Study of the kink effect in AlInAs/GaInAs/InP composite channel HFETs

A detailed study is presented on AlInAs/InGaAs/InP composite channels. These devices combine the advantages of high mobility at low voltages and high electric field operations thanks to the use of a composite channel formed by a thin InGaAs layer and a doped InP subchannel. Due to the very low gate leakage currents, it has been possible to precisely study the impact ionization contributions as a function of the temperature. Surprisingly, it is not possible from our measurements to correlate the kink effect observed in the devices with the impact ionization phenomenon. Therefore, a detailed study of the AlInAs deep traps, and the deep levels detected in the devices has been performed using DLTS, CTS, drain lag, and low frequency noise measurements. The observation of the kink effect in our HFETs has been clearly connected to an electron trap located in the AlInAs layers. In order to confirm this result, the optical properties of this deep trap have been studied by I–V measurements under optical excitation for HFETs, and by DLOS on bulk AlInAs. Our measurements show that the same defect is observed in AlInAs and in the HFETs and that it is possible to suppress the kink effect by an optical ionization of this electron trap. Finally, our electro-optical study shows the direct correlation between deep traps in the AlInAs barrier layers and the kink effect in these devices.     

Investigation of an AlInAs/GaInAs superlattice-confined emitter bipolar transistor (SCEBT)

A new AlInAs/GaInAs superlattice-confined emitter bipolar transistor (SCEBT) structure with a 20-period superlattice confinement layer has been successfully fabricated and demonstrated. In the studied SCEBT, the 20-period AlInAs/GaInAs superlattice confinement layer is employed to provide the electron injection through emitter–base (E–B) homojunction and block the hole back injection from base to emitter. An extremely small offset voltage of 61 mV and the common-emitter current gain about 25 have been obtained at room temperature. From experimental results, it is seen that the potential spike at the effective E–B junction is negligible. So, the proposed device provides a good promise for practical circuit applications.     

Effects of substrate miscut on threading dislocation distribution in metamorphic GaInAs/AlInAs buffers

Metamorphic GaInAs/AlInAs buffers with a total mismatch of 2 % with respect to GaAs have been grown by metal organic chemical vapor deposition on GaAs substrates with miscuts of 2°, 7° and 15° toward (111)A. The buffer grown on 2° miscut substrate exhibits a dislocation pile-up formation which increases the threading dislocation density greatly. However, using 7° and 15° off substrates eliminates the dislocation pile-up and decreases the dislocation density by an order of magnitude compared with that on the 2° substrate miscut, and the best crystalline quality of buffer is obtained on the 15° substrate miscut. Experimental results show that the buffers grown on the 7° and 15° miscut substrate exhibit shallower trenches on the surface and a weaker strain field in the InGaAs cap layers, and these help to suppress the occurrence of dislocation pile-up, leading to a lower threading dislocation density.     

GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence

In this report we explore the structural and optical properties of GaAs/A1GaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodo- luminescence （CL） in a scanning electron microscope （SEM）. We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy （TEM）. In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.     

VLS growth of alternating InAsP/InP heterostructure nanowires for multiple-quantum-dot structures

We investigated the Au-assisted growth of alternating InAsP/InP heterostructures in wurtzite InP nanowires on InP(111)B substrates for constructing multiple-quantum-dot structures. Vertical InP nanowires without stacking faults were obtained at a high PH(3)/TMIn mole flow ratio of 300-1000. We found that the growth rate changed largely when approximately 40 min passed. Ten InAsP layers were inserted in the InP nanowire, and it was found that both the InP growth rate and the background As level increased after the As supply. We also grew the same structure using TBAs/TBP and could reduce the As level in the InP segments. A simulation using a finite-difference time-domain method suggests that the nanowire growth was dominated by the diffusion of the reaction species with long residence time on the surface. For TBAs/TBP, when the source gases were changed, the formed surface species showed a short diffusion length so as to reduce the As background after the InAsP growth.     

Electrical breakdown and nanogap formation of indium oxide core/shell heterostructure nanowires

We report the electrical breakdown behavior and subsequent nanogap formation of In(2)O(3)/InO(x) core/shell heterostructure nanowires with substrate-supported and suspended structures. The radial heterostructure nanowires, composed of crystalline In(2)O(3) cores and amorphous In-rich shells, are grown by chemical vapor deposition. As the nanowires broke down, they exhibited two distinct current drops in the current-voltage characteristics. The tips of the broken nanowires were found to have a cone or a volcano shape depending on the width of the nanowire. The shape, the size, and the position of the nanogap depend strongly on the device structure and the nanowire dimensions. The substrate-supported and the suspended devices exhibit distinct breakdown behavior which can be explained by the diffusive thermal transport model. The breakdown temperature of the nanowire is estimated to be about 450?K, close to the melting temperature of indium. We demonstrated the usefulness of this technique by successful fabrication of working pentacene field-effect transistors.     

