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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 16 (2013) 1769–1774

1369-80http://d

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Thickness dependent physical properties of chemicallydeposited nanocrystalline MgSe thin films deposited at roomtemperature by solution growth method

Ashok U. Ubale n, Yogesh S. SakhareNanostructured Thin Film Materials Laboratory, Department of Physics, Government of Vidarbha, Institute of Science and Humanities,VMV Road, Amravati 444604, Maharashtra, India

a r t i c l e i n f o

Available online 24 July 2013

Keywords:Thin filmsSEMOptical propertiesElectrical properties

01/$ - see front matter & 2013 Elsevier Ltd.x.doi.org/10.1016/j.mssp.2013.06.015

esponding author. Tel.: +91 721 253 1706; faail address: ashokuu@yahoo.com (A.U. Ubale

a b s t r a c t

Chemical bath deposition method has been employed to deposit nanocrystalline magne-sium selenide thin films of thickness 104–292 nm onto glass substrates at room tempera-ture. The deposition bath consists of magnesium chloride, triethanolamine (TEA) andselenium dioxide. The as deposited films were characterized by X-ray diffraction, scanningelectron microscopy (SEM), atomic force microscopy (AFM), optical absorption, electricalresistivity and thermo-emf measurements. The X-ray diffraction (XRD) studies revealedthat the crystallinity of the magnesium selenide thin film increases with thickness. SEMstudies reveal that MgSe films exhibit uniform distribution of round shaped grains over theentire substrate surface.The optical band-gap and electrical resistivity of MgSe filmdecrease as the film thickness increases. Such type of dependence is attributed to thequantum size effect that is observed in nanocrystalline semiconductors.The thermo-emfmeasurement confirms its p-type conductivity.

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1. Introduction

Nanocrystalline semiconducting chalcogenides thinfilms find numerous applications in various photovoltaicand electro-optical devices [1]. Intensive research has beenperformed in the past decade on the preparation ofnanostructured semiconductors of novel electrical andoptical properties by simple and economical solutionbased methods [2,3]. MgSe is a wide band gap semicon-ductor and has technological impact due to its applicationsin optoelectronics [4] and luminescent devices [5]. Kal-pana et al. [6] have studied electrical and structuralproperties of MgSe by using the tight binding linearmuffin-tin orbital method. The electronic properties ofMgSe were reported by Chadi [7] and confirmed possibilityof obtaining large band gap alloys of n- and p-type

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conductivities. Feng et al. [8] have reported the growthof the thin layers of MgSe on relaxed 0.6 μm ZnSe epilayersand studied their electronic properties. The electronicband structure of MgSe in zinc blende epitaxial form in(001) direction has been determined. Jiang et al. [9] havesuccessfully deposited MgSe thin films on GaAs substratesby the metal organic chemical vapor deposition (MOCVD)method and their crystal structure has been investigated.The result shows that the crystal above 6500 1C growthtemperature is rock salt and between 500–550 1C it is zincblende. Frey et al. [10] have reported growth and char-acterization of binary MgSe thin films by the molecularbeam epitaxy (MBE) method. Prete et al. [11] havereported low pressure metal organic vapor phase epitaxy(LPMOVPE) method for the deposition of MgSe on (100)GaAs. The literature survey revealed that a few reports onMOCVD and MBE grown MgSe thin films are available, butno reports are published on structural, electrical, morpho-logical and optical properties of MgSe thin films grown by

Fig. 1. Variation of MgSe film thickness (nm) with deposition time(hours).

Fig. 2. X-ray diffraction pattern of MgSe thin films of various thicknesses.

A.U. Ubale, Y.S. Sakhare / Materials Science in Semiconductor Processing 16 (2013) 1769–17741770

a simple and economic solution growth method calledchemical bath deposition (CBD). The chemical bath deposi-tion method has been widely applied for fabrication ofsemiconductor layers suitable for photovoltaic applica-tions. The CBD process of thin film deposition involvesthe deposition of materials atom-by-atom, molecule-by-molecule, ion-by-ion or cluster-of-species-by-cluster-of-species and condensation [12]. The deposition normallytakes place through the adsorption of metal complexligands on the surface of the substrate which providesnucleation layer and the chemical conversion of this layerto the metal chalcogenide layer through reaction withchalcogen ions. Once this happens, the metal chalcogenidelayer acts as a catalytic surface for the ion-by-ion condensa-tion of the metal and chalcogen ions leading to the growthof thin film [13]. In this investigation thin films of MgSewith variable thickness were prepared and the effect of filmthickness on structural, electrical and optical properties isstudied. It is well known that material properties aresusceptible both to the deposition technique and prepara-tive parameters. Film thickness is one of the importantparameters, which influences material properties and canbe controlled to obtain desired properties suitable for theapplication. Consequently, the material can be tailored interms of the structure, grain size, resistivity and band gapenergy.

