Current collapse and reliability of III‐N heterostructure field effect transistors

Measurements of GaN HFET lifetime as a function of temperature show that different degradation mechanisms are involved at low temperatures (close to room temperature) and high temperatures (above 150 °C). The degradation at low temperatures is linked to the trap generation and can be explained using the current collapse model. At higher tempe‐ ratures, other degradation mechanisms become important or even dominant. The current collapse related degradation can be diminished by using improved device design, which will greatly increase the overall lifetime (up to long lifetimes obtained by extrapolating high temperature data to room temperature). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Calibrated numerical model of a GaInP–GaAs dual‐junction solar cell

In this letter a calibrated numerical model of a III–V dual‐junction solar cell including tunnel diode and Bragg reflector is presented. The quantum efficiencies of the subcells are computed by using the principle of current‐limitation in monolithic multi‐junction solar cells. A special procedure with bias‐illumination and bias‐voltage was implemented. Numerical simulations are used to study the influence of the top cell thickness on the cells' quantum efficiency and on the current‐matching condition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An Anomalous Gain Mechanism in GaN Schottky Barrier Ultraviolet Photodetectors

The gain mechanism in GaN Schottky barrier ultraviolet photodetectors is investigated by focused light beam. When the incident light illuminates the central region of the Schottky contact electrode, the responsiyity changes very little with the increase of reverse bias voltage. However, when the incident light illuminates the edge region of the electrode, the responsivity increases remarkably with the increase of reverse bias voltage, and the corresponding quantum efficiency could be even higher than 100%. It is proposed that the surface states near the edge of the electrode may lead to a reduction of effective Schottky barrier height and an enhancement of electron injection, resulting in the anomalous gain.     

Mechanism and control of current transport in GaN and AlGaN Schottky barriers for chemical sensor applications

Strong interests are recently emerging for development of integrated high-performance chemical sensor chips. In this paper, the present status of understanding and controlling the current transport in the GaN and AlGaN Schottky diodes is discussed from the viewpoint of chemical sensor applications. For this purpose, a series of works recently carried out by our group are reviewed in addition to a general discussion. First, current transport in GaN and AlGaN Schottky barriers is discussed, introducing the thin surface barrier (TSB) model to explain the anomalously large leakage currents. Following this, attempts to reduce the leakage currents are presented and discussed. Then, as an example of gas-phase sensors using Schottky barriers, a Pd/AlGaN/GaN Schottky diode hydrogen sensor developed recently by our group is presented with a discussion on the sensing mechanism and related current transport. On the other hand, in liquid-phase sensors, contact is made between liquid and semiconductor which is regarded as a kind of Schottky barrier by electrochemists. As one of such liquid-phase sensors, open-gate AlGaN/GaN heterostructure field effect transistor (HFET) pH sensor developed recently by our group is presented. Finally, a brief summary is given together with some remarks for future research.     

Effect of growth pressure on the morphology evolution and doping characteristics in nonpolar a-plane GaN

Nonpolar a-plane GaN layers grown on r-plane sapphire substrates were examined by using a two-step growth process. The higher initial growth pressure for the nucleation layer resulted in the improved crystalline quality with lower density of both threading dislocations and basal stacking faults. This was attributed to the higher degree of initial roughening and recovery time via a growth mode transition from three-dimensional (3D) to quasi two-dimensional (2D) lateral growth. Using Hall-effect measurements, the overgrown Si doped GaN layers grown with higher initial growth pressure were found to have higher mobility. The scattering mechanism due to the dislocations was dominant especially at low temperature (<200 K) for the lower initial growth pressure, which was insignificant for the higher initial growth pressure. The temperature-dependent Hall-effect measurements for the Mg doped GaN with a higher initial growth pressure yielded the activation energy and the acceptor concentration to be 128 meV and 1.2 × 10¹⁹ cm⁻³, respectively, corresponding to about 3.6% of activation at room temperature. Two-step growth scheme with a higher initial growth pressure is suggested as a potential method to improve the performance of nonpolar a-plane GaN based devices.     

