Low energy ion scattering: surface preparation andanalysis of Cu(In,Ga)Se2 for photovoltaic applicationsHelena Téllez1,2, John Druce1, Allen Hall3, Tatsumi Ishihara1, John Kilner1,2 and Angus Rockett3*1 International Institute for Carbon Neutral Energy Research (wpi-I2CNER), Kyushu University, Fukuoka 819-0395, Japan2 Department of Materials, Imperial College London, London SW7 2AZ, UK3 Department of Materials Science and Engineering, University of Illinois, 1304 W. Green St., Urbana, IL 61801, USA
chalcopyrites; low energy ion scattering; surface chemistry; epitaxial films
*Correspondence
Angus Rockett, Department of Materials Science and Engineering, University of Illinois, 1304 W. Green St., Urbana, IL 61801, USA.E-mail: [email protected]
Received 4 April 2014; Revised 29 May 2014; Accepted 16 June 2014
1. INTRODUCTION
Photovoltaics (PV) based on the chalcopyrite compounds,of which the alloy Cu(In,Ga)Se2 (CIGS) is a prototype,have the highest performance in small devices and full-scale modules of any polycrystalline thin film material[1–3]. The cost of all PV, and thin film PV in particular,has come down dramatically in recent years. However,the cost of installation of PV systems is still high. One ofthe most effective methods of reducing this cost is to in-crease the efficiency of PV modules because installationcosts scale with the number of modules installed ratherthan power output. To accomplish this, a more detailed un-derstanding of the operation of the devices is needed.
The CIGS devices consist of a thin film of CIGS depos-ited on a Mo back contact and coated with a binary com-pound, CdS in the best device, to form the heterojunctionthat collects photocurrent. It is finished with a transparentconductor. It is critical to understand the nature of theheterojunction for two reasons. First, recombination ofexcitons at the heterojunction can cause loss of currentdue to blue photons. (Excitons have no net charge; thus,
those created near the heterojunction can diffuse againstthe field and recombine there.) Second, doping of the junc-tion establishes the built-in potential that, in part, deter-mines the open circuit voltage. Both of these are key toachieving high efficiency in the resulting device. Previousstudies have shown that clean surfaces of CIGS are Cu de-ficient [4,5]. When the junction is formed with CdS, thereis evidence that Cd dopes the surface of the CIGS resultingin a heavily n-type layer [6]. These results were based onangle-resolved photoelectron spectroscopy. This is a valu-able method for studying the near-surface chemistry ofsolids but is difficult to interpret because it averages overa significant depth, 1–2 nm for CIGS.
A unique method for studying the chemistry of only thetop single monolayer of a film is low energy ion scattering(LEIS), also known as ion surface scattering [7,8]. In LEIS,a beam of positively charged inert gas ions is backscatteredfrom a solid surface. A small fraction of those ions isbackscattered in simple two-body collisions with atomsin the first atomic layer and directly detected as a functionof their scattering energy. This energy is dependent on themass of the surface atom and is defined by the Rutherford
PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONSProg. Photovolt: Res. Appl. (2014)
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2535
scattering equation. These ions are characteristic of onlythe very surface atoms because deeper penetration of theion into the solid almost always results in neutralization,giving the technique extreme surface specificity. Addition-ally, some of the neutralized primary ions penetrating fur-ther into the solid and backscattered from deeper withinthe sample may still be detectable as they are subsequentlyre-ionized in the process of leaving the solid. Ions that areproduced by re-ionization have more complex collisioncascades that do not produce backscattered particles withwell-defined energies. The results presented here exhibitboth behaviors.
