Artur Medvid

Influence of laser radiation on photoluminescence of CdTe

In the present study, we propose a new model for the process of generation of laser donor centres (LDC) and the creation of a potential barrier by laser radiation (LR) in semiconductors. According to this model the Thermogradient e7ect, created by hot electrons, has the main role in the interaction process of LR and semiconductors. The state of the CdTe(Cl)-SiO2 interface under the inuence of strongly absorbed powerful YAG : Nd laser irradiation has been studied. Such a state has been de;ned by measuring the current voltage characteristics (CVC) in the external perpendicular magnetic ;eld, the so-called magnetoconcentration e7ect, and photoluminescence method at low temperature. The rectifying type of CVC in the external magnetic ;eld are explained by the high surface recombination velocity at the interface. After laser irradiation of this structure the recti;cation factor increases with an increase in irradiation intensity, starting from I =0 : 2M W=cm 2 . This phenomenon is explained by the rise of the potential barrier at the interface owing to the generation of LD centres in the temperature gradient. The inuence of LR on the luminescence spectra has shown that at a threshold intensity of LR I =0 : 2M W=cm 2 generation of donor type A-centres begins. c � 2001 Elsevier Science Ltd. All rights reserved.

The mechanism of generation of donor centres in p-InSb by laser radiation

Abstract The laser-induced donor centers in p-InSb have been studied by magneto-concentration effect (MCE). The distribution of In vacancies in a nonuniform temperature field in InSb has been performed in term of the model of redistribution of the In vacancies under laser action. Comparison of the theoretical and experimental data has shown that the depth of the p–n junction increases with temperature and that the relatively large value of the p–n junction localization depth at intensities of laser radiation exceeding 3.5 MW cm −2 is connected with presence of the liquid phase in the laser modification process. These results are confirmed by atomic force microscopy (AFM) of the surface morphology. Experiments were performed on InSb samples in the temperature range 180–290 K. Temperature gradient was provided by YAG:Nd laser illumination ( λ =0.53 μm, τ p =15 ns). Two kinds of laser donor centers (LDC) were found: one is nonstable, annealed at room temperature with relaxation constant ∼5 s, and the other is stable, annealed at 670 K. The threshold of LDC formation is 1.5 MW cm −2 . The activation energy of the stable donor centre is ∼1.1 eV.

Formation mechanisms of nano and microcones by laser radiation on surfaces of Si, Ge, and SiGe crystals

In this work we study the mechanisms of laser radiation interaction with elementary semiconductors such as Si and Ge and their solid solution SiGe. As a result of this investigation, the mechanisms of nanocones and microcones formation on a surface of semiconductor were proposed. We have shown the possibility to control the size and the shape of cones both by the laser. The main reason for the formation of nanocones is the mechanical compressive stresses due to the atoms’ redistribution caused by the gradient of temperature induced by strongly absorbed laser radiation. According to our investigation, the nanocone formation mechanism in semiconductors is characterized by two stages. The first stage is characterized by formation of a p-n junction for elementary semiconductors or of a Ge/Si heterojunction for SiGe solid solution. The generation and redistribution of intrinsic point defects in elementary semiconductors and Ge atoms concentration on the irradiated surface of SiGe solid solution in temperature gradient field take place at this stage due to the thermogradient effect which is caused by strongly absorbed laser radiation. The second stage is characterized by formation of nanocones due to mechanical plastic deformation of the compressed Ge layer on Si. Moreover, a new 1D-graded band gap structure in elementary semiconductors due to quantum confinement effect was formed. For the formation of microcones Ni/Si structure was used. The mechanism of the formation of microcones is characterized by two stages as well. The first stage is the melting of Ni film after irradiation by laser beam and formation of Ni islands due to surface tension force. The second step is the melting of Ni and subsequent manifestations of Marangoni effect with the growth of microcones.

Application of Nd:YAG Laser in Semiconductors’ Nanotechnology

Surface nanostructures and their unique properties play a significant role in such objects as highly dispersed systems adsorbents and catalysts, fillers, composite materials, film and membrane systems (Crommie et al., 1993; Beton et al., 1995; Junno et al., 1995). Formation on the surfaces of ordered and disordered ensembles of nanoparticles allow creating materials with new unique physical properties. Nanostructuring of the surface leads to improvement of optical, electrical, thermal, mechanical and field electron emission properties of materials, for example, reducing of the work function of electron from silicon (Evtukh et al., 2010), enhancing biocompatibility with implants in living tissue and prosthetic devices used in orthopaedics and dentistry. Such materials find application in selective nanocatalyse, microelectronics, nanophotonics, photovoltaic, spectroscopy, and optics. On their base devices are created for recording and storing information with ultra-high density, as well as light-emitting devices (Vu et al., 2010).

