Germanium Thin Films Research Articles

Crystalline growth of germanium thin films on single crystal silicon substrates by solid phase crystallization

We have studied the epitaxial-like growth of germanium (Ge), due to solid phase crystallization (SPC) from amorphous Ge (a-Ge) deposited on single crystal silicon (Si) substrate. The crystalline growth of Ge following the orientation of Si substrates was successfully obtained by the SPC at 400°C or higher. The preferential growth on Si (111) substrates continues up to 10,000Å. Different orientations from the substrate orientation in XRD patterns are slightly observed in the growth on Si (100) substrates at 450°C, but the preferential growth of (100) orientation continued in the whole film thickness in TEM images. The epitaxial-like growth of Ge may be more preferable on the Si (111) substrate than the (100) one.

Characteristics of Hydrogenated Amorphous Silicon Germanium Thin Film Solar Cells by GeH4 Flow Rate

Hydrogenated amorphous silicon germanium alloys thin films had been grown by PECVD method. This article shows characteristics of hydrogenated amorphous silicon germanium thin film solar cells with GeH4 flow rates. The optical bandgap of hydrogenated amorphous silicon germanium alloy decreased from 1.78 eV - 1.54 eV with increasing in GeH4 flow rate from 0 sccm – 100sccm. The various values of Voc, Jsc, FF, and conversion efficiency were measured by the solar simulator. These results show that the conversion efficiency of the a-SiGe:H solar cells was very high when the GeH4 flow rate was 60 sccm.

Growth of Quasi-Single-Crystal Silicon–Germanium Thin Films on Glass Substrates by Continuous Wave Laser Lateral Crystallization

Growth of Quasi-Single-Crystal Silicon–Germanium Thin Films on Glass Substrates by Continuous Wave Laser Lateral Crystallization

Growth of Quasi-Single-Crystal Silicon–Germanium Thin Films on Glass Substrates by Continuous Wave Laser Lateral Crystallization

Continuous wave laser lateral crystallization (CLC) technology was applied to grow silicon–germanium (SiGe) thin films on glass substrates. The crystal structure and distribution of Ge in the films were characterized by optical and electron microscopies. The compounding of Ge with Si and the use of CLC resulted in the growth of crystals with lengths greater than 100 µm in the laser-scanning region, whereas such long crystals were not formed in pure Si films. For Si0.7Ge0.3, the orientation of the crystal surface normal was along <111>. Most crystalline boundaries were Σ3 coincidence site lattices that were electrically inactive. High-angle or random grain boundaries were rarely found. Thus, the crystallized part can be regarded as a quasi-single crystal. The growth mode and segregation of Ge are discussed in terms of a constitutional undercooling model.

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.

Low-temperature germanium thin films on silicon

We discuss thermal evaporation of Germanium thin films as a suitable route to realizing near-infrared detectors integrated on a Silicon platform. We study the structural properties of samples grown at various substrate temperatures by X-ray diffraction and transmission electron microscopy, showing that Ge thin films are amorphous when deposited below 225°C, mono-crystalline between 225 and 400°C, poly-crystalline above 450°C. We further investigate their optical and electrical properties using differential optical absorption spectroscopy, Hall and photocurrent measurements. Finally, with the evaporated Ge thin films we demonstrate near-infrared photodiodes with low dark current density and good responsivity at 1.55 μm.

Effects of temperature and near-substrate plasma density on the structural and electrical properties of dc sputtered germanium thin films

