Solar Energy Materials & Solar Cells 100 (2012) 43–47

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

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

A comparative study of silicon surface passivation using ethanolic iodine andbromine solutions

Neha Batra a, Vandana a, Sanjai Kumar b, Mukul Sharma a, S.K. Srivastava a, Pooja Sharma a, P.K. Singh a,n

a National Physical Laboratory, Dr. KS Krishnan Marg, New Delhi 110012, Indiab Central Electronics Limited, Sahibabad 201010, UP, India

a r t i c l e i n f o

Article history:

Received 31 August 2010

Received in revised form

21 December 2010

Accepted 14 April 2011Available online 14 May 2011

Keywords:

Silicon surface passivation

Minority carrier lifetime

Bulk lifetime

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.04.028

esponding author. Tel.: þ91 11 45608588; fa

ail address: pksingh@nplindia.org (P.K. Singh)

a b s t r a c t

We report the surface passivation studies made on p-type single-crystalline silicon wafers using

ethanolic solution of iodine and bromine. Minority carrier lifetime (teff) is measured by the microwave

photoconductance decay (m-PCD) method and using a Sinton’s lifetime tester. Measurements are

carried out at different molar concentrations of iodine–ethanol (I–E) and bromine–ethanol (B–E)

solutions to optimize the process parameters. It is found that good passivation (75% of measured

maximum lifetime) could be achieved for certain ranges of concentration, which in the case of I–E and

B–E are 0.07–0.12 M and 0.05–0.07 M, respectively. The effect of pre-conditioning steps on surface

passivation (silicon surfaces with and without native oxide) is investigated. It is shown that the quality

of surface passivation can be improved by an optimized wet-chemical pre-conditioning treatment. The

effect of bias light and passivation time is also studied. I–E solution provides better passivation than

B–E solution in terms of teff whereas B–E solution passivation exhibits better stability in comparison

with I–E solution. The teff measured by the Sinton method (WCT-120) and using a Semilab system

(m-PCD, WT-2000) are comparable if injection levels are matched.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The bulk minority carrier lifetime (tb) is an importantparameter that (i) limits solar cell efficiency, (ii) is used formonitoring the material quality and (iii) at times is also used asa measure of process cleanliness in semiconductor foundries.Various techniques are available for determination of lifetime ofsilicon wafers and solar cells [1–4]; however the measured valueis its effective value (teff), that has contributions from the bulk (tb)and the two surfaces (ts), and is defined by

1

teff¼

1

tbþ

1

tsð1Þ

where ts¼d/2S. Here S is surface recombination velocity and d iswafer thickness. The factor 2 accounts for the contribution of bothsurfaces. Hence, surface passivation is important to disentanglethe two components in order to determine the true value of bulklifetime. When the surface is fully passivated, teffffitb. The surfacerecombination losses can be reduced either by chemical passiva-tion or by field effect passivation linked with built-in charges. Inthe former the interface defect density is reduced by saturation ofdangling bonds either by attachment of atomic hydrogen or

ll rights reserved.

x: þ91 11 45609310.

.

halogen atoms whereas, in the later case, electron and holeconcentrations at the interface are altered by electrostatic shield-ing of minority carriers associated with fixed interface charges inSiOx, SixNy, a-Si:H and a-SiC:H films. The dielectric layers such asSiOx and SixNy are commonly used for silicon surface passivationbut the formation of such layers requires high temperatureprocessing wherein lifetime degradation associated with highthermal budget cannot be ruled out [5]. Chemical passivation isan easy route to reduce surface recombination velocity as it isfast, convenient and can be used at room temperature. Therefore,this route is more suitable to study temperature related degrada-tion of lifetime at various processing stages during device devel-opment and for evaluation of material quality in laboratory andindustrial environments. In the past, hydrofluoric acid, sulfuricacid, alcoholic solution of halogens, etc. have been used forsurface passivation [6–8] wherein the interface defect densityafter dangling bonds is reduced drastically by attachment ofatoms/groups. For example, Chabbra et al. [8] have used metha-nolic solution of quinhydrone and iodine for silicon surfacepassivation and demonstrated that the former provides higherdegree of passivation. Stephens and Green [6] showed that goodsurface passivation could be achieved by 0.08 M iodine–ethanolsolution. It was noticed that the degree of passivation provided bythe chemical routes strongly depends on the surface conditioningand its sequence [9]. Many studies related with minority carrierlifetime measurement have been reported in literature but at

N. Batra et al. / Solar Energy Materials & Solar Cells 100 (2012) 43–4744

times it is difficult to make a clear comparison because suchstudies are made on different materials (CZ and FZ, orientation,etc.), using different pre-conditioning steps and experimentalconditions such as injection level. The minority carrier lifetime hasstrong injection level (i.e., excess carrier density, Dn) dependence.

