Abruptness of a-Si:H/c-Si interface revealed by carrier lifetime measurements
Stefaan De Wolf and Michio Kondo
Citation: Appl. Phys. Lett. 90, 042111 (2007); doi: 10.1063/1.2432297
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Abruptness of a-Si:H/c-Si interface revealed by carrier
lifetime measurements
Stefaan De Wolf a͒and Michio Kondo
National Institute of Advanced Industrial Science and Technology(AIST),Central2,1-1-1Umezono,
Tsukuba,Ibaraki305-8568,Japan
͑Received27September2006;accepted15December2006;published online26January2007͒
Intrinsic hydrogenated amorphous siliconfilms can yield outstanding electronic surface passivation of crystalline silicon wafers.In this letter the authors confirm that this is strongly determined by the abruptness of the interface.For completely amorphousfilms the passivation quality improves by annealing at temperatures up to260°C,most likely byfilm relaxation.This is different when an epitaxial layer has been grown at the interface duringfilm deposition.Annealing is in such a case detrimental for the passivation.Consequently,the authors argue that annealing followed by carrier lifetime measurements allows determining whether the interface is abrupt.©2007American Institute of Physics.͓DOI:10.1063/1.2432297͔
Hydrogenated amorphous silicon͑a-Si:H͒films depos-ited on crystalline silicon͑c-Si͒surfaces have increasingly attracted attention over the past20years.Initially,it was discovered that abrupt electronic heterojunctions can be cre-ated with such structures.1Soon afterwards applications fol-lowed,including bipolar transistors,2imaging devices,3and solar cells.4For the latter it was recognized that the output parameters benefit substantially from inserting a few nano-meter thin intrinsic a-Si:H͑i͒film between the doped amor-phous emitter and c-Si substrate.For solar cells that feature a similar heterostructure back surfacefield,impressive energy conversion efficiencies exceeding21%have been reported.5 The role of the a-Si:H͑i͒buffer layer has been discussed in literature͑see,e.g.,Refs.6–12͒:It is known that suchfilms can yield outstanding surface passivation for c-Si surfaces,13 but also that growth of an epitaxial interface during a-Si:H͑i͒deposition is detrimental for heterojunction device performance.12For hot wire chemical vapor deposited ͑CVD͒a-Si:H,where no ion bombardment takes place, abrupt interfaces have been obtained either by limiting the deposition temperature T depo͑Ref.14͒or by terminating the c-Si surface with a SiN x monolayer prior to a-Si:H
deposition.15The abruptness of the interface,i.e.,whether
instant a-Si:H deposition on c-Si occurred without initial
epitaxial growth,was in these studies determined either by
transmission electron microscopy͑TEM͒͑Refs.12,14,and
15͒or by͑in situ͒spectroscopic ellipsometry͑SE͒,16for
which mirror polished surfaces are desirable.To gain know-
ledge about the electronic surface passivation properties of
these interfaces,the most straightforward technique is by
measuring the effective carrier lifetimeeff of the samples. Such measurements are known to be extremely sensitive, allowing for detection of bulk defect densities as low as 109–1011cm−3in a simple,contactless technique at room temperature.17
In this letter,we show that by low temperature͑up to
260°C͒postdeposition annealing,the surface passivation
quality of direct plasma enhanced͑PE͒CVD a-Si:H͑i͒films improves when the a-Si:H/c-Si interface is abrupt.This contrasts with the case when an epitaxialfilm has been grown at the interface,where the surface passivation quality is seen to degrade significantly by a similar annealing treat-ment.Consequently,we argue that annealing followed by carrier lifetime measurements allows accurate determination of the onset of epitaxial growth in an easy-to-use way which is not restricted to polished c-Si surfaces.
For the experiments,300m thick relatively low resistivity͑ϳ3.0⍀cm͒boron-dopedfloat zone͑100͒͑FZ͒-Si͑p͒wafers have been used.Both surfaces of the sub-strates were mirror polished to eliminate the influence of substrate surface roughness on the passivation properties18 and to allow for SE measurements.For predeposition surface cleaning,the samples werefirst immersed in a ͑H2SO4:H2O2͒͑4:1͒solution for10min to grow a chemical oxide,which was followed by a rinse in de-ionized water. The oxide was then stripped off in a dilute HF solution͑5%͒for30s.After this the samples were immediately transferred to the load lock of the deposition system.Forfilm deposi-tion,a parallel plate direct PECVD reactor operated at radio frequency͑rf͒͑13.56MHz͒power was used,in which the samples were mounted at the top electrode.The electrode distance and diameter were respectively20and230mm.An undiluted SiH4flow of20SCCM͑SCCM denotes cubic cen-timeter per minute at STP͒was used and the chamber was maintained at low pressure͑0.5Torr͒.The value for T depo was varied from105to255°C.The rf power absorbed by the plasma was5W.This is the minimal power required to maintain a stable plasma at the given deposition conditions. To evaluate the surface passivation quality,identicalfilms of about50nm thick were deposited on both wafer surfaces. After deposition,the samples were consecutively annealed in a vacuum furnace͑30min,with annealing temperatures T ann ranging from120to260°C͒.In between the annealing steps,the value foreff of the samples was measured with a Sinton Consulting WCT-100quasi-steady-state photocon-ductance system,19operated in the so-called generalized mode.Since high quality FZ-Si wafers have been used throughout the experiments,the contribution of the bulk to the total recombination expressed byeff can be neglected.In such a case,the effective surface recombination velocity S eff, which value can be regarded as a direct measure for the passivation quality of thefilms present at the surfaces,may
a͒Electronic mail:stefaan.dewolf@aist.go.jp
APPLIED PHYSICS LETTERS90,042111͑2007͒
0003-6951/2007/90͑4͒/042111/3/$23.00©2007American Institute of Physics
90,042111-1
