Nano-Cones Formation on a Surface of Semiconductors: Si,Ge, Si1-xGex , GaAs and CdZnTe Layers by Laser Radiation

.A new laser method elaborated for a cone like nanostructure formation on a surface of semiconductors is reported. Diameter of the nano-cone is increased gradually from top of cone till a substrate. Such structure has qualitative new properties in comparison with QWs and QDs. For example, nano-cone with gradually changed diameter is gradient band gap structure. Model of the nanostructures growth and optical properties of nano-cones are proposed

Mechanism of Nanocones Formation by Laser Radiation in Semiconductors: Technologies and Applications

A new laser method is elaborated for nanocones’ formation on a surface of such semiconductors as: elementary semiconductors Si and Ge and semiconductor solid solutions - Si1-x Gex /Si and Cd1-x ZnxTe. Strong change of the optical, mechanical and electrical properties of the semiconductors after irradiation by Nd:YAG laser are explained by the presence of Quantum Confinement Effect (QCE) in nanocones where radius is equal to or smaller than Bohr’ effective radius of electron, hole or exciton. “Blue shift” of photoluminescence spectra (Fig.1) and “red shift” of phonon LO line in Raman spectrum are explained by exciton and phonon QCE in nanocones, correspondently. Asymmetry of the photoluminescence band in the spectrum of Si nanocones is explained by 1D graded band gap structure. Experimental data on nanocones formation on a surface of Si, Ge and their solid solution and CdZnTe crystal and their optical properties are presented. Two-stage mechanism of nanocones’ formation on a surface of the semiconductors is proposed [1]. The first stage of the mechanism is characterized by the formation of a thin strained top layer, due to redistribution of point defects in temperature-gradient field induced by laser radiation. The second stage is characterized by mechanical plastic deformation of the stained top layer leading to arising of nanocones due to heating up of the top layer. Formation of nanocones can be applied for design of third generation solar cells, Si white light emitting diode, photodetector with selective or “bolometer” type spectral sensitivity and Si tip for field electron emitting with low work functio

Chapter 4: Nano-Cones Formed on a Surface of Semiconductors by Laser Radiation: Technology, Model and Properties

A new laser method elaborated for cone like nanostructure (diameter of the nano-cone is increased gradually from top of cone till a substrate) formation on a surface of semiconductors is reported. Model of the nanostructures formation and their optical properties are proposed

Two-Stage Mechanism of Quantum Cones Formation on a Surface of Semiconductors by Laser Radiation

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Properties of Nano-Cones Formed on a Surface of Semiconductors by Laser Radiation: Quantum Cofinement Effect of Elactrons, Phonons ans Excitons

