaxy shows that similar mechanisms are responsible for optical properties of the surfaces of films obtained under Ga-rich conditions.     

国产SiC衬底上利用AIN缓冲层生长高质量GaN外延薄膜

采用高温AlN作为缓冲层在国产SiC衬底上利用金属有机物化学气相外延技术生长GaN外延薄膜.通过优化AlN缓冲层的生长参数得到了高质量的GaN外延薄膜,其对称(0002)面和非对称(1012)面X射线衍射摇摆曲线的半峰宽分别达到130 arcsec和252 arcsec,这是目前报道的在国产SiC衬底上生长GaN最好的...     

SiO2对多层GaN外延片热应力分布影响的仿真分析

为了研究SiO2对多层结构GaN外延片的热应力的影响,以直径d为40 mm的GaN外延片为研究对象,利用有限元分析法分别对蓝宝石/AlN/GaN和蓝宝石/SiO2/AlN/GaN这两种光阴极组件外延片表面热应力进行理论计算和仿真。在其他结构参数相同的情况下,分别分析了两种光阴极组件外延片径向和厚度方向的应力分布,分析了外延片热应力分布及影响因素。分析结果显示:在1 200 ℃的生长温度下,径向区域内的热应力分布比较均匀,厚度方向的热应力均在衬底和外延层的界面上发生突变。最后分析了外延片生长温度、蓝宝石衬底和GaN、AlN过渡层厚度对表面热应力的影响。     

Comparison study of GaN films grown on porous and planar GaN templates

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition(MOCVD)-GaN template by halide vapor phase epitaxy(HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence(PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.     

AlN缓冲层对Si基GaN外延薄膜性质的影响

采用金属有机化合物化学气相沉积（MOCVD）方法制备了不同AlN缓冲层厚度的GaN样品,研究了AlN缓冲层厚度对GaN外延层的应力、表面形貌和晶体质量的影响。研究结果表明：厚度为15 nm的AlN缓冲层不仅可以有效抑制Si扩散,而且还给GaN外延层提供了一个较大的压应力,避免GaN薄膜出现裂纹。在该厚度AlN缓冲层上制备的GaN薄膜表面光亮、无裂纹,受到的张应力为0.3 GPa,（0002）和（1012）面的高分辨X射线衍射摇摆曲线峰值半高宽分别为536 arcsec和594 arcsec,原子力显微镜测试得到表面粗糙度为0.2 nm。     

Si衬底GaN基LED的结温特性

结温是发光二极管的重要参数之一,它对器件的内量子效率、输出功率、可靠性及LED的其他一些性能有很大的影响。首次报道Si衬底GaN基LED的结温特性。利用正向压降法测量Si衬底上GaN基LED的结温,通过与蓝宝石衬底上GaNLED的结温比较,发现Si衬底GaNLED有更低的结温,原因归结为Si有更好的导热性。同时也表明:用Si作GaNLED的衬底在大功率LED方面具有更大的应用潜力。     

SiC衬底上异质外延GaN薄膜XPS谱和PL谱研究

利用X射线光电子能谱,对在SiC衬底上采用MOCVD异质外延的未故意掺杂的GaN进行N、Ga组份测试,同时用光致发光技术对样品进行发光特性的研究.结果表明,随着GaN薄膜中Ga百分含量逐渐减小,室温下黄光输出峰值强度却逐渐增加.因此,在Ga含量相对低的GaN薄膜中容易形成Ga空位(即Ga空位浓度较高),而此时,黄光辐射强度单调递增证明,黄光辐射与VGa密切相关.     

Observation of ferromagnetism at room temperature for Cr ions implanted ZnO thin films

Single crystalline ZnO films were grown on c-plane GaN/sapphire (0 0 0 1) substrates by molecular beam epitaxy. Cr⁺ ions were implanted into the ZnO films with three different doses, i.e., 1 × 10¹⁴, 5 × 10¹⁵, and 3 × 10¹⁶ cm⁻². The implantation energy was 150 keV. Thermal treatment was carried out at 800 °C for 30 s in a rapid thermal annealing oven in flowing nitrogen. X-ray diffraction (XRD), atomic force microscopy, Raman measurements, transmission electron microscopy and superconducting quantum interference device were used to characterize the ZnO films. The results showed that thermal annealing relaxed the stress in the Cr⁺ ions implanted samples and the implantation-induced damage was partly recovered by means of the proper annealing treatment. Transmission electron microscopy measurements indicated that the first five monolayers of ZnO rotated an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. The magnetic-field dependence of magnetization of annealed ZnO:Cr showed ferromagnetic behavior at room temperature.     

Microstructural study of MBE-grown ZnO film on GaN/sapphire (0001) substrate

Single crystalline ZnO film is grown on GaN/sapphire (0001) substrate by molecular beam epitaxy. Ga₂O₃ is introduced into the ZnO/GaN heterostructure intentionally by oxygen-plasma pre-exposure on the GaN surface prior to ZnO growth. The crystalline orientation and interfacial microstructure are characterized by X-ray diffraction and transmission electron microscopy. X-ray diffraction analysis shows strong c-axis preferred orientation of the ZnO film. Cross-sectional transmission electron microscope images reveal that an additional phase is formed at the interface of ZnO/GaN. Through a comparison of diffraction patterns, we confirm that the interface layer is monoclinic Ga₂O₃ and the main epitaxial relationship should be and .      

