一维WO3-ZnO异质结构的合成及生长机理研究

本文采用两步水热法成功合成出一维WO3-ZnO分级异质结构.TEM表征发现ZnO纳米棒在WO3纳米带表面均匀排列,ZnO纳米棒和WO3均各自沿着[0001]方向生长,且二者之间存在特定的取向关系,即生长方向成44°或者136°角.晶体学分析表明在ZnO和WO3之间存在失配度很小的取向关系,即(0001)WO3//(1(1)02)ZnO和(11(22))WO3//(0001)ZnO,以及经过翻转后出现的(0001)WO3//(1(1)02)ZnO和(11(2)2)WO3//(0001)ZnO.根据实验和晶体学分析结果,本文认为ZnO棒在WO3纳米带上的生长机理是符合Volmer-Weber模型的外延生长.     

Selective growth of InP by MOCVD around dry-etched mesas having various patterns for photonic integrated circuits

We have investigated the selective growth ofpnp- InP by metalorganic chemical vapor deposition (MOCVD) around mesas. The mesas were formed by reactiveion-etching using SiO₂ masks having various patterns. The patterns include stripes along different crystallographic orientations, Y-bifurcations, and X-crossings. The behavior of the growth, and that of the mass transport before the growth, were found to depend remarkably on the crystallographic direction of the mesa stripe. The most notable mass transport, which transforms a nearly vertical mesa wall to a sloped one, was observed for a stripe misoriented by about 30° from the [011] direction. The growth was observed to proceed preferentially on the sloped planes, planarizing the crystal surface, for a mesa stripe directed between the [011] and [010] directions. However, for the stripe direction near [01-1], the growth was observed to creep up a nearly vertical plane on the mesa wall, and sloped planes are reluctant to form. Selective growth with sufficient planarity has been obtained for various waveguides including the Y-bifurcations and the X-crossings, which are used as the waveguide components in photonic integrated circuits, when the stripes are almost along the [011] direction.     

2D van der Waals Heterojunction of Organic and Inorganic Monolayers for High Responsivity Phototransistors

Van der Waals (vdW) heterostructures composing of organic molecules with inorganic 2D crystals open the door to fabricate various promising hybrid devices. Here, a fully ordered organic self-assembled monolayer (SAM) to construct hybrid organic–inorganic vdW heterojunction phototransistors for highly sensitive light detection is used. The heterojunctions, formed by layering MoS₂ monolayer crystals onto organic [12-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)dodecyl)]phosphonic acid SAM, are characterized by Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy. Remarkably, this vdW heterojunction transistor exhibits a superior photoresponsivity of 475 A W⁻¹ and enhanced external quantum efficiency of 1.45 × 10⁵%, as well as an extremely low dark photocurrent in the pA range. This work demonstrates that hybridizing SAM with 2D materials can be a promising strategy for fabricating diversified optoelectronic devices with unique properties.     

GaAs MESFET栅取向效应中背栅的作用

根据压电效应模型,本文详细研究了压电电荷对GaAs MESFET沟道与衬底界面耗尽层的影响。认为正压电电荷比负压电电荷所引起的阀值电压漂移大,较好解释了(100)衬底上沿[011]和[011]取向的GaAs MESFET阈值电压非对称反向漂移的现象。     

Growth effects of In0.53Ga0.47As on InP structured substrates

LPE growth studies of In0.53Ga0.47As on structured InP substrates orientated in the (100) and (111)B planes have been carried out. A reluctance to grow in the [111]A and [111]B directions has been found while nucleation readily occurs on (100) faces and rounded surfaces. Selective growth in etched channels was possible without masking.     

Analysis of non-uniform current and temperature distributions in the emitter finger of AlGaAs/GaAs heterojunction bipolar transistors

This paper develops a comprehensive and two-dimensional model for the AlGaAs/GaAs heterojunction bipolar transistor (HBT). The model takes into account the electrical-thermal interacting mechanism in both the width and length directions of the HBT emitter finger and thus is capable of describing the two-dimensional temperature and current distributions in the emitter finger of the HBT. Results produced from a three-dimensional device simulator are also included in support of the model.     

