Two-stage model of nanocone formation on a surface of elementary semiconductors by laser radiation
© Medvid' et al.; licensee Springer. 2012
Received: 12 June 2012
Accepted: 14 July 2012
Published: 31 July 2012
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.
KeywordsLaser radiation P-n junction Nanocones Thermogradient effect
Nanostructures are the most investigated object in solid state physics, especially quantum confinement effect in quantum dots, quantum wires, and quantum wells. Moreover, different shapes of nanostructures can lead to unique physical properties of material. For example, nanocones, depending on a height of structure and solid angle α at top of it, can be quantum dots, quantum wires, or quantum wells[5, 6]. The decrease of nanocone's solid angle α < 60° leads to fundamental changes of its properties. Quantum dot transforms into a quantum wire with gradually decreasing diameter from the base until the tip of the cone. This is a unique system which has wide technical applications, -for example, 1D-graded bandgap structure in elementary semiconductor is a photodetector with “bolometric” or selective type of photosensitivity, depending on irradiation side.
Our previous investigations have shown possibility to form cone-like nanostructures on a surface of elementary semiconductors – Ge, Si, and solid solutions SiGe and CdZnTe. According to our investigation, nanocone formation mechanism is characterised by two stages for SiGe and CdZnTe solid solutions. The first stage is characterized by formation of heterostructure, for example, Ge/Si from SiGe or CdTe/CdZnTe from CdZnTe solid solutions, and the second stage is characterized by formation of nanocones due to mechanical plastic deformation of the compressed Ge layer on Si or CdTe on CdZnTe, respectively. Nevertheless, the mechanism of nanocone formation for elementary semiconductors is not clear until now. Therefore, the research was aimed to study the nanocone formation mechanisms in elementary semiconductors. As a result, a new mechanism of p-n junction formation by laser radiation in elementary intrinsic semiconductor as a first stage of the process is developed.
In experiments i-Ge single crystals with resistivity ρ = 45 Ωcm; Na = 7.4 × 1011 cm-3, Nd = 6.8 × 1011 cm-3, where Na and Nd are acceptors’ and donors’ concentration respectively, and samples' size 16.0 × 3.0 × 2.0 mm3 was used. The samples were mechanically polished with diamond grease and chemically etched with H2O2 and CP-4 (HF:HNO3:CH3COOH in volume ratio 3:5:3). Commercial p- and n-type single crystal silicon substrates were investigated in the experiments as well. For the determination of conductivity type changes of Si wafer after irradiation by Nd: YAG laser radiation, Cu was electrochemically deposited from 5% CuSO4 solution on Si surface.
Different intensities and wavelengths of nanosecond Nd:YAG laser were used to irradiate the samples (pulse repetition rate 12.5 Hz, power P = 1.0 MW). The laser beam to the irradiated surface of the samples was directed normally. The diameter of the spot of the laser beam was 3 mm, and point-to-point method was used for irradiation of the samples. All experiments of nanocone formation were performed at atmospheric pressure, T = 20°C, and 60% humidity.
Current-voltage (I-V) characteristics were measured for the non-irradiated and irradiated samples. Measurements of I-V characteristics were performed by soldering 99% tin and 1% antimony alloy contacts directly on the irradiated surface of Ge and tin contacts on the opposite side. Measurements of I-V characteristics were done at room temperature and atmospheric pressure. Rectification ratio (RR) of I-V characteristics was used for the characterization of p-n junction.
The surface morphology was studied using atomic force microscope (AFM) and scanning electron microscope (SEM).
Results and discussion
It is known that Cu can be electrochemically deposited on n-type semiconductor. Therefore, it was supposed that inversion of conductivity type takes place, and 'hills' of the periodic structures are of n-type. Possibility to invert conductivity type by laser radiation was shown in several p- and n-type semiconductors: p-Si[13–15], p-CdTe, p-InSb[17, 18], p-InAs, p-PbSe, p-Ge, and n-HgCdTe. Different mechanisms have been proposed to explain the nature of inversion of conductivity type, for example, impurities' segregation, defects' generation, amorphization and oxygen-related donor generation. However, there are many contradictions in the mechanisms. For example, n-type impurities in Si irradiated by laser cannot be oxygen atoms, according to paper. Several authors have tried to explain p-n junction formation in n-type HgCdTe by defects’ generation, based on a model of defect formation related to interstitial mercury diffusion. On the other hand, the authors of those papers did not take into account the effect of temperature gradient on the diffusion of atoms in solid solution. Moreover, it is theoretically shown that the p-n junction can be formed by redistribution of impurities in co-doped Si in gradient temperature field. Thus, the mechanism of inversion of conductivity type by laser radiation is not clear until now.
The mechanism of p-n junction formation by laser radiation in elementary semiconductors is the first stage of nanocone formation.
The second stage of nanocone formation in elementary semiconductors is laser heating up of the top layer enriched by interstitial atoms with further plastic deformation due to compressive stress caused by interstitials in the top layer and vacancies in the buried layer.
Additional evidence of two-stage mechanism for elementary semiconductors is non-monotonous dependence of microhardness of Si crystal as a function of the laser intensity[6, 29] and compressive stress can be introduced in Si-SiO2 system by laser radiation.
For the first time, a new mechanism of p-n junction formation in the elementary intrinsic semiconductor by laser radiation as the first stage of nanocone formation is proposed. 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. 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.
Prof. Dr.habil.phys. AM is head of Semiconductors Laboratory at Riga Technical University. Dr.phys. PO is lead researcher in Semiconducor laboratory. ED and GM are PhD students under AM in Riga Technical University. RR is bachelor student in University of Leeds.
atomic force microscope
scanning electron microscope.
The authors gratefully acknowledges the financial support in part by Europe Project in the Framework of MATERA + project, European Regional Development Fund within the project ‘Sol-gel and laser technologies for the development of nanostructures and barrier structures’ (project number 2010/0221/2DP/220.127.116.11.0/10/APIA/VIAA/145), the ESF Project number 1DP/18.104.22.168.0/09/APIA/VIAA/142, and ESF project 'Support for the implementation of doctoral studies at Riga Technical University'.
The authors gratefully acknowledge Mr. Dmitrijs Jakovlevs (Riga Technical University) for scanning electron microscope images.
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