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Current Applied Physics 11 (2011) 547e550

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Current Applied Physics

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

Surface analysis of GeC prepared by reactive pulsed laser deposition technique

Arshad Mahmood a,*, A. Shah a, F.F. Castillon b, L. Cota Araiza b, J. Heiras b, M. Yasin Akhtar Raja c,M. Khizar c

aNational Institute of Lasers and Optronics, P.O. Nilore, Nilore, Islamabad, PakistanbCentro de Ciencias de la Materia Condensada de la UNAM, Ap. Postal 2681, BC 22800, MexicocCenter of Optoelectronic and Optical Communications, University of North Carolina, Charlotte, United States

a r t i c l e i n f o

Article history:Received 5 July 2010Received in revised form31 August 2010Accepted 13 September 2010Available online 18 September 2010

Keywords:Germanium carbideReactive pulse laser depositionThin filmsXPSEllipsometry

* Corresponding author. Tel.: þ9251 9290231x3206E-mail address: arshad_junjua@yahoo.com (A. Ma

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2010.09.011

a b s t r a c t

Amorphous germanium carbide (a-Ge1�xCx) thin films were prepared by reactive pulsed laser depositiontechnique using several methane pressures. Surface analysis was performed by X-ray photoelectronspectroscopy (XPS) to examine the composition and elemental bonding at the surface of the material.Optical analysis was carried out by spectroscopic ellipsometry to study the optical constants (n and k) andother parameters of the film. Results indicate that the carbon atoms to be incorporated in the germaniumlattice, forming a-Ge1�xCx alloy, for concentrations below about 10 atomic % where the Ge atoms areuniformly distributed. There is formation of graphitic agglomerates for higher carbon concentrations.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Considerable interest has arisen in the wide band gap materialsas possible candidates for applications in blue/ultraviolet (UV) lightemitting diodes and laser diodes. Tetrahedrally coordinated widebandgapmaterialswhich involve the elements of thefirst row in theperiodic table like Be, B, C and N have extreme properties ascompared to other conventional semiconductors also tetrahedrallybonded. Due to the excellent properties of SiC [1,2], which crystal-lizes into more than 200 polytypes, it is observed that germaniumcarbide, GeC, the other group-IV carbide,might also present peculiarproperties for opto- electronic applications due to its wide band gap[3]. In spite of receiving interest, there is still lack of experimentaland theoretical development in this important semiconductor [4].The excellent performances make Ge1�xCx films applicable fordesign and preparation of multilayer anti-reflection and protectioncoatings of IR windows [5,6]. In addition, GexC1�x filmsmay providethe apparent tunability of the band gap over a very wide range. Thisimportant characteristic can be applied for the photovoltaic appli-cations and this semiconductor has several other applications [7,8].In this work, amorphous GeeC films have beenprepared by reactivepulsed laser deposition RPLD technique under different methanepressuresduringgrowth. This techniquehas theprincipal advantage

; fax: þ9251 2208051.hmood).

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to produce very high energy particles as compared to other tech-niqueswhich can favor the incorporation of carbon in thematerial atroomtemperature. In thiswork, the influenceof the incorporationofC atoms in the Ge lattice on the surface, optical and structuralproperties of the films has been studied.

2. Experiment

All films were deposited at room temperature in the modifiedreactive PLD UHV vacuum system. In the chamber, the gas pressurewas controlled by manually adjusting the position of the gate valvebetween the chamber and the turbomolecular pump. Fine wirethermocouple was used to correlate the temperature of the surfaceof the P-etched silicon substrates to the temperature of the substrateholder. Si substrates (100) were cleaned before deposition ultra-sonically by the standard method. Target of pure Ge (99.999%) wasablated by using excimereKrF pulsed laser in the presence ofmethane as the reactive gas. Methane gas (99.999%) was used as thesource of carbon in to thefilm(s). Samplesweredeposited byvaryingthe pressure of methane from 2.5 to 75 mTorr. Excimer laser (KrFwith l¼ 248 nm)was used to ablate the Ge target (99.999%). Energyinduced by the laser source was 200mJ with repetition rate of 5 Hz.Number of pulses used for each filmwas 6000 while the depositiontime for each sample was 20 min.

The samples were analyzed using a Siemens D500 X-ray powderdiffractometer usingCuKa radiation. The surface analysiswasdone in

A. Mahmood et al. / Current Applied Physics 11 (2011) 547e550548

an analysis chamber which is equipped with an electron energyanalyzer model Mac-3 of Cameca, an e-gun and an X-ray source toperform XPS and EELS (Electron Energy Loss Spectroscopy). The XPSdata was collected after exciting the sample by the nonchromatic Ka

line (1486) of Al. The energy axis was calibrated with the 932.67 eVand 368.26 eV respectively. The quantification by XPS is based on thescheme presented [9]. With this scheme an accurate stoichiometricevaluationof thefilmcanbedonebyXPS. TheEELSdatawascollectedusing an incident e-beam of 1000 eV and a resolution of 2 eVmeasured from the FWHM of backscattered electrons. For opticalanalysis of the films, SE-850 Ellipsometer, by SENTECH instrumentsGmbH, was used. The ellipsometeric data were recorded at theincidenceangleof 70� in the spectralwavelength range370e850nm.The Cauchymodelwas used to simulate the optical constants such asrefractive index (n) and extinction co-efficient (k) of the Ge1�xCxfilms. The influence ofmethane pressurewas analyzed on the opticalconstants of the films at wavelength, l ¼ 632 nm.

