Interactions of germanium atoms with silica surfaces

GeH₄ is thermally cracked over a hot filament depositing 0.7-15 ML Ge onto 2-7 nm SiO₂/Si(1 0 0) at substrate temperatures of 300-970 K. Ge bonding changes are analyzed during annealing with X-ray photoelectron spectroscopy. Ge, GeH_x, GeO, and GeO₂ desorption is monitored through temperature programmed desorption in the temperature range 300-1000 K. Low temperature desorption features are attributed to GeO and GeH₄. No GeO₂ desorption is observed, but GeO₂ decomposition to Ge through high temperature pathways is seen above 750 K. Germanium oxidization results from Ge etching of the oxide substrate. With these results, explanations for the failure of conventional chemical vapor deposition to produce Ge nanocrystals on SiO₂ surfaces are proposed.     

Oxidation mechanism of hydrogen-terminated Ge(1 0 0) surface

Control of the surface chemistry to prepare a robust termination on the Ge surface is crucial for the development of high-end Ge devices. In this study, oxidation of a H-terminated Ge surface was studied in air ambient and H₂O using a multiple internal reflection Fourier transform infrared spectroscopy (MIR FT-IR) technique. Ge surface treated in less diluted HF exhibited a stronger Ge-H peak intensity, and the surface was easily oxidized in the air ambient. Therefore, it is believed that the treatment of the Ge surface in highly diluted HF solution has an advantage in suppressing the oxidation of Ge in the air ambient. For the oxidation of Ge(1 0 0) surface in air ambient, the Ge surface is attacked by oxidizing agents to break Ge-H and Ge-Ge bonds, and the transition GeO_x layer is first formed, followed by a layer-by-layer GeO₂ formation with the increase in exposure time. When the H-terminated Ge surface was treated in H₂O, GeO_x was mainly formed, the thickness of the oxide layer was not changed with an increase in treatment time, and the Ge surface was maintained in a suboxide state, which exhibits a different oxidation mechanism from that in air ambient.     

Characteristics improvement of HfO2/Ge gate stack structure by fluorine treatment of germanium surface

Chemical reactivity of fluorine molecule (F₂)-germanium (Ge) surface and dissociation of fluorine (F)-Ge bonding have been simulated by semi-empirical molecular orbital method theoretically, which shows that F on Ge surface is more stable compared to hydrogen. Ge MIS (metal insulator semiconductor) capacitor has been fabricated by using F₂-treated Ge(1 0 0) substrate and HfO₂ film deposited by photo-assisted MOCVD. Interface state density observed as a hump in the C-V curve of HfO₂/Ge gate stack and its C-V hysteresis were decreased by F₂-treatment of Ge surface. XPS (X-ray photoelectron spectroscopy) depth profiling reveals that interfacial layer between HfO₂ and Ge is sub-oxide layer (GeO_x or HfGeO_x), which is believed to be origin of interface state density.F was incorporated into interfacial layer easily by using F₂-treated Ge substrate. These results suggest that interface defect of HfO₂/Ge gate stack structure could be passivated by F effectively.     

Evidence of GeO volatilization and its effect on the characteristics of HfO2 grown on Ge substrate

HfO₂ films are deposited by atomic layer deposition (ALD) using tetrakis ethylmethylamino hafnium (TEMAH) as the hafnium precursor, while O₃ or H₂O is used as the oxygen precursor. After annealing at 500℃ in nitrogen, the thickness of Ge oxide's interfacial layer decreases, and the presence of GeO is observed at the H₂O-based HfO₂ interface due to GeO volatilization, while it is not observed for the O₃-based HfO₂. The difference is attributed to the residue hydroxyl groups or H₂O molecules in H₂O-based HfO₂ hydrolyzing GeO₂ and forming GeO, whereas GeO is only formed by the typical reaction mechanism between GeO₂ and the Ge substrate for O₃-based HfO₂ after annealing. The volatilization of GeO deteriorates the characteristics of the high-κ films after annealing, which has effects on the variation of valence band offset and the C–V characteristics of HfO₂/Ge after annealing. The results are confirmed by X-ray photoelectron spectroscopy (XPS) and electrical measurements.     

