Thin Solid Films 536 (2013) 220–228

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Structural and optical studies of nanostructured TiO2–Ge multi-layer thin films

Abdul Faheem Khan a,b,⁎, Mazhar Mehmood c, Turab Ali d, H. Fayaz a

a UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D UM, University of Malaya, Jalan Pantai Baharu, 59990 Kuala Lumpur, Malaysiab Department of Materials Science & Engineering, Institute of Space Technology, P.O. Box 2750, Islamabad 44000, Pakistanc National Centre for Nanotechnology & Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad 45650, Pakistand Accelerator Lab, National Center for Physics, Islamabad, Pakistan

⁎ Corresponding author. Tel.: +92 51 9075508; fax: +E-mail address: faheem_khan_1977@yahoo.com (A.

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.03.058

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 December 2011Received in revised form 26 March 2013Accepted 27 March 2013Available online 3 April 2013

Keywords:TiO2–Ge multi-layer thin filmsQuantum confinement effectsBand gap energyAnnealing

This paper reports the effects of annealing on structural and optical properties of nanostructured multi-layerTiO2–Ge thin films. These films were characterized using different techniques such as X-ray diffraction, X-rayreflectivity, Rutherford backscattering (RBS), and Fourier Transform Infrared spectroscopy. Annealing wasresponsible for pronounced changes in structural and optical properties of these films, as associated withchanges in their structures, stoichiometry and stress-state.Three sets of TiO2–Ge multi-layer films were deposited by electron beam evaporation and resistive heatingwith different Ge layer thickness (5, 10 and 15 nm), and TiO2 layer thickness was fixed to 20 nm. The filmswere annealed in air up to 500 °C for 2 h. RBS studies showed that the layer structure of TiO2–Gemulti-layer films had been formed. The absorption spectra and band gap energy showed a blue shift withdecrease in Ge layer thickness. The absorption spectra of these films suggest quantum confinement thatincreases with annealing temperature before the complete oxidation of Ge. Apparently complete oxidationresults in sudden or sharp rise in band gap energy that matches with that of TiO2. RBS study reveals thatlayered structure of TiO2–Ge multi-layer films is not destroyed by annealing, which may be due to non-wetting behavior of Ge and its oxide with TiO2. These results imply that nanostructured TiO2–Ge multi-layerthin films may be employed as heterojunctions (with tunable band gap energy) based on quantum confinementeffects for use in photovoltaics.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Germanium is an indirect band gap semiconductor of the 4thgroup in the periodic table. Due to its large Bohr radius of 25 nm (incomparison with silicon which has 5 nm), it provides with flexibilityand ease in exploiting the size-dependent quantum confinementeffects. Decreasing particle size and formation of small nanocrystalsbelow 25 nm caused changes in optical, electrical and opto-electronicproperties of Ge [1–5]. Therefore, it has tremendous potential for nano-technology based photovoltaic applications and other optical and opto-electronic devices.

Nanostructured Ge layers with thickness much greater than Bohrradius (25 nm) may exhibit quantum confinement effects [6–8] dueto their nanostructures, e.g., nanocrystallites [3,7], nanoparticles [8],and nanoporosity [1]. More pronounced quantum confinement effectsmay be possible if the thickness of Ge film would be less than that ofBohr radius of Ge. However, overall optical activity would decrease,which could be avoided by preparing several Ge layers (with a thicknessof less than 20 nm), isolated by alternative layers of a suitable

92 51 9273310.F. Khan).

rights reserved.

material with higher band gap energy. In this regard, TiO2 can beregarded as a suitable material due to its large band gap and thermo-dynamic stability.

Anatase TiO2 exhibits direct band gap in the range of 3.77 to3.85 eV and indirect band gap of about 3.23 eV [9] along with lowrefractive index and dielectric constant (~12–30) [10]. The TiO2

films deposited by electron beam evaporation of titanium dioxide inoxygen environment often exhibit non-stoichiometry, and captureoxygen during annealing in air. On the other hand, free energy offormation of TiO2 is much higher than that of germanium oxide.This aspect may be helpful in these TiO2–Ge multi-layer films as theunsaturated (non-stoichiometric) TiO2 layers can protect Ge (up tosome extent) from oxidation during annealing.

