Preferential etching of Si–Si bond in the microcrystalline silicon germanium

Hydrogenated microcrystalline silicon germanium (μc-Si_1?xGe_x:H) films were investigated as a bottom cell absorber in multi-junction solar cells. μc-Si_1?xGe_x:H films were prepared using very high frequency (VHF, 60 MHz) plasma enhanced chemical vapor deposition (PECVD) systems working pressure of about 1.5 Torr. The precursor flow rates were carefully controlled to determine the phase transition point and to improve the crystallinity of μc-Si_1?xGe_x:H. A relatively high plasma power was necessary to have the high hydrogen (H₂) dilution. Raman spectroscopy study showed transition steps from amorphous to microstructure morphology as hydrogen dilution increasing. Crystallite Si–Ge and Ge–Ge bonds were occurred at relatively higher H₂ dilution compare to crystallite Si–Si bond. The rapidly increased Ge content as increasing the H₂ dilution is believed mainly due to the different decomposition rate of silane (SiH₄) and germane (GeH₄). The other reason of high Ge content even at the low GeH₄ precursor flow rate is probably due to the preferential etching of silicon atom by H₂. The preferential etching of Si–H possibly occurred in very highly concentrated H₂ plasma due to the preferential attachment of Si–H. The compositions of μc-Si_1?xGe_x:H films measured using RBS were Si_0.83Ge_0.17, Si_0.67Ge_0.33 and Si_0.59Ge_0.41 at H₂/SiH₄ flow rate of 60, 80 and 100, respectively. μc-Si_1?xGe_x:H films showed the dark (σ_d) and photo conductivity (σ_p) of about 10^?7 and 10^?5 S/cm, respectively and photo response (σ_p/σ_d) was about 10². This study will present the comprehensive evaluation of crystallization behavior of μc-Si_1?xGe_x:H films.     

Electron–acoustic phonon interaction and mobility in stressed rectangular silicon nanowires

We investigate the effects of pre-stress and surface tension on the electron–acoustic phonon scattering rate and the mobility of rectangular silicon nanowires.With the elastic theory and the interaction Hamiltonian for the deformation potential,which considers both the surface energy and the acoustoelastic effects,the phonon dispersion relation for a stressed nanowire under spatial confinement is derived.The subsequent analysis indicates that both surface tension and pre-stress can dramatically change the electron–acoustic phonon interaction.Under a negative(positive)surface tension and a tensile(compressive)pre-stress,the electron mobility is reduced(enhanced)due to the decrease(increase)of the phonon energy as well as the deformation-potential scattering rate.This study suggests an alternative approach based on the strain engineering to tune the speed and the drive current of low-dimensional electronic devices.     

Electron–hole two-stream instability in a quantum semiconductor plasma with exchange-correlation effects

The electron–hole two-stream instability in a quantum semiconductor plasma has been studied including electrons and holes quantum recoil effects, exchange-correlation potentials, and degenerate pressures of the plasma species. Typical values of GaAs and GaSb semiconductors are used to estimate the growth rate of the two-stream instability. The effects of electron– and hole–phonon collision, quantum recoil effects, the streaming velocities, and the corresponding threshold on the growth rate are investigated numerically. Considering the phonon susceptibility allows the acoustic mode to exist and the collisional instability arises in combination with drift of the holes.     

Rigorous hole band structure calculations of p-type δ-doping superlattices in silicon

Self-consistent hole band structure calculations are performed for p-type δ-doped quantum wells and superlattices (SLs) in Si by solving the six-band Luttinger–Kohn effective mass equations, together with Poisson equation, in a plane-wave representation. Non-parabolicities due to couplings between heavy, light, and spin–orbit split bands are fully taken into consideration. Exchange and correlation effects within the multicomponent hole gas are included in the local density approximation. Results are presented for hole band structures, Fermi levels, and potentials for p-type δ-doping SLs, in which the acceptor sheet doping concentration and SL period were varied. Our results are compared with the available experimental data.     

Electron–hole superlattices in GaAs/Al x Ga1− x As multiple quantum wells

A GaAs/Al _x Ga_{1? x} As semiconductor structure is proposed, which is predicted to superconduct at T _c?≈?2?K. Formation of an alternating sequence of electron- and hole-populated quantum wells (an electron–hole superlattice) in a modulation-doped GaAs/Al _x Ga_{1? x} As superlattice is considered. This superlattice may be analogous to the layered electronic structure of high-T _c superconductors. In the structures of interest, the mean spacing between nearest electron (or hole) wells is the same as the mean distance between the electrons (or holes) in any given well. This geometrical relationship mimics a prominent property of optimally doped high-T _c superconductors. Band bending by built-in electric fields from ionized donors and acceptors induces electron and heavy-hole bound states in alternate GaAs quantum wells. A proposed superlattice structure meeting this criterion for superconductivity is studied by self-consistent numerical simulation.     

