Electronic characterization and effects of light-induced degradation on hydrogenatednanocrystalline siliconAdam F. Halverson, James J. Gutierrez, J. David Cohen, Baojie Yan, Jeffrey Yang, and Subhendu Guha Citation: Applied Physics Letters 88, 071920 (2006); doi: 10.1063/1.2175480 View online: http://dx.doi.org/10.1063/1.2175480 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Bonded hydrogen in nanocrystalline silicon photovoltaic materials: Impact on structure and defect density J. Appl. Phys. 110, 064315 (2011); 10.1063/1.3638712 Enhancement of light-induced degradation under reverse bias in hydrogenated nanocrystalline silicon solar cells J. Appl. Phys. 98, 074902 (2005); 10.1063/1.2060963 Fingerprints of two distinct defects causing light-induced photoconductivity degradation in hydrogenatedamorphous silicon Appl. Phys. Lett. 79, 3080 (2001); 10.1063/1.1413719 Effects of hydrogen on the growth of nanocrystalline silicon films by electron-beam excited plasma chemicalvapor deposition J. Appl. Phys. 88, 6848 (2000); 10.1063/1.1287754 Light-induced electron spin resonance in amorphous hydrogenated germanium Appl. Phys. Lett. 74, 3797 (1999); 10.1063/1.124183

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Electronic characterization and effects of light-induced degradationon hydrogenated nanocrystalline silicon

Adam F. Halverson, James J. Gutierrez, and J. David Cohena�

Department of Physics, University of Oregon, Eugene, Oregon 97403

Baojie Yan, Jeffrey Yang, and Subhendu GuhaUnited Solar Ovonic Corporation, 1100 W. Maple Road, Troy, Michigan 48084

�Received 22 August 2005; accepted 4 January 2006; published online 17 February 2006�

Hydrogenated nanocrystalline silicon was characterized using the transient photocapacitance �TPC�method. The TPC spectra show these materials to have a mixed-phase nature. At low temperatures,the spectra appeared very microcrystalline, whereas at moderate temperatures they appeared verysimilar to those for hydrogenated amorphous silicon. These differences are shown to result from thetemperature dependence of the minority carrier collection from the nanocrystalline component. Theeffects of light-induced degradation were also studied. This caused a substantial decrease inminority carrier collection, similar to lowering the temperature of the undegraded sample. However,no concomitant increase in dangling bond defect density was observed. © 2006 American Instituteof Physics. �DOI: 10.1063/1.2175480�

At sufficiently high levels of hydrogen dilution, plasma-enhanced CVD �PECVD� systems designed for the deposi-tion of hydrogenated amorphous silicon �a-Si:H� yield ahydrogenated nanocrystalline silicon �nc-Si:H� material, con-sisting primarily of very small crystallites of silicon �few nmto over 20 nm� with some residual a-Si:H between the crys-tallites or crystallite clusters. This nc-Si:H is being investi-gated as a possible material for use in solar cellapplications.1–3 Initially it was thought that this materialwould not degrade from light exposure as do a-Si:H cells;4

however, for many nc-Si:H based cells a similar degradationhas been observed.5

Through application of transient photocapacitance �TPC�and transient photocurrent �TPI� spectroscopies6 we havecharacterized the electronic properties of this material. ThisLetter reports what these measurements have revealed re-garding the active defects within the nc-Si:H and the natureof the degradation in material properties after prolongedexposure to light.

All of the sample devices in this study were depositedonto specular stainless-steel substrates at United Solar Ovon-ics Corporation using a radio-frequency PECVD process.7

Three “sandwich” devices consist of roughly 700 nm thicknc-Si:H layers clad between two 200 nm to 250 nm thicka-Si:H layers in a SS/n+ /a-Si:H/nc-Si:H/a-Si:H struc-ture. A semitransparent Pd Schottky barrier was deposited onthe top of each sample. Two of these �14140 and 14657�were deposited using constant hydrogen dilution �a 25%lower level for 14657�, while for the third �14661�, the hy-drogen dilution was varied during the growth to control theevolution of crystalline fraction and crystallite size.8 A fourthsample �12123� was also studied: A purely nc-Si:H device inan n-i-p structure deposited using a constant hydrogen dilu-tion and incorporating a 1000 nm thick intrinsic layer in aSS/n+ / i / p+ / indium tin oxide structure. Companion films foreach growth condition were deposited on Al foil substratesthat could be etched off to enable x-ray diffraction analysis

of the crystallite fraction and size. Crystallite fractions werereported to be: 70�+10/−5�%, 60�+10/−5�%, 69�+5�%, and68�+5�% for the four growth conditions, respectively, withcrystallite sizes varying from 22 to 26 nm.8 All the sampleswere characterized in their initial state �state A� after anneal-ing at 470 K. A couple of samples were then exposed to610 nm long-pass filtered, tungsten-halogen light at100 mW/cm2 for 100 h. These were then recharacterizedin that degraded state �State B� and in a series of partialisochronal anneal states.

