1. INTRODUCTION
The laser-driven ion sources (LIS) have been investigated and proposed for different technological applications (e.g., material modification or ion acceleration technologies). The LIS can potentially produce very high ion currents from normally solid materials, with selectable (up to very high) energy and charge states (Boody et al., 1996; Woryna et al., 2000; Láska et al., 2000; Wołowski et al., 2003).
The laser beam has many advantages and applications in terms of deposition and ion implantation of ablated and ionized material (Fernandez et al., 2005; Thareja & Sharma, 2006; Trusso et al., 2005; Wieger et al., 2006). The pulsed laser deposition (PLD) technique becomes increasingly popular as a method for producing thin films from novel materials (Veiko et al., 2006), particularly in applications for semiconductor technology, including the fabrication of semiconductor nano-crystals. The laser-induced ion implantation technique has flexibility not available in the case of other implantation methods. The application of laser-induced ion implantation is a novel method for the fabrication of semiconductor nano-crystals, competitive with conventional ion implantation techniques used for semiconductor technology. In this method, varying the laser fluence, target-to-substrate distance, parameters of ion acceleration/deflection fields, and subsequent annealing conditions, one can control the size and distribution of nano-crystals. An important advantage of the laser ion source is also the great ease of biasing the ion source or implanted sample to a high voltage for additional ion acceleration.
Application of LIS to nano-crystal formation may be attractive for direct ultra-low-energy ion beam implantation in thin SiO2 layers (Chen et al., 2003; Qi et al., 2001; Torrisi et al., 2005; Rosiński et al., 2005; Eliezer et al., 2005) and for the production of uniform size and depth-concentration of laser-produced Ge ions of different energies (Masuda et al., 2002; Wołowski et al., 2005). Although from a nano-crystal production point-of-view, the implantation of laser-produced ions appears to be quite a simple technique compared to traditional ones, there are many technological issues that should be solved before establishing a reliable and reproducible nano-structure fabrication procedure.
2. EXPERIMENTAL ARRANGEMENT FOR LASER-INDUCED GE ION PRODUCTION AND IMPLANTATION
In the deposition/implantation experiment, a repetitive Nd:glass laser (1.06 μm, 3.5 ns, <0.8 J, <10 Hz) focused on the surface of a pure Ge target that was applied for production of Ge ions and neutrals. The ion characteristics were determined with the use of ion diagnostics based on the time-of-flight method (ion collectors and electrostatic ion energy analyzer) (Woryna et al., 1996; Láska et al., 1996). The laser pulse energy was 550 mJ. The laser spots had diameters of Df = 3 mm (corresponding laser intensity was: IL = 2.5 × 109 W/cm2). For comparison, the same measurements have been performed at Df = 0.4 mm (corresponding laser intensity was: IL = 1.4 × 1011 W/cm2). A scheme of the set-up showing the ion diagnostics and samples implanted with the use of laser produced ions is presented in Figure 1.
The experimental arrangement used for characterisation of laser-produced Ge ions and their implantation/deposition into the SiO2 substrates.
The Ge ions produced in ∼1000 or in ∼3000 laser shots were deposited and implanted in SiO2 substrate of thickness of ∼20 nm, prepared on the Si single crystal. The SiO2 substrates V1B, V3B, and V5A were deposited/implanted with Ge ions produced by 1058, 3000, and 3000 laser shots, respectively. The substrate V5A has been prepared in METU, while the samples V1B and V3B have been received from a Polish semiconductor laboratory. Another set of SiO2 samples were deposited/implanted with Ge ions generated in 100, 200, 400, and 800 laser shots.
3. PARAMETERS OF LASER-PRODUCED GE IONS
The stream of Ge ions emitted along the target normal estimated on the basis of the ion collector signal for the distance of 6 cm from the laser irradiated Ge target was > 1016 ions/cm2 (for ∼1000 laser shots). The surface of the sample was also deposited by neutrals (atoms, debris, cluster) not recorded by the ion collector.
Generally, the maximum ion current density was observed in the direction perpendicular to the target surface. The maximum and the mean Ge ion energy (for IL = 2.5 × 109 W/cm2) were 1 keV and 3 keV, respectively. The streams of Ge ions at the places of the implanted samples have been determined on the basis of the ICs signals, taking into consideration the IEA spectra (for IL = 2.5 × 109 W/cm2). The total Ge ion streams on the surface of implanted substrates located at distances of 12 cm and 24 cm and at an angle of 0° were 4.5 × 1016 cm−2 and 1.2 × 1016 cm−2, respectively, while at the angle of 30° these streams were 2.7 × 1015 cm−2 and 1 × 1015 cm−2, respectively.
The maximum and mean ion charge states measured with the use of IEA at the laser intensities of 2.5 × 109 W/cm2 and IL = 1.4 × 1011 were 2+ and 9+, respectively. The examples of the IEA spectra recorded at particular deflecting voltages (fixing the Ei/z) are shown in Figure 2. The IEA spectra of ions emitted from the plasma generated by the laser beam heating a fresh place on the target shows the existence of light contaminant ions (H, C, and O) adsorbed on the Ge target surface.