In our previous work, TiO2–Ge multi-layer films were preparedwith varying thickness of the Ge layers (20 nm to 2 nm) from topto bottom. Quantum confinement effects were noticed, which werealso affected by annealing. Higher conductivity was observed underillumination. Characteristics of the heterojunctionswere also improvedwith annealing for use in solar cells [11]. In the present work, TiO2–Gemulti-layer films have been prepared such that the thickness ofTiO2 layers was 20–25 nm, while that of Ge was 5, 10, and 20 nm. Theformation of layered structure has been confirmed by Rutherford back-scattering (RBS) and X-ray reflectivity (XRR) in the case of 5 nm Ge

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films only and changes in optical and structural properties have beenstudied by UV–visible spectroscopy and XRD, respectively. These inves-tigations will help us in the optimization of structural and optical prop-erties of TiO2–Ge multi-layer films for use in photovoltaic applications.

Fig. 1. RBS spectra of as-deposited TiO2–Gemulti-layerfilms. The intended layer thicknesswas 20 nm TiO2 and that for Ge layers was (a) 5 nm, (b) 10 nm, and (c) 15 nm.

1.1. Experimental details

TiO2–Ge multi-layer thin films were fabricated by alternativeevaporation of TiO2 powder (99.99%) and Ge powder (99.999%) onsoda lime glass substrates. Electron beam evaporation was employedfor the deposition of TiO2 layers. The partial pressure of O2 wasmaintained below 2 × 10−2 Pa. The flow of oxygen was controlled bya variable leak valve. The Ge layers were deposited by resistive heatingusing tungsten crucible. The basic vacuum was pumped to less than1 × 10−3 Pa before the deposition of each Ge layer. Temperature ofthe substrate was maintained at 300 °C. It was rotated at 30 RPM andsource to substrate distance was kept at 35 cm.

In the start, a 20 nm layer of TiO2was deposited (in oxygen environ-ment) with average deposition rate of 0.45 nm s−1. A 5 nm layer of Gewas then deposited on TiO2 with a rate of 0.15 nm s−1. On Ge layer, a2nd layer of TiO2 of 20 nm was deposited and similarly 5 nm layer ofGe was deposited on this layer of TiO2 and so on. Final layer of TiO2

(20 nm) was deposited on 5 nm layer of Ge. Similarly, other sets offilms were prepared i.e., 10 and 15 nm layer thickness of Ge keepingthe TiO2 layer thickness at 20 nm. Thickness of each layer and rate ofdeposition were controlled by the quartz crystal monitor.

The films were annealed at various temperatures ranging from 100to 500 °C for a fixed time of 2 h. Optical transmittance and absorbanceof the as-deposited and annealed multi-layer films were recorded atroom temperature by a Perkin Elmer UV/VIS/NIR Lambda 19 spectro-photometer in the wavelength range 300–2500 nm. Crystallographicstructure of these films was determined by recording X-ray diffraction(XRD) patterns using Bruker D8 Discover diffractometer equippedwith Cu Kα radiations at an incident angle of 1 to 2° in parallel beamgeometry, with a Soller slit on the secondary side capable of limitingthe divergence to 0.12°. XRR was performed by using source and detec-tor slits of 0.1 mm (or in some cases of 0.15 mm). RBS was performedusing a Tandem Linear Accelerator with a beam of He2+ particles withan average energy of 2 MeV to confirm the layer structure across thethickness of the multi-layer films. The scattering angle was 170°,which is most popular for conventional RBS. However, required depthresolution was achieved by maintaining the surface normal of thesample to 70° with respect to incident beam and 70.32° with respectto the scattered beam using Cornell geometry. Fourier transform infra-red (FTIR) transmission spectra were recorded at room temperatureon a NICOLET 6700, Thermo Electron Co. USA in the range of 500–4000 cm−1.

Table 1Thickness calculated from the RBS spectra for as-deposited TiO2–Ge multi-layer films.

Sr. no. Layers 5 nm Ge layerfilms (nm)

10 nm Ge layerfilms (nm)

15 nm Ge layerfilms (nm)

01 TiO2 20.2 20.5 20.202 Ge 5.2 9.5 14.703 TiO2 20 20.7 2104 Ge 5.2 9.5 14.705 TiO2 20 21 21.506 Ge 5.4 10.2 14.507 TiO2 20 20.3 21.708 Ge 4.7 9.9 1409 TiO2 20 19.7 20.510 Ge 4.7 9.7 13.611 TiO2 20 19.7 19.712 Ge 5.2 9.7 1413 TiO2 20 20.3 2014 Ge 5.2 10 14.715 TiO2 20 20 19.516 Ge 3.6 10.2 14.717 TiO2 20 20.5 2018 Ge 3.6 10.4 14.719 TiO2 20 20.3 2020 Ge 3.6 10.9 15.421 TiO2 20 20.5 20.5

Fig. 2. Experimental (upper) and simulated (lower) reflectivity curves of TiO2–Ge multi-layer films for 5 nm Ge layer thickness.

