**3. PbSe/ZnSe composite thin films**

Co-sputtering thus employed in the above section is useful for forming a composite thin film consisting of semiconductor nanocrystals embedded in a matrix because of its simple preparation process and consequent low cost. In the material design, based on the heat of formation, nanocrystal and matrix are clearly phase-separated in spite of the co-deposition from multiple sources (Abe et al., 2008b; Ohnuma et al., 1996). However, it is generally found that sputtering techniques often damage a film due to contamination of the fed gas and high-energy bombardment of the film surface. Thermal evaporation in a high-vacuum atmosphere seems to be better as a preparation technique from the point of view of film quality. In addition, the present study focuses on the insolubility of the material system, since simultaneous evaporation from multiple sources often provides a solid solution (Nill, et al., 1973; Holloway & Jesion, 1982; Abe & Masumoto, 1999).

Fig. 3.1. HWD apparatus used in the study. It consists of four electric furnaces for substrate, wall, source-1, and source-2 (after Abe, 2011).

The PbSe-ZnSe system is a candidate for the composite. In the bulk thermal equilibrium state, the mutual solubility range is quite narrow, less than 1mol%, at temperatures below 1283K (Oleinik et al., 1982). In addition, a composite thin film of PbSe nanocrystal embedded in ZnSe matrix is capable of exhibiting the quantum size effect because of the relatively large exciton Bohr radius of 46nm in PbSe (Wise, 2000) and the relatively wide band gap of 2.67eV in ZnSe (Adachi & Taguchi, 1991). Hence, the optical gap of PbSe nanocrystals will probably be tuned to the maximum solar radiation spectrum. The

One-Step Physical Synthesis of Composite Thin Film 161

a synthesis temperature then crushed into powder for the following experiment setup. At *x*=0, all of the XRD peaks are assigned to the zinc-blend structure of ZnSe, with a lattice constant of 0.5669nm, estimated from the XRD peaks in a high-2θ range from 100º to 155º, using the Nelson-Riley function (Nelson & Riley, 1945). The XRD peak of PbSe with an NaCl structure appears at Pb concentrations exceeding 0.02. The lattice constant of the ZnSe at *x*=0.02 is the same as at *x*=0, within the precision of the experiment technique. This result indicates that the solubility range of Pb in ZnSe is negligible. In contrast, the lattice constant of PbSe is estimated to be 0.6121nm at *x*=1.0 and 0.6117nm at *x*=0.98. A slight decrease in the lattice constant is seen in PbSe, due to the difference in ionic radii of Pb and Zn. Weak XRD peaks of ZnSe are also observed at *x*=0.98 as seen in the inset for easier viewing. This result indicates that the solubility range of Zn in PbSe is less than 0.02 at 1273K. The result is in good agreement with the previous result (Oleinik et al., 1982). The phase separation of the PbSe-ZnSe system is thus also seen on the Zn-rich side in the thermal-equilibrium state. The film preparation for PbSe/ZnSe composite is next

Fig. 3.3. XRD pattern of the PbSe/ZnSe composite thin films. Dots indicate PbSe and circles

The two sources were simultaneously evaporated to prepare a PbSe/ZnSe composite thin film. In the apparatus used, thermal radiation from the wall- and the source-furnace induced an unintentional increase of the substrate temperature up to 515K without use of the substrate-furnace. The deposition rate of the film was almost the same irrespective of the substrate temperature in the range from 515K to 593K. A homogeneous color is observed visually in these films. Above a substrate temperature of 593K, the deposition rate abruptly decreased with increasing temperature, since re-evaporation of PbSe from the film surface became dominant. The films visually exhibit an inhomogeneous yellowish and metallic color, probably caused by a significant reduction in the PbSe while the ZnSe remained, due to the relatively high vapor pressure of PbSe (Mills, 1974). The wall temperature also induced similar behavior. A substrate temperature of 573K and a wall temperature of 773K

investigated based on these results.

indicate ZnSe (after Abe, 2011).

are therefore adopted throughout the present study.

dendritic PbSe nanostructure (Xue, 2009) and ZnSe nanobelt array (Liu, 2009), for instance, are hitherto investigated, but there is no report for one-step synthesis of PbSe/ZnSe composite thin film. Furthermore, an evaporation technique should be carefully selected, since the techniques involving a thermal non-equilibrium state, such as molecular beam epitaxy, increase the solubility limit (Koguchi et al., 1987). The use of HWD, which can provide an atmosphere near thermal equilibrium, is therefore indicated here (Lopez-Otero, 1978). Based on these considerations, one-step synthesis of a PbSe/ZnSe composite thin film was investigated by simultaneous HWD from multiple sources.