3. Results and discussion

The film structure was examined by the analysis of X-raydiffraction. No evidence of any crystalline phase was observed inthe entire carbon concentration range. The chemical structure ofthe films was determined by measuring the core level spectra ofGe3d, C1s and O1s. It was assumed that the peaks were composed ofseveral contributions corresponding to Ge, C and O with differentchemical states. The obtained spectra were deconvoluted and thebinding energy (BE) of each core level is presented in Table 1.

Fig.1a presents theXPS spectra of Ge3d core level of Ge1�xCxfilmsas a function of methane pressure ðPCH4

Þ. The binding energyincreases for pure Ge from 29.6 eV to 30.2 eV for the samplesprepared at pressure from0 to 75mTorr. In Fig.1b, theXPS spectra ofC1s of all films of Ge1�xCx are shown. It can be noted that there is anincrease in intensity as a function of methane pressure ðPCH4

Þ.Moreover, the binding energy of C1s increases from 283.8 eV for thepressure of less than 2.5 mTorr to 284.4 for the samples prepared atpressure of 75mTorr. Fig.1c shows the XPS spectra O1s of all films ofa-Ge1�xCx ranging from PCH4

¼ 0 � 75 mTorr. The oxygen spectra,with apeak at 534.0 eV, shows that this element ismostly presentonthe film as a surface oxide layer, for lower values of the methanepressure and that it is bonded only to germanium. When the

Table 1XPS data including binding energy and intensity of each core level of Ge1�xCx films.

Methan P (mTorr) Element Intensity Energy (eV)

0 Ge 132,278 29.60 Oxygen 50,663 533.82.5 Ge 32,238 29.82.5 C 2166 283.42.5 Oxygen 14,636 533.85 Ge 29,095 29.85 C 3238 283.85 Oxygen 12,210 534.210 Ge 28,448 29.910 C 3610 283.610 Oxygen 11,691 534.215 Ge 29,513 30.015 C 4765 283.615 Oxygen 8625 533.825 Ge 27,326 30.025 C 6410 283.825 Oxygen 11,528 534.050 Ge 26,785 30.050 C 8889 283.850 Oxygen 9829 535.075 Ge 20,599 30.275 C 9998 284.475 Oxygen 6570 535.2

methane pressure ðPCH4Þ increases, the oxide layer decreases grad-

ually and graphitic phase increases.Fig. 2 shows relative atomic concentration (RAC) of Ge, C & O

measured by XPS as a function of pressure during growth for filmsGe1�xCx prepared by RPLD. There is notable increase in RAC of Cnear 40 atomic % for methane pressure of 75 mTorr and the relativedecrease in Ge. The RAC for oxygen indicates that it appears lessthan 10 atomic % and shows direct relation with the RAC of Gewhich indicates the surface contamination of residual gases presentin the chamber during growth or the contamination of target.

Fig. 3 shows the results of electron energy loss spectroscopy(EELS) for all samples prepared by reactive pulse laser depositiontechnique fordifferentmethanepressures. Loss peaks near 10.5,15.6and 32 eV are observed. The 10.5 and 15.6 eV peaks are the surfaceand bulk plasmons, respectively. The position of the bulk plasmontends to shift to higher energy, whereas the surface plasmon peakdisappears as carbon content increases in the film. This is a clearevidenceof the formationofGe1�xCx alloybecause the incorporationof C atoms occupies the dangling bonds of Ge atoms at the surface ofthe film, forming GeeC bonds. The peak at w32 eV originates fromGe 3d core level transitions to empty dangling bond surface states[10]. Electrons contributed by the adsorbed C atoms occupy thesestates, and this peak finally vanishes at higher CH4 concentration.

Fig. 4 shows the variation of refractive index and extinction co-efficient with the partial pressures of methane which clearly indi-cates that the optical constants (refractive index and extinction co-efficient) decrease with methane pressure which confirms the XPSresults that for higher methane concentration, the graphitic phaseincreases, therefore, the optical constants decrease while opticalband gap increases.

From the above results, it is observed that the binding energy ofGe3d has increased gradually (up to 0.6 eV) as a function of methanepressure. The increase in binding energy of Ge as a function of Cconcentration can be due to the fact that the electronegativity of Cis larger in comparison to Ge. The line width of the peak remains,almost, constant during the gradual increase in binding energy ofGe3d transitionwith RAC of C which indicates that the Ge atoms aredispersed homogeneously in the Ge1�xCx films.

