Amorphous Germanium Research Articles

Solid phase crystallization of germanium films on crystalline silicon

We have investigated the lateral solid phase crystallization (SPC) of amorphous germanium (a-Ge) precursors on single crystalline silicon (c-Si) substrates with a silicon dioxide (SiO2) stripe structure. The a-Ge layers were deposited on c-Si with SiO2 stripes at 100°C and SPC was carried out at 375–385°C. The crystallization in a-Ge layers is only nucleated on c-Si substrates and thus, the preferential crystalline growth occurred along the normal orientation of the c-Si substrates, with no crystallization of the Ge layers nucleated on a-Ge layers over SiO2 layers, except near the c-Si/SiO2 boundary. As a result we have obtained lateral preferential crystalline growth of c-Ge of large grain size by SPC from a-Ge films on c-Si over SiO2 stripes, where the preferential crystalline growth distance in a-Ge on SiO2 increased from 3 to 5μm, when the annealing temperature increased from 375 to 385°C.

Transparent conductive Hf-doped In2O3 thin films by RF sputtering technique at low temperature annealing

Hf-doped In2O3 transparent conductive polycrystalline films (IHFO) were grown at a low substrate temperature by radio frequency magnetron sputtering for the applications of silicon-based solar cell. The effect of argon flow rate on the electrical and optical properties of the films was investigated. Low temperature thermal treatment improved IHFO films properties, with the optimal Hall mobility of 79.6cm2/Vs and resistivity of 3.76×10−4Ωcm. The average transmittance of the 807nm thick IHFO films in the range of 300–1500nm was above 83%. The carrier density was utilized to evaluate the plasma wavelength of IHFO conducting film which was 1.8μm. The optimized IHFO film was then applied to amorphous silicon germanium thin film solar cells as the contacting layer. Compared to the cell without such a layer, the efficiency was higher by 0.35%.

Thermal crystallization of sputter-deposited amorphous Ge films: Competition of diamond cubic and hexagonal phases

Following our previous studies on crystallization induced by electron irradiation, we have investigated the crystallization of sputter-deposited amorphous germanium films by heat treatments. On continuous heating, samples aged for 3 days and 4 months at room temperature crystallized at 500°C to form coarse spherical particles of a hexagonal structure, of about 100 nm in diameter, whereas samples aged for 7 months turned to homogeneous nanograins of the diamond cubic structure at 600°C. When the films aged for 4 months at room temperature were annealed at 350°C for 2 h and then heated, they crystallized at 550°C to form a mixture of the two microstructures, and those annealed at 350°C and further at 500°C for 1 h crystallized at 600°C mostly to nanograins. Crystallization by electron irradiation at 350°C to 4-month-aged samples has also been studied. With increasing annealing time at 350°C, coarse particles of a hexagonal structure ceased to appear, and were replaced by fine nanograins of the diamond cubic structure. These observations can be understood in terms of structural instability of sputter-deposited amorphous films. Medium-range ordered clusters must initially be present in the films and serve as nuclei of the metastable hexagonal phase. They are unstable, however, and are eliminated by annealing, resulting in the reduction in size and number of coarse particles with a metastable structure.

Thermal oxidation of amorphous germanium thin films on SiO2 substrates

In this work we report the thermal oxidation of amorphous germanium (a-Ge) thin films (140 nm thickness) in air. Following fabrication by conventional thermal evaporation on SiO2 substrates, the samples were annealed in air at different temperatures ranging from 300 to 1000 °C. By means of x-ray diffraction, x-ray reflectivity, synchrotron grazing-incidence wide-angle x-ray scattering and cross-sectional transmission electron microscopy analysis it is found that the a-Ge films abruptly crystallize at 475 °C, while simultaneously increasing the thickness of the oxide (GeO2) in a layer by layer fashion. X-ray photoemission spectroscopy reveals that the oxidation state of the Ge atoms in the GeO2 layer is 4+. However, a reaction at the GeO2/Ge interface occurs between 500 and 550 °C reducing the oxide layer to GeOx (x < 2) and containing Ge2+ and Ge+. The thickness of the oxide layer grows with the annealing temperature following an Arrhenius behavior with an activation energy of 0.82 ± 0.09 eV up to 500 °C. Remarkably, we observed simultaneous enhancement of the oxidation and crystallization of the a-Ge in the temperature interval 450 °C–500 °C, in which the oxidation rate reaches a maximum of around 0.8 nm °C−1 at around 500 °C.

