Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni(2)Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni(2)Ge nanowires exceeds 3.5 × 10(7) A cm(-2), and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni(2)Ge/Ge/Ni(2)Ge heterostructure. The interface epitaxial relationships are determined to be [Formula: see text] and [Formula: see text]. Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni(2)Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10(3) and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.     

Ag-catalyzed growth of Ge nanostructures via the thermal evaporation of Ge powder

The Ag-catalyzed growth of straight Ge nanowires (GeNWs), serrated Ge nanobelts (GeNBs), and hexagonal Ge nanotowers (GeNTs) by thermal evaporation of Ge powder at 950 degrees C in Ar was studied. The growth of GeNWs and GeNBs at 550-600 degrees C followed the top-growth mode via the vapor-solid-solid process, while that of GeNTs at 700-750 degrees C followed the bottom-growth mode via the vapor-liquid-solid process. This result shows that the growth mode of Ge nanostructures catalyzed by Ag nanoparticles is temperature-dependent. The larger size of AgGe droplets assembled at high temperatures is beneficial to the growth of GeNTs with the bottom-growth mode. In addition, the growth mechanisms of Ge nanostructures are discussed.     

Ge/GaAs中APD及薄膜性能研究

研究了一种光纤通讯用光电探测器。在ＧａＡｓ上蒸镀８００ｎｍ的Ｇｅ,并在此材料基础上提出了一种吸收倍增分离的雪崩二极管（ＳＡＭ－ＡＰＤ）的结构设计,采用ＧａＡｓ作为倍增区,Ｇｅ作为吸收区。在此结构上初步制作的二极管正向开启电压为０．２￣０．３Ｖ,反向击穿电压为２．５Ｖ,漏电不明显,ｐ－ｎ结特性良好。     

Photovoltaic properties and photoconductivity in multilayer Ge/Si heterostructures with Ge nanoislands

Interband optical transitions in multilayer heterostructures with SiGe nanoislands were investigated using photocurrent spectroscopy and photo-emf. The n-p heterostructures containing Ge nanoislands in the area of the potential barrier were prepared by molecular-beam epitaxy at the temperature about 500 °C. It was shown that electron transitions from the ground state of the valence band in a nanoislands to the conduction band of Si surrounding made the main contribution into the vertical photo-emf in the range 0.75–1.05 eV, which is below the interband absorption edge of Si. The lateral photoconductivity observed in the range 0.63–0.8 eV at 77 K can be attributed to indirect interband transitions from the ground state of a nanoisland to L-state of the conduction band of a nanoisland. Analysis of Raman scattering spectra revealed that the Ge composition x in a nanoisland is about 0.87, while elastic deformation value amounts to ε _xx  = −0.016. The calculated energies of interband transitions from the ground state of a nanoisland to the conduction band of Si surrounding (0.63 eV) and to L-state of the conduction band of a nanoisland (0.81 eV) fit the experimental data with a rather good accuracy.     

Structural perfection of the GeSi and SiGe heteroepitaxial systems

The structural perfection of heteroepitaxial systems (HESs) of the SiGe type (germanium films deposited onto silicon substrates) and of the “reverse” type (Si films on Ge substrates) has been investigated by X-ray diffraction and metallographical methods. It has been found that the real structures of these HESs are very different. The fragmentation previously observed in the SiGe HES is not found in the reverse system, but thick Ge substrates (d_Ge ～ 1 mm) appear to be plastically deformed throughout their thickness. A mutual correspondence of the damaged areas, inherent in the fragmentary structure (FS) in the SiGe system, is absent in the reverse system. Annealing does not influence the SiGe HES. The nature of the FS and the peculiarities of the deformation of both systems are discussed.     

Anisotropic thermal conductivity of Ge quantum-dot and symmetrically strained Si/Ge superlattices

We report the first experimental results on the temperature dependent in-plane and cross-plane thermal conductivities of a symmetrically strained Si/Ge superlattice and a Ge quantum-dot superlattice measured by the two-wire 3 omega method. The measured thermal conductivity values are highly anisotropic and are significantly reduced compared to the bulk thermal conductivity of the structures. The results can be explained by using heat transport models based on the Boltzmann transport equation with partially diffusive scattering of the phonons at the superlattice interfaces.     

