Ultrafast electron and hole dynamics in germanium nanowires

We present the first ultrafast time-resolved optical measurements, to the best of our knowledge, on ensembles of germanium nanowires. Vertically aligned germanium nanowires with mean diameters of 18 and 30 nm are grown on (111) silicon substrates through chemical vapor deposition. We optically inject electron-hole pairs into the nanowires and exploit the indirect band structure of germanium to separately probe electron and hole dynamics with femtosecond time resolution. We find that the lifetime of both electrons and holes decreases with decreasing nanowire diameter, demonstrating that surface effects dominate carrier relaxation in semiconductor nanowires.     

Deformation and fracture of germanium single crystals

Diamond-pyramid indentations, made with loads of 1 to 100 g at room temperature on {111}, {110} and {112} faces of germanium crystals, were studied by scanning electron microscopy. The Vickers-hardness number, which was highest for {111} faces, increased with the square root of the load by a factor of about 10 on any given surface, ranging from values as low as 60 to about 850, the latter corresponding to the VHN reported in the literature. The lengths of major cracks, generally seen on the surface of the crystal as an extension of the indentation diagonals, were found to be proportional tod-d ₀, whered ₀, equal to about 3 μm, represents a lower limiting value of the indentation diagonald, below which cracks do not seem to form. The increase of the VHN and of the crack-lengths with load is explained in terms of a tentative model involving the formation and the stress-induced growth of laminar “plastic” patches in a volume surrounding the indentation.     

Dislocation structures in deformed Cu-Ni-Zn alloy single crystals

It is known that Cu-Ni-Zn alloy has an ordered structure Cu₂NiZn (Ll₂) by annealing between 573 and 623 K. In the present experiments, the effects of annealing on the dislocation structure were studied on Cu-Ni-Zn single crystals with several compositions. Thin foils cut parallel to the {111} planes were observed in a transmission electron microscope. The results obtained are as follows. (i) In Cu-5Ni-5Zn, Cu-10Ni-10Zn and Cu-15Ni-15Zn (at%), the stress-strain behaviour, slip mode and dislocation structure did not change by annealing at 573 K. However, the slip mode became more concentrated and localized, and dislocations emitted from a source tended to stay on the same slip plane, as the nickel and zinc concentrations increased. (ii) However, those properties in Cu-20Ni-20Zn and Cu-25Ni-25Zn changed drastically by annealing. As the ordering proceeded, uniform distributions of superlattice dislocations were observed. A typical dislocation configuration, with long screw and wavy-edged superlattice dislocations, took the place of piled-up unit dislocations. (iii) The facts that edge-type superlattice dislocations formed dipoles and their clusters, and that the secondary dislocation density was much lower than the primary one, implied that the elastic interaction of the primary edge-type superlattice dislocations on the nearby parallel slip planes would control the work-hardening of ordered Cu₂NiZn alloy single crystals.     

Mobility analysis in plastically deformed CdTe single crystals

Mobility, d.c. Hall effect and d.c. conductivity studies have been done on deformed and undeformed samples of n-type CdTe in the temperature range 77 to 300 K. The Hall coefficient studies have suggested that acceptor centres are introduced during the formation and motion of dislocations. The mobility variation with temperature of the deformed samples has shown a peak at the higher temperature side. The dislocation mobility ( _D) deduced from the observed mobilities of deformed and undeformed samples has shown two regions, Region 1 being independent of temperature and Region 2 having a linear increase with temperature. The observed rate of change ofm with temperature from Region 2 and the rate deduced from the simple analysis suggested by the authors are found to be almost equal. The dislocation density at room temperature is estimated using the dislocation scattering mobility and the Dexter and Seitz model. These values are comparable with those observed from etching studies within the experimental errors.     

