Formation mechanisms of nano and microcones by laser radiation on surfaces of Si, Ge, and SiGe crystals
© Medvid et al.; licensee Springer. 2013
Received: 28 March 2013
Accepted: 6 May 2013
Published: 4 June 2013
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.
KeywordsLaser radiation Microcones Nanocones Thermogradient effect
Semiconductor nanostructures are the most investigated object in solid state physics due to their promising application in microelectronics and optoelectronics. Today we have some well-developed methods for the formation of nanostructures: MBE , CVD , ion implantation , and laser ablation . The above-mentioned methods need subsequent thermal annealing of the structures in a furnace. Nanostructure growths by these methods need a lot of time and a high-vacuum or a special environment, for example, inert Ar gas. As a result, nanocrystals grow with uncontrollable parameters, broad size distribution, and chaotically, the so-called self-assembly. Therefore, one of the important tasks for nanoelectronic and optoelectronic growth is the elaboration of new methods for the formation of nanostructures in semiconductors with controlled features.
On the other hand, laser technology is of interest both fundamentally because laser radiation of a semiconductor can lead to different and sometimes opposite results, for example, annealing defects after ion implantation or creating new additional defects and from a device viewpoint , since it can be used for annealing B/n-Si or F/p-Si structures during p-n junction formation which is appropriate for many kinds of microelectronic devices .
Moreover, our recent investigations have shown that laser radiation can be successfully applied for formation of cone-like nanostructures [7–10] with laser intensity, which do not cause melting of the material. The 1D-graded band gap structure in elementary semiconductors was formed due to quantum confinement effect . Furthermore, it has been shown that irradiation by laser of Si single crystal with intensity which exceeds melting of material leads to formation of microcones, which are possible to use for solar cells, the so-called black Si . The lack of understanding of the interaction effects of laser radiation with a semiconductor limits laser technology application in microelectronics . So the aims of this research are to show a new possibility for formation of nanocones and microcones on a surface of elementary semiconductors (Si, Ge) and their solid solution by laser radiation, and to propose the mechanism of cones formation.
Materials and methods
For the formation of nanocones in the experiments on i-type Ge single crystals with resistivity ρ = 45 Ω cm, Na = 7.4 × 1011 cm−3, Nd = 6.8 × 1011 cm−3, where Na and Nd are acceptor and donor concentrations, and samples with the size of 16.0 × 3.0 × 2.0 mm3 were used. The samples were mechanically polished with diamond paste and chemically treated with H2O2 and CP-4 (HF/HNO3/CH3COOH in volume ratio of 3:5:3).
Different intensities, pulse durations, and wavelengths of nanosecond Nd:YAG laser were used to irradiate the samples (pulse repetition rate at 12.5 Hz, power of P = 1.0 MW and wavelength of λ = 1,064 nm). The diameter of the spot of the laser beam was 3 mm, and point-to-multipoint method was used for irradiation of the samples. All experiments of nanocone formation were performed in ambient atmosphere at pressure of 1 atm, T = 20°C, and 60% humidity. Current–voltage (I-V) characteristics were measured for the nonirradiated and irradiated samples with nanocones formed on a surface of i-Ge samples. The measurements of the I-V characteristics were performed by soldering 99% tin and 1% antimony alloy contacts directly on the irradiated surface of Ge with the tin contacts on the opposite side. Measurements of I-V characteristics were done at room temperature and atmospheric pressure.
The structure consisting of Ni catalyst with thicknesses d = 30 nm deposited on commercial Si(111) single crystals were used for formation of microcones. Pulsed Nd:YAG laser for treatment Ni/Si structure with following parameters was used: wavelength of λ = 1,064 nm, pulse duration of τ = 150 ms, pulse repetition rate of 12.5 Hz, power at P = 1.0 MW, laser intensity of I = 4 MW/cm2. The threshold intensity of microcones formation is 3.15 MW/cm2. The samples were treated by laser radiation in scanning mode with step of 20 μm. All experiments of microcones formation were performed in ambient atmosphere at pressure of 1 atm, T = 20°C, and 60% humidity.
Investigations of the reflection obtained from the surface with decorated microcones structure were done with Avantes AvaSpec-2048 UV/VIS/NIR spectrometer (Avantes Inc., Apeldoorn, The Netherlands) in the wavelength range of 200 to 1,100 nm [spectrometer based on AvaBench-75 symmetrical Czerny-Turner construction (Avantes Inc., Apeldoorn, The Netherlands) with 2,048 pixel CCD detector and resolution of 1.4 nm].
Surface morphology and chemical analysis of the samples by scanning electron microscope (SEM) with integrated energy dispersive X-ray spectrometer (SEM-EDX) Hitachi S-900 (Hitachi America, Ltd., Brisbane, CA, USA) were used. Photoluminescence (PL) measurements were performed by equipment Fluorolog-3, using photo detector Hamamatsu R928 and xenon lamp (450 W) (Hamamatsu Photonics GmbH, Herrsching, Germany).
Results and discussion
The surface microstructuring of ordinary Si by pulsed femtosecond laser-induced plasma [25, 26] or chemical vapor deposition with catalytic metal on Si  is used for black Si formation. We proposed a new laser method, which is simpler and cheaper comparison with above-mentioned methods .
Experimentally, we have shown the possibility to control the size and the shape of cones both by the laser radiation and the semiconductor parameters.
Nanocone formation mechanism in semiconductors is characterized by two stages. The first stage is characterized by formation of n-p junction for elementary semiconductors or Ge/Si heterojunction for SiGe solid solution. The second stage is characterized by formation of nanocones due to mechanical plastic deformation of the compressed Ge layer on Si and in elementary semiconductor compressed n-type top layer.
The mechanism of the formation of microcones is characterized by two stages. The first stage is melting of Ni film after irradiation by laser beam and formation of Ni islands due to surface tension force. The second step is melting of Ni and subsequent manifestations of Marangoni effect with growth of microcones.
AM is the head of Semiconductors Laboratory at Riga Technical University. PO is the lead researcher in Semiconductor Laboratory at Riga Technical University. ED is a Ph D student in Riga Technical University. RJG is an associate professor at Kaunas University of Applied Sciences. IP is an associate professor at Kaunas University of Technology.
Atomic force microscope
Quantum confinement effect
Scanning electron microscope
Authors AM and PO gratefully acknowledge the financial support by the European Regional Development Fund within the project ‘Sol–gel and laser technologies for the development of nanostructures and barrier structures’, 2010/0221/2DP/22.214.171.124.0/10/APIA/VIAA/145 and Latvian Council of Science according to the grant 10.0032.6.2. ED thanks for the support of this work by the European Social Fund within the project ‘Support for the implementation of doctoral studies at Riga Technical University’. RJ thanks the Research Council of Lithuania for Postdoctoral fellowship that was funded by the European Union Structural Funds project ‘Postdoctoral Fellowship Implementation in Lithuania.’
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