Dennison, A.J.C.

Fjellvåg, Helmer

Frommen, Christoph

Hasle, Ida Margrete

Hauback, Bjørn

Haug, Halvard

Hjorvarsson, B

Marstein, Erik Stensrud

Qviller, Atle Jorstad

Vorobiev, A.

You, Chang Chuan

Østreng, Erik

NORA - Norwegian Open Research Archives

Thermal stability of photovoltaic a-Si:H determined by neutron reflectometryA. J. Qviller, A. J. C. Dennison, H. Haug, C. C. You, I. M. Hasle, E. Østreng, H. Fjellvåg, A. Vorobiev, B.Hjörvarsson, E. S. Marstein, C. Frommen, and B. C. Hauback  Citation: Applied Physics Letters 105, 231909 (2014); doi: 10.1063/1.4904340 View online: http://dx.doi.org/10.1063/1.4904340 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/23?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Role of the buffer at the interface of intrinsic a-Si:H and p-type a-Si:H on amorphous/crystalline siliconheterojunction solar cells Appl. Phys. Lett. 107, 223901 (2015); 10.1063/1.4936196  Damage free laser ablation of SiO 2 for local contact opening on silicon solar cells using an a-Si:H buffer layer J. Appl. Phys. 107, 043518 (2010); 10.1063/1.3309382  Electrical transport mechanisms in a -Si : H / c -Si heterojunction solar cells J. Appl. Phys. 107, 023711 (2010); 10.1063/1.3267316  Surface passivation of n-type crystalline Si by plasma-enhanced-chemical-vapor-deposited amorphous SiC x : Hand amorphous SiC x N y : H films Appl. Phys. Lett. 81, 4461 (2002); 10.1063/1.1527230  Sensitization of the minority carrier lifetime in hydrogenated amorphous silicon Appl. Phys. Lett. 72, 103 (1998); 10.1063/1.120658    Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions.  IP:  128.39.227.15 On: Thu, 03 Mar 2016 11:27:04Thermal stability of photovoltaic a-Si:H determined by neutron reflectometryA. J. Qviller,1,a) A. J. C. Dennison,2,3 H. Haug,1 C. C. You,1 I. M. Hasle,1 E. Østreng,4H. Fjellva˚g,4 A. Vorobiev,2,3 B. Hj€orvarsson,2 E. S. Marstein,1 C. Frommen,1and B. C. Hauback11Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway2Division for Materials Physics, Department of Physics and Astronomy, Uppsala University, Box 516,SE-751 20 Uppsala, Sweden3Institut Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France4Department of Chemistry, Centre for Materials Science and Nanotechnology, University of Oslo,P.O. Box 1033, Blindern, NO-0315 Oslo, Norway(Received 2 September 2014; accepted 1 December 2014; published online 11 December 2014)Neutron and X-ray reflectometry were used to determine the layer structure and hydrogen contentof thin films of amorphous silicon (a-Si:H) deposited onto crystalline silicon (Si) wafers for surfacepassivation in solar cells. The combination of these two reflectometry techniques is well suited fornon-destructive probing of the structure of a-Si:H due to being able to probe buried interfacesand having sub-nanometer resolution. Neutron reflectometry is also unique in its ability to allowdetermination of density gradients of light elements such as hydrogen (H). The neutron scatteringcontrast between Si and H is strong, making it possible to determine the H concentration in thedeposited a-Si:H. In order to correlate the surface passivation properties supplied by the a-Si:Hthin films, as quantified by obtainable effective minority carrier lifetime, photoconductancemeasurements were also performed. It is shown that the minority carrier lifetime falls sharply whenH has been desorbed from a-Si:H by annealing.VC 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4904340]To reduce the costs of solar electricity, much effort isdevoted to the development of more efficient crystalline sili-con (Si) solar cells, which is the most important solar celltechnology today. Key components of all high efficiency so-lar cell architectures based on Si are the surface passivationlayers: dielectric material systems that reduce minority car-rier recombination at the silicon wafer surfaces. A commonlyused material is amorphous and hydrogenated Si (a-Si:H)made by plasma-enhanced chemical vapour deposition(PECVD).1 In this material, hydrogen (H) plays an importantrole in defining the surface passivation properties. Althoughwidely used in the manufacture of heterojunction solar cells,the use of a-Si:H in other solar cell architectures is limitedby the low thermal stability of the material.2 To allow for thedevelopment of improved deposition processes for a-Si:Hresulting in films with both improved surface passivationproperties and improved thermal stability, we have investi-gated surface passivated a-Si:H films before and after ther-mal treatment by photoconductance minority carrier lifetimemeasurements, as well as by neutron reflectivity (NR) andX-ray reflectivity (XRR) measurements.Passivated a-Si:H films investigated in this study weredeposited onto n-type silicon float-zone wafers with h100iorientation. Prior to deposition, the wafers were dipped inhydrogen fluoride (HF) (5%) and rinsed in de-ionized water.Thereafter, approximately 40 nm of a-Si:H was depositedonto both sides of the wafer in an Oxford Systems PlasmaLab133 PECVD reactor at 230 C using silane (SiH4). A rapidthermal processing (RTP) system (AccuThermo AW610) wasused to anneal the wafers at specific temperatures for 1 min atsteady state. Minority carrier lifetime was measured using aWTC-120 Photoconductance Lifetime Tester from SintonInstruments. A series of such measurements for a-Si:H sam-ples annealed at different temperatures in order to desorbhydrogen is shown in Fig. 1. Two samples of a-Si:H on2 2 cm2 Si substrates were selected for XRR and NR meas-urements. One of the samples was annealed at 500 C for 1min the same day as the NR measurement took place. Theother sample was only exposed to room temperature afterdeposition and used as a pristine reference sample.FIG. 1. Minority carrier lifetime measurements on a series of 40 nm a-Si:H/Si bilayers annealed at different temperatures. The uncertainty is a fewpercent.a)Author to whom correspondence should be addressed. Electronic mail:atlejq@ife.no0003-6951/2014/105(23)/231909/3/$30.00 VC 2014 AIP Publishing LLC105, 231909-1APPLIED PHYSICS LETTERS 105, 231909 (2014) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions.  IP:  128.39.227.15 On: Thu, 03 Mar 2016 11:27:04The goal of a reflectometry experiment is to determinethe scattering length density (SLD) as a function of depth zfrom the surface. For neutrons, the SLD is defined asSLD ¼ 1/Xinibi; (1)where ni is the number of atoms in a molecule and bi is thecoherent scattering length for element i. The quantity / isthe molecular volume. For X-rays, the SLD is simplythe volume average of the elements’ electron density.Reflectometry suffers from the phase problem that is genericto all scattering techniques, meaning that different SLD pro-files may give rise to identical or almost identical reflectivityprofiles.3 Knowledge of the layer ordering and performingboth NR and XRR measurements is one way to excludeincorrect interpretation of results. XRR is sensitive to theelectron density of the material in question and thus light ele-ments, especially H, are invisible to this technique. In thepresent a-Si:H thin films, XRR has two weaknesses:First, the insensitivity to the H content; and second, the poorcontrast between Si and SiO2 resulting from surface oxida-tion. Thus, the combination of NR and XRR is highly benefi-cial as XRR gives accurate determination of the layerthicknesses, while the NR provides information about thethickness and the composition.A PANalytical X-ray reflectometer operating at the CuKa wavelength k¼ 1.542 A˚ was used for the XRR measure-ments. The reflectivity R was measured at the specular con-dition as a function of 2h in the angular range 2h ¼ 0–12for 1 h. NR was performed at the SuperADAM neutron re-flectometer situated at Institute Laue-Langevin, Grenoble,France,4 at a wavelength k¼ 4.4 A˚ and with wavelength dis-persion (Dk/k)¼ 0.007. The reflectivity R(h) was measuredfor each sample at the specular condition 2h¼ 0–8 for 8 h.Reflectivity data together with sample model fits are plottedas a function of the transferred momentum q¼ 4pk1sinh inFigs. 2(a) and 2(b), for the XRR and NR data, respectively.The data were analyzed using the GenX reflectivity fit-ting package.5 NR and XRR data were simultaneouslyrefined using the logarithm of the reflectivity as the figure ofmerit. In addition, the square root of the reflectivity and thechi square of the data were also tried. However, these figuresof merit overemphasize the region near the critical anglewhere systematic errors are known to be worst and thereforegave inferior results. A four layer model for the surfacestructure was constructed as it has proven impossible tosimultaneously fit both datasets well with just three layersdue to different q-ranges and element sensitivities of the NRand XRR measurements. In particular, the low frequency os-cillation in the XRR data is outside the q-range of the NRdata, while the internal H distribution is invisible to theXRR. In this four layer model, the contribution to the elec-tron density caused by H was assumed to be negligible. Themodel was constructed for the X-rays so that the bottom twolayers were composed of pure Si and the top two of SiO2.The density, thickness, and roughness of these layers werevaried during the fitting process. For the NR data, an addi-tional parameter was the coherent scattering length per for-mula unit.As previously mentioned, XRR has very poor contrastbetween SiO2 and Si, but NR can distinguish well betweenall different materials present in the sample. X-rays are, inparticular, powerful to determine total thickness and surfaceroughness, and XRR could resolve a thin layer of around1 nm at the solid-air interface which appeared to have anextremely low density for SiO2. The