The sandwich devices seemed superior for our studiesbecause secondary ion mass spectroscopy has shown thatimpurity diffusion into nc-Si:H can be blocked by a cappinga-Si:H layer. This sample geometry also eliminated theheavily doped n and p layers with their high deep-defectdensities—which, we were concerned, could mask the re-sponse of the intrinsic layer. On the other hand, we did notwant the a-Si:H cladding layers in the sandwich samples toinfluence our results. We considered this unlikely since, be-low room temperature, the a-Si:H layers should behave onlyas inert insulating layers in our measurements. However, tocheck this, we included one purely nc-Si:H n-i-p sampledevice in our study.

The TPC and TPI techniques have been described indetail elsewhere.6 They provide spectra that appear similar tosubband-gap optical absorption spectra, except that these sig-nals result from the charge that escapes the depletion regionin response to the monochromatic light, rather than the ab-sorbed optical energy. In the TPI method, the current tran-sient due to the motion of released carriers is monitoredwhile, in the TPC method, the change in the depletion regioncharge density is monitored by the ac junction capacitance.The TPC signal is therefore sensitive to the difference n-p�where n and p are the number of electrons and holes col-lected� while, because motion of either carrier type producesthe same sign current, the TPI signal is proportional to thesum n+ p. Comparing the two types of spectra can thus yieldthe relative numbers of majority and minority carrierscollected.

Figure 1�a� shows TPI spectra collected on one sandwicha�Electronic mail: dcohen@uoregon.edu

APPLIED PHYSICS LETTERS 88, 071920 �2006�

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geometry sample. The spectra closely resemble spectra formicrocrystalline Si obtained previously using the constantphotocurrent method.9 The spectra show a clear thresholdnear 1 eV �the crystalline silicon band gap� and some evi-dence of a defect band at lower optical energies. Despite thevery different temperatures at which they were collected,both spectra appear nearly the same.

Figure 1�b� shows TPC spectra for the same sample un-der identical conditions. The 170 K spectrum is qualitativelysimilar to the TPI spectra in Fig. 1�a�; however, the 300 Kspectrum appears much more similar to the subband-gapspectra for amorphous silicon.10 In particular, the 300 Kspectrum exhibits the familiar a-Si:H Urbach tail,11 extend-ing from 1.8 to 1.5 eV with a characteristic energy of62 meV, as well as a broad defect band at lower opticalenergies. Figure 1�c� shows TPC spectra measured at twosimilar temperatures on the n-i-p configuration device, con-taining no a-Si:H cladding layers. These spectra are verysimilar to those collected for the sandwich configuration de-vice, although with a larger Urbach energy, near 80 meV.

These results lead to a clear conclusion: These samplesare predominantly comprised of Si nanocrystallites as indi-

cated by the TPI spectra in Fig. 1�a�, but also contain asignificant component of silicon in the amorphous phase.This amorphous phase is able to dominate the TPC responseat higher temperatures but does not affect the appearance ofthe TPI spectra. We believe this temperature dependence isdue to different minority carrier collection fractions betweenthe two different silicon phases. At higher temperatures, theminority �hole� carrier collection in the nanocrystalline com-ponent is increased relative to low-temperature collection,canceling its contribution to the TPC signal. However, forthe subband-gap transitions occurring in the a-Si:H compo-nent, a much lower fraction of the photo-excited holes areable to escape, as compared to photoexcited electrons. Thus,as temperature is increased, the TPC signal �proportional ton-p� due to the amorphous phase remains relatively larger.We can also rule out that the a-Si:H-like spectra come fromthe a-Si:H cladding layers of the sandwich sample devicesbecause the spectra for the n-i-p sample device, which con-tains no a-Si:H, are qualitatively identical.

