GaN-based optoelectronic devices on sapphire and Si substrates

A recessed gate AlGaN/GaN high-electron mobility transistor (HEMT) on sapphire (0 0 0 1), a GaN metal-semiconductor field-effect transistor (MESFET) and an InGaN multiple-quantum well green light-emitting diode (LED) on Si (1 1 1) substrates have been grown by metalorganic chemical vapor deposition. The AlGaN/GaN intermediate layers have been used for the growth of GaN MESFET and LED on Si substrates. A two-dimensional electron gas mobility as high as 9260 cm²/V s with a sheet carrier density of 4.8×10¹² cm⁻² was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device on sapphire showed a maximum extrinsic transconductance of 146 mS/mm and a drain–source current of 900 mA/mm for the AlGaN/GaN HEMT with a gate length of 2.1 μm at 25°C. The GaN MESFET on Si showed a maximum extrinsic transconductance of 25 mS/mm and a drain–source current of 169 mA/mm with a complete pinch-off for the 2.5-μm-gate length. The LED on Si exhibited an operating voltage of 18 V, a series resistance of 300 Ω, an optical output power of 10 μW and a peak emission wavelength of 505 nm with a full-width at half-maximum of 33 nm at 20 mA drive current.     

Reflecting p-contact based on thin ITO films for AlGaInN flip-chip LEDs

A reflecting contact to a p-GaN layer, used in fabrication of blue flip-chip light-emitting diodes, has been produced by deposition of thin indium tin oxide (ITO) films by electron-beam evaporation. The high reflectance of the contact, which exceeds that of a Ni/Ag contact, provides a 15–20% increase in the external quantum efficiency of light-emitting crystals. The forward voltage drops for crystals with an ITO(5 nm)/Ag(220 nm) contact are comparable with the corresponding values for crystals with a Ni(1.5 nm)/Ag(220 nm) contact. The specific resistance of the contact with an ITO layer is 3.7 × 10^?3 Ω cm². It is shown that, for ITO films produced by the given method, the optimal thicknesses providing the best electrical and optical characteristics of the crystals are in the range 2.5–5.0 nm.

     

Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs

Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 10⁸ cm^?2. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.     

Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering

Progress in wide bandgap, III–V material systems based on gallium nitride (GaN) has enabled the realization of high‐power and high‐frequency electronics. Since the highly conductive, 2D electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface is based on built‐in polarization fields and is confined to nanoscale thicknesses, its charge carriers exhibit much higher mobilities compared to their doped counterparts. This study shows that such 2DEGs also offer the unique ability to manipulate electrical transport separately from thermal transport, through the examination of fully suspended AlGaN/GaN diaphragms of varied GaN buffer layer thickness. Notably, ≈100 nm thin GaN layers can considerably impede heat flow without electrical transport degradation. These achieve 4× improvement in the thermoelectric figure of merit (zT) over externally doped GaN, with state‐of‐the‐art power factors of 4–7 mW m^‐1 K^‐2. The remarkable tuning behavior and thermoelectric enhancement, elucidated here for the first time in a polarization‐based heterostructure, are achieved because electrons are at the heterostructured interface, while phonons are within the material system. These results highlight the potential for using 2DEGs in III–V materials for on‐chip thermal sensing and energy harvesting.     

Study of the characteristics of ultraviolet light-emitting diodes based on GaN/AlGaN heterostructures grown by chloride-hydride vapor-phase epitaxy

The results of work on developing and studying ultraviolet (UV) light-emitting diodes (LEDs) based on GaN/AlGaN heterostructures fabricated on Al₂O₃(0001) substrates by the chloride-hydride vaporphase epitaxy are presented. The maximum in the electroluminescence spectrum is located in the wavelength range of 360–365 nm, and its full width at half maximum is 10–13 nm. At a working current of 20 mA, the optical density and efficiency of the UV LED are 1.14 mW and 1.46%, respectively.     

The influence of OMVPE growth pressure on the morphology, compensation, and doping of GaN and related alloys

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10⁵ Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10¹³ cm⁻² and mobilities of 1500 cm²/Vs, and reduced trapping effects in fabricated devices.     