2. Experimental details

The MgSe thin films were deposited onto glass sub-strates from a solution bath containing 10 ml of 0.12 MMgCl2, 10 ml of TEA and 10 ml of 0.13 M SeO2 at roomtemperature. To prepare SeO2 solution, 1 g Se metal powderwas dissolved in HNO3 to get selenous acid. The dehydra-tion of selenous acid gives white residual powder of SeO2

which is then dissolved in 100 ml distilled water to prepare0.13 M fresh SeO2 solution. In the CBD method extremecleaning of the substrate is required before the deposition,since contaminated substrate surface provides nucleationsites facilitating growth which produces non-uniform film.In the present investigation, commercially available glassmicroslides of dimensions 26 mm�76 mm�2 mm wereboiled in chromic acid for 30 min and then washed withdouble distilled water. The substrates were again washedwith liquid detergent and rinsed in acetone. Finally slideswere ultrasonically cleaned with double distilled water for15 min prior to the actual deposition, as reported in theearlier report [14]. In order to investigate thickness depen-dent physical properties the deposition time was changedfrom 12 to 48 h to prepare films of thickness 104–292 nm.The thickness of the film was measured by the weightdifference method assuming the density of the depositedfilm to be the same as that of the bulk. The thickness of thedeposited film was measured with the help of a sensitivemicro-balance using the relation

t ¼ mρA

ð1Þ

where m is the mass of the deposited film, A is the areaof the deposited film and ρ isthe density of the depositedmaterial in the bulk from.The structural characterizations

of the as deposited films were carried out by taking XRDpatterns using a PANalytical X’Pert PROMRD X-ray dif-fractometer with CuKα radiation of wavelength 0.154 nm.Surface morphological and compositional analysis wascarried out using a scanning electron microscope, JEOL’SJSM-7600F with EDAX attachment. The atomic forcemicrographs were taken using Park Scientific Instruments.The optical measurements were carried out using a JASCOV-530 UV/visible spectrophotometer. The electrical resistiv-ities of the films having different thicknesses were mea-sured using the d.c. two-probe method. The area of thefilms was defined and silver conducting paste was appliedto ensure good electrical contacts to the film. The type ofconductivity of the MgSe films was determined by measur-ing thermo-emf generated across hot–cold junction.

3. Results and discussion

3.1. Film formation mechanism

The formation of MgSe thin films is based on the slowrelease of Mg2+ and Se2� ions in the aqueous medium andthen subsequent condensation on the substrate. The reac-tion mechanism involved in the deposition of the MgSefilm is proposed as above. The Mg salt, MgCl2 in aqueousmedium, reacts with TEA to form [Mg (TEA)]2+complex:

MgCl2 þ TEA-½MgðTEAÞ�2þ þ 2Cl� ð2Þ

A.U. Ubale, Y.S. Sakhare / Materials Science in Semiconductor Processing 16 (2013) 1769–1774 1771

The Se metal powder is dissolved in nitric acid and afterdehydration it gives SeO2 :

Se þ 2HNO3 ⟹dehydration

SeO2 þ H2 O↑þ NO↑ þ NO2↑ ð3ÞThe SeO2 prepared is then dissolved in distilled waterwhich gives SeO2�

3 as:

SeO2 þ H2 O-H2SeO3 -2Hþ þ SeO2�3 ð4Þ

Fig. 3. Scanning electron micrograph images of MgSe thin films. Films

Table 1Comparison of standard and observed JCPDS data for MgSe film.