Examples of the IEA spectra recorded at two different laser intensities: IL = 2.5 × 109 W/cm2 (left) and IL = 1.4 × 1011 W/cm2 (right), the remaining laser irradiation conditions: EL = 0.54 J, Df = 3 mm.
The typical IC signals recorded at the same distance of 70 cm but at different angles are shown in Figure 3. Generally, the maximum ion current density was observed at the direction perpendicular to the target surface. A maximum and a mean Ge ion energy (for IL = 2.5 × 109 W/cm2) were 1 keV and 3 keV, respectively.
An example of IC signals recorded by ion collectors placed at different angles. The laser irradiation conditions: EL = 0.54 J, Df = 3 mm, IL = 2.5 × 109 W/cm2.
4. THE PROPERTIES OF SIO2 SAMPLES IMPLANTED/DEPOSITED WITH LASER-PRODUCED GE IONS
The analysis of the samples was performed with the use of the XPS + AES method and ion etching (using 1-μA current of Ar+ ions having energy of 3 keV) at Warsaw University of Technology (the Faculty of Material Engineering). The etching speed was 0.0025 nm/s on a sample surface of 8 × 8 mm2. On the basis of the XPS + AES spectra (Fig. 4), the depth profiles of different elements in the SiO2 layer were estimated. In the layer of < 4 nm, the amount of deposited laser-produced Ge atoms is very high; in depth of ∼2.5 nm more than 50% Ge atoms were estimated. In this layer, there are also oxygen and carbon atoms (probably contaminants). In the SiO2 layer, the number of implanted Ge ions decreases down to several percent in depth of ∼10 nm. It was shown that laser-produced Ge ions were implanted even at the depth of 18 nm, a maximum depth investigated in this test.
An example of the XPS + AES spectra recorded at etching times of 5400 and 7200 seconds for SiO2 sample implanted with Ge ions produced by 400 laser shots. The Auger lines are marked with letter “a”.
The Raman spectroscopy measurements of the surface of substrates deposited/implanted with the use of repetitive laser were performed in METU in Ankara in the framework of IPPLM-METU cooperation. Figure 5 presents the Raman spectra of samples Numbers V1B, V3B, and V5A (described in Section 2) at different numbers of laser shots. The sharp peak at ∼300 cm−1 is due to the Ge crystallite formed probably being fast laser-produced debris or clusters striking the sample surface.
The Raman spectra of SiO2 samples implanted with the laser-produced Ge ions. The properties of samples Numbers V1B, V3B, and V5A.
The obtained Raman spectra shown in Figure 5 clearly display the band at 300 cm−1 which is the result of the scattering of Ge nano-crystallites on the SiO2 sample formed in the process of ion deposition/implantation and subsequent annealing. The line width (FWHM) of the Raman lines estimated for Ge crystalline structures of investigated samples were 7.1–10.6 cm−1. Taking into consideration the dependence of the FWHM of the Raman peak on the nano-crystal size (Wu et al., 1997) the size of investigated Ge nano-crystals was roughly estimated as 5–10 nm.
5. CONCLUSIONS
The experimental arrangement for deposition/implantation of laser-produced Ge or Si ions into semiconductor substrates using a new repetitive rate laser system (laser pulse energy, EL ≤ 0.8 J, laser pulse duration, tL = 3.5 ns, laser wavelength, λL = 1.06 μm, pulse repetition rate, ν = 10 Hz) has been completed and tested. This set-up comprises the experimental chamber, focusing optical and alignment systems, and ion diagnostic devices as well as a repetitive laser system with a Faraday rotator device and a movable target holder.
This experimental arrangement has been used at IPPLM for investigations of implantation of laser-produced Ge into the SiO2 substrate. The measurements were performed mostly at laser intensity of 2.5 × 109 W/cm2. The Ge ion stream attained maximum energy of ∼3 keV and maximum total intensity (ion fluence) of ∼5 × 1016 ions/cm2 on the SiO2 substrate. The depth profiles of different elements in the SiO2 layer of samples deposited/implanted in the IPPLM experiment were estimated on the basis of XPS + AES spectra measured at Warsaw University of Technology in collaboration with the IPPLM group. In the layer of < 4 nm, the amount of deposited laser-produced Ge atoms is very high, in depth of ∼2.5 nm more than 50% Ge atoms were estimated. It was shown that laser-produced Ge ions were implanted even at the depth of 18 nm—a maximum depth investigated in this test. At Middle-East Technical University, measurements of Raman spectra were performed on the annealed SiO2 samples deposited/implanted in the IPPLM experiment. Taking into consideration the dependence of the FWHM of the Raman peak on the nano-crystal size dimensions of investigated Ge nano-crystals was roughly estimated as 5–10 nm.
ACKNOWLEDGMENT
This work has been partially supported by the European Commission through a project called SEMINANO under the contract NMP4-CT-2004-505285.
