3.2.1 Experiments on Ge and Si crystals

All experiments for the formation of NSs were performed in an ambient atmosphere at a pressure of 1 atm, T = 20°C, and 60% humidity. Radiation from a pulsed Nd:YAG laser for Ge single crystals and Si_xGe_1−x/Si solid solution, basic frequency with the following parameters: pulse duration τ = 15 ns, wavelength λ = 1.06 μm, pulse rate 12.5 Hz, power P = 1.0 MW and energy W = 30.0 mW was used. For Si single crystals with SiO₂ natural cover layer second harmonic with τ =10 ns and λ = 532 nm were used. The SiO₂ cover layer in experiments with CdZnTe for preventing evaporation of material was used. Usually, laser beams were directed to the irradiated surface of the sample. Ge(001) i-type single crystal samples with sizes 1.0 × 0.5 × 0.5 cm³ and resistivity ρ = 45 Ω cm were used in experiments. The samples were polished mechanically and etched in CP-4A solution to ensure the minimum surface recombination velocity S_min = 100 cm/s on all the surfaces. The spot of a laser beam of 3 mm diameter was scanned over the sample surface by a two coordinate manipulator in 20 μm steps. The surface morphology was studied by an atomic force microscope (AFM) and an electron scanning microscope (ESM). Optical properties of the irradiated and non-irradiated samples were studied by using PL and back scattering Raman methods. For PL the 442 nm line of a He-Cd laser and for micro-Raman back scattering an Ar⁺ laser with λ = 514.5 nm were used.

The AFM study of Ge surface morphology after irradiation by Nd:YAG laser basic frequency is shown in Fig. 3.1. The most interesting results were found by increasing the LR intensity up to 28.0 MW/cm², when nanocones arise on the irradiated surface of Ge, which are self-organized into a 2D lattice. The 2D picture of the irradiated surface of a Ge sample, as seen under ESM, is shown in Fig. 3.2. The 2D lattice is characterized by translation symmetry along and perpendicular to periodic lines with a pattern of C_6i point group symmetry and repetition period of 1 μm. C_6i patterns are marked by white circles. The patterns’ orientation and their symmetry depending on orientation of the Ge surface were not observed. Unusual PL spectrum from the irradiated surfaces of Ge was found in the visible region of the spectrum with maximum at 1.65 eV, as shown in Fig. 3.3. The maximum band in a PL spectrum is usually situated at 0.67 eV, and the intensity of PL is very low due to the indirect band structure of Ge.

Photoluminescence in the IR, red, and blue-violet region of the spectra of Si and Ge nanoparticles implanted into SiO₂ layer has been found by several scientific groups. Strong PL in the blue-violet region was achieved, with the values of excitation and emission energy being independent of the annealing temperature and, consequently, of the cluster mean size (Rebohle et al., 2000; Werwa et al., 1996). Different explanations of this PL mechanism have been proposed. One group of scientists explain the PL by QCE (Rebohle et al., 2000) in Si and Ge nanoparticles, but another group (Fernandez et al., 2002) by local Si-O and Ge-O vibration at Si-SiO₂ and Ge SiO₂ interface. A phonon confinement effect model was developed by Campbell and Fauchet (1986) for Si nanocrystals. It showed that by simultaneously using micro-Raman back scattering spectrum and PL spectrum, it is possible to determine the origin of the PL. It means, if ‘blue shift’ in PL spectrum and ‘red shift’ of LO line in Raman back scattering spectrum are simultaneously present, then QCE takes place in NSs. Therefore, this ‘blue shift’ in PL spectrum of Ge we can explain by the presence of QCE on the top of nanocones where the radius of the balls is equal or less than Bohr’s radii of electron and hole. Our calculation of the ball diameter on the top of nanocones using formula (1) from Efors and Efors (1982) and band gap shift from PL bands with maximums at 1.65 and 1.3 eV (Fowler et al., 1966) at parameters of Ge: m_e = 0.12 m₀ and m_h = 0.379 m₀ for electron and hole effective masses, respectively, gives diameters of balls 4.0 and 6.0 nm.

[3.1]

An example of our suggestion is Raman back scattering spectra of the non-irradiated (curve 1) and of the irradiated (curve 2) surfaces of Ge crystal by the laser, as shown in Fig. 3.4. The line at 300 cm⁻¹ of the non-irradiated surface of Ge is attributed to bulk Ge (Ge–Ge vibration, LO phonon line). ‘Red shift’ of the LO phonon line in Raman back scattering spectra by 6 cm⁻¹ after irradiation of Ge surface takes place. The calculated position of line peak frequency of a Raman spectrum as a function of average crystal size (d_ave) for spherical Ge particles from Kartopu et al. (2004) corresponds to 4.0 nm diameter of Ge nanoball on the top of the nanocone. This value is in agreement with our calculation from formula [3.1] using the PL spectrum.

