149,857 research outputs found
Observation of negative differential conductance in a reverse-biased Ni/Ge Schottky diode
No full textWe report the experimental observation of negative differential conductance in a Ni/Ge Schottky diode. With the aid of theoretical models and numerical simulation we show that, at reverse bias, electons tunnel into the high electric field of the depletion region. This scatters the electrons into the upper valley of the Ge conduction band, which has a lower mobility. The observed negative differential conductance is hence attributed to the transferred-electron effect. This shows that Schottky contacts can be used to create hot electrons for transferred-electron devices
Transport properties for pure strained Ge quantum well
No full textModulation doped heterostructures consisting of a strained Ge (sGe) quantum well on a Si0.2Ge0.8 virtual substrate have been used to study enhancement of the transport properties of holes in the sGe channel due to the effective reduction of impurity scattering by placing the doping layer away from the channel.
Electrical and structural analysis was performed for sGe heterostructures produced with a range of growth parameters. The highest hole mobility was 1.34×106 cm2 /Vs at 0.5 K for a sGe quantum well in a 'normal' structure (i.e. doped above the channel) at a sheet density of 2.9×1011 cm-2, which is the largest hole mobility reported in Ge to date. 'Inverted' structures (doping layer under the channel) were also studied for different sample parameters such as channel thickness, spacer thickness, doping and different temperature growth, with a hole
mobility as high as 5.08×105 cm2 /Vs at a sheet density of 5.14×1011 cm-2 at 90 mK.
Simulations of the scattering limited mobility for inverted and normal structures were performed and showed that at low sheet density background impurity scattering limits the low temperature hole mobility. However, as the sheet density increases interface roughness scattering becomes the mobility limiting process, especially in the case of inverted structures where the resistivity and mobility anisotropy is more pronounced.
Magnetotransport measurements revealed the lowest reported effective mass for holes in Ge of 0.063±0.001 m0 for the normal structure and 0.07±0.002 m0 & 0.063±0.003 m0 for two inverted structures, and highest Dingle factors of α=78 and 33 for the normal and inverted structures, respectively. The low level of background impurities, high structural quality, and pure Ge channel revealed by structure characterisation are believed to be responsible for these exceptionally high values of mobility
Fabrication and characterisation of novel Ge MOSFETs
No full textAs high-k dielectrics are introduced into commercial Si CMOS (Complimentary Metal Oxide Semiconductor) microelectronics, the 40 year channel/dielectric partnership of Si/SiO2 is ended and the door opened for silicon to be replaced as the active channel material in MOSFETs (Metal Oxide Semiconductor Field Effect Transistor). Germanium is a good candidate as it has higher bulk carrier mobilities than silicon. In addition, Si and Ge form a thermodynamically stable SiGe alloy of any composition, allowing Ge to be implemented as a thin layer on the surface of a standard Si substrate. This thesis is a practical investigation on several aspects of Ge CMOS technology.
High-k dielectric Ge p-MOSFETs are electrically characterised. A large variation in interface state densities is demonstrated to be responsible for a threshold voltage shift and this is proportional to reciprocal peak mobility due to the Coulomb scattering of carriers by charged states. A theoretical mobility is fitted to that measured at 4.2 K and confirms that interface states are the main source of interface charged impurities.
The model demonstrates a reduction in the interface charged impurity density in p-MOSFETs that underwent a PMA (Post Metallisation Anneal) in hydrogen atmosphere and that the anneal also reduces the RMS (Root Mean Square) dielectric/semiconductor interface roughness, from an average of 0.60 nm to 0.48 nm.
High-k strained Ge p-MOSFETs are electrically characterised and have peak mobilities at 300 K (470 cm2 V-1 s-1) and 4.2 K (1780 cm2 V-1 s-1) far in excess of those measured for the unstrained Ge p-MOSFETs (285 cm2 V-1 s-1,785 cm2 V-1 s-1 respectively). Strained Ge n-MOSFETs perform significantly worse than standard Si P, - MOSFETs primarily due to a high source/drain resistance.
