Amorphous Ge Sb Research Articles

Optical study of Ge (1− x) Sb x crystallization

Optical constants of amorphous and crystalline Ge (1− x) Sb x films ( x=0.68, 0.73 and 0.84), deposited by sputtering, were calculated from reflectance and transmittance spectra. They have been also calculated during the crystallization process induced by thermal annealing for a wide range of wavelengths. Crystallization induces a semiconductor-to-metal transition. The optical gap, located in the near infrared region for amorphous films, decreases progressively to the absorption edge for Sb interband transitions, disappearing with annealing. The eutectic composition ( x=0.84) shows optical properties characteristic of crystalline Sb and may be considered as a border in Ge (1− x) Sb x behaviour system.

Dynamics of Ultrafast Phase Changes in Amorphous GeSb Films

Time resolved imaging has been used to analyze structural transformations induced by intense 100 fs laser pulses in amorphous GeSb films. Above a threshold of 19 mJ/ ${\mathrm{cm}}^{2}$ the data show the formation of a transient nonequilibrium state of the excited material within 300 fs. The results are consistent with an electronically induced, amorphous-to-crystalline phase transition.

Structural analysis and crystallization studies of germanium–antimony– tellurium sputtered films on different underlayers

Crystallization processes and structures were analyzed, by using modified dynamical reflectance spectroscopy, x-ray diffraction measurement, and transmission electron microscopy for a germanium–antimony–tellurium (Ge–Sb–Te) ternary amorphous film on different underlayer materials: a silicon nitride (Si3N4) film and a zinc sulfide–20 mol % silicon dioxide (ZnS–SiO2) film. The crystallization temperature was almost the same (about 430 K) for each sample. Above the crystallization temperature, the Ge–Sb–Te film on the Si3N4 underlayer film exhibited a face-centered-cubic (fcc) structure and grains grew gradually with transformation to a hexagonal structure, whereas the film on the ZnS–SiO2 underlayer film exhibited mixed phases of the fcc and hexagonal structure and abrupt grain growth at 520 K. These crystallization differences are thought to be caused by the interfacial atomic arrangement of amorphous Ge–Sb–Te. This arrangement depends on the chemical affinity at the interface.

Kinetics of grain-boundary reactions at semimetal-semiconductor interfaces observed duringin-situtransmission electron microscope annealing

Abstract The effects of annealing on Ge/Sb/Ge trilayer thin films consisting of amorphous Ge and polycrystalline Sb layers have been studied by in-situ heating in a transmission electron microscope. The results show that reactions are triggered at the grain boundaries (GBs) of the crystalline Sb layer and are accompanied by crystallization of the Ge and diffusion of Sb away from the GBs. The broadening of the reacted region with time t is well fitted by a t 1/2 law, leading to GB diffusion coefficients which are two orders of magnitude higher along the GBs than across them, the value across the GBs being two orders of magnitude higher than the tabulated diffusion coefficient of Sb into crystalline Ge extrapolated to the temperatures used in this work. Finally, the applicability of the Johnson–Mehl–Avrami theory to analyse the reaction kinetics in layered films will be discussed.

Existence of electronic excitation enhanced crystallization in GeSb amorphous thin films upon ultrashort laser pulse irradiation.