In this paper, we report a systematic surface passivation studymade on p-type single-crystalline silicon wafers using ethanoliciodine (I–E) and bromine (B–E) solutions. The lifetime depen-dence on I2/Br2 concentration, passivation time, effect of surfacepre-conditioning (i.e., with and without native oxide) on surfacepassivation and its role in passivation stability have also beenstudied. For the sake of comparison, measurements were made byspatially resolved microwave photoconductance decay (m-PCD;Semilab Model WT-2000 system) and using another lifetimetester (Sinton Consulting Inc., Model WCT-120) in the transientmode. In the former, injection level can be varied over a very narrowrange (i.e., 1.0�1013oDno1.5�1013 cm�3) only whereas in thelatter this parameter could be changed over a wide range (i.e.,1�1011oDno1�1016 cm�3). Therefore, WCT-120 is employed tostudy injection level dependence of minority carrier lifetime.

Fig. 1. Measured minority carrier lifetime as a function of iodine concentration

(molar) in I–E solution by m-PCD (WT-2000) for the samples with oxide (no HF

dip) and without native oxide (with HF dip). The experimental data is at low

injection level corresponding to excess carrier density, Dn¼1.2�1013 cm�3.

The symbols represent the measured values and the line gives the best fit with

the data.

2. Experimental

Float-zone (FZ) grown chemically–mechanically polished/1 0 0S p-type (boron-doped) single-crystalline silicon samples(650710 mm thick) were used in the present study. The resistiv-ity of wafers was 570.5 O cm and carrier lifetime was 500 ms ormore (as provided by the supplier [10]). In order to ensureidentical surface and electrical/electronic properties in all thesamples, 3�3 cm2 size pieces were cut from a 150 mm diameterwafer using a laser scriber. The wafers were cleaned in aH2O2:H2SO4 (1:4 by volume) solution for 10 min followed by5–6 times rinsing in DI water. As mentioned earlier, pre-con-ditioning of the surface prior to the chemical passivation is animportant step. Two sets of samples, one with native oxide(without HF dip) and the other without oxide layer (dipped in5% HF for 1 min to remove native oxide) followed by rinsing inDI water, were prepared. The measurements were carried outat different concentrations of ethanolic iodine and brominesolutions (0–0.2 M), for different passivation times (i.e., immer-sion time in the solution) varying from 0 to 300 min and atvarious bias light conditions. During the measurements a sealedacid resistant optically transparent plastic bag was used to holdsamples in the solution of desired concentration. Special care wastaken to have uniformly distributed thin solution layer over thesample. The measurements were made using a spatially resolvedmicrowave photoconductance decay system (m-PCD) where a904 nm pulsed laser (penetration depth �30 mm) and a microwavesource operating at 10 GHz are used. The former is for opticalexcitation and the latter for signal detection (where decay of excesscarrier concentration is monitored by the microwave reflectance). Inthe other lifetime tester (WCT-120) engaged for the measurements,excess carriers are generated by a flash and sheet conductivity ismeasured using an RF coil inductively coupled to the sample. Flashintensity and sheet conductivity are converted into generation rateof electron hole pairs and average excess carrier density (Dn) usingmobility models, respectively. The system operates in two decaymodes: (i) quasi-steady state photoconductance and (ii) transientphotoconductance. The former is adopted when the decay constantof the flash is at least 10 times lower than the carrier lifetime so thatexcess carrier population is in a steady state, a condition underwhich lifetime is calculated. The latter is used when the fast pulse oflight peaks and decays back in 10–20 ms and the photoconductancedecay is measured to determine effective lifetime. This mode issuitable for measurements in high lifetime materials. The transient

mode has been used in the present study as the material underinvestigation was of high lifetime. The WT-2000 (m-PCD) systemwas operated at low injection levels (i.e., excess carrier density,Dn¼1.2�1013 cm�3) whereas WCT-120 was used under highinjection conditions (Dn45�1014 cm�3) unless or otherwisespecified. The results reported here are representative for each typeof passivation route but measurements were repeated at least threetimes to ensure the reproducibility of results. The minority carrierlifetime in untreated (non-passivated) samples was 11 ms as mea-sured by the m-PCD method and 5 ms using a WCT-120 lifetimetester at matching low injection level.

