Visible InGaN/GaN Quantum-Dot Materials and Devices

General properties of III-V nitride-based quantum dots (QDs) are presented, with a special emphasis on InGaN/GaN QDs for visible optoelectronic devices. Stranski-Krastanov GaN/AlN dots are first discussed as a prototypical system. It is shown that the optical transition energies are governed by a giant quantum-confined Stark effect, which is the consequence of the presence of a large built-in internal electric field of several MV/cm. Then we move to InGaN/GaN QDs, reviewing the different fabrication approaches and their main optical properties. In particular, we focus on InGaN dots that are formed spontaneously by In composition fluctuations in InGaN quantum wells. Finally, some advantages and limitations of nitride laser diodes with active regions based on InGaN QDs are discussed, pointing out the requirements on dot uniformity and density in order to be able to exploit the expected quantum confinement effects in future devices.     

Study of the polarizations of (Al,Ga,AlGa)N nitride compounds and the charge density of various interfaces based on them

The results of a theoretical study based on ab initio calculations of the polarization properties of AlN, GaN, and AlGaN semiconductors with the wurtzite structure are presented. The values of the spontaneous and piezoelectric polarizations, as well as the piezoelectric constants, are calculated for these nitride compounds. With the aim of further considering prospective heterostructures based on (Al,Ga,AlGa)N compounds, the charge densities at the AlN/GaN, AlGaN/AlN, and AlGaN/GaN interfaces and carrier concentration at the AlGaN/GaN heterointerface is estimated and compared with the experimental data.     

Cubic group-III nitride-based nanostructures—basics and applications in optoelectronics

Molecular beam epitaxy (MBE) of cubic group-III nitrides is a direct way to eliminate the polarization effects which inherently limits the performance of optoelectronic devices containing quantum well or quantum dot active regions. In this contribution the latest achievement in the MBE of phase-pure cubic GaN, AlN, InN and their alloys will be reviewed. A new reflected high-energy electron beam (RHEED) control technique enables to carefully adjust stoichiometry and to severely reduce the surface roughness, which is important for any hetero-interface. The structural, optical and electrical properties of cubic nitrides and AlGaN/GaN will be presented. We show that no polarization field exists in cubic nitrides and demonstrate 1.55 μm intersubband absorption in cubic AlN/GaN superlattices. Further the progress towards the development and fabrication of cubic hetero-junction field effect transistors (HFETs) is discussed.     

Emerging gallium nitride based devices

Wide bandgap GaN has long been sought for its applications to blue and UV emitters and high temperature/high power electronic devices. Recent introduction of commercial blue and blue-green LED's have led to a plethora of activity in all three continents into the heterostructures based on GaN and its alloys with AlN and InN. In this review, the status and future prospects of emerging wide bandgap gallium nitride semiconductor devices are discussed. Recent successes in p-doping of GaN and its alloys with InN and AlN, and in n-doping with much reduced background concentrations have paved the way for the design, fabrication, and characterization of devices such as MESFET's, MISFET's, HBT's, LED's, and optically pumped lasers. We discuss the electrical properties of these devices and their drawbacks followed by future prospects. After a short elucidation of materials characteristics of the nitrides, we explore their electrical transport properties in detail. Recent progress in processing such as formation of low-resistance ohmic contacts and etching is also presented. The promising features of quarternaries and double heterostructures in relation to possible current injection lasers, LED's, and photodetectors are also elaborated on     

Spontaneous growth of GaN nanowire nuclei on N- and Al-polar AlN: A piezoresponse force microscopy study of crystallographic polarity

The polarity of gallium nitride (GaN) nanowire nuclei grown on AlN layers was studied by piezoresponse force microscopy (PFM). N- or Al-polar AlN layers were grown by molecular beam epitaxy (MBE) on Si (111) substrates by use of Al- or N-rich growth conditions, respectively. Short and low density GaN nanowires were then grown on each AlN polarity type. PFM measurements verified the expected AlN layer polarity and further indicated that predominantly N-polar nanowires are produced for growth on both AlN polarity types. Cross-section scanning transmission electron microscopy (STEM) images further reveal that the nanowires on Al-polar AlN films are nucleated on regions in the AlN layer that contain inversion domains, which propagate into the GaN nanowire nuclei. PFM measurements were found to be a convenient technique for mapping the polarity of a statistically significant number of individual GaN nanowires.     