In this paper, we apply LEIS to the characterization ofthe close-packed planes of CIGS exhibited by single-crystalepitaxial layers to determine the chemistry of the outermostlayer of atoms. However, the measurement is highly sensi-tive to only that layer, and the surface is easily contami-nated during air exposure by a layer of adventitiousadsorbates, which covers the outer atoms of the surface,preventing them from contributing any signal. This layeris often present in x-ray photoelectron spectroscopy(XPS) measurements, but because XPS is not as surfacespecific as LEIS, it has a less detrimental effect on themeasurements; in fact, the “adventitious carbon” peakarising from these adsorbates is often used to calibrate theenergy scale of the spectrometer. As a first step, to under-standing the CIGS surface, this paper presents preliminaryresults on methods for cleaning the CIGS sample surface inthe LEIS instrument as well as the composition of thesurface following various preparation procedures.
Here, we report preliminary experiments on surfacepreparation of chalcopyrites by three techniques: atomichydrogen, atomic oxygen, and sputter cleaning with andwithout previous atomic H (H*) and atomic O (O*) expo-sure. Treatment with O* rapidly removes hydrocarboncontamination but leaves an oxidized surface. This methodhas proven to be very effective for surface cleaning ofoxides [9]. H* can remove hydrocarbon contaminationwhile reducing the surface. Since the cleaning is based onthe chemical reaction of the active atomic species with theorganic contamination, rather than a physical process such assputtering, H* and O* methodologies minimize any physicalsurface damage, otherwise unavoidable using soft sputtering.The resulting surface compositions are consistent withprevious observations of the chemistry of the CIGS surface.
2. EXPERIMENTAL
Single-crystal epitaxial layers of CIGS were grown by ahybrid sputtering and evaporation method [10] on polished(111) GaAs wafers. This method uses two sputtering tar-gets: one pure In and the other either pure Cu, for CuInSe2(CIS) films, or Cu0.7Ga0.3 for CIGS alloy films. Se was in-troduced by evaporation from an effusion cell. The GaAssurfaces are polar, and the results presented here comparethe samples grown on the (111)A Ga-terminated (denotedas sample 269A) and (111)B As-terminated (denoted as
sample 269B) surfaces of GaAs. The growth process hasbeen described in detail elsewhere [11]. GaAs substrates~1 cmwide and 3–5 cm long were cleaved from as-received50 or 75mm diameter epi-ready wafers and mounted in thedeposition system without other cleaning. The system wasevacuated to its base pressure of ~1 × 10�6mbar and backfilled with Ar gas to the working pressure of the sputteringprocess, typically ~2mbar. The substrates were heated to~725 °C shortly before deposition of the film began. Thedeposition rate was ~1μm/h, and the total film thicknesswas typically 0.75μm. After growth, the samples werecooled under a Se flux to ~400 °C, after which the Se sourcewas shut down and the sample was allowed to cool to roomtemperature over several hours in vacuum. The composi-tions of the samples measured by energy-dispersivespectroscopy were 19.8% Cu, 15.9% Ga, 14.7% In, and49.7% Se for sample 269A, and 20.0% Cu, 15.4% Ga,14.4% In, and 50.2% Se for 269B. The difference in com-positions is presumably due to local variations in the mea-surement or local compositions, and both samples areprobably the same composition as they were grown simul-taneously. The samples had been exposed to laboratoryair for an extended time. Previous measurements showedthat the surfaces of such CIGS layers include a native oxideof 1–2 nm thickness [5,11].
The LEIS measurements were carried out in an Ion-ToFQtac100 instrument (ION-TOF GmbH, Münster, Germany)using either a 3 keV 4He+ or a 5 keV 20Ne+ ion beam. Thebase pressure in the analysis chamber is ~5 × 10�10mbar.The He+ ions were used for analysis of lower mass atomsat the surface while 20Ne+ ions were used to detect primar-ily higher mass surface species as it offers better mass res-olution in the mass range of interest. The primary ionswere incident normal to the surface and were detected ata scattering angle of 145° integrated through all azimuthalangles. The ion fluence during the surface analysis waskept below the static limit in order to reduce the surfacedamage as much as possible (maximum ion fluence2 × 1013 ions/cm2 during the analysis, implying that lessthan 1% of the surface is affected during the analysis).
