Microcrystalline Germanium Research Articles

Hydrogen-Induced Crystallization of Germanium Films at Low Temperature Using an RF-PECVD Reactor

In this paper we study the effect of H2/GeH4 dilution ratio (R) on the structural and optical properties of hydrogenated microcrystalline Germanium (Ge) embedded in amorphous matrix thin films. The thin films are prepared using standard RF-PECVD process at substrate temperature of 300 ̊ C. The effect of hydrogen dilution ratio on the optical index of refraction and the optical transmission were investigated. It was observed that by incorporating higher hydrogen flow rate in the films with low GeH4 concentration, the optical index of refraction can be tuned over a broad range of wavelengths due to the variation of crystalline properties of the produced films. By varying the hydrogen flow of µc-Ge:H samples, ~71% crystalline fraction was detected at 100sccm of H2.

The microstructure evolution of hydrogenated microcrystalline germanium promoted by power gradient method

This paper studies the microstructure evolution of hydrogenated microcrystalline germanium (μc-Ge:H) thin films deposited by plasma enhanced chemical vapor deposition (PECVD). There is an amorphous incubation layer formed in the initial deposition stage of μc-Ge:H thin film. It is demonstrated that the thickness of incubation layer can be reduced by high hydrogen dilution and high discharge power method. However, at high hydrogen dilution, the deposition rate of μc-Ge:H appears a sharply decrease. Using a high discharge power can compensate the deposition rate decrease but lead to decrease of average grain size and appearance of micro-void in the μc-Ge:H thin film. In addition, by comparing two thickness groups of μc-Ge:H thin films deposited at different discharge powers, it is noticed that the evolution process relates to the formation of crystal nucleuses. Thus, a power gradient method is proposed to understand the mechanism of nucleation and crystal growth in the initial deposition process of μc-Ge:H films. Finally, by power gradient method, the incubation layer thickness of μc-Ge:H thin films has been decreased to less than 6nm. Moreover, Raman scattering spectra shows a 38nm μc-Ge:H film has a crystal fraction (XC) of 62.4%. Meanwhile, the mobility of TFT devices shows the improved electrical property of μc-Ge:H film deposited by power gradient method.

Properties of boron-doped μc-Ge:H films deposited by hot-wire CVD

Boron-doped hydrogenated microcrystalline Germanium (μc-Ge:H) films were deposited by hot-wire CVD. H2 diluted GeH4 and B2H6 were used as precursors and the substrate temperature was kept at 300 °C. The properties of the samples were analyzed by XRD, Raman spectroscopy, Fourier transform infrared spectrometer and Hall Effect measurement with Van der Pauw method. It is found that the films are partially crystallized, with crystalline fractions larger than 45% and grain sizes smaller than 50 nm. The B-doping can enhance the crystallization but reduce the grain sizes, and also enhance the preferential growth of Ge (220). The conductivity of the films increases and tends to be saturated with increasing diborane-to-germane ratio \(R_{B_2 H_6 } \). All the Hall mobilities of the samples are larger than 3.8 cm2·V−1·s−1. A high conductivity of 41.3 Ω−1·cm−1 is gained at \(R_{B_2 H_6 } \)=6.7%.

Correlation between optical emission spectroscopy of hydrogen/germane plasma and the Raman crystallinity factor of germanium layers

Optical emission measurements were recorded during microcrystalline germanium layer growth on glass with plasma enhanced chemical vapor deposition. A significant difference for the intensities of SiH and GeH could be identified in the optical emission spectra of hydrogen/silane (H2/SiH4) and hydrogen/germane (H2/GeH4) plasma. In H2/SiH4 plasma, Si and SiH are present, whereas Ge but no GeH could be detected in H2/GeH4 plasma. The specific Raman crystallinity factor (ϕc) was evaluated for the layers after deposition. In H2/GeH4 plasma, the ratio of optical emission intensities of Hα (I(Hα), λ = 656.28 nm) and Ge (I(Ge), λ = 303.90 nm) is proportional to ϕc,Ge.

Evaluation of charge trapping properties of microcrystalline germanium thin films by Kelvin force microscopy

We have prepared highly-crystallized germanium (Ge) films on quartz and evaluated their local charge trapping and electrical conduction properties from topographic and surface potential images simultaneously taken by a conductive atomic force microscopy (AFM) during and after current application to Ge films. By applying a bias of 10V at which the current of ~8mA flows between the co-planer electrodes on Ge films, the surface potential image which was uniform before bias application shows in-plane inhomogeneity within ~1.0mV commensurate with the surface morphology. Such potential images remained inhomogeneous at zero bias for more than two hours after bias application. The inhomogeneous potential images can be interpreted in terms of the difference in electron concentration in highly-crystallized Ge films presumably caused by electron charging in the grain boundaries, indicating direct detection of electrically separated grain structures and resultant percolation current pass.