Fig. 3.2. XRD pattern of powder-synthesized Zn1-*x*Pb*x*Se with respect to *x*. Dots indicate PbSe and circles indicate ZnSe (after Abe, 2011).

A PbSe/ZnSe composite thin film was prepared by the HWD method. Figure 3-1 is a schematic diagram of the HWD apparatus used. There were four electric furnaces in the apparatus, designated as substrate, wall, source-1, and source-2. Each temperature could be controlled independently. In the HWD method, deposition and re-evaporation are continuously repeated upon a film surface, resulting in achieving a state near thermal equilibrium (Lopez-Otero, 1973). PbSe and ZnSe were used as evaporation sources and were synthesized from elements of Pb, Zn, and Se with 6N purity. The PbSe and ZnSe sources were located at furnaces of source-2 and source-1 for simultaneous evaporation to a glass substrate (Corning #7059). Here, the temperatures were kept constant at 573K for the substrate, 773K for the wall, and 973K for source-1 (ZnSe). The source-2 (PbSe) temperature was varied from 763 to 833K to provide different PbSe concentrations.

The bulk PbSe-ZnSe phase diagram is now revealed at ZnSe concentrations below 45at.% (Pb-rich side) (Oleinik et al., 1982), although the phase diagram of the Zn-rich side still remains unclear. Powder synthesis of a PbSe-ZnSe system was investigated prior to investigating the film preparation. Figure 3-2 depicts the powder XRD pattern of the Zn1 *<sup>x</sup>*Pb*x*Se system. In the powder synthesis, the bulk PbSe and ZnSe thus synthesized was used as starting materials. The desired composition of the system was prepared in an agate mortar and vacuum-sealed in a quartz tube for heat treatment at 1273K for 48h. Finally, the samples were successively water-quenched to maintain the solubility range at

dendritic PbSe nanostructure (Xue, 2009) and ZnSe nanobelt array (Liu, 2009), for instance, are hitherto investigated, but there is no report for one-step synthesis of PbSe/ZnSe composite thin film. Furthermore, an evaporation technique should be carefully selected, since the techniques involving a thermal non-equilibrium state, such as molecular beam epitaxy, increase the solubility limit (Koguchi et al., 1987). The use of HWD, which can provide an atmosphere near thermal equilibrium, is therefore indicated here (Lopez-Otero, 1978). Based on these considerations, one-step synthesis of a PbSe/ZnSe composite thin film

Fig. 3.2. XRD pattern of powder-synthesized Zn1-*x*Pb*x*Se with respect to *x*. Dots indicate PbSe

A PbSe/ZnSe composite thin film was prepared by the HWD method. Figure 3-1 is a schematic diagram of the HWD apparatus used. There were four electric furnaces in the apparatus, designated as substrate, wall, source-1, and source-2. Each temperature could be controlled independently. In the HWD method, deposition and re-evaporation are continuously repeated upon a film surface, resulting in achieving a state near thermal equilibrium (Lopez-Otero, 1973). PbSe and ZnSe were used as evaporation sources and were synthesized from elements of Pb, Zn, and Se with 6N purity. The PbSe and ZnSe sources were located at furnaces of source-2 and source-1 for simultaneous evaporation to a glass substrate (Corning #7059). Here, the temperatures were kept constant at 573K for the substrate, 773K for the wall, and 973K for source-1 (ZnSe). The source-2 (PbSe) temperature

The bulk PbSe-ZnSe phase diagram is now revealed at ZnSe concentrations below 45at.% (Pb-rich side) (Oleinik et al., 1982), although the phase diagram of the Zn-rich side still remains unclear. Powder synthesis of a PbSe-ZnSe system was investigated prior to investigating the film preparation. Figure 3-2 depicts the powder XRD pattern of the Zn1 *<sup>x</sup>*Pb*x*Se system. In the powder synthesis, the bulk PbSe and ZnSe thus synthesized was used as starting materials. The desired composition of the system was prepared in an agate mortar and vacuum-sealed in a quartz tube for heat treatment at 1273K for 48h. Finally, the samples were successively water-quenched to maintain the solubility range at

was varied from 763 to 833K to provide different PbSe concentrations.