Recovery in deformed copper and nickel single crystals

The static recovery behaviour of copper and nickel single crystals deformed in multiple slip was investigated in mechanical tests and in HVEM in situ annealing experiments. Recovery of Type I, as previously identified in aluminium was observed in both its microscopic and macroscopic features: a sharpening of the cell-wall structure without a change in substructure scale, correlated with a short transient in the strain-hardening behaviour during retesting. The substructure development with increasing recovery time and temperature is similar to that during dynamic recovery, i.e. with increasing strain in a continuous test. After longer recovery times or at higher annealing temperatures, the specimens recrystallize; after larger strains, they recrystallize dynamically. An intermediate stage akin to the Type II recovery found in aluminium was never observed, either in its macroscopic manifestation of a long reloading transient, or as a general coarsening of the subgrain structure. Examples of local sub-boundary mobility and dissolution were, however, seen in situations close to static or dynamic recrystallization. It is concluded that the fluctuations occurring during subgrain coarsening are stable in aluminium, leading to Type II recovery and extended steady-state deformation, but unstable in copper and nickel, leading to static or dynamic recrystallization.     

Creep and stress relaxation of germanium single crystals

Isothermal creep and stress relaxation were studied in germanium single crystals oriented for double slip at 400 to 700? C under compressive stresses of 180 to 490 kgf cm^?2. The curves, S-shaped in both types of experiment, were compared with correspondingones derived from a stochastic model involving a spectrum of energy barriers to dislocation movement. Agreement was satisfactory for stress relaxation; discrepancies in the case of creep point to the difficulty of correlating creep and stress relaxation when an equation of state is inadequate as an approximation for representing the plastic response. Differences between predicted and observed behaviour are, however, of diagnostic value concerning the extent of dislocation multiplication and other structural changes.     

Galvanomagnetic properties of plastically deformed InSb single crystals

The dc Hall effect, dc conductivity and mobility have been studied on deformed and undeformed samples ofn-type InSb from liquid nitrogen temperature to room temperature. These studies have shown that the Hall coefficient values of deformed samples do not differ much from undeformed sample, but a considerable amount of change was observed in mobility, suggesting that equal number of donor and acceptor type dislocations are introduced during the deformation process. In addition the mobility variation of the deformed samples with temperature has shown a peak in 170–300°K range. The dislocation mobility (μ _D) is deduced from the observed mobilities of deformed and undeformed samples. The plotμ _D vs T has two regions, region 1 being independent of temperature and region 2 having a linear increase with temperature. Theβ factor obtained from region 2 is found to be almost equal to the one calculated from Dexter and Seitz model. The dislocation densities at room temperature are also calculated for the deformed samples using the above model.     

Internal stresses and dislocation structure in germanium single crystals

The thermoelastic stress developing during the growth of disk-shaped germanium single crystals for IR applications has been evaluated theoretically. The results have been shown to correlate with the dislocation structure of large germanium single crystals grown by the Stepanov method and directional solidification.     

The Bauschinger effect in cyclically deformed niobium single crystals

The Bauschinger effect has been studied in niobium single crystals cyclically deformed to saturation at small strain amplitudes and at different temperatures. The parameters used to characterize the Bauschinger effect have been measured from the saturation hysteresis loops. The fatigued work-hardened crystals have been modelled as a composite consisting of a hard deformable phase (the regions of high dislocation density) embedded in a soft matrix (the regions of low dislocation density). The values of the mean internal stress in the matrix predicted by this simple composite model are in good agreement with those measured experimentally, using a modification of the method originally proposed by Cottrell to separate the lattice friction and internal stress components in reverse deformation.     

Interactions between dislocations and twins in deformed titanium aluminide crystals

Using scanning, transmission electron microscopy and aberration-corrected scanning transmission electron microscopy, we have studied the interactions between dislocations and twins in impact deformed polysynthetic twinned TiAl crystal. The 1/3?<?1$\overline{1}\overline{1}$] and 1/6?<?211] step dislocations on coherent twin boundaries reveal the interactions of glissile 1/2?<?101> dislocations with the coherent twin boundaries. An abnormal stacking fault was found adjacent to the coherent twin boundary. It has the same stacking sequence but different atom species in the [$\overline{1}$10] direction with an additional displacement of 1/4[$\overline{1}$10] in two neighboring {111} layers, and is likely induced by the slip of a 1/12[112] (i.e. 1/4[$\overline{1}$10] + 1/6[2$\overline{1}$1]) dislocation.     

