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Materials Chemistry and Physics 139 (2013) 17e26

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

RBS depth profiling and optical characterization of multilayers of TiO2 (20 nm)and Ge (15 nm)

Mirza Sajjad Hussain a, Mazhar Mehmood a,*, Jamil Ahmad a, M. Tauseef Tanvir a, A. Faheem Khan a,Turab Ali b, Arshad Mahmood c

aNational Centre for Nanotechnology & Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, Islamabad 45650, PakistanbNational Center for Physics, Islamabad, PakistancNILOP, PO Nilore, Islamabad, Pakistan

h i g h l i g h t s

< Multilayers of TiO2 (20 nm) and Ge (15 nm).< RBS depth profiling.< Effects of annealing.< Inter-layer diffusion and various degrees of oxidation of Ge layers.< Enhancement of quantum confinement effects and changes in optical properties.

a r t i c l e i n f o

Article history:Received 25 February 2012Received in revised form23 September 2012Accepted 25 October 2012

Keywords:MultilayersRutherfordbackscattering spectroscopy (RBS)InterfacesDiffusion

* Corresponding author.E-mail address: mazhar@pieas.edu.pk (M. Mehmo

0254-0584/$ e see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2012.10.037

a b s t r a c t

Rutherford Backscattering Spectrometry (RBS) has been employed for studying inter-layer diffusion andmixing of TiO2 and germanium in the multi-layers grown on the soda lime glass substrate. TiO2 layerswere grown by electron-beam evaporation of titanium in an oxygen atmosphere and Ge layers weregrown by resistive heating. UVevisible and Raman spectroscopy were carried out for optical charac-terization. RBS spectra were analyzed by SIMNRA which confirmed the successful formation of alternatelayers of TiO2 and Ge with a thickness of 20 nm and 15 nm, respectively. Annealing caused interfacemixing. The approximate stoichiometry of interfaces was of GeTiO2. The thickness of these layersincreased with annealing temperature. Two to five top Ge layers also underwent oxidation to form GeOand GeO2, depending on annealing temperature. Nuclear Reaction Analysis (NRA) signal of oxygen fromtop layers was also recorded to confirm the pick-up of oxygen from the annealing atmosphere. Anincrease in the transmission efficiency with blue shift of the absorption edge was also observed withannealing temperature, as associated with the decrease in the effective thickness of Ge layers. Shift ofRaman peaks confirmed the stoichiometric changes as a result of annealing.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Single crystal silicon wafer based solar cells are the mostcommonly available cells on commercial level, but their maximumefficiency (w15%) is very limited as compared to their costs [1].There are two important reasons; (i) With the technology beingmatured, their costs are dominated by the costs of their startingmaterials [2] and (ii) the single crystal silicon is very much choosyin absorbing a certain part of the entire solar spectrum because of

od).

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its unique energy band structure along withmany loss mechanismsof absorbed solar energy [1]. They are also called first generationsolar cells. The second generation solar cells based on thin films ofpolycrystalline silicon and other compound semiconductors havebeen reported to have relatively higher efficiencies and lesser costsbut their efficiencies are still constrained by the choosy absorptionof solar spectrum depending upon their unique energy bandstructures. Cadmium sulphide, amorphous silicon, copper indiumdiselenide, cadmium telluride are important semiconductors usedin such solar cells [3]. Current focus of the technology is ondeveloping such materials that are capable of absorbing most partof the solar spectrum and converting it into electron-hole pairs forconduction [4]. The advent of nanotechnology has made it possible

Fig. 1. Schematic view of 21 layers of TiO2 (20 nm) and Ge (15 nm).

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e2618

to develop thin films of nanometer thickness, nano sized particles,nano rods and sheets [5,6]. It is a known quantummechanical effectthat below a certain thickness, some properties of materials whichare otherwise independent of the physical size of the material,become size dependent. This effect is known as Quantum SizeEffect [7e10]. A few of those are energy band structure, resistivity,phonon dispersion relations, etc. Since, the absorption of solar lightdepends upon the energy band structure of absorbing material;a multilayer structure of different semiconductors can absorbentire solar spectrum if the number of layers and thicknesses ofindividual layers are adjusted so that the energy band structure ofeach layer corresponds to some portion of the solar spectrum. Solarcells based on such kind of materials are called third generationsolar cells. These solar cells are expected to attain maximum effi-ciency, allowed by thermodynamics (w93%) [2]. Because of theirapplications in high efficiency solar cells, multilayer structures havegot extensive attention by thin film technologists, from all over theworld. Many techniques are being employed for the deposition ofmultilayers including electron beam evaporation, molecular beamepitaxy and physical and chemical vapor deposition techniques[11,12]. The effects of annealing on the thicknesses of individuallayers, the formation and growth of interfaces and their composi-tions are also being investigated. The diffusion at interface betweentwo layers may also change the thickness of layers and as a resultannealing may have a pronounced effect on change in energy bandstructure and absorption and transmission peaks of multilayers.