Synthesis of aligned gallium nitride nanowire quasi-arrays

Self-aligned GaN nanowire quasi-arrays were synthesized on MgO crystal through a simple gas reaction method. They were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy and high-resolution transmission electron microscopy (HRTEM). FE-SEMimages showed that the product consisted of quasi-arrays of nanowires. XRD, EDX and HRTEM indicated that the nanowires were wurtzite GaN single crystals. Received: 19 June 2000 / Accepted: 21 June 2000 / Published online: 9 August 2000     

Morphologies of GaN one-dimensional materials

GaN one-dimensional materials with different morphologies were formed on LaAlO₃ crystal, silicon crystal and quartz glass substrates through a simple sublimation method. They were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) spectroscopy. FE-SEM images showed that the morphologies of the one-dimensional materials included straight nanorods, curved nanowires, nanoribbons, zigzag nanorods and beaded or capture-tree nanorods. XRD and EDX studies indicated that all the one-dimensional materials were wurtzite GaN. Received: 14 July 2000 / Accepted: 17 July 2000 / Published online: 20 September 2000     

Fabrication of bamboo-shaped GaN nanorods

Bamboo-shaped GaN nanorods were formed through a simple sublimation method. They were characterized by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The TEM image showed that the nanorods were bamboo-like. XRD, HRTEM and SAED patterns indicated that the nanorods were single-crystal wurtzite GaN. Received: 8 January 2001 / Accepted: 28 April 2001 / Published online: 20 December 2001     

InGaAsP/InGaAs tandem cells for a solar cell configuration with more than three junctions

Currently, triple‐junction solar cells realized from III–V semiconductor compounds hold the solar energy conversion efficiency world record. To improve the efficiency significantly, it is necessary to increase the number of junctions and to involve a sub‐cell with an absorber layer in the band gap range of 1 eV. For the realization of a stacked four‐junction device with optimised band gaps, we have grown InGaAsP/InGaAs tandem cells lattice matched to InP substrates, and investigated properties of the absorber bulk material. Time‐resolved photoluminescence of the low band gap In_0.53Ga_0.47As absorber embedded between InP barriers was measured. The InGaAs/GaAsSb tunnel diode structure used in the tandem has been processed into a separate device and I –V curves were measured. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Composite-Collector InGaAs/InP Double Heterostructure Bipolar Transistors with Current-Gain Cutoff Frequency of 242 GHz

To eliminate the conduction band spike at the base-collector interface, an InP/InGaAs double heterostructure bipolar transistor （DHBT） with an InGaAsP composite collector is designed and fabricated using the conventional mesa structure. The DHBT with emitter area of 1.6×15μm^2 exhibits current-gain cutoff frequency ft = 242 OHz at the high collector current density Jc = 2.1 mA/μm^2, which is to our knowledge the highest ft reported for the mesa InP DHBT in China. The breakdown voltage in common-emitter configuration is more than 5 V. The high-speed InP/InGaAs DHBT with high current density digital circuits. is very suitable for the application in ultra high-speed     

High-Speed InGaAs/InP Double Heterostructure Bipolar Transistor with High Breakdown Voltage

e design and fabricate an InGaAs/InP double heterostructure bipolar transistor (DHBT). The spike of the conduction band discontinuity between InGaAs base and InP collector is successfully eliminated by insertion of an InGaAs layer and two InGaAsP layers. The current gain cutoff frequency and maximum oscillation frequency are as high as 155 and 144GHz. The breakdown voltage in common-emitter configuration is more than 7V. The high cutoff frequency and high breakdown voltage make high-speed andhigh-power circuits possible     

High-Breakdown-Voltage Submicron InGaAs/InP Double Heterojunction Bipolar Transistor with ft=170GHz and fmax=253GHz

The layer structure of InGaAs/InP double heterojunction bipolar transistor (DHBT) is designed to enhance the frequency performance and breakdown voltage. The composition-graded base structure is used to decrease the base transit time. The InGaAs setback layer and two highly doped InGaAsP layers are used to eliminate the conduction band spike of the collector. The submicron-emitter InGaAs/InP DHBT is fabricated successfully. The base contact resistance is greatly decreased by optimization of contact metals. The breakdown voltage is more than 6V. The current gain cutoff frequency is as high as 170GHz and the maximum oscillation frequency reached 253GHz. The DHBT with such high performances can be used to make W-band power amplifier.     

Comparison of annealing effects on Zn-doped GaMnAs and undoped GaMnAs epilayers

To compare the annealing effects on GaMnAs-doped with Zn (GaMnAs:Zn) and undoped GaMnAs (u-GaMnAs) epilayers, we grew GaMnAs thin films at 200 °C by molecular beam epitaxy (MBE) on GaAs substrates, and they were annealed at temperatures ranging from 220 °C to 380 °C for 100 min in air. These epilayers were characterized by high-resolution X-ray diffraction (XRD), electrical, and magnetic measurements. A maximum resistivity at temperatures T_m close to the Curie temperatures T_c was observed from the measurement of the temperature-dependent resistivity ρ(T) for both the GaMnAs:Zn and the u-GaMnAs samples. We found, however, that the maximum temperature T_m observed for GaMnAs:Zn epilayers increased with increasing annealing temperature, which was different from the result with the u-GaMnAs epilayers. The formation of GaAs:Zn and MnAs or Mn-Zn-As complexes with increasing annealing temperature is most likely responsible for the differences in appearance.     

