Krasimira Shtereva | University of Ruse (original) (raw)

Papers by Krasimira Shtereva

2004 24th International Conference on Microelectronics (IEEE Cat. No.04TH8716), 2004

Thin film palladium (Pd) and indium tin oxide (ITO) microelectrodes modify by mercury (Hg) were d... more Thin film palladium (Pd) and indium tin oxide (ITO) microelectrodes modify by mercury (Hg) were developed for heavy metal analysis by anodic stripping voltammetry. In case of Pb2+ determination the sensitivity of ITO-Hg array of disc electrodes was higher (130 nA/μM) in comparison with Pd-Hg interdigitated array of electrodes (60 nA/μM).

2008 26th International Conference on Microelectronics, 2008

Aluminum - doped ZnO thin films (ZnO:Al) and Al - N co-doped (ZnOALN) thin films were prepared by... more Aluminum - doped ZnO thin films (ZnO:Al) and Al - N co-doped (ZnOALN) thin films were prepared by RF diode sputtering from a ceramic target (ZnO+2 wt. % AI2O3) in Ar and Ar/25divide75 %N2 atmosphere. The ZnOAl films exhibited n-type conduction with a minimum resistivity (2times10-3 Omegacm), a high visible transmittance (&amp;amp;amp;amp;amp;gt; 82 %, including Corning glass substrate) and an

Micro Electronic and Mechanical Systems, 2009

ECS Transactions, 2009

Nitrogen-doped zinc oxide thin films (ZnO:N), and aluminumnitrogen co-doped zinc oxide thin films... more Nitrogen-doped zinc oxide thin films (ZnO:N), and aluminumnitrogen co-doped zinc oxide thin films (ZnO:Al:N), were deposited on Corning glass substrates by RF diode sputtering. The nitrogen (N 2 ) content in the working gas varied from 0 to 100 %. XRD patterns revealed improving of the crystalline structure and stronger expressed c-axis preferential orientation of the aluminumnitrogen co-doped films compared to nitrogen-doped films. The estimated crystallite size varied from 140 to 8 nm, depending on the N 2 content. The transmittance spectra of ZnO:N and ZnO:Al:N films were acquired over the wavelength range of 200 < < 1000 nm. All nitrogen-doped and Al-N 2 co-doped films were colored as a result of the increase absorption near the band edge. Optical band gap (E g ) of the ZnO:N and ZnO:Al:N thin films decreases with increase of the N 2 content in the working gas.

2006 International Conference on Advanced Semiconductor Devices and Microsystems, 2006

A p-type ZnO thin film was prepared by RF diode sputtering and nitrogen doping. Deposition in pla... more A p-type ZnO thin film was prepared by RF diode sputtering and nitrogen doping. Deposition in plasma N2 gas source increases the N solubility and thus the incorporation of No acceptor that is responsible for p-type conductivity of the ZnO films. Raman analyses performed in back scattering configuration proved the incorporation of the nitrogen acceptor No into ZnO:N. Raman spectra

Vacuum, 2005

We have developed a mercury-plated microelectrode array based on Indium Tin Oxide (ITO) films (60... more We have developed a mercury-plated microelectrode array based on Indium Tin Oxide (ITO) films (600 nm) prepared by rf reactive sputtering on a silicon substrate. Microelectrode behaviour of the disc arrays has been characterised by cyclic voltammetry (CV). Electroplating of mercury onto the ITO microelectrode array was carried out in a solution of Hg(NO 3 ) 2 and KNO 3 by applying a potential of À1500 mV vs. Ag/AgCl/Cl À for varying charge (350-1050 mC). Mercury plating and Pb 2+ pre-concentration conditions were optimised for ex situ determination of Pb 2+ in aqueous solution by anodic stripping voltammetry (ASV). The sensitivity of ITO-Hg microelectrode array to Pb 2+ was 130 nA(mol/l) À1 .