Since, the energy band structure of the individual layers ina multilayer structure plays most important role in defining theabsorption of solar spectrum, the measurement and the controlover the thicknesses of individual layers is also the most importantand key parameter in the third generation solar cell technology.Rutherford Backscattering Spectrometry (RBS) is the most efficientnon-destructive technique for depth profiling among nano-scalecharacterization tools for thin films. It provides the compositionand the relative amounts of elements as a function of depth. It canalso provide the information about diffusion of materials at inter-faces of two layers present at depth resulting from annealing.

In this paper, we have investigated the RBS depth profiling ofmultilayers of titania and germanium grown alternatively on sodalime glass. The first and the last layer being of TiO2, with a constantthickness of 20 nm, while Ge having a constant thickness of 15 nm.The effect of annealing on the thickness of individual layers, as wellas, on the interfaces was also studied using RBS. UVevisible spec-trometric study was also performed to correlate the effects ofannealing, predicted by RBS, with the change in transmission effi-ciencies and blue shift of absorption edge of multilayers. NuclearReaction Analysis (NRA)was performed to confirm the oxygen pick-up from the atmosphere associated with the oxidation of germa-nium. Raman spectroscopy was also performed, which confirmedthe changes in the local environment of the multilayers as a resultof annealing.

2. Experimental

2.1. Sample preparation

Multilayers were prepared using alternate evaporation of TiO2(99.99%) and germanium (99.999%) powders. TiO2 was depositedusing electron beam evaporation technique while germanium wasdeposited using resistive heating technique. During deposition ofTiO2 the partial pressure of oxygen (O2) was maintained below2 � 10�4 mbar; while during the deposition of germanium, thevacuum was kept higher than 1 � 10�5 mbar. The substrate wasa 0.5 mm thick sheet of soda lime glass and was held at a temper-ature of 300 �C and rotated at 30 rpm during the deposition. The

source to substrate distance was 35 cm. The deposition rate of TiO2was 0.45 nm s�1 and that of germanium was 0.15 nm s�1. Thedeposition rate was controlled by a quartz crystal monitor. Theintended thickness of TiO2 was 20 nm, while that of germaniumwas 15 nm. A schematic view of the sample is shown in Fig. 1.

2.2. Annealing

The samples were annealed at 300 �C, 400 �C and 500 �C for 2 h.Annealing was carried out in a programmable Muffle furnace witha ramp of 10 �C min�1 for reaching the dwell temperature atatmospheric pressure.

2.3. Rutherford backscattering spectrometry

RBS data was collected using a tandem linear accelerator witha beam of He2þ particles with average energy of 2.07 MeV. A solid-state detector placed in Cornell geometry with sample, havinga solid angle of 2.74 milli steradians, was used to collect theparticles backscattered from the sample at an angle of 170� withincident normal. The distance between sample and detector was13 cm. The charge collection was 20 mC for all the samples. Theangle of incident beamwith the sample’s normal was kept 55�. Thisangle was optimized to get information from all the layers withsufficient depth resolution.

RBS analysis was done using SIMNRA. The effects of energystraggling with depth and multiple scattering were taken intoaccount during simulation.

Calibrationwas performed using thin films of gold and silver; inaddition to silicon substrate covered by gold film (thinner than thedetector’s resolution) at an incident angle of zero degree withsample’s normal. For standard samples, energy of the scatteredparticles was calculated using well-known kinematic factors(K ¼ Escattered/Eincident) [13], and was observed to be linearly relatedwith channel scale for reliable calibration.

The full width half maximum (FWHM) of the detector was takenequal to the FWHM of RBS peak from a very thin gold film of knownthickness (5 nm), which was 22 keV. This was justified by the factthat the calculated energy loss of He2þ in thin gold film wassignificantly less than the FWHM of the measured peak. Thesimulated peaks of germanium and titaniumwere best fitted on theexperimental peaks by assuming no diffusion between individuallayers of germanium and titania, in case of as-deposited sample.Any diffusion between individual top layers, if present, could havebeen suppressed in the FWHM of the detector and layers appearedsmooth. In this way, diffusion reported in annealed samples wasautomatically calibrated with the (presumed) zero diffusion for theas-deposited sample.