Growth of InAs/InAsSb heterostructured nanowires

We report the growth of InAs/InAs(1-x)Sb(x) single and double heterostructured nanowires by Au-assisted chemical beam epitaxy. The InAs(1-x)Sb(x) nanowire segments have been characterized in a wide range of antimony compositions. Significant lateral growth is observed at intermediate compositions (x ~ 0.5), and the nucleation and step-flow mechanism leading to this lateral growth has been identified and described. Additionally, CuPt ordering of the alloy has been observed with high resolution transmission electron microscopy, and it is correlated to the lateral growth process. We also show that it is possible to regrow InAs above the InAsSb alloy segment, at least up to an intermediate antimony composition. Such double heterostructures might find applications both as mid-infrared detectors and as building blocks of electronic devices taking advantage of the outstanding electronic and thermal properties of antimonide compound semiconductors.     

Investigation of indium oxide as a self-catalyst in ZnO/ZnInO heterostructure nanowires growth

We investigated the self-catalytic role of indium oxide in the growth process of ZnO/ZnInO heterostructure nanowires on Si(111). The prepared nanowires had hexagonal cross sections and were tapered with tip diameters of 90 ± 5 nm and base diameters of 230 ± 5 nm. Energy dispersive X-ray and field emission Auger spectroscopies indicated that the grown nanowires were heterostructures of ZnO and ZnInO. Analysis of the early growth process revealed that indium may play a self-catalytic role. Therefore, the vapor-liquid-solid mechanism is likely to be responsible for growth of ZnO/ZnInO nanowires. X-ray diffraction and room temperature photoluminescence (PL) data demonstrated that the presence of indium results in a decrease in nanowires' crystallinity. These wires produced a large PL emission peak in the ultraviolet (UV) region and a smaller peak in the green region of the electromagnetic spectrum. The UV peak of the ZnO/ZnInO nanowires is blue-shifted with respect to that of pure ZnO nanowires.     

Directional emission from beryllium doped GaAs/AlGaAs nanowires

The emission directionality of self-catalytic GaAs nanowires in an AlGaAs shell, produced by molecular-beam epitaxy with a varied level of beryllium doping, is studied. It is shown that an undoped sample possesses pronounced waveguide properties along the growth direction. With increasing doping level, the intensity of the emission directed perpendicular to the lateral nanowire walls grows.     

Optical emission of InAs nanowires

Wurtzite InAs nanowire samples grown by chemical beam epitaxy have been analyzed by photoluminescence spectroscopy. The nanowires exhibit two main optical emission bands at low temperatures. They are attributed to the recombination of carriers in quantum well structures, formed by zincblende-wurtzite alternating layers, and to the donor-acceptor pair. The blue-shift observed in the former emission band when the excitation power is increased is in good agreement with the type-II band alignment between the wurtzite and zincblende sections predicted by previous theoretical works. When increasing the temperature and the excitation power successively, an additional band attributed to the band-to-band recombination from wurtzite InAs appears. We estimated a lower bound for the wurtzite band gap energy of approximately 0.46?eV at low temperature.     

MWCNT/ZnO纳米线复合阴极薄膜的场发射特性

实验提出以多壁碳纳米管(MWCNT)/ZnO纳米线复合材料作为场发射阴极薄膜,研究其图形化制备工艺以及其场发射特性.用丝网印刷工艺制备图形化MWCNT/ZnO纳米线复合阴极薄膜,实验获得合适的浆料配比以及适合的烘烤和烧结温度.对MWCNT/ZnO纳米线样品进行SEM分析和场发射特性测试,发现图形化阴极设计提高了场发射电流,并且改善场发射发光均匀度;材料组分的低维化明显降低场发射开启电压;加电老练处理有效改善场发射特性.     

Temperature dependence of AlInAs band gap energy and AlInAs/InP band offsets

Abstract

The temperature variations of the AlInAs photoluminescence (PL) transition energies and the AlInAs/InP interface staggered lineup luminescence (SLL) energy are reported. The S shape appearing from 4 to 90 K on the energy v. temperature curves of these PL energies are owing to extrinsic recombinations. In particular, the S shape of the SLL energy curve v. temperature is probably a result of acceptor impurities localised in AlInAs at the interface (on edge impurities). The band offsets were determined by solving the Schrödinger and Poisson equations with a self consistent calculation program. At 4.5 K, the conduction and valence band offsets are 0.384 and 0.295 eV respectively. Their temperature variation is shown to be important: 35 and 23 meV at 4.5 and 300 K respectively. The Van Vechten and Malloy model (following a thermodynamic approach) for the temperature variation of the band offsets is compared to the results in the present work.     