A Thermal Conductivity Model for Micro-Nanoscale Diamond Thin Films Using Dispersion Curve Data

Thermal conductivity is investigated for a cubic C (diamond). Boundary scattering, Umklapp processes, normal processes, and the presence of impurities are the mechanisms considered for heat flow resistance. Three symmetry directions [001], [110], [111], and three polarizations for each direction in the first Brillouin zone are considered for the material. The main purpose of this study is to analyze the effect of the curvature of phonon dispersion curves on the thermal conductivity, and develop an accurate model. The model incorporates the effects of impurity and impurity concentration, film thickness, and crystal orientation on thermal conductivity. The model is validated by comparing results with experimental data for diamond in the [111] direction. The model is then used for the other symmetry directions using the appropriate dispersion curves. The results show that the curvature of the dispersion curves dramatically affects the thermal conductivity. A sensitivity analysis is conducted to study the effect of boundary scattering as the film decreases in thickness and the effect of impurities.     

[110] strained-SOI n-MOSFETs with higher electron mobility

We have recently developed [110]-surface strained silicon-on-insulator (SOI) n-MOSFETs. The strained-silicon (Si) layer with the strain of about 0.6% has been fabricated on a relaxed SiGe-on-insulator (SGOI) structure with the germanium (Ge) content of 25%. The electron mobility characteristics along the various current directions have been experimentally studied and compared to those of [100]- and [110]-surface unstrained-bulk MOSFETs. We have demonstrated, for the first time, that the electron mobility of [110] strained-SOI MOSFETs is enhanced, compared to that of [110] unstrained-bulk MOSFETs. The electron mobility enhancement depends on the current-flow directions, and the maximum enhancement factor amounts to 23% along the <001> direction. As a result, the electron mobility ratio of [110] strained-SOI MOSFETs to [100] universal mobility is 81% at maximum, whereas the ratio of [110] unstrained-bulk MOSFETs is only 66%. Therefore, [110] strained-SOI devices are also promising candidates for future high-performance CMOS.     

Toward Faster Organic Photodiodes: Tuning of Blend Composition Ratio

The ability of a light-sensor to detect fast variation in incident light intensity is a vital feature required in imaging and data transmission applications. Solution-processed bulk heterojunction (BHJ) type organic photodiodes (OPDs) have gone through key developments, including dark current mitigation and longer linear dynamic range. In contrast, there has been less focus on increasing OPD response speed (f_–3dB). Here, bulk heterojunction OPDs based on electron-donating polymer poly[thiophene-2,5-diyl-alt-5,10-bis((2-hexyldecyl)oxy)dithieno[3,2-c:3′,2′-h][1,5]naphthyridine-2,7-diyl] (or PTNT) and electron-accepting phenyl-C₇₁-butyric acid methyl ester (or PC₇₁BM) are reported. The intrinsic charge transport characteristics required for fast speed OPDs are discussed, and an analytical model for the same is developed. The OPDs present 0.8 MHz f_–3dB under no applied voltage bias for a typical blend ratio of 1:1 by weight. It is shown that balanced electron and hole mobility is a critical criterion for faster speed OPDs, which can be realized by tuning the composition ratio of the bulk heterojunction. By tuning PTNT and PC₇₁BM blend ratio, the f_–3dB was successfully raised by more than quadruple to 4.5 MHz. The findings provide a tool to set device architecture for faster next-generation light sensors.     

High mobility ambipolar organic field-effect transistors with a nonplanar heterojunction structure

Ambipolar organic field-effect transistors (OFETs) based on a bilayer structure of highly crystalline small molecules, n-type α,ω-diperfluorohexylquaterthiophene (DFH-4T) and p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are investigated. By employing DFH-4T/DNTT as the bottom/top layers and appropriate high work function (WF) electrodes in a bottom-gate, top-contact configuration, the superior ambipolar characteristics with matched electron and hole mobilities of 1–1.1 cm² V⁻¹ s⁻¹ are achieved. Intriguingly, this high-performance device exhibits a unique feature of an extremely rough, nonplanar heterojunction in the DFH-4T/DNTT combination and a large electron injection barrier from the high WF electrodes to DFH-4T, suggesting some underlying mechanisms for the effective charge transport and injection. The electrical and structural analyses reveal that the crystal packing of the bottom DFH-4T layer supports the growth of a high-quality DNTT crystal network for high-mobility hole transport upon the nonplanar heterojunction, and also enables the formation of an enlarged organic/metal contact surface for efficient electron injection from the high WF electrodes, as the key attributes leading to an overall excellent ambipolar behavior. The effect of intrinsic charge accumulation at the heterojunction interface on the ambipolar conduction is also discussed. Furthermore, a complementary-like inverter constructed with two DFH-4T/DNTT ambipolar OFETs is demonstrated, which shows a gain of 30.     

