Bulk Ge Research Articles

Crystal structure of the intermetallic compound lithium aluminum germanium, Li16Al4Ge7

AUGe7Lii6, tetragonal, I4\/amd (No. 141), a = 6.193(2) Â, c = 12.267(3) A, V= 470.5 Â, Z = 1, Rgt(F) = 0.034, wRTef(F) = 0.040, T = 293 K. Source of material The compound Lii6AUGe7 was prepared from Li and Al bulk metals and Ge powder (atomic proportion 15:3:6) inserted in a Ta reactor weld-sealed under argon and protected in a stainless container. The product was obtained by reacting the elements for 11 h at 873K, homogenizing for 5 h at 1173K, and single crystals were formed by cooling the melt at the rate of 25 K/h to ambient temperature. The air-sensitive crystals were selected in a glove box, inserted into Lindemann capillaries. Cell parameters and space group were initially determined by oscillation and Weissenberg techniques. The Al:Ge ratio of nearly 0.6:1 has been confirmed from elemental analysis (SEM) of single crystals. The best diffracting crystal was used for data collection. Table 1. Data collection and handling. Crystal: dark grey platelet, size 0.02 χ 0.06 χ 0.09 mm Wavelength: Mo Ka radiation (0.71069 A) μ· 111.86 cm 1 Diffractometer, scan mode: Nonius CAD4, ω/20 73.98° N(hkl)m 2 σ(lobs), 131 N(param)Krmoi· 15 Programs: SHELXS-97 [4], SHELXL-76 [5], DIAMOND [6], CRYSTALS [7] Table 2. Atomic coordinates and displacement parameters (in A). Atom Site Occ. * y ζ Uiso Li(3) 8e 0.38(6) 0 1/4 0.118(4) 0.013(8) * Correspondence author (e-mail: mtillard@univ-montp2.fr) Unauthenticated Download Date | 2/16/16 5:16 AM

Pulsed laser annealing of Si–Ge superlattices

Si5Ge5 superlattices (SL) were treated by 80-ns pulses of a ruby laser in a wide range of energy densities. The induced structural and electronic changes were monitored in situ by time-resolved reflectivity (TRR) and ex situ by scanning electron microscopy (SEM), Raman scattering and atomic force microscopy (AFM). The SL starts to melt at energy densities typical of bulk Ge (less than 0.4 J/cm2). At ≧0.7 J/cm2, a self-organization phenomenon is observed: a system of quasiregular rectangular grains with linear dimensions of about 100 nm is developed on the sample surface.

Ferromagnetism in Cr-doped Ge

We have fabricated Cr-doped bulk Ge single crystal using the vertical gradient solidification method. The material shows ferromagnetic ordering at 126 K, as determined from temperature-dependent magnetization and resistance measurements. A sample with x=0.01 was p type with nh=3×1017 cm−3 at 350 K. The measured magnetic moment per Cr was 0.83μB at 5 K.

Synthesis of large-area germanium cone-arrays for application in electron field emission

High-vacuum electron-beam evaporation was used to synthesize germanium (Ge) cone-arrays on N+-type (100) Si. The surface morphology of Ti nanocrystals’ catalyst and Ge cone-arrays was investigated by scanning electron microscopy and atomic force microscopy. The lattice constant of the Ge cones determined from x-ray diffraction was nearly identical to that of bulk Ge. The mechanism of Ge cone-arrays’ formation was investigated. The electron field emission properties of the as-prepared Ge cone-arrays were studied based on current–voltage measurements and the Fowler–Nordheim equation.

Electronic susceptibility in thin films and interfaces

We use tight-binding theory to investigate the electronic contribution to dielectric susceptibility in thin films and interfaces of covalent materials. We begin by describing the effects of an electric field on the elemental unit of a covalent material, the bond. Then, we show how the responses of individual bonds can be added up to obtain an estimate of the susceptibility of a bulk material. In doing so, we see that the polarization of a material can be viewed as arising from the transfer of charge from one side of the system to the other, and that this viewpoint leads naturally to a local definition of susceptibility in semiconductors. Using this concept, we examine dielectric susceptibility in thin films and interfaces, with a Si/Ge/Si heterostructure serving as an example. The interesting feature of thin films and interfaces is that they exhibit spatial variations in susceptibility, which we attribute to: (i) elastic distortions; (ii) the creation of bonds at an interface which are of a type not found in either bulk material; and (iii) the coupling of a bond to neighboring antibonds different than those in the bulk material. We then ask what error is introduced by neglecting these local variations when calculating the capacitance of a multilayer dielectric. For the Si/Ge/Si heterostructure, we find that effect (iii) introduces only small errors, even for very thin Ge layers, because the decrease in susceptibility on the Ge side of an interface is offset by the increase in susceptibility on the Si side. Similarly, effect (ii) is small because the polarizability of the Si–Ge bonds at the interface is very nearly the average of that for Si and Ge. On the other hand, effect (i) does lead to noticeable errors, but these errors can be removed almost entirely by choosing the permittivity of the Ge layer to be that of bulk Ge under the same state of strain as the Ge layer in the heterostructure. We conclude by interpreting recent experiments on “high-k” dielectrics in term of what we have learned here. [C. M. Perkins et al., Appl. Phys. Lett. 78, 2357 (2001); M. Koyama et al., Tech. Dig. Int. Electron Devices Meet., 459 (2001); W.-J. Qi et al., ibid., 145 (1999)].