In Fig. 2, we show similar TPC spectra collected at twodifferent temperatures for a second sandwich device. Thedecreased signal at 240 K in the 1.1 to 1.8 eV region isagain attributed to an increase in minority carrier collection.We then examined this temperature dependence in detail at1.47 eV optical energy, between 170 K and 300 K, and theresults are shown in Fig. 3. The linear Arrhenius dependence

FIG. 1. Comparison of �a� TPI and �b� TPC spectra for sandwich sample 14 661 at two measurement temperatures, along with the �c� TPC spectra for the n-i-psample �12 123�. Note the microcrystalline Si appearance of the TPI, as well as the lower-temperature TPC spectra, but the much more a-Si:H-like spectrafor the TPC spectra at higher temperature. Spectra have been aligned in low-energy regions.

FIG. 2. Comparison of TPC spectra for sample 14 657 in State A at 180 Kand 240 K, along with the TPC spectrum at 240 K after light exposure�State B�. The State B hole collection is reduced to a level comparable tothat of State A at 180 K. Also, note that the defect band region below 1 eVis unchanged between the 240 K data for States A and B.

FIG. 3. Detailed temperature dependence in States A and B of the TPCsignal magnitude at 1.47 eV. The fraction of holes collected appears ther-mally activated with nearly the same energy in both cases, but is more thanten times higher in State A.

071920-2 Halverson et al. Appl. Phys. Lett. 88, 071920 �2006�

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indicates that the hole collection is a thermally activated pro-cess. The deduced activation energy may imply a hole traplying 0.22 eV above the nanocrystalline phase valence band.We note that in both the high- and low-temperature extremes,the trap signal reaches a limiting value. At high temperatures,it approaches that of the a-Si:H phase that dominates theTPC signal response, while in the low-temperature limit, itreaches a maximum when the hole collection is negligiblecompared to the electron collection.

We also examined the effects of light exposure on thesesamples. The TPC spectrum obtained at 240 K on the 100 hdegraded “State B” of sample 14657 is included in Fig. 2.From the large increase in signal in the 1.4 to 1.6 eV opticalenergy range, it is clear that light exposure has substantiallyreduced the hole collection in the sample. Indeed, the changeis by almost the same factor as lowering the temperature inState A from 240 K to 180 K.

In the degraded state, we again measured the tempera-ture dependence of the TPC signal in detail at 1.47 eV �theseresults are included in Fig. 3� and observed activated holecollection with a characteristic energy very close to that ob-tained in State A. However, the overall TPC signal is muchlarger, by more than a factor of 10 compared to State A.These results strongly suggest that the same defects act ashole traps for both States A and B, but that there are simplymore of them in the degraded state.

This increased hole trapping does not appear to becaused by any increase in deep defect density. No such in-crease is observed in a series of TPC spectra obtained duringa stepwise isochronal annealing process that took the samplefrom State B back to State A. In Fig. 4, we display theresultant series of TPC spectra obtained at 180 K. At this lowtemperature, because the minority carrier collection is al-ready too small to affect the TPC signal, the spectra above1 eV look essentially identical regardless of their degrada-tion state. What is surprising, however, is that there is nosignificant change in the magnitude of the deep �danglingbond� defects, below 1.0 eV. The corresponding spectra ob-tained at 240 K �shown in Fig. 2 for States A and B�, in spiteof the large differences in the 1.3–1.8 eV energy range �dueto the changes in the hole collection� again indicate no dis-cernable difference in the deep defect energy regime below1.0 eV. In addition, no significant light-induced changeswere observed in the drive-level capacitance profiles forthese samples as reported previously.12

This implies that the degradation cannot be attributed toincreases in the dangling bond defects within the a-Si:Hcomponent; rather, it has an as yet unidentified origin. Somepossibilities include other types of bulk light-induced defect,

defects at grain boundaries �or amorphous region/crystalliteboundaries� that do not give rise to strong optical signals, oran increase in potential fluctuations as a result of light-induced charge separation across the different components ofthese mixed phase nc-Si:H materials.

This work was funded under NREL subcontracts XAF-8-17619-05 at the University of Oregon and ZDJ-2-30630-19at United Solar.

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FIG. 4. TPC spectra for a series of metastable states produced by step-wisepartial anneals of a fully light soaked state at the temperatures indicated. Asshown, no change was observed during annealing in the deep defect regionof the spectra.

071920-3 Halverson et al. Appl. Phys. Lett. 88, 071920 �2006�

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