Nanostructured Si/SiO 2 Quantum Wells Nanostructured Si/SiO 2 Quantum Wells

The motivation for developing light-emitting devices on an indirect transition semicon - ductor such as silicon has been widely discussed for Si/SiO 2 nanostructures. In this chapter, we report on the fabrication of Si/SiO 2 quantum-confined amorphous nanostructured films and their optical properties. The Si/SiO 2 nanostructures comprising amorphous Si, SiO 2 , and Si/SiO 2 multilayers are grown using ultrahigh vacuum radio frequency magnetron sputter - ing. Optical absorption coefficients of the Si/SiO 2 nanostructures are evaluated with regard to tentative integrated Si thicknesses. Optical energy band gaps of the Si/SiO 2 multilayer films are in accordance with the effective mass theory and described as E 0 = 1.61 + 0.75d −2 eV at the Si layer-integrated thicknesses ranging from 0.5 to 6 nm. Quantum confinement effects in the Si/SiO 2 nanostructures are inferred from optical transmittance and reflectance spectra. The rapid-thermal-annealed Si/SiO 2 multilayer films demonstrate the intensified photoluminescence at ~1.45 eV due to the formation of nanocrystalline silicon. The tem perature dependence of the nanocrystalline luminescence intensity shows the nonmonoto - nous behavior which is interpreted invoking the Kapoor model.


Si/SiO 2 layer films preparation
Si/SiO 2 QWs films are synthesized in an ultrahigh vacuum (UHV; 3 × 10 −8 Pa) RF MS system at a very small deposition rate (from 0.005 to 0.5 nm/s). The schematics of the UHV RF magnetron sputtering systems are shown in Figure 1. The ultrahigh vacuum chamber is equipped with two AJA A300 UHV RF magnetron sputtering guns connected to argon and oxygen gas lines, sputter ion guns, and 5 N Si and 5 N fused quartz SiO 2 targets. Preparation temperatures are controlled at the substrate holder. Transparent substrates are used for optical measurements; crystalline and amorphous substrates are used to test the influence of substrate crystallinity on the film growth. All depositions are operated at room temperature on both transparent sapphire A and opaque Si (100) substrates. The polished sapphire substrates are etched in dilute HF and put into the UHV chamber. The base pressure of the chamber is 10 −7 Pa, and the sputtering gun pressure during the plasma operation in argon is 2 × 10 −1 Pa. The deposition process is a repetition of Si and

Crystallinity
The XRD spectra (CuKα source) of 180-nm thick Si single layer and 150-nm thick SiO 2 single layer prepared on the sapphire substrates at room temperature do not show the crystallinity of the samples. Both spectra reveal the noncrystalline characteristics of Si and SiO 2 films.

Density-of-states structures
Atomic constitutions of each layer are evaluated with XPS on binding energy of Si2p and O1s electrons. The bulk Si2p core-level binding energy for Si(111) is ~99.3 eV and the bulk Si2p oxide binding energy value for SiO 2 is ~103.7 eV referring to Keister [66]. PHI 500 Versa Probe II scanning XPS microprobe is designed to take out a 10-degree signal, enable slow speed (0.01 nm/s SiO 2 ) area etching, and equipped with a monochromatic AlKα X-ray source. The depth profiles are characterized by using a low-energy argon ion gun to avoid selective etching. The binding energy dependence of the core densities of states at each etched depth suggests periodic distributions of each atomic composition. This analytical technique has particular applicability to the evaluation of the density of states with atomic contributions.  Figure 3(a), at the depth of 0.54 nm, 103.2 eV intensity peaks and 98.9 eV show a minimum. The density-of-states depth profiles explain the presence of the Si/SiO 2 -layered amorphous nanostructure fabricated using the UHV RF magnetron sputtering method at the atomic scale precision.