ExperimentalSimulated

Channel

1,6001,4001,2001,000800600400

1,4501,4001,3501,3001,2501,2001,1501,1001,0501,000

950900850800750700650600550500450400350300250200150100500

500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Energy [keV]

Fig. 2. Experimental spectrum of as-deposited twenty-one alternate layers of TiO2 and Ge.

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e26 19

2.4. Nuclear reaction analysis (NRA)

NRA signal of oxygen was also recorded at an incident energy ofapproximately, 3.033MeV. All the parameters of experiment (excepttheenergyof incidentHeþ2 particles)were identical to thoseused forRBS.

Fig. 3. Simulation of RBS spectrum of as deposited twenty-one alternate layers of TiO2 and Gof the references to color in this figure legend, the reader is referred to the web version of

2.5. Raman spectroscopy

Raman spectra of the as-deposited and annealed samples werealso acquired at room temperature using confocal mode Micro-Raman-Spectrometer. The wavelength of incident laser was532 nm.

e at 300 �C (experimental in red color and simulation in blue color). (For interpretationthis article.)

Table 1The depth profile of as-deposited twenty-one alternating layers of TiO2 and Ge.(*Formula units and thickness in nm for non-stoichiometric diffused layers are notdefined.)

Originallayernumber

Layerstoichiometry

Thickness(1015atoms cm�2)

Thickness(1015formulaunits cm�2)

Thickness(nm)

1 TiO2 197 65.6 20.12 Ge 63 63 14.33 TiO2 210 70 21.74 Ge 63 63 14.35 TiO2 205 68.3 21.06 Ge 63 63 14.37 TiO2 200 66.6 20.58 Ge 66 99 15.09 TiO2 205 68.3 21.010 Ge 64 64 14.511 TiO2 195 65 19.912 Ge 65 65 14.713 TiO2 195 65 19.914 Ge 64 64 14.515 TiO2 192 64 19.616 Ge 62 62 14.017 TiO2 200 66.6 20.518 Ge 60 60 13.519 TiO2 157 52.3 16.0Diffused layer GeTiO2 50 12.5 *20 Ge0.99Ca0.01 35 * *Diffused layer (GeTiO2) 0.09 Ca0.01 50 12.5 *21 Ti0.32 O0.67Ca0.01 157 * *

1120 1160 1200 1240 1280 1320 1360 1400 1440 1480 1520 1560 1600-200

0

200

400

600

800

1000

1200

1400

1600

Yiel

d

Channels

as-deposited annealed at 300°C annealed at 400°C annealed at 500°C

500 1000 1500-200

0

200

400

600

800

1000

1200

1400

1600

as-d

epos

ited

Channels

as-deposited annealed at 300°C annealed at 400°C annealed at 500°C

a

b

Fig. 5. a: Comparison of experimental spectra of as-deposited and annealed samples in

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e2620

2.6. UVevisible spectroscopy

Transmission spectra were collected in the wavelength range of190e900 nm using a Labomed UVevisible spectrophotometer.

3. Results and discussion

3.1. RBS analysis

Fig. 2 shows the experimental RBS spectrum of the as-depositedsample. The peaks are well resolved which confirms the successful

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Depth (1015 atoms/cm2)

O

2

21191715131197531

Rel

ativ

e C

once

ntra

tion

Ti

21191715131197531

14Ge

Fig. 4. Concentration versus thickness (atoms cm�2) plots for Ge, Ti and O in as-deposited sample. Layer numbers are marked from left to right.

the channel range of 1100e1600. b: Comparison of experimental spectra of as-deposited and annealed samples in the complete channel range.

200 400 600 800 1000

Yiel

d

Channels

as-deposited (3.036 MeV)

annealed at 3000C (3.034 MeV)

annealed at 4000C (3.033MeV)

annealed at 5000C (3.035 MeV)

Fig. 6. NRA spectra of as-deposited and annealed samples. The incident energies ofincident Heþ2 particles are also mentioned for each sample.

Table 2Relative height of the peaks of Ge and O in NRA spectra.

Height ofoxygen peak

Average heightof Ge peaks

Ratio

As-deposited 1333 642 2.07Annealed at 300�C 1737 603 2.8Annealed at 400�C 1910 549 3.47Annealed at 500�C 1816 493 3.68

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e26 21

formation of layered structure of the multilayer. The startingpositions of signals of oxygen, titanium and germanium are markedwith sign “/”. The spectrum can be divided into four distinctregions. The regions marked by R1, and R3, originate by scatteringof He2þ by Ge atoms from a few top germanium layers in thesample, and by Ti atoms of a few bottom TiO2 layers, respectively.The region R2 is a result of the overlap of scattering response fromremaining Ge and TiO2 layers. The peaks of the Ti in region R3 arenot well resolved because of multiple scattering and energystraggling effects. The region R4 includes the signals from thesubstrate, as well as oxygen of the deposited multi-layer film. It canbe seen that the height of Ti peaks in region R3 is lower than that ofgermanium peaks in region R1. This is partly related with the factthat Ti has one-third atomic concentration in its compound TiO2 ascompared to pure Ge.