5 MeV Proton irradiation effects on 200 GHz silicon–germanium heterojunction bipolar transistors

The total dose effects of 5?MeV proton and Co-60 gamma irradiation in the dose range from 1 to 100?Mrad on advanced 200?GHz Silicon–Germanium heterojunction bipolar transistors (SiGe HBTs) are investigated. The SRIM simulation study was conducted to understand the energy loss of 5?MeV proton ions in SiGe HBT structure. Pre- and post-radiation DC figure of merits such as forward- and inverse-mode Gummel characteristics, excess base current, DC current gain and output characteristics were used to quantify the radiation tolerance of the devices. The results show that the proton creates a significant amount of damages in the surface and bulk of the transistor when compared with gamma irradiation. The SiGe HBTs shows robust ionizing radiation tolerance even up to a total dose of 100?Mrad for both radiations.     

Double Electron–Electron Resonance Between Trapped Electron and Hole in a Semiconductor

We demonstrate the spin interactions between dispersedly trapped electrons and holes in a semiconductor using the double electron–electron resonance (DEER) method of the pulsed electron paramagnetic resonance (EPR) techniques. An aluminum-doped titanium dioxide crystal is adopted as a spin system, in which optically generated electrons and holes are trapped, to reveal EPR signals that appear close to each other at a selected crystal orientation under an external magnetic field. We used the four-pulse DEER method by applying two microwave frequencies to a microwave cavity for pumping electrons and probing holes at the optimum temperature of 32 K. The dipolar modulation in the probed signal by pumping interacting spins was successfully detected. The observed non-oscillating decay shape indicates that the detected interaction is caused by widely distributed trapped electron and hole spins over long distances. We were able to extract a spin-pair distribution function by the first derivative of a background-corrected curve, referring to a previously reported method.     

Spatial heterogeneity in liquid–liquid phase transition

Molecular dynamics simulations are performed to investigate the liquid–liquid phase transition(LLPT) and the spatial heterogeneity in Al–Pb monotectic alloys. The results reveal that homogeneous liquid Al–Pb alloy undergoes an LLPT,separating into Al-rich and Pb-rich domains, which is quite different from the isocompositional liquid water with a transition between low-density liquid(LDL) and high-density liquid(HDL). With spatial heterogeneity becoming large, LLPT takes place correspondingly. The relationship between the cooling rate, relaxation temperature and percentage of Al and the spatial heterogeneity is also reported. This study may throw light on the relationship between the structure heterogeneity and LLPT, which provides novel strategies to control the microstructures in the fabrication of the material with high performance.     

Investigation of muon states in silicon and germanium by field-quenching and RFμSR

The temperature dependence of the three states of positive muons in the semiconductors with diamond structure ( ⁺ in diamagnetic states ^d and paramagnetic muonium Mu and Mu^*) have been investigated on six Si (pure, B and P doped) and four Ge (ultrapure, CZ-grown undoped, Ga and Sb doped) single crystals by longitudinal field-quenching and radio-frequency ⁺SR. Clear evidence for the transition Mu^{* d} is found. The influence of light-induced charge-carriers is shown to be quite different in p- and n-type material.The work has been supported by the Bundesministerium für Forschung und Technologie in Bonn, Germany, under contract no. 03-SE3STU.     

Electron spin resonance in InGaAs/GaAs heterostructures with a manganese δ layer

The static and high-frequency dynamic magnetic properties and photoluminescence of two-dimensional semiconductor GaAs heterostructures containing an InGaAs quantum well and a thin manganese layer (δ layer) are studied. It is found that the Curie temperature is T _C ≈ 35 K and the magnetic anisotropy field of the ferromagnetic manganese δ layer is H _a ≈ 600 Oe. The spin resonance spectrum exhibits a line in weak fields (from −50 to 100 Oe), which is observed in the same temperature interval T < 40 K where the ferromagnetic ordering of the manganese δ layer occurs. This line is probably caused by the nonresonance contribution of the spin-dependent scattering of charge carriers to the negative magnetic resistance. The dependence of the degree of polarization of photoluminescence on the magnetic field also points to the ferromagnetic behavior of the manganese δ layer.     