Dispersion of optical and acoustic phonons in diamond and germanium crystals

The dispersion laws of the optical and acoustic branches in diamond-like crystals have been derived using idealized models of crystal lattices. Two types of analytical expressions have been proposed, whose parameters can be derived from inelastic neutron scattering experiments and the properties of acoustic waves. The calculation results are in satisfactory agreement with experimental data for the crystallographic directions [100], [110], and [111]. The group velocities of optical and acoustic phonons have been determined, and expressions for the effective mass of optical phonons have been derived.     

Microstructural characterization of calcium flouride single crystals deformed in steady state

Calcium fluoride single crystals have been deformed in compression to conditions of steady-state deformation in the temperature range 590 to 907° C (0.53 to 0.72T/T _m). The deformation microstructures have been characterized using cold-stage transmission electron microscopy. The microstructure of deformed samples is seen to consist of dislocation tangles, networks and subgrain boundaries. Dislocation structures in the subgrain boundaries have been characterized and the effect of the temperature of deformation on the subgrain boundary structure has been established. The flow stress,σ, during steady-state deformation, has been found to be proportional tod ^−1.14, whered is the subgrain size. The steady-state deformation behaviour is believed to be controlled by the mechanisms of obstacle-limited glide of dislocations and power-law creep. During characterization of the deformation microstructures, regions of non-uniform cell or subgrain boundary structure have been observed. It has been suggested that such regions arise from either non-uniform deformation or recovery and recrystallization. Despite the presence of such regions, subgrain strengthening appears to be a viable means of improving the flow stress of calcium fluoride single crystals.     

TEM study of the recovery process of cyclically deformed MgO single crystals

Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470° C a large number of cycles could be obtained before failure, and the cyclic stress-strain response showed a period of rapid hardening followed by a period of decreasing hardening rate. TEM investigations of the lower temperature samples show structures of isolated dislocation dipoles, multipoles and debris. At 470° C dense bundles of dislocations were observed aligned perpendicular to the Burgers vector direction. The regions between the bundles were relatively dislocation free, but they contained a high density of debris. Bowed out screw dislocations are observed between the edge dislocation bundles, suggesting that screw dislocations were largely mobile. Comparisons are made with the cyclic deformation and structure of fcc metals and other NaCl structure single crystals.     

Etch-pit investigations of deformed stoichiometric and non-stoichiometric Mn-Zn ferrite single crystals

Etching solutions both for stoichiometric and non-stoichiometric Mn-Zn ferrite crystals have been developed. The hot phosphoric acid method is suitable for nearly stoichiometric compositions and shows an etch memory effect for dislocations nearly parallel to the crystal surfaces. From the etch-pit arrays the slip system of Mn-Zn ferrite crystals is concluded to be of {111}1 0 type independent of the chemical composition and the Schmid factor.     

Two-phonon Raman scattering of light in rare gas crystals

In this paper we calculate the two-phonon efficiency for light scattering from rare gas crystals at all temperatures. We use a Lennard-Jones potential and the Lorentz-Lorenz model following previous theoretical work and use the first-order self-consistent phonon theory, supplemented by a simple model giving the phonon anharmonic widths and shifts. Our results, though larger than the experimental results, agree well with earlier calculations at 0 K and related molecular dynamics work. The calculated spectra show a rich structure that varies with temperature. We suggest that further experiments could lead to significant new insights into the nature of excitations in rare gas solids as a function of temperature.Work submitted in partial fulfillment of the requirements of the Ph.D. degree at Rutgers University.