was investigated by simultaneous HWD from multiple sources.

and circles indicate ZnSe (after Abe, 2011).

a synthesis temperature then crushed into powder for the following experiment setup. At *x*=0, all of the XRD peaks are assigned to the zinc-blend structure of ZnSe, with a lattice constant of 0.5669nm, estimated from the XRD peaks in a high-2θ range from 100º to 155º, using the Nelson-Riley function (Nelson & Riley, 1945). The XRD peak of PbSe with an NaCl structure appears at Pb concentrations exceeding 0.02. The lattice constant of the ZnSe at *x*=0.02 is the same as at *x*=0, within the precision of the experiment technique. This result indicates that the solubility range of Pb in ZnSe is negligible. In contrast, the lattice constant of PbSe is estimated to be 0.6121nm at *x*=1.0 and 0.6117nm at *x*=0.98. A slight decrease in the lattice constant is seen in PbSe, due to the difference in ionic radii of Pb and Zn. Weak XRD peaks of ZnSe are also observed at *x*=0.98 as seen in the inset for easier viewing. This result indicates that the solubility range of Zn in PbSe is less than 0.02 at 1273K. The result is in good agreement with the previous result (Oleinik et al., 1982). The phase separation of the PbSe-ZnSe system is thus also seen on the Zn-rich side in the thermal-equilibrium state. The film preparation for PbSe/ZnSe composite is next investigated based on these results.

Fig. 3.3. XRD pattern of the PbSe/ZnSe composite thin films. Dots indicate PbSe and circles indicate ZnSe (after Abe, 2011).

The two sources were simultaneously evaporated to prepare a PbSe/ZnSe composite thin film. In the apparatus used, thermal radiation from the wall- and the source-furnace induced an unintentional increase of the substrate temperature up to 515K without use of the substrate-furnace. The deposition rate of the film was almost the same irrespective of the substrate temperature in the range from 515K to 593K. A homogeneous color is observed visually in these films. Above a substrate temperature of 593K, the deposition rate abruptly decreased with increasing temperature, since re-evaporation of PbSe from the film surface became dominant. The films visually exhibit an inhomogeneous yellowish and metallic color, probably caused by a significant reduction in the PbSe while the ZnSe remained, due to the relatively high vapor pressure of PbSe (Mills, 1974). The wall temperature also induced similar behavior. A substrate temperature of 573K and a wall temperature of 773K are therefore adopted throughout the present study.

One-Step Physical Synthesis of Composite Thin Film 163

0.27eV (Zemel et al., 1965). Another TEM image also indicates the presence of relatively large PbSe crystals of approximately 100nm, even with a small amount of 5mol%PbSe (not shown here). Hence, the mean grain size of the PbSe is bimodally distributed in the composite. These large-scale PbSe grains probably dominate the full width at half maximum value of the XRD peak, resulting in no obvious relation between the optical absorption shift and the PbSe grain size. The size control of the nanocrystalline PbSe is therefore insufficient in the present study. The substrate temperature thus adopted seems to assist in the aggregation of PbSe nanocrystals. However, a one-step synthesis of the composite package

has the potential to lead to low-cost production of next-generation solar cells.

Fig. 3.4. Direct observation of PbSe/ZnSe composite thin film containing 5mol%PbSe. (a) Bright-field TEM image. (b) Bright-field image of STEM mode. (c) Elemental mapping of Zn

(red), (d) Se (blue), and (e) Pb (green) (after Abe, 2011).

Figure 3-3 depicts the XRD pattern for the PbSe/ZnSe composite thin films. The weak XRD peak of PbSe at 1mol% is enlarged in the inset for easier viewing. At a PbSe concentration of 0mol% (i.e., pure ZnSe), polycrystalline ZnSe with a zinc-blend structure is observed, with PbSe phase appearing at concentrations exceeding 1mol%. The solubility range of Pb in ZnSe is therefore found to be quite narrow, less than 1mol%, corresponding well to the bulk result (Fig. 3-2). The composite films thus deposited on a glass substrate exhibit a reasonably polycrystalline structure, but dominant (111) growth is seen in the ZnSe phase irrespective of *x*. At 1mol%, the lattice constant at the PbSe (220) peak is estimated to be 0.6118nm, close to that of the bulk result (Fig. 3-2). This result suggests that there is also a narrow solubility range on the Pb-rich side. The phase-separating PbSe-ZnSe system is therefore maintained not only in the bulk product, but also in the film thus obtained, despite the simultaneous evaporation from multiple sources. This result demonstrates that an atmosphere near thermal equilibrium was achieved in the HWD apparatus used.

