Biphasic GaN nanowires: Growth mechanism and properties

Catalyst-free vapor-solid nanowire growth has been used to produce novel multiphase zinc-blende/wurtzite gallium nitride nanowires. Multiphase nanowire growth occurred at nanoscale nucleation sites on platelets of gallium nitride. Growth temperature has been shown to exert a strong influence on nucleation site formation. Scanning electron microscopy (SEM) was used to characterize the matrix from which the nanowires grew. Nanowires were characterized with transmission electron microscopy (TEM).     

Ion implantation doping of OMCVD grown GaN

Doping by ion implantation using Si, O, Mg, and Ca has been studied in single crystal semi-insulating and n-type GaN grown on a-sapphire substrates. The n-and p-type dopants used in this study are Si and O; Mg and Ca, respectively. Room temperature activation of Si and O donors has been achieved after 1150°C annealing for 120 s. The activation of Mg and Ca acceptors is too low to measure at both room temperature and 300°C. Using higher doses to achieve a measurable p-type conduction increases the amount of damage created by the implantation. Rutherford back scattering measurements on this material indicate that the damage is still present even after the maximum possible heat treatment. Secondary ion mass spectrometry measurements have indicated a redistribution in the measured profiles of Mg due to annealing.     

SiNx 掩膜对GaN 外延薄膜性质的影响

研究了纳米量级的多孔 SiNx 插入层的生长位置对高质量GaN外延薄膜性质的影响。测量结果表明：当把SiNx 插入层生长在GaN 粗糙层上,能够得到最好的晶体质量;SiNx 插入层的生长位置对GaN 薄膜的应变大小基本没有影响;然而,插入层的位置改变了薄膜中的本征载流子浓度。     

Optical Properties of Nanocrystalline GaN Films Prepared by Annealing Amorphous GaN in Ammonia

Nanocrystalline GaN films were prepared by thermal treatment of amorphous GaN films under flowing NH₃ at a temperature of 600°C to 950°C for 1 h to 2 h. X-ray diffraction and field-emission scanning electron microscopy confirmed the formation of high-crystal-quality hexagonal GaN films with preferential (002) orientation. The photoluminescence spectrum showed a sharp peak near the band gap emission located at 368 nm and a broad blue peak centered at 430 nm. Five first-order Raman modes near ∼143 cm⁻¹, 535 cm⁻¹, 555 cm⁻¹, 568 cm⁻¹, and 731 cm⁻¹ with two new additional Raman peaks at 257 cm⁻¹ and 423 cm⁻¹ were observed. The origin of these new Raman peaks is discussed briefly.     

Photoluminescence properties of GaN with dislocations induced by plastic deformation

Fresh (a/3)[1120] dislocations on the (1100) prismatic plane were introduced into GaN bulk crystals by plastic deformation at 950–1000°C. In photoluminescence studies at 11 K, the near-band-edge (3.48 eV) luminescence intensity decreased remarkably in the deformed GaN, which was attributed to the introduction of high-density nonradiative recombination centers during plastic deformation. The yellow-band luminescence (2.22 eV) decreased due to plastic deformation, while several luminescence bands centered at 1.79, 1.92, and 2.4 eV developed. The dependence of PL features on deformation and annealing suggests that yellow luminescence is not related to the native structure of edge dislocations in GaN.     

Effects of vacuum annealing on electrical properties of GaN contacts

To understand formation and deterioration mechanisms of Ta/Ti ohmic contacts that were previously developed for p-GaN, the electrical properties of the Ta/Ti contacts, which were deposited on undoped GaN substrates and subsequently annealed in vacuum (where a slash (/) sign indicates the deposition sequence), were studied. The Ta/Ti contacts displayed good ohmic behavior after annealing at a temperature of 800°C for 10 min in vacuum, although the undoped GaN substrates were used. However, deterioration of the present ohmic contacts was observed during room-temperature storage. These contact properties were similar to those observed in the Ta/Ti contacts prepared on p-GaN. Hall-effect measurements revealed that thin n-type conductive layers were found to form on surfaces of both the undoped GaN and p-GaN substrates after annealing at 800°C in vacuum.     

Detailed structural analysis of semiconductors with X-ray scattering

Some of the most important material systems, GaInN alloys and quantum dot structures create interesting and complex challenges for structural analysis. This paper concentrates on the interpretation of the microstructure of both these materials, the former to assess the defect separation and the latter to obtain the shape and composition of the quantum dots. The methods used are based on mapping the X-ray intensity in reciprocal space and simulating proposed models to achieve good agreement with the experimental results. An indication of the reliability of these methods is presented.The simulation of the 0002 reciprocal space map of a 0 0 0 1 orientated InGaN/GaN sample yielded a range of dimensions of the perfect regions between defects of 60 and 220 μm. This comes from careful fitting of the tails of the scattering parallel to the surface plane. The average composition within an InGaAs quantum dot has been determined to a reliability of ±3% and the dimensions of these buried dots evaluated from simulating the reciprocal space maps using the in-plane scattering geometry.     

