Full-Band Monte Carlo investigation of hole mobilities in SiGe, SiC and SiGeC alloys

Hole transport properties in relaxed and biaxially strained Si_1-xGe_x, Si_1-yC_y and Si_1-x-yGe_xC_y alloys are investigated using a Full-Band Monte Carlo simulator. Our results allow the extraction of bulk, in-plane and out-of-plane hole drift mobilities. Doping effects in SiGeC alloys are taken into account through an efficient ionized-impurity scattering model. This model is based on inverse momentum relaxation times derived from phase-shift theory.¹ A new alloy scattering model relevant to the case of random ternary alloys is presented. It involves two effective alloy potential parameters, which account for the respective scattering strengths of Ge and C in the crystal lattice. From our mobility results we have derived an analytical hole mobility model which includes dependencies upon doping concentration, doping type, Ge content and C content.     

P-type InP grown by liquid phase epitaxy with rare earths: not intentional Ge acceptor doping

InP single crystal layers were grown by liquid phase epitaxy (LPE) on semi-insulating InP:Fe substrates with praseodymium added to the melt. Room temperature Hall effect measurements revealed p-type conductivity of the layers with the hole concentration 6×10¹⁴ cm⁻³ and mobility 150 cm² V⁻¹ s⁻¹. By measuring temperature dependence of the hole concentration the binding energy of the dominant acceptor was determined as 223 meV. A photoluminescence line was found at 1.195 eV, close to the previously estimated no-phonon line of Ge acceptor transitions in Ge doped n-type InP. It was concluded that Ge acceptors cause the p-type conductivity of the grown layers.     

Electrical properties of evaporated polycrystalline Ge thin-films

Electrical properties of Ge thin films evaporated on Si₃N₄ CVD-coated Si substrate were improved by introducing a heat treatment after the deposition of Ge films. Evaporation conditions were optimized by changing the substrate temperature and deposition rate, and then, heat treatment was performed. At substrate temperatures during the evaporation lower than 300 °C and higher than 400 °C, deposited films were amorphous and polycrystalline, respectively. At substrate temperatures lower than 400 °C, Ge films were evaporated without degrading the surface roughness. The Hall mobility of films evaporated at room temperature increased with increasing the substrate and heating temperature and showed about 400 cm² V⁻¹ s⁻¹ for the hole concentration of 4 × 10¹⁷ cm⁻³ at the heating temperature of 900 °C. This value was almost comparable to that of p-type Ge single crystal.     

On the particle–particle contact effects on the hole cleaning process via a CFD–DEM model

ABSTRACT

The accurate and precise computational models in order to predict the hole cleaning process is one of the helpful assets in drilling industries. Besides the bulk properties such as the flow velocity, particles average size, cleaning fluid properties, etc., that will affect the cleaning process, there is an unanswered question about the microscopic properties of the particles, particularly those which determines the contact characteristics: Do those play a major role or not? The rudimentary answer is not. The first purpose of the present work is to answer this question via a developed computational fluid dynamics coupled with discrete element method (CFD–DEM) in which the six unknown rolling and sliding friction coefficients of particle–particle contact, particle–wall contact, and particle–drill contact are considered as the main microscopic properties of the contacts. The second purpose is to search for optimum values of these coefficients in order to calibrate the CFD–DEM model with the experimental data for a near horizontal well cleaning available in the literature. The verification of the calibrated CFD–DEM model is checked by simulation of the hole cleaning process for different inclination angles of the deviated well. The results indicate the pivotal role of the microscopic properties of the particles on the characteristics of the particle transport mechanism.     

Fabrication and characteristics of porous germanium films

Abstract

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm^?3) and a maximum of Hall mobility at ～225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Fabrication and characteristics of porous germanium films

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm⁻³) and a maximum of Hall mobility at ∼225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Investigation of the transport properties of InSb single-crystal films obtained by directional crystallization

Both n-type and p-type InSb films (with a wide range of carrier concentration) were obtained by the directional crystallization method from the melt on large areas of mica, quartz and sapphire substrates. The p-type films were doped with germanium. It is shown that, depending on the crystallization conditions, one can obtain films of different structure: dendritic films, films containing macrodefects and homogeneous single-crystal films. The optimal growth conditions of single-crystal films having transport properties close to those of bulk material are given. The investigation of some transport properties in single-crystal p-type InSb films was carried out in the temperature range 77–600 K.A hole mobility in the range from 180 cm² V^-1 s^-1 to 5×10³ cm² V^-1 s^-1 in films with a concentration p=1.2×10¹⁶?5×10¹⁸cm^-3 of uncompensated acceptors was observed.An investigation of the concentration and temperature dependence of the hole mobility was carried out. Experimental results are in good agreement with the theory if a combined impurity, acoustic and optical mode scattering is taken into account.The phonon drag effect in p-type InSb films was observed. The temperature dependence of the thermoelectric power shows fair agreement with Herring's theory.     