Microcrystalline germanium thin films prepared by reactive RF sputtering

We study microcrystalline germanium (μc-Ge) film as narrow gap semiconductor materials for infrared absorbers by reactive sputtering with inert gas/mixtures. H 2 mixed with Ne, Ar and Xe was used as sputtering gases, in order to research effects of the ion damage. A higher deposition rate is obtained by using inert gases with a larger mass. But the crystallinity becomes lower by the damage due to larger mass ions. In the Ar/H 2 mixtures, the structure changes from crystalline to amorphous with increase in the Ar/H 2 flow rate ratio. The damage of Xe ion is too large to crystallize the films, but the influence of Ne on the crystallinity is not significant. The photo-sensitivity is obtained in the mixed structures between crystalline and amorphous given by proper ion damages. The amorphous parts probably contribute suppression of the grain-boundary defects. The observation of photo-sensitivity indicates the possibility of μc-Ge as a narrow gap material for PV cells.

Microcrystalline germanium thin films prepared by the reactive RF sputtering method

We focus on thin film microcrystalline germanium (μc-Ge) as narrow gap semiconductor materials for high infrared sensitivity and consider applying it to thermo-photo-voltaic (TPV). The μc-Ge films were prepared on glass substrates by the reactive RF sputtering method with Ar and H2 gas mixtures. We could successfully produce photosensitive μc-Ge films. Higher crystallinity structures do not always result in better carrier properties. Probably, the amorphous portions between crystalline grains have important roles to suppress the grain boundary defects. We applied the μc-Ge to i-layers of pin structure devices, and observed the photovoltaic effect for the first time.

Preparation of Microcrystalline Germanium Carbon Thin Films by Hot-Wire Chemical Vapor Deposition Using Dimethylgermane

Microcrystalline germanium carbon (µc-Ge1-xCx) thin films were deposited by hot-wire chemical vapor deposition using dimethylgermane. We investigated the influence of the hydrogen dilution ratio on the deposition rate and film structure. The deposition rate and crystal orientation of the films were strongly affected by the hydrogen dilution ratio. The conductivity of the films was changed with the structure of the films from amorphous to crystalline. The conductivity of the amorphous film was 3.04×10-5 S/cm and that of the microcrystalline films was up to 0.1 S/cm. The carbon concentrations of the films were about 2 at. %, as estimated by X-ray photoelectron spectroscopy. The energy at which the absorption coefficient becomes 104 cm-1 for the µc-Ge0.98C0.02 film is 1.30 eV, which is 0.25 eV higher than that of c-Ge.

Effect of Boron Doping on Microcrystalline Germanium Carbon Thin Films

AbstractEffects of boron doping on microcrystalline germanium carbon alloy (μc-Ge1-xCx:H) thin films have been investigated. We deposited boron-doped p-type μc-Ge1-xCx:H thin films by hot-wire chemical vapor deposition technique using hydrogen diluted monomethylgermane (MMG) and diborane (B2H6). A dark conductivity of 1.3 S/cm and carrier concentration of 1.7 x 1020 cm-3 were achieved with B2H6/MMG ratio of 0.1. Furthermore, the activation energy decreased from 0.37 to 0.037 eV with increasing B2H6/MMG ratio from 0 to 0.1. We also fabricated p-type μc-Ge1-xCx:H/n-type c-Si heterojunction diodes. The diodes showed rectifying characteristics. The typical ideality factor and rectifying ratio were 1.4 and 3.7 x 103 at ¡Ó 0.5 V, respectively.

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.

Deposition and characterization of μc-Ge 1− xC x thin films grown by hot-wire chemical vapor deposition using organo-germane

Microcrystalline germanium carbon (μc-Ge 1− x C x ) films prepared by hot-wire chemical vapor deposition were characterized by Raman, Fourier transform infra-red and X-ray photoelectron spectroscopy. μc-Ge 1− x C x films ( x = 0.02 to 0.03 by using dimethylgermane and x = 0.07 to 0.08 by using monomethylgermane) were successfully deposited using organo-germane and hydrogen. Raman scattering measurements reveal that the microcrystalline films could be obtained with high hydrogen dilution conditions. X-ray photoelectron spectroscopy measurements suggest the possible bonding of carbon with Ge atoms in the deposited thin films. The conductivity of the films were 10 − 3 ∼10 − 1 S/cm for microcrystalline films and 10 − 8 ∼10 − 5 S/cm for amorphous films. The dissociation efficiency of monomethylgermane is found to be higher than that of dimethylgermane.

Formation of microcrystalline germanium (μc-Ge:H) films from inductively coupled plasma CVD

Inductively coupled RF plasma of H 2-diluted GeH 4 gas was applied to the growth of hydrogenated microcrystalline germanium (μc-Ge:H) films on quartz in a reactor with an external single-turn antenna placed on quartz plate window parallel to the substrate. The deposition rate, the crystallinity and the thickness of an amorphous incubation layer formed in the early stages of the film growth were evaluated as functions of GeH 4 concentration, gas flow rate, substrate temperature and the distance between the antenna and the grounded substrate susceptor. We demonstrated the growth of highly crystalized Ge films at a rate as high as 0.9 nm/s at 250 °C using a 8.3% GeH 4 diluted with H 2.

