Fabrication and characteristics of porous germanium films

Abstract

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.     

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.     

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.     

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.     

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.     

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 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.     

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.     

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.     

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.     

An infrared study of the structure of GeO2-CeO2 thin films

The infrared spectra of amorphous thin films consisting of GeO₂ co-evaporated with CeO₂ are presented and interpreted in relation to the spectrum of the pure amorphous GeO₂ film. The lower frequency side of the broad absorption band within this spectrum peaking at 730 cm^–1 is believed to be due to defect centres similar to the O ₁ ^– and O ₃ ⁺ centres found in a-SiO₂. Absorption at higher frequencies within this band is due to the O stretch vibrations of the Ge-O-Ge linkage. After considering the vibrations of the O ₁ ^– and O ₃ ⁺ centres in detail, it is shown that the band at 495 cm^–1 cannot be due to either of these centres and must therefore by caused by some other reactive defect centres. The variation of the position of the 730 cm^–1 peak within the series of spectra is noted and probable explanations are offered. The optical absorption edge of a-GeO₂ thin film is compared with that of a-SiO and a possible explanation of the basic differences is proposed.     

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.     

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.     

Electron spin resonance and some electrical and optical properties of GeO2/SiO x thin films

Electron spin resonance measurements show a reduction in the spin density associated with unpaired electron dangling bonds of a mixed complex of co-evaporated GeO₂/SiO _x films compared with that of SiO _x films investigated under similar conditions. Unlike the behaviour of BaO/GeO₂ complex films, these samples show a decrease in the optical gap as the thickness increases for films of fixed compositions. However, it increases as the SiO percentage increases in films of the same thickness. D.C. measurements at low applied fields below 10⁵ V cm^–1 agree well with the spin density results.     

Observations on crystalline transformation in amorphous Ge thin films

The crystalline transformation of amorphous Ge films as a function of film thickness and environment has been studied using electron diffraction, electron microscope and electron microprobe techniques. Crystallization heat treatments were carried out in an electron diffraction unit on two series of films, one examined immediately following deposition and the other after being exposed to air at room temperature for several weeks. It was found in both series that crystallization is independent of film thickness. The transformation occurred at 375° ± 10°C over a broad range of thicknesses from 40 Å to 260 Å. On heating the films to 700°C, however, a difference in structure between the two film series was observed. Films exposed to air showed the formation of GeO₂ in addition to Ge. This difference, analyzed by transmission electron microscopy and the electron microprobe, could be attributed to a surface oxide and suggests that surface contamination does occur on aging at room temperature but does not alter the crystalline transformation of the amorphous material.     

Removal of stacking faults in Ge grown on Si through nanoscale openings in chemical SiO2

Nucleation and eventual coalescence of Ge islands, grown out of 5 to 7 nm diameter openings in chemical SiO₂ template and epitaxially registered to the underlying Si substrate, have been shown to generate a low density of threading dislocations (?10⁶ cm^− 2). This result compares favorably to a threading dislocation density exceeding 10⁸ cm^− 2 in Ge films grown directly on Si. However, the coalesced Ge film contains a relatively high density of stacking faults (5 × 10⁷ cm^− 2), and subsequent growth of GaAs leads to an adverse root-mean-square roughness of 36 nm and a reduced photoluminescence intensity at 20% compared to GaAs grown on Ge or GaAs substrates. Herein, we find that annealing the Ge islands at 1073 K for 30 min before their coalescence into a contiguous film completely removes the stacking faults. However, the anneal step undesirably desorbs any SiO₂ not covered by existing Ge islands. Further Ge growth results in a threading dislocation density of 5 × 10⁷ cm^− 2, but without any stacking faults. Threading dislocations are believed to result from the later Ge growth on the newly exposed Si where the SiO₂ has desorbed from areas uncovered by Ge islands. The morphology and photoluminescence intensity of GaAs grown on the annealed Ge is comparable to films grown on GaAs or Ge substrates. Despite this improvement, the GaAs films grown on the annealed Ge/Si exhibit a threading dislocation density of 2 × 10⁷ cm^− 2 and a minority carrier lifetime of 67 ps compared to 4 to 5 ns for GaAs on Ge or GaAs substrates. A second oxidation step after the high temperature anneal of the Ge islands is proposed to reconstitute the SiO₂ template and subsequently improve the quality of Ge film.     