AIN、GaN立方晶体的静态性质和AIN/GaN异质结的价带偏移

采用LMTO能带从头计算方法,计算了闪锌矿（立方）结构AIN和GaN的静态性质;用平均键能方法,预言了AlN与GaN自由应变生长、以AlN为衬底和以GaN为衬底等三种不同应变状态下AlN／GaN应变层异质结的△Ev值;最后,采用超原胞（AIN）n（GaN）n（001）,（n=1,3,5）界面自洽计算方法,考察了超晶格中平均键能Em的“对齐”程度和验证了价带偏移△Ev计算结果的准确性。     

基于宽禁带氮化物的微腔光频梳进展（特邀）

微腔光频梳在光谱测量、微波光子学、光学原子钟和相干光通信等领域具有重要的应用。宽禁带氮化物半导体材料,如氮化铝（AlN）和氮化镓（GaN）等属于非中心对称晶体,具有二阶和三阶光学非线性系数,宽带的透明窗口以及与蓝宝石衬底较高的折射率差,使其成为研究非线性光子器件的理想平台。文中介绍了氮化物微腔的特性,同时对基于氮化物微腔光梳的相关研究进展,包括AlN微腔中的宽谱光频梳产生和光学参量振荡、GaN微腔中的孤子光频梳产生等进行了介绍和展望。     

Blue/green lasers focus on the market

The authors present a review of past, present, and future blue/green laser technologies. They discuss: II-VI semiconductor materials and alternate approaches (including the use of SiC and III-V nitrides, e.g. GaN, AlN); laser diode design; multiple quantum well approaches; contacting to p-ZnSe; and remaining challenges     

Surface-layer damage and responsivity in sputtered-ITO/p-GaN Schottky-barrier photodiodes

It is postulated that donor-like nitrogen vacancies, caused by the sputtering of a Schottky-barrier metal onto p-type gallium nitride, diffuse into the GaN and form a surface layer in which both the minority-carrier lifetime and mobility are drastically reduced. Such a damaged surface layer is shown to reduce the responsivity of p-GaN Schottky-barrier photodiodes, thereby offering an explanation for the responsivity values in the range of 0.03–0.04 A/W that have been measured in experimental ITO/p-GaN devices. On making allowance for the damaged surface layer, an electron diffusion length of around 300 nm can be inferred for the undamaged p-GaN region.     

以金属为缓冲层在Si(111)上分子束外延GaN及其表征

用电子束蒸发方法在Si(111)衬底上蒸发了Au/Cr和Au/Ti/Al/Ti 两种金属缓冲层,然后在金属缓冲层上用气源分子束外延(GSMBE)生长GaN. 两种缓冲层的表面都比较平整和均匀,都是具有Au(111)面择优取向的立方相Au层. 在Au/Cr/Si(111)上MBE生长的GaN,生长结束后出现剥离. 在Au/Ti/Al/Ti/Si(111)上无AlN缓冲层直接生长GaN,得到的是多晶GaN;先在800℃生长一层AlN缓冲层,然后在710℃生长GaN,得到的是沿GaN(0001)面择优取向的六方相GaN. 将Au/Ti/Al/Ti/Si(111)在800℃下退火20min,金属层收缩为网状结构,并且成为多晶,不再具有Au(111)方向择优取向.     

Ion implantation and rapid thermal processing of Ill-V nitrides

Ion implantation doping and isolation coupled with rapid thermal annealing has played a critical role in the realization of high performance photonic and electronic devices in all mature semiconductor material systems. This is also expected to be the case for the binary III-V nitrides (InN, GaN, and A1N) and their alloys as the epitaxial material quality improves and more advanced device structures are fabricated. In this article, we review the recent developments in implant doping and isolation along with rapid thermal annealing of GaN and the In-containing ternary alloys InGaN and InAlN. In particular, the successful n- and p-type doping of GaN by ion implantation of Si and Mg+P, respectively, and subsequent high temperature rapid thermal anneals in excess of 1000°C is reviewed. In the area of implant isolation, N-implantation has been shown to compensate both n- and p-type GaN, N-, and O-implantation effectively compensates InAlN, and InGaN shows limited compensation with either N- or F-implantation. The effects of rapid thermal annealing on unimplanted material are also presented.     