The energy density crystallization threshold of amorphous GeSb films has been studied for the first time as a function of the laser pulse duration in the range from 170 fs to 8 ns. The results obtained provide evidence of the existence of enhanced crystallization upon irradiation with pulses shorter than 800 fs, which is most likely related to electronic excitation effects. The study of the interaction of ultrashort laser pulses with solids has raised some important questions concerning the nature of the transformations occurring at the material surface in the presence of very strong levels of electronic excitation. The existence of electronic excitation induced phase transitions has been already demonstrated in several materials (Si [1], GaAs [2‐ 4], graphite [5]). In the case of crystalline semiconductors, excitation with pulses shorter than the longitudinal optical phonon emission time [6] may lead to a structural instability of the lattice, leading to the formation of a metastable transient phase with (semi)metallic character. It is not clear, however, whether the presence of such transient strong electronic excitation and/or transient phases may influence the final state or structure induced since, once the energy of the excited carriers has been transferred to the lattice, the structural transformation path is thermal in nature. Different nonthermal mechanisms which could lead to enhanced crystallization and to crystal damage recovery upon pulsed laser irradiation were proposed during the early 1980s [7,8]. More recently, both photonic effects and electron-hole plasma induced network softening have been invoked to explain structural changes induced in amorphous Si and Ge upon ns and ps laser pulse irradiation [9 ‐ 11]. High Sb-content GeSb amorphous and crystalline thin films have been shown to both crystallize and amorphize upon irradiation with subnanosecond laser pulses. Both processes show optical contrast [12] making this material a potential candidate for applications in ultrashort laser pulse driven phase change optical recording [13,14]. These characteristics also make this material interesting for the study of possible high electronic excitation induced and enhanced crystallization processes. The aim of this work is to elucidate whether the high electronic excitation, induced by irradiation with subnanosecond laser pulses, can induce or enhance the crystallization process of the amorphous phase. Such an enhancement process should lead to appreciable changes in the minimum energy density required for crystallization as the laser pulse duration changes, according to the differences in the excited carrier populations induced. We have performed what we believe to be the first experiment to determine the minimum energy density required for crystallization of an amorphous phase upon irradiation with laser pulses whose durations extend over 3 orders of magnitude, covering the range 170 fs to 8 ns. The samples used in the experiment were 50 nm thick, Ge 0.07Sb 0.93 films grown at room temperature in a multitarget magnetron dc sputtering system from Ge and Sb (99.999% purity) targets onto glass, carbon-coated mica, and Si(100) substrates. In order to improve the sensitivity of the material and to enhance the crystallization rate upon pulsed laser irradiation, two different kinds of samples were grown: (a) fully amorphous layers with the composition Ge 0.07Sb 0.93 and (2) bilayered films formed by an amorphous surface layer of similar composition and an underlayer slightly richer in Sb and therefore polycrystalline. Since the bilayered films showed smaller transformation times and improved sensitivity upon pulsed laser irradiation with 10 ps laser pulses at 583 nm, they were chosen for the present work. The samples on mica and Si were used, respectively, to prepare planar and cross-section transmission electron microscopy (TEM) specimens in order to characterize film structure and composition. The latter was determined by energy dispersive x-ray spectroscopy on planar TEM specimens. An ultrashort CPA laser pulse source (described in Ref. [15]) was used to irradiate the samples on glass. The experimental configuration is shown in Fig. 1. A commercial fs Ti:Al 2O 3 oscillator providing 150 fs duration pulses at a repetition rate of 82 MHz at 830 nm was used to seed an Ar 1 ion pumped Cr-LiSAF solid state regenerative amplifier. Before seeding the amplifier, the oscillator pulses were stretched to approximately 300 ps with a diffraction grating pulse stretcher. After amplification the pulses were recompressed with a grating pair compressor. The typical output of the laser system consisted of a 10 kHz repetition rate pulse train with energies per pulse in the order of 5 mJ. A second Pockels cell, synchronized to the one in the regenerative amplifier, was used

Investigation of Electronic Excitation Crystallization Enhancement in Amorphous Gesb Films Upon Ultrashort Laser Pulse Irradiation

ABSTRACTPulsed laser induced crystallization of amorphous GeSb films has been studied as a function of the laser pulse duration. The energy density crystallization threshold has been determined for pulses in the range from 400 fs to 8 ns. The threshold is observed to increase substantially for pulse durations in the ns range due to the existence of heat flow through the substrate while the pulse is still being absorbed. The energy density crystallization threshold remains constant within the experimental resolution as the pulse duration is decreased down to 800 fs. For the shortest pulse length used, (400 fs), a decrease in the threshold is observed, suggesting the possible existence of electronic excitation enhanced crystallization.

Structural ripple formation in Ge/Sb multilayers induced by laser irradiation

Thin multilayer films (Ge/Sb/Ge/Sb/Si substrate) have been irradiated with single nanosecond laser pulses (λ=193 nm). Real-Time Reflectivity (RTR) measurements have been used to follow the transformation in situ and cross-sectional transmission electron microscope analysis was used to study both the microstructure and the composition profile before and after irradiation. Melting and mixing are both found to nucleate at preferential sites in the upper Ge/Sb interface. During this process the film surface topography changes in a way not previously seen, and rippling of the film is observed due to lateral mass flow induced in the Sb layer underneath the surface, most probably arising from volume changes upon melting. For the highest irradiation energy densities, melting of the whole multilayer configuration takes place, the ripples are no longer observed, and following cooling and solidification, a mixed amorphous GeSb film is formed.

Laser induced interface reactions in Sb/Ge multilayer thin films: a study by RBS and CS-TEM

Abstract Thin films consisting of alternate layers of Ge and Sb are laser irradiated by single nanosecond pulses from an excimer laser. RBS and CS-TEM are used to characterize the induced transformations upon irradiation. It is shown that the interdiffusion process occurs within the liquid phase and causes, by reactions at the first interface, the growth of a buried amorphous GeSb layer with a nonconstant stoichiometry. The high cooling rates (2 × 10 10 K s −1 ) and liquid/solid interface velocities (0.8 ms −1 ) achieved during solidification, together with the absence of a crystalline nucleus at the interface to initiate crystallization, explain this behaviour.

