Oxygen adsorption on Ge(111) surface: II. Alkali metal-covered surfaces

Oxygen adsorption on an alkali metal (a.m.)-covered Ge(111) surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (ELS), thermal desorption (TD), and work function measurements (WF). It was found that the presence of a.m. results in enhancement of the oxygen adsorption rate. The initial values of the sticking coefficient, S₀, are exponential functions of the work function changes caused by the a.m. adsorption. It was shown that no germanium oxide phases are formed on an alkali-covered Ge surface at 300 K. The oxidation rate at high temperatures is limited by the rearrangement processes taking place in the surface GeO layer. The results obtained show that the alkali metal perturbs the GeO bond to a certain extent but no alkali oxide formation was observed at a.m. covertages under investigation.     

Protoluminescence study of ultra-high vacuum cleaved germanium

We have studied the photoluminescence of Ge samples cleaved in ultra-high vacuum to better understand the role of intrinsic surface states in radiative and non-radiative recombination. We find that the surface states associated with the as-cleaved surface (2 × 1 reconstructed 〈111〉) are very efficient in quenching the band-to-band recombination at T ～ 90 K. The transitions through surface states are predominantly non-radiative. The adsorption of oxygen, up to a half monolayer, markedly reduces this quenching effect by removing the surface state bands. Some preliminary experimental results on the question of radiative transitions involving the surface states is presented. No luminescence unambiguously attributable to surface state transitions is observed for wavelengths shorter than 5 μm.     

Charge transfer in the atomic structure of Ge (1 0 5)

The atomic structure and charge transfer on the Ge (1 0 5) surface formed on Si substrates are studied using scanning tunneling microscopy and spectroscopy (STM and STS). The bias-dependent STM images of the whole Ge (1 0 5) facets formed on a Ge “hut” structure on Si (0 0 1) are observed, which are well explained by the recently confirmed structure model. The local surface density of states on the Ge (1 0 5) surface is measured by STS. The localization of the electronic states expected from charge transfer mechanism is observed in the dI/dV spectra. The surface band gap is estimated as 0.8-0.9 eV, which is even wider than the bulk bandgap of Ge, indicating the strong charge transfer effect to make the dangling bonds stable. The shape of normalized tunnel conductance agrees with the theoretical band structure published recently by Hashimoto et al.     

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.     

Green function theory of chemisorption on sp-hybrid crystals

A self-consistent Green function calculation of the adatom charge transfer (Δq) and the chemisorption energy (ΔE) is performed for hydrogen and the halogens on Si and Ge substrates, which are modeled by sp-hybrid orbital chains. The effect of Shockley surface states on the hydrogen chemisorption process is studied in some detail. From the chemisorption point of view, the behaviour of Si resembles that of Ge, the (111) surfaces having a smaller value of |ΔE| than the (100) ones for all the adsorbates considered. In the case of the halogens, the strong reactivity of fluorine is confirmed by the large values obtained for Δq and |ΔF|.     

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.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.     

The effect of non topological short range disorder in the density of states of germanium

We study the effect of non topological short range disorder in the density of states n(E) of germanium. The method used approximates it by the average of local densities of states of systems with a few disordered sites surrounded by an ordered, crystalline structure. Changes in bond lengths and bond angles compatible with the radial distribution function of amorphous Ge and keeping the topology of diamond give densities of states very similar to that of crystalline Ge and not to the amorphous material. We thus conclude that non topological disorder alone does not explain the n(E) of amorphous Ge.     

Adsorption of sodium and its effect upon some electric properties of clean germanium (111) surfaces

The effect of adsorbed Na on the surface conductivity, Δσ, and surface recombination velocity, S, of a clean (114)Ge surface is studied. The surface conductivity is a complicated function of the surface Na concentration, N_Na; at N_Na ≈ 1.5 × 10¹³ atoms/cm², it has a minimum; at ca. (3–5) × 10¹⁴atoms/cm², it has a maximum. For a monolayer coverage (ca. 7.2 × 10¹⁴atoms/cm²) the values of Δσ are not much different from those of a clean Ge surface. The surface recombination velocity is a three-valued function of the surface potential, U_S (calculated from the Δσ values), depending on the Na overlayer coverage and heat treatment of the sample. Three different surface structures (LEED data) were found to correspond to the three S versus U_S curves reported here. Thermal desorption studies show that Na is desorbed in a wide temperature interval. Two peaks have been isolated, studied and discussed. At low coverages a single peak is found to exist, which obeys the first-order desorption kinetics, with a desorption energy of (52 ± 3)kcal/mol. This peak is attributed to the surface defects. For coverages close to$\text{1}\text{4}$ monolayer a new peak was observed in the spectrum. The desorption energy of this binding state exceeds that of all the other states. When the overlayer coverage is increased, this peak is shifted to higher temperatures, as predicted for a half-order desorption kinetics. By comparing also with LEED data, it may be concluded that this most tightly bound sodium has formed on the Ge(111) surface patches of an ordered structure in which one Na atom is bonded to three Ge atoms.     

Structure of69Ge

The beta decay of⁶⁹As is reinvestigated. Its decay half-life is measured to be 15.1±0.3 m. Using beta, gamma and conversion electron spectroscopy techniques, a decay scheme comprising 68 transitions among 28 excited states in⁶⁹Ge is proposed. Spin-parity assignments are made to many states andB(M1) andB(E2) are deduced for some transitions. The structure thus obtained for⁶⁹Ge is compared with that from other works and discussed in the light of nuclear models.     