Highly Conductive,Bendable, Embedded Ag Nanoparticle Wire Arrays Via Convective Self‐Assembly: Hybridization into Ag Nanowire Transparent Conductors

The optoelectrical properties of Ag nanowire (NW) networks are improved by incorporating the NWs into highly conductive ordered arrays of Ag nanoparticle wires (NPWs) fabricated via surfactant‐assisted convective self‐assembly. The NPW–NW hybrid conductor displays a transmittance (T) of 90% at 550 nm and a sheet resistance (R _s) of 5.7 Ω sq^?1, which is superior to the corresponding properties of the NW network showing a R _s of 14.1 Ω sq^?1 at a similar T. By the modified wettability of a donor substrate and the capillarity of water, the sintered NPW–NW hybrid conductors are perfectly transferred onto an UV‐curable photopolymer film, and the embedded hybrid conductors exhibit excellent electromechanical properties. The R _s and T of the NPW arrays can be predicted by using a simple model developed to calculate the width and height of the hexagonal close‐packed particles formed during the convective self‐assembly. The numerical analysis reveals that the maximum Haacke figure of merit of the NW networks is increased considerably from 0.0260 to 0.0407 Ω^?1 by integration with the NPW array. The highly conductive NPW arrays generated using a simple, low‐cost, and nonlithographic process can be applied to enhancing the performances of other transparent conductors, such as carbon nanotubes, metal oxides, and graphenes.     

Screen‐Printed Poly(3,4‐Ethylenedioxythiophene):Poly(Styrenesulfonate) Grids as ITO‐Free Anodes for Flexible Organic Light‐Emitting Diodes

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) grids have been successfully constructed by roll‐to‐roll compatible screen‐printing techniques and have been used as indium tin oxide (ITO)‐free anodes for flexible organic light‐emitting diodes (OLEDs). The grid‐type transparent conductive electrodes (TCEs) can adopt thicker PEDOT: PSS grid lines to ensure the conductivity, while the mesh‐like grid structure can play an important role to maintain high optical transparency. By adjusting grid periods, grid thickness and treatment of organic additives, PEDOT: PSS TCEs with high optical transparency, low sheet resistance, and excellent mechanical flexibility have been achieved. Using the screen‐printed PEDOT: PSS grids as the anodes, ITO‐free OLEDs achieved peak current efficiency of 3.40 cd A^?1 at the current density of 10 mA cm^?2, which are 1.56 times better than the devices with ITO glass as the anodes. The improved efficiency is attributed to the light extraction effect and improved transparency by the grid structure. The superior optoelectronic performances of OLEDs based on flexible screen‐printed PEDOT: PSS grid anodes suggest their great prospects as ITO‐free anodes for flexible and wearable electronic applications.     

Specific contact resistance and noise in contacts on thin layers

A method is presented for the calculation of the specific contact resistance and noise of contacts on thin layers. The contribution to the contact resistance and noise of the interface between the contact and the layer is determined from experimental results. Using the developed method on thin Cu_xS layers with a sheet resistance of 42.5 Ω values for the specific contact resistance of 4 × 10^?2 Omega; cm² and 2.5 Ω cm² were observed for two different contact technologies.     

Transparent Ultrathin Oxygen‐Doped Silver Electrodes for Flexible Organic Solar Cells

An effective method for depositing highly transparent and conductive ultrathin silver (Ag) electrodes using minimal oxidation is reported. The minimal oxidation of Ag layers significantly improves the intrinsic optical and structural properties of Ag without any degradation of its electrical conductivity. Oxygen‐doped Ag (AgO_x) layers of thicknesses as low as 6 nm exhibit completely 2D and continuous morphologies on ZnO films, smaller optical reflections and absorbances, and smaller sheet resistances compared with those of discontinuous and granular‐type Ag layers of the same thickness. A ZnO/AgO_x/ZnO (ZAOZ) electrode using an AgO_x (O/Ag = 3.4 at%) layer deposited on polyethylene terephthalate substrates at room temperature shows an average transmittance of 91%, with a maximum transmittance of 95%, over spectral range 400?1000 nm and a sheet resistance of 20 Ω sq^?1. The average transmittance value is increased by about 18% on replacing a conventional ZnO/Ag/ZnO (ZAZ) electrode with the ZAOZ electrode. The ZAOZ electrode is a promising bottom transparent conducting electrode for highly flexible inverted organic solar cells (IOSCs), and it achieves a power conversion efficiency (PCE) of 6.34%, whereas an IOSC using the ZAZ electrode exhibits a much lower PCE of 5.65%.     

