Ge Crystals Research Articles

Influence of Ge and GeO2 on the microstructure and varistor properties of TiO2–Ta2O5–CaCO3 ceramics

This study investigated the effect of elemental crystal Ge or/and GeO2 doping on the microstructure and varistor properties of TiO2–Ta2O5–CaCO3 varistor ceramics, which were prepared via the traditional ball milling–molding–sintering process. X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy demonstrated that co-doping with Ge and GeO2 changed the microstructure of TiO2–Ta2O5–CaCO3 ceramics, thereby increasing the nonlinear coefficient and decreasing the breakdown voltage. The optimum doping concentrations of Ta2O5, CaCO3, Ge, and GeO2 exhibited the highest nonlinear coefficient (α=14.6), a lower breakdown voltage (EB=18.7Vmm−1), the least leakage current (JL=10.5μAcm−2), and the highest grain boundary barrier (ΦB=1.05eV). In addition, Ge and GeO2 function as sintering aids, which reduce the sintering temperature because of their low melting points.

High dopant activation of phosphorus in Ge crystal with high-temperature implantation and two-step microwave annealing

In this letter, high-temperature ion implantation and low-temperature microwave annealing were employed to achieve high n-type active concentrations, approaching the solid solubility limit, in germanium. To use the characteristics of microwave annealing more effectively, a two-step microwave annealing process was employed. In the first annealing step, a high-power (1200 W; 425 °C) microwave was used to achieve solid-state epitaxial regrowth and to enhance microwave absorption. In the second annealing step, contrary to the usual process of thermal annealing with higher temperature, a lower-power (900 W; 375 °C) microwave process was used to achieve a low sheet resistance, 78Ω/◻, and a high carrier concentration, 1.025 × 1020 P/cm3, which is close to the solid solubility limit of 2 × 1020 P/cm3.

Electrical properties of Ge crystals and effective Schottky barrier height of NiGe/Ge junctions modified by P and chalcogen (S, Se, or Te) co-doping

The electrical properties of Ge crystals and the effective Schottky barrier height (SBH) of NiGe/Ge diodes fabricated by P and/or chalcogen (S, Se, or Te) doping were investigated for Ge n-channel metal–oxide–semiconductor field-effect transistors with a NiGe/n+Ge junction. The electron concentration in Ge was increased more by co-doping with chalcogen and P than by doping with P alone. Moreover, SBH values were decreased in NiGe/nGe diodes and increased in NiGe/pGe diodes compared with undoped NiGe/Ge by both P doping and P and chalcogen co-doping. Co-doping with Te and P was most effective in modifying the SBH.

(Invited) Application of DFT Calculation for the Development of High Quality Si and Ge Substrates: From Ultra Large Diameter Crystal Pulling to Metal Gettering