Structure and effects of annealing in colloidal matrix‐free Ge quantum dots

The structure of small (2–5 nm) Ge quantum dots prepared by the colloidal synthesis route is examined. Samples were synthesized using either GeO₂ or GeCl₄ as precursor. As‐prepared samples were further annealed under Ar or H₂/Ar atmosphere at different temperatures in order to understand the effect of annealing on their structure. It was found that as‐prepared samples possess distinctly different structures depending on their synthesis route as indicated by their long‐range ordering. An appreciable amount of oxygen was found to be bound to Ge in samples prepared with GeO₂ as a precursor; however, not for GeCl₄. Based on combined transmission electron microscope, Raman, X‐ray diffraction and X‐ray absorption measurements, it is suggested that as‐prepared samples are best described by the core‐shell model with a small nano‐crystalline core and an amorphous outer layer terminated either with oxygen or hydrogen depending on the synthesis route. Annealing in an H₂Ar atmosphere leads to sample crystallization and further nanoparticle growth, while at the same time reducing the Ge—O bonding. X‐ray diffraction measurements for as‐prepared and annealed samples indicate that diamond‐type and metastable phases are present.     

Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level E_F and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C₆₀) and fullerene (C₆₀) films on the surface of the real germanium oxide ((GeO₂)Ge) have been presented. The content of the oxide on the (GeO₂)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C₆₀ and C₆₀ coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C₆₀ and APP-C₆₀ films has made it possible to reveal the energy range (6–10 eV above the Fermi level E_F) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C₆₀ and unsubstituted C₆₀ films. The formation of the interfacial potential barrier upon deposition of APP-C₆₀ and C₆₀ on the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface E_vac–E_F by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C₆₀ and C₆₀, the work function of the surface changes only slightly.     

磁控共溅射法制备的Zn2GeO4多晶薄膜结构及其光致发光研究

用射频磁控共溅射方法在不同温度的单晶硅基片上生长薄膜,然后在800℃真空环境下对薄膜进行退火处理,成功获得了结晶状态良好的Zn₂GeO₄多晶薄膜.利用X射线衍射(XRD),X射线光电子能谱(XPS)和原子力显微镜(AFM)对薄膜进行了结构、成分和形貌分析,研究了基片温度对三者的影响. 结果显示,当基片温度升高到400℃以上时,薄膜中的Zn₂GeO₄晶粒在(220)方向上显示出了明显的择优取向. 当基片温度在500—600℃范围内,有利于GeO₂结晶相的形成. XPS显示薄膜中存在着Zn₂GeO₄,GeO₂,GeO,ZnO四种化合态. 同时,随着基片温度的升高,晶粒尺寸增大且薄膜表面趋于平整. 薄膜的光致发光在绿光带存在中心波长为530和550nm两个峰,应该归因于主体材料Zn₂GeO₄中两个不同的Ge²⁺的发光中心. 关键词： 射频磁控溅射 2GeO₄')" href="#">Zn₂GeO₄ 荧光体     

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E_F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO₂)Ge). The concentration of oxide on the (GeO₂)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO₂)Ge surface to 6 nm was studied. At energies below 9 eV above E_F, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface, E_vac–E_F, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.     

On the origin of the 2.2-2.3 eV photoluminescence from chemically etched germanium

The photoluminescence (PL) at ∼2.2-2.3 eV from Ge-based nanocrystalline materials is described in the literature as nanocrystal size-independent. We have observed visible luminescence from two different types of stain-etched Ge samples, one prepared after Sendova-Vassileva et al. (Thin Solid Films 255 (1995) 282) in a solution of H₂O₂:HF at 50:1 volume ratio, and the other in a solution of HF:H₃PO₄:H₂O₂ at 34:17:1 volume ratio. Energy dispersive X-ray analysis (EDX), Raman and FTIR spectroscopy, and the near edge X-ray absorption structure (XANES), indicate that the chemically etched Ge layers of the former type of samples are composed of non-stoichometric Ge oxides, i.e. GeO_x (0<x<2), and free from any Ge nanoconstructions. It is also suggested from XANES that the latter type of chemically etched Ge samples comprise 8-9 nm nanocrystals of Ge, surface-covered with mainly oxygen. Photoluminescence occurred at ∼2.3 eV for all samples. The PL behavior of the latter type of chemically etched Ge on annealing in different chemical environments (air or H) allowed us to conclude that the PL from these materials, as well as that from those Ge-based nanocrystalline materials reported in the literature, is from GeO_xs.     

Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films

We have investigated cathodeluminescence (CL) of Ge implanted SiO₂:Ge and GeO₂:Ge films. The GeO₂ films were grown by oxidation of Ge substrate at 550 °C for 3 h in O₂ gas flow. The GeO₂ films on Ge substrate and SiO₂ films on Si substrate were implanted with Ge-negative ions. The implanted Ge atom concentrations in the films were ranging from 0.1 to 6.0 at%. To produce Ge nanoparticles the SiO₂:Ge films were thermally annealed at various temperatures of 600-900 °C for 1 h in N₂ gas flow. An XPS analysis has shown that the implanted Ge atoms were partly oxidized. CL was observed at wavelengths around 400 nm from the GeO₂ films before and after Ge⁻-implantation as well as from SiO₂:Ge films. After Ge⁻-implantation of about 0.5 at% the CL intensity has increased by about four times. However, the CL intensity from the GeO₂:Ge films was several orders of magnitude smaller than the intensity from the 800 °C-annealed SiO₂:Ge films with 0.5 at% of Ge atomic concentration. These results suggested that the luminescence was generated due to oxidation of Ge nanoparticles in the SiO₂:Ge films.     

