N Tzenov

Vibrational properties and structure of undoped and Al-doped ZnO films deposited by RF magnetron sputtering

Highly conductive and transparent in the visible range Al-doped ZnO (ZnO:Al) and undoped ZnO films have been deposited by RF magnetron sputtering. Reflection high-energy electron diffraction observations characterized them as textured. The habitus of the microcrystallites forming the texture depends on the Al doping. The layer texture of undoped ZnO films has texture axis parallel to the substrate. The ZnO:Al films, instead, show a columnar texture with texture axis perpendicular to the substrate. The Raman spectra of the films obtained by non-resonant excitation are completely different from those of the target material which is polycrystalline ZnO. For the interpretation of the different bands in the Raman spectra the existence of a depletion region near the grain boundaries has been assumed. The most intensive band in the Raman spectra at approximately 570 cm−1 has been assigned to electric field-induced Raman scattering on longitudinal optical phonons. The built-in electric field in the depletion region induces the Raman activity of the B2 modes and a band at 276 cm−1 appears in the spectra. Phonon modes highly localized near the grain boundaries have been detected at 516 cm−1 and 468 cm−1 which are well pronounced in the Raman spectra for the doped samples. Localized modes were observed also in the infrared reflection spectra of the doped films. Surface enhanced Raman scattering has been applied and the band in the range 830–920 cm−1 has been interpreted as due to adsorbates from the ambient air. It has been shown that the non-resonant Raman scattering can be used for qualitative study of some details of the microstructure of the zinc oxide films like the built-in electric field and the adsorbates in the films.

Preparation of thin porous silicon layers by stain etching

Abstract A method of preparation of thin (1000 A) porous silicon layers by chemical etching of c-Si in HF:HNO 3 :H 2 O=1:3:5 solution is reported wherein a thin Al film is deposited by evaporation on the silicon surface prior to etching. The method is characterized by no “incubation” period to the onset of the stain etching. The reaction between Al and HNO 3 produces the required holes for the very fast start of the chemical etching of Si. Using Al patterns deposited through a mask or prepared by photolithography it is possible to achieve selective formation of porous silicon. The porous silicon layers exhibit visible photoluminescence and when prepared on p-type c-Si and a shallow p–n c-Si homojunction the resulting heterostructures also display electroluminescence and photovoltaic properties.

Optical and electrical properties of R.F. magnetron sputtered ZnO:Al thin films

Optical and electrical properties of Al-doped ZnO films, deposited by R.F. magnetron sputtering, have been investigated as a function of preparation conditions in an attempt to develop transparent films with low electrical resistivity. The electrical resistivity as well as the Hall mobility of sputtered films depend on the R.F. power density and thickness of the films when it is less than about 300 nm. The free carrier concentration is almost independent on the film thickness and the R.F. power. The optical transmission of the films in the visible range does not depend on the thickness as well as on the R.F. power and is about 90% from the substrate transmission there. In the near-infrared, where the absorption is due to free carriers, the transmission depends strongly on the film thickness and on the preparation conditions.

Modification of the structure of ZnO:Al films by control of the plasma parameters

Abstract ZnO:Al films were deposited by RF magnetron sputtering in triode configuration applying an external DC electric field to the substrates. Reflection high-energy electron diffraction measurements characterized the different films as consisting of randomly-oriented zinc blende crystallites or randomly and texture-oriented wurtzite crystallites, as well as of the amorphous phase. The non-resonant Raman spectra are strongly influenced by the presence of a built-in electric field at the grain boundaries and they do not depend on the symmetry of the microcrystallites. The Raman spectra taken at resonant excitation are more sensitive to the presence of the amorphous phase in the films.

Electrical, photoelectrical and electroluminescent properties of porous Si–c-Si heterojunctions

Abstract Porous silicon–c-Si heterojunctions have been formed by the method of stain etching. A ZnO highly conductive thin film was used as a front transparent contact. The transport mechanism, photovoltaic and electroluminescent properties have been studied. The heterojunctions show a wide spectral response, from 400 to 1000 nm. The spectral dependence of the photocurrent in the region 400–600 nm depends on the reverse bias. Electroluminescence from the device structure in the visible region has been observed under forward bias. A model based on tunneling of minority carriers through a narrow energy barrier between ZnO and porous Si and the presence of a spike barrier and a conduction band discontinuity at the interface porous Si/c-Si is suggested for describing the properties of the heterojunctions.

Correlation between the photoluminescence and chemical bonding in porous silicon

Abstract The changes in the visible photoluminescence and Raman spectra of n-type porous silicon as a result of rapid thermal annealing and chemical treatment have been studied. Modification of the chemical bonds due to these treatments has been observed by X-ray photoemission spectroscopy. Our results show that the change in the luminescence spectra of porous silicon is related to the modification of the chemical bonds and that complexes such as the terminations in siloxene as well as silicate molecules play a role in determining the optical properties of this new photonic material. The surface containing siloxene-like bonding exhibits very intensive photoluminescence. The presence of silicates leads to a decrease in luminescence intensity and to the appearance of a weak high energy band in the photoluminescence spectra.

Surface‐enhanced Raman scattering of amorphous silicon‐carbon films

Surface‐enhanced Raman scattering has been used to study the surface of magnetron sputtered amorphous silicon‐carbon alloys applying the silver overlayer method. The presence of clusters from the sputtered material and different types of carbon–carbon bond configurations has been detected on the film surface. It has been shown that structural transformations which are not related to the hydrogen in the plasma take place on the surface.

Visible luminescence from C-containing silicon oxide films

Abstract Luminescent a-Si:O:C layers were obtained by magnetron co-sputtering and annealing. The maximum photoluminescence intensity is comparable with that of porous silicon. The luminescence maximum is between 1.8 eV and 2.2 eV. X-ray photoelectron spectroscopy finds both CSi and CC bonding and maximum luminescence is obtained when the amount of the two kinds of bonding is comparable.

Modification of magnetron sputtered a-Si1−xCx:H films by implantation of Ge+

Abstract Implantation of Ge+ ions into a-Si1−xCx:H films deposited by RF reactive magnetron sputtering of silicon and graphite, was carried out in order to obtain optical contrast in the layers. The expected optical effect which is a transmission edge shift to the lower photon energies accompanied by a decrease of the transmission coefficient was observed. This effect is more pronounced with higher implantation doses. Ion implantation of Ge+ considerably increases the absorption coefficient (α) even for the lowest dose which is of the order of 1015 cm−2. This increase is most pronounced in a narrow energy region around 2.2 eV (1.8–2.4 eV). Raman, infrared (IR) and photoelectron spectroscopy (XPS) measurements were used to study the structure and bond configurations of the implanted films. These measurements reveal that ion implantation introduces an additional disorder in the films as well as leads to a chemical modification of the films, which could be related to the changes of the optical properties.

Influence of the alloying on the vibrational properties of amorphous silicon in the frequency range 200–450 cm−1: Raman studies

Raman spectra of a large variety of amorphous silicon–carbon films prepared by magnetron sputtering at different technological conditions are studied. Great emphasis is given to the bands in the region 200–450 cm−1. The position of the well‐defined dip between the bending mode band and the bands in the central region is used as a quantitative measure of the position of the bands in the central region. The relative carbon content in the films is estimated by infrared spectroscopy. It is shown that the changes in the central region of the Raman spectrum with the carbon atoms incorporation are due to the formation of a new bond type, but not due to increased disorder. It was found that the position of the above‐defined dip could be used for comparison of the alloying atom content of nonhydrogenated amorphous silicon alloys. This is also applicable for hydrogenated samples having approximately one and the same hydrogen content.