The role of hydrogenated amorphous silicon oxide buffer layer on improving the performance of hydrogenated amorphous silicon germanium single-junction solar cells

Hydrogenated amorphous silicon oxide (a-Si1−xOx:H) film was used as a buffer layer at the p-layer (μc-Si1−xOx:H)/i-layer (a-Si1−xGex:H) interface for a narrow band gap hydrogenated amorphous silicon germanium (a-Si1−xGex:H) single-junction solar cell. The a-Si1−xOx:H film was deposited by plasma enhanced chemical vapor deposition (PECVD) at 40 MHz in a same processing chamber as depositing the p-type layer. An optimization of the thickness of the a-Si1−xOx:H buffer layer and the CO2/SiH4 ratio was performed in the fabrication of the a-Si1−xGex:H single junction solar cells. By using the wide band gap a-Si1−xOx:H buffer layer with optimum thickness and CO2/SiH4 ratio, the solar cells showed an improvement in the open-circuit voltage (Voc), fill factor (FF), and short circuit current density (Jsc), compared with the solar cells fabricated using the conventional a-Si:H buffer layer. The experimental results indicated the excellent potential of the wide-gap a-Si1−xOx:H buffer layers for narrow band gap a-Si1−xGex:H single junction solar cells.

Elastic Measurements of Amorphous Silicon Films at mK Temperatures

The low temperature properties of glass are distinct from those of crystals due to the presence of poorly understood low-energy excitations. The tunneling model proposes that these are atoms tunneling between nearby equilibria, forming tunneling two level systems (TLSs). This model is rather successful, but it does not explain the remarkably universal value of the mechanical dissipation $Q^{-1}$ near 1 kelvin. The only known exceptions to this universality are the $Q^{-1}$ of certain thin films of amorphous silicon, carbon and germanium. Recently, it was found that $Q^{-1}$ of amorphous silicon (a-Si) films can be reduced by two orders of magnitude by increasing the temperature of the substrate during deposition. According to the tunneling model, the reduction in $Q^{-1}$ at 1 kelvin implies a reduction in $P_{0}\gamma ^{2}$, where $P_{0}$ is the density of TLSs and $\gamma $ is their coupling to phonons. In this preliminary report, we demonstrate elastic measurements of a-Si films down to 20 mK. This will allow us, in future work, to determine whether $P_{0}$ or $\gamma $ is responsible for the reduction in $Q^{-1}$ with deposition temperature.

Effects of hydrogen incorporation on structural, optical and electrical properties of germanium carbon films prepared with RF magnetron co-sputtering

Hydrogenated amorphous germanium carbon (a-Ge1-xCx:H) films were deposited by using magnetron co-sputtering starting from a mixed target of germanium and graphite in Ar and H2 mixture atmosphere. The structural, bonding, optical and electrical properties of the films were investigated as functions of H2 flow rate with the C and Ge contents in the films respectively kept constant. Both electrical resistivity and optical band gap of the a-Ge1-xCx:H films increased with increasing H2 flow rate from 0 to 15 sccm, while the refractive index stayed around 3.6. X-ray diffraction patterns and Raman spectra confirmed that the structural disorder degree of the a-Ge1-xCx:H films had declined with the rise in H2 flow rate. The results of X-ray photoelectron spectroscopy showed that the formation of sp3 hybridized C and CGe bonds in the films had been promoted by incorporation of hydrogen into the films. The relations between the chemical bonding and the structural, optical and electrical properties of the films were discussed.

Crystallization of sputter-deposited amorphous Ge films by electron irradiation: Effect of low-flux pre-irradiation

We investigated the effect of low-flux electron irradiation with 125 keV to sputter-deposited amorphous germanium on the amorphous structure and electron-induced crystallization microstructure by TEM following our previous study on the effect of aging at room temperature. In samples aged for 3 days, coarse, spherical particles about 100 nm in diameter appear dominantly. By low-flux pre-irradiation to the samples, a reduction in the size and number of coarse particles, embedded in the matrix with fine nanograins of the diamond cubic structure, was noted with the increase in fluence. The crystal structure of these coarse particles was found to be not cubic but hexagonal. In samples aged for 4 months, a similar tendency was observed. In samples aged for 7 months, on the other hand, the homogeneous diamond cubic structured nanograins were unchanged by pre-irradiation. These results indicate that pre-irradiation as well as aging modifies the amorphous structure, preventing the appearance of a hexagonal phase. The elimination of a certain amount of medium-range ordered clusters by pre-irradiation, included in as-deposited samples and the samples aged for 4 months, apparently gives rise to a reduction in the size and number of coarse particles with a metastable hexagonal structure.

