Germanium Thin Films Research Articles

Low-temperature metal-induced crystallization of hydrogenated amorphous Si1-xGex (0.25≤x≤1) thin films with Au solution

Low-temperature (∼ 400 °C) metal-induced crystallization of hydrogenated amorphous silicon–germanium thin films using Au solution has been investigated by X-ray diffraction, Raman spectra, scanning electron microscopy and atomic force microscopy. It was shown that Au solution significantly promotes the crystallization of the films at low temperatures. The effects of annealing temperature, Ge fraction in the films and the concentration of Au solution on the structure and morphology of the films were analyzed. An increase in crystallinity was observed on increasing the annealing temperature and concentration of Au solution. High-frequency shifts of Ge-Ge and Si-Ge modes and a low-frequency shift of the Si-Si mode were found on increasing the Ge fraction. Compared with Au-induced crystallization, the Au solution induced crystallization tends to give large crystal grains.

RETRACTED ARTICLE: Hydrogenated nanocrystalline silicon germanium thin films

Hydrogenated nanocrystalline silicon germanium thin films (nc-SiGe:H) is an interesting alternative material to replace hydrogenated nanocrystalline silicon (nc-Si:H) as the narrow bandgap absorber in an a-Si/a-SiGe/nc-SiGe(nc-Si) triple-junction solar cell due to its higher optical absorption in the wavelength range of interest. In this paper, we present results of optical, structural investigations and electrical characterization of nc-SiGe:H thin films made by hot-wire chemical vapor deposition (HW-CVD) with a coil-shaped tungsten filament and with a disilane/germane/hydrogen gas mixture. The optical band gaps of a-SiGe:H and nc-SiGe:H thin-films, which are deposited with the same disilane/germane/hydrogen gas mixture ratio of 3.4 : 1.7 : 7, are about 1.58 eV and 2.1 eV, respectively. The nc-SiGe:H thin film exhibits a larger optical absorption coefficient of about 2–4 in the 600–900 nm range when compared to nc-Si:H thin film. Therefore, a thinner nc-SiGe:H layer of ∼500 nm thickness may be sufficient for the narrow bandgap absorber in an a-Si based multiple-junction solar cell. We enhanced the transport properties as measured by the photoconductivity frequency mixing technique. These improved alloys do not necessarily show an improvement in the degree of structural heterogeneity on the nanometer scale as measured by smallangle X-ray scattering. Decreasing both the filament temperature and substrate temperature produced a film with relatively low structural heterogeneity while photoluminescence showed an order of magnitude increase in defect density for a similar change in the process.

On the Two to Three Dimensional Growth Transition in Strained Silicon Germanium Thin Films

On the Two to Three Dimensional Growth Transition in Strained Silicon Germanium Thin Films

Low-temperature Al-induced crystallization of hydrogenated amorphous Si 1− xGe x (0.2 ≤ x ≤ 1) thin films

Low-temperature Al-induced crystallization of hydrogenated amorphous silicon–germanium thin films has been investigated by X-ray diffraction, Raman spectra and scanning electron microscopy measurements. It was shown that the Al-induced layer exchange significantly promotes the crystallization of the films. The influence of the annealing temperature and the Ge fraction on X-ray diffraction patterns and Raman spectra was analyzed. The increase in Raman peak intensity was observed with the increase of the annealing temperature, and the high-frequency shifts of Ge–Ge and Si–Ge peaks were found with the increase of the Ge fraction. There is an enhancement in film crystallinity and grain size with the increase of the Ge fraction and annealing temperature.

Silicon-based thin films as bottom electrodes in chalcogenide nonvolatile memories

The effect of the electrical resistivity of a silicon–germanium (SiGe) thin film on the phase transition in a GeSbTe (GST) chalcogenide alloy and the manufacturing aspect of the fabrication process of a chalcogenide memory device employing the SiGe film as bottom electrodes were investigated. While p-type SiGe bottom electrodes were formed using in situ doping techniques, n-type ones could be made in a different manner where phosphorus atoms diffused from highly doped silicon underlayers to undoped SiGe films. The p–n heterojunction did not form between the p-type GST and n-type SiGe layers, and the semiconduction type of the SiGe alloys did not influence the memory device switching. It was confirmed that an optimum resistivity value existed for memory operation in spite of proportionality of Joule heating to electrical resistivity. The very high resistivity of the SiGe film had no effect on the reduction of reset current, which might result from the resistance decrease of the SiGe alloy at high temperatures.