Figure 3-4(a) presents a bright-field TEM image of the PbSe/ZnSe composite thin film containing 5mol%PbSe. Dark isolated grains with sizes of 25nm to 50nm are seen dispersed along the grain boundary of the bright area. Figures 3-4(b-e) present an scanning transmission electron microscopy (STEM) - energy dispersive spectroscopy (EDX) elemental mapping of the sample through X-ray detection of Zn K (red), Se K (blue), and Pb L (green). Similar morphology is also seen in the bright-field STEM image [Fig. 3-4(b)]. The dark grains indicate the absence of elemental Zn [Fig. 3-4(c)] and the presence of Se and Pb [Figs. 3-4 (d and e)]. It is thus determined that the dark grains are nanocrystalline PbSe. The other region is widely covered with the elements Zn and Se [Figs. 3-4 (c and d)], reasonably assumed to compose ZnSe. It is therefore determined that isolated PbSe nanocrystals are dispersed in the ZnSe matrix. The nanocrystals are estimated to be sufficiently small to exhibit the quantum-size effect because of the exciton Bohr radius of 46nm in PbSe (Wise, 2000).

Figure 3-5 depicts optical absorption spectra for the PbSe/ZnSe composite thin films. For comparison, the spectrum of a pure ZnSe thin film is also presented in the figure. PbSe and ZnSe have direct band structures (Theis, 1977; Zemel et al., 1965) and the absorbance squared is employed here. At a 0mol%PbSe, the optical absorption edge of ZnSe is clearly observed at 2.7eV. Weak absorption then broadly appears at a PbSe concentration of 1mol% in the visible region, together with the optical absorption edge of ZnSe. Such multiple absorptions are also seen in the spectra at concentrations up to 7mol%, indicating the obvious phase separation of the PbSe-ZnSe system. The broad absorption edge shifts toward the lower-energy region as the PbSe content increases. In particular, onset absorption can be confirmed at approximately 1.0eV at 16mol%PbSe, favorably covering the desirable energy region for high conversion efficiency (Loferski, 1956). Therefore, it should be noted that the PbSe/ZnSe composite thin film exhibits the valuable characteristic of vis-NIR absorption.

However, it is unclear whether the shift of the optical absorption edge is due to the PbSe nanocrystals, since the mean grain size of the PbSe remains almost the same at 27nm irrespective of the PbSe content, according to the XRD result (Fig. 3-3) using Scherrer's equation (Scherrer, 1918). A PbSe-ZnSe solid-solution system cannot provide continuous change of the energy band gap because of the quite narrow solubility range (Fig. 3-3). In contrast, the PbSe nanocrystals are sufficiently smaller than the exciton Bohr radius of PbSe (Fig. 3-4). Therefore, this obvious shift is assumed to be responsible for the quantum-size effect of the PbSe nanocrystals embedded in the ZnSe matrix. The minimal appearance of infrared absorption at 16mol%PbSe strongly suggests that relatively large-scale PbSe grains are partially involved in the composite film, since the energy band gap of bulk PbSe is

Figure 3-3 depicts the XRD pattern for the PbSe/ZnSe composite thin films. The weak XRD peak of PbSe at 1mol% is enlarged in the inset for easier viewing. At a PbSe concentration of 0mol% (i.e., pure ZnSe), polycrystalline ZnSe with a zinc-blend structure is observed, with PbSe phase appearing at concentrations exceeding 1mol%. The solubility range of Pb in ZnSe is therefore found to be quite narrow, less than 1mol%, corresponding well to the bulk result (Fig. 3-2). The composite films thus deposited on a glass substrate exhibit a reasonably polycrystalline structure, but dominant (111) growth is seen in the ZnSe phase irrespective of *x*. At 1mol%, the lattice constant at the PbSe (220) peak is estimated to be 0.6118nm, close to that of the bulk result (Fig. 3-2). This result suggests that there is also a narrow solubility range on the Pb-rich side. The phase-separating PbSe-ZnSe system is therefore maintained not only in the bulk product, but also in the film thus obtained, despite the simultaneous evaporation from multiple sources. This result demonstrates that an atmosphere near