Fig. 3 shows the simulation of RBS spectrum of as-depositedsample. The simulated (blue) and experimental (red) spectraare very well overlapped. Thickness of each deposited layer alongwith stoichiometry has been presented in Table 1. Comb-like plotin Fig. 4 shows profile of relative concentration of Ti, O and Ge asa function of depth from the top surface. Some interface mix-ing between three bottom layers seems due to substrate effectsthat may profoundly influence the initial morphology during

Fig. 7. RBS spectrum of annealed twenty one alternating layers of TiO2 and Ge at 300 �Creferences to color in this figure legend, the reader is referred to the web version of this ar

deposition. The observed variation of thickness is �1.5 nm forgermanium and is �1.7 nm for titania, which seems to lie withinexperimental limitations.

Fig. 5a and b compares the experimental spectra of as-depositedand annealed samples. It may be noticed that the separationbetween germanium peaks increases as a result of annealing.Another aspect is the decrease in the height of these peaks, moreprominently seen in the relatively outer layers of Ge in the samplesannealed at 400 �C and 500 �C. The overall width of the spectraemerging from Ge and Ti is also accompanied by left shift of thecounts coming from the substrate. The former observation maypossibly be associated with interlayer diffusion, while the laterobservation clearly suggests an overall increase in the mass(content) of film that gives rise to enhanced energy loss for theparticles scattered from the relatively low lying layers as well as thesubstrate. The added mass may at least partly be due to intake ofoxygen from the atmosphere related with oxidation of germanium.

This aspect was further confirmed by employing the He2þ

particles of average incident energy of 3.03 MeV. As a result anintense additional peak is seen around channel number 370 and isshown in Fig. 6. This is related with considerably high scatteringcross-section for oxygen at an energy of 3.03 MeV by inelasticscattering (a resonant scattering). The calculated energy loss ofoxygen peak (w30 keV) occurs approximately up to the thirdlayer of titania and second layer of Ge. The intensity of this peakgradually increases with annealing temperature (relative heights ofoxygen and germanium peaks are compared in Table 2) clearlysuggesting that the amount of oxygen in these top layers isincreasing. Hence, oxidation of the multi-layers due to environ-mental oxygen is clearly evident.

Fig. 7 shows the RBS spectrum of multilayers annealed at 300 �C.Simulated spectrum (blue) has been successfully overlapped onexperimental spectrum (red). Table 3 presents the depth profile of

(experimental in red color and simulation in blue color). (For interpretation of theticle.)

Table 3The depth profile of sample annealed at 300 �C. (*Formula units and thickness in nm for non-stoichiometric diffused layers are not defined.)

Original layernumber

Layerstoichiometry

Thickness(1015atoms cm�2)

Thickness(1015formulaunits cm�2)

Thickness(nm)

Original layernumber

Layer stoichiometry Thickness(1015

atoms cm�2)

Thickness(1015formulaunits cm�2)

Thickness(nm)

1 TiO2 186 62 19.1 11 TiO2 150 50 15.4Diffused layer TiGeO2 10 2.5 * Diffused layer TiGeO2 50 12.5 *2 Ge0.6O0.4 97 * * 12 Ge 40 40 9.1Diffused layer TiGeO2 10 2.5 * Diffused layer Ti0.2Ge0.3O0.5 50 5 *3 TiO2 195 65 19.9 13 TiO2 118 39.3 12.1Diffused layer TiGeO2 10 2.5 * Diffused layer Ti0.3Ge0.2O0.5 50 5 *Partial oxidation Ge0.8O0.2 58 * * 14 Ge 38 38 8.64 Ge 29 29 6.6 Diffused layer Ti0.2Ge0.3O0.5 60 6 *Diffused layer TiGeO2 10 2.5 * 15 TiO2 110 36.6 11.35 TiO2 190 63.3 19.5 Diffused layer Ti0.3Ge0.2O0.5 49 4.9 *Diffused layer TiGeO2 10 2.5 * 16 Ge0.98Ca0.02 39 * *6 Ge 58 58 13.1 Diffused layer Ti0.19Ge0.29O0.5Ca0.02 49 * *Diffused layer TiGeO2 10 2.5 * 17 Ti0.32O0.66Ca0.02 132 * *7 TiO2 185 61.6 18.9 Diffused layer Ti0.28O0.65Ca0.02Si0.05 53 * *Diffused layer TiGeO2 10 2.5 * 18 Ge0.93Ca0.02Si0.05 36 * *8 Ge 61 61 13.8 Diffused layer Ti0.20Ge0.32O0.41Ca0.02Si0.05 80 * *Diffused layer TiGeO2 10 2.5 * 19 Ti0.31O0.59Ca0.02Si0.05 98 * *9 TiO2 195 65 19.9 Diffused layer Ti0.27Ge0.18O0.45Ca0.02Si0.05 80 * *Diffused layer TiGeO2 10 2.5 * 20 Ge0.90Ca0.02Si0.08 23 * *10 Ge 61.7 61.7 13.9 Diffused layer Ti0.19Ge0.25O0.46Ca0.02Si0.08 100 * *Diffused layer TiGeO2 10 2.5 * 21 Ti0.31O0.57Ca0.02Si0.10 145 * *

