Thermal stability and surface passivation of Ge nanowires coated by epitaxial SiGe shells

Epitaxial growth of a highly strained, coherent SiGe alloy shell around a Ge nanowire core is investigated as a method to achieve surface passivation and carrier confinement, important in realizing nanowire devices. The high photoluminescence intensity observed from the core-shell nanowires with spectral features similar to that of bulk Ge indicates effective surface passivation. Thermal stability of these core-shell heterostructures has been systematically investigated, with a method demonstrated to avoid misfit strain relaxation during postgrowth annealing.     

Superior Thermoelectric Performance of SiGe Nanowires Epitaxially Integrated into Thermal Micro-Harvesters

Semiconductor nanowires have demonstrated fascinating properties with applications in a wide range of fields, including energy and information technologies. Particularly, increasing attention has focused on SiGe nanowires for applications in a thermoelectric generation. In this work, a bottom-up vapour-liquid-solid chemical vapour Deposition methodology is employed to integrate heavily boron-doped SiGe nanowires on thermoelectric generators. Thermoelectrical properties –, i.e., electrical and thermal conductivities and Seebeck coefficient – of grown nanowires are fully characterized at temperatures ranging from 300 to 600 K, allowing the complete determination of the Figure-of-merit, zT, with obtained values of 0.4 at 600 K for optimally doped nanowires. A correlation between doping level, thermoelectric performance, and elemental distribution is established employing advanced elemental mapping (synchrotron-based nano-X-ray fluorescence). Moreover, the operation of p-doped SiGe NWs integrated into silicon micromachined thermoelectrical generators is shown over standalone and series- and parallel-connected arrays. Maximum open circuit voltage of 13.8 mV and power output as high as 15.6 µW cm⁻² are reached in series and parallel configurations, respectively, operating upon thermal gradients generated with hot sources at 200 °C and air flows of 1.5 m s⁻¹. These results pave the way for direct application of SiGe nanowire-based micro-thermoelectric generators in the field of the Internet of Things.     

核壳结构SiC/SiO2纳米线的低温合成与表征

以酚醛树脂(PF)作为碳源, 纳米SiO₂为硅源, 在1300℃氩气气氛下通过碳热还原反应, 制备出具有核壳结构的SiC/SiO₂纳米线。采用X射线分析衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、高分辨率透射电子显微镜(HRTEM)、拉曼光谱(Raman)对产物的组成、形貌、微观结构等进行了表征。结果表明; SiC/SiO₂纳米线长可达数毫米, 单根SiC/SiO₂纳米线由直径30 nm的β-SiC晶体为内核和厚度约12 nm的无定形SiO₂壳层组成; 室温下SiC/SiO₂纳米线的PL发光峰与β-SiC单晶的发光特征峰相比有蓝移。最后, 讨论了核壳结构SiC/SiO₂纳米线的生成机制。     

Polarization-induced pn diodes in wide-band-gap nanowires with ultraviolet electroluminescence

Almost all electronic devices utilize a pn junction formed by random doping of donor and acceptor impurity atoms. We developed a fundamentally new type of pn junction not formed by impurity-doping, but rather by grading the composition of a semiconductor nanowire resulting in alternating p and n conducting regions due to polarization charge. By linearly grading AlGaN nanowires from 0% to 100% and back to 0% Al, we show the formation of a polarization-induced pn junction even in the absence of any impurity doping. Since electrons and holes are injected from AlN barriers into quantum disk active regions, graded nanowires allow deep ultraviolet LEDs across the AlGaN band-gap range with electroluminescence observed from 3.4 to 5 eV. Polarization-induced p-type conductivity in nanowires is shown to be possible even without supplemental acceptor doping, demonstrating the advantage of polarization engineering in nanowires compared with planar films and providing a strategy for improving conductivity in wide-band-gap semiconductors. As polarization charge is uniform within each unit cell, polarization-induced conductivity without impurity doping provides a solution to the problem of conductivity uniformity in nanowires and nanoelectronics and opens a new field of polarization engineering in nanostructures that may be applied to other polar semiconductors.     