An AFM 3D image of the Si surface after irradiation by second harmonic of Nd:YAG laser at I = 2.0 MW/cm² of SiO₂/Si structure, and the same AFM 3D image of Si surface after subsequent wet etching by HF are shown in Figs. 3.5a and 3.5b respectively. Photoluminescence spectra of the irradiated (curves 1 and 2) and non-irradiated (curve 3) surface of SiO₂/Si at intensity of LR up to 2.0 MW/cm² are shown in Fig. 3.6. The surface morphology of SiO₂ layer is smooth and ‘stone-block’ like, but in reality, under the SiO₂ layer are very sharp Si nanocones, as shown in Fig. 3.5b, which arise on the SiO₂/Si interface after irradiation by the laser.

Photoluminescence of the SiO₂/Si structure in the visible range of a spectrum with maximum at 2.05 eV obtained after irradiation by the laser at intensity I = 2.0 MW/cm², as shown in Fig. 3.6, is unusual. The maximum band in PL spectrum is usually situated at 1.0 eV and the intensity of PL is very low due to the indirect band structure of Si crystals. PL of this structure after removal of the SiO₂ layer by wet etching in HF acid is similar and is obtained in the same region of the spectrum and having the same positions of maximums. It means that PL is not connected with local Si-O vibration at the Si–SiO₂ interface (Fernandez et al., 2002). Therefore, we explain our results by the QCE in nanocones. Decrease of the PL intensity can be explained by increase of reflection index of the structure after removal of the SiO₂ layer. We can see that the visible PL spectrum of the SiO₂/Si structure is wide and asymmetric with gradual decrease of intensity in IR range of the spectra. It is unique to PL spectrum typical for graded band gap structure. These results present a dramatic rise of PL intensity with a much higher band gap than for indirect Si. A schematic image of a nanocone with a gradual decrease in diameter from p-Si substrate till top is shown in Fig. 3.7. An increase of energy of a radiation quantum from substrate till top of the Si single crystal at PL of nanocone takes place due to the QCE in nanowire, according to this formula (Li and Wang, 2004):

[3.2]

where , ( and are electron and hole effective-masses, respectively) and d is the diameter. For QWs, ζ = 2.4048. In our case, the diameter of nanocones/nanowires is a function of height d(z), therefore it is a graded band gap semiconductor. Our calculation of the Si band gap as a function of nanowires d from PL spectrum using formula (2) from Li and Wang (2004) is shown in Fig. 3.7. We can see that the dependence is nonlinear and that the function of diameter is decreasing, especially rapidly at small diameters. In our case the maximum band gap is 2.05 eV which corresponds to the minimal diameter 2.3 nm on the top of nanocones/nanowires.

We have found for the first time a method of forming a 1D graded band gap semiconductor structure. The graded change of band gaps arises due to the Quantum Confinement Effect. Usually, a 2D graded band gap semiconductor structure is formed by conventional methods, for example, by MBE (Voitsekhovskii et al., 2008), changing molecular components’ concentration layer by layer.

3.2.2 Experiments on Si_1−x Ge_x/Si (X = 0.3; 0.4) and Cd_1−x Zn_xTe (X = 0.1) solid solutions

A 200 nm thick crystalline Si_1−xGe_x alloy layer was grown on Si(100) wafers with a thickness of 150 μm by MBE on top of Si. Alloys containing 30% and 40% Ge were used in the experiments. The surface of an Si Ge/Si structure was irradiated by basic frequency of the Nd:YAG laser. The three-dimensional surface morphology of an Si_1−xGe_x/Si hetero-epitaxial structure recorded by AFM measurements after irradiation by the Nd:YAG laser at intensities of 7.0 MW/cm² (a) and 20.0 MW/cm² (b) is shown in Fig. 3.8. Figure 3.8a shows the nanocones with an average height of 11 nm formed by LR at the intensity of 7.0 MW/cm². Similar nanocones with an average height of 27 nm shown in Fig. 3.8b have been obtained by irradiation intensity of 20 MW/cm². Due to higher irradiation intensity, they are higher and more compact in diameter. After irradiation of the Si Ge/Si hetero-epitaxial structure by the laser at an intensity of 7.0 MW/cm² the surface structure begins to look like spots on unwetting material, for example, they look like water spots on a glass or ‘quick silver’ on a floor, as shown in Figs. 3.8c and 3.8d, respectively. It means that LR induces segregation of Ge phases at the irradiated surface of the sample due to drift and concentration of Ge atoms. This conclusion is in agreement with data from Kamenev et al. (2005) and Hartmann et al., (2005). It shows that the Ge phase starts to form at 50% concentration of Ge atoms in an SiGe solid solution.