A 10 nm thick SiGe-01 (On Insulator) layer with a Ge composition of 58% is obtained from a 55 nm Si0_88Ge1o2. initial layer on 100 nm Si-Ol substrate via the germanium condensation technique. For the first time, germanium is demonstrated to diffuse through the BOX (Buried OXide) during Ge-condensation and into the underlying Si substrate. An order of magnitude increase in the calculated ITOX (Internal Thermal OXidation) rate of the BOX in the final stages of Ge-condensation is hypothesised to be responsible for stopping this diffusion
Electrodeposited Ni/Ge and germanide schottky barriers for nanoelectronics applications
No full textIn recent years metal/semiconductor Schottky barriers have found numerous applications in nanoelectronics. The work presented in this thesis focuses on the improvement of a few of the relevant devices using electrodeposition of metal on Ge for Schottky barrier fabrication. This low energy metallisation technique offers numerous advantages over the physical vapour deposition techniques. Electrical characteristics of the grown diodes show a high quality rectifying behaviour with extremely low leakage currents even on highly doped Ge. A non-Arrhenius behaviour of the temperature dependence is observed for the grown Ni/Ge diodes on lowly doped Ge that is explained by a spatial variation of the barrier heights. The inhomogeneity of the barrier hights is explained in line with an intrinsic surface states model for Ge. The understanding of the intrinsic surface states will help to create ohmic contacts for doped n-MOSFETs. NiGe were formed single phase by annealing. Results reveal that by using these high-quality germanide Schottky barriers as the source/drain, the subthreshold leakage currents of a Schottky barrier MOSFET could be minimised, in particular, due to the very low drain/body junction leakage current exhibited by the electrodeposited diodes. The Ni/Ge diodes on highly doped Ge show negative differential conductance at low temperature. This effect is attributed to the intervalley electron transfer in Ge conduction band to a low mobility valley. The results show experimentally that Schottky junctions could be used for hot electron injection in transferred-electron devices. A vertical Co/Ni/Si structure has been fabricated for spin injection and detection in Si. It is shown that the system functions electrically well although no magnetoresistance indicative of spin injection was observed
High quality Schottky contacts for limiting leakage currents in Ge-based Schottky barrier MOSFETs
No full textSchottky barrier (SB) Ge channel MOSFETs suffer from high drain-body leakage at the required elevated substrate doping concentrations to suppress source-drain leakage. Here we show that electrodeposited Ni-Ge and NiGe/Ge Schottky diodes on highly doped Ge show low off current, which might make them suitable for SB p-MOSFETs. The Schottky diodes showed rectification of up to 5 orders in magnitude. At low forward biases the overlap of the forward current density curves for the as deposited Ni/n-Ge and NiGe/n-Ge Schottky diodes indicates Fermi-level pinning in the Ge band gap. The SB height for electrons remains virtually constant at 0.52 eV (indicating a hole barrier height of 0.14 eV) under various annealing temperatures. The series resistance decreases with increasing annealing temperature in agreement with four point probe measurements indicating the lower specific resistance of NiGe as compared to Ni, which is crucial for high drive current in SB p-MOSFETs. We show by numerical simulation that by incorporating such high quality Schottky diodes in the source/drain of a Ge channel PMOS, highly doped substrate could be used to minimize the subthreshold source to drain leakage current
Low bandgap α-(Si,Ge):H and α-(Ge):H devices
No full textIn this research, alpha-(Si,Ge):H and alpha-(Ge):H devices are grown by electron cyclotron resonance plasma enhanced CVD technique and the electrical and optical properties of these devices under different plasma conditions are investigated;The ion bombardment during the growth can be enhanced by the low pressure, inert gas and substrate bias. The conclusion that low pressure leads to better quality of the material under hydrogen plasma because of higher ion bombardment is further verified by the space charge limited current (SCLC) method. The midgap defect density of states for the low pressure sample is lower than the one deposited under high pressure. The film properties for He-ECR films did not degrade as the pressure changed from 10mTorr to about 25mTorr, but at higher pressure (35mTorr). It's because the ion energy of He plasma is higher than that of H plasma;We also found that the negative substrate bias can provide higher ion bombardment. The effects of substrate bias on the growth rate are different for the high Ge content sample and the low Ge content sample. Under certain deposition environment, the performance of the devices can be improved when negative substrate voltage is applied. But under some conditions that the ion bombardment is already high, the effect is not so obvious;Most researchers concentrates on the alloy with Ge content >50%. In this research, we explored the properties of alpha-(Si,Ge):H alloy with Ge content <50 and found that device quality material can be obtained under the combination of low pressure, high temperature, high hydrogen dilution and ppm boron doping in i layer even for the bandgap as low as 1.16ev. Fourier transformed infrared absorptance spectroscopy (FTIR) measurement showed that the preference of Si-H to Ge-H is broken up, low pressure and high hydrogen dilution is beneficial to reduce the Ge-Ge clustering and increase the Ge-H bond so that the material is grown more homogeneously. The quality of the alpha-(Ge):H material can be further improved by provides some graded boron doping in I layer and higher ion bombardment. A fill factor of 55% and urbach energy of 40mev are obtained for alpha-(Ge):H solar cell. This is the first time ever that a good alpha-(Ge):H solar cell has been made.</p
