The Self-assembled Deposition on the Surface of Mono-crystalline Silicon Induced by High-Current Pulsed Electron Beam

Introduction

The preparation of thin films containing nano-crystalline silicon (nc-Si) has attracted much attention due to its entirely different function comparing with the indirect band-gap for bulk material. In recent years, some researchers reported that nano-particles with diameters less than the Bohr radius of exciton (~5 nm) have been made to exhibit intense visible photoluminescence (PL) owing to the quantum size effect. The silicon nanoparticles constitute a promising route to electroluminescent devices [1, 2, 3, 4]. They can be obtained by a number of techniques such as evaporation deposition [5, 6], sputtering deposition [7, 8] and laser ablation [9]. The higher-energy acted on surface results in the occurrence of evaporation-deposition for the material surface [10]. So the evaporated features can be used to prepare the nc-Si films.

Under the action of HCPEB, a high energy (10⁸–10⁹ W/cm²) is deposited only in a very thin layer(less than tens microseconds) and causes superfast heating and cooling, even melting, evaporation, and solidification on the treated surface of material. As a result, abundant metastable microstructures are formed in the irradiated surface layer [11, 12, 13, 14]. The combination of the above-mentioned factors produced by the HCPEB irradiation would remarkably influence the physical mechanical properties of the irradiated surface, which is mainly investigated in our previous work.

To our knowledge, HCPEB technique has been proved to be a powerful tool for surface modification and nano material preparation [15, 16]. The surface of material is modified under three modes including “heating”, “melting” and “evaporating” due to the different electron beam energy. Under the type of evaporation mode, the deposited phenomenon of material surface produced after HCPEB irradiation is reported rarely. In addition HCPEB technique has been investigated by many researchers with different kinds of metals, steels, super alloys and etc. [17,18, 19], while few studies are concentrated on nonmetal.

In the present work, we prepare the regular pattern of nc-Si under the evaporation mode after HCPEB irradiation. The abundant of dislocations of the irradiated surface layer were characterized in detail. In particular, the evaporation-deposition mechanism between evaporation silicon particles and microstructures was also discussed.

Experimental details

Si wafers (10×10×0.5 mm) with resistivity of 1–50 Ω cm were subjected to HCPEB irradiation. Prior to HCPEB treatment, samples were ultrasonically rinsed with ethanol and acetone for 1 h, respectively. The direction of fixed silicon wafer was perpendicular to the electron beam. The HCPEB irradiations were carried out using “Nadezhda-2” source, Figure 1 shows the schematic diagram of HCPEB irradiation experiment. The bombardment was carried out under the following conditions: the vacuum 10⁻⁵ torr, an accelerating voltage 27 keV, and energy density 4 J/cm², the repetition interval being 10 s, the number of pulses: 1, 5 and 10 respectively. More details about the principle of the HCPEB system are in ref. [14,15]. The surface morphologies were investigated by using MFP-3D Atomic Force Microscope (AFM) and JEOL JSM-7001F scanning electron microscope (SEM). The transmission electron microscope (TEM) observation was performed by JEM-2010 type TEM, which operated at 200 kV acceleration voltages. The ultrathin silicon samples were prepared by Ar ion beam milling in Gatan model 691 precision ion-polishing system after mechanical polishing.

Figure 1:

Schematic diagram of HCPEB irradiated experiment.

Result and discussion

The AFM measurement was adopted to study the surface morphology of Si (100) substrate by HCPEB irradiation, as shown in Figure 2. Figure 2(a) shows the surface morphology, which was not completely flatness, presented low density of nc-Si particles with 1 pulse. Figure 2(b) shows the surface morphology after 10-plused irradiation. One can see that the density of nc-Si particles, which size was increased slightly, have significantly improved comparing with Figure 2(a). It is worthy noting that the sediment nanometer particles arranged in grid array form. Upon rising the pulses to 20, very dense island-like nc-Si particles were formed on the Si wafer surface, the size of which increased slowly, as show in Figure 2(c). The corresponding size, density and the height of the nc-Si particles of single crystal silicon (100) wafer by HCPEB irradiation can be estimated using the Nanoscope analysis software, as shown in Figure 2(d). It follows that the size and density of nc-Si particle were increased with the increment of the pulse number. Beside the height was still about 20 nm after the 10-pulsed irradiation.

Figure 2:

Si (100) self-assembled nanostructures prepared by HCPEB irradiation (a) 1 pulse, (b) 5 pulses, (c) 10 pulses, (d) structural information of Si dots.

Figure 3 shows the AFM images of Si (111) substrate by the HCPEB irradiation. The statistics of three dimensional structure are similar with single crystal silicon (100).

Figure 3:

Si (111) self-assembled nanostructures prepared by HCPEB irradiation (a) 1 pulse, (b) 5 pulses, (c) 10 pulses, (d) Structural information of Si dots.