During the last decade, considerable progress has been made in understanding the properties and behavior of the vacancy V and self-interstitial I in silicon (Si) and germanium (Ge) crystals. This is to a large extent due to the maturing of density functional theory (DFT) calculation techniques and the increase of computing power enabling to calculate not only the formation and migration energies of V and I, but also the interaction with impurities and with crystal surfaces. Furthermore, the impact of internal and external stress on formation and migration enthalpies of both intrinsic point defects has been clarified recently. In this paper an overview will be given of recent results obtained by the author’s group calculating the interaction of V and I with the melt-solid interface, and the impacts of stress and doping on V and I properties. The useful application of DFT calculations for research and process development on the control and engineering of intrinsic point defects in Si and Ge single crystal growth from a melt are presented in focusing on (i) the difference of the dominant point defect in Si and Ge, (ii) the impact of thermal stress on intrinsic point defects in large-diameter Si crystal and (iii) the dopant impact on intrinsic point defects in Si. The results indicate that DFT calculation will be a necessary tool for the cost-effective development of the advanced crystal growth processes that will be needed for the new family of 450 mm diameter Si crystals or for V-poor, large diameter Ge crystals [1]. Besides the crystal growth, DFT calculations are also useful to for the development of “impurity gettering” technology in large-diameter Si wafers. Very recently, a C3H5 cluster ion implantation technique for proximity gettering has been reported with the low energy of around 1015/cm2 dose without recovery heat treatments. The main feature of this technique is that the gettering efficiency is higher than that by C monomer implantation, even though irradiation defects are too small to clarify by TEM observation. In the present work, we evaluate the binding energies of metal atoms with candidate gettering sites of C, H, intrinsic point defects and related complexes in Si wafers induced by C3H5 cluster ion implantation or different methods, for example, H implantation etc. by using DFT calculations. In addition to C and H atoms, we consider donor P and O atoms contained in an n- CZ-Si wafer for use in a CMOS image sensor. The simplest complexes of substitutional/interstitial C (Cs/Ci), Hi, Ps, Oi, and incorporated intrinsic point defects (vacancy (V) and self-interstitial Si (I)) by C3H5 implantation were also considered. We found that Cs-I (= Ci), Ci-Ci, Hi-I, VHn (n =1 to 3), and VO complexes are the best candidates for gettering sites. Gettering by C3H5 cluster ion implantation is more effective than that by C monomer implantation due to the formation of VHn (n = 1–3) and Hi-I complexes, which provides more effective gettering sites [2]. An approach based on statistical thermodynamics and DFT calculations to predict properties of materials composed of different types of atoms is presented. The key point of the “Hakoniwa” method is to take into account all possible structural supercells constructed by the fixed number of atoms of each species according to the composition of the target material [3]. The conservation of the total number of atoms enables calculating the average value of a material property for a given temperature by applying statistical thermodynamics to the material property values obtained for each of the possible supercells. The applications of the Hakoniwa method to the crystal growth and metal gettering are mentioned.

Low-Temperature Formation of Sn-Doped Ge on Insulating Substrates by Metal-Induced Crystallization

Low-temperature formation of Sn-doped Ge on insulator is desired to realize next generation flexible electronics. To achieve this, metal-induced crystallization (MIC) of a-GeSn is investigated. By MIC of a-GeSn with initial Sn concentration of 5%, Sn-doped Ge is obtained at low temperatures (<250℃). The Sn concentration (0.5-2.0%) in the grown layers can be controlled by modulating the annealing temperatures in the range of 150-250℃. Introduction A technique for low-temperature (≤250°C) formation of high-quality crystalline Ge films on insulator is expected to realize advanced high performance thin film transistors (TFT) on flexible plastic substrates (softening temperature: ~300oC). To improve quality of Ge crystals, Sn-doping (<4%) is very effective, because it enables passivation of point defects in Ge [1]. This triggered the recent research of solid-phase crystallization of Sn-doped Ge (Sn concentration: 2%) on SiO2layers [2]. However, the growth temperature (425°C) reported to date is higher than the softening temperatures of plastic substrates. To decrease the growth temperature of Sn-doped Ge, in the present study, we investigate metal-induced crystallization (MIC) of a-GeSn. This successfully decreases the growth temperature below 250°C. Experimental Procedure Fig.1 schematically shows the procedure of sample preparation. Amorphous Ge1-xSnx layers (x: 0-0.2, thickness: 100 nm) were deposited on quartz substrates at room temperature. Then, Au layer patterns were formed on the amorphous GeSn layers. The samples were annealed at 150-250oC for 10-60 min to induce lateral growth in a N2ambient. The growth characteristics were analyzed by Nomarski optical microscopy and microprobe Raman spectroscopy. Results and Discussion Figs. 2(a) and 2(b) show Nomarski optical micrographs of Ge1-xSnx (x: 0.05 and 0.2, respectively) samples after annealing at 250oC for 10 min. Au pattern regions are located in the left side areas of the micrographs. In the case of the low Sn concentration (5%), different contrast regions are observed around the Au patterns, as indicated by the broken lines. On the other hand, the sample with the high Sn concentration (20%) shows no change around the Au pattern after annealing. The Raman spectra obtained from these samples are summarized in Fig. 2(c). In Raman spectra #1 for the low Sn concentration (5%), a sharp peak due to Ge-Ge bonding in c-Ge is clearly observed. On the other hand, no peaks are detected in spectra #3 for the sample with the high Sn concentration (20%). These results indicate that lateral growth is generated from the Au patterns for samples with the low Sn concentration (5%), while it is not generated for the high Sn concentration (20%).The lateral growth lengths at 150 and 250oC for samples with various Sn concentrations were estimated using theNomarski optical microscopy combined with line-scanning of microprobe Raman measurements. The results are summarized as a function of the annealing time in Fig. 3. In the case of low Sn concentrations (<5%), lateral growth lengths increase in a short annealing time (10 min). In addition, the growth lengths increase with increasing annealing temperature and exceed 20 μm at 250°C, which is sufficient for device fabrication.The substitutional Sn concentrations in grown layers for samples with the initial Sn concentration of 5% are evaluated by peak shift of Raman spectra. Substitutional Sn concentrations of 0.5−2.0% are obtained in grown layers, which is useful to passivate point defects in Ge [1]. The substitutional Sn concentration depended on the annealing temperature. This indicates that the substitutional Sn concentration can be controlled by modulating annealing temperature. This technique will be useful to realize next generation flexible electronics.