Microstructural properties of GaN grown on a Si(110) substrate by gas-source molecular beam epitaxy: Dependence on the ammonia flux

The microstructural properties of a GaN thin film grown on a Si(110) substrate under various ammonia (NH₃)-flux conditions were observed to study growth mode and defect evolution. The surface flatness of GaN thin films was improved with the increase of the NH₃ flux while the thickness was decreased by increasing the NH₃ flux. In addition, the crystalline quality of the GaN film grown under the lower NH₃ flux (100 sccm) was better than that of the film under the higher NH₃ flux (400 sccm). The different dislocation behaviors depending on NH₃ fluxes were observed; the low density of dislocations was measured and most of dislocations penetrating the thin film was mixed- and edge-type dislocations when GaN was grown under the low NH₃ flux condition while the high density of dislocation and many mixed- and screw-type dislocations penetrating the film were observed in the GaN film grown under the high NH₃ flux. These phenomena are demonstrated by using a kinetic model related to the role of NH₃.     

Polarity control and residual strain in ZnO epilayers grown by molecular beam epitaxy on (0001) GaN/sapphire

We report on the polarity control of ZnO grown by plasma assisted molecular beam epitaxy on Ga polar (0001) GaN/sapphire templates simply via the oxygen‐to‐Zn (VI/II) ratio during the growth of a thin nucleation layer at 300 °C. Following Zn pre‐exposure, the ZnO layers nucleated with low VI/II ratios (<1.5) exhibited Zn‐polarity. Those nucleated with VI/II ratios above 1.5, exhibited O‐polarity. Supported by scanning transmission electron microscopic imaging, we have unequivocally demonstrated that polarity inversion takes place without formation of any vertical inversion domains and within one monolayer of presumably non‐stoichiometric GaO_x formed at the ZnO/GaN interface. A direct correlation between polarity and strain sign of ZnO layers has been found. The Zn‐polar ZnO layers were under tensile biaxial strain, whereas the O‐polar material exhibited compressive strain. Moreover, the amount of residual strain varied linearly with VI/II ratio used during the low‐temperature nucleation layer growth. Strain control with VI/II ratio has been explained by the potential formation of Zn interstitials.     

Fabrication of freestanding GaN microstructures using AlN sacrificial layers

The fabrication of freestanding GaN microstructures using AlN sacrificial layers (SLs) is reported. GaN layers were grown by plasma assisted molecular beam epitaxy (PAMBE) on polycrystalline AlN sacrificial layers that have been deposited at 600 °C. Isotropic wet chemical etching of the AlN film released GaN microbridges and – cantilevers. The stress gradient and the compressive stress in the GaN‐film was extracted by analysis of the relation between beam geometry and displacement. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Metal catalyst-assisted growth of GaN nanowires on graphene films for flexible photocatalyst applications

We report the growth of high-areal-density GaN nanowires on large-size graphene films using a nickel (Ni) catalyst-assisted vapor-liquid-solid (VLS) method. Before the nanowire growth, the graphene films were prepared on copper foils using hot-wall chemical vapor deposition and transferred onto SiO₂/Si substrates. Then, for catalyst-assisted VLS growth, Ni catalyst layers with thickness of a few nanometers were deposited on the graphene-coated substrates using a thermal evaporator. We investigated the effect of the Ni catalyst thickness on the formation of GaN nanowires. Furthermore, the structural and optical characteristics of GaN nanowires were investigated using X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The GaN nanowires grown on graphene films were transferred onto polymer substrates using a simple lift-off method for applications as flexible photocatalysts. Photocatalysis activities of the GaN nanowires prepared on the flexible polymer substrates were investigated under bending conditions.     

Effect of thickness on the microstructure of GaN films on Al2O3 (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 110-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.     

Effect of ZnO sacrificial layer thickness on the structure and optical properties of GaN nanoparticles prepared by RF magnetron sputtering

GaN nanoparticles were prepared on sapphire (0001) substrates with ZnO sacrificial layers by self assembly of Ga₂O₃ films in their reaction with NH₃. ZnO sacrificial layers with different thicknesses and Ga₂O₃ films were deposited on sapphire substrates in turn by a radio frequency (RF) magnetron sputtering system. Nitridation of the Ga₂O₃ films was then carried out in a quartz tube furnace. The effect of ZnO sacrificial layer thickness on the structure and optical properties of nanoparticles prepared by RF magnetron sputtering were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL). GaN nanoparticles with ZnO sacrificial layers of different thicknesses possess hexagonal wurtzite crystal structure and have a preferred orientation with c axis perpendicular to the sapphire substrates. XRD, SEM, and AFM results reveal that the better-crystallinity, uniform, and well-dispersed GaN nanoparticles (～30 nm) without agglomeration were obtained with a ZnO sacrificial layer 300-nm thick. The PL result reveals that the optical properties of the GaN nanoparticles are improved with a ZnO sacrificial layer 300-nm thick. Therefore, we suggest that a ZnO sacrificial layer 300-nm thick is the most suitable condition for obtaining better-quality GaN nanoparticles with good luminescence performance. Moreover, the mechanism of the formation of GaN nanoparticles with ZnO sacrificial layers is also discussed.