Optical properties of Si/SiO 2 layer films
Optical transmittance spectra and reflectance spectra are measured with the help of JASCO V-670 visible and ultraviolet optical photometer at room temperature. Optical properties of an amorphous Si/SiO 2 nanostructure film show the higher optical transmittance and wide optical window effects. Unique optical properties are a candidate for solar windows in solar cells or filters of ultraviolet light. The parameters characterizing the Si/SO 2 film structures are the well layer thickness, the barrier layer thickness, and the number of periodicity. Figure 4 displays the optical reflectance and transmittance spectra of amorphous Si/SiO 2 -nanostructured layer films of various period numbers. The well thickness of the samples changes from 2 to 24 nm, while the barrier thickness is fixed at 4.8 nm. As the period's number of layers increases, the optical reflection decreases and the optical transmittance increases markedly, although the onset energy of transmittance and the absorption edge wavelength show the constant values. The increasing period number enhances optical transmittance and decreases optical reflectance. The spectra are saturating at 8-12 barrier layers. Increasing the period number does not change the absorption edge energy. Nanostructure effects observed on the 12-layer Si/SiO 2 film as the optical transmittance and reflectance effects are saturating. Figure 5 exhibits the optical reflectance (a) and transmittance spectra (b) of Si/SiO 2 films as a function of the Si well thickness at the constant 12-period numbers and the constant barrier thickness of 4.8 nm. The increasing Si well thickness increases the reflectance and decreases the transmittance as expected. Also, the absorption edge energy shows the constant values. Figure 6 shows the barrier thicknesses dependence of optical reflectance (a) and optical transmittance (b) spectra for the constant 12-layer period and 2 nm well thicknesses. The increasing barrier thicknesses diminish the optical reflectance and enhance the optical transmittance.

Absorption coefficient
Absorption coefficients α (λ) are used as the index of intrinsic properties of thin film materials. Absorption coefficient α (λ) at a wave length λ is evaluated from Eq. (1) for the sample thickness d with the optical transmittance T(λ) and reflectance R(λ). On the Si/SiO 2 nanostructure multilayer films, the integrated thickness of the Si layer, the reduced film thickness in Eq. (2) is used as the tentative thicknesses.
( αhν ) 1/2 = β (hν − E 0 ) α > 10 3 (3) Figure 7 shows the dependence of the absorption coefficients on the Si well thickness of 12-layer Si/SiO 2 films. The photon energy dependences of absorption coefficient show a sharp rise in the energy of absorption edges above 1000/cm. The dependence of the absorption edge energies on the Si well layer thickness is measured from 0.5 to 6 nm at the SiO 2 barrier layer fixed at 2.4 nm. In Figure 8, (αhν) 1/2 vs. photon energy is plotted. The absorption coefficients of amorphous films are related in Eq. (3) known as an amorphous plot to obtain the absorption edge energy.

Quantum confinement
In Figure 9, the absorption edge energy is plotted vs. the tentative well thicknesses in a Si/SiO 2 multilayer structure and compared with the effective-mass theoretical estimations. Two types of the absorption edge energy evaluated from Figures 7 and 8 are indicated. The absorption edge energy becomes larger as the QW thickness gets smaller. The blue shifts of the absorption energy are impressive in Figure 9. The absorption edge energy values E 0 are evaluated for each well thickness following the effective-mass theory, Eq. (4) [9]. The Si layer thickness dependency of absorption edge energy is in accordance with the effective-mass theory for thicknesses 0.5 < d < 6 nm in Eq. (4). Therefore, a good agreement is obtained with the effective mass theory assuming infinite potential barriers [34]. The thickness variation of the absorption edge energy shown in Figure 9 demonstrates a remarkable blue shift of the spectra as the  Although quantum confinement is obtained from the optical absorption measurements, the recombination mechanism is still indistinct. To elucidate the latter, we investigate PL spectra of the Si/SiO 2 multilayer nanostructures.

