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Applied Surface Science 296 (2014) 185–188

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

ultilayer Si/Ge thin films with quantum confinement effectsor photovoltaic applications

alman Ali Shaha, Abdul Faheem Khana,b,∗, AsadUllah Khana,.A. Rahimb, Mazhar Mehmoodc

Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, PakistanUM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D UM, University of Malaya, Jalan Pantai Baharu, 59990 Kuala Lumpur,alaysia

Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan

r t i c l e i n f o

rticle history:eceived 3 November 2013eceived in revised form 9 January 2014ccepted 13 January 2014vailable online 25 January 2014

eywords:ulti-layer Si/Ge thin films

aman spectroscopyutherford backscattering

a b s t r a c t

Multilayer thin films of Si and Ge were grown by e-beam evaporation and resistive heating techniques.Each Si-layer has a thickness of 15 nm and that of Ge is 20 nm. In this way, three sets of multilayerstructures with 2, 4 and 6 layers of Si and Ge in an alternative way were deposited on glass substrates.Structural and electrical properties of these multi-layer films were studied using Raman spectroscopy,Rutherford backscattering, Fourier transform infrared spectroscopy, and electrical resistivity measure-ments. Raman spectra of these multilayer thin films exhibit peaks shift towards lower wavelength (incomparison with bulk Ge) demonstrating that the films consist of nanostructures and also represent quan-tum confinement effect in Ge. The quantum confinement effect increases with the increase in number oflayers. Raman spectra also reveal the formation of silicon oxide and germanium oxide. FTIR spectroscopy

ourier transform infrared spectroscopy also confirms the presence of oxides in these multilayer films. The layer thickness and composition wasdetermined using Rutherford backscattering spectroscopy. The DC-conductivity measurement of Si/Gemultilayer thin films shows gradual increase in conductivity with the increase in number of layers. Thesemulti-layer Si/Ge films show high electrical conductivity in comparison with pure Ge and Si films due tothe SiGe alloy phase formed at the interface during deposition. These investigations suggest that Si/Ge

qua

multilayer thin films with

. Introduction

Thin films fabricated in the form of multi-layers at nanometercale give unusual properties due to their reduced dimensions. Theroperties of multi-layer films strongly depend on its thickness.n extremely thin film/layer is characterized by a high surface-to-olume ratio, and its small size perpendicular to the film givingise to quantum confinement of electrons. Novel properties canlso be obtained in rather thicker nanostructured films composedf multi-layers, nanoclusters, nanocomposites, etc., by interfacialhenomenon and quantum confinement effects. Nanostructuredemiconducting materials, particularly 2-D thin films are of primemportance due to their structure dependent optical and electrical

roperties such as absorption, transmission, conductivity, mobil-

ty, etc. that can be used to design and create new generation oflectronic devices predominantly photovoltaic [1–9].

∗ Corresponding author at: UM Power Energy Dedicated Advanced CentreUMPEDAC), Level 4, Wisma R&D UM, University of Malaya, Jalan Pantai Baharu,9990 Kuala Lumpur, Malaysia. Tel.: + 603 2246 3457; fax: + 603 2246 3257.

E-mail address: faheem khan 1977@yahoo.com (A.F. Khan).

169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2014.01.075

ntum confinement effects can be used for solar cells.© 2014 Elsevier B.V. All rights reserved.

The concept of tandem thin film photovoltaic (third genera-tion photovoltaics) possessing cascading band gaps and exploitingquantum confinement effect has become the focus of research now-a-days. The high efficiency of third generation photovoltaic is due tothe reduction in loses by absorption of wide spectrum of solar radi-ation by different semiconductor absorber stacked over each otherin decreasing band gap order [1,2,10]. Quantum confinement hasnoticeable effect on the structural, electrical and opto-electronicsproperties of the nanomaterials or nanostructured thin films caus-ing size-tunable properties [1–3].

Ge has a Bohr radius of 25 nm while Si has 5 nm. The larger Bohrradius of Ge in comparison with Si makes it easier to tune the elec-tronic properties of Ge and consequently, exhibits more prominentquantum confinement effects. The indirect band gap of silicon isabout 1.1 eV, while Ge has 0.67 eV at room temperature. Ge (con-taining nanoparticles) in the Si matrix shifts the absorption edgeof germanium towards higher energies subject to the particle size[11]. This effect can be used to increase the conversion efficiency

of these Si/Ge films for solar devices. Nevertheless, the ability todeposit multi-layer films using PVD techniques with nanoscale pre-cision is of prime importance for the fabrication of photovoltaicdevice. In this respect, it is worthwhile to fabricate the multi-layer

1 face Science 296 (2014) 185–188

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lms in the form of two, four and six layers as they provide bet-er understanding of interfacial reactions/properties and growthinetics and their morphology.