Characterization for Sulfurized Polyethylene Glycol As Electrode Material for Li-S Battery

Sulfur is one of the promising electrode materials because of a large theoretical capacity (1672 mAh g-1). However, both ionic and electronic conductivities of sulfur are poor and the lithium polysulfide (Li2Sx) dissolves into the electrolyte solution during charge/discharge process [1]. To improve the ionic and electronic conductivities, the composites with carbon, such as nano-carbon architectures [2, 3], graphene [4-6], and carbon nanotubes [7, 8], have been investigated as positive electrode materials. Recently, it was reported that the sulfurized carbon composite prepared by heat treatment of polyacrylonitrile with sulfur exhibits a high capacity and long cyclability [9-11]. In this study, we propose a novel sulfurized carbon composite (here after denoted as SPEG), which is synthesized by heat treatment of polyethylene glycol (PEG: a representative polymer) with sulfur. SPEG showed a second discharge capacity of 488 mAh g(SPEG)-1 and was maintained at 433 mAh g(SPEG)-1 even after the 10thcycle. Although SPEG has high capacity and long life-cycle, crystallographic structure of SPEG itself is not clear. In this paper, we investigate the local structure and chemical states of S in SPEG by X-ray absorption spectroscopy and examined the chemical state of SPEG by Raman spectroscopy. SPEG was prepared through refluxing 25.0 g of polyethylene glycol (PEG; Kishida Chemical, mean molecular weight: 200 g mol-1) and 51.6 g sulfur (Kanto Chemical) in an alumina tube (Nikkato) in an electric furnace. The mixture of PEG and S was heated up to 673 K, then cooled. The obtained crude product was heated again at 573 K under nitrogen flow (100 ml min-1) in order to remove the residual molecular sulfur. Composite obtained (8.9 g) was pulverized using mortar and pestle. The composite powder is a black powder and contained about 60 wt.% of S in the sample. The local structure and chemical states of S in SPEG were examined by S K-edge X-ray absorption fine structure (XAFS, BL-10 of the Synchrotron Radiation Center, Ritsumeikan University). The total electron yield method was used and the incident X-ray beam was monochromatized with a Ge(111) crystal (2d = 0.6532 nm) pair. The photon energy was calibrated with the strong resonance of K2SO4 appearing at 2481.7 eV dipole transition. The XANES spectrum of this composite shows three distinct peaks at 2469, 2472 and 2473 eV, which however do not correspond to profiles of pure S and Li2S. Therefore, this suggests that free S is absent in the SPEG. The chemical states of composites were investigated by Raman spectroscopy (ALMEGA XR, Thermo Fisher SCIENTIFIC) at room temperature. The excitation source is a 532 nm laser. The wavenumber of monochromator was calibrated by a silicon plate at 520 cm-1. The spectrum was recorded by accumulation of 12 scans (each of 5 s) in the range of 100 < ν/cm-1 < 3000. The highest Raman peak of SPEG appears at 1441 cm-1 between G (1600 cm-1) and D (1300 cm-1) bands of graphite. The other peaks appear at 1279, 1066, 846, and 772 cm-1. Those peaks are not assigned to various vibration modes of pure S and carbon. Thus, the new chemical states are formed between S and C in this composite. This result is consistent with result of XAFS. The chemical states in SPEG are related to the electrochemical properties. We will discuss a possible structural model of SPEG and also the correlation between the electrochemical properties and the electronic/local structural changes in SPEG.