Hole transport properties of p-type polycrystalline ZnO film using a dual-acceptor doping method with lithium and nitrogen

Hole transport properties of p-type polycrystalline ZnO film using a dual-acceptor doping method with lithium and nitrogen (denoted as ZnO:(Li, N)) were investigated by the temperature-dependent Hall-effect measurements. The hole mobility of the ZnO:(Li, N) firstly increases with increasing temperature from 85 to 140 K, and then decreases from 140 to 300 K. The comparison of experimental results and theoretical models shows that the mobility at temperature below 140 K is mainly affected by the grain boundary scattering, whereas the hole mobility above 140 K is dominated by mixed scatterings, involving lattice vibration, dislocation, and ionized impurity.     

Radiation damage of Si1-xGex S/D p-type metal oxide semiconductor field effect transistor with different Ge concentrations

The 2-MeV electron radiation damage of Si_1-xGe_x source/drain (S/D) p-type metal oxide semiconductor field effect transistor (p-MOSFET) with different Ge concentrations is studied. After irradiation at fluences below 2 × 10¹⁷ e/cm², the drain current and the maximum hole mobility decrease with increasing electron fluence for all Ge concentrations. It suggests that lattice defects are introduced by electron irradiation. In the case of Si_1-xGe_x S/D p-MOSFET, there are two locations for lattice defects, namely, the Si channel and SiGe stressor regions (S/D). Below 2 × 10¹⁷ e/cm² irradiation, no clear correlation between the radiation degradation and Ge concentration has been observed. It suggests that this degradation is mainly due to lattice defects in the Si channel, and the effects of the compressive-strain induced by the SiGe stressors on the enhancement of the hole mobility still remains after irradiation at 2 × 10¹⁷ e/cm². In the case of 5 × 10¹⁷ e/cm² irradiation, the drain current drastically decreases after irradiation for all Ge concentrations. Moreover, after 5 × 10¹⁷ e/cm² irradiation, the maximum hole mobility of x = 0.2 is close to x = 0, and in the case of x = 0.3, the maximum hole mobility drastically decreases. This fact suggests the contributions of the lattice defects, which are in the SiGe stressors, are prominent after 5 × 10¹⁷ e/cm² irradiation and dependent on Ge concentration. In addition, it provides evidence that the compressive strain in the Si-channel is relaxed by high fluence electron irradiation.     

High-performance single layered WSe₂ p-FETs with chemically doped contacts

We report high performance p-type field-effect transistors based on single layered (thickness, ～0.7 nm) WSe(2) as the active channel with chemically doped source/drain contacts and high-κ gate dielectrics. The top-gated monolayer transistors exhibit a high effective hole mobility of ～250 cm(2)/(V s), perfect subthreshold swing of ～60 mV/dec, and I(ON)/I(OFF) of >10(6) at room temperature. Special attention is given to lowering the contact resistance for hole injection by using high work function Pd contacts along with degenerate surface doping of the contacts by patterned NO(2) chemisorption on WSe(2). The results here present a promising material system and device architecture for p-type monolayer transistors with excellent characteristics.     

Impact of Solvent Additive on Carrier Transport in Polymer:Fullerene Bulk Heterojunction Photovoltaic Cells

The effects of a solvent additive, 1,8‐diiodooctane (DIO), on both hole and electron transport are investigated in a state‐of‐the‐art bulk‐heterojunction (BHJ) system, namely PTB7:PC₇₁BM. For a polymer:fullerene weight ratio of 1:1.5, the electron mobility in the blend film increases by two orders of magnitude with the DIO concentration while almost no change is found in the hole mobility. For lower DIO concentrations, the electron mobility is suppressed because of large, but poorly connected PC₇₁BM domains. For higher concentrations of DIO, the electron mobility is improved progressively and the hole mobility becomes the limiting factor. Between 1 and 5 vol%, the electron and hole mobilities are balanced. Using the Gaussian disorder model (GDM), we found that the DIO concentration modifies fundamentally the average hopping distances of the electrons. In addition, there exist alternative donor–acceptor ratios to achieve optimized PTB7:PC₇₁BM based solar cells. It is demonstrated that the fullerene content of the BHJ film can be significantly reduced from 1:1.5 to 1:1 while the optimized performance can still be preserved.     