Resolving degradation mechanisms in ultra-high performance N-p-nheterojunction bipolar transistors

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown Al_xGa_1-xAs/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBTs). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current ratio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V _BE uniformity and long-term reliability of HBTs     

Investigation of short-term current gain stability of GaInP/GaAs-HBTs grown by MOVPE

Initial current gain variation in GaInP/GaAs-HBTs stressed at high-current density was investigated in dependence on base–dopant concentration and hydrogen passivation. It is shown that current-induced acceptor activation results in short-term current gain reduction, but does not affect emitter–base heterojunction properties. The incorporation of hydrogen during growth and the resulting passivation of carbon acceptors in the base layer depends mainly on carbon doping level and the precursors present during growth and cool-down. Under the growth conditions used, in-diffusion of hydrogen within AsH₃-rich growth steps turned out to be the major source of base–dopant passivation. Utilizing the blocking effect of n-type layers for H⁺ in-diffusion we were able to reduce the base passivation ratio [H]/[C] to 0.1 leading to significant improvements in gain stability. It is shown that current gain instabilities can be completely suppressed by using a high-current density pre-stress treatment.     

TEM study of the effect of growth interruption in MBE of InGaP/GaAs superlattices

The influence of growth interruption during the MBE growth of (100) In_0.5Ga_0.5P/GaAs superlattices is investigated by cross-sectional TEM. A roughening of the growth front is observed during an interruption after the exchange of the group-V molecular beams. The roughening of growth front occurs due to a spontaneous change in the growth orientation of the superlattice from [100] to 〈311〉 directions. This change in growth orientation is characterized by an initial formation of V-shaped grooves with {311} facets on the GaAs growth front which eventually lead to the formation of regions of {311} superlattice structures. The direction of V-shaped grooves is along the [011] axis, which is parallel to the surface dangling bonds of the group V atoms in the unreconstructed (100) plane. The most critical stage for the spontaneous change of the growth orientation is the interruption after the growth of a GaAs layer with the P₂ flux.     

掺In半绝缘GaAs衬底上外延GaAs的晶格失配研究

本文报道了在掺In半绝缘GaAs衬底上的液相和汽相外延生长,并用x射线双晶衍射和光学显微等方法研究外延层和衬底之间的晶格失配.结果表明,当衬底中In组分x<0.004时,外延层失配应力主要由弹性形变调节,不出现失配位错,并可得到很好的表面形貌;当x≥0.006时,外延层产生失配位错,失配应力主要由失配位错调节,液相外延层表面出现沿[110]和[110]方向的十字网络.当外延层产生范性形变时衬底中的临界In组分x_c在0.004和0.006之间.     

Preparation and optical property of porous ZnO nanobelts

Porous ZnO nanobelts self-assembled by nanoparticles were fabricated via a two-step process. First, the precursors were prepared via a simple, rapid, controllable solution route. Then, ligand molecules and water molecules can be removed from the precursors during calcinations in air, as a result, porous ZnO nanobelts are formed. In the edge of the nanobelts and the narrow nanobelts, ZnO nanoparticles align wall growth along [0001] directions were observed. In the middle of the wide belts, some nanoporous wall grows along [10−10] and [2−1−10]. The growth mechanism is discussed in detail from beltlike precursor to ZnO nanobelts. The porous ZnO nanobelts exhibit a strong UV emission.     