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e2622

the sample. Profile of relative concentration as a function of depthfrom the top surface is presented in Fig. 8.

Two major changes have been observed by the analysis of RBSspectra. Top germanium layer (Layer #2) has been oxidized toa non-stoichiometric Ge0.6O0.4, while the 2nd layer (Layer #4)partially oxidized to a non-stoichiometric Ge0.8O0.2. Rest of Gelayers remains intact. In addition, a mixing at the interfaces of Geand TiO2 layers has been noticed. A phase of approximate stoichi-ometry of GeTiO2 is formed as a result of diffusion at the interfaces.New phase formation at interfaces in multilayers as a result ofannealing has also been reported in literature [13e17]. Table 3shows that the thickness and stoichiometry (GeTiO2) of thediffused layers remain constant up to layer number 11. The

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Depth (1015 atoms/cm2)

5 7 9 11 13 15 17 19 21

19

O

Con

cent

ratio

n

1 3 5

6

7

8 12 14 16 18 20

9 11 13 15 17 21

C a

C a

S i

S i

T i

C a

S i

G e

24

10

Fig. 8. Concentration versus thickness (atoms cm�2) plots for Ge, Ti and O in sampleannealed at 300 �C. Layer numbers are marked from left to right.

thickness of diffused layers between deeper layers increasesaccompanied by a little deviation from the stoichiometry of GeTiO2

as well as diffusion of substrate material to these deeper layers. Thereason for this extraordinary diffusion is difficult to explain withcertainty. It is because that, this information is obtained by thefitting of that part of the RBS spectra in which the peaks are not asresolved as they are for top layers.

Fig. 9 shows the RBS spectrum of sample annealed at 400 �C.The simulation (blue) fits over the experimental (red) spectrumvery well. The thickness of each layer along with the stoichi-ometry of diffused layers is presented in Table 4. Profile of rela-tive concentration as a function of the depth from the top surfaceis presented in Fig. 10. A clear decrease in the height and increasein the width of top five peaks in RBS spectrum, as compared tothe spectra of as-deposited sample and annealed samples at300 �C, have been observed which infer enhanced addition ofexternal material from top at this annealing temperature. RBSsimulation of spectrum also confirms the oxidation of top five Gelayers into oxides, out of which four Ge layers are oxidized to anapproximately stoichiometric of GeO2 while the fifth one toGe0.5O0.5 or GeO. The germanium layers lying underneath remainintact. This observation suggests that oxidation proceeds fromtop layers, and as the overlying germanium layers are oxidized,access of oxygen to underneath germanium layers also becomespossible. Increase in oxidation rate at increasing temperatureresults in the complete oxidation of larger number of germaniumlayers at the top.

Interface mixing between TiO2 and Ge layers is also observed atannealing temperature of 400 �C. The interlayer has the same stoi-chiometry of GeTiO2 as in case of annealing temperature of 300 �C,butwith an increased thickness. At annealing temperature of 300 �Cthe thickness of the interlayer between top layerswas of the order of2.5 � 1015 formula units cm�2; while in the case of the annealingtemperature of 400 �C, the interlayer’s thickness between the toplayers increased up to about 7.5 � 1015 formula units cm�2. At thisannealing temperature the diffusion length was approximatelyuniform throughout the multilayer (up to the 19th layer). The masstransfer from the substrate to themultilayer has also been observed.

