Crystallization Of Amorphous Germanium Research Articles

Crystallization of Ge in ion-irradiated amorphous-Ge/Au thin films

Herein, the structural, optical, and electrical properties of Au-induced crystallization in amorphous germanium (a-Ge) thin films are presented for future solar energy material applications.

A novel liquid-mediated nucleation mechanism for explosive crystallization in amorphous germanium

We report a novel mechanism for explosive crystallization in amorphous germanium (a-Ge), which operates through liquid-mediated nucleation occurring under extreme thermal gradient conditions. The crystallization kinetics of sputter-deposited films with thicknesses ranging from 30 to 150 nm were characterized using in situ movie-mode dynamic transmission electron microscopy (MM-DTEM). After localized heating from a short laser pulse, explosive liquid phase nucleation (LPN) was observed to occur during the early stage (<2 μs) of crystallization in the thicker (>50 nm) films deposited on silicon nitride substrates. The crystallization front propagated at ∼12–15 m/s and produced nanocrystalline microstructure with ∼50 nm grains. A mechanism involving the existence of a relatively thick (>100 nm) transient liquid layer and a high nucleation rate is proposed to explain the behavior. The key thermodynamic and kinetic features as well as the feasibility of the mechanism are further explored by employing parametric and systematic phase-field modeling and simulations.

Effects of gold‐induced crystallization process on the structural and electrical properties of germanium thin films

Gold‐induced (Au‐) crystallization of amorphous germanium (α‐Ge) thin films was investigated by depositing Ge on aluminum‐doped zinc oxide and glass substrates through electron beam evaporation at room temperature. The influence of the postannealing temperatures on the structural properties of the Ge thin films was investigated by employing Raman spectra, X‐ray diffraction, and scanning electron microscopy. The Raman and X‐ray diffraction results indicated that the Au‐induced crystallization of the Ge films yielded crystallization at temperature as low as 300°C for 1 hour. The amount of crystallization fraction and the film quality were improved with increasing the postannealing temperatures. The scanning electron microscopy images show that Au clusters are found on the front surface of the Ge films after the films were annealed at 500°C for 1 hour. This suggests that Au atoms move toward the surface of Ge film during annealing. The effects of annealing temperatures on the electrical conductivity of Ge films were investigated through current‐voltage measurements. The room temperature conductivity was estimated as 0.54 and 0.73 Scm−1 for annealed samples grown on aluminum‐doped zinc oxide and glass substrates, respectively. These findings could be very useful to realize inexpensive Ge‐based electronic and photovoltaic applications.

In situ investigation of explosive crystallization in a-Ge: Experimental determination of the interface response function using dynamic transmission electron microscopy

The crystallization of amorphous semiconductors is a strongly exothermic process. Once initiated the release of latent heat can be sufficient to drive a self-sustaining crystallization front through the material in a manner that has been described as explosive. Here, we perform a quantitative in situ study of explosive crystallization in amorphous germanium using dynamic transmission electron microscopy. Direct observations of the speed of the explosive crystallization front as it evolves along a laser-imprinted temperature gradient are used to experimentally determine the complete interface response function (i.e., the temperature-dependent front propagation speed) for this process, which reaches a peak of 16 m/s. Fitting to the Frenkel-Wilson kinetic law demonstrates that the diffusivity of the material locally/immediately in advance of the explosive crystallization front is inconsistent with those of a liquid phase. This result suggests a modification to the liquid-mediated mechanism commonly used to describe this process that replaces the phase change at the leading amorphous-liquid interface with a change in bonding character (from covalent to metallic) occurring in the hot amorphous material.

Complex crystallization dynamics in amorphous germanium observed with dynamic transmission electron microscopy

Crystallization of amorphous germanium (a-Ge) by laser or electron beam heating is a remarkably complex process that involves several distinct modes of crystal growth and the development of intricate microstructural patterns on the nanosecond to ten microsecond time scales. Here we use dynamic transmission electron microscopy (DTEM) to study the fast, complex crystallization dynamics with 10 nm spatial and 15 ns temporal resolution. We have obtained time-resolved real-space images of nanosecond laser-induced crystallization in a-Ge with unprecedentedly high spatial resolution. Direct visualization of the crystallization front allows for time-resolved snapshots of the initiation and roughening of the dendrites on submicrosecond time scales. This growth is followed by a rapid transition to a ledgelike growth mechanism that produces a layered microstructure on a time scale of several microseconds. This study provides insights into the mechanisms governing this complex crystallization process and is a dramatic demonstration of the power of DTEM for studying time-dependent material processes far from equilibrium.