Macroinhomogeneity of the properties of semiconductor crystals due to the specifics of the melt’s behavior under microgravity conditions

The effect of convective flows caused by the changing direction of the residual gravitational vector relative to the solidification front on the radial inhomogeneity (macrosegregation) of the dopant distribution in semiconductor crystals is studied within the context of mathematical and physical simulation for terrestrial and space conditions. Theoretical calculations and experimental research are carried out by the example of the growth of Ge crystals and their heavy doping with Ga (1019 сm–3). The velocities of convective flows near the solidification front which lead to radial macroinhomogeneity in the grown crystals are theoretically calculated. The requirements to obtain homogeneous semiconductor crystals under real microgravity conditions are formulated.

(Invited) Unseeded Growth of Poly-Crystalline Ge with (111) Surface Orientation on Insulator by Pulsed Green Laser Annealing

Thin-film transistors (TFTs) with a high channel mobility for both electrons and holes are required for the three-dimension large scale integrations or system-on-panels. Lower temperature process is more favorable for TFT fabrication because the application of plastic substrate to system-on-panels is expected to add the flexibility function. Germanium (Ge) has higher hole and electron mobilities than Si, suggesting that large-grain poly-crystalline or single-crystalline Ge thin films lead to high channel mobility TFTs. This paper describes unseeded growth of poly-crystalline Ge with well-oriented large-size grains on insulator from amorphous Ge by scanning pulsed green laser annealing. In experimental, a 50-nm-thick amorphous Ge layer was deposited on a Si substrate with a 50-nm-thick SiO2 film. The Ge layer was patterned to narrow stripes of 2-µm-width. Then a capping layer of 650-nm-thick SiO2 was deposited. For this processed template, a scanning pulse green laser annealing was performed. Q-switch Nd:YAG laser beam with the following properties was used for annealing; a Gaussian line beam of 70 x 1400 µm2 at a laser wavelength of 532 nm, a pulse frequency of 7.5 kHz, a scanning velocity of 150 µm/s, and energy densities from 0.5 to 0.7 J/cm2. The Ge stripes were crystallized by a single scanning, where the beam was scanned perpendicular to the longitudinal direction of the beam. The Ge stripes crystallized at the optimum energy of 0.56 J/cm2 by the scanning pulsed laser annealing was characterized by electron back scattering diffraction patterns (EBSD). The EBSD mapping of normal direction to the sample surface indicates that the crystallized Ge film has (111) surface orientation in the 600 µm-long stripe. The (111) Ge crystal begins within only 2 µm after the starting point of the laser scanning. It was observed that the crystal orientation was recovered to (111) even if the prior part of crystalized Ge film was rotated from (111) surface. In the mapping of rolling direction, which corresponds to the laser scanning direction, two types of grains are dominant. One shows (101) and the other shows (112) for the laser scanning direction. From the normal direction and rolling direction images of EBSD, the Ge stripes were crystallized with the grains of 2 µm wide and ~10 µm long. The maximum grain length of 20 µm was also observed. Raman scattering was measured to evaluate the crystalline quality of the (111) surface-oriented Ge stripes. The full-width at half-maximum of 2.35 cm-1 shows good crystal quality of the film, which is smaller than the values of 3.2-3.3 cm-1 for the single crystalline Ge film obtained by lateral liquid-phase epitaxy from a Si seed. For laser fluence dependence, Ge stripes were not crystallized when the fluence was smaller than 0.56 J/cm2, and Ge stripes show non-oriented crystal with a long grain boundary in the center of stripe width when the fluence was larger than 0.67 J/cm2. Simulation analyses of thermal diffusion reveal the crystallization process. The pulse laser irradiation heats the Ge stripe and melts it. Simultaneously the underlying Si in the outside region of the Ge stripe patterns absorbs the laser energy and is also heated up. During the cooling period after each pulse a temperature gradient between the edge and the center is produced in the molten Ge stripe, because of suppression of cooling at the edge. Then the temperature at the center of the Ge stripe drops below the solid point first rather than the center. Crystal growth occurs from the center to the edges, resulting that random nucleation at the edge is suppressed and crystal growth proceeds with controlled orientation in the Ge stripe. This crystallization method is promising to realize high-mobility (111) Ge TFTs and easily extended to the glass or plastic substrate by making amorphous Si layer, which works as a laser absorbing layer, and SiO2, which works as a heat insulating layer, on the substrate.