Germanium thin films were deposited by dc reactive magnetron sputtering as a function of substrate temperature and ion flux using an unbalanced magnetron with an external magnetic field. The ion flux and energy distribution were measured using a retarding field energy analyzer (RFEA), a flat probe with a guard ring, and cylindrical Langmuir probes. The RFEA ion flux, the flat probe saturation currents, and the ion densities inferred from the cylindrical probe data are in very good agreement over a wide range of plasma densities, which were varied both by the external coil current and discharge power. The RFEA ion energy distributions are in good agreement with the plasma potentials inferred from the cylindrical probes, and suggest that the nonuniformity of the plasma in the vicinity of the substrate holder should be considered in the interpretation of probe results in these systems. The deposited films were characterized by x-ray diffraction, Raman spectroscopy, optical transmission, resistivity, and Hall effect measurements. Under low ion bombardment conditions, an abrupt onset of the crystalline phase with respect to temperature is observed in the Raman and x-ray diffraction measurements, and the crystal quality increases with increasing temperature above the transition temperature. The transition is also accompanied by a sharp decrease in film resistivity. The microcrystalline films have a strong preferential orientation in the (220) direction, and are p type with carrier densities in the range 1018 cm−3 and mobilities in the range 15–30 cm2/V-s. The structural and electronic properties of the films are sensitive to the ion flux. Near the transition temperature the effects of increasing ion flux can be attributed to the small temperature rise that accompanies the higher plasma density. At higher temperatures the increased ion flux results in a more random crystallographic orientation, with significantly lower carrier concentrations and only slightly lower carrier mobilities, implying either a reduction in acceptor defect density and/or the creation of compensating n-type defects.

Micro to nano scale thermal energy conduction in semiconductor thin films

We study the effects of phonon boundary scattering on the transport of thermal energy in semiconductor thin films across multiple length scales and temperatures. We use a model based on the kinetic theory of transport processes that accurately calculates the reduction of the phonon mean free paths by including the effects of spatial location and propagation direction of phonons. We investigate how the effective phonon mean free paths and the resultant thermal conductivities are reduced by the film length scale and surface roughness. The thermal conductivities of silicon and germanium thin films are calculated for temperatures between 4 K and 500 K and thicknesses from nano to micro and good agreement is obtained with experimental measurements. The theoretical study in this paper helps to understand and quantitatively predict the transport of thermal energy in nanoscale materials, which can be used to improve the efficiency of optoelectronic devices and thermoelectric materials.

Modeling magnetotransport in inhomogeneous (Ge,Mn) films

We have performed numerical simulations using the finite elements method in order to investigate magnetotransport in manganese doped germanium thin films. Up to now, several groups have reported similar transport measurements on (Ge,Mn) films obtained either by low temperature molecular beam epitaxy or by manganese implantation of germanium wafers. However, different physical interpretations have been proposed, including the existence of a diluted carrier-mediated ferromagnetic phase. Here, by carefully taking into account the morphology of (Ge,Mn) films (Mn-rich ferromagnetic inclusions in a highly diluted germanium matrix), we can reproduce some experimental findings. In particular, in order to observe high positive magnetoresistance and Hall angles, there are two requirements: (i) a strong anomalous Hall effect within Mn-rich inclusions and (ii) a conductivity contrast between these inclusions and the germanium matrix. Finally, we conclude that (Ge,Mn) films behave as granular ferromagnetic systems.

High quality tensile-strained n-doped germanium thin films grown on InGaAs buffer layers by metal-organic chemical vapor deposition

We show that high quality tensile-strained n-doped germanium films can be obtained on InGaAs buffer layers using metal-organic chemical vapor deposition with isobutyl germane as germanium precursor. A tensile strain up to 0.5% is achieved, simultaneously measured by x-ray diffraction and Raman spectroscopy. The effect of tensile strain on band gap energy is directly observed by room temperature direct band gap photoluminescence.

Room temperature luminescence from Germanium

We analyze the band to band radiative transitions in germanium thin films deposited on silicon and compare them with those in bulk material. A significant down shift of the direct transition related peak was observed from the thin films samples, caused by the preparation formed tensile strain in the film. A comparison between the ratios of the direct to indirect transitions peak intensities showed that those are very similar for the thin films and the bulk material, when the self absorption of the emission is accounted for. The observed similarity in the spectral shape indicates that the strong direct transition luminescence detected in strained germanium films is mainly due to the improved light extraction in the energy range of about 0.8 eV, rather than an increase in the probability of band to band direct transition. We find a feature at around 0.72 eV in the spectrum of the germanium luminescence, which correlates with the presence of dislocations in the crystal. We discuss the origin of this feature in view of one dimensional dislocation bands, split off from the Gamma valley of the conduction band due to the dislocations local strain field.