Figure 3-4(a) presents a bright-field TEM image of the PbSe/ZnSe composite thin film containing 5mol%PbSe. Dark isolated grains with sizes of 25nm to 50nm are seen dispersed along the grain boundary of the bright area. Figures 3-4(b-e) present an scanning transmission electron microscopy (STEM) - energy dispersive spectroscopy (EDX) elemental mapping of the sample through X-ray detection of Zn K (red), Se K (blue), and Pb L (green). Similar morphology is also seen in the bright-field STEM image [Fig. 3-4(b)]. The dark grains indicate the absence of elemental Zn [Fig. 3-4(c)] and the presence of Se and Pb [Figs. 3-4 (d and e)]. It is thus determined that the dark grains are nanocrystalline PbSe. The other region is widely covered with the elements Zn and Se [Figs. 3-4 (c and d)], reasonably assumed to compose ZnSe. It is therefore determined that isolated PbSe nanocrystals are dispersed in the ZnSe matrix. The nanocrystals are estimated to be sufficiently small to exhibit the

quantum-size effect because of the exciton Bohr radius of 46nm in PbSe (Wise, 2000).

Figure 3-5 depicts optical absorption spectra for the PbSe/ZnSe composite thin films. For comparison, the spectrum of a pure ZnSe thin film is also presented in the figure. PbSe and ZnSe have direct band structures (Theis, 1977; Zemel et al., 1965) and the absorbance squared is employed here. At a 0mol%PbSe, the optical absorption edge of ZnSe is clearly observed at 2.7eV. Weak absorption then broadly appears at a PbSe concentration of 1mol% in the visible region, together with the optical absorption edge of ZnSe. Such multiple absorptions are also seen in the spectra at concentrations up to 7mol%, indicating the obvious phase separation of the PbSe-ZnSe system. The broad absorption edge shifts toward the lower-energy region as the PbSe content increases. In particular, onset absorption can be confirmed at approximately 1.0eV at 16mol%PbSe, favorably covering the desirable energy region for high conversion efficiency (Loferski, 1956). Therefore, it should be noted that the PbSe/ZnSe composite thin film exhibits the valuable characteristic of vis-NIR absorption. However, it is unclear whether the shift of the optical absorption edge is due to the PbSe nanocrystals, since the mean grain size of the PbSe remains almost the same at 27nm irrespective of the PbSe content, according to the XRD result (Fig. 3-3) using Scherrer's equation (Scherrer, 1918). A PbSe-ZnSe solid-solution system cannot provide continuous change of the energy band gap because of the quite narrow solubility range (Fig. 3-3). In contrast, the PbSe nanocrystals are sufficiently smaller than the exciton Bohr radius of PbSe (Fig. 3-4). Therefore, this obvious shift is assumed to be responsible for the quantum-size effect of the PbSe nanocrystals embedded in the ZnSe matrix. The minimal appearance of infrared absorption at 16mol%PbSe strongly suggests that relatively large-scale PbSe grains are partially involved in the composite film, since the energy band gap of bulk PbSe is

thermal equilibrium was achieved in the HWD apparatus used.

0.27eV (Zemel et al., 1965). Another TEM image also indicates the presence of relatively large PbSe crystals of approximately 100nm, even with a small amount of 5mol%PbSe (not shown here). Hence, the mean grain size of the PbSe is bimodally distributed in the composite. These large-scale PbSe grains probably dominate the full width at half maximum value of the XRD peak, resulting in no obvious relation between the optical absorption shift and the PbSe grain size. The size control of the nanocrystalline PbSe is therefore insufficient in the present study. The substrate temperature thus adopted seems to assist in the aggregation of PbSe nanocrystals. However, a one-step synthesis of the composite package has the potential to lead to low-cost production of next-generation solar cells.

Fig. 3.4. Direct observation of PbSe/ZnSe composite thin film containing 5mol%PbSe. (a) Bright-field TEM image. (b) Bright-field image of STEM mode. (c) Elemental mapping of Zn (red), (d) Se (blue), and (e) Pb (green) (after Abe, 2011).