For the analysis of the near-surface composition depthprofile, the instrument is equipped with a low energy sput-ter beam incident at 59° relative to the surface normal.Depth profiling analyses were performed in a dual-beammode, alternating between the analytical beam (5 keVNe+
beam at normal incidence) to obtain a surface spectrum andthe sputtering beam (500 eVAr+ beam at 59° incidence angle)to erode the surface.
In order to remove surface contamination, a sourcecapable of producing (neutral) atomic hydrogen (H*) oroxygen (O*) (MPS-ECR, Specs Surface Nano AnalysisGmbH, Germany) is located in a loading chamber. Thatchamber has a base pressure of ~1 × 10�9 mbar, reaching~2 × 10�5mbar during the cleaning process. The Ar sputtergun was also used for cleaning in addition to depth profil-ing. Cleaning processes were 10min of H* at an indicatedpressure of 10�4mbar (note that the cold cathode gageused to monitor the pressure may overestimate the pressure
of hydrogen), 10min exposure to O* at an indicated pressureof 5 × 10�5 mbar, and sputter cleaning with 500 eV Ar+ ionsat an ion beam current density of 48 nA/cm2 for 40 s. Theeffectiveness of the cleaning treatments was assessed byanalyzing the surface after each cleaning cycle with the3 keV He+ beam, and the cleaning procedure was repeateduntil no carbon peak was detected and the surface spectrumdid not change with further cleaning. Depth profile analysesof the samples were performed after the surface wascompletely cleaned.
3. RESULTS
Scanning electron microscope images of the two filmsstudied here are shown in Figure 1. Gross film surface mor-phologies are similar showing very flat surfaces with trian-gular surface islands as observed previously [11]. On theGaAs(111)A substrate, the film 296A was found to growwithout rotational growth twins evident, while for growthon GaAs(111)B, film 296B showed twinned domainswhere the triangular features on the surface rotate 180° rel-ative to each other. Although surface roughness decreasesthe absolute signal intensity in LEIS, it does not influencethe ratios of signal intensities [12]. The surfaces are suffi-ciently flat that gross surface roughness should not haveaffected any of the measurements reported here.
As-received air-exposed films show the spectra (Figure 2)exhibiting a high scattering signal between ~1100 and2100 eV when analyzed with 4He+ ions. This is attributedto ions that penetrated into the sample, were backscatteredby atoms beneath the surface, and were subsequently ionizedas they left the sample. This signal is typically found to beenhanced by the presence of electronegative species on thesurface (e.g., O) that promote the re-ionization of neutralized4He [8].
In the low energy range of the spectrum, there is anexponential decay of the signal that corresponds to positive
secondary ions sputtered from the surface. It is an indica-tion of species adsorbed on the surface, typically hydrocar-bons and other atmospheric contaminants, and also othersurface components with high positive sputter yields (e.g.,Na and Ca) [9,13]. The adsorbed species prevent the detec-tion of any surface atoms other than a weak oxygen peak(Figure 2). The spectra in Figure 2 are typical of such con-taminated surfaces and were expected for the as-receivedsamples. Both samples showed identical backscatteringspectra as-received.
The result of treating the as-received surfaces with H* isshown in Figure 3 for both samples. The component attrib-uted to sputtered ions is significantly reduced, indicatingthe removal of much of the surface contamination. Someof the remaining signal may be attributed to H adsorbedon the surface after the H* treatment being sputtered asH+. After the removal of the surface contamination, the4He+ spectra show high intensity surface peaks with energyonsets at 2428 and 2643 eV, corresponding to masses of70 and 115 u (i.e., Ga and In, respectively). Subsequentanalyses with 20Ne+ confirm the assignment of these peaksto those elements and show a weak Se signal. The theoret-ical peak positions of the atoms for the scattering of the dif-ferent primary beams used in this work are summarized inTable I. We note that even after H* treatment, there is aweak O peak in the backscattered spectrum (Figure 3).