Low-energy photoluminescence of structures with GeSi/Si(001) self-assembled nanoislands

The photoluminescence spectra of structures with self-assembled GeSi/Si(001) islands are investigated as functions of the growth temperature. It is shown that the shift of the peak of photoluminescence from islands toward lower energies on decreasing the growth temperature is due to the suppression of Si diffusion into islands and an increase in the fraction of Ge in islands. A photoluminescence signal from the GeSi islands is found in the region of energies down to 0.6 eV, which is considerably smaller than the band-gap width in bulk Ge. The position of the peak of photoluminescence from islands is described well by the model of a real-space indirect optical transition with account of the real composition and elastic strains of the islands. Mono-and multilayer structures are obtained with self-assembled GeSi/Si(001) nanoislands exhibiting a photoluminescence signal in the region 1.3–2 μm at room temperature.

Auger spectroscopy thermodesorption of Sb on Si 1− xGe x layers grown on Si( [formula omitted]) substrates

Desorption of Sb from Si 1− x Ge x layers ( x=0, 0.05, 0.1, 0.2, 1) grown by molecular beam epitaxy (MBE) on Si(1 0 0) substrates is investigated using Auger electron spectroscopy thermodesorption (TD-AES). Sb desorption process on Si 1− x Ge x is well described by a first-order reaction. No extra TD-AES peaks are observed on Si 1− x Ge x compared to Si. For 1 ML of Sb coverage the TD-AES peak shifts to lower temperature when Ge bulk concentration increases. The Sb monolayer desorbs at 801, 752, 740, 715, and 706 °C for x=0, 0.05, 0.1, 0.2 and 1, respectively. We explain the non-linear decrease of the Sb desorption energy when x increases by the strong Ge surface segregation during the MBE growth of Si 1− x Ge x layers, resulting in an almost pure Ge surface even for low x.

Ultrahigh room-temperature hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures

We have obtained ultrahigh room-temperature (RT) hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures with very small parallel conduction. Reducing parallel conduction was achieved by employing Sb doping in Si0.3Ge0.7 buffer layers, which drastically increased RT hole Hall mobility up to 2100 cm2/V s in the strained Ge channel modulation-doped structures and improved device characteristics of the p-type metal–oxide–semiconductor field-effect transistors with the strained Ge channel. The peak effective mobility reached to 2700 cm2/V s at RT, which was much higher than the bulk Ge drift mobility.

Ferromagnetism in Mn-doped Ge

We have successfully fabricated highly (up to 6%) Mn-doped bulk Ge single crystals. The lattice constant increases linearly with Mn concentration due to the larger Mn atomic radius compared with Ge, strongly indicating that Mn ions are being incorporated into the host Ge lattice. Alloys with lower Mn concentrations showed paramagnetism due to localized magnetic ions. ${\mathrm{Ge}}_{0.94}{\mathrm{Mn}}_{0.06}$ showed ferromagnetic ordering at \ensuremath{\sim}285 K, as determined from temperature-dependent magnetization and resistance measurements. The coersive field was 1260 Oe at 250 K.

Scanning tunneling microscopy of SiGe alloy surfaces grown on Si [formula omitted] by molecular beam epitaxy

An investigation into the surface structure and morphology that results from the growth of SiGe alloys with 0%, 5%, 10%, and 20% Ge bulk concentration at 500 and 800 °C on Si(1 0 0) is presented. Stages of alloy growth are reported from the initial Si–Ge intermixing to the segregation of Ge from the alloy to the final 2D island surface morphology. Surface Ge produces a “2× n” surface reconstruction, which is shown to result from a mixture of local atomic orderings of c(4×2) and p(2×2) that varies with alloy concentration. The alloy surfaces are characterized by 2D island growth, where the surface roughness is observed to increase with increasing Ge concentration and with increasing growth temperature. A discussion of the effects of adatom mobility, compressive strain, step-edge attachment, and surface reconstruction involved in 2D island growth are presented.