3. Results and discussion

3.1. Iodine–ethanol solution

Fig. 1 shows the effective minority carrier lifetime at variousconcentrations (up to 0.2 M) of iodine–ethanol solution for twodifferent surface pre-conditioned samples (i.e., with and withoutnative oxide). Measured effective lifetime teff increases with theincrease in iodine concentration initially, attains a maximum andthereafter decreases with further increase in concentration. Theteff in the samples without native oxide (HF dip samples) is abouttwo times that of samples with native oxide (without HF dip) atall concentrations. This is a clear indication of more effective andhigher degree of passivation in the samples with no oxide layer. Itcan be noticed that good passivation (75% of the highestmeasured lifetime, tmax) in I–E solution could be realized for arange of iodine concentration (i.e., 0.07–0.12 M) for both with andwithout native oxide samples rather than at a fixed concentration.

To see the effect of passivation time (tin-soln, i.e., immersiontime of sample in solution), the measurements were carried out atdifferent time intervals while the sample was kept in the solution.Fig. 2 shows the dependence of teff on passivation time for a fixedI–E concentration (0.12 M solution). As can be seen from thefigure the carrier lifetime decreases from its initial value of 171 msand stabilizes after 60 min to �125 ms. Similar trend is seen forall I–E concentrations with two different pre-conditioned surfacesnamely with (not shown in the figure) and without native oxide.The results show that dangling bonds saturation occurs almostinstantaneously (o2 min) in the I–E solution. The decrease in teff

with tin-soln may be attributed to oxidation of silicon surface(as will be discussed later) and thereby virtual reduction in theeffectiveness of passivation.

Fig. 2. Minority carrier lifetime versus passivation time curves for 0.12 M I–E

solution at Dn¼1.2�1013 cm�3 (main figure). Measurements were done by m-

PCD for the samples without native oxide. The inset shows carrier lifetime

dependence on bias light intensity of the same sample. The symbols represent

the measured values and the line gives the best fit with the data.

Fig. 3. Measured minority carrier lifetime as a function of bromine (molar)

concentration in B–E solution by m-PCD (at injection level corresponding to excess

carrier density, Dn¼1.2�1013 cm�3) for the samples with oxide (no HF dip) and

without native oxide (with HF dip). The symbols represent the measured values

and the line gives the best fit with the data.

Fig. 4. Minority carrier lifetime dependence on bias light intensity in B–E solution

(for 0.08 M) by m-PCD for the samples with oxide (no HF dip) and without native

oxide (with HF dip). The symbols represent the measured values and the line gives

the best fit with the data.

Fig. 5. Measured minority carrier lifetime as a function of passivation time for the

samples with oxide (no HF dip) and without native oxide (with HF dip) for B–E

solution (in 0.08 M) by m-PCD (WT-2000). All the data is for Dn¼1.2�1013 cm�3.

The symbols represent the measured values and the line gives the best fit with

the data.

N. Batra et al. / Solar Energy Materials & Solar Cells 100 (2012) 43–47 45

The basic idea of bias light in lifetime measurement is togenerate a large number of excess carriers, which are continu-ously generated and recombine due to the surface states andtherefore surface states are continuously occupied. Thus, excesscarriers generated by excitation pulse have little probability ofrecombination at the surface states and hence measured teff hasreduced surface contribution. A linear dependence of teff valuewith bias light intensity (varied from 0–1.8 SUN) was observed ascan be seen from the inset of Fig. 2. The linear dependence is seenin both types of surface pre-conditioned samples.