Surface studies by modulation spectroscopy

Surface barrier and electronic surface states are important parameters for characterizing semiconductor surfaces. Qualitative information on the surface electric field can be deduced from electroreflectance studies. Energetic positions of surface states have been determined by spectroscopic methods using effect modulation techniques. Besides the field effects of surface conductivity and absorption, which are limited to low densities of surface states, new information on surface states was gained by investigating the spectral dependence of surface photoconductivity. Also surface phonons were detected in “spectral oscillations” of photoconductivity. The measurement of surface photovoltage at photon energies where the bulk is transparent promises a new tool for surface state research in the future. To demonstrate these modulation techniques examples are given for Ge, Si, ZnO, and CdS surfaces.     

Atomic and electronic structures of thin NaCl films grown on a Ge(0 0 1) surface

Density functional theory (DFT) with LDA and GGA have been employed to study the interface and thin film properties of NaCl on a Ge(0 0 1) surface. The atomic and electronic structures of thin NaCl films from one to ten monolayers were analyzed. The layer adsorption energies show that a quasi-crystalline (0 0 1) fcc NaCl film is built up via a layer-by-layer growth mode with NaCl thickness above 2 ML. Simulated STM images show a well-resolved (1 × 1) NaCl atomic structure for sample bias voltage V_s < −2.5 V and the bright protrusions should be assigned to the Cl⁻ ions of the NaCl film. The Ge substrate dimer is reserved and buckled like a clean Ge(0 0 1)-p(2 × 2) surface as the result of weak interface interaction between the dangling bonds coming from valence π states of the Ge substrate and the 3p states of the interfacial Cl⁻ ion. These results are consistent with the experiments of STM, LEED and EELS.     

New insight into the optical properties of amorphous Ge and Si

A short range disorder model, unlike present long range disorder theories, has been able to account well for both the density of states and the optical properties of amorphous Ge and Si. Our results indicate that the imaginary part of the dielectric function for amorphous Ge and Si has the same form as an averaged gradient matrix element as a function of energy. This conclusion should be valid for all tetrahedrally bonded amorphous solids.     

X-ray photoelectron and auger spectroscopy analysis of Ge:Mn-based magnetic semiconductor layers

Thin (～60 nm) germanium layers supersaturated with a manganese impurity of 10–16 at % have been studied by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The layers have been fabricated by pulsed laser deposition onto a semi-insulating single-crystal GaAs substrate. The results of XPS analysis of the Ge:Mn layers reveal a change in the line shape of germanium and manganese (2p) in the surface region compared to deeper layers, which indicates a transition from the oxidized form of the base material (Ge²⁺ and Ge¹⁺) and impurity (Mn²⁺) near the surface to the unoxidized state of germanium (Ge ⁰) and manganese (Mn⁰) in the interior of the layer. The XPS spectra of the valence electrons of the Ge:Mn structure indicate that the density of states in the valence band of the ferromagnetic Ge:Mn structures is caused not only by mechanical mixing of germanium and manganese. The composition of heterogeneous inclusions in Ge:Mn films has been studied using scanning Auger microscopy.     

Unoccupied surface and conduction band states on Ge (111) from energy loss spectroscopy

Unoccupied surface states are observed on Ge (111) by measuring 3d-core level to conduction-band transitions using electron energy loss spectroscopy. These experiments also yield values for conduction band density-of-states features.     

Tight-binding calculations of (111) surface densities of states of Ge and GaAs

We have used the tight-binding method to calculate the local densities of states of unreconstructed Ge (111) and GaAs (111), ($\text{111}$) surfaces. In the unrelaxed surface configuration we find two types of states for each surface. The effects of relaxation on Ge surface states are also discussed.     

Kinetics of residual gas adsorption on Ge(1 1 1) surface in low-energy electron backscattering

Low-energy (0.4-1.2 eV) electron backscattering is applied for the investigation of kinetics of residual gas adsorption effect on the concentration and energy positions of surface electron states of Ge(1 1 1) surface. Chemosorption of residual gas molecules on Ge(1 1 1) at P ∼ 10⁻⁷ Pa and room temperature is shown to be most active during the first 48 h. Low concentration of dangling valence bonds on the reconstructed Ge(1 1 1) (2 × 8) surface is shown to determine its low activity to chemosorption.     

Ge/Si quantum dots in external electric and magnetic fields

Electric field-induced splitting of the lines of exciton optical transitions into two peaks is observed for Ge/Si structures with quantum dots (QDs). With increasing field, one of the peaks is displaced to higher optical transition energies (blue shift), whereas the other peack is shifted to lower energies (red shift). The results are explained in terms of the formation of electron-hole dipoles of two types differing in the direction of the dipole moment; these dipoles arise due to the localization of one electron at the apex of the Ge pyramid and of the other electron under the base of the pyramid. By using the tight-binding method, the principal values of the g factor for the hole states in Ge/Si quantum dots are determined. It is shown that the g factor is strongly anisotropic, with the anisotropy becoming smaller with decreasing QD size. The physical reason for the dependence of the g factor on quantum-dot size is the fact that the contributions from the states with different angular-momentum projections to the total wave function change with the QD size. Calculations show that, with decreasing QD size, the contribution from heavy-hole states with the angular-momentum projections ±3/2 decreases, while the contributions from light-hole states and from states of the spin-split-off band with the angular-momentum projections ±1/2 increase.     

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.