Fabrication and characteristics of porous germanium films

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm⁻³) and a maximum of Hall mobility at ∼225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Characterisation of thin surface films on germanium in various solvents by ellipsometry

Ellipsometric techniques were used to investigate the thin films that formed on cleaved germanium samples in air, deionised water, and 30% hydrogen peroxide. In all three cases the films were less than 100 Å in thickness. The films formed in air and water had a refractive index of 1.63±0.02 which is close to that of glassy GeO₂. An increase of refractive index to about 1.74 was observed when the samples were immersed in a solution of 30% H₂O₂. The thickness of this film (18Å) remained relatively constant in a period of 5 min to 6 h. The film is attributed to a layer of Ge(OH)₂ ²⁺ surface complexes, and a thick reaction envelope of metagermanic acid (H₂GeO₃). After rinsing surfaces covered with glassy GeO₂ with a 48% HF solution, the original oxide was removed; however, within 5 min a 50Å thick film of glassy GeO₂ had formed.Also affiliated with the Department of Physics.Also affiliated with the Department of Geochemistry and Mineralogy.     

In situ observation of electron beam irradiation effects in oxidized polycrystalline Si1−xGex films

This study examined the morphological and compositional changes that occur in oxidized poly-Si_1−xGe_x film during electron-beam irradiation in a transmission electron microscope. Before irradiation, the oxide layer was composed of a mixture of SiO₂ and GeO₂ phases. However, during electron-beam irradiation, there were significant changes in the microstructure and elemental distribution. For the oxidized poly-Si_0.6Ge_0.4 films, the agglomeration of GeO₂ was observed at the surface region. On the other hand, in the case of the oxidized poly-Si_0.4Ge_0.6 films, the crystallization of GeO₂ occurred in the oxide layer. Ge lattice fringes and twinning were also observed in the oxide layer.     

Atomic layer deposition of HfO2 on self-assembled monolayer-passivated Ge surfaces

HfO₂ films are not easily deposited on hydrophobic self-assembled monolayer (SAM)-passivated surfaces. However, in this study, we deposited HfO₂ films on a tetradecyl-modified SAM with a Ge surface using atomic layer deposition at 350 °C. A slightly thinner HfO₂ film thickness was obtained on the tetradecyl-modified SAM passivated samples than that typically obtained on GeO_x-passivated samples. The resulting electrical properties are explained by the physical thickness and stoichiometry of the interfacial layer.     

Effects of Heat Treatment on the Microstructure and Optical Properties of Sputtered GeO2 Thin Films

Microstructure and composition significantly influence the physical properties of thin films. Therefore, these can be adapted to enhance the functionality of thin films for practical applications. Herein, the anomalous microstructural evolution of sputtered GeO₂ thin films based on postdepositional heat treatments is reported. Temperature-dependent microstructural variations are investigated systematically via a combinatorial postdepositional heat treatment employing a natural thermal gradient in a tube furnace. Heat treatment under an oxidizing atmosphere results in a transition from the amorphous phase to the quartz phase, and subsequent heat treatments under a reducing atmosphere cause H₂O-incorporated chemical reactions. Hence, these conditions create unique microstructural features and yield optical transmittance variations in the GeO₂ thin films. The phase transition induces the formation of spherulitic hexagonal GeO₂ crystallites, and further increase in the temperature promotes the agglomeration of crystallites in the amorphous matrix. The incorporation of H₂O results in the growth of the microstructure, and the chemical reduction to Ge metal begins to generate small granules from the boundary of the microstructures. The experimental results and proposed mechanisms presented herein for the microstructural and compositional changes serve as references for designing the physical properties of thin films.     

Phase transformation and optical characteristics of porous germanium thin film

In this study, we proposed a method to prepare GeO₂ by treating porous Ge thin film with thermal annealing in O₂ 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 O₂ ambient were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectra. Initial Ge composition was gradually oxidized to GeO₂ with increasing annealing time. Comparing the photoluminescence (PL) results between Ge and GeO₂, 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.     