Catalyst-free synthesis of GeO2 nanowires using the thermal heating of Ge powders

Germanium dioxide (GeO₂) nanowires have been synthesized by means of the simple evaporation of solid Ge powders, without using metal catalysts. The nanowires, with a diameter of about 90–200 nm, were characterized using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The obtained GeO₂ nanowires were crystalline with a hexagonal structure. The growth mechanism was discussed with respect to the vapor–solid process. The photoluminescence measurement revealed two emission peaks at about 2.45 eV and 2.91 eV at room temperature, opening up a route to potential applications in future optoelectronic nanodevices. Raman measurement of as-synthesized GeO₂ nanowires was made at room temperature.     

Physico-chemical and electrical properties of rapid thermal oxides on Ge-rich SiGe heterolayers

Rapid thermal oxidation of high-Ge content (Ge-rich) Si_1−xGe_x (x = 0.85) layers in dry O₂ ambient has been investigated. High-resolution X-ray diffraction (HRXRD) and strain-sensitive two-dimensional reciprocal space mapping X-ray diffractometry (2D-RSM) are employed to investigate strain relaxation and composition of as-grown SiGe alloy layers. Characterizations of ultra thin oxides (∼6-8 nm) have been performed using Fourier transform infrared spectroscopy (FTIR) and high-resolution X-ray photoelectron spectroscopy (HRXPS). Formation of mixed oxide i.e., (SiO₂ + GeO₂) and pile-up of Ge at the oxide/Si_1−xGe_x interface have been observed. Enhancement in Ge segregation and reduction of oxide thickness with increasing oxidation temperature are reported. Interface properties and leakage current behavior of the rapid thermal oxides have been studied by capacitance-voltage (C-V) and current-voltage (J-V) techniques using metal-oxide-semiconductor capacitor (MOSCAP) structures and the results are reported.     

Optical and structural properties of Ge-ion-implanted fused silica after annealing in different ambient conditions

Ge+ ions are implanted into fused silica glass at room temperature and a fluence of 1×10 17 cm-2 . The as-implanted samples are annealed in O2, N2 and Ar atmospheres separately. Ge0 , GeO and GeO2 coexist in the as-implanted and annealed samples. Annealing in different atmospheres at 600℃ leads each composite to change its content. After annealing at 1000℃, there remains some amount of Ge 0 in the substrates. However, the content of Ge decreases due to out-diffusion. After annealing in N2 , Si–N composite is formed. The absorption peak of GeO appears at 240 nm after annealing in O2 atmosphere, and a new absorption peak occurs at 418 nm after annealing in N2 atmosphere, which is attributed to the Si–N composite. There is no absorption peak appearing after annealing in Ar atmosphere. Transmission electron microscopic images confirm the formation of Ge nanoparticles in the as-implanted sample and GeO 2 nanoparticles in the annealed sample. In the present study, the GeO content and the GeO2 content depend on annealing temperature and atmosphere. Three photoluminescence emission band peaks at 290, 385 and 415 nm appear after ion implantation and they become strong with the increase of annealing temperature below 700℃, and their photoluminescences recover to the values of as-grown samples after annealing at 700℃. Optical absorption and photoluminescence depend on the annealing temperature and atmosphere.     

Quasi-direct optical transitions in Ge nanocrystals embedded in GeO<Subscript>2</Subscript> matrix

GeO₂ films with germanium nanocrystals (NCs) were deposited from supersaturated GeO vapor with subsequent dissociation on Ge:GeO₂. The films were studied using photoluminescence (PL), Raman scattering, IRspectroscopy techniques. Ge NCs in initial film have sizes about 6–8 nm and have no visible PL signal. The broad green-red PL peak was detected in Ge:GeO₂ films after thermal annealings. According to effective mass approach, maximum of PL signal from such relatively big Ge NCs should be in IR region. The experimentally observed PL signal is presumably originated due to quasi-direct L ₁-L ₃’ optical transitions “folded” in germanium NCs. The article is published in the original.     