A new laser method elaborated for a cone like nanostructure formation on a surface of semiconductors is reported. Diameter of the nanocone is increased gradually from top of cone till a substrate. Such structure has qualitative new properties in comparison with QWs and QDs. For example, nanocone with gradually changed diameter is gradient band gap structure [1]. Model of the nanostructures growth and optical properties of nanocones are proposed. Optical properties of elementary semiconductors such as Ge, Si and Si1-xGex solid solution after irradiation by Nd:YAG laser are investigated. Nanocones on the surface of Ge single crystal were formed by basic frequency of Nd:YAG laser radiation at intensity of 30.0 MW/cm2. This structure is characterized by patterns related to C6i point group symmetry covering all the surface of the sample and having translations symmetry. In the case of n- and p-type SiO2 /Si single crystals nanocones were formed by the second harmonic of Nd:YAG laser radiation at intensity of 2.0 MW/cm2. The same nanocones were induced on the surface of SixGe1-x/Si heterostructures with x = 0.3 and 0.4 by basic frequency of Nd:YAG laser radiation at intensities from 2.0 till 20.0MW/cm2 [2]. The same methods for formation of nanocones on a surface of compound GaAs and ternary compound Cd1-xZnxTe semiconductors were studied. Nanocones were formed on the surface of GaAs and Cd1-xZnxTe with x = 0.1 [3] by the second harmonics of Nd:YAG laser radiation at intensity within 4.0 - 12.0MW/cm2. The optical properties and surface morphology of nanostructures' formed by the laser radiation on the surface of semiconductors using Atomic force microscope, Electron scanning microscope, Photoluminescence (PL) and Raman back scattering methods were studied. Unusual photoluminescence spectrum from the irradiated surfaces was found in the visible range of spectrum. Photoluminescence from Ge, Si, SiGe/Si and GaAs nanocones can be explained by Quantum confinement effect. A shift of micro-Raman scattering spectra in Ge and GaAs is a good evidence of this suggestion. Asymmetric of photoluminescence spectra of the irradiated SiO2/Si structure is explained by Quantum confinement effect in nanocones with a gradually decrease of diameter toward the top of nanocone. The following mechanism of nanocones formation in Si1-xGex/Si structure by laser radiation is proposed: irradiation of SiGe/Si heterostructure by Nd:YAG laser initiates Ge atoms drift to the irradiated surface due to gradient of temperature - Thermogradient effect (TGE). This process is characterized by positive feedback: after every laser pulse gradient of temperature increases due to increase of Ge atoms concentration at the irradiated surface and new Ge phase formation occurs at the end of the process. Ge atoms are localized at the surface of Si like a thin film. A mismatch of Si and Ge crystal lattices leads to compressive stress of Ge layer. This stress relaxation takes place by plastic deformation of the top layer and creation of nanocones on the irradiated surface according to the modified Stransky-Krastanov' mode. The "blue shift" on 0.23mV of exciton bands in PL spectra of the irradiated Cd1-xZnxTe is explained by Exciton Quantum confinement effect in nanocones. A new PL band at 1.88 eV is found. Appearance of the PL band is explained by formation of CdTe/Cd1-xZnxTe heterostructure in the bulk of the semiconductor with x=0.18 due to TGE. For the first time was shown the possibility of 1D gradient band gap structure formation in elementary semiconductors. Thermogradient effect has a main role in initial stage of nanocones and gradient band gap structure formation by laser radiation in semiconductors. [1] Artur Medvid’,”Nano-cones Formed on a Surface of Semiconductors by Laser Radiation: Technology, Model and Properties”, in Book “Nanowires Science and Technology” edited by: Nicoleta Lupu, INTECH, Viena, pp.61-82, 2010. [2] A. Medvid', P. Onufrijevs, K. Lyutovich, M.

Nano-Cones Formed on a Surface of Semiconductors by Laser Radiation: Technology, Model and Properties

The new laser method for nano structures formation on a surface of semiconductors Si, Ge, GaAs and SiGe, CdZnTe solid solutions is proposed. For the first time was shown the possibility of graded band gap structure formation in elementary semiconductors. Thermogradient effect has a main role in initial stage of nano-cones and graded band gap structure formation by laser radiation in semiconductors

Increase of Solar Cell Efficiency in Graded Band Gap Structure

The photo current-voltage characteristic of a solar cell with graded band gap is calculated numerically based on the drift-diffusion equation and Poisson equation. The calculated efficiency of the CdTe solar cell with p-n junction located in 1μm depth increases remarkably when the band gap of the front n-type layer is graded. The effect is strong for high surface recombination velocity S and is remarkable even at S=0 : the calculated efficiency increases from 19.6% to 24.3%

Формирование наноструктуры на поверхности CdZnTe с помощью лазерного излучения

Проведен эксперимент по облучению твердого раствора CdZnТe лазерным излучением. Согласно результатам эксперимента, после облучения поверхность образца представляла собой структуру, состоящую из наноконусов высотой 10 нм и диаметром в основании ≈ 10 нм. На спектрах Фотолюминесценции облученного образца, измеренных при температуре 5 К, наблюдались две новые полосы, соответствующие экситонным линиям А0Х и D0Х, смещенными в коротковолновую область спектра на 0,23эВ – экситонный квантовый размерный эффек