Film thickness (nm) Standard Observed hkl

2θ (deg) d Å 2θ (deg) d Å

104 – – – – –

211 28.30 3.1508 28.08 3.1767 11132.80 2.7280 32.33 2.7682 20047.00 1.9316 46.99 1.9331 22068.70 1.3650 68.77 1.3645 400

280 28.30 3.1508 28.09 3.1756 11132.80 2.7280 32.74 2.7344 20068.70 1.3650 68.68 1.3661 400

292 28.30 3.1508 28.46 3.1298 11132.80 2.7280 32.83 2.7271 200

In the presence of hydrazine hydrate SeO2�3 reduces to

Se2� :

SeO2�3 þ 6Hþ þ 2e�-Se2� þ 3H2O ð5Þ

thickness: (A) 104 nm, (B) 211 nm, (C) 280 nm and (D) 292 nm.

Fig. 4. The EDAX spectrum of MgSe thin film of thickness 211 nm,deposited on glass substrate.

A.U. Ubale, Y.S. Sakhare / Materials Science in Semiconductor Processing 16 (2013) 1769–17741772

The Mg2+ ions released from Mg-complex react with Se2�

to from MgSe:

½Mg TEAð Þ�2þ þ Se2�-MgSe↓ þ TEA ð6ÞThe deposition of MgSe occurs when ionic product of

Mg2+ and Se2� ions exceeds the solubility product ofMgSe. The variation of MgSe film thickness with deposi-tion time is shown in Fig. 1. It was observed that asdeposition time increases, film thickness increases, attainsmaximum thickness at around 48 h deposition time andremains constant thereafter.

3.2. Structural properties

X-ray diffraction (XRD) is an efficient tool for thestructural analysis of crystalline materials. Fig. 2 showsXRD patterns of the MgSe thin films deposited by varyingdeposition time. The film of thickness 104 nm deposited at12 h deposition time is amorphous in nature. The broadhump observed in XRD pattern is due to the amorphous

Fig. 5. AFM image of MgSe thin films. Film thickness: (A) 104 nm and (B)211 nm.

glass substrate. However, at higher thickness it showsnanocrystalline nature with cubic phase (JCPDS Card no.18-0777). The (111) plane due to cubic lattice is found to beprominent at higher thickness. The (220) orientationobserved for the film of thickness 211 nm disappeared infilm with thickness above 280 nm. The earlier reports onMgSe deposited by physical methods like MBE andMOCVD show zinc blende, wurtzite and rock-salt struc-tures [15–17]. The average crystallite size has been calcu-lated from the recorded XRD patterns using the wellknown Scherrer Equation [18]

d¼ 0:9λβ cos θ

ð7Þ

where d is the average crystallite size, λ is the wave-length of incident X-ray, β is the full width at halfmaximum (FWHM) of the peak in radian and θ is Bragg’sangle of diffraction peak. The average grain size of MgSethin film of thickness 211 nm is 36 nm and it increases to43 nm when thickness rises to 292 nm (Table 1). The grainsize of MgSe film increases with thickness i.e. withdeposition time. This may be attributed to the decreasein imperfections in the film with increase in film thicknessand quantum size effect observed in semiconductingnanostructured thin films [19].

Fig. 6. Plots of optical absorption (αt) versus wavelength λ (nm) for MgSefilms. Film thickness: (A) 104 nm, (B) 211 nm, (C) 280 nm and (D)292 nm.

Fig. 7. Plots of (αhυ)2 versus hυ for MgSe films.

Fig. 8. Variation of optical band gap energy (eV) with MgSe filmthickness (nm).

Fig. 9. Variation of log (ρ) versus (1/T)�103(K�1) for MgSe films. Filmthickness: (A) 104 nm (B) 211 nm (C) 280 nm and (D) 292 nm.

A.U. Ubale, Y.S. Sakhare / Materials Science in Semiconductor Processing 16 (2013) 1769–1774 1773

3.3. Surface morphology

The SEM images of CBD deposited MgSe thin films areshown in Fig. 3. The SEM micrographs reveal that thesubstrates are well covered by uniformly distributed roundgrains. At 12 h deposition time, tiny grain growth isobserved on homogenous background. As deposition timeincreases grain growth is improved. The composition ofthe film deposited at 24 h deposition time was investi-gated using an energy dispersive analysis setup attachedwith SEM system. Fig .4 shows a typical EDAX pattern andthe elemental analysis was carried out only for Mg and Se.The average atomic percentage of Mg:Se was 47.27:52.73,showing that the sample was slightly selenium rich.Theother peaks are due to glass substrate. Fig. 5 shows AFMimage of MgSe film of thickness 104 nm and 211 nm.A well-defined granular structure is observed. The surfaceroughness increases with deposition time.