Table 2Thickness of as-deposited TiO2–Ge multi-layer films with Ge layer of 5 nm determinedfrom reflectivity curve.

Layer Material Thickness (nm) Roughness (nm)

1 TiO2 — Anatase 20 0.992 Ge 5 0.993 TiO2 — Anatase 19.9 0.964 Ge 4.9 0.985 TiO2 — Anatase 19.9 0.976 Ge 5 0.987 TiO2 — Anatase 20 0.988 Ge 5 0.999 TiO2 — Anatase 20 0.9810 Ge 5 0.97

Fig. 4. Optical transmission spectra of as-deposited TiO2–Ge thin film.

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2. Results and discussion

2.1. As-deposited TiO2–Ge films

Fig. 1 shows typical RBS spectra of as-deposited TiO2–Gemulti-layerfilms with intended TiO2 layer thickness of 20 nm and Ge layer thick-ness of 5 nm (a), 10 nm (b) and 15 nm (c). The kinematic factor of Ge

Fig. 3. XRD patterns of as-deposited TiO2–Ge multi-layer films.Fig. 5. Plots of absorption coefficient, α, as a function of photon energy, hν, ofas-deposited multi-layer TiO2–Ge films. Inset is a plot of band gap energy.

Fig. 6. XRD patterns of annealed TiO2–Ge multi-layer with Ge layer thickness of 5 nm.

Fig. 7. RBS spectra of annealed TiO2–Ge multi-layer films with Ge layer thickness of 5 nm.

Fig. 8. Transmission spectra of annealed TiO2–Gemulti-layerfilmswithGe layer thicknessof 5 nm as a function of wavelength.

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Fig. 9. Plot of band gap energy as a function of annealing temperature of annealedTiO2–Ge multi-layer films with Ge layer thickness of 5 nm.

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is higher than that of titanium, as well as the scattering cross-sectiondue to larger atomic mass (and atomic number). Accordingly, thepeak for the 1st Ge layer appears at relatively higher energy (channel# of about 1700), while that of the 1st layer of TiO2, which is at thetop of the film appears at the absorption edge of titanium (channelnumber of about 1500/about 1.5 MeV) giving rise to an increase inoverall yield. Substrate has not been combined in smiluated spectra.Some discrepancy between the measured data and fitted data may bedue to non-stoichiometry of TiO2 in the film, which can give rise to anincreased yield (or the background counts.). For instance, a hump hasbeen observed for the film with Ge layer thickness of 5 nm as shownin Fig. 1a.

Table 1 shows the thickness of individual layers as estimated fromthe RBS spectra. Some discrepancy in the intended layer thickness andthe estimated film thickness may be due to the limitations of the fittingparameters or the deposition technique. Nevertheless, RBS spectra(Fig. 1) clearly reveal the formation of the layered structure.

Nanostructured TiO2–Ge multi-layer thin films containing 10 layersof Ge and 11 layers of TiO2 is a complex structure. Thickness and surfaceroughness have been measured by two different techniques due to its

Fig. 10. FTIR spectra of 50 nm Ge films, as deposited and annealed at 300 °C asindicated.

complex structure at lower thickness. However, our main focus wason RBS technique. For this reason, XRR results of only 5 nm Ge layersamples are presented here. Fig. 2 shows XRR curves for the filmswith Ge layer thickness of 5 nm. Layer thickness and interface rough-ness calculated from Fig. 2 are shown in Table 2. As the results of fittingat low 2θ angles are only affected by a few top layers, hence thicknessand interface roughness can be more reliable only for these top 5–10layers. It may be noticed that thickness of Ge layers is 5 nm and thatof TiO2 layer is also about 20 nm with interface roughness of about1 nm. It can be seen in Table 2 that the top 10 layers are in good agree-ment with the intended thickness of 5 nm for Ge and 20 nm for TiO2.However, both of the techniques clearly suggest that the layered struc-ture has been effectively formed with the thickness of layers close tothe intended ones with small discrepancy for the films with Ge layerthickness of 5 nm.

Fig. 3 shows the XRD patterns of the as-deposited TiO2–Ge films forGe layer thickness of 5, 10 and 15 nm. Crystallinity of the as-depositedfilms seems to depend on layer thickness and synergistic effects. Forinstance, TiO2 and Ge exhibit only slight crystallinity with Ge layerthickness of 5 nm, while the crystallinity gradually improved with anincrease in the layer thickness of 10 and 15 nm. Degree of crystallinityof these films depends on overall stress state and synergistic effectsbetween the adjacent layers during deposition.