According to the TGE (Medvid’, 2002), it is supposed that LR initiates the drift of Ge atoms toward the irradiated surface of the hetero-epitaxial structure (Medvid’ et al., 2009). PL spectra of the Si_1−xGe_x/Si hetero-epitaxial structures with the maxima at 1.60–1.72 eV obtained after laser irradiation at intensities of 2.0, 7.0 and 20.0 MW/cm² are shown in Fig. 3.9. The spectra are unique and unusual for the material because, depending on Ge concentration, the band gap of SiGe is situated between 0.67 and 1.12 eV (Sun et al., 2005). As seen in Fig. 3.9, the Si_1−xGe_x structure emits light in the visible range of spectrum and the intensity of PL increases with the intensity of irradiation. The maximum of the PL band at 1.70 eV is explained by the QCE (Efors and Efors, 1982). Position of the observed PL peak compared with the bulk material shows a significant ‘blue shift’. The maxima of PL spectra of the Si_1−xGe_x/Si hetero-epitaxial structure slightly shifts to higher energy when the laser intensity increases from 2.0 to 20.0 MW/cm², which is consistent with the QCE too. Our suggestions concerning Ge phase formation are supported by the back scattering Raman spectra as shown in Fig. 3.10. After laser irradiation at the intensity of 20.0 MW/cm², a Raman band at 300 cm⁻¹ appears in the spectrum. This band is attributed to the Ge–Ge vibration and is explained by formation of a new Ge phase (Kamenev et al., 2005) in the Si_1−xGe_x/Si hetero-epitaxial structure. It is proposed to use modified formula (3) from Efors and Efors (1982) for determination of concentration x in the nanocones of Si_1−xGe_x/Si hetero-epitaxial structure formed by LR using PL spectra.

Nanocone formation on a surface of Cd_xZn_1−xTe solid solution with x = 0.1 after irradiation by strongly absorbed Nd:YAG LR with intensity I = 12.0 MW/cm² was observed using AFM, as shown in Fig. 3.11. Studying the nanocones’ optical properties, a new exciton band at energy up to 1.87 eV in the PL spectrum was seen for the first time. At the same time, a shift of A⁰X and D⁰X exciton lines on 3.1 and 2.5 meV correspondingly, toward the higher energy of quantum, that is the so-called ‘blue shift’, was observed, as shown in Fig. 3.12. Appearance of a new PL band is explained by Exciton Quantum Confinement (EQC) effect in nanocones and the ‘blue shift’ of the exciton bands – basically by mechanical compressive stress of the sample top layer formed on the irradiated surface of the samples. This process is described in the following way: irradiation of the Cd_1−xZn_xTe solid solution by the laser leads to the drift of Cd atoms toward irradiated surface and of Zn atoms – in the bulk of the semiconductor due to high gradient of temperature, the so-called TGE (Medvid’, 2002). As a result, formation of a Te hetero-structure, where x₁ > x, takes place due to replacement of Zn atoms by Cd atoms at the irradiated surface. At the same time, the opposite process takes place under the top layer, in the buried layer of the semiconductor – Zn atoms replace Cd atoms. At least three factors determine A⁰X and D⁰X exciton lines position in PL spectrum. They are: concentration of Zn atoms in CdTe top layer and in CdZnTe buried layer (Reno and Jones, 1992), 2D EQC effect in the CdTe top layer when its thickness is comparable with Bohr’s radius of exciton, and mechanical compressive stress of the CdTe top layer due to mismatch of CdTe and CdZnTe crystalline lattice. Decrease of the Zn atoms concentration in the top layer with an increase of LR intensity, according to the proposed model, leads to the ‘red shift’ of the exciton bands in PL spectra, as shown in Medvid’ et al. (2007b), but increase of the Zn atoms concentration in buried CdZnTe layer manifests in ‘blue shift’ of the PL spectrum, as shown in Fig. 3.13. These effects do not compensate each other because they take place in different layers. This unusual situation can be explained by different input of these layers in the intensity of PL spectrum. If the top layer is excited by short wavelength light, then the ‘red shift’ of PL spectrum will be mostly observed, but if mainly the buried layer is excited, for example due to small thickness or transparency of top layer, then the ‘blue shift’ will be observed.