Phase transformation and optical characteristics of porous germanium thin film

In this study, we proposed a method to prepare GeO 2 by treating porous Ge thin film with thermal annealing in O 2 ambient. After annealing, the morphological transformation from porous thin film to an island structure was observed. The crystallization and composition of the porous Ge thin film prepared using different annealing time in O 2 ambient were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectra. Initial Ge composition was gradually oxidized to GeO 2 with increasing annealing time. Comparing the photoluminescence (PL) results between Ge and GeO 2, it was found that the visible photoluminescence originated from the germanium oxide. Photoluminescence measurements obtained at different temperatures exhibited a maximum integrated PL intensity at around 200 K. A possible explanation for this behavior might be the competition between radiative recombination and nonradiative hopping process.

Electron-stimulated desorption of sodium atoms from germanium-coated tungsten

The yield of sodium atoms and energy distribution upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a germanium thin film are measured under variations in the electron energy, the sodium coverage, and the surface temperature by the time-of-flight method with the use of a surface ionization detector. It is revealed that the electron-stimulated desorption of sodium atoms occurs via three channels, namely, a channel involving ionization of adsorbed sodium; the most efficient channel, which is produced by the germanium ionization; and a channel associated with the formation of tungsten excitons, which brings about desorption of NaGe molecules.

Changes in surface topography of amorphous silicon germanium films after light soaking

Light-induced metastable degradation of hydrogenated amorphous silicon and silicon germanium thin films (a-SiGe:H) is conjectured to be accompanied by structural changes but there has not been a direct measurement of the same. We measure the surface topography of these films in the annealed and the light soaked state using atomic force microscopy. We quantified the surface topography in terms of surface roughness and find that the surface roughness increases after light soaking. Our results provide direct evidence of the light-induced structural changes in these films.

Diffraction of strongly convergent X-rays from picosecond acoustic transients

Following femtosecond laser excitation, large transient changes in the X-raydiffraction efficiency have been observed in thin crystalline films of germanium. This behavior is explained in terms of the thermo-acoustic response of the impulsively heated material, when an inhomogeneously strained surface layer is probed by divergent X-ray beams.

Study of a Fabrication Process and Characterization of One Dimensional Array of Un-cooled Micro-bolometers Based on Germanium Films Deposited by Plasma

AbstractIn our previous works we have studied the fabrication process and characterization of single cell micro-bolometers based on germanium thin films deposited by low frequency (LF) PECVD technique at low temperature and fully compatible with the IC fabrication technology. We have demonstrated promising properties of those devices for further development of IR imaging systems.In this work we report the study of a fabrication process and characterization of one dimensional array of 32 un-cooled micro-bolometers. We have used surface micro-machining techniques for the array fabrication onto a silicon wafer. The micro-bolometers in the array have a “bridge type” configuration, in which a SiNx supporting film is suspended 2.5 μm from the substrate by two legs forming the bridge in order to provide sufficient thermo-isolation for thermo-sensing layer, which is deposited by LF PECVD over it. The a-GexSiy:H film used in this devices showed high activation energy Ea= 0.34 eV, providing high thermal coefficient of resistance, TCR=α=0.043 K-1 and improved but still high resistance. We studied the effect of include boron in the a-GexSiy:H film deposition process in order to reduce its undesirable high resistance and we used this layer (a-GexBySiz:H) as thermo-sensing film in the micro-bolometers arrays also. The active area of the cells in the array is Ab=70×66 μm2 and the area of the array including interconnection lines and pads is A=1600×3120 μm2. The temperature dependence of conductivity σ(T), current-voltage characteristics I(U) and spectral noise density have been measured in the micro-bolometers in the array in order to characterize and compare their performance characteristics, such as responsivity and detectivity.