Non-selective growth of SiGe heterojunction bipolar trasistor layers at 700 °C with dual control of n- and p-type dopant profiles

This paper describes the growth of the collector, base, and emitter layers of a SiGe HBT in a single epitaxy process. A non-selective SiGe heterojunction bipolar transistor growth process at 700 °C has been developed, which combines n-type doping for the Si collector, p-type doping for the SiGe base and n-type doping for the Si emitter cap. Control of the collector doping concentration by varying the growth conditions is shown. The boron tailing edge from the SiGe base into the Si emitter layer was removed by interrupting the growth process with a hydrogen flow after the SiGe base growth but before the Si emitter growth to remove the dopant gas from the chamber. The layer thicknesses are compared using three different analytical techniques–secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and spectroellipsometry. A good agreement was obtained for the three different methods.     

Characterization of luminescence properties of V2O5-capped ZnSe nanowires

ZnSe-core/V2O5-shell nanowires were synthesized by the thermal evaporation of ZnSe powders on gold-coated Si (100) substrates followed by the sputter depositon of V2O5. Scanning electron microscopic images showed that the core-shell nanowires were a few tens to a few hundreds of nanometers in diameter and a few hundreds of micrometers in length. Transmission electron microscopy and X-ray diffraction analyses revealed that the core and shell of the core-shell nanowires were single crystal wurtzite-structured ZnSe and amorphous V2O5, respectively. Photoluminescence measurement showed that the core-shell nanowires as-synthesized or annealed in an oxidative atmosphere had a green emission band centered at around 520 nm whereas the as-synthesized ZnSe nanowires and the ZnSe-core/V2O5-shell nanowires annealed in a reducing atmosphere had a yellow emission band centered at around 590 nm. Our results also showed that V2O5 capping with an optimal thickness and subsequent annealing in a reducing atmosphere could significantly enhance the emission intensity of the ZnSe nanowires. In addition, the origins of the enhancement in intensity and the blue shift of the major emission by V2O5 capping are discussed.     

High quality GaAs nanowires grown on glass substrates

We report for the first time the growth of GaAs nanowires directly on low-cost glass substrates using atmospheric pressure metal organic vapor phase epitaxy via a vapor-liquid-solid mechanism with gold as catalyst. Substrates used in this work were of float glass type typically seen in household window glasses. Growth of GaAs nanowires on glass were investigated for growth temperatures between 410 and 580 °C. Perfectly cylindrical nontapered nanowires with a growth rate of ~33 nm/s were observed at growth temperatures of 450 and 470 °C, whereas highly tapered pillar-like wires were observed at 580 °C. Nanowires grew horizontally on the glass surface at 410 °C with a tendency to grow in vertically from the substrate as the growth temperature was increased. X-ray diffraction and transmission electron microscopy revealed that the nanowires have a perfect zinc blende structure with no planar structural defects or stacking faults. Strong photoluminescence emission was observed both at low temperature and room temperature indicating a high optical quality of GaAs nanowires. Growth comparison on impurity free fused silica substrate suggests unintentional doping of the nanowires from the glass substrate.     

Core-shell morphology and characterization of carbon nanotube nanowires click coupled with polypyrrole

Core-shell nanowires having multiwalled carbon nanotubes (MWNT) as a core and polypyrrole (PPy) as a shell were synthesized using Cu(I)-catalyzed azide-alkyne cycloaddition click chemistry. According to transmission electron microscopy measurements, the uniform PPy layers of 10-20 nm in thickness were formed well on the MWNT's surface. In particular 'grafting from' click coupling was more effective in obtaining uniform and stable core-shell nanowires as well as in the reaction yield, compared to 'grafting to' click coupling. This is due to chemical bond formation between PPy and MWNT in equal intervals along the longitudinal direction of the MWNT, achieved by 'grafting from' click coupling. As a result, the core-shell nanowires were very stable even in the sonication of nanowires and showed an enhanced electrical conductivity of 80 S cm(-1), due to the synergetic interaction between MWNTs and PPy, which is higher than the conductivity of pure MWNTs and pure PPy. In addition, the core-shell nanowires could show better NO2 gas sensing properties compared to pure MWNTs and pure PPy as well as MWNT/PPy composites prepared by in situ polymerization. The synthesized core-shell nanowires would play an important role in preparing electrical and sensing devices.     