Of course, it is possible to observe both PL spectra simultaneously at intermediate situations. Exactly such a situation is observed in the PL spectrum shown in Fig. 3.12, after destruction of the CdTe top layer and formation of nanocones on the irradiated surface of the sample. Relaxation of the mechanical compressive stress in the CdTe layer comes to expression as self-assembly of nanocones on the irradiated surface of the structure like Stranski–Krastanov’s mode, and simultaneous appearance of a new exciton band at 1.872 eV in PL spectrum at high intensity of LR takes place. Reconstruction of this band shows that it consists of three lines which look like A⁰X, D⁰X and A⁰X–LO lines (the distance between the lines and their relation intensities are the same) in the non-irradiated PL spectrum of the semiconductor. Therefore, we connect the appearance of both the new band in PL spectrum and the nanocones’ formation on the irradiated surface of the semiconductor with EQC in nanocones and denote them as A⁰XQC and D⁰XQC lines. Evidence of the mechanical stress relaxation process in the CdTe layer is non-monotonic dependence of the ‘blue shift’ as function of LR intensity, as shown in Fig. 3.13. Such non-monotonic dependence has been observed in p- and n-type Si after studying mechanical microhardness of Si after irradiation by Nd:YAG laser (Medvid’ et al., 2007b). Calculation of the mechanical compressive stress in the CdTe top layer using the maximum of the ‘blue shift’ of A⁰X exciton line from Fig. 3.13 and dE_g/dP = 10 eV/Pa (Thomas, 1961), where E_g and P are the band gap of the CdTe crystal and mechanical stress respectively, gives P = 4.62 × 10⁵ Pa. This value corresponds to the ultimate strength limit of CdTe (Yonenaga, 2005). Calculation of the quantum dot diameter using the formula from Kayanuma (1988) and the ‘blue shift’ of A⁰XQC line in PL spectrum on 0.27 eV gives the diameter of the quantum dots up to 10.0 nm. This data corresponds to the size of nanocones (height and diameter of the bottom of the cones are 10.0 nm) measured using 3D image of AFM. Evidence of the presence of the EQC effect in nanocones is the decrease of LO phonon energy by 0.7 meV in the PL spectrum, that is the so-called phonon QCE (Campbell and Fauchet, 1986). Our calculation of the Zn atom’s distribution depending on intensity of LR using the thermo-diffusion equation has shown that the process of Te heterostructure formation is characterized by gradual increase of Zn atom’s concentration in the buried layer with intensity of LR up to 8% (x₁ = 0.18). The thickness of the CdTe layer after irradiation by a laser with intensity of I = 12.0 MW/cm² becomes 10 nm. Therefore, we explain that the A⁰X and D⁰X lines do not return to their initial position in the PL spectrum by increase of Zn atoms concentration in buried layer till x = 0.18.

We connect the appearance of both the new band in PL spectrum and the nanocones formation on the irradiated surface of the semiconductor with EQC effect in nanocones and denote them as A⁰XQC and D⁰XQC lines. The process of nanocone formation is characterized by LR threshold intensity up to I = 9.0 MW/cm², as we can see in Fig. 3.13, maximum of ‘blue shift’ position VS the laser intensity.

1. Non-monotonic dependence of Si crystal micro-hardness as a function of the laser intensity. There is an increase of the crystal micro-hardness at the LRA stage of laser irradiation and decrease of it at the LSA stage, as shown in Fig. 3.14, III and IV bands respectively.

2. An example of the presence of the first stage is the appearance and growth of LO phonon line intensity with a frequency of 300 cm⁻¹ in the Raman back scattering spectrum of Ge_xSi_1−x solid solution after irradiation by the laser. It means formation of a new Ge phase on the irradiated surface of Ge_xSi_1−x (Medvid’ et al., 2010a), as shown in Fig. 3.10.

3. The shift of the PL spectrum of a Cd_xZn_1−xTe solid solution at a low intensity of LR toward lower energy of quantum – the ‘red shift’ (Medvid’ et al., 2007b) – is the next evidence of the first stage of thin Cd_xZn_1−xTe layer formation.

4. Appearance of nanocones on the irradiated surface of Ge_xSi_1−x solid solution and their growth with the laser intensity has been found by measurements of the irradiated surface morphology by AFM, as shown in Fig. 3.8.

5. The shift of bands in the PL spectrum of a Cd_xZn_1−xTe solid solution at a high intensity of LR toward higher energy of quantum – the ‘blue shift’ – and appearance of a new PL band at higher energy of quantum – EQC Effect, as shown in Fig. 3.12, are evidence of the second stage of the mechanism.

6. The ‘blue shift’ of the PL spectra and increase of the PL bands intensity of Si, Ge, Ge_xSi_1−x and Cd_xZn_1−xTe crystals with increasing intensity of the laser due to QCE, for example as shown in Fig. 3.9 for Ge_xSi_1−x crystal, is the next evidence of the LSA stage.

7. Evidence of the first stage presence at nanocones formation process is the redistribution of intensity of LO-ZnTe, TO-CdTe and LO-CdTe phonon lines in Raman back scattering spectra, as shown in Fig. 3.15. It means that before irradiation of the sample by the laser intensity of LO-ZnTe, the phonon line was 3–4 times higher than the intensity of TO and LO-CdTe phonon lines but after irradiation, the opposite is observed in Raman spectra.