Fig. 4 shows the transmission spectra of the as-deposited TiO2–Gemulti-layer films. It has been noticed from Fig. 4 that the absorptionedge shifts towards smaller wavelength as the Ge layer thicknessdecreases. This may be attributed to the quantum confinement effectsof nanostructured Ge layers. Hence, apart from the formation of nano-structures within the film/layer, one dimensional confinement in thelayers has also been responsible for enhanced quantum confinementeffects with decrease in Ge layer thickness. Fig. 5 shows the plot ofabsorption coefficient as a function of photon energy (hν). Band gapenergy has been determined from the graph and is plotted in the insetfor the as-deposited TiO2–Ge films with Ge layer thickness of 5, 10and 15 nm. It can be seen that the 5 nm Ge layer film has a band gapof 1.48 eV, while 10 and 15 nm Ge layer films have 1.38 and 1.22 eV,respectively.

2.2. Effects of annealing on TiO2–Ge films

2.2.1. Films with Ge layer thickness of 5 nmFig. 6 shows typical XRDpatterns of thefilmswithGe layer thickness

of 5 nm. The XRD patterns do not show any reflections up to anannealing temperature of 300 °C. After annealing at 400 and 500 °C,

Fig. 11. X-ray diffraction patterns of annealed TiO2–Ge multi-layer films with Ge layerthickness of 10 nm.

Fig. 12. RBS spectra of annealed TiO2–Ge multi-layer films with Ge layer thickness of 10 nm.

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Fig. 13. Transmission spectra of annealed TiO2–Ge multi-layer films with Ge layerthickness of 10 nm as a function of wavelength. Fig. 15. XRD patterns of annealed TiO2–Ge multi-layer films with Ge layer thickness of

15 nm.

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X-ray reflections of anatase TiO2 appear without any reflections ofcrystalline Ge.

Fig. 7 shows typical RBS spectra of annealed TiO2–Ge multi-layerfilms with Ge layer thickness of 5 nm. It is clear from the figure thatthe layer structure of the films does not annihilate. This is true evenafter crystallization. It has been shown by Alonso et al. [12] that Gelayer exhibits non-wetting behavior (higher interfacial energy) withsilica layers that result in the isolation of Ge and silica after crystalli-zation. The fact that the layer structure does not break down aftercrystallization in the case of TiO2–Ge multi-layer films appears to berelated with non-wetting behavior similar to Ge-silica films. It can beseen that the overall width of the spectra and individual peak widthsincrease with increasing annealing temperature, which suggests theabsorption of oxygen.

Fig. 8 shows the transmission curves of annealed TiO2–Ge multi-layer films with Ge layer thickness of 5 nm. The transmission spectrashift towards lower wavelengths with the rise of annealing tempera-ture up to 200 °C, which may be due to enhanced quantum confine-ment effects. Significant left shift of the absorption spectra (Fig. 8)and increase in band gap energy (Fig. 9) after annealing at 300 °Cand above seem to be due to oxidation of Ge layers.

Fig. 14. Plot of band gap energy as a function of annealing temperature of annealedTiO2–Ge multi-layer films with Ge layer thickness of 10 nm.

To prove the oxidation of Ge layers, fresh Ge films of about 50 nm(as the total thickness of Ge in 5 nm Ge layer films was about 50 nm)were prepared with the same deposition parameters as described inthe “Experimental details” section. The films were analyzed by FTIR inas-deposited and annealed condition and results are plotted in Fig. 10.A single peak is visible at about 862 cm−1for the film annealed at300 °C, which can be assigned to Ge–O stretching vibrations. Defectsconsisted of agglomerated oxygens were formed during thermalannealing and supersaturated oxygens tend to form clusters or non-stoichiometric GeO2 precipitates. However, no Ge–O vibration wasdetected for the as-deposited film. Oxidation of thicker Ge (about100 nm) has already been proved in the temperature range of 400–500 °C [6].

2.2.2. Films with Ge layer thickness of 10 nmFig. 11 shows typical XRD patterns of the films with Ge layer thick-

ness of 10 nm. It has already been described earlier that these filmsexhibit crystallineGe and TiO2 in the as-depositedfilms. After annealingat 400 °C and above, the X-ray reflections of crystalline Ge almostcompletely vanish, which is due to oxidation of Ge (Fig. 10). Neverthe-less, the layer structure does not annihilate completely as shown in theRBS spectra (Fig. 12).