3.2. Bromine–ethanol solution

Fig. 3 shows the teff variation for different molar concentra-tions of B–E solution where an opposite behavior compared to theI–E case is seen with respect to the two different pre-conditionedsurfaces. The measured teff in samples without oxide (HF dipped)is lower by a factor of 0.2-0.3 as compared to values obtained inthe samples with oxide layer. The maximum lifetime values �50and 120 ms are obtained, respectively, in without and with nativeoxide samples (in 0.06 M solution). It is also found that goodpassivation with bromine could be attained in a narrower range ofconcentration (0.05–0.07 M). Fig. 4 shows the teff variation withbias light intensity for 0.08 M B–E solution where the measuredteff increases with increase in bias light intensity for both the

cases of pre-conditioned surfaces, a trend similar to the oneobserved with I–E solutions. Fig. 5 depicts teff dependence ontin-soln data for 0.08 M concentration of B–E solution, which is notas strong as seen in the I–E case. The measured teff increasesgradually with time and after a certain time its value decreasesslowly in both with and without native oxide samples. However,the rate of teff increase is marginally different in the two cases. Itcan be remarked that surface passivation is more stable in B–Esolution compared to the I–E case, where a sharp decrease withpassivation time is observed.

3.3. Injection level dependence

The results described so far were obtained by m-PCD(WT-2000) at low injection level. It is known that teff has stronginjection level dependence and hence the large lifetime values(in ms) reported in the literature [8] are, generally, at highinjection level (excess carrier density DnZ1015 cm�3) usingSinton’s method (WCT-120). To compare m-PCD measured teff

with that by some other technique (about this very little informationis available in literature) and also to study the injection leveldependence, measurements were carried out using a WCT-120lifetime tester, which has provision to vary injection level and hencethe excess minority carriers density could be changed over a widerange (1011oDno1015 cm�3).

N. Batra et al. / Solar Energy Materials & Solar Cells 100 (2012) 43–4746

The carrier lifetime measured using WCT-120 for differentiodine concentrations (I–E) up to 0.1 M in a sample withoutnative oxide has been shown in Fig. 6. The measured teff increaseswith the increase in iodine concentration initially, then remainsnearly constant at a certain concentration range and thereafterdecreases with further increase in concentration. The maximumteff value (within 75%) was obtained between 0.02 and 0.08 Msolution, which is consistent with the concentration range(though narrower) obtained with WT-2000 (Fig. 1). An increaseor decrease of iodine concentration outside this range did notimprove teff value. Further, the highest teff obtained by the twomethods were 5–6 times different. To explain this the measure-ments were carried out at different injection levels on the samesample and the results are shown in Fig. 7, where an increase in

teff with Dn could be seen. Furthermore, teff has linear depen-

dence with Dn in the 5�1012–5�1014 cm�3 range on semi-logscale. This trend has been observed at other concentrations andalso with passivation time for I–E solutions. The m-PCD measured

teff (¼171 ms) value for Dn¼1.2�1013 cm�3 is also shown inFig. 7. This value is slightly small compared to the values obtainedwith WCT-120 at the matching injection levels. It is to be notedhere that the teff values measured with WCT-120 were generallyhigher compared to the values obtained with WT-2000 for thesame excess carrier densities.

For an illustration, measurements were done after passivationusing a quinhydrone–methanol (Q–M) solution on the same

Fig. 6. Measured minority carrier lifetime as a function of iodine concentration

(molar) in I–E solution using Sinton’s lifetime tester (WCT-120) for the sample

without native oxide at an injection level corresponding to excess carrier density,

Dn¼5�1014 cm�3. The symbols represent the measured values and the line gives

the best fit with the data.

Fig. 7. Injection level dependence of effective minority carrier lifetime measured

using WCT-120 for without native oxide sample in I–E (0.08 M) solution. The

m-PCD measured teff (at Dn¼1.2�1013 cm�3) in the same I–E solution is also

shown (by a filled circle). The symbols represent the measured values and the line

gives the best fit with the data.

sample. The best measured teff was within 73% of the valueobtained with I–E passivation but only after prolonged exposure(more than 1 h) to the Q–M solution. For example teff �785 ms atDn¼1.2�1015 cm�3. These results indicate that both iodine–ethanol and quinhydrone–methanol systems provide equallygood passivation but the former is less time consuming and,therefore, more suitable for quick evaluation of the bulk lifetime.

It was noticed that surface passivation for both I–E and B–Ecases was not stable when samples were removed from thesolution and exposed to ambient condition and consequentlyminority carrier lifetime values dropped from its in solutionvalues. This observation is consistent with the reports on siliconsurface passivation using different chemical routes [8].