3.4.1 High intensity Si source of white light and photon detector with selective or ‘bolometer’ type of spectral sensitivity on the base of nanocones

A result of investigation into PL spectra of Si and Ge_xSi_{1 – x} crystals has shown, in Figs. 3.2 and 3.9, that nanocones radiate a wide spectrum of light quantum like the sun or a glow lamp. At the same time, this structure can be used as a photon detector with a ‘bolometer’ type of spectral photo sensitivity (Capasso, 1987), as shown in Fig. 3.16, curve 1. If irradiation of the structure takes place from the wide band gap part of a semiconductor with a gradient band gap structure, it means from the top of cones, or as a photon detector with selective type of spectral photo sensitivity, if irradiation of the structure takes place from narrow band gap part of graded band gap structure, as shown in Fig. 3.16, curve 2. Schematically graded band gap structure is shown in Fig. 3.16.

3.4.2 X- and γ-ray detector with increased radiation hardness

This part of the work deals with studying the possibility of increasing the radiation hardness of CdZnTe crystals using LR (Medvid’ et al., 2010b). An Nd:YAG laser with the following parameters – second harmonic and intensity range I = 0.5–2.0 MW/cm^2– – was used as a source of light. This method of PL was used in the experiments for estimation of crystalline lattice damage after irradiation by gamma rays. Cd_0.9Zn_0.1Te crystal was the object of investigation. It is known that γ-radiation causes intrinsic defect generation in a semiconductor. The effect consists of an increase of the A⁰X exciton line in the PL spectrum of CdZnTe by ten times after γ-irradiation by ⁶⁰Co source (E = 1.2 MeV) at room temperature with a dose of 5 × 10⁵ Rad = 5 kGy, as shown in Fig. 3.17, curves 1 and 3. Experimental results showed that this effect is suppressed by five times in CdZnTe crystals after preliminary irradiation by the laser at an intensity of more than 2.0 MW/cm², as shown in Fig. 3.17, curve 4. The decrease of the A⁰X exciton line intensity in the PL spectrum indicates the decrease of generation of Cd vacancies in CdZnTe after preliminary irradiation by the laser. The mechanism of this effect is explained in the following way: γ radiation leads to generation of additional Cd vacancies near the surface layer (Xia and Yang, 2003), which causes an increase of the A⁰X exciton line in the PL spectrum. Laser radiation has the opposite effect on Cd_0.9Zn_0.1Te crystals: interstitial Cd atoms are concentrated near the irradiated surface layer, but vacancies in the bulk of the semiconductor (Medvid’, 2002). This leads to an A⁰X line decrease and increase of the D⁰X exciton line in the PL spectrum, as shown in Fig. 3.18. An increase in Cd_i atom concentration near the surface layer leads to an increase of the materials radiation hardness due to two facts: that Cd atomic weight is larger compared to other atoms – Zn and Te, and ‘healing’ of the crystal lattice by Cd_i atoms – recombination V_cd and Cd_i.

3.4.3 Electron field emitter on the base of Si nanocones

Nanocones have been formed on the irradiated surface of p-Si crystals by the second harmonic of Nd:YAG laser with an intensity of 2.0 MW/cm². The measurements of electron field emissions were performed with a flat diode configuration with a glass spacer (Evtukh et al., 2010). Distance between the cathode (silicon wafer) and the anode was 0.8 mm. The applied field emission setup allows achieving a vacuum of 1 × 10⁻⁵ Pa. The applied voltage varied between 1500 and 3600 V. The current–voltage characteristics of field emission current and corresponding curves in Fowler–Nordheim plots (lgI/E² – 1/E) are shown in Fig. 3.19. The electron field emission from such nanocones have some peculiarities, namely: (i) decrease of the threshold field from E_th = 4 × 10⁴ V/cm at the first measurement to E_th = 3.5 × 10⁴ V/cm in subsequent measurements, (ii) two slopes of Fowler–Nordheim curves (higher slope at low fields and lower slope at high fields) (Fig. 3.19). Analysis of the SEM micrographs and electron field emission curves allows us to estimate (i) the electron field enhancement coefficient, β≈100, (ii) work functions (Φ₁ = 6.8 eV at the first measurement and Φ₂ = 3.9 eV, Φ₃ = 2.38 eV from the two slopes in subsequent measurements), (iii) effective emission area, α = 3 × 10⁻⁸ – 1.8 × 10⁻⁵ cm². The obtained experimental results were explained in the frame of the proposed model which takes into account formation of native oxide and/or positive dipoles (Si⁺–O⁻) due to oxygen adsorption on the Si surface. An improvement in the field emission after the first measurement may be due to the oxygen thermal field desorption or the transformation of the native oxide caused by a stress-induced leakage current process. As in metal-insulator-Si structures, the resistance of the thin oxide is reduced as a result of the current being passed through. The lower work function in relation to the known value for high doped n-type silicon Φ₀ = 4.15 eV is explained by an increase of Si band gap on the top of nanocones, as shown in Fig. 3.7.