Characterization of high refractive index semiconductor films by surface plasmon resonance

Si-based surface plasmon resonance (SPR) in the Kretschmann-Raether geometry is considered as a platform for the optical measurement of high refractive index films. The implementation of the SPR effect becomes possible due to the relatively high index of refraction of Si compared to most materials. As examples we study the SPR responses for some important semiconductor-based films, including laser-ablated porous silicon and thin germanium films. Using SPR data, we determine the refractive indices of these films for different parameters (thickness and porosity) and ambiences. We also discuss novel SPR biosensor architectures with the use of these solid films.

Bismuth doping of hydrogenated amorphous germanium thin films

The optoelectronic properties of Bi-doped hydrogenated amorphous germanium (a-Ge:H), with relative impurity concentrations [Nimp/NGe] ranging between 8×10−6 and 5.5×10−3, are reported. The incorporation of Bi produces small changes in the dark conductivity of a-Ge:H. For a three orders of magnitude change in impurity concentration the room-temperature conductivity changes by just one order of magnitude. Within this doping range no, or small, changes were measured in the values of the pseudo-gap, the Urbach energy, and the hydrogen content. The Fermi level always remains far from the conduction band edge, shifting by only 0.1 eV for the sample with the largest doping concentration. The main conclusion is that Bi is a very inefficient active donor in a-Ge:H. The likely reasons for such behavior are discussed.

Simulation of Interfacial Phonon Transport in Si–Ge Heterostructures Using an Atomistic Green’s Function Method

An atomistic Green’s function method is developed to simulate phonon transport across a strained germanium (or silicon) thin film between two semi-infinite silicon (or germanium) contacts. A plane-wave formulation is employed to handle the translational symmetry in directions parallel to the interfaces. The phonon transmission function and thermal conductance across the thin film are evaluated for various atomic configurations. The contributions from lattice straining and material heterogeneity are evaluated separately, and their relative magnitudes are characterized. The dependence of thermal conductance on film thickness is also calculated, verifying that the thermal conductance reaches an asymptotic value for very thick films. The thermal boundary resistance of a single Si∕Ge interface is computed and agrees well with analytical model predictions. Multiple-interface effects on thermal resistance are investigated, and the results indicate that the first few interfaces have the most significant effect on the overall thermal resistance.

Influences of GeF4 on poly-SiGe films prepared by reactive thermal CVD

Polycrystalline silicon–germanium (Poly-SiGe) thin films are prepared by reactive thermal chemical vapor deposition (RTCVD) at substrate temperature of 450°C using Si2H6 and GeF4 as source gases diluted in Helium. The influences of GeF4 on the properties of poly-SiGe films have been investigated by varying the flow rates of GeF4. The results show that GeF4 plays a very important role in poly-SiGe growth. Introduction of a small flow rate of GeF4 can effectively promotes the nuclei formation on the substrate and decreases the incubation time. Ge content increases with GeF4 flow rate and high Ge content results in an increase of the film surface roughness. It is also found that the film conductivity increases and activation energy decreases with the increase of Ge content.

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25–4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.

Successive segregation in laser-crystallized poly-SiGe thin films

Polycrystalline silicon–germanium (poly-SiGe) thin films on glass substrates, crystallized with multiple number of laser pulses, were characterized with energy dispersive X-ray and Raman spectroscopy. Strong lateral segregation was observed for laser-crystallized poly-Si1−xGex with 0.33<x<0.7, causing the local Ge content to differ by as much as 40% from the average value. The segregation manifests itself in the appearance of well-resolved peaks in the Raman phonon modes. It is shown that the segregation in the samples evolves successively with increasing number of laser pulses.

Dielectric function and electric properties of germanium thin films prepared by gold mediated crystallization

Crystallization of hydrogenated amorphous germanium (a-Ge:H) thin films deposited by plasma enhanced chemical vapor deposition using the GeH4 and H2 precursors has been investigated. A comparative analysis of the kinetics of the thermal crystallization by annealing to 650°C and of the gold-mediated crystallization (Au-MMC) is carried out. The impact of the Au-MMC on the microcrystalline Ge film microstructure and electrical properties is discussed. The Au thin layer results in a more dense and ordered structure with lower roughness of the microcrystalline Ge films. In order to describe the Ge crystallization kinetics, the dielectric functions of a-Ge:H and microcrystalline germanium μc-Ge have also been determined by spectroscopic ellipsometry in the range of 0.75−6.0eV and parametrized using the Tauc-Lorentz dispersion equation.