In this paper, structural and electrical properties of Si/Ge multi-ayer thin films deposited by physical vapor deposition have beeneported. Si and Ge were grown alternatively in the form of 2, 4nd 6 layers. The key focus of this research work is to evaluate theffect of number of layers on the structural and electrical proper-ies of nanostructured multilayer Si/Ge thin films due to quantumonfinement effects.

. Experimental details

Multi-layer Si/Ge thin films were grown on BK7 glass substratessing e-beam evaporation of high purity Si powder (99.999% pure)nd resistive heating of Ge powder with 99.999% purity. Sub-trates were cleaned with acetone and isopropanol alcohol usingltrasonic bath for 30 min. The deposition was performed usingdwards Coating System. For better uniformity and homogeneouslms, the substrates temperature was kept at 300 ◦C and rota-ion was maintained at 30 rpm. The substrates were fixed at aistance of 35 cm from source. Germanium layers were depositedy tungsten crucible and Si layers with silica coated graphite cru-ible with an evaporation rate of about 0.1 nm/s. Before deposition,he vacuum was kept to less than 1 × 10−5 mbar. Crystal quartz

onitor was used to control the thickness of the films duringeposition.

First, a layer of 20 nm of Ge was grown onto bare glass sub-trate. Then a 15 nm layer of Si was grown on 20 nm Ge layer.n the Si layer, a second layer of Ge of 20 nm was deposited and

imilarly third layer of Ge was deposited on Si layer of 15 nm.hus forming 2, 4 and 6 layer structure with total thickness of5 nm, 70 nm and 105 nm, respectively. The Raman spectroscopicesults were achieved by confocal mode of Micro-Raman spec-rometer (MST-4000A, DongWoo Optron Co. Ltd., South Korea)t room temperature. HeCd laser beam of 442 nm were usedor this purpose. Fourier transform infrared (FTIR) spectra werechieved by NICOLET 6700, Thermo Electron Co., USA in theange of 400–4000 cm−1 at room temperature. The thickness andomposition of these multi-layers films were measured by Ruther-ord backscattering spectroscopy (RBS). The measurements waserformed using 5UDH-2 Pelletron (5 MV Pelletron Tandem Accel-rator) with He2+ beam. The average energy of beam was 2 MeV.he analysis was performed by keeping the scattering angle at70◦. The samples were placed at 70◦ with respect to incidenteam and 70.32◦ with respect to the scattered beam. All RBS mea-urements were made using Cornell geometry. Electrical resistivityeasurements were achieved at room temperature using 4-probeethod.

. Results and discussion

Fig. 1(a–c) shows the typical RBS spectra of Si/Ge multi-layerhin films along with fitted spectra. These films were depositedith intended Ge layer thickness of 20 nm and Si layer thickness of

5 nm. The kinematic factor of Ge is about 0.833, which is higherhan that of Si (0.566) at 170◦ (and Ge (0.983) and Si (0.993) at20◦) [12] due to larger atomic mass. Consequently, the peak ofilicon appears at lower energy ∼1121 (though it was at the top)n comparison with Germanium (energy ∼1668). Intended layerhickness and calculated layer thickness for 2, 4, and 6 layers Si/Ge

ulti-layers from RBS spectra along with error limits are shownn Table 1. However, it is clearly visible from Fig. 1 that the layertructure has been effectively formed in all 2, 4, and 6 number ofayers of Si/Ge films. Moreover, absorption of oxygen (from atmo-

Fig. 1. RBS spectra of multi-layer Si/Ge thin films (a) 2-layers, (b) 4-layers, and (c)6-layers.

sphere) can also be seen (Fig. 1) due to slightly increase in the widthof individual peak and overall spectra with the increase of numberof layers.