Deposition of III-N thin films by molecular beam epitaxy

Fabrication of optoelectronic devices from III-N materials, operable in the blue and ultraviolet regions of the spectrum, has been a goal of many groups since the first infrared and red devices were commercially produced. Commercially viable blue-light-emitting diodes have now been achieved in these materials, and numerous investigators are now entering this field. Microelectronic devices are also of considerable interest.

While much of the original research on GaN and InN centered around films with the wurtzite structure, the zincblende structure nitrides have recently been receiving increasing interest because of their potential inherent advantages, e.g. increased carrier mobility. Recent results from several groups around the world have successfully addressed some of the traditional problems associated with growth of these materials.

Chemical vapor deposition (CVD) has been a traditional growth technique for nitride growth and continues to be very successful. However, many researchers, including those growing III–V nitrides, have turned to plasma-enhanced molecular beam epitaxy (MBE) for its important advantages in purity and in situ analytical capabilities. This paper reviews the recent developments in MBE growth of cubic boron nitride, aluminum nitride and the two poly-types of gallium nitride.     

AlN阻挡层对AlGaN/GaNHEMT器件的影响

利用低压MOCVD技术在蓝宝石衬底上生长了AlGaN/GaN异质结和AlGaN/AlN/GaN异质结二维电子气材料,采用相同器件工艺制造出了AlGaN/GaN HEMT器件和AlGaN/AlN/GaN HEMT器件.通过对两种不同器件的比较和讨论,研究了AlN阻挡层的增加对AlGaN/GaN HEMT器件性能的影响.     

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.     

Specific features of kinetics of molecular beam epitaxy of compounds in the GaN-AlN system

The effect of growth conditions (V/III ratio, substrate temperature) on the properties of materials in AlN-GaN systems is discussed. A concept of the growth of the AlN/AlGaN/GaN multilayer heterostructure, which provides the improvement of crystal quality and surface morphology of the layers, is suggested and realized. The improvement of the properties of GaN in the AlN/AlGaN/GaN/AlGaN multilayer heterostructure is confirmed by a considerable increase in electron mobility in the two-dimensional electron gas formed at the upper heterointerface GaN/Al_0.3Ga_0.7N.     

Digitally Alloyed Modulated Precursor Flow Epitaxial Growth of Ternary AlGaN with Binary AlN and GaN Sub-Layers and Observation of Compositional Inhomogeneity

We report the growth of ternary aluminum gallium nitride (AlGaN) layers on AlN/sapphire template/substrates by digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG), which combined an MPEG AlN sub-layer with a conventional metalorganic chemical vapor deposition (MOCVD)-grown GaN sub-layer. The overall composition in DA-MPEG Al _x Ga_1−x N was controlled by adjustment of the growth time (i.e., the thickness) of the GaN sub-layer. As the GaN sub-layer growth time increased, the Al composition in AlGaN decreased to 50%, but the surface morphology of the AlGaN layer became rough, and a three-dimensional structure with islands appeared for the DA-MPEG AlGaN with relatively thick GaN sub-layers, possibly resulting from the Ga adatom surface migration behavior and/or the strain built up from lattice mismatch between AlN and GaN sub-layers with increasing GaN sub-layer growth time. Through strain analysis by high-resolution x-ray diffraction, reciprocal space mapping, and scanning transmission electron microscopy, it was found that there was compositional inhomogeneity in the DA-MPEG AlGaN with AlN and GaN binary sub-layers for the case of the layer with relatively thick GaN sub-layers.     