High-performance multilayer WSe2 p-type field effect transistors with Pd contacts for circuit applications

WSe₂ is thought to be one of the best emerging p-type transition metal dichalcogenide (TMD) materials for potential low-power complementary metal oxide semiconductor (CMOS) circuit applications. However, the contact barrier and the interface quality hinder the performance of p-type field effect transistors (FETs) with WSe₂ films. In this work, metals with different work functions—Pd, Pt, and Ag—were systematically investigated as contacts for WSe₂ to decrease the contact resistances at source/drain electrodes and potentially improve transistor performance. Optimized p-type multilayer WSe₂ FETs with Pd contacts were successfully fabricated, and excellent electrical characteristics were obtained: a hole mobility of 36 cm²V^?1 s^?1; a high on/off ratio, over 10⁶; and a record low sub-threshold swing, SS?=?95 mV/dec, which may be attributed to the small Schottky barrier height of 295 meV between Pd and WSe₂, and strong Fermi-level pinning near the top of the valence band at the interface. Finally, a full-functional CMOS inverter was also demonstrated, consisting of a p-type WSe₂ FET together with a normal n-type MoS₂ FET. This confirmed the potential of TMD FETs in future low-power CMOS digital circuit applications.

     

Preparation of β-FeSi2 substrates by molten salt method

Large-sized β-FeSi₂ substrates were successfully prepared for the first time from the silicide bulk crystal grown by the molten salt method. The structural, electrical and optical properties of the as-grown β-FeSi₂ bulk crystals were also investigated. The crystal is single phase β-FeSi₂, and polycrystalline with no preferable growth crystallographic orientations. It was also determined that the β-FeSi₂ shows a p-type conduction, and the hole concentration and the Hall mobility at room temperature were about 10¹⁷ cm^− 3 and 10 cm²/Vs, respectively. In addition, the PL emission around 0.8 eV was realized from the β-FeSi₂ bulk crystal. This simple vacuum-free growth technique of β-FeSi₂ and the large-sized substrate preparation procedure encourage us to develop future silicide-based electronics.     

P-type SnO thin films prepared by reactive sputtering at high deposition rates

SnO is an ideally suitable p-type conductive material, with large hole mobility, and has attracted great interest in connection with next-generation electronic applications. In the present work, tin oxide (SnO_x) thin films were deposited on unheated soda lime glass substrates by reactive DC sputtering from a pure Sn target. The structural, optical and electrical properties of the films were analysed as a function of the oxygen partial pressure in the sputtering atmosphere and of the post-deposition annealing temperature in air. A structural analysis was carried out using Raman spectroscopy and X-ray diffraction. Optical and electrical characterizations were performed using photo-spectrometry and Hall effect measurements, respectively. The films grown at room temperature and low oxygen pressures reached high deposition rates of above 45 nm/min, showing poorly crystalline SnO and low transparency. Subsequent heating to 350 °C allowed to achieve a more crystalline tetragonal SnO with an average visible transmittance of 65%, a p-type conductivity of 0.8 S/cm, and a hole mobility of 3.5 cm²/(V s).     

2.5 Behaviour of amorphous Ge contacts to monocrystalline silicon

The performance of evaporated amorphous Ge films (thickness $\text{～-500}\text{A}\text{?}$) contacts to etched n-and p-type silicon crystals of different resistivities are discussed. The Ge was 3 Ωcm n-type was also used but gave no difference), and as external contact to the Ge film an Au layer was evaporated. The behaviour of the aGeSi junction seems to be largely governed by interface effects (and thus depends on surface preparation), as is often the case with metal-Si junctions, but Ge gives more reproducible and less time-varying results.In the process of clarifying the function of the contact the following structures were investigated (1) aGe_pⁿSiIn (Hg), where the latter is an ohmic contact, (2) amSi_pⁿSi-metal where amSi is a surface region of the crystal which has been rendered amorphous by ion bombardment, (3) aGe_pⁿSi-metal. I-V and C-V measurements were performed. From the results we conclude that aGeSi junctions act as low-resistance contacts when fed by electron or hole currents from the crystal. The currents (holes and electrons) that are injected into the crystal from the film are limited by barriers to small current densities, usually in the range 10^?6 A cm^?2. It is suggested that the small hole currents are explained by an increase in the hole barrier, effected by positive charges at the interface or in the Ge film, which are built up when positive carriers (holes) are injected by the contact.     