First principles study of electronic and structural properties of the Ge/GeO2 interface

The electronic and structural properties of the Ge/GeO₂ interface are addressed through a density functional simulation scheme which includes the use of hybrid functionals for achieving accurate band gaps, band offsets and defect levels. The present work discusses the germanium dangling bond levels, the thermodynamics of GeO_x, the stability of the oxygen vacancy across Ge/HfO₂ interfaces, the atomic structure of GeO_x, electron and hole trapping in GeO_x, and the band alignment at the Ge/GeO₂ interface.     

Ellipsometry of GeO<Subscript>2</Subscript> films with Ge nanoclusters: Influence of the quantum-size effect on refractive index

Using the methods of scanning and spectral laser ellipsometry and Raman scattering spectroscopy, GeO₂ films containing Ge nanoclusters with a Ge/GeO₂ mole ratio of 1: 1 are studied. A substantial difference is found between the experimental spectral dependence of the complex permittivity of the films and the one calculated for the effective medium in the Bruggeman model. The distinction can be qualitatively explained by the influence of the quantum-size effect. With the use of theoretical models for quantitative analysis, this approach will make it possible to determine the phase composition and dimensions of the nanoclusters of germanium in a contactless way without destructing the film.     

Effect of chemical surface treatments on interfacial and electrical characteristics of atomic-layer-deposited Al2O3 films on Ge substrates

The Ge surfaces were cleaned and passivated by two kinds of chemical pretreatments: conventional combination of HF + (NH₄)₂S, and new one of HBr + (NH₄)₂S. The chemical states and stability at passivated Ge surfaces were carefully characterized. The influence of chemical surface treatments on the interface and electrical properties of Al₂O₃ gate dielectric films on Ge grown by atomic layer deposition (ALD) has been investigated deeply. It is found that the combination of HBr and (NH₄)₂S can remove more Ge-O bonds on the Ge surface compared to that of conventional HF and (NH₄)₂S with excellent stability. X-ray photoelectron spectroscopy (XPS) reveals that HBr and (NH₄)₂S treated Ge surface has a mixture states of GeO_x (9.25%) and GeS (7.40%) while HF and (NH₄)₂S treated Ge surface has a mixture states of GeO_x (16.45%) and GeS (3.37%). And the Ge-S peak from the surface of Ge substrates decreases a little after the HBr and (NH₄)₂S treated Ge surface was exposed in the ambient for 300 min, which suggests the Ge surface is stable to oxidants. The Al₂O₃ films on HBr and (NH₄)₂S treated Ge substrates exhibits better electrical properties such as large capacitance, decreased leakage current density by ∼two orders of magnitude, and less C-V hysteresis. This indicates that a reduction in charge traps possibly at the interface and more interface traps are terminated by sulfur. The surface treatment of HBr and (NH₄)₂S seems to be very promising in improving the quality of high-k gate stack on Ge substrates.     

Transitions of microstructure and photoluminescence properties of the Ge/ZnO multilayer films in certain annealing temperature region

The Ge/ZnO multilayer films have been prepared by rf magnetron sputtering. The effects of annealing on the microstructure and photoluminescence properties of the multilayers have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectrometry and photoluminescence (PL) spectrometry. The investigation of structural properties indicates that Zn₂GeO₄ has been formed with (2 2 0) texture and Zn deficiency from Ge/ZnO multilayer films in the process of annealing. However, lower Zn/Ge ratio can be improved by annealing. The annealed multilayers show three main emission bands at 532, 700, and 761 nm, which originate from the transition between oxygen vacancy () and Zn vacancies (V_Zn), the radiative recombination of quantum-confined excitons (QCE) in Ge nanocrystals, and the optical transition in the GeO color centers, respectively. Finally, the fabrication of thin film Zn₂GeO₄ from Ge/ZnO multilayer films by annealing at low temperature provides another approach to prepare the green-emitting oxide phosphor film:Zn₂GeO₄:Mn.     

Coordination changes in crystalline and vitreous GeO2

Abstract

High pressure x ray absorption spectroscopy (XAS) has been performed up to 29 GPa on crystalline and amorphous GeO₂. The modification of the x ray absorption near edge structure (XANES) as well as the variation of the Ge-O distance, indicate that the coordination of Ge changes from 4 to 6 above 6.5 GPa. The transition is confirmed by Raman spectroscopy.     

First-principles study of transport properties of endohedral Li@C20 metallofullerene

The transport properties of the endohedral Li@C₂₀ metallofullerene are studied using density functional non-equilibrium Green’s function method. The equilibrium conductance of Li@C₂₀ metallofullerene becomes larger than that of the empty C₂₀ fullerene molecule. The I–V curve under low-bias voltage shows the characteristic of metallic behavior; another, the novel negative differential resistance behavior is also observed. It is found that the doping effect of Li atom significantly changes the transport properties of C₂₀ fullerene.