Journal of Physics: Conference Series, 2008

High transparent and conductive, aluminium - doped zinc oxide thin films (ZnO:Al), were prepared ... more High transparent and conductive, aluminium - doped zinc oxide thin films (ZnO:Al), were prepared by radio - frequency (RF) diode sputtering from ZnO+2 wt. % Al2O3 target on Corning glass 7059 substrates. The RF power and the substrate temperature modified their structure and surface morphology, electrical and optical parameters. The XRD patterns reveal only one sharp (002) diffraction line, providing

2013 8TH INTERNATIONAL SYMPOSIUM ON ADVANCED TOPICS IN ELECTRICAL ENGINEERING (ATEE), 2013

ABSTRACT This paper presents the efforts to introduce integrated circuit (IC) design to Electroni... more ABSTRACT This paper presents the efforts to introduce integrated circuit (IC) design to Electronics engineering undergraduate students, using the LASI program. The professional Computer-aided Design (CAD) systems for IC design are expensive in terms of hardware, CAD tools and CAD licenses. The next problem connected to their use for educational purposes is the need of training on both the CAD software and the UNIX system. The Windows LASI system is a freeware CAD environment for a full-custom Very Large Scale Integration (VLSI) microelectronic circuits design, intended for educational purposes. The LASI tool is employed for teaching IC design in Electronics engineering undergraduate program at the Department of Electronics, Ruse University, Bulgaria. The paper provides description of the Microelectronics courses, LASI system, technology, and an example design.

ECS Journal of Solid State Science and Technology, 2012

ABSTRACT The electrical characteristics of gallium-doped zinc oxide (ZnO:Ga) thin films prepared ... more ABSTRACT The electrical characteristics of gallium-doped zinc oxide (ZnO:Ga) thin films prepared by rf diode sputtering were altered via nitrogen implantation by performing two implants at 40 keV and 80 keV with doses of 1 x 10(15) and 1 x 10(16) cm(-2) to achieve a p-type semiconductor. An implantation of 1 x 10(15) cm(-2) N+-ions yielded a p-type with hole concentrations 10(17)-10(18) cm(-3) in some as-implanted samples. The films annealed at temperatures above 200 degrees C in O-2 and above 400 degrees C in N-2 were n-type with electron concentrations 10(17)-10(20) cm(-3). The higher nitrogen concentration (confirmed by SRIM and SIMS), in the films implanted with a 1 x 10(16) cm(-2) dose, resulted in lower electron concentrations, respectively, higher resistivity, due to compensation of donors by nitrogen acceptors. The electron concentrations ratio n((1) (x) (1015))/ n((1 x 1016)) decreases with increasing annealing temperature. Hall measurements showed that 1 x 10(16) cm(-2) N-implanted films became p-type after low temperature annealing in O-2 at 200 degrees C and in N-2 at 400 degrees C with hole concentrations of 3.2 x 10(17) cm(-3) and 1.6 x 10(19) cm(-3), respectively. Nitrogen-implanted ZnO:Ga films showed a c-axes preferred orientation of the crystallites. Annealing is shown to increase the average transmittance (>80%) of the films and to cause bandgap widening (3.19-3.3 eV).

2006 25th International Conference on Microelectronics, 2006

Nitrogen doped zinc oxide (ZnO:N) thin films were prepared by RF diode sputtering from ZnO target... more Nitrogen doped zinc oxide (ZnO:N) thin films were prepared by RF diode sputtering from ZnO target in different ratio of Ar/N2 gas mixture. The p-type features of ZnO:N thin films have been caused by the incorporation of the nitrogen acceptor NO into ZnO what was proven by second ion mass spectroscopy (SIMS) analysis. The minimum value of resistivity of 790

Vacuum, 2012

ABSTRACT We investigated the possibility of achieving p-type zinc oxide (ZnO) by RF diode sputter... more ABSTRACT We investigated the possibility of achieving p-type zinc oxide (ZnO) by RF diode sputtering and gallium–nitrogen co-doping. ZnO:Ga:N thin films were prepared with a different N2 content in Ar/N2 working gas, ranging from 0 to 100%, and at a varying substrate temperature, from room temperature (RT) to 300 °C. A hole conduction with maximum carrier concentration of 2.6 × 1018 cm−3, mobility of 2 cm2/Vs and resistivity of 1.5 Ω cm resulted from deposition at RT with 100% N2. It arose from N incorporation and formation of NO acceptors. In the secondary ion mass spectrometry (SIMS) depth profiles of the co-doped films were observed NO/NO2 negative ions. Average transmittance (including Corning glass substrate) across the visible spectrum varied (60 ÷ 66%) with increasing nitrogen content (50 ÷ 100% N2). As the substrate temperature increased (RT – 300 °C), highly transparent (T ∼72–83%) and conductive (electron concentrations of 1017–1019 cm−3) n-type ZnO:Ga:N films were attained. Reduction of optical band gap (Eg) (∼3.13–3.08 eV) was observed for co-doped ZnO films. Atomic force microscopy (AFM) images revealed that the films grown at RT have roughness of approximately 5.3 nm while roughness of those grown at 300 °C is approximately 3.9 nm.

