On the formation and photoluminescence of Si(1-x)Ge(x) nanoparticles

Si(1-x)Ge(x) nanoparticles were prepared from two annealed alloy ingots at the compositions of Si:Ge = 9.5:0.5 and 9:1 using a vapor condensation technique under Ar atmosphere. These nanoparticles are all spherical, and increasing the working pressure leads to an increased particle size and size dispersion. Comparing to the alloy ingots, the nanoparticles have a higher average content of Ge. In addition, increasing the working pressure also causes the Si(1-x)Ge(x) nanoparticles to become more Ge-rich. This can be ascribed to the lower melting point and higher kinetic energy of Ge than Si during the evaporation process. The photoluminescence of Si(1-x)Ge(x) nanoparticles ranges from visible light to infrared region, and the luminescence peak exhibits a red shift as the Ge content in the nanoparticles increases. This indicates that the incorporation of Ge into Si has a dominant effect in the radiative recombination process, in comparison with the constant luminescence peak position in the case of pure Si nanoparticles with similar size distribution.     

Interface charge induced p-type characteristics of aligned Si(1-x)Gex nanowires

This study reports the electrical transport characteristics of Si(1-x)Gex (x=0-0.3) nanowires. Nanowires with diameters of 50-100 nm were grown on Si substrates. The valence band spectra from the nanowires indicate that energy band gap modulation is readily achievable using the Ge content. The structural characterization showed that the native oxide of the Si(1-x)Gex nanowires was dominated by SiO2; however, the interfaces between the nanowire and the SiO2 layer consisted of a mixture of Si and Ge oxides. The electrical characterization of a nanowire field effect transistor showed p-type behavior in all Si(1-x)Gex compositions due to the Ge-O and Si-O-Ge bonds at the interface and, accordingly, the accumulation of holes in the level filled with electrons. The interfacial bonds also dominate the mobility and on- and off-current ratio. The large interfacial area of the nanowire, together with the trapped negative interface charge, creates an appearance of p-type characteristics in the Si(1-x)Gex alloy system. Surface or interface structural control, as well as compositional modulation, would be critical in realizing high-performance Si(1-x)Gex nanowire devices.     

Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

Ge oxidation in the remaining cores of Si(1-x)Ge(x) nanowires after prolonged oxidation

For this investigation of the Ge behavior of condensed Si(1-y)Ge(y) (y > x) cores during the oxidation of Si(1-x)Ge(x) nanowires, Si(1-x)Ge(x) nanowires were grown in a tube furnace by the vapor-liquid-solid method and thermally oxidized. The test results were characterized using several techniques of transmission electron microscopy. The two types of Ge condensation are related to the diameter and Ge content of the nanowires. The consumption of Si atoms in prolonged oxidation caused the condensed SiGe cores to become Ge-only cores; and the continuous oxidation resulted in the oxidation of the Ge cores. The oxidation of Ge atoms was confirmed by scanning transmission electron microscopy.     

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.     

Electrical transport of bottom-up grown single-crystal Si(1-x)Ge(x) nanowire

In this work, we fabricated an Si(1-x)Ge(x) nanowire (NW) metal-oxide-semiconductor field-effect transistor (MOSFET) by using bottom-up grown single-crystal Si(1-x)Ge(x) NWs integrated with HfO(2) gate dielectric, TaN/Ta gate electrode and Pd Schottky source/drain electrodes, and investigated the electrical transport properties of Si(1-x)Ge(x) NWs. It is found that both undoped and phosphorus-doped Si(1-x)Ge(x) NW MOSFETs exhibit p-MOS operation while enhanced performance of higher I(on)～100?nA and I(on)/I(off)～10(5) are achieved from phosphorus-doped Si(1-x)Ge(x) NWs, which can be attributed to the reduction of the effective Schottky barrier height (SBH). Further improvement in gate control with a subthreshold slope of 142?mV?dec(-1) was obtained by reducing HfO(2) gate dielectric thickness. A comprehensive study on SBH between the Si(1-x)Ge(x) NW channel and Pd source/drain shows that a doped Si(1-x)Ge(x) NW has a lower effective SBH due to a thinner depletion width at the junction and the gate oxide thickness has negligible effect on effective SBH.     

Metastable Ge1-xCx alloy nanowires

Carbon-containing alloy materials such as Ge(1-x)C(x) are attractive candidates for replacing silicon (Si) in the semiconductor industry. The addition of carbon to diamond lattice not only allows control over the lattice dimensions, but also enhances the electrical properties by enabling variations in strain and compositions. However, extremely low carbon solubility in bulk germanium (Ge) and thermodynamically unfavorable Ge-C bond have hampered the production of crystalline Ge(1-x)C(x) alloy materials in an equilibrium growth system. Here we successfully synthesized high-quality Ge(1-x)C(x) alloy nanowires (NWs) by a nonequilibrium vapor-liquid-solid (VLS) method. The carbon incorporation was controlled by NW growth conditions and the position of carbon atoms in the Ge matrix (at substitutional or interstitial sites) was determined by the carbon concentration. Furthermore, the shrinking of lattice spacing caused by substitutional carbon offered the promising possibility of band gap engineering for photovoltaic and optoelectronic applications.     