Effect of the interface on the local structure of Ge–Si nanostructures

We first discuss the limitations of Raman scattering as applied to Ge/Si nanostructures. We further summarize our recent efforts to investigate the local structure of various Ge nanostructures, namely, Ge quantum dots grown by molecular beam epitaxy (MBE) on bare Si(100), on Si(111) with a 0.3 nm SiO2 coverage, and nanocrystals embedded in SiO2, by x-ray absorption fine structure spectroscopy. In particular, the MBE growth of Ge dots on bare Si(100) has been studied as a function of the growth conditions: in most cases strong alloying with Si takes place. Ge nanoislands on Si(111) with SiO2 coverage, on the other hand, may retain the local structure of bulk Ge and be very stable against oxidation. The Ge nanocrystals embedded in SiO2 possess the structure of relaxed bulk Ge without any Ge–Si bonding. The latter two kinds of Ge nanostructures possess visible photoluminescence.

Pressure-induced resonant Raman scattering in Ge/Si islands

Self-assembled Ge islands grown by solid-source molecular-beam epitaxy were investigated by resonant Raman scattering under hydrostatic pressure at room temperature. We utilize the effect of pressure to tune the electronic transition through laser excitation energies in the Ge islands. The pressure coefficient of this resonating electronic transition thus obtained is ∼2.7±0.5 meV/kbar, which is significantly smaller than the pressure shift of the El transition in bulk Ge. This is attributed to the fact that the Ge islands are strongly constrained by the surrounding Si lattice, leading to a smaller deformation as compared to the bulk Ge, when subjected to the same pressure.

Nanoscale electrodeposition of germanium on Au(111) from an ionic liquid: an in situ STM study of phase formation

The electrodeposition of germanium on Au(111) from the GeCl4 saturated ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was investigated with in situ scanning tunneling microscopy and in part, qualitatively, with in situI/U tunneling spectroscopy. Ge deposition starts in the UPD regime with the decoration of the gold steps by Ge at +1000 mV vs. Ge, then small islands that have initial heights of about 150 pm on average start growing at +950 mV vs. Ge. At +750 mV vs. Ge islands with average heights of 250 pm form, they can be removed reversibly at higher electrode potentials and holes with apparent depths between about 30 and 100 pm remain in the gold surface. Before bulk deposition of germanium starts, a thin rough layer with metallic behaviour is deposited on the surface. It can be stripped reversibly upon increasing the electrode potential to higher values. The overpotential deposition on this layer starts with nanoclusters of 10–40 nm in diameter and some nanometres in height. They can be stabilized for hours by proper selection of the electrode potential. In situI/U tunneling spectroscopy of about 100 nm thick Ge films clearly shows semiconducting behaviour on the whole of the surface in the scanned area. The band gap of the bulk Ge determined by in situI/U tunneling spectroscopy is 0.7 ± 0.1 eV, in good agreement with the theoretical value of 0.67 eV for intrinsic microcrystalline germanium at 300 K. On a layer of approximately 20 nm in thickness we found both sites with semiconducting behaviour and sites with rather metallic behaviour.

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.

Stable Structure of the Sb Monolayer on a Strained Ge(111) Substrate

Using an ab initio total energy and force method, we haveinvestigated the stability of different structures ofGe(111):Sb(1 ML) as a function of the lateral lattice constant. Wefind that the (2×1)-reconstruction of Ge(111):Sb,experimentally found to be stable at the equilibrium lattice constantof Ge, is also the stable structure for slightly dilated Ge films(< 1%), while for larger dilatations the (1×1)-structurebecomes stable. For compressed Ge films the((3)1/2×(3)1/2)T4-structure (found experimentallyon Si(111):Sb) becomes competitive and it is stable for the latticeconstants compressed by more than 5%. Furthermore, we find that foreach structure, the equilibrium lattice constant is different fromthe bulk Ge crystal. Our results are helpful for the understanding ofsurfactant mediated island growth on strained films.