SrTiO3/TiO2 heterostructure nanowires with enhanced electron--hole separation for efficient photocatalytic activity

Heterostructure is an effective strategy to facilitate the charge carrier separation and promote the photocatalytic performance. In this paper, uniform SrTiO₃ nanocubes were in-situ grown on TiO₂ nanowires to construct heterojunctions. The composites were prepared by a facile alkaline hydrothermal method and an in-situ deposition method. The obtained SrTiO₃/TiO₂ exhibits much better photocatalytic activity than those of pure TiO₂ nanowires and commercial TiO₂ (P25) evaluated by photocatalytic water splitting and decomposition of Rhodamine B (RB). The hydrogen generation rate of SrTiO₃/TiO₂ nanowires could reach 111.26 mmol·g⁻¹·h⁻¹ at room temperature, much better than those of pure TiO₂ nanowires (44.18 mmol·g⁻¹·h⁻¹) and P25 (35.77 mmol·g⁻¹·h⁻¹). The RB decomposition rate of SrTiO₃/TiO₂ is 7.2 times of P25 and 2.4 times of pure TiO₂ nanowires. The photocatalytic activity increases initially and then decreases with the rising content of SrTiO₃, suggesting an optimum SrTiO₃/TiO₂ ratio that can further enhance the catalytic activity. The improved photocatalytic activity of SrTiO₃/TiO₂ is principally attributed to the enhanced charge separation deriving from the SrTiO₃/TiO₂ heterojunction.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.     

锐钛矿/TiOz(B)异质纳米线制备、表征及其光催化性能研究

通过简单的水热反应,和后续的退火处理得到锐钛矿/TiO2(B)异质结构纳米线.通过XRD、SEM和TEM对其进行表征.并对锐钛矿/TiO2 (B)进行甲基橙紫外光降解性能测试,探究和讨论了H2O2对其光催化性能的影响,实验表明当加入1.6mL H2O2时对体系光催化促进效果最优,只需要8min分解率达到99％,降解时间只为无H2O2时的1/5.     

Characteristics of stimulated emission from an optically pumped GaN/AlGaN double heterostructure

The luminescence properties of a GaN/Al_0.1Ga_0.9N double heterostructure grown by vapor-phase deposition from organometallic compounds are studied. When luminescence is observed from the end, the radiation intensity shows a sharply defined threshold dependence on the pump density. The threshold excitation density at T=77 K was ∼40 kW/cm² and the wavelength of the stimulated emission was λ=357 nm. The long-wavelength shift of the emission line at high pump densities may be attributed to renormalization of the band gap caused by many-particle interactions in the electron-hole plasma. Pis’ma Zh. Tekh. Fiz. 23, 53–59 (August 12, 1997)     

Diameter dependent growth rate and interfacial abruptness in vapor-liquid-solid Si/Si1-xGex heterostructure nanowires

A strong diameter dependence is observed in the interfacial abruptness and growth rates in Si/Si 1- x Ge x axial heterostructure nanowires grown via Au-mediated low pressure CVD using silane and germane precursors. The growth of these nanowires has similarities to that of heterostructure thin films with similar compositional interfacial broadening, which increases with and is on the order with diameter. This broadening may reveal a fundamental challenge to fabrication of abrupt heterostructures via VLS growth.     

生长温度对InAs/GaAs横向异质结构纳米线形貌及晶体结构的影响

低维半导体材料因其超常的物理性能而受到了广泛关注和研究。本文采用金属有机物化学气相沉积（MOCVD）技术,利用金作催化剂制备了InAs/GaAs横向异质结构纳米线,并讨论了不同生长温度情况下InAs横向异质材料对纳米线形貌及晶体结构的影响。提高InAs材料的生长温度,可以有效地抑制纳米线的纵向生长,使其实现横向异质结构的生长。在异质结构纳米线横向生长时发生了侧面晶面旋转的现象,这是纳米线表面重构后侧面趋向能量更低的晶面的结果。本文的研究工作为推动微纳技术的发展提供了相应的理论基础和科学依据。      

二维层状WS2/MoS2异质结中镧系近红外光子发射与能量转移（英文）

镧系离子由于其独特的光子特性而备受关注.二维层状范德华异质结的光电特性和器件性能受到界面耦合的极大影响,该异质结通常是由两层或多层过渡金属二硫化物(TMD)堆叠而成.本文通过两步合成构建了镧系离子掺杂的层状WS2/MoS2异质结.所制备的掺杂薄膜是在晶圆衬底上生长的高度织构纳米片.更重要的是,由于两个TMD层中镧系离子之间的能量转移,层状异质结的结构减少了因均匀掺杂或浓度猝灭而引起的无益交叉松弛,所制备的堆叠异质结能够在近红外通讯窗口产生高效的光子发射.镧系掺杂和能量转移的研究结果表明,镧系离子可以有效地扩展TMD薄膜的发射波段及其异质结构.本工作所发展的镧系掺杂TMD异质结有助于进一步研究原子级超薄近红外光子器件.