Higher teff observed with I–E solution in comparison with B–Esolution indicates that iodine containing solution produces highersaturation of dangling bonds than bromine, which may beexplained as follows. The Br–Br bond strength (¼193 kJ/mol) ismuch higher than the I–I (¼151 kJ/mol) bond strength andtherefore the possibility of iodine dissociation in the solution ishigher, which may result in better passivation than with bromine.Furthermore, enhanced surface passivation by I–E may also bedue to the lower oxidation tendency of iodine compared tobromine in the alcoholic solution or to the lesser disturbance ofthe silicon back bond electron density due to the higher atomicnumber element, i.e., iodine [11]. The observed teff decay withpassivation time in I–E solution may be due to halogen oxidation.

As shown earlier the measured teff for the samples withoutnative oxide (after HF dip) is �2� that of samples with nativeoxide at all I–E concentrations but an opposite behavior is seenwith B–E solution, where higher lifetime is observed in thesamples with native oxide layer (0.2–0.3teff of the samples with-out native oxide layer). It has been reported that HF treatedsilicon surfaces are atomically rough and exhibit all possiblehydride terminations [12]. The silicon surface has –H and –OHterminations for with and without HF treatment, respectively;therefore, the surface chemistry in the two cases is entirelydifferent. Lower teff values obtained in the presence of nativeoxide confirm that oxidized silicon surface has less tendency ofiodination in comparison with hydrogenated surface. This may bedue to higher bond dissociation energy for Si–O (464 kJ/mol) thanSi–H (314 kJ/mol). We have found that B–E results are opposite tothose of I–E for the two pre-conditioned surfaces. Furthermore,the passivation stability features are also different. It is knownthat Si–Br bond is stronger than Si–I bond due to higher electro-negativity of bromine as compared to iodine (as confirmed by 368and 293 kJ/mol bond enthalpies for Si–Br and Si–I bonds, respec-tively), which may be responsible for the superior stability in B–Etreated surfaces.

To see the effect of oxidation on both I–E and B–E, the sampleswere taken out of solution and were subjected to XPS investiga-tions. Fig. 8a and b shows the silicon core level XPS spectra for I–Eand B–E passivated silicon surfaces where a clear broad peakcorresponding to 103.2 eV (a peak corresponding to Si–Ox) isobserved in the case of I–E passivated samples, which is absent inthe B–E solution. This shows that in I–E solution, the sampleoxidizes quickly when taken out of solution, which furthersupports the arguments given above. In order to finish compar-ison of the results, XPS spectra of Q–M passivated silicon surfaceis shown in Fig. 8c where a peak though weak corresponding toSi–Ox could be seen. A comparison of the results described aboveshows various level of Si–O bonding as the wafer is exposed toambient condition and that may be the cause of varying degreesof surface passivation in different systems. However, the effectivebonding of bromine with silicon surface provides the bestchemical screening against oxidation and the passivation stabilityorder may be as bromine4quinhydrone4 iodine.

Fig. 8. Si 2p core level XPS spectra of (a) I–E (0.08 M), (b) B–E (0.12 M) and (c) Q–M solutions for the samples prepared without native oxide (after HF dip). Clear broad

peaks at 103.2 eV (corresponding to Si–Ox) can be seen in I–E (strong) and Q–M (faint) passivated samples, which are absent in B–E solution based passivation results.

N. Batra et al. / Solar Energy Materials & Solar Cells 100 (2012) 43–47 47

Finally, the disadvantage associated with the chemical passi-vation is that the passivation itself does not remain stable whenexposed to ambient condition and the best effective minoritycarrier lifetime values could be achieved when the sampleremains immersed in the solution. This limits their applicabilityin devices.

4. Conclusions

Ethanolic solutions of iodine (I–E) and bromine (B–E) havebeen used to passivate the silicon surface. The I–E solutionexhibits better passivation than B–E solution. The effect of surfacepre-conditioning (surface with and without native oxide ) prior tosurface passivation has been investigated. It has been shown thatthe degree of surface passivation depends on the treatment priorto surface passivation. It has been found that I–E and B–Esolutions show good passivation effect in certain concentrationranges (0.07–0.12 and 0.05–0.07 M for I–E and B–E, respectively)rather than at a particular concentration. The teff measured by theSinton method (WCT-120) and using a Semilab (m-PCD, WT-2000)system are comparable if injection levels are matched.

Acknowledgement

The authors would like to thank Mr. Praveen Kumar for XPSmeasurements and Dr. S.N. Singh for useful discussions. The workwas carried out under SIP-17 grant from Council of Scientific andIndustrial Research, India.

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