Charge capture by surface states in heat-oxidized germanium

It is shown that heat-oxidized germanium surfaces are distinguished by low density of fast surface states, low surface-recombination rates, and high stability. The dependence of the charge of heat-oxidized surface on heating temperature in vacuum is interpreted on the basis of the heterojunction model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.11, pp. 68–72, November, 1975.In conclusion the authors wish to thank V. F. Kiselev and Yu. F. Novototskii-Vlasov for useful discussions and valuable remarks.     

Charging of pre-existing slow states of the real surface of epitaxial CdSe films during adsorption of acceptor molecules

The influence of chemisorption of NO₂ on the electrical conductivity of epitaxial CdSe films treated in various ways was investigated. It was found that the profile of the (t) kinetic curves of the CdSe films during chemisorption is determined by the concentration and nature of the biographical (pre-existing) slow states on their surface. It was found that the effective relaxation time _s does not depend on the thickness d of the oxide film, and is entirely determined by the nature and concentration of the biographical slow states. It is shown that chemisorption of gases can be used as a method to investigate the biographical slow states of the real surface of CdSe films.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 53–56, March, 1981.We are very grateful to Professor V. F. Kiselev and Doctor Yu. A. Zarif'yants (Moscow State University) for their constant interest in the work and discussion of the results.     

Distribution of fast and slow states by photoelectric measurements on real InSb and Si surfaces

At T = 79 K, X rays change the surface properties of n and p type InSb single cryslals as well as the properties of Si-MOS structures. In order to study the influence of surface parameters the distributions of fast and slow surface states were measured at various stages of irradiation. The distribution of fast states on real InSb surfaces has been investigated by techniques similiar to those employed in measurements of current voltage characteristics. By using a combination of integral ac field effect with differential ac field effect the continuous spectrum of fast states was studied not only within the depletion region but also under strong accumulation and inversion. Analysis of photodischarging spectroscopy gives information on the energetic location of slow surface states on real Si surfaces.     

Optical charging of real surface of germanium over a wide range of temperatures

A study is made of the charging of a real germanium surface under the effect of light. Mechanisms of electron and hole transport from germanium to the oxide layer are discussed. Conclusions are drawn as to the nature of the deep electron and hole traps in the oxide.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 36–41, May, 1979.In conclusion the authors wish to thank V. F. Kiselev for his discussion of the paper and his useful comments.     

Laser excitation of surface electronic states for a one-dimensional semiconductor

The electronic band structure of a finite one-dimensional semiconductor is examined. The surface states are identified and the absorption cross-section for transitions to these states is calculated. The use of a laser to enhance surface charge density is discussed.     

Model studies of the Tamm-like and field-sustained surface states of germanium

The effects of varying the intensity of an applied positive electric field on the Tamm-like and field-sustained surface states of a semi-infinite 6-well model of germanium are studied. The effects of a neon image gas atom on such states are also examined. It is found that there are ranges of the field intensity for which field-sustained surface states do not exist. The localization properties of electrons in the various surface states are discussed with the help of relative probability density functions. As is well-known, electrons in Tamm-like states in the first forbidden energy gap tend to be strongly localized near the surface and inside the crystal. However those in Tamm-like states in higher gaps do not exhibit any pronounced localization. Electrons in field-sustained surface states tend to be strongly localized outside the crystal.     

The influence of different forms of surface oxide on the fast states of germanium

The alternating field effect was used to study the kinetics of the influence of water molecules on the fastN _t centres on the surface of germanium. It was observed that the time dependence in the adsorption of water and in the change in concentration of the fast centres caused by this adsorbed water differs, indicating that the water diffuses through the oxide to the fast centres with a diffusion constantD approximately equal to 10^–15-10^–14 cm²/sec. The magnitude of the diffusion constantD constantly increased after annealing the sample in a wet atmosphere, and temporarily decreased after annealing in a dry atmosphere. The influence of annealing on the concentration of the fast states and the penetrability of the water through the oxide layer of germanium was also studied.The author thanks S. Koc, C.Sc., for advice and the interest he showed in the work.     

一种锗晶体光码盘的分划制作工艺研究

为了在锗晶体光坯上制作出满足要求的码盘图案,分别采用正性光刻胶和负性光刻胶,先镀膜后照相和先照相后镀膜的工艺路线进行工艺实验,并对实验结果进行分析,得出结论:用负性光刻胶按先照相后镀膜的工艺路线为锗晶体光码盘分划制作工艺的最佳方案。实验结果表明:加工出的图案明暗对比度大、边缘不均匀性小于0.008 mm,可实际应用于锗晶体上分划图案的制作。     

Photoemission yield spectroscopy of electronic surface states on germanium (111) surfaces

The electronic surface states of cleaved and annealed Ge(111) surfaces have been investigated by photoemission yield spectroscopy and contact potential measurements on a set of differently doped samples. On the 2 × 1 cleaved surface, a surface state band centered about 0.7 eV below the valence band maximum is found. The variations of the work function with the doping level show that an empty surface state band exists above the Fermi level. After annealing at temperatures of the order of 350°C, this surface exhibits a 2 × 8 superstructure. A new surface state band is then found closer to the valence band maximum. This variation of the surface state distribution is correlated to a change in the surface potential. The variation of the electronic characteristics upon oxygen adsorption are also reported and an evaluation of the sticking coefficient is made for both structures.     

Field ion microscopy of germanium: Field ionization and surface states

Field evaporated Ge tips were imaged with Ar and Ne at 80° K. The ions of these gases display identical field ion images. A striking correlation between the regional brightness of FIM and FEM patterns was found. Electron shadow microscopy and the behavior of FIM images above BIV indicate no local field enhancements, thus this correlation must be explained by the electronic structure of the surface. Explanation is furnished by a model, where electrons of the image gas tunnel preferentially into surface states. This model, based on the known fact that surface states of Ge contribute in field emission, is supported by the observation that in regions of low index poles, oxygen adsorption decreases the surface density of centers above which field ionization occurs.     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

Investigation of oxygen states and reactivities on a nanostructured cupric oxide surface

Nanostructured copper (II) oxide was formed on clean copper foil at room temperature using activated oxygen produced by RF discharge. CuO particles of approximately 10-20 nm were observed on the surface by Scanning Tunneling Microscopy (STM). The copper states and oxygen species of the model cupric oxide were studied by means of X-ray Photoelectron Spectroscopy (XPS). These oxide particles demonstrated abnormally high reactivity with carbon monoxide (CO) at temperatures below 100 °C. The XPS data showed that the interaction of CO with the nanostructured cupric oxide resulted in reduction of the CuO particles to Cu₂O species. The reactivity of the nanostructured cupric oxide to CO was studied at 80 °C using XPS in step-by-step mode. The initial reactivity was estimated to be 5 × 10⁻⁵ and was steadily reduced down to 5 × 10⁻⁹ as the exposure was increased. O1s spectral analysis allowed us to propose that the high initial reactivity was caused by the presence of non-lattice oxygen states on the surface of the nanostructured CuO. We established that reoxidation of the partially reduced nanostructured cupric oxide by molecular oxygen O₂ restored the highly reactive oxygen form on the surface. These results allowed us to propose that the nanostructured cupric oxide could be used for low temperature catalytic CO oxidation. Some hypotheses concerning the nature of the non-lattice oxygen species with high reactivity are also discussed.