Method of forming microstructured and heavily doped layer on silicon surface

FIELD: chemistry.

SUBSTANCE: invention can be used in producing photosensitive cells for solar power generation and night vision devices. A dry silicon surface is irradiated with multiple focused ultra-short femto- or short picosecond laser pulses for ablation microstructuring thereof. In order to dope the surface layer of silicon with sulphur atoms, the microstructured surface is treated with multiple ultra-short pulses under a thin layer of a carbon sulphide liquid phase, for which a silicon target is immersed therein. Sulphur atoms formed from decomposition of carbon sulphide diffuse into the volume of the condensed silicon phase.

EFFECT: invention enables to form a microstructured layer on a silicon surface which is heavily doped with sulphur atoms - up to 5 at %.

2 cl, 4 dwg

 

The invention relates to the field of forming microstructured and vysokodobrotnykh thin layers on the silicon surface, absorbing well not only in the UV, visible, and near infrared ranges, the photosensitive elements of solar power and night vision devices.

Known methods of directional create irregular microtexture of the surface of various materials by means of optical and electron beam lithography, chemical and electrochemical etching, ion sputtering or electron beam [1]. Common disadvantages of these methods are the need of evacuating the samples at a fairly low speed and high cost of fabrication, in the case of lithography - the need for the use of the resist and its subsequent chemical treatment. In the case of chemical and electrochemical etching, there is a need to use aggressive chemical agents.

Also known methods of doping semiconductor surface layer, which is the most effective method of ion implantation. However, the maximum degree of doping, achieved by ion implantation, a relatively small - less than 10-1atomic % (concentration of impurities of the order of 1020cm-3), which is associated with dispersion implantirovannogo� layer by ion beam, as well as amorphization of the material when the impurity concentration of more than 1021cm-3. In the case of using low-intensity beams of the implantation time becomes unnecessarily large.

In total, there are known methods of microstructurally surface compatible with the known methods of doping (mainly by ion implantation) only as a separate stage of processing, both even necessary for the formation of relatively subtopian layers.

At the same time there is also a way to simultaneously microstructurally and strong doping (the degree of doping is several orders of magnitude higher than when ion implantation up to several atomic percent) silicon surface under the action of multiple femtosecond laser pulses, when the silicon sample is placed in a chamber with gaseous sulfur-containing compounds [2] or the surface napylyaetsya nanometer layer of selenium [3], which does not have the above disadvantages (prototype). The essence of this method consists in decomposition of sulfur-containing compounds on heated molten by laser pulses of a silicon surface and subsequent diffusion of sulfur atoms in the volume of the condensed phase, despite the fact that the movement of the molten material on the surface forms a visually "black" structure microcones (Fig.1, wholesale�ical (a) and TEM (b) images of a microstructured silicon surface), which defines the almost complete absorption of electromagnetic radiation with a wavelength of less than 10 microns due to his entanglement in the valleys of this structure [4]. Ablation of a silicon surface in certain modes, there is also the structure formation of microcones, shown in Fig.1, the movement of the melt or propylene on growing microcones substances, remote (absorbance) from the valleys between the cones, so that repeated Cycling doping atoms of sulfur in the zone of laser irradiation on the surface after each laser pulse occurs due to the decomposition of sulfur-containing compounds on heated surface or by interaction with ablative torch with subsequent precipitation of sulfur atoms or sulfur-containing intermediates on the surface of the microcones patterns. Unlike the valley, the ablation of themselves microUSB hardly occurs due to oblique incidence of the IES, which reduces the laser energy density on the slopes of microcones below the threshold for ablation. Similarly, when processing - heating, melting and ablation under the action of the IES privateline deposited on the surface of nanometer silicon film solid selenium occurs and the introduction of selenium atoms in the surface layer, and the formation of structure microcones silicon. As a result of sorbic�rogo (picosecond) melting thin dotiruemaja and microstructuring surface layer of the silicon target under the action of the IES, and also very fast (within a few nanoseconds) of its solidification during cooling by conduction, evaporation and radiation losses it can enter the high nonequilibrium concentration of sulfur or selenium, unattainable by ion implantation.