One-Step Physical Synthesis of Composite Thin Film 165

Abe, S. & Masumoto, K. (1999). Compositional plane and properties of solid solution

Abe, S, Ohnuma, M, Ping, D. H., Ohnuma, S. (2008a). Single dominant distribution of Ge

Abe, S, Ohnuma, M, Ping, D. H., Ohnuma, S (2008b). Anatase-Dominant Matrix in Ge/TiO2

Abe, S, Ohnuma, M, Ping, D. H., Ohnuma, S (2008c). Preparation of Ge nanogranules

Abe, S (2009). Solubility Range and Energy Band Gap of Powder-Synthesized Ti1-*x*Ge*x*O2 Solid

Abe, S (2011). One-step synthesis of PbSe/ZnSe composite thin film, *Nanoscale Research Letters*

Adachi, S. & Taguchi, T. (1991). Optical properties of ZnSe,: *Physical Review B*, Vol. 43, pp.

Chatterjee, S. Goyal, A. & Shah, I. (2006). Inorganic nanocomposites for next generation

Chatterjee, S. & Chatterjee, A. (2008). Optoelectronic properties of Ge-doped TiO2 nanoparticles. *Japanese Journal of Applied Physics*, Vol. 47, pp. 1136-1139. Chatterjee, S. (2008). Titania-germanium nanocomposite as a photovoltaic material, *Solar* 

Holloway, H. & Jesion, G. (1982). Lead strontium sulfide and lead calcium sulfide, two new

Hoyer, P. & Könenkamp, R. (1995). Photoconduction in porus TiO2 sensitized by PbS quantum

Kitiyanan, A. Kato, T. Suzuki, Y. & Yoshikawa, S. (2006). The use of binary TiO2-GeO2 oxide

Koguchi, N. Kiyosawa, T.& Takahashi, S. (1987). Double hetero structure of Pb1-*x*Cd*x*S1-*y*Se*y* lasers grown by molecular beam epitaxy, *Journal of Crystal Growth*, Vol. 81, pp. 400-404. Kolodinski, S. Werner, J. H. Wittchen, T. & Queisser, H.J. (1993). Quantum efficiencies

Kubelka, P. & Munk, F. (1931). Ein Beitrag zur Optik der Far-banstriche, *Zeitschrift Technische* 

Landsberg, P. T. Nussbaumer, H. & Willeke, G. (1993). Band‐band impact ionization and solar

Liu, D. & Kamat, P. V. (1993). Photoelectrochemical behavior of thin CdSe and coupled

Liu, J. & Xue, D. (2009). Solution-based route to semiconductor film: Well-aligned ZnSe

Loferski, J. J. (1956). Theoretical considerations covering the choice of the optinum

TiO2/CdSe semiconductor films, *Journal of Physical Chemistory*, Vol. 97, pp. 10769-10773.

semiconductor for photovoltaic solar energy conversion, *Journal of Applied Physics*,

electrodes to enhanced efficiency of dye-sensitized solar cells, *Journal of Photochemistry* 

exceeding unity due to impact ionization in silicon solar cells, *Applied Physics Letters*,

alloy semiconductors, *Physical Review B*, Vol. 26, pp. 5617-5622.

Kubachevski, O & Alcock, C. B. (1979). *Metallurgical Thermochemistry*, Pergamon.

cell efficiency , *Journal of Applied Physics*, Vol. 74, pp. 1451-1452.

nanobelt arrays, *Thin Solid Films*, Vol. 517, pp. 4814-4817.

*International Conference on Thin Films (ICTF 14)*, pp.101-104.

photovoltaics, *Materials Letters.* Vol.60, pp. 3541-3543.

dots, *Appied Physics Letters*, Vol. 66, pp. 349-351.

*and Photobiology A*, Vol. 179, pp. 130-134.

Solution , *Japanese Journal of Applied Physics*, Vol. 48, pp. 081605 1-3

semiconductor Pb1-*x*Ca*x*S1-*y*Se*y* for mid-infrared lasers, *Journal of Crystal Growth*,

nanogranule embedded in Al-oxide thin film, *Journal of Aplied Physics*, Vol. 104, pp.

Thin Films Prepared by RF Sputtering Method , *Applied Physics Express*, Vol. 1, pp.

embedded in Anatase-dominant TiO2 thin films by RF sputtering, *Proceedings of 14th*

**6. References** 

Vol.204, pp. 115-121.

104305 1-3.

095001 1-3.

Vol. 6, pp.324 1-6.

*Energy*, Vol.82, pp. 95-99.

Vol. 63, pp. 2405-2407.

Vol. 27, pp.777-784.

*Physik* Vol. 12, pp. 593-601.

9569-9577.

Fig. 3.5. Optical absorption spectra for PbSe/ZnSe composite thin films (after Abe, 2011).