Band-gap modulation in single-crystalline Si1-xGex nanowires

We report the energy band-gap modulation of single-crystalline Si1-xGex (0 相似文献   

Atomically controlled processing in silicon-based CVD epitaxial growth

One of the main requirements for Si-based ultrasmall device is atomic-order control of process technology. Here, we show the concept of atomically controlled processing for group IV semiconductors based on atomic-order surface reaction control in Si-based CVD epitaxial growth. Self-limiting formation of 1-3 atomic layers of group IV or related atoms after thermal adsorption and reaction of hydride gases on Si(1-x)Gex(100) (x = 0-1) surface are generalized based on the Langmuir-type model. Moreover, Si-based epitaxial growth on N, P or C atomic layer formed on Si(1-x)Gex(100) surface is achieved at temperatures below 500 degrees C. N atoms of about 4 x 10(14) cm(-2) are buried in the Si epitaxial layer within about 1 nm thick region. In the Si(0.5)Ge(0.5) epitaxial layer, N atoms of about 6 x 10(14) cm(-2) are confined within about 1.5 nm thick region. The confined N atoms in Si(1-x)Gex preferentially form Si-N bonds. For unstrained Si cap layer grown on top of the P atomic layer formed on Si(1-x)Gex(100) with P atomic amount of below about 4 x 10(14) cm(-2) using Si2H6 instead of SiH4, the incorporated P atoms are almost confined within 1 nm around the heterointerface. It is found that tensile-strain in the Si cap layer growth enhances P surface segregation and reduces the incorporated P atomic amount around the heterointerface. Heavy C atomic-layer doping suppresses strain relaxation as well as intermixing between Si and Ge at the nm-order thick Si(1-x)Gex/Si heterointerface. These results open the way to atomically controlled technology for ULSIs.     

Demonstration of Defect-Free and Composition Tunable Ga(x)In(1-x)Sb Nanowires

The Ga(x)In(1-x)Sb ternary system has many interesting material properties, such as high carrier mobilities and a tunable range of bandgaps in the infrared. Here we present the first report on the growth and compositional control of Ga(x)In(1-x)Sb material grown in the form of nanowires from Au seeded nanoparticles by metalorganic vapor phase epitaxy. The composition of the grown Ga(x)In(1-x)Sb nanowires is precisely controlled by tuning the growth parameters where x varies from 1 to ～0.3. Interestingly, the growth rate of the Ga(x)In(1-x)Sb nanowires increases with diameter, which we model based on the Gibbs-Thomson effect. Nanowire morphology can be tuned from high to very low aspect ratios, with perfect zinc blende crystal structure regardless of composition. Finally, electrical characterization on nanowire material with a composition of Ga(0.6)In(0.4)Sb showed clear p-type behavior.     

Contact printing of compositionally graded CdS(x)Se(1-x) nanowire parallel arrays for tunable photodetectors

Spatially composition-graded CdS(x)Se(1-x) (x = 0-1) nanowires are grown and transferred as parallel arrays onto Si/SiO(2) substrates by a one-step, directional contact printing process. Upon subsequent device fabrication, an array of tunable-wavelength photodetectors is demonstrated. From the spectral photoconductivity measurements, the cutoff wavelength for the device array, as determined by the bandgap, is shown to cover a significant portion of the visible spectrum. The ability to transfer a collection of crystalline semiconductor nanowires while preserving the spatially graded composition may enable a wide range of applications, such as tunable lasers and photodetectors, efficient photovoltaics, and multiplexed chemical sensors.     

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.     

放电等离子烧结Gd/Gd5Si2Ge2复合材料磁热效应研究

以高纯钆和Gd5Si2Ge2合金为原料,采用放电等离子烧结技术制备了两组元Gdx(Gd5Si2Ge2)1-x(x=0,0.33,0.5,0.7,1)层状复合磁制冷材料.通过自制的磁热效应测量仪器直接测量了复合材料在外加磁场1.5 T下的磁热效应(ΔTad).随着复合比例的变化,材料的最大绝热温变(ΔTad)从x=0.3时的1.6 K增加到x=0.7时的2.0 K,而最大绝热温变峰的位置从286K变到了293 K.同时,与单组元的Gd5Si2Ge2合金相比,随着钆的含量增加时,复合材料的最大绝热温变峰变宽.当x=0.7时,层状复合磁制冷材料在外加磁场1.5 T下的最大绝热温变(ΔT)在260-310K范围里从1.1 K变到2.0 K,这种材料非常适合作为室温磁制冷材料.     