Potential of amorphous silicon for solar cells

This paper reviews recent developments in the field of amorphous-silicon-based thin-film solar cells and discusses potentials for further improvements. Creative efforts in materials research, device physics, and process engineering have led to highly efficient solar cells based on amorphous hydrogenated silicon. Sophisticated multijunction solar cell designs make use of its unique material properties and strongly suppress light induced degradation. Texture-etching of sputtered ZnO:Al films is presented as a novel technique to design optimized light trapping schemes for silicon thin-film solar cells in both p-i-n and n-i-p device structure. Necessary efforts will be discussed to close the efficiency gap between the highest stabilized efficiencies demonstrated on lab scale and efficiencies achieved in production. In case of a-Si:H/a-Si:H stacked cells prepared on glass substrates, significant reduction of process-related losses and the development of superior TCO substrates on large areas promise distinctly higher module efficiencies. A discussion of future perspectives comprises the potential of new deposition techniques and concepts combining the advantages of amorphous and crystalline silicon thin-film solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 14 June 1999     

Theoretical Analysis of Current Crowding Effect in Metal/AIGaN/GaN Schottky Diodes and Its Reduction by Using Polysilicon in Anode

There exists a current crowding effect in the anode of AIGaN/GaN heterojunction Schottky diodes, causing local overheating when working at high power density, and undermining their performance. The seriousness of this effect is illustrated by theoretical analysis. A method of reducing this effect is proposed by depositing a polysilicon layer on the Schottky barrier metal. The effectiveness of this method is provided through computer simulation. Power consumption of the polysilicon layer is also calculated and compared to that of the Schottky junction to ensure the applicability of this method.     

Raman-scattering spectrum of GaN straight nanowires

The Raman spectrum of GaN straight nanowires deposited on a LaAlO₃ crystal substrate was studied. The E₂ (high) phonon frequency at 560 cm^-1 shows a 9 cm^-1 shift compared with the calculated value. The low-energy shift and band broadening of the Raman modes result from the nanosize effect. The unique property of the low intensity ratio of I_E2/I_A1(LO) on the Raman spectrum from the GaN straight nanowires was observed. Received: 5 June 2000 / Accepted: 7 June 2000 / Published online: 2 August 2000     

GaN nanotweezers

A new form of GaN nanomaterial (nanotweezers) has been obtained by chemical vapor deposition on an etched cubic MgO (100) plane. The nanotweezers consist of a bottom rod and two arms. The bottom rods have diameters of about 100–150 nm and lengths of about 200–500 nm, on which two arms grow out. The bottoms of the arms are about 40–70 nm and the tops are about 15–30 nm in diameter, and 0.8–1.5 μm in length. X-ray and electron diffractions indicate the nanotweezers are zinc blende gallium nitride. We infer that the fabrication of the GaN nanotweezers is associated with small convex hillocks on the surface of the etched cubic MgO (100) single-crystal substrates and that the nanotweezers grow by a growth mechanism that is similar to vapor-phase heteroepitaxy. Received: 23 April 2002 / Accepted: 25 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: xlchen@aphy.iphy.ac.cn     

Synthesis and Raman scattering of GaN nanorings, nanoribbons and nanowires

Low-dimensional GaN materials, including nanorings, nanoribbons and smooth nanowires have been synthesized by reacting gallium and ammonia using Ag particles as a catalyst on the substrate of MgO single crystals. They were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). EDX, XRD indicated that the low-dimensional nanomaterials were wurtzite GaN. New features are found in Raman scatterings for these low-dimensional GaN materials, which are different from the previous observations of GaN materials. Received: 16 November 2000 / Accepted: 17 November 2000 / Published online: 21 March 2001     

Buried emitter solar cell structures: Decoupling of metallisation geometry and carrier collection geometry of back contacted solar cells

We present a novel solar cell structure, the “buried emitter solar cell”. This concept is designed for decoupling the metallisation geometry from the geometry of the carrier collecting p–n junction in back‐contacted (and in particular back‐junction) solar cells without requiring electrical insulation by dielectric layers. The most prominent features of this device structure are a carrier collecting emitter that covers close to 100% of the total cell area and an effective electrical insulation between emitter and base metallisation via a p⁺–n⁺ junction. The experimental results presented in this paper report a 19.5% efficient “buried emitter solar cell”, where 50% of the solar cell's rear side exhibit a p⁺–n⁺ junction. This preparation technique implies covering a boron‐doped p‐type emitter with an n‐type surface layer that can be efficiently surface‐passivated by thermal oxidation. All structuring of this cell has been performed by laser processing without any photo‐lithography. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)