Nanoscale interfacial engineering to grow Ge on Si as virtual substrates and subsequent integration of GaAs

We have demonstrated the scalability of a process previously dubbed as Ge “touchdown” on Si to substantially reduce threading dislocations below 10⁷/cm² in a Ge film grown on a 2 inch-diameter chemically oxidized Si substrate. This study also elucidates the overall mechanism of the touchdown process. The 1.4 nm thick chemical oxide is first formed by immersing Si substrates in a solution of H₂O₂ and H₂SO₄. Subsequent exposure to Ge flux creates 3 to 7 nm-diameter voids in the oxide at a density greater than 10¹¹/cm². Comparison of data taken from many previous studies and ours shows an exponential dependence between oxide thickness and inverse temperature of void formation. Additionally, exposure to a Ge or Si atom flux decreases the temperature at which voids begin to form in the oxide. These results strongly suggest that Ge actively participates in the reaction with SiO₂ in the void formation process. Once voids are created in the oxide under a Ge flux, Ge islands selectively nucleate within the void openings on the newly exposed Si. Island nucleation and growth then compete with the void growth reaction. At substrate temperatures between 823 and 1053 K, nanometer size Ge islands that nucleate within the voids continue to grow and coalesce into a continuous film over the remaining oxide. Coalescence of the Ge islands is believed to result in the creation of stacking faults in the Ge film at a density of 5 × 10⁷/cm². Additionally, coalescence results in films of 3 µm thickness having a root-mean-square roughness of 8 to 10 nm. We have found that polishing the films with dilute H₂O₂ results in roughness values below 0.5 nm. However, stacking faults originating at the Ge-SiO₂ interface and terminating at the Ge surface are polished at a slightly reduced rate, and show up as 1 to 2 nm raised lines on the polished Ge surface. These lines are then transferred into the subsequent growth morphology of GaAs deposited by metal-organic chemical vapor deposition. Room temperature photoluminescence shows that films of GaAs grown on Ge-on-oxidized Si have an intensity that is 20 to 25% compared to the intensity from GaAs grown on commercial Ge or GaAs substrates. Cathodoluminescence shows that nonradiative defects occur in the GaAs that spatially correspond to the stacking faults terminating at the Ge surface. The exact nature of these nonradiative defects in the GaAs is unknown, however, GaAs grown on annealed samples of Ge-on-oxidized Si, whereby annealing removes the stacking faults, have photoluminescence intensity that is comparable to GaAs grown on a GaAs substrate.     

Structural analysis and luminescent study of thin film zinc germanate doped with manganese

Thin films of zinc germanate doped with manganese (Zn₂GeO₄:Mn) were fabricated by radio frequency magnetron sputtering, and their structural characteristics and luminescent properties were studied. The Zn₂GeO₄:Mn films exhibited a pronounced absorption edge at around 271 nm and a high optical transparency in the visible wavelength region with a peak transmittance of 0.927 at 691 nm. While the as-deposited Zn₂GeO₄:Mn films had an amorphous structure, the annealed films possessed a rhombohedral polycrystalline structure with a random crystallographic orientation of grains. The broad-band photoluminescence (PL) emission was observed from the annealed Zn₂GeO₄:Mn films. The PL emission spectrum showed a peak maximum at around 537 nm in the green range, which was accounted for by the intrashell transition of 3d⁵ orbital electrons from the ⁴T₁ lowest excitation state to the ⁶A₁ ground state in the divalent manganese ions. Two discrete peaks were observed in the PL excitation spectrum at 256 and 296 nm, which are considered to be associated with the band-to-band absorption of the host and the sub-band absorption from defect states, respectively. The green cathodoluminescence (CL) emission was obtained from the annealed Zn₂GeO₄:Mn films with a peak centered at around 534 nm, analogous to the PL emission spectrum.