Low-temperature metal-induced crystallization of Mn-containing amorphous Ge thin films

This work focuses on the crystallization of amorphous germanium (a-Ge) thin films induced by manganese species. A series of Mn-containing a-Ge films ([Mn] ~0−3.7at.% range) was deposited at 150°C by the cosputtering technique. After deposition, all films were submitted to isochronal thermal annealing treatments up to 600°C and analyzed by Raman scattering, optical transmission spectroscopy and electrical resistivity measurements. The experimental results indicate that: (a) Mn impurity lowers the crystallization temperature of a-Ge in ~100°C, as confirmed by the Raman analyses, (b) the optical properties of the films are affected by both the insertion of Mn and the temperature of thermal treatment, with the optical bandgap staying in the range of ~0.7−1eV, and (c) the electrical resistivity of the samples is also influenced by the Mn concentration and by the temperature of annealing, varying between ~1.0×101 and 1.6×104Ω cm. These experimental observations were systematically studied and the possible reasons associated to them are presented and discussed.

Pulsed laser annealing of semiconductor structures for functional devices

AbstractWe review our progress on the laser processing of semiconductor microstructures and nanostructures for functional devices. Fundamental research conducted to understand the melt‐mediated phase transformations induced by nanosecond laser irradiation in thin semiconductor films is presented. A detailed experimental study analyzed the physical mechanisms of the explosive crystallization in amorphous germanium that produces large area self‐sustained crystal growth. The double laser crystallization method that combines a nanosecond laser pulse and a modulated microsecond laser beam was shown to produce ultra‐large grain polycrystalline silicon, enabling fabrication of thin film transistor devices of high performance from amorphous silicon films. The crystal growth process was imaged by temporally resolved photography. Non‐melt Excimer Laser annealing of thin silicon‐on‐insulator films for dopant activation was demonstrated. Applying multiple laser pulses below the melting threshold effected solid phase annealing of the single crystalline silicon films. Semiconductor nanowires are one‐dimensional nanostructures that have displayed the potential to be used with low‐cost flexible plastic substrates for applications such as large‐area displays and sensor arrays. The excimer laser annealing of silicon nanowires is demonstrated as an alternative to conventional thermal annealing for dopant activation. The optical absorption of the nanowires is discussed and the effect of parameters such as fluence and number of pulses is investigated. The interaction of laser pulses with silicon nanowires is investigated through numerical simulations. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Low Temperature Copper-induced Crystallization Technique for Germanium on Flexible PET, by Means of Mechanical Compressive Stress

AbstractLow temperature copper-induced crystallization of amorphous germanium (a-Ge) has been significantly enhanced by applying mechanical compressive stress during thermal post-treatment. Manipulation of this technique, alongside with proper patterning of the a-Ge layer before thermo-mechanical process, has led to growth of device-quality poly-Ge layer on flexible PET substrate at temperatures as low as 130°C. Flexibility of the substrate allows the efficient application of uniaxial compressive stress by bending the PET sheets inward. Effects of compressive stress and ultimate crystallization of the Ge layer has been verified by electrical sheet resistance and Hall mobility measurements, and analyzed by XRD, SEM, TEM and RAMAN spectroscopy.

Aluminium Induced Crystallization of Amorphous Germanium

Crystallization morphologies and kinetic processes have been studied in amorphous germanium (a-Ge) films in contact with aluminium (Al) layers of various thickness. The well-known spherulitic crystallization has been observed in a-Ge/Al/a-Ge trilayers with Al layer thickness above 10 nm. The kinetics of this growth was followed by electrical conductivity measurement of the a-Ge/Al bilayer. In the case of the thinnest Al layer (below 4 nm) fibre-like Ge crystal growth was found. The measurement results suggest that the crystallization of the amorphous Ge layers was controlled by diffusion processes.