Ambient‐Dried Cellulose Nanofibril Aerogel Membranes with High Tensile Strength and Their Use for Aerosol Collection and Templates for Transparent,Flexible Devices

The application potential of cellulose nanofibril (CNF) aerogels has been hindered by the slow and costly freeze‐ or supercritical drying methods. Here, CNF aerogel membranes with attractive mechanical, optical, and gas transport properties are prepared in ambient conditions with a facile and scalable process. Aqueous CNF dispersions are vacuum‐filtered and solvent exchanged to 2‐propanol and further to octane, followed by ambient drying. The resulting CNF aerogel membranes are characterized by high transparency (>90% transmittance), stiffness (6 GPa Young's modulus, 10 GPa cm³ g^?1 specific modulus), strength (97 MPa tensile strength, 161 MPa m³ kg^?1 specific strength), mesoporosity (pore diameter 10–30 nm, 208 m² g^?1 specific surface area), and low density (≈0.6 g cm^?3). They are gas permeable thus enabling collection of nanoparticles (for example, single‐walled carbon nanotubes, SWNT) from aerosols under pressure gradients. The membranes with deposited SWNT can be further compacted to transparent, conductive, and flexible conducting films (90% specular transmittance at 550 nm and 300 Ω ?^{? 1} sheet resistance with AuCl₃‐salt doping). Overall, the developed aerogel membranes pave way toward use in gas filtration and transparent, flexible devices.     

Lasing in the vertical direction in InGaN/GaN/AlGaN structures with InGaN quantum dots

InGaN/GaN structures with dense arrays of InGaN nanodomains were grown by metallorganic chemical vapor deposition. Lasing in vertical direction occurs at low temperatures, indicating ultrahigh gains (～ 10⁵ cm^?1) in the active region. Fabrication of an effective AlGaN/GaN distributed Bragg reflector with reflectivity exceeding 90% enables vertical lasing at room temperature in structures with a bottom distributed Bragg reflector, despite the absence of a well-reflecting upper mirror. The lasing wavelength is 401 nm, and the threshold excitation density is 400 kW/cm².     

用633nm的激光器监控峰值波长为 530nm的AlGaN/GaN DBR的生长

本文从理论和实验上研究了用633nm激光器干涉仪监控峰值反射波长为530nm的1/4光学厚度的AlGaN/GaN 分布布拉格反射器（DBR）的生长。首先采用传输矩阵法从理论上研究了不同周期厚度的AlGaN/GaN DBR的实时反射率随DBR生长厚度的变化。接着采用金属有机化合物气相外延法生长了两个与模拟结构相同的DBR样品。仿真结果和实验结果表明能够从DBR实时反射率随生长厚度变化的曲线形状判断DBR的结构参数。最后通过激光干涉仪实时监控生长了DBR发光二极管,光致发光实验证明DBR对发光二极管出射光的加强作用在期望波长范围内。     

High‐Performance Hazy Silver Nanowire Transparent Electrodes through Diameter Tailoring for Semitransparent Photovoltaics

Solution‐processed metal nanowire networks have attracted substantial attention as clear transparent conductive electrodes (TCEs) to replace metal oxides for low‐cost and flexible touch panels and displays. While targeting photovoltaic applications, TCEs are expected to be more hazy for enhancing light absorption in the active layer, but are still required to retain high transmittance and low sheet resistance. Balancing these properties (haze, transmittance, and conductivity) in TCEs to realize high performance but high haze simultaneously is a challenge because they are mutually influenced. Here, by precisely tailoring the diameter of thick–long silver nanowires using rapid radial electrochemical etching, high hazy flexible TCEs are fabricated with high figure of merit of up to 741 (4 Ω sq^?1 at 88.4% transmittance with haze of 13.3%), surpassing those of commercialized brittle hazy metal oxides and exhibiting superiority for photovoltaic applications. Laminating such TCEs onto the perovskite solar cells as top electrodes, the obtained semitransparent devices exhibit power efficiencies up to 16.03% and 11.12% when illuminated from the bottom and top sides, respectively, outperforming reported results based on similar device architecture. This study provides a simple strategy for flexible and hazy TCEs fabrication, which is compatible with mild solution‐processed photovoltaic devices, especially those containing heat‐sensitive or chemical‐sensitive materials.     

AlGaN中Mg掺杂对蓝光发光二极管光电性质的影响

采用金属有机化学气相沉积技术生长了GaN基多量子阱(MQW)蓝光发光二极管外延片,并采用高分辨率X射线衍射仪(HRXRD)和光致光谱仪(PL)表征晶体质量和光学性能,其他的光电性能由制成芯片后测试获得,目的是研究外延片p型AlGaN电子阻挡层Mg掺杂的优化条件.结果表明,在生长p型AlGaN电子阻挡层的Cp2Mg流量为300 cm3/min时,蓝光发光二极管获得最小正向电压VF,而且在此掺杂流量下的多量子阱蓝光发光二极管芯片发光强度明显高于其他流量的样品.因此可以通过优化AlGaN电子阻挡层的掺杂浓度,来显著提高多量子阱蓝光发光二极管的电学性能和光学性能.     

