study of pbse layer oxidation and oxide dissolution

Ž .Applied Surface Science 141 1999 157–163

Study of PbSe layer oxidation and oxide dissolution

C. Gautier ), M. Cambon-Muller, M. AverousGroupe d’Etude des Semiconducteurs UMR 5650, UniÕersite Montpellier II, Place Eugene Bataillon, F34095 Montpellier Cedex 5, France´ `

Received 20 August 1998; accepted 20 October 1998

Abstract

The presence of contamination at the Pbrp-PbSe interface corresponding to a Schottky contact leads to the formation ofan n-type inversion layer resulting in an ohmic behaviour. In this work, the PbSe oxidation kinetics by air exposure isstudied using XPS and AES measurements. We found that an oxide layer is formed at the surface, containing PbO and SeO2

compounds. Secondly, the oxide layer dissolution in KOH-based solutions is investigated. A C- and O-decontaminated PbSesurface is obtained after a few minutes soaking in a bath containing ethylene glycol as solvent. Auger microanalysisdemonstrates that the decontaminated surface is made of Pb Se with x-1, corresponding to a lead deficit. The surfacex

morphology does not appear to be modified, as seen by AFM, but the root mean square roughness calculation shows a weakimprovement. q 1999 Elsevier Science B.V. All rights reserved.

PACS: 81.60.Cp; 82.80.Pv; 68.55.Bd; 81.15.Ef

Keywords: PbSe; Oxide; Dissolution; KOH; XPS; AES

1. Introduction

Ž .The lead salts such as PbSe are direct narrowgap semiconductors that allow efficient infrared de-

Ž .tection. The heteroepitaxy of lead selenide on 111silicon substrate is used for the realisation of pho-todetectors operating in the atmospheric window of

w x3–5.5 mm 1 . Detecting devices are mainly consti-tuted of a Schottky rectifying contact obtained bydepositing lead on p-PbSe.

However, the Pbrp-PbSe interface quality is anessential parameter to achieve high-performancephotodiodes. In fact, the presence of contamination

) Corresponding author. Tel.: q33-4-67-14-46-02; Fax: q33-4-67-14-37-60; E-mail: [email protected]

on the PbSe surface, essentially carbon and oxygen,leads to the formation of an n-type inversion layerresulting in an ohmic behaviour of the metalrsemi-

w xconductor contact 2 . During the device realisation,the lead selenide layer is brought in contact with airand, as we showed in a previous paper, the oxidationof the PbSe layer surface occurs after air exposurew x w x3 as it was previously reported for PbTe 4 .

In this work, we first studied the PbSe layeroxidation kinetics using XPS and AES analysis.Secondly, the oxide layer dissolution in KOH solu-tions was investigated vs. solvent, reagent concentra-tion and treatment time. The surface composition isstudied by XPS measurements, AES semiquantitativeanalysis and Auger microanalysis. The surface mor-phology is observed by AFM.

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00606-0

( )C. Gautier et al.rApplied Surface Science 141 1999 157–163158

2. Experimental

2.1. Growth of the PbSe layer

Ž .Heteroepitaxy of a PbSe layer on silicon 111substrate is carried out by molecular beam epitaxyŽ .MBE in a 2300 RIBER system. First, the siliconsubstrate is cleaned using a modified Shiraki method

w xdescribed elsewhere 5 . After its introduction intoŽ y10 .the MBE system P;10 Torr , a thermal an-

nealing is performed at 6508C in order to obtain a Sisurface free from contaminations. Secondly, a thinCaF buffer layer is grown, which minimises thermal2

defects in the active layer, caused by the differenceof thermal expansion coefficients between Si andPbSe. After a second thermal annealing under sele-

w xnium flux 6 , the p-type PbSe layer is grown bysimultaneous evaporation of Se and PbSe materials.Adjustment of the flux ratio allows to control thecarrier concentration in the PbSe layer to about 1017

cmy3, yielding a 1.5=103 to 2=104 cm2 Vy1 sy1

Hall mobility at 4 K according to the p-PbSe layerw xquality 7 .

2.2. Oxide dissolution treatment

In a previous paper, we studied the chemicalsulfur treatment of a PbSe layer using the Na SP2

w x9H O reagent in ethylene glycol as solvent 3 . The2

intensity of the surface reactions convinced us that a

chemical treatment is sufficient to dissolve the oxidelayer.