H-Bridged Structures for Tetrahedranes A<sub>4</sub>H<sub>4</sub> (A = C, Si, Ge, Sn, and Pb)
No full textAb initio MO studies at the HF, MP2, and
Becke3LYP levels on H-bridged tetrahedranes of the group
14
elements (A4H4, A = C, Si, Ge, Sn, Pb) with
the 6-31G* basis set for C and Si and LANL1DZ and
quasirelativistic
pseudopotential basis sets for Ge, Sn, and Pb are reported. As
expected, the classical Td structure
1 is more stable
than all the other tetrahedrane alternatives for
C4H4. The triply hydrogen bridged
structure 2
(C3v) is found to
be
more stable for Si, Ge, and Sn. Pb4H4
prefers the four H-bridged structure 8
(Cs). However, the calculations
with
the quasirelativistic pseudopotential basis set show the quadruply
H-bridged D2d structure
7 to be the most stable
structure for Ge, Sn, and Pb. Thus the structures derived from the
transition metal organometallic chemistry are
competitive for heavier elements (Si to Pb). The periodic behavior
begins only with the second period; Li to Ne, are
the exceptions. Suggestions for the realization of these H-bridged
structures for Pb from NaPb are discussed
Laser-vibrometric ultrasonic characterization of resonant modes and quality factors of Ge membranes
No full textThe vibrations of a single-crystal germanium (Ge) membrane are studied in air and vacuum using laser vibrometry, in order to determine mechanical properties such as Q-factors, tensile stress, anisotropy, and robustness to shock. Resonance modes up to 3:2 are identified, giving a residual stress measurement of 0.22 GPa, consistent with the value obtained from x-ray relaxation studies. The membrane is found to be isotropic, with Q-factors ranging from around 40 at atmospheric pressure to over 3200 at 5 x 10-4 mbar, significantly lower than those found in polycrystalline Ge micromechanical devices. The robustness to shock is explained through the high resonance mode frequencies and the dissipation mechanism into the substrate, which is a direct consequence of having a high quality film with low residual tensile stress, giving the potential for such films to be used in optoelectronic devices
Ge1-ySny (y=0.01-0.10) alloys on Ge-buffered Si: Synthesis, microstructure, and optical properties
No full textabstract: Novel hydride chemistries are employed to deposit light-emitting Ge [subscript 1- y] Sn [subscript y] alloys with y ≤ 0.1 by Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) on Ge-buffered Si wafers. The properties of the resultant materials are systematically compared with similar alloys grown directly on Si wafers. The fundamental difference between the two systems is a fivefold (and higher) decrease in lattice mismatch between film and virtual substrate, allowing direct integration of bulk-like crystals with planar surfaces and relatively low dislocation densities. For y ≤ 0.06, the CVD precursors used were digermane Ge [subscript 2]H[subscript 6] and deuterated stannane SnD[subscript 4]. For y ≥ 0.06, the Ge precursor was changed to trigermane Ge [subscript 3]H[subscript 8], whose higher reactivity enabled the fabrication of supersaturated samples with the target film parameters. In all cases, the Ge wafers were produced using tetragermane Ge [subscript 4]H[subscript 10] as the Ge source. The photoluminescence intensity from Ge [subscript 1− y] Sn [subscript y] /Ge films is expected to increase relative to Ge [subscript 1− y] Sn [subscript y] /Si due to the less defected interface with the virtual substrate. However, while Ge [subscript 1− y] Sn [subscript y] /Si films are largely relaxed, a significant amount of compressive strain may be present in the Ge [subscript 1− y] Sn [subscript y] /Ge case. This compressive strain can reduce the emission intensity by increasing the separation between the direct and indirect edges. In this context, it is shown here that the proposed CVD approach to Ge [subscript 1− y] Sn [subscript y] /Ge makes it possible to approach film thicknesses of about 1 μm, for which the strain is mostly relaxed and the photoluminescence intensity increases by one order of magnitude relative to Ge [subscript 1− y] Sn [subscript y] /Si films. The observed strain relaxation is shown to be consistent with predictions from strain-relaxation models first developed for the Si[subscript 1− x] Ge [subscript x] /Si system. The defect structure and atomic distributions in the films are studied in detail using advanced electron-microscopy techniques, including aberration corrected STEM imaging and EELS mapping of the average diamond–cubic lattice.Copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. along with the following message: The following article appeared in 116, 13 (2014) and may be found at http://dx.doi.org/10.1063/1.489678
Mechanism of vertical Ge nanowire nucleation on Si (111) during subeutectic annealing and growth
No full textThe direct integration of Ge nanowires with silicon is of interest in multiple applications. In this work, we describe the growth of high-quality, vertically oriented Ge nanowires on Si (111) substrates utilizing a completely sub-Au-Si-eutectic annealing and growth procedure. With all other conditions remaining identical, annealing below the Au-Si eutectic results in successful heteroepitaxial nucleation and growth of Ge nanowires on Si substrate while annealing above the Au-Si eutectic leads to randomly oriented growth. A model is presented to elucidate the effect of the annealing temperature, in which we hypothesized that sub-Au-Si-eutectic annealing leads to the formation of a single and well-oriented interface, essential to template heteroepitaxial nucleation. These results are critically dependent on substrate preparation and lead to the creation of integrated nanowire systems with a low thermal budget process
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