SiGe nanowire growth and characterization

Single-crystal SiGe nanowires were synthesized via the vapour-liquid-solid (VLS) growth mechanism using disilane and germane as precursor gases. We have investigated the effect of temperature, pressure, and the inlet gas ratio on the growth and stoichiometry of Si(x)Ge(1-x) nanowires. The nanowires were characterized using scanning and transmission electron microscopies and energy dispersive x-ray analysis. It was found that nanowires with a Si:Ge ratio of about 1 had smooth surfaces, whereas departure from this ratio led to rough surfaces. Electrical properties were then investigated by fabricating back-gated field effect transistors (using a focused ion beam system) where single SiGe nanowires served as the conduction channels. Gated conduction was observed although resistance in the undoped devices was high.     

Direct growth of core-shell SiC-SiO(2) nanowires and field emission characteristics

A simple, direct synthesis method was used to grow core-shell SiC-SiO(2) nanowires by heating NiO-catalysed silicon substrates. A carbothermal reduction of WO(3) provided a reductive environment and carbon source to synthesize crystalline SiC nanowires covered with SiO(2) sheaths at the growth temperature of 1000-1100?°C. Transmission electron microscopy showed that the SiC core was 15-25?nm in diameter and the SiO(2) shell layer was an average of 20?nm in thickness. The thickness of the SiO(2) shell layer could be controlled using hydrofluoric acid (HF) etching. Field emission results of core-shell SiC-SiO(2) and bare SiC nanowires showed that the SiC nanowires coated with an optimum SiO(2) thickness (10?nm) have a higher field emission current than the bare SiC nanowires.     

On the p-type character of Cd-and Zn-doped InAs nanowires

InAs nanowires are potential materials for high speed nanoelectronic devices due to their high electron mobility among the semiconductor nanostructures. One of the main challenges, however, is to obtain a p-type InAs material, since the Fermi level is usually pinned at the conduction band, leading to an intrinsic n-type behaviour. Here we show through first principles calculations that InAs nanowires, doped with Cd or Zn substitutional impurities, can behave as p-type materials. Differently from other III-V nanowires, these impurities introduce shallow acceptor levels. We show that the Zn impurity can be equally distributed along the nanowire radius, naturally compensating the surface levels. On the other hand, the Cd impurity is preferentially found in the core region, requiring a surface treatment to eliminate the surface pinning levels. These results explain the available experiments and show how and why p-type InAs nanowires can be obtained.     

The growth and characterization of ZnO/ZnTe core-shell nanowires and the electrical properties of ZnO/ZnTe core-shell nanowire field effect transistor

Vertically aligned ZnO/ZnTe core-shell nanowires were grown on a-plane sapphire substrate by using chemical vapor deposition with gold as catalyst for the growth of ZnO core and then followed by growing ZnTe shell using metal-organic chemical vapor deposition (MOCVD). Transmission electron microscope (TEM) and Raman scattering indicate that the core-shell nanostructures have good crystalline quality. Three-dimensional fluorescence images obtained by using laser scanning confocal microscope demonstrate that the nanowires have good optical properties. The core-shell nanowire was then fabricated into single nanowire field effect transistor by standard e-beam photolithography. Electrical measurements reveals that the p-type ZnO/ZnTe FET device has a turn on voltage of -1.65 V and the hole mobility is 13.3 cm2/V s.     

Controlled growth of ternary alloy nanowires using metalorganic chemical vapor deposition

We report the growth and characterization of ternary AlxGa1- xAs nanowires by metalorganic chemical vapor deposition as a function of temperature and V/III ratio. Transmission electron microscopy and energy dispersive X-ray spectroscopy show that, at high temperatures and high V/III ratios, the nanowires form a core-shell structure with higher Al composition in the nanowire core than in the shell. We develop a growth model that takes into account diffusion of reactants and decomposition rates at the nanowire catalyst and stem to describe the compositional difference and the shell growth rate. Utilizing this model, we have successfully grown compositionally uniform Al0.16Ga0.84As nanowires. The ability to rationally tune the composition of ternary alloy nanowires broadens the application range of nanowires by enabling more complex nanowire heterostructures.     