Influence of peripheral vibrations and traveling magnetic fields on VGF growth of Sb-doped Ge crystals

We performed 3D numerical and experimental studies to assess the potential of peripheral low frequency mechanical vibrations for improving the homogeneity of Sb-doped 4″ Ge crystals grown by vertical gradient freeze (VGF). For this study, a novel bell-shaped graphite vibrator was developed for the generation of the axial vibrations in the direction of three-phase junction. Melt stirring by downward traveling magnetic field (TMF) was used as a benchmark.The results showed superiority of peripheral vibrations to TMF stirring concerning radial and longitudinal doping distribution and initial stirring rate. Experimentally observed standing free surface waves in Ge were caused by shielding effect of the vibrator on TMF.

Disentangling nonradiative recombination processes in Ge micro-crystals on Si substrates

We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in μm-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission.

Junction-less poly-Ge FinFET and charge-trap NVM fabricated by laser-enabled low thermal budget processes

A doping-free poly-Ge film as channel material was implemented by CVD-deposited nano-crystalline Ge and visible-light laser crystallization, which behaves as a p-type semiconductor, exhibiting holes concentration of 1.8 × 1018 cm−3 and high crystallinity (Raman FWHM ∼ 4.54 cm−1). The fabricated junctionless 7 nm-poly-Ge FinFET performs at an Ion/Ioff ratio over 105 and drain-induced barrier lowering of 168 mV/V. Moreover, the fast programming speed of 100 μs–1 ms and reliable retention can be obtained from the junctionless poly-Ge nonvolatile-memory. Such junctionless poly-Ge devices with low thermal budget are compatible with the conventional CMOS technology and are favorable for 3D sequential-layer integration and flexible electronics.

Closely packed Ge quantum dots in ITO matrix: influence of Ge crystallization on optical and electrical properties

In the present work, a method for the low-temperature production of the material consisting of closely packed Ge QDs embedded in ITO matrix is described. The films are produced by magnetron sputtering deposition followed by thermal annealing. It is shown that the conductivity and optical properties of the films depend on the structure, Ge content in the ITO matrix as well as on the annealing conditions. The conductivity of the films changes up to seven orders of magnitude in dependence on the annealing conditions, and it shows transformation from semiconductor to metallic behavior. The optical properties are also strongly affected by the preparation and annealing conditions, so both conductivity and optical properties can be controllably manipulated. In addition, the crystallization of Ge is found to occur already at 300 °C, which is significantly lower than the crystallization temperature of Ge produced by the same method in silica and alumina matrices.