Optical absorbers based on strong interference in ultra‐thin films (Laser Photonics Rev. 10(5)/2016)

Structures that enhance the absorption of electromagnetic waves find uses in a variety of applications, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, thermal light sources, and color coatings. Recent efforts have sought to reduce the footprint of optical absorbers by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large-scale optical and optoelectronic devices. The cover article reviews recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These absorbers have a small physical footprint and require no nanoscale patterning, making them ideal for large-area applications. On the cover is a photograph of color images lithographically printed on a glass slide, using ultra-thin layers of lossy amorphous germanium on top of gold films. The different colors are generated by germanium thickness variations of about four nanometers from one region to another, which dramatically change the absorption spectrum, and hence the color. Photograph courtesy of M. Kats and L. Liu. (Picture: Mikhail A. Kats and Federico Capasso, 10.1002/lpor.201600098 pp. 735–749, in this issue)

Silicon germanium active layer with graded band gap and µc-Si:H buffer layer for high efficiency thin film solar cells

We investigated solar cells with graded band gap hydrogenated amorphous silicon germanium active layer and hydrogenated microcrystalline silicon buffer layer at the interface of intrinsic and n-type doped layer. A significantly improved, 10.4% device efficiency was observed in this type of single junction solar cell. The intrinsic type microcrystalline silicon buffer layer is thought to play dual roles in the device; as a crystalline seed-layer for growth of n-type hydrogenated microcrystalline silicon layer and helping efficient electron collection across the i/n interface. Based on these, an enhancement in cell parameters such as the open-circuit voltage (Voc), and fill factor (FF) was observed, where the FF and Voc reaches up to 69% and 0.85 V respectively. Our investigation shows a simple way to improve device performance with narrow-gap silicon germanium active layer in solar cells in comparison to the conventionally constant band gap device structure.

Very thin and stable thin-film silicon alloy triple junction solar cells by hot wire chemical vapor deposition

We present a silicon-based triple junction solar cell that requires a deposition time of less than 15 min for all photoactive layers. As a low-bandgap material, we used thin layers of hydrogenated amorphous silicon germanium with lower band gap than commonly used, which is possible due to the application of hot wire chemical vapor deposition. The triple junction cell shows an initial energy conversion efficiency exceeding 10%, and with a relative performance stability within 6%, the cell shows a high tolerance to light-induced degradation. With these results, we help to demonstrate that hot wire chemical vapor deposition is a viable deposition method for the fabrication of low-cost solar cells.

Resonant-cavity-enhanced a-Ge:H nanoabsorber solar cells for application in multijunction devices

A resonant-cavity-enhanced solar cell with a 25nm thick amorphous germanium nanoabsorber is demonstrated. The device is specifically designed for use as bottom cell in multijunction devices. In this case, instead of relying on broadband absorption, only red light needs to be absorbed in the bottom cell. This allows exploiting the narrow resonance patterns of long Fabry-Pérot cavities to achieve efficient absorption inside the nanoabsorber. It is shown that this approach can only be integrated into multijunction devices when suppressing inter-subcell coherence. To achieve this, the intermediate reflector of the tandem device is textured using electrochemically grown zinc oxide nanorods.

A method for distinguishing between propagons, diffusions, and locons

The majority of intuition on phonon transport has been derived from studies of homogenous crystalline solids, where the atomic composition and structure are periodic. For this specific class of materials, the solutions to the equations of motions for the atoms (in the harmonic limit) result in plane wave modulated velocity fields for the normal modes of vibration. However, it has been known for several decades that whenever a system lacks periodicity, either compositional or structural, the normal modes of vibration can still be determined (in the harmonic limit), but the solutions take on different characteristics and many modes may not be plane wave modulated. Previous work has classified the types of vibrations into three primary categories, namely, propagons, diffusions, and locons. One can use the participation ratio to distinguish locons, from propagons and diffusons, which measures the extent to which a mode is localized. However, distinguishing between propagons and diffusons has remained a challenge, since both are spatially delocalized. Here, we present a new method that quantifies the extent to which a mode's character corresponds to a propagating mode, e.g., exhibits plane wave modulation. This then allows for clear and quantitative distinctions between propagons and diffusons. By resolving this issue quantitatively, one can now automate the classification of modes for any arbitrary material or structure, subject to a single constraint that the atoms must vibrate stably around their respective equilibrium sites. Several example test cases are studied including crystalline silicon and germanium, crystalline silicon with different defect concentrations, as well as amorphous silicon, germanium, and silica.

Highly retarded crystallization in hydrogenated amorphous germanium; emergence of a porous nanocrystalline structure

The thermal crystallization of heavily hydrogenated, amorphous Ge (a-Ge:H) and ultra-pure amorphous Ge (a-Ge) thin films was investigated, on a comparative basis, by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) performed during in situ heating. It was found that the crystallization process in a-Ge:H is pronouncedly retarded, due to the initial presence of hydrogen-induced nanovoids. The crystallization of a-Ge:H initiates only after considerable reduction of the amount of nanovoids in the amorphous matrix by thermally induced coarsening of the nanovoids leading to nanopores in the a-Ge:H. Such retarded crystallization leads to the formation of nanocrystalline, porous Ge with nanocrystal sizes below 20nm, in contrast with the fast formation of very large-grained (grain sizes of hundreds of nanometer) polycrystalline Ge upon thermal crystallization of pure a-Ge.