Microstructural characterisation and mechanical properties of Ni–Ge alloys containing Ni3Ge

Abstract

Six Ni–Ge alloys were prepared and equilibrated at 1000°C. The microstructures, compositions of phases, solubilities in various phases, volume fractions of phases, lattice parameters of Ni₃ Ge, and microhardness values of phases were determined. Two alloys contained Ni₃Ge and α, one alloy was single phase Ni₃Ge, and three alloys contained Ni₃Ge and Ni₅Ge₃. The volume fractions of second phase were 6 and 33%α and 37 and 54%Ni₅Ge₃. The amount of second phase in the third Ni₃Ge–Ni₅Ge₃ alloy was too low to be determined accurately. The two phase alloys containing 6%α, 37%Ni₅Ge₃, or 54%Ni₅Ge₃ were tested in compression. A yield strength maximum was observed in the three alloys. The deformation behaviour of the alloy containing 6%α was similar to that of single phase Ni₃Ge: crack formation along Ni₃Ge grain boundaries and very low plastic strain were revealed. In alloys containing Ni₃Ge and an appreciable percentage of Ni₅Ge₃, crack formation was not observed along Ni₃Ge grain boundaries. Instead, at low temperatures, deformation and crack growth occurred within Ni₅Ge₃ grains and, at high temperatures, decohesion occurred along the Ni₃Ge/Ni₅Ge₃ interphase. The plastic strain in the alloy containing 54%Ni₅Ge₃ increased to about 40%.

MST/1724     

Ge/Si纳米多层膜的埋层调制结晶研究

采用磁控溅射设备,当衬底温度为500℃时,在Si(100)基片上磁控溅射生长Ge/Si多层膜样品.使用Raman,AFM和低角X射线技术对样品进行检测和研究,结果表明通过控制Ge埋层的厚度,可以调制Ge膜的结晶及晶粒尺寸,获得晶粒平均尺寸和空间分布较均匀的多晶Ge/Si多层膜.     

Ge quantum dot memory structure with laterally ordered highly dense arrays of Ge dots

This work was devoted to the development of a Ge quantum dot memory structure of a MOSFET type with laterally ordered Ge quantum dots within the gate dielectric stack. Lateral ordering of the Ge dots was achieved by the combination of the following technological steps: (a) use of a focused ion beam (FIB) to create ordered two-dimensional arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunneling oxide of homogeneous thickness on the patterned silicon surface, and (e) self-assembly of the two-dimensional arrays of Ge dots on the patterned tunneling oxide. The charging properties of the obtained memory structure were characterized by electrical measurements. Charging of the Ge quantum dot layer by electrons injected from the substrate resulted in a large shift in the capacitance-voltage curves of the MOS structure. Charges were stored in deep traps in the charging layer, and consequently the erasing process was difficult, resulting in a limited memory window. The advantages of controlled positioning of the quantum dots in the charging layer will be discussed.     

Oxidation resistance of thin boron carbo-nitride films on Ge(100) and Ge nanowires

Chemical vapor deposition of thin (< 10 nm) films of amorphous boron carbo-nitride (BC_0.7N_0.08, or BCN) on Ge(100) and Ge nanowire (GeNW) surfaces was studied to determine the ability of BCN to prevent oxidation of Ge. X-ray photoelectron spectroscopy was used to track Ge oxidation of BCN-covered Ge(100) upon exposure to ambient, 50 °C deionized water, and a 250 °C atomic layer deposition HfO₂ process. BCN overlayers incorporate O immediately upon ambient or water exposure, but it is limited to 15% O uptake. If the BCN layer is continuous, the underlying Ge(100) surface is not oxidized despite the incorporation of O into BCN. The minimum continuous BCN film thickness that prevents Ge(100) oxidation is ~ 4 nm. Thinner films (≤ 3.2 nm) permitted Ge(100) oxidation in each of the oxidizing environments studied. GeNWs with a 5.7 nm BCN coating were resistant to oxidation for at least 5 months of ambient exposure. High resolution transmission electron microscopy images of HfO₂/BCN/Ge(100) cross-sections and BCN-coated GeNWs reveal clean, abrupt BCN-Ge(100) interfaces.     

Si versus Ge for future microelectronics

The quest for higher performance of scaled down technologies resulted in the use of high-mobility substrates and strain engineering approaches. The development of advanced processing modules, based on low temperature processing and deposited (MBE, ALD, epitaxially grown, etc.) gate stacks, has triggered the interest of exploring Ge for sub 32 nm technology nodes. A comparison between Si and Ge for future microelectronics has to take into account a variety of materials, processing and performance aspects. Here special attention will be given to passivation and gate stack formation in relation to device performance, including leakage current and reliability aspects. The potential of Ge-based device structures and the monolithic integration of Ge and III-V devices on silicon are highlighted.     

Interface engineering for Ge metal-oxide-semiconductor devices

High mobility semiconductors such as Ge with high-k gates may be required to enhance performance of future devices. One of the biggest challenges for the development of a Ge metal-oxide-semiconductor (MOS) technology is to find appropriate passivating materials and methodologies for the Ge/high-k interfaces. Germanium oxynitride is frequently used as a passivating interlayer in combination with HfO₂ and is found to be necessary for the fabrication of functional devices. However, it is also considered to be insufficient since electrical characteristics in capacitors are non-ideal and field effect transistors underperform, probably due to a the high density of interface defects. We show that alternative passivating rare earth oxide layers prepared by molecular beam deposition produce improved electrical characteristics and a significant reduction of the density of interface states. In the case of CeO₂, a thick interfacial layer is spontaneously formed containing oxidized Ge, which is considered to be the key for the observed improvements.     