Study of structural and optical properties of Ge doped ZnO films

The Ge doped ZnO films were deposited on quartz substrates by radio frequency magnetron sputtering. The effects of doping and substrate temperature on the structural and optical properties of the Ge doped ZnO films were investigated by means of X-ray diffraction (XRD), UV-visible transmission spectra, X-ray photoelectron spectroscopy and photoluminescence (PL) spectra. The XRD patterns showed that Zn₂GeO₄ phases were formed in the films. With the increase of substrate temperature the crystallization of Zn₂GeO₄ was improved, and that of ZnO phases turned worse, and no diffraction peak of ZnO was observed when the substrate temperature was 700 °C. Obvious ultraviolet (UV) light emission was found due to ZnO grains, and it was much stronger than that of un-doped ZnO films. The enhancement of UV light emission at about 380 nm may be caused by excitons which were formed at the interface between Zn₂GeO₄ and ZnO grains. In the visible region of the PL spectra, the green light emission peak of samples at about 512 nm was associated with defects in ZnO. A red shift of the green light emission peak was observed which can be explained by the fact that there is a luminescence center at about 548 nm taking the place of the defect emission of ZnO with the increase of substrate temperature. The red shift of the green light emission peak and the 548 nm green light emission peaks of the PL spectrum show that some Ge²⁺ should replace the Zn²⁺ positions during the Zn₂GeO₄ grains growth and form the Ge²⁺ luminescence centers in Zn₂GeO₄ grains.     

Phase transition and optical properties of Ge doped ZnO films synthesised by sputtering

Ge doped ZnO films were deposited on Si substrates by sputtering technique. With the increasing annealing temperature, the crystal quality of samples becomes gradually better and the phase transition can be observed at annealing temperature of 600°C. X-ray photoelectron spectroscopy results show the incorporation of Ge into the ZnO films with 14·81 at-%Ge content. Fourier transform infrared spectroscopy absorption spectra of samples annealed at above 600°C display vibration mode of ν (ZnO₄) and ν (GeO₄) in Zn₂GeO₄. The enhancement of ultraviolet emission intensity should be attributed to the yielded mass holes caused by Ge doping and the rising crystal quality. The sample annealed at 800°C displays the strongest blue emission due to the native defects in Zn₂GeO₄ films or/and surface defects.     

Modification of germanium nanoclusters in GeO x films during isochronous furnace and pulse laser annealing

We have studied the formation and modification of germanium nanoclusters in GeO _x films under the action of pulse laser and isochronous furnace annealing. Pulse treatments were effected using either a Tisapphire laser operating at a wavelength of λ = 800 nm and a pulse duration τ of about 30 fs or a KrF excimer laser with λ = 248 nm and τ = 25 ns. The pulse annealing stimulated both the crystallization of initial amorphous Ge nanoclusters in GeO _x matrix and the formation of new nanoclusters. In order to prevent the evaporation of films under the action of laser radiation, the samples were covered with a protective layer of SiN _x O _y . The proposed approach can be used for the modification of dimensions and phase composition of Ge clusters in GeO _x films.     

Improvement of Al2O3/Ge interfacial properties by O2-annealing

The electrical properties of an Al₂O₃/Ge gate stack structure were improved by O₂-annealing. The interface state density can be decreased by O₂-annealing without the formation of a GeO₂ interfacial layer. X-ray photoelectron spectroscopy measurements revealed that Ge diffusion into the Al₂O₃ layer occurs and Ge is uniformly distributed in the oxide layer after O₂-annealing. Crystallization of the Al₂O₃ film was observed after O₂-annealing at 550 °C and was identified as an Al-Ge-O compound using cross sectional transmission electron microscopy and transmission electron diffraction measurement.     

Structure and superconductivity of Gex Cu1−x alloy films

Structural studies of Ge_x - Cu_1?x alloy films show that for x >0.80 300 K deposited films are found to be amorphous. On in situ heating these samples to ～ 100°C, a discrete, crystalline, second phase particles of Cu₃ Ge (?) compound appear in the amorphous matrix of Ge. For x ? 0.8 crystallites of Cu₃ Ge can be detected in amorphous Ge at room temperature. At higher Cu concentration (x < 0.4) in addition to crystalline Cu₃ Ge and Ge, free copper is detected. These structural studies suggest that the superconductivity behavior observed in the quenched alloy films presumably arise from small crystallites of Cu₃ Ge embedded in the amorphous matrix of Ge.     

Aerosol assisted chemical vapour deposition of germanium thin films using organogermanium carboxylates as precursors and formation of germania films

Diethyl germanium bis-picolinate, [Et₂Ge(O₂CC₅H₄N)₂], and trimethyl germanium quinaldate, [Me₃Ge(O₂CC₉H₆N)], have been used as precursors for deposition of thin films of germanium by aerosol assisted chemical vapour deposition (AACVD). The thermogravimetric analysis revealed complete volatilization of complexes under nitrogen atmosphere. Germanium thin films were deposited on silicon wafers at 700°C employing AACVD method. These films on oxidation under an oxygen atmosphere at 600°C yield GeO₂. Both Ge and GeO₂ films were characterized by XRD, SEM and EDS measurements. Their electrical properties were assessed by current?Cvoltage (I?CV) characterization.     