Lattice bending in three-dimensional Ge microcrystals studied by X-ray nanodiffraction and modelling

Extending the functionality of ubiquitous Si-based microelectronic devices often requires combining materials with different lattice parameters and thermal expansion coefficients. In this paper, scanning X-ray nanodiffraction is used to map the lattice bending produced by thermal strain relaxation in heteroepitaxial Ge microcrystals of various heights grown on high aspect ratio Si pillars. The local crystal lattice tilt and curvature are obtained from experimental three-dimensional reciprocal space maps and compared with diffraction patterns simulated by means of the finite element method. The simulations are in good agreement with the experimental data for various positions of the focused X-ray beam inside a Ge microcrystal. Both experiment and simulations reveal that the crystal lattice bending induced by thermal strain relaxation vanishes with increasing Ge crystal height.

From plastic to elastic stress relaxation in highly mismatched SiGe/Si heterostructures

We present a detailed experimental and theoretical analysis of the epitaxial stress relaxation process in micro-structured compositionally graded alloys. We focus on the pivotal SiGe/Si(001) system employing patterned Si substrates at the micrometre-size scale to address the distribution of threading and misfit dislocations within the heterostructures. SiGe alloys with linearly increasing Ge content were deposited by low energy plasma enhanced chemical vapour deposition resulting in isolated, tens of micrometre tall 3D crystals. We demonstrate that complete elastic relaxation is achieved by appropriate choice of the Ge compositional grading rate and Si pillar width. We investigate the nature and distribution of dislocations along the [001] growth direction in SiGe crystals by transmission electron microscopy, chemical defect etching and etch pit counting. We show that for 3 μm wide Si pillars and a Ge grading rate of 1.5% μm−1, only misfit dislocations are present while their fraction is reduced for higher Ge grading rates and larger structures due to dislocation interactions. The experimental results are interpreted with the help of theoretical calculations based on linear elasticity theory describing the competition between purely elastic and plastic stress relaxation with increasing crystal width and Ge compositional grading rate.

Phase formation and texture of thin nickel germanides on Ge(001) and Ge(111)

We studied the solid-phase reaction between a thin Ni film and a single crystal Ge(001) or Ge(111) substrate during a ramp anneal. The phase formation sequence was determined using in situ X-ray diffraction and in situ Rutherford backscattering spectrometry (RBS), while the nature and the texture of the phases were studied using X-ray pole figures and transmission electron microscopy. The phase sequence is characterized by the formation of a single transient phase before NiGe forms as the final and stable phase. X-ray pole figures were used to unambiguously identify the transient phase as the ϵ-phase, a non-stoichiometric Ni-rich germanide with a hexagonal crystal structure that can exist for Ge concentrations between 34% and 48% and which forms with a different epitaxial texture on both substrate orientations. The complementary information gained from both RBS and X-ray pole figure measurements revealed a simultaneous growth of both the ϵ-phase and NiGe over a small temperature window on both substrate orientations.

Growing Single Crystals with a Low Melt Height and an Axial Vibration

Single crystal growth of antimony-doped germanium is investigated by the vertical Bridgman (VB), axial heat processing (AHP), and axial vibrational control (AVC) methods. In the VB method, the existence of a radial temperature gradient in the melt leads to an inhomogeneous solute distribution which affects the quality of the grown crystal. However, in the AHP growth, the melt height above the interface and the radial temperature gradient in the melt can be reduced by immersing a baffle with a high thermal conductivity. Consequently, the radial solute segregation can be reduced. In order to provide even a better solute homogeneity, the immersed baffle can be vibrated axially which forms the AVC growth. Seven Sb-doped Ge crystals are grown with the three mentioned methods in order to investigate the effect of the melt height, the pulling velocity, and amplitude and frequency of the vibration on the homogeneity of the solute distribution, the achieved single crystal length, the morphological stability, and t...