3.4.4 Third generation solar cells on the base of ITO/Si/Al structure

ITO/p-Si/Al structures, where the ITO top layer was 70 nm thick, an Si layer with a thickness of 500 μm and an Al back layer with a thickness of 100 nm in the experiments were studied. The structure was irradiated from the ITO side by an Nd:YAG LR second harmonic with a wavelength of λ = 512 nm and a pulse duration of τ = 10 ns. The diameter of the laser beam spot was 0.9 mm. Irradiation of the samples in the scanning regime using a two coordinate manipulator with a 20 μm step was carried out. Experiments in ambient atmosphere at a pressure of 1 atm, T = 20°C, and 60% humidity were performed.

The irradiated structures were studied using AFM and PL spectroscopy methods. The measurements of current–voltage (I–V) characteristics to observe the changes of electrical parameters for ITO/p-Si/Al structure after irradiation by laser were carried out. ITO/p-Si/Al structure has been irradiated by an Nd:YAG laser with the aim of forming a p-n junction on the interface of ITO/Si and to grow nanocones on the interface of ITO/Si in which QCE takes place (Medvid’ et al., 2011). The n-type Si layer was formed on p-type Si due to drift of interstitial Si atoms toward the irradiated surface, as a result of huge gradient of temperature induced by LR (Medvid’, 2002).

Nanocones were formed on the irradiated Si surface with an average height of 30 nm by LR with an intensity of up to 2.83 MW/cm² as shown in Fig. 3.20. PL spectra of the ITO/p-Si structures with the maxima at 575 and 490 nm obtained after laser irradiation at intensities of 1.13 and 2.83 MW/cm² are shown in Fig. 3.21. Position of the observed PL peak compared with the bulk Si shows a significant ‘blue shift’. The maxima of the PL band at 575 and 490 nm are explained by presence of the QCE (Medvid’ et al., 2011) on the top of the nanocones. After irradiation of the ITO/p-Si/Al structure by the laser, photocurrent power increased twofold in comparison to the non-irradiated structure. This effect is explained by QCE in nanocones. It means that the Si band gap is increased with the increase of the laser intensity. It is found that the dark I–V characteristic becomes diode-like with a rectification coefficient of K = 105 at 5 V caused by the laser irradiation with a intensity of I₂ = 2.83 MW/cm² as shown in Fig. 3.22. The improvements of the ITO/p-Si/Al structure as a solar cell can be explained by the increase of potential barriers between ITO and p-Si layers.

Alivisatos, A.P. Semiconductor clusters, nanocrystals, and quantum dots. Science. 1996; 271:933–937.

Beigelsen, D.K., Rozgonyi, G.A., Shank, C.V.Energy beam–solid interactions and transient thermal processing. Pittsburgh: Material Research Society, 1985.

Brus, L.E. Electron–electron and electron–hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. Journal of Chemical Physics. 1984; 80:4403–4409.

Campbell, H., Fauchet, P.M. The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors. Solid State Communications. 1986; 58:739–774.

Capasso, F., Graded-gap and superlattice devices by bandgap engineeringDingle, R., eds. Semiconductors and semimetals; Volume 24. At&T Bell Laboratories Murray Hill, New Jersey, 1987:319–395.

Carini, G.A., Bolotnikov, A.E., Camarda, G.S., James, R.B. High-resolution X-ray mapping of CdZnTe detectors. Nuclear Instruments and Methods in Physics Research A. 2007; 579:120–124.

Efors, A.L., Efors, A.L. Band-band absorption of light in semiconductor ball. Physics and Technics of Semiconductors. 1982; 16:1209–1214. [(in Russian)].

Emel’yanov, V.I., Panin, I.M. Defect-deformational self-organization and nanostructuring of solid surfaces. Solid State Physics. 1997; 39:2029–2035.

Evtukh, A., Medvid’, A., Onufrijevs, P., Okada, M., Mimura, H. Electron field emission from the Si nanostructures formed by laser irradiation. Journal of Vacuum Science & Technology B. 2010; 28:C2B11. [C2B13].

Fernandez, B.G., Lopez, M., Garcia, C., Perez-Rodriguez, A., Morante, J.R., Bonafos, C., Carrada, M., Claverie, A. Influence of average size and interface passivation on the spectral emission of Si nanocrystals embedded in SiO₂. Journal of Applied Physics. 2002; 91:789–793.

Fowler, A.B., Fang, F.F., Howard, W.E., Stiles, P.J. Magneto-oscillatory conductance in silicon surfaces. Physical Review Letters. 1966; 16:901–903.

Fujisawa, I. Type conversion of InSb from p to n by ion bombardment and laser irradiation. Japanese Journal of Applied Physics. 1980; 19:2137–2140.