Metastable phase formation and structural change characteristics of vapor deposited semiconductor films

AbstractReported here are studies about the thermal and laser induced metastable phase formation in amorphous GeSb2Te4 thin films prepared by RF‐magnetron sputtering. The general structural properties of this most promising optical phase change recording material are discussed from the point of view of fast structural phase transformation.

Simultaneous crystallization of both elements in amorphous GeSb and GeAl eutectic alloys

Electron microscopic investigations have been carried out on the early stages of crystallization of amorphous eutectic GeSb and GeAl alloys with no intermediate compound. The GeSb and GeAl films, prepared by vacuum coevaporation on the substrates at room temperature, are amorphous for Ge concentrations larger than 0.1 and 0.5, respectively. The crystallization of these films are characterized by simultaneous formation of both constituents in their crystalline forms, adjacent each other with specific orientation relationships: (246)Ge  (003)Sb and [03 2] Ge  [110]Sb for GeSb, and (111)Ge  (224)Al and [1 21] Ge  [1 10] Al for GeAl. The crystallization temperature decreases monotoneously with increasing Sb or Al content, and is well fitted to 2 3 values of the liquids temperature in Kelvin in the case of GeSb.

Selective Etching and Photo—Bleaching in Thin Amorphous Ge—Sb—S Films

ABSTRACTA mechanism is proposed for the selective etching effects in thin amorphous Ge—Sb—S films based on the combined action of photostructural changes and alkaline solvent with a surface active substance. Evidence is given in support of the assertion thatthe same photostructural changes (namely, the breaking of Ge—Sb bonds under the action of UV light) are responsible for the irreversible photo—bleaching of these layers. Four different compositions in the system Ge—Sb—S have been studied under different preparation conditions (thermal evaporation and laser—beam sputtering).Photostructural changes were induced by UV light irradiation. Infrared spectra of the layers are presented in support of the proposed model.

A.c. conduction in the amorphous Ge—Sb—Se system

Abstract A.c. and d.c. conductivities of amorphous films of Ge—Sb–Se have been investigated for different Sb: Se ratios, keeping the Ge content constant at 20 at. %. The a.c. conductivity [sgrave](ω, T) in the frequency range 0•5-10•0 kHz is found to obey the law [sgrave](ω, T) = Aω3. The exponent s is found to decrease with increasing temperature and Sb content, which is inconsistent with the quantum-mechanical tunnelling model. A strong temperature dependence of the a.c. conductivity [sgrave](ω, T) and exponent s in the entire range of temperatures and frequencies is reasonably interpreted by the correlated barrier hopping model, taking into account the contribution of both single polarons and bipolarons. The values of the energy of bound states, the correlation energy and the Fermi energy are estimated from the best fit between theoretical and experimental results. It is observed that the correlation energy increases while the energy of bound states decreases with increasing Sb content. The decrease in...

Thickness dependence of DC conductivity of amorphous Se and binary amorphous SeTe, SeGe, and SeSb films

The dc conductivity of a-Se and binary amorphous SeTe, SeGe, and SeSb films is measured as a function of temperature (80 to 300 K) and film thickness (500 to 2000 A). It is found that except for SeSb films, the conduction in all the samples takes place in the tails of localized states via thermally activated tunneling in the entire range of temperature (80 to 300 K). For SeSb films, in the high temperature region (200 to 300 K) the conduction mechanism remains the same while at lower temperatures (80 to 200 K), the conduction is due to variable range hopping in the localized states near the Fermi level. An increase in the film thickness for the SeSb system is found to result in the decrease of the activation energy while a reverse effect is observed for the conductivity. Es wird die Gleichstromleitfahigkeit von a-Se und binaren amorphen SeTe-, SeGe- und SeSb-Schichten als Funktion der Temperatur (80 bis 300 K) und Schichtdicke (500 bis 2000 A) gemessen. Es wird gefunden, das auser fur SeSb-Schichten, die Leitfahigkeit in allen Proben im gesamten Temperaturbereich (80 bis 300 K) in den Auslaufern der lokalisierten Zustande uber einen ther-misch aktivierten Tunnelmechanismus stattfindet. Fur SeSb-Schichten bleibt im Hochtempera-turbereich (200 bis 300 K) der Leitfahigkeitsmechanismus derselbe, wahrend bei tieferen Tem-peraturen (80 bis 200 K) die Leitfahigkeit durch Hopping mit veranderlicher Reichweite in den lokalisierten Zustilnden in der Nahe des Ferminiveaus erfolgt. Es wird gefunden, das das Anwach-sen der Schichtdicke fur das SeSb-System zu einer Verringerung der Aktivierungsenergie fuhrt, wiihrend ein umgekehrter Effekt fur die Leitfiihigkeit beobachtet wird.