In a prior study, Liao et al. [11] showed that the surfaceof similar samples was covered by approximately threemonolayers of group III oxide. In related work, angle-resolved XPS suggested that the oxide on CIGS consists ofa Ga-rich outer layer and an In-rich inner oxide [11,14]. Thisstudy also found little Se or Cu present in the oxide. We pro-pose that the H* treatment results in a reduction of at least theouter layers of oxide, resulting in a Ga-rich In–Ga atomicmixture on the sample surface with no Cu and little Se
1 µm
a) b)
1 µm
Figure 1. The surface morphology of the film grown on (a) theGa-terminated (sample 296A)GaAssurface and (b) theAs-terminatedGaAs surface (sample 296B). The CIGS is expected to maintain
this polarity in the grown film.
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Scattering Energy (eV)
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500 1000 1500 2000 2500 3000
Reionizedbackscattered 4He+
Sputtered positive ions
16O
296A before H296A before O296B before H296B before O
Figure 2. The spectra obtained in several measurements on bothsamples as-received. Sample 296A was grown on Ga-terminatedGaAs (111) while sample 296B was grown on As-terminated
present. This is also consistent with the volatility of hydro-gen selenides. The absence of Cu is harder to explain but isconsistent with the absence of Cu from the native oxide.
A conventional approach to the removal of organic con-tamination on air-exposed samples in preparation for LEISis treatment with O*. This is known to convert hydrocar-bons quickly to volatile species. In the current case, O*treatment did remove the hydrocarbon layer quickly, elim-inating the low energy decay arising from sputtering ofthese species as positive ions and giving rise to clear atom-ically derived peaks in the spectrum (Figure 4). In all
surfaces treated with O*, the dominant features in the ionscattering spectra are the O peak and an increase in the sig-nal due to ions backscattered after penetrating to the deeperlayers of the sample and subsequently re-ionized on theiroutward trajectory. This signal is enhanced by the in-creased density of electronegative species in the sample,especially on the immediate surface.
A careful analysis of the data for the O*-treated surfacereveals four peaks in the 4He+ spectra, consistent with O,Cu, Se, and In (see Table I for expected peak positions).The Ga peak observed for the H*-treated surface at2428 eV does not appear significant in the O*-treatedsurface. Similar results were obtained with the 20Ne+ beamfor the heavier elements (Figure 4). The spectrum aftercleaning is consistent with the bulk composition of thetwo samples although, perhaps surprisingly, the low con-centration Ga is not observed. We conclude that O* oxi-dizes the sample approximately uniformly and brings allof the bulk species into the surface oxide layer rather thanresulting in a Ga-enriched surface as was the case for H*cleaning.
Finally, CIGS samples have been prepared previouslyfor photoemission and inverse photoemission by sputteringwith Ar [5,15,16]. This preparation method was found toresult, after annealing the sputtered surfaces, in valenceband spectra, conduction band spectra, and energy gapsconsistent with the alloys studied. Therefore, it is of
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20Ne Scattered Energy (eV)
296A 5 kV Ne
296B5 kV Ne
In
Se
GaCu
O
296B 3 kV He
296A 3 kV He
Figure 3. Results for atomic H* treatment of the samples. TheNe spectra are shown as insets that provide higher mass resolu-tion for higher mass species. Note the residual O, which is stron-ger on the metal-terminated surface. The data show no Cu and
little Se on both surfaces.
Table I. Energies for a scattering angle of 145° of the differentprimary ions used in this work.
Figure 4. Results for O* treatment of the samples. The Nespectra are shown as insets that provide higher mass resolutionfor higher mass species. Both samples show strong O peaks as
well as Cu, Se, and In signals with little or no Ga detected.
interest to consider gentle Ar ion milling of the surface. Inthe current study, 500 eV Ar+ ions were incident at a 59°angle. Typical ion damage depths for this type ofsputtering are of the order of the primary ion range in thesample. In this case, the projected range for low-energyAr+ sputtering in CIGS was estimated to be 1.4 nm accord-ing to SRIM software [17] (Table II). Therefore, we mayexpect ion mixing of a few atomic layers, consistent withtypical displacement cascade lengths for low energy ions.In addition, because the technique measures surface com-position, the higher sputtering rate of Se and Cu relativeto In and Ga could have impacted the results. However,the results here do not appear to have been significantly af-fected by preferential sputtering as the trends run generallycontrary to the trends that would have resulted from prefer-ential sputtering.