Preferential growth of μc-Ge: H along the crystallographic axes of Si and Ge substrates by ECR plasma CVD

Hydrogenated microcrystalline germanium (μc-Ge : H) films have been grown on crystalline (c-) Si and Ge substrates in a temperature range from 25 to 313 °C, by means of electron-cyclotron-resonance plasma chemical vapor deposition. An increase in substrate temperature induced an amorphous to microcrystalline phase transition at around 50 °C, and increased the crystalline volume fraction of μc-Ge : H on both c-Si and c-Ge. A volume fraction of nearly 100% was realized at 200 °C. Above this, (1 0 0) preferential growth of μc-Ge : H was observed on both c-Si(1 0 0) and c-Ge(1 0 0). For c-Si(1 1 1) and c-Ge(1 1 1) substrates, the growth direction was mainly (1 1 1), but containing (1 1 0) and (3 1 1) components, attributed to twin formation. The dependence of the film-surface roughness on the substrate temperature was also examined. The peak energy and magnitude of the photoluminescence decreased with increasing substrate temperature.

Laser Interference Structuring of a-GeN for the Production of Optical Diffraction Gratings

AbstractIn this work, we study the pulsed laser crystallization of hydrogen-free amorphous germanium-nitrogen alloys (a-GeN). We discuss the role of nitrogen during phase transitions and the possible application of the resulting structure as an optical diffraction grating. The crystallized region results of pure microcrystalline germanium (μc-Ge). An indication that Ge-N bonds have broken and nitrogen outdiffused of the film is obtained from infrared spectroscopy and confirmed by Raman spectra. A pattern of alternating a-GeN and μc-Ge lines with a period of about 4 μm acts as an optical diffraction grating due to the difference in optical properties between the two materials, and the three dimensional surface profile, caused by N2 effusion, that is formed on the sample.

Optical diffraction gratings produced by laser interference structuring of amorphous germanium–nitrogen alloys

We use the interference of two pulsed laser beams (wavelength=355 nm) to produce an optical diffraction grating in amorphous germanium–nitrogen alloy (a-GeN). At the constructive maxima of the interference pattern, the absorption of light leads to crystallization. The crystallized region results of pure microcrystalline germanium (μc-Ge). An indication that Ge–N bonds have broken and nitrogen outdiffused of the film is obtained from infrared spectroscopy and confirmed by Raman spectra. A pattern of alternating a-GeN and μc-Ge lines with a period of about 4 μm acts as an optical diffraction grating due to the difference in optical properties between the two materials, and the three dimensional surface profile, caused by N2 effusion, that is formed on the sample.

Spatially Selective Formation of Microcrystalline Germanium by Laser-Induced Pyrolysis of Organogermanium Nanocluster Film

Abstract A new method to prepare microcrystalline Ge film by laser-induced pyrolysis of spin-coated organogermanium nanocluster film was developed, which is applicable to the spatially selective formation of microcrystalline Ge micropatterns.

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.

Preparation of polycrystalline and microcrystalline germanium composite films by codeposition of active additives

It is shown experimentally that as-prepared polycrystalline Ge films of various crystallite sizes can be tailored by codepositing SiO x of different concentrations. The formation mechanisms of these structures are discussed by taking into account the theoretical structure zone models of polycrystalline films, interpreting the development of the tissue phases covering the grain boundaries. The key phenomenon of these mechanisms is the process-induced segregation of SiO x species on the growing surface of Ge crystals resulting in the formation of an SiO x tissue phase on the crystal surfaces and at the grain boundaries. This kinetic segregation model is proposed for the interpretation of the growth of nanocrystalline Si in case of chemical vapour deposition at high H 2 dilution of SiH 4.

Origins of the Ductile Regime in Single‐Point Diamond Turning of Semiconductors

A germanium surface and the chips produced from a single‐point diamond turning process operated in the “ductile regime” have been analyzed by transmission electron microscopy and parallel electron‐energy‐loss spectroscopy. Lack of fracture damage on the finished surface and continuous chip formation are indicative of a ductile removal process. Periodic thickness variations perpendicular to the machining direction also are observed on these chips and are identified as ductile shear lamellae. The chips consist of an amorphous, elemental germanium matrix containing varying amounts of microcrystalline germanium fragments. The relative orientation of machining marks and crystallographic fragment texture are used to position individual chips with respect to the initial angular cutting zone on the wafer. Chips with high fragment content correlate directly to cutting zones subject to the highest resolved tensile stress on cleavage planes. These findings are explained in the context of a high‐pressure metallization (brittle‐to‐ductile) transformation with ductility limited by the onset of classical brittle fracture.

Electron-flux induced growth of microcrystalline germanium by ECR plasma

An amorphous to microcrystalline phase transition is observed by controlling the electron irradiation of the growth surface of Ge films while keeping the incident ion flux constant in a hydrogen-diluted germane ECR plasma. Changes in the structural and optoelectronic properties of the deposited films are also clearly associated with the phasse transition. A high-quality a-Ge:H is obtained at a low electron irradiation level; i.e., an electron-flux to ion-flux ratio in the range of 0.3 to 1.0. This effect is interpreted in terms of electron-induced hydrogen etching of weak or strained Ge bonds.