All experiments were performed at room tempera-ture and the various baths were suitably stirred. Thetreatments were carried out by immersing the sam-ples in various solutions containing KOH as reagentand different solvents. The effect of reagent concen-tration and soaking time on the PbSe layer surfacecomposition was investigated. Afterward, the sam-

Žples were rinsed in deionised water resistivity;18.MV cm and dried with N gas.2

3. Results and discussion

3.1. Air exposed surfaces

XPS measurements were carried out with a stan-dard V-G Escalab MK2. X-ray photoelectrons were

Ž .excited by the Mg Ka line 1253.6 eV , using adouble pass cylindrical mirror analyser.

In order to determine the nature of interactionsbetween oxygen and PbSe layers, XPS analysis wasperformed on a sample which has been exposed to

Ž .air for 1 month Fig. 1a and on similar sample on˚which 100 A of the surface has been removed by

q Ž .Ar sputtering Fig. 1b , in the range of 45–65 eVand 130–150 eV. In the first case, the Se 3d and5r2

3d transitions, respectively observed at 53.7 eV3r2

and 54.3 eV, correspond to Se2y in PbSe. A second

˚Ž . Ž .Fig. 1. XPS spectra of Se 3d and Pb 4f transitions recorded on PbSe surfaces: a after air exposure, b after 100 A sputtering.

( )C. Gautier et al.rApplied Surface Science 141 1999 157–163 159

Table 1Semiquantitative XPS analysis of oxidation degrees for selenideand lead atoms

Elements

Se Pb2y 4q 2q 2qOxidation degree Se Se Pb Pb

Ž . Ž . Ž . Ž .PbSe SeO PbSe PbO2Ž . Ž .Quantity % "2% 70 30 60 40

set of two peaks appeared at 59 eV and 59.6 eV,4q w xwhich is attributed to Se in SeO 8 . In addition,2

the Pb 4f and 4f transitions at 137.8 eV and7r2 5r2

143 eV corresponding to Pb2q in PbSe exhibited ashoulder on the high energy side. For each transition,this shoulder was assigned to another peak at 138.8eV and 144 eV respectively, corresponding to Pb2q

˚ qw xin PbO 4,8 . After a 100-A Ar sputtering, it wasseen that the SeO and PbO associated transitions2

disappeared. In conclusion, PbSe layers oxidised byair contain at their surface the SeO and PbO com-2

pounds. The semiquantitative XPS analysis leads tovalues presented in Table 1, showing that the PbOquantity is more important than the SeO one. It2

appears that lead is more oxidised than selenium.The formation of oxide compounds means that

Pb–O and Se–O bonds were realised to the detri-ment of Pb–Se bonds. In order to determine if suchreactions are thermodynamically favoured, the chem-

Ž 0 .ical bonds dissociation energy at 298 K D are298

presented in Table 2. We can see that Pb–Se bondrequires less energy to be broken than the Pb–O andSe–O ones, which are stable thermodynamically.This is in good agreement with our XPS results.

The oxidation kinetics was studied by AES mea-surements in the MBE system with a primary elec-tron beam energy of 3 keV. Fig. 2 a displays aspectrum recorded immediately after the PbSe layer

Ž .growth. In addition to the lead peak NOO transition ,

Table 2Dissociation energies of various chemical bonds at 298 K

0 y1Ž .Chemical bonds D kJ mol298

Pb–Se 302.9"4Pb–O 382.0"12.6Se–O 464.8"21.3

Ž .Fig. 2. AES spectra taken on PbSe surfaces: a immediately afterŽ . Ž .layer growth, b after air exposure for a few minutes, c after air

exposure for 1 month.