Similarly, it is found that C1s peak shifts gradually toward higherbinding energy side as methane pressure increases. Here in thiscase the binding energy of C1s increases from 283.8 eV to 284.4 eV

Sensib. Int./Sens. Conc. % Conc. %(w/o oxygen)

0.31 426,703 86 1000.71 71,356 140.31 103,994 78 920.23 9417 7 80.71 20,614 150.31 93,855 75 870.23 14,078 11 130.71 17,197 140.31 91,768 74 850.23 15,696 13 150.71 16,466 130.31 95,203 74 820.23 20,717 16 180.71 12,148 90.31 88,148 67 760.23 27,870 21 240.71 16,237 120.31 86,403 62 690.23 38,648 28 310.71 13,844 100.31 66,448 56 600.23 43,470 36 400.71 9254 8

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

80

90

100

Rel

ativ

e At

omic

Con

cent

ratio

n (%

)

Methane Depositing Pressure (mTorr)

Ge C O

Fig. 2. The relative atomic concentration (RAC) of Ge, C & O measured by XPS asa function of pressure.

0 10 20 30 40 50 60

75502515

105

CH4(mTorr)

2.5

0

EELS results of GeCx

Inte

nsity

(arb

. uni

ts)

Electron loss energy (eV)

Fig. 3. The electron energy loss spectroscopy (EELS) of Ge1�xCx films.

24 26 28 30 32 34 36 38

Ge 3dIn

tens

ity (a

rb. u

nits

)

Binding energy (eV)

0 mTorr 2.5 mTorr 5 mTorr 10 mTorr 15 mTorr 25 mTorr 50 mTorr 75 mTorr

278 280 282 284 286 288 290 292

C 1s

Inte

nsity

(arb

. uni

ts)

Binding energy (eV)

2.5 mTorr

5 mTorr 10 mTorr

15 mTorr

25 mTorr

50 mTorr

75 mTorr

528 530 532 534 536 538 540 542

O 1s

Inte

nsity

(arb

. uni

ts)

Binding energy (eV)

0 mTorr

2.5 mTorr

5 mTorr 10 mTorr

15 mTorr

25 mTorr

50 mTorr 75 mTorr

a

b

c

Fig. 1. a The transition Ge3d of Ge1�xCx films as a function of methane pressure. b Thetransition C1s of Ge1�xCx films as a function of methane pressure. c The XPS spectra ofO1s of a-Ge1�xCx films as a function of methane pressure.

A. Mahmood et al. / Current Applied Physics 11 (2011) 547e550 549

as the CH4 concentration increases. The C1s peak at lower bindingenergy (283.8 eV) corresponds to sp3 CeGe bonds [11], while athigher binding energy side (284.4 eV) it may correspond to sp2 CeCbonds [12]. This is due to the fact that for lower methane pressure,the majority of C atoms are incorporated in Ge lattice while forhigher methane pressure the lower mixing between Ge and Catoms introduces the formation of graphitic type agglomerates ofcarbon. Furthermore the increase of the relative intensity of the

peak with higher binding energy confirms the formation ofgraphitic agglomerates or CeC bond. From XPS, it is revealed thatthe presence of GeeO, CeC and GeeC bonding, in the near-surfaceregion of the coatings, has strong dependence upon the methanepressure. It is important to emphasize that the increase in thepercent of GeeC and CeC bonding while the correspondingdecrease in the percents of GeeO suggest that at higher CH4

pressure, the oxygen is first incorporated in GeeO bonds which arelater broken in the process of incorporation of carbon.

Moreover, it is known that carbon atoms have ability to incor-porate in the lattice with sp2 and sp3 hybridizations. In Ge-richfilms, carbon atoms mainly bond to germanium atoms in sp3

hybridization. On the other hand, carbon atoms favor to combinewith carbon atoms forming sp2 hybridization. In this contest, atlower methane pressure, there is sufficient amount of Ge atomsbonded with tetrahedral carbon atoms in the dominant formationof sp3 hybridization which is mostly due to GeeC bonds while athigh methane pressure, the sp3 carbon atoms are decreasing whichmeans the fraction of sp2 carbon atoms forming CeC bonds rapidlyincreases while GeeC bonds are decreasing. The films prepared atlower methane pressure present good optoelectronic properties

Fig. 4. The refractive index and extinction co-efficient verses partial pressures ofmethane.

A. Mahmood et al. / Current Applied Physics 11 (2011) 547e550550

and high infrared absorptionwhich is mainly due to the increase insp3 GeeC bonds and carbon is incorporated substitutionally in Genetwork to form an alloy of Ge1�xCx.

4. Conclusion

It is observed that methane pressure has a strong influence onthe surface and optical properties of films prepared by Reactive

Pulsed Laser Deposition technique at different methane pressuresand have been characterized by several diagnostic techniques likeX-ray photoelectron spectroscopy, spectroscopic ellipsometry andelectron energy loss spectroscopy which re-in force each other.

The substitutional incorporation of C atoms in the Ge lattice byforming an alloy of a-Ge1�xCx has been observed for lowerconcentration which is app.10 atomic % where the Ge atoms werefound to be distributed uniformly.

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