The oxides growth during high-temperature oxidation of Si1-xGe(x) nanowires

Concerning the oxidation behavior of Si1-xGe(x) (x = 0.15, 0.3) nanowires at high temperature, Si1-xGe(x) nanowires were thermally oxidized for various lengths of time compared with Si nanowires, Si and Si1-xGe(x) thin films. The structural and compositional properties of the oxidized nanowires were characterized using several transmission electron microscopy (TEM) techniques including energy dispersive X-ray spectroscopy (EDS), which confirm that the oxidation rates of Si1-xGe(x) and Si (silicon) nanowires were saturated with increasing oxidation time due to retarding behavior, while the oxidation rate of Si1-xGe(x) nanowires were faster than that of Si nanowires. In addition, the differences in Ge (germanium) content and stress distribution contribute to the observed differences in oxidation behavior.     

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by μRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650?°C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.     

In(x)Ga(₁-x)As nanowires on silicon: one-dimensional heterogeneous epitaxy, bandgap engineering, and photovoltaics

We report on the one-dimensional (1D) heteroepitaxial growth of In(x)Ga(1-x)As (x = 0.2-1) nanowires (NWs) on silicon (Si) substrates over almost the entire composition range using metalorganic chemical vapor deposition (MOCVD) without catalysts or masks. The epitaxial growth takes place spontaneously producing uniform, nontapered, high aspect ratio NW arrays with a density exceeding 1 × 10(8)/cm(2). NW diameter (～30-250 nm) is inversely proportional to the lattice mismatch between In(x)Ga(1-x)As and Si (～4-11%), and can be further tuned by MOCVD growth condition. Remarkably, no dislocations have been found in all composition In(x)Ga(1-x)As NWs, even though massive stacking faults and twin planes are present. Indium rich NWs show more zinc-blende and Ga-rich NWs exhibit dominantly wurtzite polytype, as confirmed by scanning transmission electron microscopy (STEM) and photoluminescence spectra. Solar cells fabricated using an n-type In(0.3)Ga(0.7)As NW array on a p-type Si(111) substrate with a ～ 2.2% area coverage, operates at an open circuit voltage, V(oc), and a short circuit current density, J(sc), of 0.37 V and 12.9 mA/cm(2), respectively. This work represents the first systematic report on direct 1D heteroepitaxy of ternary In(x)Ga(1-x)As NWs on silicon substrate in a wide composition/bandgap range that can be used for wafer-scale monolithic heterogeneous integration for high performance photovoltaics.     

Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x? = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.     

Mutual passivation of electrically active and isovalent impurities

The alloy GaN(x) As(1-x) (with x typically less than 0.05) is a novel semiconductor that has many interesting electronic properties because of the nitrogen-induced dramatic modifications of the conduction band structure of the host material (GaAs). Here we demonstrate the existence of an entirely new effect in the GaN(x) As(1-x) alloy system in which the Si donor in the substitututional Ga site (Si(Ga)) and the isovalent atom N in the As sublattice (N(As)) passivate each other's electronic activity. This mutual passivation occurs in Si-doped GaN(x) As(1-x) through the formation of nearest-neighbour Si(Ga) -N(As) pairs and is thermally stable up to 950 degrees C. Consequently, Si doping in GaN(x) As(1-x) under equilibrium conditions results in a highly resistive GaN(x) As(1-x) layer with the fundamental bandgap governed by a net 'active' N, roughly equal to the total N content minus the Si concentration. Such mutual passivation is expected to be a general phenomenon for electrically active dopants and localized state impurities that can form nearest-neighbour pairs.     

Diffusion in Si(x)Ge(1-x)/Si nanowire heterostructures

Si0.48Ge0.52/Si tip/nanowire heterostructures were grown by pulsed laser vaporization (PLV) at a growth temperature of 1100 degrees C. Ge diffusion in [111]-growth Si nanowires was studied for different post-synthesis annealing temperatures from 200 degrees C to 800 degrees C. Ge composition profiles were quantified by energy-dispersive X-ray spectroscopy in a transmission electron microscope. The compositional profiles were modeled by a limited-source diffusion model to extract temperature-dependent diffusion coefficients. The Ge diffusion coefficients followed an Arrhenius relationship with an activation energy of 0.622 +/- 0.050 eV. This rather low activation energy barrier is similar to the previously reported activation energy barrier of 0.67 eV for Ge surface diffusion on Si, suggesting that surface diffusion may dominate in nanowires at this length scale.     

GexSi1-x/Si中应变的会聚束电子衍射研究

介绍了会聚束电子衍射（ＣＢＥＤ）技术与计算机模拟相结合测定ＧｅｘＳｉ１－ｘ／Ｓｉ化学梯度层中应变分布的实验结果,提供了一种高空间分辨率,高灵敏度,且适用于任何材料系中微区晶格常数测定及应变分布研究的技术途径。