Ž . Ž .both selenium signals at low MNN and high LMMenergies are observed. The presence of a Se low-en-

Ž .ergy peak MNN transition indicates that the layersurface is stabilised with Se atoms. Moreover, notrace of carbon and oxygen appears. This PbSe layeris exposed to air for a few minutes. The correspond-ing AES spectrum, in Fig. 2b, shows the presence ofoxygen atoms and a clear decrease of the Se-peakintensity. This phenomenon cannot be explained byoxygen atoms screening, since the intensity of thelead peak is not modified. Thus, we may think thatoxygen atoms are not only adsorbed but also substi-tuted to selenium ones. This is in good agreementwith semiquantitative AES measurements presented

Ž .in Table 3. After air exposure for 1 month Fig. 2c ,the PbSe layer surface contained a lot of carbon andoxygen contamination. We can also notice the disap-pearance of the Se low-energy signal and a cleardecrease of the lead peak intensity. Moreover, thedefinite doublet structure observed for the lead peak


Table 3Surface composition obtained by the semiquantitative AES analy-sis from peak intensities of spectra presented in Fig. 2

Ž . Ž . Ž .I Pb I O I SeŽ . Ž . Ž ."0.01 "0.005 "0.01

Ž .a 0.34 – 0.66Ž .b 0.35 0.030 0.62Ž .c 0.27 0.060 0.67

Ž .line on a clean surface first spectrum disappears,w xindicating lead oxide presence 9 . We have alsoŽ .observed, on the spectrum E) N E , the rounded

shape of carbon signal showing that C atoms areonly adsorbed at the surface. This is not any moretrue after air exposure for several months: the sharpshape of the carbon signal shows that C atoms arebonded with the layer. The semiquantitative resultsobtained from spectra of the Fig. 2 are reported in

Ž .Table 3. In the case of spectrum c , it is impossibleto determine in a correct way the surface carbonquantity; this leads to a misevaluation of the Pb andSe concentrations. First, the Pb peak is located atlow energy and so will be screened by carbon atoms.Secondly, the weak sensitivity coefficient of sele-nium and the normalisation procedure lead to aweaker Pb quantity and a higher Se concentration.However, the oxygen value is reliable and shows thatoxidation kinetics is rapid since the oxygen quantityreaches, in a few minutes, half of its final value,which is obtained for exposure times beyond 1 month.

The penetration depth of oxygen atoms is esti-mated by Auger microanalysis. In contrast with thesurface measurements for which oxygen concentra-tion remains constant, Table 4 demonstrates that itsquantity varies with depth. Actually, the oxide layerthickness increases with air exposure time, which isconfirmed by ellipsometry measurements.

Table 4Variation of oxide layer thickness with air exposure time

˚Ž .Exposition time Thickness A˚Ž . Ž .months "1 A

4 126 209 36

Thus, initially, it seems that oxide layer formationcomes from substitution of selenium atoms by oxy-gen ones. Afterwards, the adsorption and diffusion ofoxygen atoms takes place and, in the long-term, thecarbon behaves similarly.

We think that, in all the cases, the oxide layerthickness is sufficiently weak to be removed by achemical treatment.

3.2. Oxide dissolution

The oxide dissolution experiments were per-formed on a PbSe sample which had been exposed toair for several months. Fig. 3a and b correspondrespectively to XPS and AES spectra recorded on aPbSe layer after 10 min chemical treatment in 0.1 MKOH–ethylene glycol solution. In the first case,transitions corresponding to Se4q of SeO and Pb2q

2

of PbO have disappeared. AES spectrum confirmsthe surface oxygen disappearance and also shows theabsence of carbon atoms, within AES detection lim-its. The appearance of the selenium low energytransition and the definite doublet structure of thelead peak indicates a deoxidised surface.

Ž . Ž .Fig. 3. a XPS, b AES spectra of a treated PbSe layer surfacefor 10 min in a KOH–ethylene glycol solution.


Table 5Surface composition of PbSe obtained by semiquantitative AESanalysis after chemical treatment in KOH solutions, using varioussolvents during 10 min

Ž . Ž . Ž .Solvent I Pb I O I Se CŽ . Ž . Ž ."0.01 "0.005 "0.01

Water 0.22 0.010 0.77 YesEthylene glycol 0.24 – 0.76 NoEthanol 0.29 0.050 0.66 YesIsopropanol 0.31 0.060 0.63 No

The same experiments are carried out in KOH-based solutions containing various solvents. AESsemiquantitative measurements are reported in Table5. We observed an equal quantity of oxygen on PbSesurface treated in baths containing ethanol and iso-propanol as solvents. Thus, it seems that PbO andSeO compounds are insoluble in these electrolytes.2