The main disadvantage of this method of forming microstructured and vysokodobrotnykh sulfur or selenium layer on the silicon surface is the limitation on the concentration doperalski agent (the degree of doping), which you can enter in the silicon target to 0.7 at.% [2, 3]. Doping plays a crucial role in creating in the bandgap below the bottom of the conduction band of silicon deep donor States [5] (in this case sulfur or selenium), determining uncharacteristic of pure silicon doped absorption material in the near-IR region [6]. The degree of doping determines the number (density) of the donor States and, as a result, the absorption coefficient of the doped material in the near-IR region. In the case of doping from the gas phase (for example, sulfur-containing compounds) [2], in the absence of a preliminary condensation deruosi compounds before exposure UKI effective collisions of the molecules containing deruosi element with the target surface or particles ablative torch cyclically occur after exposure�of each third IES in a certain time window, which is determined by the cooling time of the surface or the lifetime (expansion, cooling and condensation) ablative torch. At an initial low density of molecules containing deruosi element in the reaction gas mixture at a pressure of <1 ATM irradiated over multiple IES the surface of silicon and narrow - on the order of nanoseconds - the time window for his doping kinetics is rather slow due to relatively low average flow doperalski agent and a small time of its introduction into a target. As a result, to achieve high degrees of doping requires too much time. In a more advantageous case when processing (heating, melting and ablation) under the action of the IES pre-deposited on the surface of nanometer silicon film solid selenium originally deruosi agent is present in excess on the surface of silicon and the kinetics of its implementation is determined only by the specified time window. However, in the latter case, the ablation of the target starts with this film Selena and therefore immediately starts irreplaceable consumption (removal of the external environment) this doperalski agent and then introduced into the target amount of selenium in the subsequent microstructurally target can only decrease due to partial irreversible (BAA� resultant deposition on the microcones) ablative removal of already doped target material and is, in the end, ~0.1%. This disadvantage is avoided by using the proposed invention, including a new method of forming microstructured and vysokodobrotnykh gray layer on the surface of silicon.

The problem solved by the invention is to eliminate the disadvantage of the prototype, i.e. in multiple increase in the degree of doping of the surface layer of the silicon atoms of sulfur in the process of microstructurally under the action of the IES.

To solve this problem it is proposed to choose a special type of active reaction medium is a liquid phase sulfur-containing compounds with a high sulfur content, as well as the mode of action of the IES, the parameters of which are selected so that radiation UKI penetrated to the target through a liquid phase sulfur-containing compounds, and the energy, repetition rate, and focusing UKI provided ablative microstructurally silicon surface [7].

The solution of this problem is demonstrated by the following examples. Plate undoped silicon with a polished surface of optical quality is first irradiated in the scan mode of the focused radiation UKI titanium-sapphire laser with a Central wavelength of 744 nm, duration 100-110 FS and energy of 0.3-5 MJ, to provide intense ablation and microstructurally silicon surface (Fig.1) when density improvement�and energy of the IES in the range ≈0.3-0.7 j/cm 2depending on the number of laser pulses (typically in the range 102-103) falling in each point of the surface. Then, this silicon wafer with a microstructured surface is immersed in the cell with a sulfur-containing compound is a liquid carbon disulphide CS2- at a depth of 3-4 mm, and a microstructured surface is re-exposed under the same conditions for ablative doping already prepared microstructured surface. High-temperature vaporization of the liquid carbon disulphide and thermal decomposition of the molecules of CS2at least until diatomic molecules CS and sulfur atom in interaction with the heated surface of a solid or molten silicon [8] or with atomic and cluster-drip components ablative torch silicon provide high - close to solid-phase concentration of sulfur atoms on the silicon surface, resulting in extremely high speed and the resulting record of the degree of doping (up to 5%), according to energy dispersive x-ray analysis of the doped layer. The spectrum in (a) and table (b) with the results of the analysis on the content of silicon, oxygen, and sulfur in the surface layer of the irradiated material are shown in Fig.2. Otherwise microstructurally silicon under the action of IES at once - at one stage - Prov�be carried out in liquid carbon disulfide for simultaneous doping his microstructuring surface.