Characterization of cross-hatch morphology of MBE (211) HgCdTe

We present the results of a detailed study of the nature and origin of cross-hatch patterns commonly observed on (211) HgCdTe epilayers deposited by molecular beam epitaxy. Cross-hatch patterns were examined using x-ray topography as well as Nomarski, interferometric, and atomic force microscopies. Cross-hatch patterns were generally comprised of three sets of lines, parallel to the [231],, [213], and [011] directions. The lines parallel to the [011] direction exhibited distinct properties compared to the two sets of lines parallel to [231] and [213]. Under growth conditions characterized by excessive Hg flux (low temperature), lines parallel to [011] were periodic and tended to dominate the cross-hatch pattern. In some cases, bands of dislocations, 10–100 m in width, formed parallel to [011]. Under optimized growth conditions, on very closely lattice-matched substrates, (dislocation densities <10⁵ cm⁻²) lines parallel to [011] vanished entirely, and lines parallel to [231] and [213] became sparse. The remaining lines were typically fragments terminated by either a single dislocation, a cluster of dislocations (micro-void), or the wafer's edge. The density of these line fragments tended to decrease as the dislocation density decreased. Under the best growth conditions on very closely lattice-matched substrates we have achieved dislocation densities of 5 10⁴ cm⁻², which is comparable to the dislocation density of the CdZnTe substrate.     

Special features of structural defects in undoped CdTe textured ingots produced by free growth from a gasdynamic vapor flow

The structural features of undoped CdTe ingots grown in a gasdynamic flow at 620°C were studied by selective etching and X-ray diffractometry. It is found that the samples grown at a deposition rate of up to 500 μm/h consist of independently growing rods with both [111] A and [111] B directions. This indicates that the vapor composition in the growth region is almost stoichiometric. Both rod types exhibited transverse striations due to rotation twins. The twin boundaries in rods with the growth direction [111] A were shown to be also small-angle boundaries with additional misorientation of separate twins of 0.2°–0.3°. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 129–133. Original Russian Text Copyright ? 2003 by Klevkov, Martovitskii, Medvedev.     

In Situ Growth of [hk1]‐Oriented Sb2S3 for Solution‐Processed Planar Heterojunction Solar Cell with 6.4% Efficiency

Binary compound antimony sulfide (Sb₂S₃) with its nontoxic and earth‐abundant constituents, is a promising light‐harvesting material for stable and high efficiency thin film photovoltaics. The intrinsic quasi‐1D (Q1D) crystal structure of Sb₂S₃ is known to transfer photogenerated carriers rapidly along the [hk1] orientation. However, producing Sb₂S₃ devices with precise control of [hk1] orientation is challenging and unfavorable crystal orientations of Sb₂S₃ result in severe interface and bulk recombination losses. Herein, in situ vertical growth of Sb₂S₃ on top of ultrathin TiO₂/CdS as the electron transport layer (ETL) by a solution method is demonstrated. The planar heterojunction solar cell using [hk1]‐oriented Sb₂S₃ achieves a power conversion efficiency of 6.4%, performing at almost 20% higher than devices based on a [hk0]‐oriented absorber. This work opens up new prospects for pursuing high‐performance Sb₂S₃ thin film solar cells by tailoring the crystal orientation.     

The second-order magnetization precession in an anisotropic medium. Part 2: The cubic anisotropy

The second-order precession of the magnetization vector in a normally magnetized magnetic plate with the cubic anisotropy is considered for the [001], [011], and [111] orientations of the axes of a cubic cell along the static field. The precession pattern of the forced oscillation in the form of a large ring and small rings located along the envelope is obtained. A relationship between the observed high- and low-density groups of small rings and the spatial position of the [111] easy magnetization axes is revealed and explained on the basis of the energy model of the potential. It is found that the phenomenon is highly sensitive to the orientation of cubic axes, the anisotropy constant, and the intensities and directions of the static and alternating fields. It is reported that the observed phenomena can be used in practice.     

Coalescence of InP Epitaxial Lateral Overgrowth by MOVPE with V/III Ratio Variation

The effects of V/III ratio and seed window orientation on the coalescence of epitaxial lateral overgrowth InP over SiO₂ using metal organic vapor-phase epitaxy with tertiary butyl phosphine were investigated. Parallel lines having θ = 60° and 30° off [0[`1] \bar{1} 1] were coalesced, and their lateral growth rate variation with V/III was measured. Coalescence of lines separated by narrow angles in a star-like pattern was also studied. We find the greatest extent of coalescence to occur when the window stripe is oriented just off of the ⟨010⟩ directions. V/III ratio strongly affects the extent of coalescence, showing an alternating enhancement or inhibition depending on which side of the ⟨010⟩ direction the stripes are oriented. The variation in quality of coalesced material between stripes separated by narrow angles is examined with cross-sectional transmission electron microscopy, illustrating the most problematic growth directions under two V/III ratio conditions.