Influence of an increase in the implantation dose of erbium ions and annealing temperature on photoluminescence in AlGaN/GaN superlattices and GaN epitaxial layers

Room-temperature photoluminescence (PL) has been studied in AlGaN/GaN superlattices and GaN epitaxial layers implanted with 1-MeV erbium at a dose of 3 × 10¹⁵ cm^?2 and annealed in argon. The intensity of PL from Er³⁺ ions in the superlattices exceeds that for the epitaxial layers at annealing temperatures of 700–1000°C. The strongest difference (by a factor of ～2.8) in PL intensity between the epitaxial layers and the superlattices and the highest PL intensity for the superlattices are observed upon annealing at 900°C. On raising the annealing temperature to 1050°C, the intensity of the erbium emission from the superlattices decreases substantially. This circumstance may be due to their thermal destruction.     

Pulse‐Mode Dynamic Ron Measurement of Large‐Scale High‐Power AlGaN/GaN HFET

We propose pulse‐mode dynamic R_on measurement as a method for analyzing the effect of stress on large‐scale high‐power AlGaN/GaN HFETs. The measurements were carried out under the soft‐switching condition (zero‐voltage switching) and aimed to minimize the self‐heating problem that exists with the conventional hard‐switching measurement. The dynamic R_on of the fabricated AlGaN/GaN MIS‐HFETs was measured under different stabilization time conditions. To do so, the drain‐gate bias is set to zero after applying the off‐state stress. As the stabilization time increased from 0.1 μs to 100 ms, the dynamic R_on decreased from 160 Ω to 2 Ω. This method will be useful in developing high‐performance GaN power FETs suitable for use in high‐efficiency converter/inverter topology design.     

Low‐Temperature‐Processed Printed Metal Oxide Transistors Based on Pure Aqueous Inks

Additive patterning of transparent conducting metal oxides at low temperatures is a critical step in realizing low‐cost transparent electronics for display technology and photovoltaics. In this work, inkjet‐printed metal oxide transistors based on pure aqueous chemistries are presented. These inks readily convert to functional thin films at lower processing temperatures (T ≤ 250 °C) relative to organic solvent‐based oxide inks, facilitating the fabrication of high‐performance transistors with both inkjet‐printed transparent electrodes of aluminum‐doped cadmium oxide (ACO) and semiconductor (InO_x ). The intrinsic fluid properties of these water‐based solutions enable the printing of fine features with coffee‐ring free line profiles and smoother line edges than those formed from organic solvent‐based inks. The influence of low‐temperature annealing on the optical, electrical, and crystallographic properties of the ACO electrodes is investigated, as well as the role of aluminum doping in improving these properties. Finally, the all‐aqueous‐printed thin film transistors (TFTs) with inkjet‐patterned semiconductor (InO_x ) and source/drain (ACO) layers are characterized, which show ideal low contact resistance (R _c < 160 Ω cm) and competitive transistor performance (µ _lin up to 19 cm² V^?1 s^?1, Subthreshold Slope (SS) ≤150 mV dec^?1) with only low‐temperature processing (T ≤ 250 °C).     

The influence of composition and unintentional doping on the two-dimensional electron gas density in AlGaN/GaN heterostructures

We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2–3×10¹³ cm^?2 were measured. A one-dimensional Schrödinger-Poisson model was used to describe the heterostructure. The calculations gave two-dimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 10¹⁵ cm^?3 to 10¹⁸ cm^?3. The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.     

Catalytic materials manufactured by the polyol process for monolithic dye‐sensitized solar cells

Three types of screen‐printable catalytic pastes were successfully prepared to be used as counterelectrode for monolithic dye solar cells encapsulated with glass frit. The electroless bottom‐up method or so‐called polyol process has been applied to fabricate thermally stable SnO₂:Sb/Pt and carbon black/Pt nanocomposites. The catalytic and electric properties of these materials were compared with a new platinum‐free type of carbon counterelectrode. The layers containing low platinum amounts (less than 5 µg/cm²) exhibit a very low charge transfer resistance of about 0·4 Ω · cm². Also the conductive carbon layer shows an acceptable charge transfer resistance of 1·6 Ω · cm². Additionally the catalytic layer containing porous carbon black reveals excellent sheet resistance below 5 Ω/□; this feature has enabled to work out a low cost counterelectrode which combined suitable catalytic and conductive properties. The layers have been characterized using following methods: electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE‐SEM), energy filter transmission electron microscopy (EF‐TEM) and inductively coupled plasma mass spectroscopy (ICP‐MS). Copyright © 2008 John Wiley & Sons, Ltd.