Furukawa, S., Miyasato, T. Quantum size effects on the optical band gap of microcrystalline Si:H. Physical Review B. 1988; 38:5726–5729.

Gnatyuk, V.A., Aoki, T., Hatanaka, Y., Vlasenko, O.I. Defect formation in CdTe during laser-induced doping and application to the manufacturing nuclear radiation detectors. Physica Status Solidi (c). 2006; 1:121–124.

Green, M.A.Third generation photovoltaics: Advanced solar emerge conversion. Berlin: Springer-Verlag, 2004.

Gupta, A., Viral, P., Compaan, L., Alvin, D. High efficiency ultra-thin sputtered CdTe solar cells. Solar Energy Materials & Solar Cells. 2006; 90:2263–2271.

Hartmann, J.M., Bertin, F., Rolland, G., Semeria, M.N., Bremond, G. Effects of the temperature and of the amount of Ge on the morphology of Ge islands grown by reduced pressure–chemical vapor deposition. Thin Solid Films. 2005; 479:113–120.

Kamenev, B.V., Baribeau, J.-M., Lockwood, D.J., Tsybekov, A. Optical properties of Stranski-Krastanov grown three-dimensional Si/Ge nanostructures. Physica E. 2005; 26:174–178.

Kartopu, G., Bayliss, S.C., Hummel, R.E., Ekinci, Y. Report on the origin of the orange PL emission band. Journal of Applied Physics. 2004; 957:2466–2472.

Kayanuma, Y. Quantum-size effects of interacting electrons and holes in semiconductor microcrystals with spherical shape. Physical Review B. 1988; 38:9797–9805.

Kurbatov, L., Stojanova, I., Trohimchuk, P.P., Trohin, A.S. Laser annealing of A^IIIB^V compound. Report Academy of Science USSR. 1983; 268:594–597.

Li, J., Wang, L.W. Comparison between quantum confinement effects of quantum wires and quantum dots. Chemistry of Materials. 2004; 16:4012–4015.

Mada, Y., Ione, N. p-n junction formation using laser induced donors in silicon. Applied Physics Letters. 1986; 48:1205–1207.

Medvid’, A. Redistribution of the point defects in crystalline lattice of semiconductor in nonhomogeneous temperature field. Defects and Diffusion Forum. 2002; 210–212:89–101.

Medvid’, A., Fedorenko, L. Generation of donor centers in p-InSb by laser radiation. Materials Science Forum. 1999; 297–298:311–314.

Medvid’, A., Lytvyn, P. Dynamics of laser ablation in SiC. Materials Science Forum. 2004; 457–460:411–414.

Medvid’, A., Dmytruk, I., Onufrijevs, P., Pundyk, I. Quantum confinement effect in nanohills formed on a surface of Ge by laser radiation. Physica Status Solidi (c). 2007; 4:3066–3069.

Medvid’, A., Fedorenko, L., Korbutjak, B., Kryluk, S., Yusupov, M., Mychko, A. Formation of graded band-gap in CdZnTe by YAG:Nd laser radiation. Radiation Measurements. 2007; 42:701–703.

Medvid’, A., Fedorenko, L., Snitka, V. The mechanism of generation of donor centres in p-InSb by laser radiation. Applied Surface Science. 1999; 142(142):280–285.

Medvid’, A., Fukuda, Y., Michko, A., Onufrievs, P., Anma, Y. 2D lattice formation on a surface of Ge single crystal by YAG:Nd laser. Applied Surface Science. 2005; 244(1–4):120–123.

Medvid’, A., Hatanaka, Y., Korbutjak, D., Fedorenko, L., Krilyuk, S., Snitka, V. Generation of the A-centres at the surface of CdTe(Cl) by YAG:Nd laser radiation. Applied Surface Science. 2002; 197–198:124–129.

Medvid’, A., Kaupuzs, J., Madzulis, I., Blms, J. Buried layer formation in silicon by laser-radiation. Journal of Applied Physics. 1996; 79:9118–9122.

Medvid’, A., Knite, M., Kaupuzs, J., Frishfelds, V. Mechanism of recording and erasing of optical information by laser radiation on SiO₂-(Co+Si)-SiO₂-Si multilayer structure. Applied Surface Science. 1997; 115:393–398.

Medvid’, A., Litovchenko, V.G., Korbutjak, D., Fedorenko, L.L., Hatanaka, Y. Influence of laser radiation on photoluminescence of CdTe. Radiation Measurements. 2001; 33:725–729.

Medvid’, A., Mychko, A., Strilchyuk, O., Litovchenko, N., Naseka, Y., Onufrijevs, P., Pludonis, A. Exciton quantum confinement effect in nanostructures formed by laser radiation on the surface of CdZnTe ternary compound. Physica Status Solidi (c). 2008; 6:209–212.