density of this layer canbe explained as being a non-Gaussian roughness at this inter-face and the value being composed of the SLD for both airand SiO2. Below this layer, the model includes a slightlythicker layer of SiO2 with the same density and thus similarX-ray SLD as the a-Si:H layers.A broad interface is observed in the pristine andannealed samples underneath the oxide layers. Ascertainingthe composition of this layer is, however, complicated by thepossible presence of the three elements Si, O, and H. Sincethe value of the neutron SLD of this layer lies between thatfor pure Si and SiO2, we would hypothesize that this inter-face does contain some oxygen. X-ray and neutron SLD pro-files are shown in Fig. 3.Hydrogen has a negative scattering length b and thermalprocesses resulting in desorption of H will therefore cause anincrease of the a-Si:H SLD. When regarding the neutronSLDs in Fig. 3, it is clearly seen that the neutron SLD of theinnermost a-Si:H layer in the annealed sample is higher thanthe innermost layer in the pristine sample. Good fits to theneutron reflectivity data are obtained with a SLD¼ 1.597 106 A˚2 of this innermost a-Si:H layer in the pristinesample and a SLD¼ 1.744 106 A˚2 of the correspondinga-Si:H layer in the annealed sample. The respective densitiesof these layers, as obtained from XRR are q¼ 2.160 g/cm3and q¼ 2.119 g/cm3, while the crystalline Si substrate has adensity of q¼ 2.330 g/cm3.The definition of the SLD, Eq. (1), may be rearranged inorder to solve for the number of H atoms per Si atom in theinnermost a-Si:H layer. In order to do this, one uses / ¼FIG. 2. (a) XRR profiles and model fits for the pristine sample and the sam-ple annealed at 500 C. (b) NR profiles and model fits. The graphs are offsetby a factor of 0.1 for clarity. In parts of the figures, the statistical error barsare smaller than the model fit lines.231909-2 Qviller et al. Appl. Phys. Lett. 105, 231909 (2014) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions.  IP:  128.39.227.15 On: Thu, 03 Mar 2016 11:27:04MR=qNA for the molecular volume with Si formula massMR¼ 28.09, Si density q, and Avogadro’s number NA. It isalso assumed that H does not contribute to the density q andformula mass MR. By setting nSi¼ 1, Eq. (1) is then rear-ranged intonH ¼ SLD  / bSibH; (2)where nH is now the number of H atoms per Si atom. Inaddition to the a-Si:H layer SLD, the molecular volume/ calculated with q obtained from the XRR data and tabu-lated coherent scattering lengths bSi¼ 4.1491 fm andbH¼3.7390 fm are used in Eq. (2) to determine this ratiofor both samples. For the pristine sample, the calculationyields nH¼ 0.18, while the result for the annealed sample isnH¼ 0.08. Thus, approximately half of the H content beendesorbed from the a-Si:H thin films as a result of the thermalprocessing. It is clearly seen from Fig. 1 that this is stronglycorrelated with significant reduction of minority carrier life-time of the sample depleted of H after annealing at 500 C.In this work, a clear connection between long minoritycarrier lifetimes and the presence of H in thin films of a-Si:H/Si deposited on Si has been shown by NR, XRR, andphotoconductance measurements. The results are consistentwith the assumption that H plays a significant role in passiva-tion. Further work will focus on the intermediate temperatureregime and different a-Si:H layer thicknesses.Financial support was acknowledged from The ResearchCouncil of Norway through the SYNKNØYT program,Project No. 218418. A.J.C.D. and A.J.Q. would like to thankMatts Bj€orck for assistance and useful discussions regardingthe GenX fitting package. A.J.C.D. was funded by theSwedish Research Council VR 2009–6232.1S. De Wolf, A. Descoeudres, Z. C. Holman, and C. Ballif, “High-efficiencysilicon heterojunction solar cells: A review,” Green 2(1), 7–24 (2012).2H. Li and S. R. Wenham, “Influence of a-Si:H deposition power on surfacepassivation property and thermal stability of a-Si:H/SiNx:H stacks,” AIPAdv. 2(2), 022106 (2012).3J. Daillant and A. Gibaud, X-ray and Neutron Reflectivity (Springer, 2010),p. 117.4A. Devishvili, K. Zhernenkov, A. J. C. Dennison, B. P. Toperverg, M.Wolff, B. Hj€orvarsson, and H. Zabel, Rev. Sci. Instrum. 84(2), 025112(2013).5M. Bj€orck and G. Andersson, J. Appl. Cryst. 40, 1174 (2007).FIG. 3. X-ray and neutron SLD profiles of the 40 nm pristine sample and the40 nm sample annealed at 500 C. The X-ray SLDs are multiplied with a fac-tor of 0.1 to fit into the same figure as the neutron SLDs.231909-3 Qviller et al. Appl. Phys. Lett. 105, 231909 (2014) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions.  IP:  128.39.227.15 On: Thu, 03 Mar 2016 11:27:04

Thermal stability of photovoltaic a-Si:H determined by neutron reflectometry

Thermal stability of photovoltaic a-Si:H determined by neutron reflectometry

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