RBS spectrum of sample annealed at 500 �C overlapped withsimulated spectrum has been shown in Fig. 11. The thickness of

Fig. 9. RBS spectrum of annealed twenty-one alternating layers of TiO2 and Ge at 400 �C (experimental in red color and simulation in blue color). (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e26 23

layers and stoichiometry of inter-layers are presented in Table 5.Fig. 12 shows the relative concentration depth profile of thesample. It is found that at the annealing temperature of 500 �C, thetop five germanium layers are completely oxidized into GeO2,which further confirms that the rate of oxidation increases withannealing temperature. The decrease in germanium concentrationfor top five layers from 1.0 to 0.33 relates with the formation ofGeO2 (Fig. 10). Interface mixing results in the formation of diffusedlayers between Ge and TiO2 layers but there is no significant

Table 4The depth profile of sample annealed at 400 �C. (*Formula units and thickness in nm for

Original layernumber

Layerstoichiometry

Thickness(1015 atoms cm�2)

Thickness(1015 formulaunits cm�2)

Thickness(nm)

Originum

1 TiO2 174 58 17.8 DiffuDiffused layer TiGeO2 30 7.5 * 122 GeO2 145 48.3 * DiffuDiffused layer TiGeO2 30 7.5 * 133 TiO2 164.529 54.84 16.8 DiffuDiffused layer TiGeO2 30 7.5 * 144 GeO2 145 48.3 * DiffuDiffused layer TiGeO2 30 7.5 * 155 TiO2 159 53 16.3 DiffuDiffused layer TiGeO2 30 7.5 * 166 GeO2 145 48.3 * DiffuDiffused layer TiGeO2 30 7.5 * 177 TiO2 154.45 51.48 15.8 DiffuDiffused layer TiGeO2 30 7.5 * 188 GeO2 144 48 * DiffuDiffused layer TiGeO2 30 7.5 * 199 TiO2 149 49.6 19.9 DiffuDiffused layer TiGeO2 30 7.5 * 2010 GeO 98 49 * DiffuDiffused layer TiGeO2 30 7.5 * 2111 TiO2 144 48 14.7

difference in the thickness of diffused layers for annealingtemperatures of 400 �C and 500 �C.

It is important to note that even if the thickness of diffusedlayers and the number of oxidized germanium layers increasewith annealing temperature, the stoichiometry of diffused layersremains GeTiO2. This shows that a stable phase forms at theinterface of Ge and TiO2 layers instead of surface roughening etc.Mass transfer from substrate has also been observed at thisannealing temperature.

non-stoichiometric diffused layers are not defined.)

nal layerber

Layerstoichiometry

Thickness(1015

atoms cm�2)

Thickness(1015 formulaunits cm�2)

Thickness(nm)

sed layer TiGeO2 30 7.5 *Ge 50 50 11.3

sed layer TiGeO2 30 7.5 *TiO2 155 51.6 15.9

sed layer TiGeO2 30 7.5 *Ge 49 49 11.1

sed layer TiGeO2 30 7.5 *TiO2 152 50.6 15.6

sed layer TiGeO2 30 7.5 *Ge0.93Ca0.02Si0.05 50 * *

sed layer Ti0.24Ge0.24.O0.45Ca0.02Si0.05 30 * *Ti0.3O0.63Ca0.02Si0.05 150 * *

sed layer Ti0.24Ge0.24.O0.45Ca0.02Si0.05 30 * *Ge0.93Ca0.02Si0.05 47 * *

sed layer Ti0.20Ge0.32O0.41Ca0.02Si0.05 30 * *Ti0.30O0.59Ca0.03Si0.08 90 * *

sed layer Ti0.23Ge0.23O0.46Ca0.03Si0.08 106 * *Ge0.92Ca0.03Si0.08 8 * *

sed layer Ti0.20Ge0.22O0.50Ca0.03Si0.08 145 * *Ti0.29O0.62Ca0.03Si0.08 106 * *

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Depth (1015 atoms/cm2)

21191715131197531

O

T i

Con

cent

ratio

n

2018161412

10864

C a

S i

C a

S i

C a

S i

G e

2

Fig. 10. Concentration versus thickness (atoms cm�2) plots for Ge, Ti and O in sampleannealed at 400 �C. Layer numbers are marked from left to right.