Figure 5 shows the result of Ar ion cleaning of the as-received samples. Both samples show nearly identicalspectra under 20Ne+ ions. The peaks correspond to Cu,Ga, Se, and In. The 4He+ spectra show the same elementsbut with lower resolution. No lighter mass peaks, where4He+ is particularly sensitive (C, O, for example), wereobserved. Note that the shape of the Se peak reflects therange of common isotopes for Se covering the mass rangefrom 74 to 82. The signal attributed to ions scattered fromdeeper in the sample and subsequently re-ionized is muchlower in these spectra, consistent with the removal of theO and the relatively weak contribution to re-ionization ofthe Ne by Se atoms in the solid. Although we would expectpossible preferential sputtering of Se and Cu, both ofwhich have high sputtering yields as estimated usingSRIM (Table II), the spectra of the Ar+-cleaned surfacesshow general peak intensity ratios qualitatively consistentwith the bulk composition, although they suggest largeramounts of In and lower amounts of Cu. (Note thatbecause the peak intensities were not calibrated againststandards, we do not attempt to give specific compositionvalues for the surface.) In light of this apparent differ-ence between the cleaned surface composition and thebulk composition, it is interesting to investigate the in-depth composition of the samples. However, beforeperforming this depth profiling analysis, we must assessthe effects of Ar sputtering on the H*-treated and O*-treated surfaces, with compositions previously described(Figures 3 and 4).
Table II. Sputtering yields and projected ranges for primary ions and target atoms used in this work, as estimated using SRIM [17].
Figure 5. 20Ne+ spectra obtained for both as-received specimensafter a brief sputter with a dose of 1.2×1015cm2 500eV Ar ions in-cident at an angle of 59° relative to the surface normal. No differentor additional featureswere observed under the 4He+ analysis beam.
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20Ne+ Scattered Energy (eV)
0
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40
60
80
100
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500 1000 1500 2000 2500 3000
269A H* 269B H*
Cu
Ga Se
In
Figure 6. 20Ne+ LEIS spectra for the two samples after initial H*(lower) and O* (upper) treatment and subsequently sputteredwith1.2×1015cm2 500eV Ar+. The spectra resemble the Ne spectra inFigures 3 and 4 indicating that the sputter cleaning has not altered
the compositions produced by these preparation methods.
The 20Ne+ spectra for Ar sputtered specimens previouslytreated with H* and O* are shown in Figure 6. The spectrashow very similar atomic compositions as were found priorto Ar sputtering. The brief Ar sputter has apparentlyremoved a small amount of the surface material but hasnot changed the overall composition of the surfaces.
To determine how these surface preparation methodsrelate to the bulk composition of the samples, we per-formed depth profile analyses in a dual-beam mode. In thismode, we alternate the 5 keV Ne+ static analysis andsputtering cycles using the low energy Ar beam in orderto minimize the layer mixing from the higher energy Ne+
Inte
gra
ted
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)
Estimated depth (nm)
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Cu In Ga Se
Sample 269 B
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Sample 269 A
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Figure 7. Depth profiles for the as-received sample 269B (anion terminated) for (a) O* cleaned (b), H* cleaned, and (c) sputter cleanedand sample 269A (cation terminated) for (d) O* cleaned (e), H* cleaned, and (f) sputter cleaned.
analysis beam (note that the projected range (Rp) for 5 keVNe+ beam at normal incidence is almost 10 times largerthan the Rp for the sputter beam, indicating a higher mixinglength for the analysis beam; Table II). The depth profilesfor the H*, O*, and 500V Ar+ sputter cleaned surfaces ofthe two samples are shown in Figure 7. Some clear differ-ences and similarities appear.