Fig. 2 shows the Raman spectra of Si/Ge multi-layer thin filmswith 2, 4, and 6 number of layers. A sequence of peaks is evident inthe spectrum of multi-layer films for 2 layers at about 288, 386, 808,885 and 945 cm−1. Raman peak at about 288 cm−1 can be assignedto Ge. The peak at 288 cm−1 is left-shifted with respect to the peakof bulk Ge (i.e. 302 cm−1). In 4-layer and 6-layer Si/Ge films, this

peak appears at 285 and 283 cm−1, respectively. The shifting ofthe peak is due to nanostructures indicating quantum confinementeffects due to localization of phonons in the disordered structure[13]. The smaller the size of nanostructure, the more is the shifting

S.A. Shah et al. / Applied Surface Science 296 (2014) 185–188 187

Table 1Intended layer thickness and calculated layer thickness for Si/Ge multi-layer thin films.

Layers Intended layer thickness (nm) Si/Ge-2 layers Si/Ge-4 layers Si/Ge-6 layersCalculated thickness (nm) Calculated thickness (nm) Calculated thickness (nm)

Ge 20 17 ± 0.61 21 ± 0.72 19 ± 0.69Si 15 13 ± 0.41 14 ± 0.45 13 ± 0.49Ge 20 20 ± 0.59 18 ± 0.80

13 ± 0.54 13 ± 0.5019 ± 0.5514 ± 0.60

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Fig. 3. FTIR spectra of multi-layer Si/Ge thin films with 2, 4, and 6-layers.

Si 15

Ge 20

Si 15

owards lower wavenumbers [8]. The peak at about 386 cm−1 cane assigned to SiGe for 2-layer Si/Ge films while this peak appearst about 393 and 384 cm−1 for 4-layer and 6-layer films, respec-ively. This peak seems to be related with the interface mode andrises due to SiGe vibrations at the interface [14]. It is also notablehat there is practically some detectable intermixing between theonstituent layers with the increase of number of layers leadingo alloy formation at the interface [15]. Peak at about 464 cm−1

as been related to amorphous Si for the 6-layer Si/Ge films [7].ue to lower thickness, this peak is not visible in 2-layer and 4-

ayer films. Peaks at 808 and 905 cm−1 can be assigned to SiO andeak at 885 cm−1 is related to GeO2. The presence of small quan-ity of oxide at grain boundaries causes pronounced effects on theuantum confinement of Si/Ge multi-layer films.

Fig. 3 shows the FTIR spectra of Si/Ge multi-layer thin filmsith 2, 4, and 6 number of layers. Two peaks are visible at 762

nd 914 cm−1 in all spectrums. The peak at 762 can be assignedo stretching vibrations of Si-O [16] and peak at 914 cm−1 can beelated with non-symmetric stretching vibrations of Ge O [17].ormally, defects are formed due to agglomerated oxygen dur-

ng deposition of Si/Ge thin films leading to non-stoichiometriceO and SiO [2]. The formation of GeOx and SiOx preferably at

he grain boundaries (in small quantities) enhances the quantumonfinement effects, which has an advantage for the fabrication ofptoelectronic devices particularly photovoltaic.

Fig. 4 shows the plot of conductivity as a function of numberf layers for Si/Ge multi-layer thin films. Conductivity of the filmsncreases with the increase of number of layers. The conductivity for-layers films was about 3.42 × 10−3 � cm−1, which increases for

and 6-layer films to 6.65 × 10−3 � cm−1 and 7.39 × 10−3 � cm−1,espectively. The conductivity of Si/Ge films is higher than pure Ge

lms [2] and Si films [18]. The increase of conductivity is primarilyttributed due to increase in number of charge carrier with thick-ess, which appears to be due to synergistic effects at the interface.

Fig. 2. Raman spectra of multi-layer Si/Ge thin films with 2, 4, and 6-layers.

Fig. 4. Plot of dc-conductivity vs. number of layers of multi-layer Si/Ge thin films.

Moreover, the SiGe phase formed at the interface during depositionalso facilitate for the enhancement of conductivity. Another possi-ble reason for the rise in conductivity with number of layers mayalso be related with variations in the properties of heterojunctioncomposite film [1,15].

4. Conclusion

The as-deposited Si/Ge films show quantum confinement

effects, which increases with the increase in number of layers.Raman and FTIR results confirm the presence of non-stoichiometricGeOx and SiOx. Raman spectroscopy also confirms the presenceof SiGe at the interface. These multi-layer Si/Ge films show high

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lectrical conductivity in comparison with pure Ge and Si films dueo the SiGe phase formed at the interface during deposition. Electri-al conductivity of the present films also increases with the increasen number of layers. Consequently, these multi-layer Si/Ge filmsppear to be a suitable candidate for photovoltaic applications.

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