Stacked Ge islands for photovoltaic applications

Stacked Ge islands formed via the Stranski–Krastanov growth mode were incorporated into the intrinsic layer of Si-based pin diode to improve the performance of the solar cells in the near-infrared regime. The onset of the external quantum efficiency was extended up to around 1.4 mm for the solar cells with stacked Ge islands. The quantum efficiency was found to increase with increasing number of stacking, and the onset of the photocurrent response was in good agreement withroom-temperature photoluminescence energy of the Ge islands. These results manifest that the Ge islands did play a role toincrease the quantum efficiency. Furthermore, a part of electron-hole pairs generated within Ge islands was separated by the internal electric field and contribute to the photocurrent.     

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.     

Low Temperature Heat Capacity of Layered Superconductors SrNi2Ge2 and SrPd2Ge2

Low-temperature heat capacity C(T) of the weakly electron-correlated SrNi₂Ge₂ 122-layer compound undergoes a superconducting transition with onset at 1.4 K and a bulk T _c =0.75 K, where heat-capacity jump ratio ΔC(T _c )/γT _c =0.88–1.05. A small average superconducting energy gap E _g (ave)=2.21 kT _c =0.14 meV is derived for this multi-gap superconductor. Similar results for isostructural SrPd₂Ge₂ include T _c (onset)=3.5 K, bulk T _c of 2.92 K, ΔC(T _c )/γT _c =0.70 and E _g (ave)=2.54 kT _c =0.64 meV. The higher T _c onset could be associated with stoichiometric 1:2:2 grains in the polycrystalline samples. In addition, deviations of E _g /kT _c from the BCS ratio of 3.5 suggest that, just like their iron-based counterpart, these 122-layer germanides may also exhibit an unconventional, fully-opened multi-gap s-wave superconductivity.     

Ge掺杂碳化硅晶体的生长缺陷

采用物理气相传输法(PVT)制备了2英寸Ge掺杂和非掺SiC晶体, 并使用二次离子质谱仪(SIMS)、显微拉曼光谱(Raman spectra)仪、体式显微镜、激光共聚焦显微镜(LEXT)和高分辨X射线衍射(HRXRD)仪等测试手段对其进行了表征。结果表明, Ge元素可以有效地掺入SiC晶体材料中, 且掺杂浓度达到2.52×10¹⁸/cm³, 伴随生长过程中Ge组份的消耗和泄漏, 掺杂浓度逐渐降低; 生长初期高浓度Ge掺杂会促使6H-SiC向15R-SiC晶型转化, 并随着生长过程中Ge浓度的降低快速地转回6H-SiC稳定生长。用LEXT显微镜观察发现, 生长初期过高的Ge掺杂导致空洞明显增多, 位错密度增加, 掺杂晶体中位错密度较非掺晶体增大一倍。HRXRD分析表明掺Ge能增大SiC晶格常数, 这将有利于提高与外延III族氮化物材料适配度, 并改善器件的性能。     

One-step Ge/Si epitaxial growth

Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Ge films on silicon wafer with a Ge(x)Si(1-x) (0 < x < 1) graded buffer layer was demonstrated through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO(2) powder under hydrogen atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diffraction analysis shows that the Ge film with an epitaxial relationship is along the in-plane direction of Si. The successful growth of epitaxial Ge films on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.     

Interfacial effects on the optical behavior of Ge:ITO and Ge:ZnO nanocomposite films

Nanophase semiconductors are of interest for their unique, size-tunable solar spectral absorption characteristics as well as their potential to contribute to the improved energy conversion efficiency of photovoltaics (PV). Embedding these nanoparticles within electrically active transparent conductive oxides (TCO) can also provide an opportunity for enhanced, long-range carrier transport. However, differences in the atomic and electronic structure, dielectric behavior, and chemistry between the matrix and semiconductor phases highlight the influence of interfacial effects on the optical absorption properties of the composite. In this work, nanocomposites of Ge:indium tin oxide (Ge:ITO) and Ge:ZnO were fabricated with sequential RF-magnetron sputtering and annealed at temperatures from 310 to 550?°C to investigate the impact of matrix identity on this interface and its contribution to nanostructure-mediated optical absorption. Transmission electron microscopy showed a decrease in Ge nanocrystal size relative to the initial semiconductor domain size in both matrices that was correlated with an increase in absorption onset energy after annealing. The effect was particularly pronounced in Ge:ITO composites in which Raman spectroscopy indicated the presence of germanium oxide at the semiconductor-ITO interface. These results support the primary contribution of carrier confinement in the Ge nanophase to the shifts in absorption onset energies observed.