All samples show a Cu-deficient surface relative to thebulk, even for the O*-cleaned samples. The H*-cleanedsamples both show a significant loss of Se near the surface.The O*-cleaned samples show a similar, but smaller, loss ofSe that persists over a shorter distance than the H*-cleanedsamples. The sputter-cleaned samples show no significantSe deficiency near the surface. The H*-cleaned samples
show a very strong In peak below the surface with a Gapeak closer to the surface. In addition, there is a significantCu accumulation below the group III-rich layers. A smallerCu accumulation below the group III-rich region is also ob-served for the other preparation methods. Eventually, all ofthe profiles settle to nearly identical bulk behaviors, and thedepth profiles of the A and B samples are nearly identical.Note that the changes in composition near the surfaceshould have been reversed if they were dominated by pref-erential sputtering effects. The higher sputtering rates of Seand Cu should have resulted in reduced concentrations ofthese elements with greater depth profiling times as thepreferential sputtering would have reduced their surfaceconcentrations. This is opposite to what was observed.
Inte
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/nC
)
Estimated depth (nm)
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Sputter cleanedSample 269 A
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Figure 7. Continued.
Organic Contamination
Ga oxide
In oxide
CIGS
As-received
Ga
In
CIGS
Cu-rich Layer
CIGS
Slightly In-rich CIGS
Multi-componentOxide
CIGS
Slightly In-rich CIGS
Ar sputtered
Figure 8. A schematic illustration of the results of this study. The schematic exaggerates some aspects tomake the general features clear,so the reader should realize that the Ga and In layers in the H*-treated surface contain some oxides, and so on. See the text for details.
Although we did not observe the effect of surface polarity(cation versus anion termination) strongly, there is someevidence of the difference. Minor enhancements of In con-centration are found in some of the spectra for the cation-terminated films compared with the anion-terminatedsurfaces. However, it is apparent from the measurementsto date that the surface preparation method is having astronger effect on the observed composition than the resid-ual surface composition has.
4. CONCLUSION
The nature of the surface and bulk regions of the twosamples based on the LEIS measurements conducted herefor each of the surface preparation methods is shownschematically in Figure 8. The as-received samples havea surface layer of adventitious organic contaminationthat is substantially removed by all of the preparationmethods. There is a significant surface oxide present onthe samples that is reduced by H* treatment leaving agroup III elemental bilayer on the samples, with Ga segre-gated to the surface and In below. Under this layer is aregion of enhanced Cu concentration. Both the O* andsputtering methods produce compositions closer to thebulk composition. The presence of an In-rich region nearthe surface could be due to the often proposed ordereddefect compound [18]. However, this may be the resultof the surface preparation. More experiments are neededto verify if a group III-rich layer is present in clean, undam-aged layers.
The sample cleaning procedures employed to date maybe significantly improved by annealing the samples aftertreatment. We conclude based on this work that the optimalpreparation process involves 500 eV Ar sputtering ofthe sample at a low angle followed by annealing at atemperature in excess of 400 °C, where Cu becomesmobile to restore the surface stoichiometry. If the annealis conducted at a low enough temperature, Se loss maynot occur. We will explore the effect of annealing in futureexperiments.
ACKNOWLEDGEMENTS
H. T., J. D., J. K., T. I., and A. R. gratefully acknowledgethe support of the International Institute for Carbon NeutralEnergy Research (wpi-I2CNER), sponsored by the WorldPremier International Research Center Initiative (WPI),MEXT, Japan. Additionally, H. T. was also funded by aMarie-Curie Intra-European fellowship (PIEF-GA-2010-274999). A. R. gratefully acknowledges the supportof the US National Science Foundation under projectDMR 13-12539. The samples were grown by D. Liaoduring his time as a member of the Rockett group, whichis much appreciated.
REFERENCES
1. Emery K. Best record cell efficiencies. 2014. Availablefrom: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg [accessed on 25 May 2014].