Triggering of diffusion and crystallization of amorphous germanium in contact with antimony

Abstract The diffusion and crystallization of amorphous Ge films in contact with Sb has been studied in bilayer Ge–Sb films. Under prolonged annealing in vacuum, the Sb layer sublimes and, similarly to what happens under etching, the reactions which had occurred at the interface are “recorded” at the surface of the residual Ge film. The results show that the bulk crystallization of the Ge layer occurs at 75% of the eutectic temperature (in degrees Kelvin) of the Ge-Sb system although the trigger of preferential diffusion and crystallization at the defects of the Sb grains is observed at lower temperatures. A comparison of the results obtained in a-Ge films grown onto a Sb layer and films covered by an Sb layer show that the interface in the former film configuration is more active than in the latter and leads to a different crystallization mechanism.

Crystallization of Amorphous Germanium in a Silver Germanium Layered System

ABSTRACTWe studied crystallization of amorphous germanium (a-Ge) induced in a Ag/a-Ge layered system, using in situ transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Amorphous Ge was found to crystallize at about 270°C with the heat of reaction of 10±lkJ/mol. In situ TEM revealed that Ag grains migrate into the a-Ge phase leaving the crystalline Ge (c-Ge) phase behind. From this observation, we propose a model whereby the Ag provides the fastest path for the Ge atoms to diffuse from a-Ge to c-Ge phases.

Multiple Twinning in the Solid Phase Crystallization of Amorphous Germanium

Multiple Twinning in the Solid Phase Crystallization of Amorphous Germanium

Effects of Ion Dose on the Morphology and Crystallization of Amorphous Germanium

ABSTRACT1.5 MeV Kr+ irradiation of polycrystalline Ge at room temperature, and subsequent annealing were carried out with in situ TEM observations. After a Kr+ dose of 1.2x1014 ions/cm2, Ge in the electron transparent region was completely amorphized. Continuous irradiation of the amorphized Ge resulted in a high density of small cavities. These cavities, first observed after 7x1014 ions/cm2 with an average diameter of ∼3 nm, grew into large (∼50 nm) irregular-shaped holes, transforming the irradiated Ge into a sponge-like material after 8.5x1015 ions/cm2. The crystallization temperature and the morphology of the crystallized Ge after annealing were found to be dependent on the Kr+ dose. The sponge-like structure was retained after crystallization at ∼600°C.

Amorphous-crystalline interfaces after laser induced explosive crystallization in amorphous germanium

Unsupported thin a-Ge films crystallized by pulsed laser irradiations have been observed in TEM. We show the influence of the initial film structure on the crystallization process and HREM observations provide informations on the structure of amorphous-crystalline interfaces inside or at the laser impact periphery.

Amorphous-crystalline interfaces after laser induced explosive crystallization in amorphous germanium

Amorphous-crystalline interfaces after laser induced explosive crystallization in amorphous germanium

Growth rate of crystallization in amorphous germanium produced by ion implantation a Raman spectroscopy study

Growth rate of crystallization in amorphous germanium produced by ion implantation a Raman spectroscopy study

Electrical conductivity and crystallization in amorphous germanium

A simple model is presented which interprets conductivity variations due to crystallization in amorphous layers and allows the determination of the growth rate of crystallization. The validity of this model is demonstrated in the case of germanium.

Lattice Defects and Crystallization of Amorphous Germanium

In-situ measurements of resistivity during annealing cycles have been made on a-Ge films with thicknesses from 500Å to 6000Å. Dynamics of annealing depends strongly on film thickness, which may be partly associated with the anisotropy of conductivity. Ambient oxygen adsorbed on the film surface seems to have no significant influence on annealing kinetics of defects. Elimination of oxygen adsorption causes the low temperature crystallization near 250°C. This is explained in terms of the stabilization of the surface structure by adsorbed oxygen. It is found that progressive decrease of resistivity at the crystallazation temperature is expressed by the exponential function of the annealing time.

Displacement spike crystallization of amorphous germanium during irradiation

Displacement spike crystallization of amorphous germanium during irradiation