It is important to note that an increase of treatmenttime does not modify these results. In contrast, PbSesurfaces treated in aqueous bath or in ethylene gly-col-based solutions do not contain oxides any more.However, an important quantity of carbon is ob-served on samples soaked in aqueous bath, when Catoms cannot be detected on PbSe layer immersed inethylene glycol electrolyte. Moreover, in these twocases, the measurements indicate that selenium andlead concentrations are, respectively, higher andlower than on as-grown PbSe layer surface given inTable 3. This Se excess or Pb deficit at the surfacemeans that the oxide dissolution leads to the hypoth-

Fig. 4. Depth profile of PbSe layer after oxide dissolution, ob-tained by Auger microanalysis. The treatment is carried out for 10min in a KOH–ethylene glycol solution.

esis of point defects creation, that are probably leadvacancies. This result is in agreement with semiquan-titative XPS analysis corresponding to a higher PbOsurface concentration compared to SeO . It is impor-2

tant to notice that Pb deficit or Se atoms excess onthe surface has the great advantage to avoid theformation of an n-type PbSe inversion layer.

In addition, Auger microanalyses were performedon treated samples. The depth profile presented inFig. 4 displays a progressive variation versus depth,indicating that the Se excess is not limited to thesurface, in agreement with the previous hypothesis.The region close to the surface can be described as aPb depletion layer, designed as Pb Se, with x vary-x

ing from 0.9 at the surface to 1 in PbSe layer. It isimportant to note that the Pb depletion layer thick-ness varies according to oxide layer thickness and,therefore, to air exposure time.

Fig. 5. Semiquantitative AES analysis: variation of surface com-Ž .position a vs. soaking time in a 0.1 M KOH–ethylene glycolŽ .solution, b vs. KOH reagent concentration for 10 min of soak-

ing.


Moreover, a systematic study was performed ver-sus soaking time and reagent concentration using the

Ž .semiquantitative AES analysis Fig. 5 . To facilitatecomparisons, we plot the initial oxidised surfacecomposition at ts0 min in Fig. 5a and at Cs0molrl in Fig. 5b. The composition variation showsthat, for a 0.1 molrl reagent concentration, twominutes are sufficient to observe the disappearanceof C and O contamination on the surface. This resultmeans that oxide dissolution is carried out with arapid kinetics as compared to the case of sulfidation

w xreaction 3 . The same kind of results is observed onsamples treated in more diluted solutions during 10

Ž .Fig. 6. Surface morphology observed by AFM a after air expo-Ž .sure b after chemical treatment in 0.1 M KOH–ethylene glycol

solution for 10 min.

minutes. This reagent concentration decrease, ac-companied by a weak pH decrease, seems to have noinfluence on oxides solubility. It is then not neces-sary to use high reagent concentration.

The surface morphology observed on air-exposedPbSe layers before and after KOH treatment is shownin Fig. 6. No clear modification appears on micro-

Žgraphs the photograph blur in Fig. 2a is due to the.AFM picture instability , which leads us to think that

oxide dissolution is carried out in a homogeneousway on the sample. Moreover, the root mean squareroughness calculation shows a weak improvementafter 10 min treatment.

4. Conclusion

First, we have shown that PbSe layer oxidationunder air exposure is a fast reaction, leading to theformation of an oxide layer which contains PbO andSeO compounds; its thickness depends on air-ex-2

posure time. Secondly, we studied the oxides’ disso-lution in KOH-based solutions. The C and O contam-ination disappears after treatment in a KOH bathcontaining ethylene glycol as solvent during a fewminutes. The semiquantitative AES measurementsshow the presence, at the surface, of a seleniumexcess or lead deficit. In fact, a thin Pb Se layer isx

formed, with x varying from 0.9 at the surface to 1inside PbSe layer. This result has a great technologi-cal interest, since an n-type inversion layer will beavoided at the Pbrp-PbSe interface. However, themeasured deviation from stoichiometry towards a Pbdepletion, leading to a p2q behaviour can be sostrong. In order to quantify the treatment efficiency,electrical measurements will be performed on Schot-tky diodes, realised using the deoxidation processdescribed above.

Acknowledgements

We would like to thank G. Chatainier for XPSmeasurements, J-P. Palmari for Auger microanalysisand V.D. Ribes for the technical assistance.


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study of pbse layer oxidation and oxide dissolution

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