Surface structure of high-T c Nb3Ge films

We have studied the surface structure of rf-sputtered Nb₃Ge films by means of reflection electron diffraction and Auger electron spectroscopy. It is found that a tetragonal Nb₅Ge₃ phase exists, being partly disordered, at the surface of high-T _c films, whereas A15 phase is predominant in the body of the film as proved by x-ray diffraction. The tetragonal surface phase is replaced by an amorphous phase in thin films of <1000 Å, where reducedT _c 's as compared with those of thick films are found. Single A15 phase occurs at the surface of Ge-deficient films which have lowT _c 's. Auger analysis has shown a pronounced Ge-rich layer with a depth of 60–100 Å, depending on film composition and thickness. At the top of the surface layer, Ge is enriched beyond a composition corresponding to Nb₅Ge₃. It is inferred that the Ge-rich layer is responsible for formation of the tetragonal phase.Work supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.     

Particularities of the photoluminescence spectra of Cr4+:Ca2GeO4 single crystal films

It was found that the room-temperature photoluminescence (PL) spectra of Cr⁴⁺:Ca₂GeO₄ single crystal films grown by magnetron sputtering on Ca₂GeO₄ bulk single crystal substrates exhibit a sharp emission band in the region of ～1.17 μm with satellites at ～1.21 and 1.27 μm. Particularities of the PL spectra are interpreted within the framework of a model based on the interaction between electrons of the Cr⁴⁺ center and the oscillations excited in the film.     

A Chemical Route to Monolithic Integration of Crystalline Oxides on Semiconductors

This work demonstrates the growth of crystalline SrTiO₃ (STO) directly on germanium via a chemical method. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin film of STO (2 nm) is deposited by atomic layer deposition (ALD) at 225 °C. Following post‐deposition annealing at 650 °C for 5 min, the STO film becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. Thicker STO films (up to 15 nm) are then grown on the crystalline STO seed layer. The crystalline structure and orientation are confirmed via reflection high‐energy electron diffraction, X‐ray diffraction, and transmission electron microscopy. Electrical measurements of a 15‐nm thick epitaxial STO film on Ge show a large dielectric constant (k ≈ 90), but relatively high leakage current of ≈10 A/cm² for an applied field of 0.7 MV/cm. To suppress the leakage current, an aluminum precursor is cycled during ALD growth to grow crystalline Al‐doped STO (SrTi_1‐x­Al_xO_3‐δ) films. With sufficient Al doping (≈13%), the leakage current decreases by two orders of magnitude for an 8‐nm thick film. The current work demonstrates the potential of ALD‐grown crystalline oxides to be explored for advanced electronic applications, including high‐mobility Ge‐based transistors.     

2.5 Behaviour of amorphous Ge contacts to monocrystalline silicon

The performance of evaporated amorphous Ge films (thickness $\text{～-500}\text{A}\text{?}$) contacts to etched n-and p-type silicon crystals of different resistivities are discussed. The Ge was 3 Ωcm n-type was also used but gave no difference), and as external contact to the Ge film an Au layer was evaporated. The behaviour of the aGeSi junction seems to be largely governed by interface effects (and thus depends on surface preparation), as is often the case with metal-Si junctions, but Ge gives more reproducible and less time-varying results.In the process of clarifying the function of the contact the following structures were investigated (1) aGe_pⁿSiIn (Hg), where the latter is an ohmic contact, (2) amSi_pⁿSi-metal where amSi is a surface region of the crystal which has been rendered amorphous by ion bombardment, (3) aGe_pⁿSi-metal. I-V and C-V measurements were performed. From the results we conclude that aGeSi junctions act as low-resistance contacts when fed by electron or hole currents from the crystal. The currents (holes and electrons) that are injected into the crystal from the film are limited by barriers to small current densities, usually in the range 10^?6 A cm^?2. It is suggested that the small hole currents are explained by an increase in the hole barrier, effected by positive charges at the interface or in the Ge film, which are built up when positive carriers (holes) are injected by the contact.     