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e2624

3.2. Transmission efficiency in UVevisible range

Fig. 13 shows the comparison between transmission intensitiesof as-deposited and annealed samples in UVevisible range ofspectrum (190e900 nm). It can be clearly seen that the trans-mission increases with the oxidation of germanium layers. Thenumber of oxidized germanium layers increases gradually withincrease in annealing temperature causing significant changes in

Fig. 11. RBS spectrum of annealed twenty-one alternating layers of TiO2 and Ge at 500 �

references to color in this figure legend, the reader is referred to the web version of this a

transmission efficiencies with the increase of annealing tempera-ture. A change in the band structure with increase in annealingtemperature is also inferred by the shift of absorbtion edge towardslower wavelengths. This effect is related with the alteration of bandgap structure due to decrease in Ge layer thickness as a result of theformation of GeTiO2 at the interfaces of pure layers, with annealing.The change in transmission efficiency from annealing temperatureof 300e400 �C is more than that from 400 �C to 500 �C, whichconfirms the results of RBS that the major change in depth profilehas occurred at 400 �C.

3.3. Raman spectroscopy

Fig. 14 shows the Raman spectra of as-deposited TiO2eGemultilayer thin films and annealed films at various temperaturesranging from 300 �C to 500 �C for 2 h. Raman spectroscopy providesinformation about the dependence of nature of bonds on the localenvironment of the atoms e.g. physical structure and bond stressesand strains.

In as-deposited spectrum, three peaks are clear at 294, 417 and615 cm�1. First peak can be assigned to germanium, while the nexttwo peaks are related to TiO2. After annealing at 300e500 �C, thefirst peak of TiO2 shifts toward lower wave numbers in comparisonwith as-deposited films, which may be due to increase in thecrystallinity of the films while the last peak of TiO2 shifts towardhigher wave numbers, which may be due to some structuralchanges. At annealing temperature of 400 �C and 500 �C, anotherintermediate peak becomes prominent which is also related withTiO2. This peak may also be observed in the as-deposited films andannealed films at 300 �C with lower intensity.

The Raman peak for bulk Ge usually appears at 302 cm�1 [18].The appearance of Ge peak at 294 cm�1 is due to quantumconfinement in Ge (as Ge shows quantum confinements due to

C (experimental in red color and simulation in blue color). (For interpretation of therticle.)

Table 5The depth profile of sample annealed at 500 �C. (*Formula units and thickness in nm for non-stoichiometric diffused layers are not defined.)

Original layernumber

Layerstoichiometry

Thickness(1015

atoms cm�2)

Thickness(1015 formulaunits cm�2)

Thickness(nm)

Original layernumber

Layer stoichiometry Thickness(1015 atoms cm�2)

Thickness(1015 formulaunits cm�2)

Thickness(nm)

1 TiO2 174 58 17.8 Diffused layer TiGeO2 30 7.5 *Diffused layer TiGeO2 30 7.5 * 12 Ge0.75O0.25 64 * *2 GeO2 145 48.3 * Diffused layer TiGeO2 30 7.5 *Diffused layer TiGeO2 30 7.5 * 13 TiO2 150 50 15.43 TiO2 164.529 54.843 16.8 Diffused layer TiGeO2 30 7.5 *Diffused layer TiGeO2 30 7.5 * 14 Ge 49 49 11.14 GeO2 145 48.3 * Diffused layer TiGeO2 30 7.5Diffused layer TiGeO2 30 7.5 * 15 TiO2 146 48.6 14.95 TiO2 159 53 16.3 Diffused layer TiGeO2 30 7.5 *Diffused layer TiGeO2 30 7.5 * 16 Ge0.93Ca0.02Si0.05 52 * *6 GeO2 145 48.3 * Diffused layer Ti0.24Ge0.24.O0.45Ca0.02Si0.05 30 * *Diffused layer TiGeO2 30 7.5 * 17 Ti0.32O0.61Ca0.02Si0.05 142 * *7 TiO2 154.45 51.4 15.8 Diffused layer Ti0.26Ge0.26O0.39Ca0.04Si0.05 30 * *Diffused layer TiGeO2 30 7.5 * 18 Ge0.91Ca0.04Si0.05 45.5 * *8 GeO2 145.420 48.4 * Diffused layer Ti0.20Ge0.32O0.41Ca0.02Si0.05 30 * *Diffused layer TiGeO2 30 7.5 * 19 Ti0.32O0.59Ca0.04Si0.05 103 * *9 TiO2 159.5 53.16 16.3 Diffused layer Ti0.23Ge0.23O0.46Ca0.03Si0.08 100 * *Diffused layer TiGeO2 30 7.5 * 20 Ge0.89Ca0.04Si0.07 12 * *10 GeO2 149 49.67 * Diffused layer Ti0.24Ge0.24O0.43Ca0.04Si0.07 120 * *Diffused layer TiGeO2 30 7.5 * 21 Ti0.25O0.61Ca0.04Si0.1 108 * *11 TiO2 153 51 15.7

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e26 25

lower thickness than Bohar radius (25 nm)). The peak positiongradually shifts towards lower values due to enhanced quantumconfinement effects. These results are consistent with the RBSresults.