Thermal annealing of Si/SiO 2 layer films
Since the as-deposited samples show very weak photoluminescence, two experimental efforts are made to improve the PL intensity. The first is the increasing the well number of Si/SiO 2 films layered with 0.5-15 nm (Si) QWs from 10 to 50 periods. The second is the thermal annealing of Si/SiO 2 films in nitrogen. In our work, RTA in nitrogen was performed at 700 and 1100°C for 30 min. Figure 10 shows the cross-sectional view of an RTA treated Si/SiO 2 film. Apparently, the Si QW layers are changed revealing partially dark spots and eroded SiO 2 barrier layers. Figure 11 shows the XRD spectra of a 10-layer Si/SiO 2 film on a sapphire substrate which is rapid-thermal annealed at 700 and 1100°C. The crystallization is clearly identified at 700°C by the splitting the (111), (220), and (311) Braggs peaks indicating that the amorphous Si layers are crystallized as the nanocrystal Si.

Photoluminescence of Si/SiO 2 layer films
Photoluminescence spectra of as-deposited amorphous 10 layers Si/SiO 2 films are excited at 325 nm by a He-Cd laser. The highest energy peaks at 2.35, 2.05, 1.81 eV, with subpeak at 1.45 eV are observed. The improved PL is observed upon crystallization of Si after subjecting the 50-layer Si/SiO 2 multilayer nanostructures to RTA at 700 and 1100°C as shown in Figure 12. The spectra show a broadband peak and shoulders. The main peak energies are 1.62, 1.68, and 1.45 eV. In Figure 13, photoluminescence spectra of Si/SiO 2 QWs annealed at 1100°C for 30 min in nitrogen are shown for the QW thickness ranging from 1.2 to 2.5 nm. The intensity becomes higher for the thinner QWs. Figure 13 presents the well thickness dependence of PL spectra taken on the 50-layer Si/SiO 2 structure upon RTA in nitrogen at 1100°C. The strongest PL is observed for the thinnest Si QW (1.2 nm), fading as the QW thickness increases. Figure 14 displays the temperature dependences of photoluminescence spectra. Among the three temperatures, the 80 K spectrum is the

The Kapoor model
The temperature dependence of the photoluminescence intensity peaks observed at 80, 4, and 293 K are analyzed using the Kapoor empirical models [45,67]. The simulation of the Si/SiO 2 sample comprising 50 quantum wells (1.2 nm well width) annealed at 1100°C for 30 min in nitrogen is performed following Eq. (5). Figure 15 presents the result, which evidences a reasonable agreement between the experimental and simulated results using T r = 70 K, T B = 80 K, and υ 0 = 0.1.

Summary
Amorphous nanostructured Si/SiO 2 films are smartly fabricated using a UHV RF magnetron sputtering system at room temperature. Absorption coefficients are evaluated considering the tentative well Si thickness and energy band gap energy of the Si/SiO 2 layers. The photon energy dependence of absorption coefficient on the quantum well thickness is simulated taking into account the quantum-confined properties. The choice of the Si layer thicknesses interfacing the SiO 2 barrier layer of the constant thickness (4.8 nm) mainly determines the blue shift of the absorption energy. Assuming the infinite potential SiO 2 barriers, the effective-mass theory provides the fitted absorption coefficient edge energy in accordance with E (eV) = 1.61 + 0.75 d −2 (eV) for one-dimensionally confined amorphous Si (d: nm). The amorphous Si/SiO 2 nanostructure films show the quantum confinement. Thermal annealing of the Si/SiO 2 films affects the improvement of photoluminescence intensity. Anomalous temperature dependence of photoluminescence is attempted to be explained based on the Kapoor model. Future work is expected to resolve many more research questions. Waseda University, for many impressive discussions on quantum confinement. One of the authors (T. T.) expresses deep thanks to Dr. Tadashi Takahashi, Emeritus Professor at Tohoku University, for his encouragements and instructions.

Author details
Toshio Takeuchi 1 * and Yoshiji Horikoshi 2 *Address all correspondence to: toshio-takeuchi@ve.cat-v.ne.jp 1 Sendai National College of Technology, Sendai, Japan 2 School of Science and Engineering, Waseda University, Tokyo, Japan