Arthur Medvid

Properties of Nanostructure Formed on SiO2/Si Interface by Laser Radiation

The aim of this work is to study optical properties of Si nanohills formed on the SiO2/Si interface by the pulsed Nd:YAG laser radiation. Nanohills which are self-organized on the surface of Si, are characterized by strong photoluminescence in the visible range of spectra with long wing in the red part of spectra. This peculiarity is explained by Quantum confinement effect in nanohillsnanowires with graded diameter. We have found a new method for graded band gap semiconductor formation using an elementary semiconductor. Graded change of band gap arises due to Quantum confinement effect.

Laser Induced Nanostructure Formation on a Surface of CdZnTe Crystal

For the first time, a new PL band at 1.8718 eV is observed after irradiation of Cd1-xZnxTe (x = 0.1) crystal by Nd:YAG laser at intensity 12.0 MW/cm2. The origin of this PL band we connect with formation of strongly enriched layer by Zn atoms at the irradiated surface of the sample due to a themogradient effect. Self-organizing structures of nanometer size are observed on the surface of a CdZnTe crystal (x = 0.1) irradiated by strongly absorbed Nd:YAG laser radiation at intensities 4.0-12.0 MW/cm2. The effect of exciton quantum confinement manifested by a shift to higher energies of the A0,X exciton line in the photoluminescent spectrum is present in structures of 10–15 nm in diameter at the top of nanohills. A graded band gap structure with optical window is formed on the top of nanohills.

Dynamic of Laser Ablation in SiC

Experimentally it has been found that under irradiation of 6H-SiC with a focused N2 pulsed laser beam the process of laser ablation (LA) is characterised by the repeated unusual stages: 1. production of liquid phase under irradiated C surface due to the lowering of the melting temperature of the crystal lattice caused by the enhancing from pulse to pulse concentration of the impurity atoms in the region of focused spot accumulation effect; 2. extrusion of liquid phase on the surface in the form of nano-hills by pressure of the laser beam through a colder surface layer, which has been created due to atom sublimation by laser radiation (LR) – “a lid effect”. This process leads to production of a cone crater takes place as a result of multiple extrusions of liquid substance onto the surface by laser pulses. The repetition of the described stage process was observed. Peculiarities of LA in 6H-SiC, a threshold character of the effect, the effect of accumulation and photoluminescence quenching are explained by Thermogradient effect at the initial stage of the LR interaction with a semiconductor and they speak in favour of the hydrodynamics model of LA.

Laser-induced reversible change of electrical resistivity of CoSi2 thin-film sensors

Abstract Formation of thin crystalline layers of cobalt suicide, CoSi 2 , deposited by coevaporation of cobalt and silicon mixtures on SiO 2 /Si substrates, has been attained by Q-switched YAG:Nd and CO 2 lasers. The electrical and optical properties of the CoSi 2 layers have been studied by simultaneous laser irradiation. It is shown that high-quality resistive crystalline CoSi 2 films can be obtained by treatment with CO 2 laser radiation of 2 to 8 MW cm −2 intensity. Besides, the resistance of the layer decreases by a factor of nine. When such a layer is subjected to Q-switched YAG:Nd laser radiation of 20–35 MW cm −2 intensity, the magnitude of the resistance increases by a factor of three due to the formation of clusters of vacancies in CoSi 2 . The CoSi 2 phase and rise of concentration of vacancies are determined by X-ray diffraction. A correlation between changes in optical and electrical properties as well as structural changes of the suicide layer is found. It is shown that the change in optical parameters of the resistive film subjected to laser treatment can be used for both remote resistivity measurements and quality control.

Interaction of Point Defects with Impurities in the Si-SiO2 System and its Influence on the Properties of the Interface

The results of investigation of the point defect generation and interaction with impurities in the Si-SiO2 system during the process of its formation by means of electron paramagnetic resonance (EPR) and nucleous magnetic resonance (NMR) technique are presented. It has been shown that the diference in point defects interaction with hydrogen at the Si-SO2 interface with n- and p-type conductivity are connected with the sign of hydrogen ions incorporation dependence on the Fermi level position in accordance with the proposed model. The interface properties may be improved by laser irradiation.