Investigation of three-step epilayer growth approach of GaN films to minimize compensation

The heteroepitaxial growth of gallium nitride (GaN) films using three distinct growth steps is investigated in terms of improving the electrical properties of the layer. The first step involves the deposition of a fixed quality aluminum nitride (AlN) layer on an a-plane sapphire substrate. The second step is aimed at maximizing the GaN grain size initially grown on the AlN. The third step is aimed at optimizing the surface morphology of the GaN layer. The means of transitioning the growth between steps is investigated and an optimum transition method is reported. Growth parameters investigated include pressure, trimethylgallium molar flow rate, and ratio of group V to group III precursor molar flows. Carrier statistics show a lower level of compensation in films grown at a slower growth rate in the second step and a moderate rate in the third step.     

N-type implantation doping of GaN

Doping characteristics of N/Si and N/Ge co-implanted GaN have been systematically investigated. N-type regions were produced in undoped GaN films by the co-implantation and subsequent annealing with an SiO₂ encapsulation layer at high temperatures. The annealing procedures above 1100 and 1200°C were required to achieve an n-type activation for N/Si and N/Ge co-implanted GaN, respectively. The both samples show effective activation efficiencies of 50% after annealing at 1300°C. However, actual Si activation seems to be much higher than the Ge activation due to the different behaviors of implantation-induced damage.     

The effect of substrate misorientation on the optical, structural, and electrical properties of GaN grown on sapphire by MOCVD

We report the growth and characterization of unintentionally doped GaN on both exact and vicinal (0001) sapphire substrates. The GaN heteroepitaxial layers are grown by metalorganic chemical vapor deposition on c-plane A1₂O₃ substrates either on-axis or intentionally misoriented 2° toward the a-plane (1120) or 5 or 9° toward the m-plane (10 10). The samples are characterized by 300K photoluminescence, cathodoluminescence, and Hall-effect measurements as well as by triple-axis x-ray diffractometry to determine the effect of the misorientation on the optical, electrical, and structural properties of heteroepitaxial undoped GaN. Ten different sample sets are studied. The data reveal enhanced photo-luminescence properties, increased electron mobility, a reduced n-type background carrier concentration, and a somewhat degraded surface morphology and crystalline quality for the misoriented samples compared to the on-axis samples.     

Synthesis of GaN nanowires and nanorods via self-growth mode control

The synthesis of hexagonal wurzite one-dimensional (1D) GaN nanostructures on c-Al₂O₃ substrates was investigated using a thermal chemical vapor deposition (CVD) process. The diameter of the GaN nanostructures was controlled by varying the growth time using a mixture of GaN powder and Ga metal with the ammonia gas reaction. The morphologies of the GaN nanowires and nanorods were confirmed by field emission scanning electron microscopy. The micro-Raman spectroscopy and X-ray scattering measurements indicated that the GaN nanostructures had a hexagonal wurzite structure without any oxide phases. We investigated the difference in the structural properties between the GaN nanowires and nanorods. Deep-level emission bands were not observed in cathodoluminescence measurements from either the GaN nanowires or nanorods, indicating the incorporation of low-level impurities into our 1D GaN nanostructures.     

A面和C面AlGaN/GaN异质结的比较研究

采用MOCVD技术在R面和C面蓝宝石上生长非极性A面和极性C面AlGaN/GaN异质结。分别用X射线衍射仪和原子力显微镜比较了两种材料的结构特性及表面形貌,通过电容-电压测试比较了两种材料的电学特性。研究结果表明,较高浓度的二维电子气的存在使得极性材料在微波功率器件方面更有优势,而非极性材料可以消除与极化相关的电场,更适合应用于光电器件领域。     

A study of temperature field in a GaN heterostructure field-effect transistor

This paper presents a three-dimensional finite element based heat transfer model for a Gallium Nitride-based Heterostructure Field-Effect Transistor (henceforth referred to as GaN HFET). Analyses were carried out to study the distribution of temperature in the HFET under steady-state conditions for two different steady-current inputs. Two different substrates for the HFET, sapphire and silicon carbide (SiC), were studied. The paper discusses the effect of using a heat sink and also that of using reasonable contact resistances on the substrate side of the HFET, on the temperature profile. In all cases, the gate region of the HFET was found to attain the highest temperature. Subsequent experiments to validate the results of the computational analysis were carried out at the Oakridge National Laboratories, Knoxville, and are also presented in this paper.     

Phonon–plasmon coupling in Si doped GaN nanowires

The vibrational properties of silicon doped GaN nanowires with diameters comprised between 40 and 100 nm are studied by Raman spectroscopy through excitation with two different wavelengths: 532 and 405 nm. Excitation at 532 nm does not allow the observation of the coupled phonon–plasmon upper mode for the intentionally doped samples. Yet, excitation at 405 nm results in the appearance of a narrow peak at frequencies close to that of the uncoupled A₁(LO) mode for all samples. This behavior points to phonon–plasmon scattering mediated by large phonon wave-vector in these thin and highly doped nanowires.     