The sulfur doping leads to the appearance in the IR spectrum of the treated material (symbol "exp") is compared with the tabulated spectrum of transmittance of undoped crystalline silicon (symbol "tabul") is characterized by the failure of the transmittance in the region of 1.4-2 µm, is marked by an arrow in Fig.3, the appearance of which was consistent with the formation of deep donor S-sulfur centers in silicon at a depth of 0.7 eV below the bottom of the conduction band [5]. Processing the IR spectrum for sulfur doped silicon for the characteristic depth of the doped layer ~100 nm, measured by us by return the Rutherford scattering of α particles shows more substantial than has been achieved previously [2, 3, 6], the absorption coefficient of silicon in the infrared region in the range of 1.4 to 4 μm. The corresponding spectra of the absorption coefficient for undoped crystalline silicon (solid curve) and its doped layer with the degree of doping of ≈5 at.% (the dotted curve indicated by the absorption band of deep donor S-sulfur centers) are shown in Fig.4.

Thus, the proposed by the present invention multiple (almost an order of magnitude) increase in the degree of doping of the surface layer of the silicon atoms of sulfur in the process of microstructurally under the action of the IES being implemented in practice and involves significant higher�s IR photosensitivity of silicon for possible applications for example, in solar energy and optoelectronics night vision devices.

Literature

1. N. C. Lindquist, P. Nagpal, K. M. McPeak, D. J. Norris, S.-H. Oh, Engineering metallic nanostructures for plasmonics and nanophotonics, Rep.Prog. Phys. 75, 036501 (2012).

2. C. H. Crouch, J. E. Carey, M. Shen, E. Mazur, and F. Y. Genin, Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation, Appl. Phys. A 79. 1635-1641 (2004).

3. M. J. Smith, M. Winkler, M.-J. Sher, Y.-T. Ling, E. Mazur, S. Gradecak, The effect of a thin dopant precursor on the structure and properties of femtosecond-laser irradiated silicon, Applied Physics A 105, 795-800 (2011).

4. P. G. Maloney, P. Smith, V. King, C. Billman, M. Winkler, E. Mazur, Emissivity of microstructured silicon. Applied Optics 49, N7, 1065-1068 (2010).

5. P. Yu, M. Cardona, fundamentals of physics of semiconductors, Moscow, Fizmatlit, 2002, Chapter 4.

6. M. A. Sheehy, L. Winston, J. E. Carey, C. M. Friend, and E. Mazur, the Role of the background gas in the morphology and optical properties of laser-microstructured silicon, Chem. Mater. 17, 3582-3586 (2005).

7. E. B. Votes, A. A. Ionin, Yu. R. Kolobov, S. I. Kudryashov, A. E. Ligachev, S. V. Makarov, Yu. N.Novoselov, L. V. Seleznev, D. V. Sinitsyn. The formation of quasi-periodic nano - and microstructures on silicon surface under the action of IR and UV femtosecond laser pulses. Quantum. El-ka 41 (9), 829-834 (2011).

8. A. A. Ionin, S. I. Kudryashov, L. V. Seleznev, D. V. Sinitsyn, A. F. Bunkin, V. N. Lednev, S. M. Pershin, thermal melting and ablation of silicon surface by femtosecond laser radiation, JETP 143, №3, 403-422 (2013).

1. Method of forming microstructured, vysokoperedelnogo sulfur atoms in the surface layer of silicon, based on the irradiation surface�ti silicon multiple focused ultrashort - Femto - or short picosecond - laser pulses (IES) in a chemically active medium sulfur-containing compounds from the decomposition of sulfur-containing compounds on heated or melted by the laser to the silicon surface and subsequent diffusion of sulfur atoms in the volume of the condensed phases of silicon, or laser ablation of the surface of the silicon UKI and decomposition of carbon disulphide in the interaction with ablative torch, accompanied by the precipitation and decomposition of sulfur-containing intermediates on the heated or melted by the laser to the silicon surface, followed by diffusion of sulfur atoms in the volume of the condensed phases of silicon,
characterized in that the chemically active medium is chosen liquid-phase compound with high content of sulfur, carbon, in which immerse the irradiated silicon target, and choose this mode of impact on the target of the IES that the radiation UKI penetrated through the liquid carbon disulfide to the silicon target, and the energy, repetition rate, and focusing UKI provided ablative microstructurally silicon surface.

2. A method according to claim 1, characterized in that the first perform ablative microstructurally dry silicon surface under the action of multiple IES, and then a microstructured surface process multiple IES padonkam layer of the liquid phase of carbon disulphide for doping the surface layer of the silicon atoms of sulfur.



 

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FIELD: chemistry.

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