2. Krum S. First solar sets thin-film module efficiencyworld record of 17.0 percent. 2014. First Solar. Avail-able from: http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=833971 [accessed 2 April 2014].
3. Green MA, Emery K, Hishikawa Y, Warta W, DunlopED. Solar cell efficiency tables (version 43). Progressin Photovoltaics 2014; 22(1): 1–9.
4. Mauch RH, Hedstrom J, Lincot D, RuckhM, Kessler J,Klinger R, Stolt L, Vedel J, Schock HW. Optimizationof windows in ZnO-CdS-CuInSe2 heterojunctions. InConference Record of the Twenty Second IEEE Photovol-taic Specialists Conference - 1991 (Cat. No.91CH2953-8),Las Vegas, NV, USA, IEEE, 7-11 Oct. 1991.
5. Liao D, Rockett A. Cu depletion at the CulnSe2 surface.Applied Physics Letters 2003; 82(17): 2829–2831.
6. Liao D, Rockett A. Cd doping at the CuInSe2/CdSheterojunction. Journal of Applied Physics 2003; 93(11): 9380–9382.
7. Brongersma HH. Low-energy ion scattering. In Char-acterization of Materials, Kaufmann EN (ed.). JohnWiley & Sons: New York, 2012; 2024–2044.
8. Brongersma HH, Draxler M, de Ridder M, Bauer P.Surface composition analysis by low-energy ion scat-tering. Surface Science Reports 2007; 62(3): 63–109.
9. de Ridder M, van Welzenis RG, Brongersma HH. Sur-face cleaning and characterization of yttria-stabilizedzirconia. Surface and Interface Analysis 2002; 33(4):309–317.
10. Rockett A, Lommasson TC, Yang LC, Talieh H,Campos P, Thornton JA. Deposition of CuInSe2 bythe hybrid sputtering-and-evaporation method. InConference Record of the Twentieth IEEE PhotovoltaicSpecialists Conference - 1988 (Cat. No.88CH2527-0),Las Vegas, NV, USA, IEEE, 26-30 Sept. 1988.
11. Liao D, Rockett A. The structure and morphology of(112)-oriented Cu(In,Ga)Se2 epitaxial films. Journalof Applied Physics 2008; 104(9): 094908–094908-8.
12. Jansen WPA, Knoester A, Maas AJH, Schmit P,Kytokivi A,Von der GonAW,BrongersmaHH. Influenceof compaction and surface roughness on low-energy ionscattering signals. Surface and Interface Analysis 2004;36(11): 1469–1478.
13. Tellez H, Aguadero A, Druce J, Burriel M, Fearn S,Ishihara T, McPhail DS, Kilner JA. New perspectivesin the surface analysis of energy materials by com-bined time-of-flight secondary ion mass spectrometry(ToF-SIMS) and high sensitivity low-energy ion scat-tering (HS-LEIS). Journal of Analytical Atomic Spec-trometry 2014. DOI: 10.1039/C3JA50292A
14. Liao D, Rockett A. The nanochemistry of oxides onCuInSe2 epitaxial layers. Unpublished.
15. Elson AJ, Gebhard S, Rockett A, Colavita E, EngelhardtM, Hochst H. Synchrotron-radiation photoemission-study of CdS/CuInSe2 heterojunction formation. Physi-cal Review B 1990; 42(12): 7518–7523.
16. Weinhardt L, Fuchs O, Gross D, Storch G, Umbach E,Dhere NG, Kadam AA, Kulkarni SS, Heske C. Bandalignment at the CdS/Cu(In,Ga)S2 interface in thin-
film solar cells. Applied Physics Letters 2005; 86(6):62109-1.
17. Ziegler JF, Biersack JP, Littmark U. The Stopping andRange of Ions in Solids, Vol. 1. Pergamon: New York,US, 1985.
18. Schmid D, Ruckh M, Grunwald F, Schock HW. Chal-copyrite/defect chalcopyrite heterojunctions on thebasis of CuInSe2. Journal of Applied Physics 1993;73(6): 2902–2909.