3.4. Optical studies

The optical properties of MgSe thin films of differentthicknesses deposited by chemical bath deposition techni-ques on glass substrates were investigated. Fig. 6 showsvariation of optical absorption with wavelength for MgSethin films. At incident wavelength 375 nm the absorptioncoefficient of the film of thickness 104 nm is 4.47�104 cm�1

and it increases to 2.58�105 cm�1 at 292 nm thickness.From the optical absorption data the optical band gapenergies were calculated using the relation [20–24]

αhυ¼ AðEg�hυÞn ð8Þwhere hυ is the photon energy, Eg is the band gap

energy and A and n are constants. For allowed directtransition n¼1/2 and for allowed indirect transitionn¼2. The optical band gap of MgSe was estimated byextrapolating the straight portion of (αhυ)2 against hυ plotto hυ axis (Fig. 7). The optical energy band gap value ofMgSe thin film decreases from 2.7 eV to 2.49 eV as the filmthickness increases from 104 to 292 nm (Table 2; Fig. 8).The shift observed in band gap is associated with thesmaller grain size of nanostructured material as a result ofvarious size effects.

3.5. Electrical resistivity measurement

The electrical resistivities of MgSe thin films weremeasured by the conventional d.c. two-probe method inthe temperature range 300–500 K. Fig. 9 shows the variationof dark electrical resistivity (log ρ) with temperature (1000/T). The electrical resistivity of MgSe thin film (at tempera-ture 373 K) decreases from 43�103 to 33�102 Ω cm as film

Table 2Grain size, electrical resistivity at 373 K, band-gap energy and activation energy

Films Deposition time (h) Thickness (nm) Band gap energy (eV)

A 12 104 2.7B 24 211 2.65C 36 280 2.56D 48 292 2.49

thickness increases from 104 to 292 nm (Fig. 10). Thedecrease in resistivity with increases in thickness is due toincreased grain size and porous nature of the MgSe film.This high value of resistivity at lower thickness is due to thefine grains i.e. nanocrystalline nature of the MgSe film. Theactivation energy was calculated using the relation [25]

ρ¼ ρoEaKT

� �ð9Þ

Where ρ is the resistivity at temperature T, ρo is aconstant, K is the Boltzmann constant and Ea is activationenergy. Fig. 11 shows the variation in activation energy ofMgSe with film thickness. It decreases from 0.17 to 0.12 eVas film thickness increases from 104 to 292 nm.

3.6. Thermoelectric properties

In the measurement of thermo-emf it was found thatvoltage developed across the junction rises from 1.4 to1.9 mV as film thickness varies from 104 to 292 nm at

values of MgSe thin films.

Activation energy (eV) Grain size (eV) ρ�106 at 373 K (Ω cm)

0.17 – 430.14 36 410.13 41 360.12 43 33

Fig. 12. Plots of thermo-emf. versus temperature differences for differentthicknesses of MgSe thin films. Film thickness: (A) 104 nm, (B) 211 nm,(C) 280 nm and (D) 292 nm.

Fig. 11. Variation of activation energy (eV) of MgSe with film thickness(nm).

Fig. 10. Variation of electrical resistivity (Ω cm) of MgSe at 373 K withthickness (nm).

A.U. Ubale, Y.S. Sakhare / Materials Science in Semiconductor Processing 16 (2013) 1769–17741774

220 K temperature difference applied across the junction.The thickness dependence of thermo-emf generatedacross the junction for MgSe is shown in Fig. 12. Themagnitude of thermo-emf developed increases with thick-ness, which may be due to the decrease in electricalresistivity and improvement in the crystalline nature ofMgSe thin film. The thermo-emf measurement confirmsits p-type conductivity.

4. Conclusions

Nanocrystalline MgSe thin films of thickness 104–292 nmwere prepared onto glass substrates by the CBD method. TheX-ray diffraction study reveals that films are nanocrystallinein nature with cubic lattice. The films deposited at lowerthickness show amorphous nature. The surface analysisshows circular rounded type islands on homogenous back-ground. The optical band gap value of MgSe thin film variedfrom 2.70 eV to 2.49 eV as the deposition time was increasedfrom 12 to 48 h. The decrease in resistivity with increase inthickness may be due to increased grain size and porousnature of the MgSe films. The thermo-emf measurementconfirms its p-type conductivity.

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