Marian Tzolov

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.

Optical and transport studies on thin microcrystalline silicon films prepared by very high frequency glow discharge for solar cell applications

The initial growth stage of phosphorus doped microcrystalline silicon films prepared by plasma enhanced chemical vapor deposition with different plasma excitation frequencies in the range 13.56–116 MHz was studied by Raman and infrared spectroscopy, optical transmission and reflection, and conductivity measurements. The sensitivity of Raman spectroscopy and optical reflection on Si crystallites in the initial growth regime is compared and optical reflection at 4.5 eV is proposed as an easy and reliable tool for this investigation. While the crystallite formation on amorphous silicon substrates at 13.56 MHz is delayed in comparison with glass, SiO2 and chromium substrates, nucleation of the crystalline phase on amorphous silicon is found to be greatly enhanced at higher plasma excitation frequencies. On the other hand, for deposition on glass, SiO2, and chromium at frequencies equal to or higher than 70 MHz, increased porosity is found in the initial growth region. The results are interpreted within a mode...

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.

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.

Optical characterization of chemically doped thin films of poly(p-phenylene vinylene)

Abstract Films of precursor route poly( p -phenylene vinylene) (PPV) have been doped by immersion in FeCl 3 solution. The conductivity changes by ca. seven orders of magnitude upon doping and the photoluminescence is quenched. Investigations by photothermal deflection spectroscopy (PDS) have shown the development of broad subbandgap absorption in the spectral range 1.5–2.2 eV and at ca. 0.6 eV upon doping. Similar absorption bands have been detected also for doping with InCl 3 . The results for undoped samples have shown that the surface of the undoped films is a source of a featureless subgap absorption. The changes in the Raman and infrared spectra with the doping have been investigated. The similarity with the results for films doped with other dopants suggests that the changes are due to transformations of the polymer chains and are independent on the nature of the dopant. It has been shown that the used doping process leads to nonuniform doping of the films. The nonuniformity depends on the film thickness and the concentration of the solution as well.

Raman scattering from monoalkali (Na-Sb and K-Sb), bialkali (Na-K-Sb) and multialkali (Na-K-Sb-Cs) photocathodes

Abstract The Raman spectra of monoalkali (Na-Sb and K-Sb), bialkali (Na-K-Sb) and multialkali (Na-K-Sb-Cs) photocathodes were measured at room temperature. Three Raman lines were detected in Na-Sb at 45 cm -1 , 128 cm -1 and 187 cm -1 . The latter line was assigned to A 1g vibrations of sodium atoms at the (f) sites of the hexagonal ( D 4 6h ) lattice of Na 3 Sb. The only Raman-active mode ( F 2g ) in cubic ( O 5 h ) K 3 Sb was found at 145 cm -1 . The Raman line at 180–185 cm -1 in the spectra of the bialkali and multialkali photocathodes was assigned to the F 2g vibration of sodium atoms at the (c) sites of the cubic Na 2 KSb lattice. It was established that in the latter type of photocathode a definite amount of the cubic K 3 Sb phase is also present.

Electron transport characteristics of the carbon nanotubes/Si heterodimensional heterostructure

The properties of nanosize heterojunctions are of increasing interest as the trend of scaling down the size of electronic devices continues. We present here the direct growth of carbon nanotubes on a silicon substrate to form a heterodimensional heterojunction. Current-voltage measurements reveal the characteristics of a Schottky diode. However, a close examination of the data suggests that the device is limited in the forward bias direction by space charge limited current. In the reverse direction, it is functionally altered by the heterodimensionality of the junction and its associated enhancement of field emission.

Growth and Structure of Microcrystalline Silicon Prepared with Glow Discharge at Various Plasma Excitation Frequencies

Microcrystalline silicon was prepared with glow discharge deposition from silane/hydrogen mixtures at plasma excitation frequencies in the range 13.56 MHz - 116 MHz. The influence of the plasma excitation frequency on the growth and the structural properties of the material is investigated. At high excitation frequencies, higher growth and etching rates, larger grain sizes with less disorder within the grains, higher crystalline volume fractions, a reduced amorphous but more porous interface layer on glass and quartz substrates, and faster nucleation on amorphous silicon substrates are obtained. The results are discussed within a schematical growth model.