Formation of heavily boron-doped hydrogenated polycrystalline germanium thin films by co-sputtering for developing p + emitters of bottom cells

Properties of heavily boron-doped hydrogenated polycrystalline germanium (Ge) films sputter-deposited on glass are investigated for developing p + emitters of the bottom cells of low cost monolithic tandem solar cells. The films were deposited and in-situ doped by co-sputtering Ge with boron at various power levels ( P B) in a mixture of argon and hydrogen at 500 °C, and then followed by a rapid thermal anneal process at 550 °C for 120 s or 600 °C for 60 s, respectively. The dependence of the structural properties of the films on the various boron incorporations as well as annealing conditions was explored by Raman and X-ray diffraction measurements. We find that the films as deposited at 500 °C are polycrystalline with strong (2 2 0) preferential orientation, which are normally of columnar structure, confirmed by cross-sectional transmission electron microscopy. Revealed by Hall measurements, a boron activation level of 6.24×10 19 atoms/cm 3, well above reported maximum solid solubility in crystalline Ge, was obtained in the as deposited film at P B=55 W. Annealing at 600 °C enhanced the concentration to 1.21×10 20 atoms/cm 3. A fact that boron hardly diffuses in Ge at temperatures below 800 °C makes the resultant material an excellent candidate for thin p + emitters of the bottom cells of monolithic tandem solar cells.

Thermal conductivity and its anisotropy research of germanium thin film

Germanium is an important element in semiconductor and microelectronics industries. Its thermal conductivity is an invariable and isotropic parameter in germanium crystal in the same temperature and its thermal conductivity is difficult to be measured in thin film. In the present paper, using the non-equilibrium molecular dynamics simulation method and Stillinger–Weber potential model, the normal and tangential thermal conductivity of germanium thin film are studied. There are remarkable anisotropy and size effect of thermal conductivity in normal and tangential direction of germanium thin film. In the theoretical analysis, Boltzmann transport theory is used to analyze the phonon transport in germanium thin film. Theoretical results further demonstrate the anisotropy and size effect of thermal conductivity in thin film, which indicate the significant decrease of phonon mean free path and predominant boundary phonon scattering in germanium thin film.

Hydrogenated amorphous silicon–germanium thin films with a narrow band gap for silicon-based solar cells

Hydrogenated amorphous silicon–germanium (a-Si x Ge 1− x :H) thin films were grown using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD, 27.13 MHz). The films were used in a silicon-based thin film solar cell. The films were grown from a gas mixture of silane (SiH 4) and tunable germane (GeH 4) gas that was diluted in hydrogen (H 2). The results show that the optical band gap ( E g), optical adsorption coefficient ( α), grain size and chemical composition depend on the germane contents of the films. The relationships among optical properties, structural properties, and chemical structural features were characterized by UV–visible spectroscopy (UV–vis), scanning electron microscopy (SEM), X-ray photoelectron spectrometer (XPS), and Raman spectroscopy. a-Si x Ge 1− x :H solar cells with an efficiency of 5.59% were obtained the films.

Micro-Raman characterization of Germanium thin films evaporated on various substrates

We perform an extensive micro-Raman analysis of Germanium thin films physically evaporated on several substrates including silicon, silicon oxide and glass. We investigate the dependence of crystal quality on thin film deposition parameters such as substrate temperature and growth rate. We also study the continuous transitional change of the material structure from amorphous to crystalline phases. Ge films obtained by this simple and low cost technique are a viable solution towards the realization of virtual substrates and devices.