Ion Beam Analysis of Amorphous and Nanocrystalline Group III-V Nitride and ZnO Thin Films

The ion beam analysis (IBA) techniques of Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), nuclear reaction analysis (NRA), and particle-induced x-ray emission (PIXE) have been used to quantitatively determine composition, uniformity, impurity, and elemental depth profiles of major, minor, and trace elements of group III-V nitride and zinc oxide (ZnO) thin films prepared by various growth techniques. The IBA revealed that an amorphous GaN film prepared by ion beam assisted deposition (IBAD) has large variations in film thickness and composition coupled with typically 10–20% oxygen that was found to be essential to stabilize their amorphous structure. The IBA characterization of plasma-assisted molecular beam epitaxy (PAMBE) grown GaN, InN, and InCrN films revealed composition, impurity, and uniformity information of the films. The IBA of ZnO films prepared by radio frequency (RF) sputtering showed that the Zn/O ratio often varied significantly over the film thickness. Hydrogen was found to be a major impurity in the films with around one present in the as-deposited ZnO films. It is clearly shown that the nondestructive, quantitative, and rapid IBA measurements are very useful to develop and optimize growth protocols in respect to film thickness, stoichiometry, and especially in regard to hydrogen and oxygen impurities for group III-V nitride and ZnO thin films prepared by various growth techniques.     

Ammonothermal synthesis of aluminum nitride crystals on group III-nitride templates

Polycrystalline aluminum nitride (AlN) crystals were synthesized using the ammonothermal technique at temperatures between 525°C and 550°C. The growth of AlN was conducted in alkaline conditions with potassium azide (KN₃) as the mineralizer. The growth mechanism was found to be reversegradient soluble, necessitating the placement of the GaN and AlN seeds at a higher temperature than the aluminum metal source. Growth on the GaN seeds varied from 100 to 1500 μm in thickness at a gestation period of 21 days. Additionally, scanning electron micrographs revealed varying microstructure ranging from pointed hexagonal rods, which are approximately 5 μm wide and 20 μm long, to highly densified and contiguous films. Formation of hexagonal AlN was verified using x-ray powder diffraction measurements. Oxygen was detected at 3.7 at.% by inert gas fusion analysis on AlN nucleated on the walls of the autoclave and a qualitative indication of unintentionally incorporated impurities in the AlN grown on the GaN seeds was obtained using energy-dispersive x-ray analysis. Photoluminescence spectroscopy conducted at 20 K revealed a deep-level emission at 3.755 eV due to unintentionally incorporated impurities.     

Progress and prospects of group-III nitride semiconductors

We review recent progress in the group-III nitride and related materials, and electronic and optical devices based on them. Blue and UV (e.g. ultra violet) emitters and detectors, and high temperature/high power electronics which has long been coveted are beginning to be realized either in the laboratory or in the commercial arena, due in part to the breathtaking progress made in the last few years in the art and science of GaN, InN, AlN and their salloys. With brief references to the historical aspect of the relevant developments, this review concerns itself primarily with the current status of wide bandgap gallium nitride and related semiconductors from both the materials and devices points of view. Following a discussion of the structural properties of these materials, their electrical and optical properties are described in detail. The available data on metal contacts, the properties of which are indeed very conducive for the devices mentioned, from the points of view of ohmic contacts and Schottky barriers, are elaborated on. Recent progress on processing issues such as etching are reviewed. The review then embarks on an indash;depth discussion and analysis of field effect transistors, bipolar transistors, light emitting diodes, laser and photo detectors.     

Progress in wide bandgap technology

The developing list of wide gap substrates for device production is remarkable compared with a few years ago and continues to provide new device design possibilities. For GaN it ranges from the largest volume (and hetero) materials, sapphire and SiC (both available in 2″–4″ diameters and used for commercial devices) to homo and hetero substrates that now include four compound materials; aluminium nitride (AIN), gallium nitride (GaN), silicon carbide (SiC), and zinc oxide (ZnO). These are all available commercially, but in varying stages of size, unit volumes, surface and defect qualities. Additionally, a fifth substrate has been announced in the form of HVPE-grown aluminium gallium nitride (AlGaN) available in development quantities. Their production processes include several types of vapiur phase epitaxy, vapour phase transport, crystallisation from the melt and solution growth. Since they are all wide gap materials, these production methods are all high temperature processes and some require high pressures.