SUPPRESSION OF PORES FORMATION ON A SURFACE OF P-SI BY LASER RADIATION

The influence of strongly absorbing N¬2 laser radiation on pores formation on a surface of Si single crystal has been investigated using optical microscope, atomic force microscope and photoluminescence. After irradiation by the laser and subsequent electrochemical etching in HF acid solution morphological changes of the irradiated parts of a surface of Si were not observed. At the same time, pores formation on the non-irradiated parts of Si surface took place. The porous part of the Si surface is characterized by strong photoluminescence in red part of spectra with maximum at 1.88 eV and intensity of photoluminescence increases with current density. Suppression of the pores formation by the laser radiation is explained with inversion of Si type conductivity from p-type to n-type. This fact is explained by Thermogradient effect – generation and redistribution of the intrinsic defects in gradient of temperature. It was shown that the depth of n-Si layer on p-Si substrate depends on intensity of laser radiation and it increases with intensity of laser radiation. The results of the investigation can be used for optical recording and storage of information on surface of semiconductors.

Hydrogen Interaction with Point Defects in the Si-SiO2 Structures and its Influence on the Interface Properties

The type and density of the point defects that are generated in the Si surface layer during thermal oxidation depend on the oxidation condition: temperature, cooling rate, oxidation time, impurity content. Interaction between the point defects with extended defects and impurities affects the SiO2 structure and Si-SiO2 interface properties. Hydrogen adsorption on n- and p- type wafers is different. One possible reason for that can be the strength of the magnetic interaction between the hydrogen and paramagnetic impurities of the adsorbent. The influence of point defects and impurities may be diminished and the interface properties improved by an appropriate choice of the oxidation conditions and postoxidation laser irradiation.

Laser Assisted Formation of SiC Nano-Tips for Field Emission Application

Nano-tip formation was performed on the surface of 6H-SiC(N) ( ND-NA ~ 1-3 10 cm) by an N2 laser ( λ = 0.337 μm, tp = 7 ns) focused beam in air at room temperature. The nano-tips are of submicron size. The average intensity of this threshold of laser irradiation ( LI) (<Ith>) is the same for two kinds of operation mode and estimated as 5 GW cm. Results of the photoluminescence and studies of AFM in the friction mode suggest an increase of the nitrogen concentration near the surface. The emission current was measured in the vacuum diode struct ure. In the measurement conditions electron field emission from single crystal SiC were not observed. After laser irradiation of the surface and the creation of nano-tips the field emission appear ed (th eshold voltage 1000 V, emission current ~7 10 A, effective work function φef ≈ 4.65 eV). The phenomenon observed can become a basis for the development of an alternative cold technology of forming nano-size structures on a surface of SiC. Laser assisted formation of Si C nano-tips is a promising method for the creation of electron field emission devices.

Laser pulse induced change of CoSi-SiO-Si structure

Abstract The Q-switched YAG : Nd and CO 2 laser initiated formation has been studied of thin CoSi 2 crystalline layers deposited by coevaporation of the Co+Si mixture on SiO 2 \Si substrates. The electrical and optical properties of the CoSi 2 layers were also determined. High quality resistive crystalline CoSi 2 films were obtained by treating the layer with CO 2 laser radiation of 2 MW\cm 2 to 8 MW\cm 2 intensity. The electrical resistance of the layer decreased by a factor of 9. When this layer is subjected to a Q-switched YAG : Nd laser radiation of the intensity of 20 MW\cm 2 to 35 MW\cm 2 , the magnitude of the resistance increases by a factor of 3 due to clusters of vacancies in the CoSi 2 . The phase of the CoSi 2 and the build-up of the concentration of vacancies were determined by the X-ray diffraction. A correlation between changes in the optical and electrical properties as well as structural changes of the silicide layer was found. It is shown that change in the optical parameters of the resistive films subjected to laser treatment can be used for both remote resistivity control and quality check.

Stress Relaxation Mechanism by Strain in the Si-SiO2 System and its Influence on the Interface Properties

The results of the investigation of stresses relaxation by strain by means of EPR spectra, IR absorption spectra, SEM and samples deflection are presented. It has been shown that stresses relaxation mechanism depended on the oxidation conditions: temperature, cooling rate, oxide thickness. In the Si-SiO2-Si3N4 system the stresses relaxation by the strain occur due to the opposite sign of the thermal expansion coefficient of Si-SiO2 and Si3N4 on Si. Laser irradiation allows to modify the system stresses.