Medvid’, A., Mychko, A., Gnatyuk, V.A., Levytskui, S., Naseka, Y. Mechanism of nanostructure formation on a surface of CdZnTe crystal by laser irradiation. Journal of Automation, Mobile Robotics & Intelligent Systems. 2009; 3:127–129.

Medvid’, A., Onufrijevs, P., Chiradze, G., Muktupavela, F. Impact of laser radiation on microhardness of a semiconductor. AIP Conference Proceedings, 30th International Conference on the Physics of Semiconductors, July 25–30 338, Coex, Seoul, Korea, 2010.

Medvid’, A., Onufrievs, P., Dauksta, E., Barloti, J., Ulyashin, A., Dmytruk, I., Pundyk, I. P-n junction formation in ITO/p-Si structure by powerful laser radiation for solar cells applications. Advanced Materials Research. 2011; 222:225–228.

Medvid’, A., Onufrijevs, P., Dmitruk, I., Pundyk, I. Properties of nanostructure formed on SiO₂/Si interface by laser radiation. Solid State Phenomena. 2008; 131–133:559–562.

Medvid’, A., Onufrijevs, P., Lyutovich, K., Oehme, M., Kasper, E., Dmitruk, N., Kondratenko, O., Dmitruk, I., Pundyk, I. Self-assembly of nanohills in Si_xGe_1-x/Si by laser radiation. Journal of Nanoscience and Nanotechnology. 2010; 10:1094–1098.

Mooney, P.M., Jordan-Sweet, J.L., Ismail, K., Chu, J.O., Feenstra, R.M., LeGoues, F.K. Relaxed Si0.7Ge0.3 buffer layers for high-mobility devices. Applied Physics Letters. 1995; 67:2373–2375.

Morales, A.M., Lieber, C.M. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science. 1998; 279:208–211.

Rebohle, L., von Borany, F.J.H., Skorupa, W. Blue photo- and electroluminescence of silicon dioxide layers ion-implanted with group IV elements. Applied Physicas B: Lasers Optics. 2000; B70:131–134.

Reno, J., Jones, E. Determination of the dependence of the band-gap energy on composition for Cd_1–x Zn_xTe. Physical Review B. 1992; 45:1440–1442.

Sun, K.W., Sue, S.H., Liu, C.W. Visible photoluminescence from Ge quantum dots. Physica E. 2005; 28:525–530.

Talochkin, A.B., Teys, S.A., Suprun, S.P. Resonance Raman scattering by optical phonons in unstrained germanium quantum dots. Physical Review B. 2005; 72:115416–115423.

Thomas, D.G. Excitons and band splitting produced by uniaxial stress in CdTe. Journal of Applied Physics. 1961; 32:2298–2304.

Vigil-Galán, O., Arias-Carbajal, A., Mendoza-Pérez, R., Santana-Rodríguez, G., Sastre-Hernández, J., Alonso, J.C., Moreno-García, E., Contreras-Puente, G., Morales-Acevedo, A. Improving the efficiency of CdS/CdTe solar cells by varying the thiourea/CdCl₂ ratio in the CdS chemical bath. Semiconductor Science and Technology. 2005; 20:819–822.

Voitsekhovskii, A.V., Grygor’ev, D.V., Smith, R. Radiation defect formation in graded-band-gap epitaxial structures Hg_1–xCd_xTe after boron ion implantation. Semiconductor Science and Technology. 2008; 23:1–7.

Vorobyev, L.E., Semiconductor parametersLevinshtein, M., Rumyantsev, S., Shur, M., eds. Handbook Series on Semiconductor Parameters; Vol. 1. World Scientific, London, 1996:33–57.

Werwa, E., Seraphin, A.A., Chiu, L.A., Zhou, C., Kolenbrander, K.D. Synthesis and processing of silicon nanocrystallites using a pulsed laser ablation supersonic expansion method. Applied Physics Letters. 1996; 64:1821–1825.

Xia, Y., Yang, Y. Chemistry and physics of nanowires. Advanced Materials. 2003; 15:351–353.

Yonenaga, I. Hardness, yield strength, and dislocation velocity in elemental and compound semiconductors. Materials Transactions. 2005; 46:1979–1985.

Yoshida, T., Yamada, Y., Orii, T. Electroluminescence of silicon nanocrystallites prepared by pulsed laser ablation in reduced pressure inert gas. Journal of Applied Physics. 1998; 83(10):5427–5432.

Zhao, Z.M., Yoon, T.S., Feng, W., Li, B.Y., Kim, J.H., Liu, J., Hulko, O., Xie, Y.H., Kim, H.M., Kim, K.B., Kim, H.J., Wang, K.L., Ratsch, C., Caflisch, R., Ryu, D.Y., Russell, T.P. The challenges in guided self-assembly of Ge and InAs quantum dots on Si. Thin Solid Films. 2006; 508:195–199.