Ge layer transfer to Si for photovoltaic applications

We have successfully used hydrophobic direct-wafer bonding, along with H-induced layer splitting of Ge, to transfer 700-nm-thick, single-crystal Ge (100) films to Si (100) substrates without using a metallic bonding layer. The metal-free nature of the bond makes the bonded wafers suitable for subsequent epitaxial growth of triple-junction GaInP/GaAs/Ge solar cell structures at high temperatures, without concern about metal contamination of the active region of the device. Contact-mode atomic force microscopy images of the transferred Ge surface generated by hydrogen-induced layer-splitting reveals root mean square (rms) surface roughness of between 10 and 23 nm. Electrical measurements indicate ohmic I– V characteristics for as-bonded Ge layers bonded to silicon substrates with ∼400 Ω cm −2 resistance at the interface. Triple-junction solar cell structures grown on these Ge/Si heterostructure templates by metal–organic chemical vapor deposition show comparable photoluminescence intensity and minority carrier lifetime to a control structure grown on bulk Ge. An epitaxial Ge buffer layer is grown to smooth the cleaved surface of the Ge heterostructure and reduces the rms surface roughness from ∼11 to as low as 1.5 nm, with a mesa-like morphology that has a top surface roughness of under 1.0 nm, providing a promising surface for improved GaAs growth.

Stress induced surface melting during the growth of the Ge wetting layer on Si(0 0 1) and Si(1 1 1)

We compute the isotropic stress for a Ge film deposited on Si(0 0 1) and Si(1 1 1) at T=870 K. By direct comparison with experimental phase diagrams measured for bulk Ge we deduce that Ge melts—at least partially—when deposited on Si surfaces at this temperature, due to the high heteroepitaxial stress (in the range 1–10 GPa), which has also been reported in experimental studies. Surface melting (complete or incomplete) induced by heteroepitaxial stress is consistent with a recent theoretical model proposed in the literature, and may account, at least in part, for the large values of Ge/Si intermixing reported in other studies.

Resonant Raman Scattering by Strained and Relaxed Ge Quantum Dots

ABSTRACTFundamental vibrations in Ge/Si structures with strained and relaxed Ge quantum dots (QDs) grown by molecular beam epitaxy were investigated using resonant Raman spectroscopy. Transmission electron microscopy experiments show that the strained Ge QDs are typical “hut clusters” with base size of 15nm and a height of 2nm. A two mode distribution in size (100–200nm and 3–6nm) is found for relaxed QDs. The Raman efficiencies of the Ge optical phonons as a function of excitation energy reveal maxima at 2.35–2.41eV attributed to the E0 resonance in Ge QDs due to electronic confinement. The frequency positions of optical phonons localized in Ge “hut clusters” under non-resonant conditions correspond to fully strained Ge QDs while the frequency position of optical phonons in relaxed Ge QDs corresponds to the value in bulk Ge. With increasing excitation energy (2.5–2.7eV) the position of the Ge optical phonons shifts downwards due to size-confinement effect of optical phonons in strained and relaxed Ge QDs, indicating the presence of a QD size distribution in Ge dot structures.

Ab initioinvestigation of point defects in bulk Si and Ge using a cluster method

Atomic and electronic structures of various charge states $(2+,+,0,\ensuremath{-},2\ensuremath{-})$ of monovacancies V and divacancies ${V}_{2}$ in crystalline Si and Ge are calculated from first principles. The calculations are performed in real space on bulk-terminated spherical and prolate clusters that are passivated by hydrogens at the boundaries. Defect-induced Jahn-Teller distortions, Jahn-Teller and relaxation energies, vacancy wave function characters, and hyperfine parameters are calculated and compared with available experimental data. The magnitudes of Jahn-Teller distortions and energies are found to be smaller in Ge compared to Si for both V and ${V}_{2}.$ Unlike in Si, the pairing type distortions induced by a divacancy in Ge are not large enough to result in a deep level crossing inside the band gap. Furthermore, the relaxed atomic configurations of the divacancy in Ge with resonant bond type distortions are found to be slightly lower in energy than those with pairing type distortions. The effect of the lattice constant at which the calculations are performed (experimental versus theoretical) is also examined, and found to be quite important, especially for Ge.

Anomalous Elastic Properties of Si/Ge Superlattices: The Role of Interfaces

The elastic constant tensors of Sim/Gen superlattices with different modulation wavelengths have been determined by Brillouin light scattering spectroscopy, and their Young's moduli measured by the nanoindentation technique. Two sets of samples have been studied: m = n and m = 4n. The C33 elastic constant obtained for Sin/Gen superlattices (where bilayers are built-up by an equal number of Si and Ge monolayers) are some 13% above the predictions of the continuum elasticity theory, using the Cij values of bulk Si and Ge. On the other hand, the elastic constants tensor of Si4n/Gen superlattices (where bilayers are built-up by four times more Si monolayers than Ge ones) matches perfectly the predictions. A thorough structural analysis by low-angle X-ray reflectivity, high-angle diffraction and X-ray absorption spectroscopy has been carried out and a deep characterization of the microstructure is presented here. The unexpected value of C11 in Sin/Gen superlattices can be attributed to the compressed Si–Ge alloy found at the wide interfaces present in that set of superlattices.