Simultaneous optical second harmonic and sum frequency intensity image observation of hydrogen deficiency on a H–Si(1 1 1) 1 × 1 surface after IR light pulse irradiation
Introduction
The hydrogen desorption promoted by rapid temperature rise on a Si surface after light pulse irradiation is known as laser-induced thermal desorption (LITD). The LITD process is utilized in the optical chemical vapor deposition (CVD) growth technique and has been studied well [1], [2]. In the practical optical CVD growth of a uniform Si film, spatial uniformity of coverage and the orientation of H–Si bonds are important. However, because LITD is an indirect process, the spatial distribution changes dramatically as a function of power density, wavelength, and duration of the incident light pulse [1], [3]. One must also consider the effect of electron–hole plasma excitation on the hydrogen desorption [1].
For the application of the LITD process, it is important to understand the spatial distribution of the coverage and the orientation of the Si–H bonds on the Si surface after pulse light irradiation [4], [5], [6]. However, there have been only a few reports on the spatial distribution of hydrogen coverage on a Si surface [3], [6], [7], and none concerning the spatial distribution of the orientation and vibrational mode of the Si–H bonds. One reason why it is difficult to determine the chemical bonding state during the LITD process may be the lack of techniques for monitoring a two-dimensional distribution image of hydrogen on a Si surface.
In order to establish a technique for such observations, we have developed a visible–IR sum frequency microscope operating in ultra-high vacuum (UHV) conditions. The SF microscopy enables us to observe a resonant vibrational image on a Si(1 1 1) surface. It is expected to be useful for detecting the spatial variation of the orientation and the vibrational mode of the Si–H bonds. The peak wavenumber of the SF response reflects the species of the hydrides. We have further added the function of detecting optical second harmonic generation (SHG) and have created a system for observing SF and SH images simultaneously.
SHG and SFG are the lowest-order nonlinear optical processes. They occur in media without inversion symmetry and have high surface sensitivity [8]. SHG has been used as a tool to investigate the surface structures [9] and surface electronic states associated with dangling bonds on a Si surface. It was used to estimate the speed of hydrogen adsorption [10] and the diffusion constant of hydrogen [11] on a Si(1 1 1) surface. Recently, it has been applied to microscopy [12] for monitoring the spatial distribution of dangling bonds on a Si(1 1 1) surface [3], [6].
SFG is a useful tool for studying Si–H bonds [13] and identifying hydride species [14] on a Si surface. In order to analyze the non-uniform reaction of hydrogen on a Si surface with complicated spatial structures – e.g., in integrated circuits during fabrication – it is necessary to observe the two-dimensional distribution of vibration in hydrides. This is difficult to do with conventional vibrational microscopy, such as Raman and IR microscopy, because these techniques are not sufficiently sensitive. Thus it will be advantageous to use SFG to observe the contrasts in both the SF and SH intensity images, to distinguish clearly between the H–Si(1 1 1) surface areas irradiated and not irradiated by the light pulses.
By simultaneous SF and SH microscopy, one can evaluate the coverage, the orientation and the vibrational mode of Si–H bonds more systematically than by separate observations. As a demonstration of this system, we observed the hydrogen deficiency of a H–Si(1 1 1) surface irradiated by IR light pulses with a temporal width of ∼6 μs (hereafter we call this light pulse a desorption-inducer IR light pulse). The hydrogen desorption process promoted by the desorption-inducer IR light pulse in this study is a LITD process, as we have confirmed by LITD model simulation (not shown).
In the SH intensity images obtained in this study, we first confirmed that dangling bonds were formed after hydrogen desorption in the area irradiated by the desorption-inducer IR light pulses. Then, in the SF intensity images, non-resonant signals were observed in the irradiated area. We also found that a boundary area occurs between the areas with resonant and non-resonant SF signals in some irradiation light conditions. Both the resonant and non-resonant signals were very weak in this area. This last result suggests the occurrence of a special Si–H bonding state at the edges of the focused light spot on the surface. It can be observed only by SF microscopy.
Section snippets
Experimental
The system for observing simultaneous SF and SH intensity images in UHV conditions is shown in Fig. 1. In this SF microcopy system, we used doubled frequency output from a mode-locked Nd3+:YAG laser as the visible light at wavelength 532 nm, and output (∼4.8 μm) from an optical parametric generator and amplifier system (OPG/OPA) as the wavelength-tunable infrared light (IR probe light). The spectral bandwidth of the IR probe light was 6 cm−1. The pulse energies of the IR probe and visible light
Results and discussion
We irradiated the H–Si(1 1 1) 1 × 1 surface with the desorption-inducer IR light pulses, drawing a circle with a diameter of ∼0.2 mm and a dot at the center of the circle, and observed the SF and SH intensity images. Fig. 2a shows a SH intensity image of the H–Si(1 1 1) surface soon after irradiation by the desorption-inducer IR light pulses. The bright dots represent SH photons, and the scale bar corresponds to 200 μm. The SH signals at the irradiated area have a circular shape with a diameter of ∼0.2
Conclusions
In this study, we have developed a microscope for the simultaneous observation of SF and SH images, and have observed a H–Si(1 1 1) surface after irradiation by desorption-inducer IR light pulses. We have obtained the first SF images of a solid surface in an ultra-high vacuum condition. We have found that after irradiation by the desorption-inducer IR light pulses, the SH signals were enhanced due to the formation of dangling bonds after hydrogen desorption. Non-resonant SF signals appeared after
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