Photoluminescence of Si<Subscript>0.9</Subscript>Ge<Subscript>0.1</Subscript>O<Subscript>2</Subscript> and GeO<Subscript>2</Subscript> films irradiated with silicon ions

We have studied the photoluminescence (PL) of GeO₂ and 90 mol % SiO₂-10 mol % GeO₂ films synthesized by method of RF magnetron sputtering and then irradiated with silicon ions and annealed. The PL of silicon-implanted GeO₂ films, related to the presence of Si nanocrystals (nc-Si), was observed for the first time. It is established that the transformation of the defect centers responsible for the PL in the spectral range 350–600 nm, as well as the formation of nc-Si emitting in the region of 700–800 nm, significantly depend on the matrix type. In particular, the PL intensity at 700–800 nm in 90 mol % SiO₂-10 mol % GeO₂ films is weak. The role of the isovalent substitution of Si and Ge atoms in the transformation of defect centers and the formation of nc-Si is discussed.     

Preparation of Nb3Ge films by chemical transport reaction and their critical properties

Niobium-germanium films have been deposited on sapphire substrates at 900 C by a chemical transport reaction method. The highest superconducting transition onset temperature T _C,on of 22.4 K is observed for a nearly stoichiometric Nb₃Ge film with the A15-type structure (thickness ~ 93.5 m). Lattice constants for the Nb₃Ge phase formed in the Nb-Ge films with both T _C,on above 22 K and T _C,midpoint above 21 K are found to extend from 5.143 to 5.153 ». Deposition rates for the obtained films are in the range of 2–10 m/min. Critical current densities for the Nb₃Ge film with the highest T _C,on value are observed to be relatively low (~ 10³ A/cm² at 19 K at self-field). This is due to the coarse grain structure of the film or the low density of effectual pinning centers in the film. Field variations of the pinning forces operating in this film in magnetic fields both parallel to the film surface and perpendicular to the film surface are found to follow closely b ^1/2(1\s-b)², to which the pinning force for flux pinning at the surface of normal regions, such as grain boundaries, film surfaces, etc., is proportional, and where b is the reduced magnetic induction (B/B _C2).A small increase in J _C at low fields is caused by the presence of a small amount of the Nb₅Ge₃ phase in a Nb₃Ge film, and seems attributable to additional flux pinning on Nb₅Ge₃-phase particles in the film.Supported by Deutsche Forschungsgemeinschaft.     

Spectral narrowing of self-trapped exciton emission in anatase thin films

Self-trapped excition (STE) emission of TiO₂ (anatase) thin films grown by atomic layer deposition technique on single crystal α-Al₂O₃ (0 1 2) substrates were studied at temperatures 5–120 K. An in-plane preferential orientation of anatase crystallites was detected in the films by using the dependence of the emission polarisation on the observation angle. The STE emission recorded in the direction perpendicular to the film surface had a wide spectrum with the maximum at approximately 2.3 eV. The spectrum recorded from a cleaved edge in the direction nearly parallel to the film surface, showed two sharp peaks (spectral width as low as 0.03 eV) on the top of the broad STE emission band. The beam divergence determined at the wavelengths of these peaks was 6–10°. The effect was interpreted as a constructive interference of the emission leaving the film at the angle that is close to the angle of total internal reflection.     

New intelligent multifunctional SiO2/VO2 composite films with enhanced infrared light regulation performance,solar modulation capability,and superhydrophobicity

Abstract

Highly transparent, energy-saving, and superhydrophobic nanostructured SiO₂/VO₂ composite films have been fabricated using a sol–gel method. These composite films are composed of an underlying infrared (IR)-regulating VO₂ layer and a top protective layer that consists of SiO₂ nanoparticles. Experimental results showed that the composite structure could enhance the IR light regulation performance, solar modulation capability, and hydrophobicity of the pristine VO₂ layer. The transmittance of the composite films in visible region (T_lum) was higher than 60%, which was sufficient to meet the requirements of glass lighting. Compared with pristine VO₂ films and tungsten-doped VO₂ film, the near IR control capability of the composite films was enhanced by 13.9% and 22.1%, respectively, whereas their solar modulation capability was enhanced by 10.9% and 22.9%, respectively. The water contact angles of the SiO₂/VO₂ composite films were over 150°, indicating superhydrophobicity. The transparent superhydrophobic surface exhibited a high stability toward illumination as all the films retained their initial superhydrophobicity even after exposure to 365 nm light with an intensity of 160 mW^.cm^?2 for 10 h. In addition, the films possessed anti-oxidation and anti-acid properties. These characteristics are highly advantageous for intelligent windows or solar cell applications, given that they can provide surfaces with anti-fogging, rainproofing, and self-cleaning effects. Our technique offers a simple and low-cost solution to the development of stable and visible light transparent superhydrophobic surfaces for industrial applications.