P-N Junction Formation in ITO/p-Si Structure by Powerful Laser Radiation for Solar Cells Applications

The research report is devoted to the development of a new method of nanostructures formation in ITO/p-Si/Al structure with powerful laser radiation and study of its optical and electrical properties for solar cells applications. It was shown that after the structure irradiation by Nd:YAG laser second harmonic, dark current voltage characteristics become diode-like. Increase of ITO/p-Si/Al solar cell efficiency after irradiation by the laser, using photocurrent voltage characteristic method, was shown.

Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation

The investigation of surface morphology using atomic force microscope has shown self-organizing of the nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an intensity of 12.0 MW/cm2. The formation of nanocones is explained by the presence of a thermogradient effect in the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which is explained by the exciton quantum confinement effect in nanocones.

Two-stage model of nanocone formation on a surface of elementary semiconductors by laser radiation

In this work, we study the mechanism of nanocone formation on a surface of elementary semiconductors by Nd:YAG laser radiation. Our previous investigations of SiGe and CdZnTe solid solutions have shown that nanocone formation mechanism is characterized by two stages. The first stage is characterized by formation of heterostructure, for example, Ge/Si heterostructure from SiGe solid solutions, and the second stage is characterized by formation of nanocones by mechanical plastic deformation of the compressed Ge layer on Si due to mismatch of Si and Ge crystalline lattices. The mechanism of nanocone formation for elementary semiconductors is not clear until now. Therefore, the main goal of our investigations is to study the stages of nanocone formation in elementary semiconductors. A new mechanism of p-n junction formation by laser radiation in the elementary semiconductor as a first stage of nanocone formation is proposed. We explain this effect by the following way: p-n junction is formed by generation and redistribution of intrinsic point defects in temperature gradient field – the thermogradient effect, which is caused by strongly absorbed laser radiation. According to the thermogradient effect, interstitial atoms drift towards the irradiated surface, but vacancies drift to the opposite direction – in the bulk of semiconductor. Since interstitials in Ge crystal are of n-type and vacancies are known to be of p-type, a n-p junction is formed. The mechanism is confirmed by the appearance of diode-like current–voltage characteristics after i-Ge irradiation crystal by laser radiation. The mechanism in Si is confirmed by conductivity type inversion and increased microhardness of Si crystal. The second stage of nanocone formation is laser heating up of top layer enriched by interstitial atoms with its further plastic deformation due to compressive stress caused by interstitials in the top layer and vacancies in the buried layer.

“Black Silicon” Formation by Nd:YAG Laser Radiation

The possibility to form “black silicon” on the surface of Si structure by Nd:YAG laser radiation has been shown. The shape and height of micro-cone structure strongly depends on Nd:YAG laser intensity and number of laser pulses. Light is repeatedly reflected between the cones in the way that most of it is absorbed. Si micro-cone structure spectral thermal radiation is close to black body spectral radiance, which makes this structure useful for solar cells application. The micro-chemical analysis performed by SEM has shown that the microstructures contain NiSi2. This was approved by presence of LO phonon line in Raman back scattering spectrum. The control of micro-cone shape and height was achieved by changing the laser intensity and number of pulses.

Laser-induced self-organization of nano-wires on SiO2/Si interface

Original observation of new graded band gap structures formed on the surface of elementary Si semiconductor at studying the optical properties of Si nano-hills formed at the SiO2/Si interface by pulsed Nd:YAG laser irradiation is reported. The self-organized nano-hills on Si surface are characterized by a strong photoluminescence in the visible range of spectrum with a shoulder extended to the long-wave part of the spectrum. The feature is explained by the quantum confinement effect in nano-hills-nano-wires of gradually changing diameter.

Influence of irradiation with γ-ray photons on the photoluminescence of Cd0.9Zn0.1Te crystals preliminarily subjected to the intense radiation of a neodymium laser

The effect of the preliminary treatment of Cd0.9Zn0.1Te crystals with high-power pulses of neodymium laser radiation (the power density is ≤1.8 MW/cm2, at a wavelength of 532 nm) on the low-temperature (5 K) photoluminescence induced by γ-ray radiation (the dose was Φγ = 5 kGy) is studied. The luminescence bands are related to radiation-stimulated donor-acceptor pairs, which include shallow neutral donors and neutral cadmium vacancies stimulated by γ-ray irradiation, the transition of free electrons to neutral cadmium vacancies formed by radiation, and the annihilation of excitons bound to the above vacancies. It is shown that, in the crystals preliminarily treated with laser radiation, the intensity of the γ-ray-stimulated luminescence bands is significantly lower than in crystals not subjected to laser radiation. This fact is accounted for by a decrease in the concentration of cadmium vacancies generated by the γ-ray radiation as a result of their annihilation during the course of their interaction with laser-stimulated defects, in particular, as a consequence of their recombination at laser-stimulated interstitial cadmium atoms.