4. Discussion

Successful formation of multilayers of Ge and TiO2 is evidentfrom RBS spectra. The simulated spectra infer a layer thickness of15 � 1.5 nm for germanium and 20 � 1.7 nm for TiO2. Annealing ofthe multilayers of Ge and TiO2 at different temperatures (300 �C,400 �C and 500 �C) has remarkably affected the RBS spectra.Interlayer mixing forming an interface with stoichiometry ofGeTiO2 is obvious. The increase in diffusion length as a result ofincrease in annealing temperature can be attributed to the fact that

0 500 1000 1500 2000 2500 3000 35000.00.20.40.60.81.0

0.00.20.40.60.81.0

0.00.20.40.60.81.0

21191715131197531

Depth (1015 atoms/cm2)

21191715131197531

Rel

ativ

e C

once

ntra

tion

2018161412

10864

C aS i

C aS i

C a

S i

O

T i

G e

2

Fig. 12. Concentration versus thickness (atoms cm�2) plots for Ge, Ti, O in sampleannealed at 500 �C. Layer numbers are marked from left to right.

at higher temperatures, more energy is available for the atoms atinterfaces to diffuse into each other.

Shift of the absorbtion edge toward lower wavelength seems tobe related with the enhanced quantum confinement effects due todecrease in the thickness of Ge layers as a result of the formationGeTiO2 at the interfaces of Ge and TiO2 layers. A shift in the peak ofGe towards lower wave number in the Raman spectra also relateswith the enhanced quantum confinement effects due to interfacediffusion and decrease in the thickness of pure Ge, as a result ofannealing.

Partial or complete oxidation of Ge layers as a result of annealingis clearly evident by RBS of multilayers at incident energy of Heþ2

particles equal to 3.03MeV, which is a resonant energy for the largescattering cross-section of oxygen. Scattering cross-sectiondecreases to normal value at a depth where the energy of inci-dent particles becomes less than the resonant energy. The resonantsignal of oxygen represents the oxygen from a few top layers (4e5)

Fig. 13. UVevisible spectra of as-deposited and annealed samples in transmissionmode.

400 600 800 1000

518 630

289

291annealed at 500 °C

531400

628annealed at 400 °C

413

620

292

Ra

ma

n In

te

ns

ity

(a

.u

)

annealed at 300 °C294

417

615

Raman Shift (cm )

as-deposited

Fig. 14. Raman spectra of as-deposited and annealed samples.

M.S. Hussain et al. / Materials Chemistry and Physics 139 (2013) 17e2626

and its height increases with annealing temperature inferring thatthe oxygen for the oxidation of Ge layers is picked up from theannealing environment. The simulation of RBS spectra showsa gradual increase in the number of oxidized layers. At 300 �C, firstand second germanium layers oxidize in the stoichiometry ofGe0.6O0.4 and Ge0.8O0.2, respectively.While at 400 �C the top four Gelayers oxidize to form GeO2 and the fifth Ge layer partially oxidizesinto a stoichiometry of Ge0.5O0.5. At 500 �C top five Ge layerscompletely oxidize into GeO2 while sixth layer partially oxidizeswith stoichiometry of Ge0.75O0.25. More oxygen intake in themultilayers is possible at higher annealing temperatures.

5. Conclusion

Periodic nano-multilayers of Ge (15 nm) and TiO2 (20 nm) weredeposited and annealed at temperatures of 300e500 �C. It wasconcluded that these multilayers are potential candidates for theiruse as absorbing medium in third generation photovoltaic devices.The number of layers and the thicknesses of individual layers can beadjusted during deposition to obtain a material of desired energyband structure. Also the energy band structure of a multilayer canbe engineered by annealing at suitable temperatures. As a result ofannealing, new inter-layers emerge. These inter-layers alter the

energy band structure of multilayers, in one way, by reducing thethicknesses of their parent layers and, in another way, by contrib-uting their own quantum confinement effects. The thicknesses ofthese inter-layers depend upon the annealing temperature and inthis way, the annealing plays an important role in the band-gapengineering of multilayers. Annealing environment also playsa role in the oxidation of layers of pure elements (depending upontheir oxidation temperatures). The chemistry and number ofoxidized layers also depend upon the annealing temperature. In ourcase, controlling the annealing temperature, oxidation of germa-nium layers and the thickness of diffused layers can be controlled totune the optical and photovoltaic properties of multilayers.

Acknowledgment

We are thankful to Accelerator Lab, National Centre for Physics,Islamabad for providing us the facility of RBS installed at TandemAccelerator.

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