Electrical Characterization of p-Type Pb1−x Eu x Te

The transport properties of p-type Pb_1?x Eu _x Te epitaxial layers were studied as a function of Eu content, temperature, and magnetic field. The low-temperature hole mobility is drastically reduced when the Eu concentration is increased from 0 to 6%, while the hole concentration remains almost constant. A metal-insulator transition was observed for x ≈ 0.04, which is probably induced by the disorder caused by the introduction of Eu. For temperatures down to 10 K, only positive magnetoresistance has been observed at low magnetic fields. An anomalous behavior of the resistivity as a function of temperature has been detected for a Eu content about 5%, which is attributed to the resonance between the localized Eu 4f level and the valence band maximum.     

Electrical properties of conductive Ge nanocrystal thin films fabricated by low temperature in situ growth

Thin films composed of Ge nanocrystals embedded in an amorphous SiO(2) matrix (Ge-NC TFs) were prepared using a low temperature in situ growth method. Unexpected high p-type conductivity was observed in the intrinsic Ge-NC TFs. Unintentional doping from shallow dopants was excluded as a candidate mechanism of hole generation. Instead, the p-type characteristic was attributed to surface state induced hole accumulation in NCs, and the hole conduction was found to be a thermally activated process involving charge hopping from one NC to its nearest neighbor. Theoretical analysis has shown that the density of surface states in Ge-NCs is sufficient to induce adequate holes for measured conductivity. The film conductivity can be improved significantly by post-growth rapid thermal annealing and this effect is explained by a simple thermodynamic model. The impact of impurities on the conduction properties was also studied. Neither compensation nor enhancement in conduction was observed in the Sb- and Ga-doped Ge-NC TFs, respectively. This could be attributed to the fact that these impurities are no longer shallow dopants in NCs and are much less likely to be effectively activated. Finally, the photovoltaic effect of heterojunction diodes employing such Ge-NC TFs was characterized in order to demonstrate its functionality in device implementation.     

A Triode Device with a Gate Controllable Schottky Barrier: Germanium Nanowire Transistors and Their Applications

Electrical contacts often dominate charge transport properties at the nanoscale because of considerable differences in nanoelectronic device interfaces arising from unique geometric and electrostatic features. Transistors with a tunable Schottky barrier between the metal and semiconductor interface might simplify circuit design. Here, germanium nanowire (Ge NW) transistors with Cu₃Ge as source/drain contacts formed by both buffered oxide etching treatments and rapid thermal annealing are reported. The transistors based on this Cu₃Ge/Ge/Cu₃Ge heterostructure show ambipolar transistor behavior with a large on/off current ratio of more than 10⁵ and 10³ for the hole and electron regimes at room temperature, respectively. Investigations of temperature‐dependent transport properties and low‐frequency current fluctuations reveal that the tunable effective Schottky barriers of the Ge NW transistors accounted for the ambipolar behaviors. It is further shown that this ambipolarity can be used to realize binary‐signal and data‐storage functions, which greatly simplify circuit design compared with conventional technologies.     

Electrical Characterization of p-Type Pb1?xEuxTe

The transport properties of p-type Pb_1–x Eu _x Te epitaxial layers were studied as a function of Eu content, temperature, and magnetic field. The low-temperature hole mobility is drastically reduced when the Eu concentration is increased from 0 to 6%, while the hole concentration remains almost constant. A metal-insulator transition was observed for x 0.04, which is probably induced by the disorder caused by the introduction of Eu. For temperatures down to 10 K, only positive magnetoresistance has been observed at low magnetic fields. An anomalous behavior of the resistivity as a function of temperature has been detected for a Eu content about 5%, which is attributed to the resonance between the localized Eu 4f level and the valence band maximum.     

Overlayer stress effects on defect formation in Si and Ge

Point-defect formation energies in bulk crystalline materials such as Si and Ge are material specific quantities defined for the case of formation at a free surface, but in many cases of technological interest, point defects are formed at the interface between the crystalline substrate and a strained material overlayer. Here the energy cost of generating a bulk point defect at the overlayer/substrate interface is modified by the stress interaction during defect formation, leading to an effective supersaturation or undersaturation in the bulk, relative to the ‘equilibrium’ concentration expected for the case of a free surface. This in turn impacts on diffusion, defect formation and activation of dopant impurities in the substrate. We present current experimental evidence for this phenomenon, based on studies of B diffusion under tensile-strained nitride layers, and discuss the likely implications for dopant activation in Si and Ge.