Structural, Vibrational, and Optical Characterization of Anodically Electroplated Germanium Thin Films

Germanium thin films were anodically electrodeposited from ethylenediamine solutions of which contain the well known deltahedral Zintl ions. The anodes were -doped Si(100) wafers or ITO-covered glass slides. The films were studied for their optical and structural properties by Raman spectroscopy, TEM, and luminescence measurements. The Raman spectra of the films show peaks shifted to lower frequencies compared to the same peaks in bulk germanium and indicate nanocrystalline structure for the films. This is further corroborated by the TEM observations. Laser-induced crystallization at selected areas of the films results in significant growth of the nanocrystals. The films exhibit photoluminescence in the visible light range. This has been attributed to a combination of nanocrystallinity and surface defects. Overall, the optical and structural properties of the anodically electrodeposited thin germanium films can be effectively adjusted by tuning the electrodeposition parameters.

Interface-controlled layer exchange in metal-induced crystallization of germanium thin films

Low-temperature synthesis of polycrystalline germanium (poly-Ge) thin films is of great interest in thin-film photovoltaic and electronics applications. We demonstrate metal (Al)-induced crystallization to form poly-Ge thin films on both glass and polymer substrates at temperatures as low as 200 °C. An interfacial diffusion control layer, intentionally interposed between the Al and the underlying amorphous Ge (a-Ge) layer, is found to achieve layer exchange while suppressing uncontrolled Ge crystallization within the bilayer samples. Germanium thin films with micron-size grains and (111)-preferred orientation are prepared by controlled Ge nucleation and Ge lateral overgrowth of Al during a-Ge crystallization.

Germanium–single-wall carbon nanotube anodes for lithium ion batteries

High-capacity thin-film germanium was coupled with free-standing single-wall carbon nanotube (SWCNT) current collectors as a novel lithium ion battery anode. A series of Ge–SWCNT compositions were fabricated and characterized by scanning electron microscopy and Raman spectroscopy. The lithium ion storage capacities of the anodes were measured to be proportional to the Ge weight loading, with a 40 wt% Ge–SWCNT electrode measuring 800 mAh/g. Full batteries comprising a Ge–SWCNT anode in concert with a LiCoO2 cathode have demonstrated a nominal voltage of 3.35 V and anode energy densities 3× the conventional graphite-based value. The higher observed energy density for Ge–SWCNT anodes has been used to calculate the relative improvement in full battery performance when capacity matched with conventional cathodes (e.g., LiCoO2, LiNiCoAlO2, and LiFePO4). The results show a &gt;50% increase in both specific and volumetric energy densities, with values approaching 275 Wh/kg and 700 Wh/L.

Structural dependence of electrical properties of Ge films prepared by RF magnetron sputtering

This paper investigates the electrical properties of non-hydrogenated and hydrogenated germanium thin films deposited on silicon nitride coated glass in order to develop a material for the bottom cells of low cost monolithic tandem solar cells. Films were deposited by RF magnetron sputtering over a series of substrate temperatures up to 500°C. A structure-dependent conduction property of germanium films was found. As the substrate temperature increased from 255 to 400°C, both series of films first showed n-type conductivity with progressively increasing room-temperature dark resistivity that peaks around the type switch. Upon attaining p-type character the resistivity decreased rapidly with further increase in T s. Accompanying these trends, the film grain orientation evolved from predominantly (220) to (111).

In situ X-ray absorption spectroscopy of germanium evaporated thin film electrodes

The understanding of germanium Li-ion insertion/extraction reaction mechanism is drawing more and more attention in the field of Li-ion batteries. When a germanium thin film electrode is inserted with Li ions, the material remains amorphous until it crystallizes into Li 15Ge 4 as evidenced by X-ray diffraction. The local coordination environment of the Ge atoms of the intermediate amorphous phases was investigated by in situ X-ray absorption spectroscopy. Li-ion insertion and extraction were electrochemically controlled by continuous and intermittent galvanostatic methods. The evolution of the coordination number and interatomic distance of Ge–Ge and Ge–Li shells was determined as a function of Li composition. From a short range ordering perspective, it was observed that the first Ge–Ge interatomic distance increases and the Ge–Ge coordination number decreases with increasing Li content. The opposite is observed for the first Li–Ge interaction. Moreover, it was found that electrochemical lithiation is reversible at the atomic scale.