Scalable physical coloration

Methods to produce colored surfaces using physical mechanisms are proposed, with aluminum used as a demonstration vehicle. Our approach aims to achieve tunable, vibrant colors through the interaction of three well-established physical mechanisms of color formation, namely (1) resonant scattering and interference effects in a nano-structured surface (e.g. via anodization of aluminum), (2) plasmonic resonances in metal nanoparticles, and (3) spectrally selective absorption enhancement in ultrathin films, such as amorphous germanium and carbon. We provide proof-of-concept by fabricating and characterizing representative aluminum-based samples showing vivid colors throughout the visible spectral range.

Structural transition in sputter-deposited amorphous germanium films by aging at ambient temperature

The structure of amorphous Ge (a-Ge) films prepared by sputter-deposition and the effects of aging at ambient temperature and pressure were studied by pair-distribution-function (PDF) analysis from electron scattering and molecular dynamics simulations. The PDFs of the as-deposited and aged samples for 3–13 months showed that the major peaks for Ge-Ge bonds decrease in intensity and broaden with aging for up to 7 months. In the PDFs of a-Ge of molecular dynamics simulation obtained by quenching liquid at different rates, the major peak intensities of a slowly cooled model are higher than those of a rapidly cooled model. Analyses on short- and medium-range configurations show that the slowly cooled model includes a certain amount of medium-range ordered (MRO) clusters, while the rapidly cooled model includes liquid-like configurations rather than MRO clusters. The similarity between experimental and computational PDFs implies that as-deposited films are similar in structure to the slowly cooled model, whereas the fully aged films are similar to the rapidly cooled model. It is assumed that as they undergo room-temperature aging, the MRO clusters disintegrate and transform into liquid-like regions in the same matrix. This transition in local configurations is discussed in terms of instability and the non-equilibrium of nanoclusters produced by a vapor-deposition process.

Effect of medium range order on pulsed laser crystallization of amorphous germanium thin films

Sputter deposited amorphous Ge thin films had their nanostructure altered by irradiation with high-energy Ar+ ions. The change in the structure resulted in a reduction in medium range order (MRO) characterized using fluctuation electron microscopy. The pulsed laser crystallization kinetics of the as-deposited versus irradiated materials were investigated using the dynamic transmission electron microscope operated in the multi-frame movie mode. The propagation rate of the crystallization front for the irradiated material was lower; the changes were correlated to the MRO difference and formation of a thin liquid layer during crystallization.

Ultrathin tandem solar cells on nanorod morphology with 35-nm thick hydrogenated amorphous silicon germanium bottom cell absorber layer

We demonstrate single junction and tandem solar cells using an ultrathin (35nm) hydrogenated amorphous silicon germanium absorber which is deposited by hot wire chemical vapor deposition within only 90s. We take advantage of the extremely small thickness of the absorber layer and the capability of hot wire chemical deposition to create conformal layers, by fabricating solar cells on morphologies consisting of self-assembled zinc oxide nanorods. We created solar cells in tandem configuration using the hydrogenated amorphous silicon germanium absorber layer in the bottom cell. Our cells include a cell structure that is “vertical”, which has great potential for high photocurrent generation. We show that the nanorod morphology particularly improves the generation of current in the bottom cell.

Formation of large-grain crystalline germanium on single layer graphene on insulator by rapid melting growth

We demonstrate the crystallization of thermally deposited amorphous germanium (Ge) microstrips on single layer graphene (SLG) by rapid melting growth. Lateral growth of large grain crystalline Ge was successfully obtained over entire microstrip structure. SLG has shown its capability to suppress the spontaneous nucleation in the melting Ge, where no or less intermixing of C and Ge atoms has been detected. The interaction of C atoms from the graphene and Ge atoms at the interface is the possible reason for the observation of large compressive strain generated in the Ge strip grown on SLG. This technique provides an innovative breakthrough towards the realization of single-crystalline Ge-on-insulator (GOI) structure on SLG to facilitate the next-generation ultra-large-scale integrated circuits (ULSIs) with multifunctionalities.

Effect of substrate temperature and radio frequency power on compositional, structural and optical properties of amorphous germanium carbide films deposited using sputtering

Fabrication of GexC1−x has been a big challenge because of the solubility of C in Ge. Only a small percentage of GeC bonds (11.6%) have been introduced so far. In this work, a-GexC1−x with GeC content up to 21% has been fabricated with 50W RF power at 250°C by reactive sputtering methods. The effects of the radio frequency power and substrate temperature on the yield of GeC were analysed in detail. The GeC percentage by volume was found to first increase and then decrease with increasing substrate temperature. Introduction of C into the Ge matrix seems to tune the optical bandgap over a range of 2.7eV to 1.0eV depending on the combination of substrate temperature and radio frequency power.