Figure 4(a) represents the surface SEM image of the single crystal silicon (100). It can be seen that nearly spherical particles with size of about 800 nm were distributed on the top surface of the sample. Besides, the corresponding EDS analysis shows that the element of these nanoparticles was identified as single element of Si (Figure 4(b)), and matrix (Figure 4(a)) are the same, no trace of any other elements. It reveals that the nc-Si particles were indeed deposited on the surface of single crystal silicon by HCPEB irradiation. In other words, the phenomenon of self-deposition have been induced by HCPEB irradiation on the surface of sample.

Figure 4:

SEM and EDS of Si (100) after 10 pulses irradiated by HCPEB (a)SEM image, (b) EDS of Si nanoparticle, (c) EDS of Si base.

Previous research results show that the surface of material was modified under three modes including “heating”, “melting” and “evaporating” depending on the temperature reached at the surface of the material [11,19,20]. When the electron beam energy was high enough, the surface of material reached the boiling point and then the evaporation phenomenon was occurred. In case of silicon, the temperature of surface was estimated by the calculative method, which provided by Proskurovsky et al. using the parameters employed for the experiment [20]. The estimated temperature, which reached about 3,000 K, was higher than the boiling point (2,628 K) of silicon. In this condition, the evaporation phenomenon took place in the surface of Si wafer. Under the type of evaporation mode, the vaporing droplets were deposited on the Si substrate surface in the process of cooling, then the self-deposited nanostructures were formed, as shown in Figures 2 and 3.

In order to obtain more information between microstructures (especially defect structures) and self-deposited nanostructures, the thin foils for TEM observation were prepared. TEM investigations reveal that significant deformed microstructures were induced on the material surface after HCPEB irradiation. Figure 5 shows the network of dislocations with 5-pulsed irradiation. The array of dislocations in Si (100) direction is rectangular network, and in Si (111) direction is approximate hexagonal network, as shown in Figure 5(a) and (b) respectively. These dislocation network was formed due to dislocation reaction induced by HCPEB irradiation.

Figure 5:

Rectangular dislocation and hexagon dislocation induced by HCPEB irradiation in Si (a) Si (100) Net dislocation, (b) Si (111) hexagon dislocation.

It is noted that the geometric configuration of self-deposited nanostructures are similar with regular array dislocations network induced by HCPEB irradiation, as shown in Figure 6. Figure 6(a) shows that the length of rectangular network in the embedded TEM graph is about 700nm. The one side value of self-deposited nanostructure was same. It is possible that the different areas are observed by TEM and AFM, respectively. But the measured values of two different structures are close. Thus they have direct relationships between the self-deposited nanostructure and regular array network. As shown in Figure 6(b), the three-dimensional height reveals that four nanoparticles, the height of which on vertex is always higher than the around particles, are distributed on each side of rectangular network. So the self-assembly nanometer structures were formed by deposition.

Figure 6:

Si (100) wafer irradiated by HCPEB (a) self-assembled net nano-array, (b) the three-dimensional height of Si (100) nano-array.

Figure 7 shows the self-deposited nanostructures in the orientation Si (111). The geometric configuration of dislocation networks also was consistent with self-deposited nanostructures. This fact further demonstrates that the deposited Si droplets were favor to deposit at the crystal defects such as dislocation configurations.

Figure 7:

Si (111) self-assembled hexagon nano-array prepared by HCPEB irradiation.

Based on the above results, we therefore conclude the growth mechanisms of self-deposited nanostructure produced by HCPEB irradiation. In the irradiation experiments, the phenomenon of evaporation was occurred on the surface of material. Note that the gravity force cannot be responsible for the re-deposition of the evaporated matter, because the irradiated direction of HCPEB was perpendicular to gravity. Evaporated droplets, which were charged by negative charge, were drawn back by target (as anode material). In the following irradiation at every turn, deposited droplets were acted as more stable nucleus during the process of evaporation and deposition. Eventually nanoparticles were formed for the merging between the nucleus, as shown in Figures 6 and 7. Wang et al. [21] investigated that deposited-particles were not homogeneous distributed on the surface of matrix in the process of deposition. The positions with high energy and lower deposited activation (crystal boundary, dislocation etc.) on surface were priority to attract evaporated particles. It is clear that amount of crystal defects including dislocations were induced by HCPEB irradiation from TEM investigations. During deposited process, the evaporation particles were attracted by the area with crystal defects to preferentially deposit.

Conclusion

In the present work, the surface microstructure and the distribution of Si nanoparticles, before and after HCPEB irradiation with various pulses, were characterized. Combining with all the experimental results, the main conclusions can be summarized as follows:

1. After HCPEB treatment, evaporated droplets were deposited to form island-like nc-Si particles. Deposited nc-Si particles on the surface of material were arrayed orderly.

2. TEM investigations reveal that a significant dislocation structures were induced on material surface after HCPEB irradiation. The dislocations in Si (100) direction are rectangular network, and in Si (111) direction are approximate hexagonal network.

3. Under the type of evaporation, the evaporated droplets preferred to gather in the area with dislocations (high energy and lower deposited activation) and eventually regular self-deposited structure were formed on the surface of material.