Quantum degeneracy and Bose–Einstein condensation in low-dimensional trapped gases

At present, there are significant efforts to create 2D or 1D trapped gases by (tightly) confining the particle motion to zero point oscillations in one or two directions. The goal of this article is to show that the reduction of the spatial dimensionality in trapped Bose gases drastically changes the nature of the Bose-condensed state. In 2D and 1D gases, one can get a peculiar Bose-condensed state (quasicondensate) where the density fluctuations are suppressed, but the phase still fluctuates. The quasicondensate has the same density profile and local correlation properties as true condensates. However, it is very different with regard to phase coherence properties. We discuss how the phase coherence can be studied in experiments with 2D and 1D trapped gases.     

How does tetrahedral structure grow in liquid silicon upon supercooling?

We present an extensive set of isothermal-isobaric first-principles molecular-dynamics simulations of liquid silicon over a temperature range of 950-1700 K. We find that the tetrahedral order gradually grows upon cooling to approximately 1200 K, but that the growth accelerates significantly below approximately 1200 K. This growth process gives rise to anomalous changes in density and liquid structure upon supercooling. In particular, we find that the atomic coordination number remains constant to approximately 1200 K and then begins to decrease below approximately 1200 K, which resolves the existing controversy regarding liquid structure in the supercooled regime [T. H. Kim, Phys. Rev. Lett. 95, 085501 (2005)10.1103/PhysRevLett.95.085501].     

Generation of far-infrared radiation by hot holes in germanium and silicon inE ⊥H fields

The experimental results obtained by the investigation of stimulated FIR emission from dopedp-type germanium andp-type silicon by hot holes in crossedE andH fields at = 10 and 80 K are reported. The analysis of the emission intensity fromp-type germanium as a function ofE andH fields permits us to draw a conclusion about the important role of quantization of the energy spectrum of light holes and the contribution of light hole transitions with n = 2 to the amplification of FIR radiation. A new region of generation is demonstrated inp-type germanium under uniaxial stress. The first experimental results on stimulated FIR emission fromp-type silicon are reported.     

In-situ monitoring and control of hydrogenated amorphous silicon–germanium band-gap profiling during plasma deposition process

In-situ germanium content monitoring and its characteristics in SiH₄/GeH₄/H₂ plasmas was studied during hydrogenated amorphous silicon–germanium (a-SiGe:H) film depositions. Since an appropriate band-gap profiling in a-SiGe:H deposition is very important to achieve high efficiency solar cell, the accurate monitoring and control of Ge contents are required. In this work, we found the spectral intensity ratio of silicon atom (288.2 nm) and germanium atom (303.9 nm) emission has strong relation with Ge content in plasmas. In typical, band-gap energy of films was decreased with the increasing of gas flow ratio GeH₄/SiH₄. However, at different total flow rate of GeH₄, the band-gap was different for same gas flow ratio cases because the Ge content in plasmas was changed due to the changes of electron temperature by hydrogen dilution. On the other hand, the emission intensity ratio Ge/Si detected the band-gap variation. Using this method, therefore, we measured and control Ge/Si to make a U-shape band-gap profile which was proved by an ellipsometer and Auger electron spectroscopy depth profile analysis.     

Electron–phonon heat exchange in quasi-two-dimensional nanolayers

We study the heat power P transferred between electrons and phonons in thin metallic films deposited on free-standing dielectric membranes. The temperature range is typically below 1 K, such that the wavelengths of the excited phonon modes in the system is large enough so that the picture of a quasi-two-dimensional phonon gas is applicable. Moreover, due to the quantization of the components of the electron wavevectors perpendicular to the metal film’s surface, the electrons spectrum forms also quasi two-dimensional sub-bands, as in a quantum well (QW). We describe in detail the contribution to the electron–phonon energy exchange of different electron scattering channels, as well as of different types of phonon modes. We find that heat flux oscillates strongly with thickness of the film d while having a much smoother variation with temperature (T _e for the electrons temperature and T _ph for the phonons temperature), so that one obtains a ridge-like landscape in the two coordinates, (d, T _e ) or (d, T _ph ), with crests and valleys aligned roughly parallel to the temperature axis. For the valley regions we find P ∝ T _e ^3.5 – T _ph ^3.5 . From valley to crest, P increases by more than one order of magnitude and on the crests P cannot be represented by a simple power law. The strong dependence of P on d is indicative of the formation of the QW state and can be useful in controlling the heat transfer between electrons and crystal lattice in nano-electronic devices. Nevertheless, due to the small value of the Fermi wavelength in metals, the surface imperfections of the metallic films can reduce the magnitude of the oscillations of P vs. d, so this effect might be easier to observe experimentally in doped semiconductors.