Vacuum, 2007

We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide ... more We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide (ZnO) target and the crystalline, electrical and optical properties of n-/p-type ZnO thin films. The properties of the ZnO thin films depended on RF power, substrate temperature and, particularly, on working gas mixtures of Ar/O 2 and of Ar/N 2 . Sputtering in Ar+O 2 working gas (up to 75% of O 2 ) improved the structure of an n-type ZnO thin film, from fibrous ZnO grains to columnar crystallites, both preferentially oriented along the c-axis normally to the substrate (/0 0 2S direction). These films had good piezoelectric properties but also high resistivity (rE10 3 O cm). ZnO:N p-type films exhibited nanograin structure with preferential /0 0 2S orientation at 25% N 2 and /1 0 0S orientation for higher N 2 content. The presence of nitrogen N O at O-sites forming N O -O acceptor complexes in ZnO was proven by SIMS and Raman spectroscopy. A minimum value of resistivity of 790 O cm, a p-type carrier concentration of 3.6 Â 10 14 cm À3 and a Hall mobility of 22 cm 2 V À1 s À1 were obtained at 75% N 2 .

Thin Solid Films, 2007

ABSTRACT The relations between the sputtering parameters and the crystalline microstructure of Zn... more ABSTRACT The relations between the sputtering parameters and the crystalline microstructure of ZnO thin films are presented. The energetic bombardment of substrate by neutral atoms, ions and electrons during sputtering is characterized by total energy flux density which affects the film. This parameter can be estimated by RF power, substrate bias voltage and concentration of reactive gases. Substrate temperature and total energy flux density are the major parameters which have a significant influence on ZnO thin film crystalline structure.

IOP Conference Series: Materials Science and Engineering, 2012

Acceptor doping of zinc oxide thin films prepared by radio-frequency diode sputtering, using nitr... more Acceptor doping of zinc oxide thin films prepared by radio-frequency diode sputtering, using nitrogen as a doping source, was realized during sputtering in Ar/N 2 gas mixture or via ion implantation of 180 keV N ions. Effects of N doping and (Al,Ga):N codoping on the films' properties have been studied. Highly resistive ZnO:N films with p-type carrier concentrations ranging 10 14 ÷10 15 cm -3 and more random orientation of the crystallites were obtained. The co-doped ZnO:Al:N and ZnO:Ga:N films maintained a c-axis texture in the direction of the surface normal. The carrier concentration of the p-type films was on the order of 10 17 ÷10 18 cm -3 . 180 keV N ions were implanted in sputtered ZnO:Ga thin films. The implant doses varied 10 15 ÷2x10 16 cm -2 . Annealing studies were performed under O 2 and N 2 ambient at different temperatures and times. N-implanted films were polycrystalline with a preferred caxis orientation of the crystallites. Post-implantation annealing caused the increase of the (002) diffraction line integrated area and reduction in resistivity and the biaxial stress. After annealing in N 2 , a p-type conduction was observed with a carrier concentration and average transmittance on the order of 2.8x10 19 cm -3 and 81 % respectively.

Applied Surface Science, 2008

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode spu... more Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential h0 0 1i orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400-800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing. #

Here, we report on experimental studies of the role of doping and co-doping on the properties of ... more Here, we report on experimental studies of the role of doping and co-doping on the properties of ZnO thin films deposited by radio-frequency diode sputtering at varying nitrogen content ͑0 ÷ 100%͒ in the sputtering Ar/N 2 gas. Co-doping improved the crystalline structure, and ZnO:Al:N films maintain a c-axis texture in the direction of the surface normal. Depending on the N 2 content, the estimated crystallite size varies from 8 to 37 nm for ZnO:N and 21-33 nm for ZnO:Al:N. Nitrogen doping results in an increased absorption around the bandedge and the bandgap narrowing ͑E g Ͻ 3.2 eV͒.