As far as the absorption spectra are concerned, blue shift has beenobserved up to an annealing temperature of 300 °C. The absorptionspectra of the films annealed at 400 and 500 °C exhibit the signaturesof complete oxidation of Ge layers (Fig. 13). Significant change inband gap energy after annealing at 400 and 500 °C is clearly depictedby Fig. 14. Comparing between Figs. 13 & 14, it may be noticedthat the temperature at which complete oxidation of Ge takes placedepends on the Ge layer thickness of the multi-layer films (Fig. 10)[6].

2.2.3. Films with Ge layer thickness of 15 nmFig. 15 shows typical XRD patterns of the films with Ge layer thick-

ness of 15 nm. These films do not allow oxidation of Ge even atannealing temperature of 500 °C.

It is known that oxidation results in the formation of nanometricscale oxide layers. The growth rate of these thin oxides decreaseslogarithmically if the previously formed oxide covers the completesurface and may not allow further oxidation. Due to reasonably largethickness of 15 nm and restricted access of oxygen in the presence ofTiO2 layers, complete oxidation does not take place.

Now if a few topmost layers of Ge remain intact, these may protectthe underlying layers; and if they are oxidized, they may allow thediffusion of oxygen for the oxidation of the underlying layers as

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well. This may be the reason that Ge (with an accumulative thicknessof 50 nm) is completely oxidized at 300 °C in case of the Ge layerthickness of 5 nm, and at 400 °C for the Ge layer thickness of 10 nm(with an accumulative thickness of 100 nm). This critical temperatureseems higher than 500 °C for Ge layer thickness of 15 nm. Below the

Fig. 16. RBS spectra of annealed TiO2–Ge multi-la

critical temperatures, oxide formation may not consume top layers ofGe (depending on the Ge layer thickness), which then protect thewhole underlying Ge in the multi-layer films.

RBS spectra (Fig. 16) again demonstrate that layered structure almostremain intact. Thewidth of the peaks undergoes slightmodificationwith

yer films with Ge layer thickness of 15 nm.

Fig. 17. Transmission spectra of annealed TiO2–Ge multi-layer films with Ge layerthickness of 15 nm as a function of wavelength.

Fig. 18. Plot of band gap energy as a function of annealing temperature of annealedTiO2–Ge multi-layer films with Ge layer thickness of 15 nm.

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annealing temperature as shown in Fig. 16. These films have beenextremely useful to understand the effects of annealing on the absorptionspectra (without oxidation). As can be seen in Figs. 17 and 18, a pro-nounced increase of the band gap energy in the range of 1.2 to 1.6 eV ispossible with annealing. This is related with quantum confinementeffects. Possibly clustering and partial conversion of 2-D structure into3-D structures, for instance, due to interface roughening and crystalliza-tion, may be responsible for enhanced quantum confinement effects.Another possible reason may be the partial oxidation of Ge at grainboundaries, pore surfaces, etc., that can provide with high band gapobstacles resulting in enhanced quantum confinement effects withinGe nanostructures causing an increase in the band gap energy.

3. Conclusions

Three sets of TiO2–Ge multi-layer films have been deposited byelectron beam evaporation and resistive heating. Thickness of eachTiO2 layer was fixed to 20 nm while the Ge layer thickness was 5, 10,and 15 nm in respective films. RBS and XRR (in case of 5 nm Ge films)studies show that the layer structure of the TiO2–Ge multi-layers filmshas been effectively formed. The absorption spectra andband gap energyshowa blue shiftwith decrease in the thickness of Ge layer. As-depositedfilms with 5 nm Ge layer thickness are predominantly amorphous.While the filmswith Ge layer thickness of 10 and 15 nm are crystalline,as XRD patterns revealed the formation of anatase TiO2 and cubic Ge.

Annealing of thesemulti-layer films up to 500 °C is performed in air.The films with Ge layer thickness of 5 nm oxidize at lower annealingtemperature of 300 °C while the films with higher Ge layer thickness

of 10 nm oxidize at about 400 °C. Films with Ge layer thickness of15 nm do not completely oxidize up to 500 °C. The absorption spectraof these films suggest quantum confinement that increases withannealing temperature before complete oxidation of Ge. It causes asudden or sharp rise in the band gap energy that matches with that ofTiO2. RBS study reveals that the layered structure of TiO2–Ge multi-layer films is not destroyed by annealing, which may be due to non-wetting behavior of Ge and its oxide with TiO2.

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