Catalyst-free growth of GaN nanowires

We have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity greater than that of free-standing HVPE-grown GaN, relaxed lattice parameters, and the tendency of nanowires dispersed in solvents to align in response to electric fields. The wires were well separated, 50–250 nm in diameter, and grew to lengths ranging from 2 μm to 7 μm. Transmission electron microscopy indicated that the wires were free of defects, unlike the surrounding matrix layer.     

GaN薄膜表面缺陷密度的提取

在GaN薄膜制备中，薄膜表面缺陷密度提取是改进质量的重要依据。文章通过对若干幅GaN薄膜缺陷图像的分析．提出一种GaN薄膜缺陷密度的提取方法。该方法首先果用闯值分割法二值化背影复杂的GaN薄膜表面图像：然后基于数学形态学方法提取出薄膜表面缺陷的密度；最后给出了薄膜表面缺陷粒径的分布模型。实验结果表明此方法使GaN薄膜表面缺陷提取简单且易于测量，为分析缺陷原因提高薄膜质量起到重要的指导作用。     

A study of cracking in GaN grown on silicon by molecular beam epitaxy

It is observed that GaN layers grown on silicon substrates often crack. The crack characteristics in hexagonal GaN films on Si(111) has been characterized using scanning electron microscopy and Nomarski optical microscopy. The effects of growth temperature, layer thickness, and V/III ratios on the cracking have been analyzed. The critical thickness for crack initiation was estimated using a simple theoretical model and is shown to have good agreement with experimental results. Crack-free GaN on Si(111) of thicknesses greater than one micron is possible by using low growth temperatures.     

Model development of GaN MOVPE growth chemistry for reactor design

The metalorganic vapor phase epitaxy of GaN is complicated by the extensive and pervasive complex gas phase chemistry within the growth system. This gas phase chemistry leads to the high sensitivity of the material properties on the detailed fluid dynamics within the system. Computational fluid dynamics (CFD) based reactor modeling combined with gas phase kinetics studies was used to determine the transport and reaction behavior within a high performance vertical MOVPE reactor. The complexity of the growth chemistry model was increased in a step-wise fashion. At each step, the concentration profiles were determined using available recent kinetic data. The high gas flow rate typically employed in GaN MOVPE results in a very thin high-temperature flow sheet above the growth front, leading to an extremely high thermal gradient. Within this thin high-temperature flow sheet, a stratified chemical structure is formed as a result of the unique thermal fluid environment. This stratified structure is closely related to the transport and reaction behavior within GaN MOVPE processes and forms part of the engineering guidelines for GaN MOVPE reactor design.     

Growth of GaN by nitridation of seed/catalyst free electrodeposited Ga-based compound materials on graphene on insulator

The nitridation of the electrochemically deposited Ga-based compound material on graphene on insulator towards the formation of GaN/graphene hybrid structure was studied by varying the nitridation time and temperature. First, the growth of Ga-based compounds which contains GaON and Ga₂O₃ on multi-layer graphene on SiO₂/Si using a mixture of NH₄NO₃ and Ga(NO₃)₃ by a simple two terminal electrochemical deposition at room temperature was performed. Then, the conversion of the grown structures to the crystalline GaN structure was carried out by nitridating the grown structures in NH₃ gas. The properties of the grown structures were critically influenced by the studied nitridation parameters. The complete transformation to hexagonal GaN was achieved at nitridation temperature of 1100 °C and time of above 60 min due to the observation of significant diffraction peaks which correspond to hexagonal GaN planes. Meanwhile no diffraction peaks of GaON and Ga₂O₃ structure were observed. Temperature and time are the key parameters in a nitridation process where the ammoniation rate of GaON and deoxidization rate of Ga₂O₃ to generate gaseous Ga₂O, increase with temperature and time. It was speculated that a complete transformation can not be realized without a complete ammoniation of GaON and deoxidization of Ga₂O₃. The change of morphological structures was also observed due to both reactions. The presented method demonstrates the feasibility to realize GaN/ graphene hybrid structure on insulator which is highly demanded in fabricating optoelectronic and sensing devices.     

Selective Angle Electroluminescence of Light‐Emitting Diodes based on Nanostructured ZnO/GaN Heterojunctions

Selective angle electroluminescence of violet light with a peak wavelength of 405 nm from light‐emitting diodes based on nanostructured p‐GaN/ZnO heterojunctions is reported. The fabrication of well‐aligned nanobottles with excellent crystalline quality is achieved by chemical vapor deposition at temperatures as low as 450 °C with a specially designed upside‐down arrangement of substrate configuration. Selective angle light sources are essential in our daily life. With the geometry of the nanobottle waveguides, it is very easy to realize such a practical application. Therefore, the discovery reported here should be very useful for the future development of many unique optoelectronic devices.