Resonances in the cesium atom yield in electron-stimulated desorption from tungsten coated with a germanium monolayer

The yield and energy distributions of Cs atoms emerging from cesium layers, which are adsorbed on tungsten coated with a thin germanium film (1-to 2-monolayers thick), have been measured as a function of the incident electron energy, the amount of adsorbed cesium, and the substrate temperature. The measurements were performed by the time-of-flight technique with a surface ionization detector. At low cesium coverages (Θ 0.14. For Θ ≥ 0.15, the cesium atom appearance threshold shifts to ∼30 eV, which corresponds to the Ge 3d-level ionization energy and the plateau is replaced by a resonance peak at ∼38 eV, which can be identified with the ionization energy of the W 5p3/2 level. This peak is observed only for Θ < 0.3 and T = 160 K. For Θ ≥ 0.3, there appears a resonance peak at ∼50 eV, and for Θ ≥ 0.5, another resonance peak appears at ∼80 eV. These peak positions correlate with the ionization energies of the W 5p1/2 and W 5s levels, and their intensity is maximum at Θ = 1. The Cs atom energy distributions for Θ < 0.15 consist of a bell-shaped peak with a maximum at ∼0.55 eV, and those for Θ ≥ 0.15 contain two nearly resolved maxima, a broad one peaking at ∼0.5 eV and a narrow one at ∼0.35 eV. The above results argue for the existence of three channels of Cs atom desorption. One channel involves reverse motion of the Cs2+ ion; another channel, neutralization of the adsorbed Cs+ ion following the Auger decay of a vacancy in the Ge atom; and the third channel involves desorption of a CsGe molecule as it is repelled from a W core exciton.

Characterization of KrF excimer laser annealed PECVD Si xGe 1− x for MEMS post-processing

This work studies the possibility to treat plasma enhanced chemical vapor deposited (PECVD) silicon germanium (Si 1− x Ge x ) thin films grown at 400 °C or lower with a pulsed excimer laser for obtaining good MEMS structural layers. The main advantage of using PECVD is that a high growth rate (∼35 nm/min) can be achieved at low temperatures (≤370 °C). It is demonstrated that optimizing the pulse fluence, number and rate yields high quality films characterized by a low defect density (∼10 2 defect/cm 2), large grains (∼300 nm), a low mean stress and a low stress gradient. Furthermore, the electrical resistivity of the as grown material, deposited at 210 °C, is reduced from 12 kΩ cm to 1.3 mΩ cm after laser annealing.

Polycrystalline Si1-xGex thin film deposition by rapid thermal chemical vapor deposition

Polycrystalline silicon germanium (poly-Si1−xGe x ) thin films on a-Si film have been deposited by rapid thermal chemical vapor deposition (RTCVD) with SiH4–GeH4–H2. Effect of GeH4/SiH4 and deposition temperature on stoichiometry (x), Si-Ge binding character, composition, hydrogen configuration, crystallinity, preferred orientation, grain size, and surface roughness of poly-Si1−xGe x films has been investigated. Poly-Si1−xGe x deposited on the substrate with amorphous silicon buffer layer on oxide shows better crystallinity and contains the less amount of oxygen than the one deposited directly on the oxide surface. At low temperature region, the Ge–H bond with the small amount of Si–H2 bond is dominant but all hydrogen bonds are desorbed at high temperature. All films have polycrystalline phase and the grain size and (111) orientation increased with increasing deposition temperature in which Ge content also increases at the fixed gas flow rate of GeH4 to total source gas. Poly-Si1−xGe x /Si thin film transistors (TFT) are fabricated and hydrogen during post-hydrogenation process preferentially is attached to Ge dangling bond and the TFT characteristics could be improved.