Electronic level scheme in boron- and phosphorus-doped silicon nanowires

We report the first observation of the electronic level scheme in boron (B)- and phosphorus (P)-doped nanowires (NWs). The NWs' morphology dramatically depends on the doping impurity while a few deep electronic levels appear in both kinds of nanowires, independently of the doping type. We demonstrate that the doping impurities induce the same shallow levels as in bulk silicon. The presence of two donor levels in the lower half-bandgap is also revealed. In both kinds of NWs, B- and P-doped, the donor level (0/+) at E(v) + 0.36 eV of the gold-hydrogen complex is observed. This means that the gold diffusion from the NW tip introduces an electronically active level, which might negatively affects the electrical characteristics of the NWs. In P-doped NWs, we observed a further donor level at 0.26 eV above the valence band due to the phosphorus-vacancy pairs, the E-center, well-known in bulk silicon. These findings seriously question both diffusion modeling of impurities in NWs and the technological aspects arising from this.     

Vertically aligned ZnO/amorphous-Si core-shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.     

Role of confinement on carrier transport in Ge-Si(x)Ge(1-x) core-shell nanowires

We examine the impact of shell content and the associated hole confinement on carrier transport in Ge-Si(x)Ge(1-x) core-shell nanowires (NWs). Using NWs with different Si(x)Ge(1-x) shell compositions (x = 0.5 and 0.7), we fabricate NW field-effect transistors (FETs) with highly doped source/drain and examine their characteristics dependence on shell content. The results demonstrate a 2-fold higher mobility at room temperature, and a 3-fold higher mobility at 77K in the NW FETs with higher (x = 0.7) Si shell content by comparison to those with lower (x = 0.5) Si shell content. Moreover, the carrier mobility shows a stronger temperature dependence in Ge-Si(x)Ge(1-x) core-shell NWs with high Si content, indicating a reduced charge impurity scattering. The results establish that carrier confinement plays a key role in realizing high mobility core-shell NW FETs.     

Tuning the surface charge properties of epitaxial InN nanowires

We have investigated the correlated surface electronic and optical properties of [0001]-oriented epitaxial InN nanowires grown directly on silicon. By dramatically improving the epitaxial growth process, we have achieved, for the first time, intrinsic InN both within the bulk and at nonpolar InN surfaces. The near-surface Fermi-level was measured to be ～0.55 eV above the valence band maximum for undoped InN nanowires, suggesting the absence of surface electron accumulation and Fermi-level pinning. This result is in direct contrast to the problematic degenerate two-dimensional electron gas universally observed on grown surfaces of n-type degenerate InN. We have further demonstrated that the surface charge properties of InN nanowires, including the formation of two-dimensional electron gas and the optical emission characteristics can be precisely tuned through controlled n-type doping. At relatively high doping levels in this study, the near-surface Fermi-level was found to be pinned at ～0.95-1.3 eV above the valence band maximum. Through these trends, well captured by the effective mass and ab initio materials modeling, we have unambiguously identified the definitive role of surface doping in tuning the surface charge properties of InN.     

Doping of Si into GaN nanowires and optical properties of resulting composites

Doping of Si into GaN nanowires has been successfully attained via thermal evaporation in the presence of a suitable gas atmosphere. Analysis indicated that the Si-doped GaN nanowire is a single crystal with a hexagonal wurtzite structure, containing 2.2 atom % of Si. The broadening and the shift of Raman peak to lower frequency are observed, which may be attributed to surface disorder and various strengths of the stress. The band-gap emission (358 nm) of Si-doped GaN nanowires relative to that (370 nm) of GaN nanowires has an apparent blue shift (approximately 12 nm), which can be ascribed to doping impurity Si.     

几种Si、Ge纳米线导热系数的原子模拟

采用非平衡分子动力学方法模拟了Si纳米线、Ge纳米线、核-壳结构的Si/Ge纳米线及超晶格结构的Si/Ge纳米线的导热系数,给出了纳米线的温度与导热系数关系曲线,对比了几种纳米线导热特性的差异,研究结果表明,随着温度的升高,各纳米线的导热系数降低;相同温度下,纳米线导热系数的大小顺序为：核-壳结构的Si/Ge纳米线、超晶格结构的Si/Ge纳米线、Si纳米线、Ge纳米线。     

Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires

Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.