Positronium production in cryogenic environments

We report measurements of positronium (Ps) formation following positron irradiation of mesoporous ${\mathrm{SiO}}_{2}$ films and Ge(100) single crystals at temperatures ranging from 12\char21{}700 K. As both of these materials generate Ps atoms via nonthermal processes, they are able to function as positron-positronium converters at cryogenic temperatures. Our data show that such Ps formation is possibly provided the targets are not compromised by adsorption of residual gas. In the case of ${\mathrm{SiO}}_{2}$ films, we observe a strong reduction in the Ps formation efficiency following irradiation with UV laser light ($\ensuremath{\lambda}=243.01$ nm) below 250 K, in accordance with previous observations of radiation-induced surface paramagnetic centers. Conversely, Ps emission from Ge is enhanced by irradiation with visible laser light ($\ensuremath{\lambda}=532$ nm) via a photoemission process that persists at cryogenic temperatures. Both mesoporous ${\mathrm{SiO}}_{2}$ films and Ge crystals were found to produce Ps efficiently in cryogenic environments. Accordingly, these materials are likely to prove useful in several areas of research, including Ps mediated antihydrogen formation conducted in the cold bore of a superconducting magnet, the production of Rydberg Ps for experiments in which the effects of black-body radiation must be minimized, and the utilization of mesoporous structures that have been modified to produce cold Ps atoms.

Nanosecond pulsed laser ablation of Ge investigated by employing photoacoustic deflection technique and SEM analysis

Nanosecond pulsed laser ablation phenomena of single crystal Ge (100) has been investigated by employing photoacoustic deflection as well as SEM analysis techniques. Nd: YAG laser (1064nm, 10ns, 1–10Hz) at various laser fluences ranging from 0.2 to 11Jcm−2 is employed as pump beam to ablate Ge targets. In order to evaluate in-situe ablation threshold fluence of Ge by photoacoustic deflection technique, Continuous Wave (CW) He–Ne laser (632nm, power 10mW) is employed as a probe beam. It travels parallel to the target surface at a distance of 3mm and after passing through Ge plasma it causes deflection due to density gradient of acoustic waves. The deflected signal is detected by photodiode and is recorded by oscilloscope. The threshold fluence of Ge, the velocity of ablated species and the amplitude of the deflected signal are evaluated. The threshold fluence of Ge comes out to be 0.5Jcm−2 and is comparable with the analytical value. In order to compare the estimated value of threshold with ex-situe measurements, the quantitative analysis of laser irradiated Ge is performed by using SEM analysis. For this purpose Ge is exposed to single and multiple shots of 5, 10, 50 and 100 at various laser fluences ranging from 0.2 to 11Jcm−2. The threshold fluence for single and multiple shots as well as incubation coefficients are evaluated. It is observed that the value of incubation co-efficient decreases with increasing number of pulses and is therefore responsible for lowering the threshold fluence of Ge. SEM analysis also reveals the growth of various features such as porous structures, non-uniform ripples and blisters on the laser irradiated Ge. It is observed that both the fluence as well as number of laser shots plays a significant role for the growth of these structures.

Enhancement of Au-induced lateral crystallization in electron-irradiated amorphous Ge on SiO2

We have investigated the acceleration energy (0.5–2.0 MeV)-modulated electron irradiation effect of Au-induced lateral crystallization for amorphous Ge on SiO2. As a result, low-energy electron (≤1.0 MeV) irradiation was not effective for Au-induced lateral crystallization. On the other hand, when high-energy electron (2.0 MeV) irradiation was utilized, the lateral growth velocity was significantly enhanced (∼1.8 times). We have confirmed that the Au-induced lateral growth is enhanced by electron irradiation, which is due to the introduction of point defects into amorphous Ge, allowing the easy diffusion of Au atoms.

Modeling the concentration profiles of aluminum and indium impurities in crystals of germanium–silicon solid solutions

A one-dimensional problem of the axial distribution of Al and In impurities in uniform crystals of Ge–Si solid solutions grown by double feeding of the melt method has been solved in the Pfann approximation. The mathematical modeling results suggest that the impurity concentration profile in Ge–Si crystals of constant composition can be varied in a wide range through appropriate changes in the relationship between the crystal growth rate and the rates of feeding the melt using germanium and silicon rods. We demonstrate the possibility of growing completely homogeneous Ge–Si kAl,Inl crystals, in terms of both the concentrations of their major components and the impurity concentration profile.