interface and bulk defect density in a-Si:H by means of photothermal deflection spectroscopy and photoconductivity

Journal of Non-Crystalline Solids 97&98 (1987) 731-734 North-Holland, Amsterdam

73 ]

STUDY OF SURFACE/INTERFACE AND BULK DEFECT DENSITY IN a-Si:H BY MEANS OF PHOTOTHERMAL DEFLECTION SPECTROSCOPY AND PHOTOCONDUCTIVITY M. FAVRE, H. CURT1NS and A.V. SHAH lnstitut de Microtechnique, Universit~ de Neuch~tel, Breguet 2, CH-2000 Neuch~tel, Switzerland From comparison of PDS (Photothermal Deflection Spectroscopy) spectra with photoconductivity (mobility-lifetime product) data over four decades of sample thickness we find experimental evidence for the presence of material inhomogeneities along the film growth axis. 1.INTRODUCTION The determination of both surface/interface (Ns) and bulk defect density (Nb) in amorphous silicon (a-Si:H) films is of great importance to applications such as thin film transistors, superlattice devices and solar cells. In a recent paper Smith et al. 1 pointed out the use of PDS (Photothermal Deflection Spectroscopy) and CPM (Constant Photocurrent Method) as complementary techniques by which N s and N b can be obtained separately. These two quantities have also been determined from the evolution of PDS spectra as function of sample thickness d 2-4. In latter studies film thicknesses d ranging from -4).1 to -10 ~tm are required: Films with d larger than a few txm are, however, in general difficult to prepare due to the presence of internal stress and/or low adherence (film cracking, peeling off the substrate, etc.). Another important problem in connection with the determination of N s and N b is related to a possible spatial variation of material properties (defect density, transport parameters, composition, etc.) along the film growth axis z due to inhomogeneities of the material. This problem is of great importance to device applications. Such inhomogeneities may result from initial film nucleation mechanisms, structural and/or stress relaxation phenomena, impurity incorporation profiles, etc. Spear 5 has reported a mobility-lifetime product (measured with different methods in devices with sandwich stmcture) for minority and majority carriers varying para" olically with d (for d<6 I.tm). The changes in latter study, however, are not explained in terms of a gradual improvement of the material properties along z, but are attributed to non-uniformities of the internal field. The aim of this paper is to deduce a defect density profile riD(Z) (determined via PDS spectra) and compare it with the profile for the mobility-lifetime product ErBtx(z) (determined by photoconductivity) for film thicknesses d ranging from -0.01 to -100 p.m. 2. DEFECT DENSITY DISTRIBUTION MODEL nl)(Z) In studies involving the thickness dependence of the PDS defect density N D, one in general assumes a defect density distribution nD(z) of the form: nD,(Z ) = N s t~(z) + N b

(1)

where Ns=sUrface defect dcnsity, N b =bulk defect density, 5(z)--delta function and z denotes the axis of film growth. PDS 'sees' an average defect density N D defined as: 1 fd N~ ND(d)= ~- o nD(z)dz = ~ - + Nb

(2)

Eqn.2 is physically only meaningful (and applicable) ifd is much more larger than the thickness do of the high defect density layer associated with N s (mathematically do=O, it .eality do=finite). To 0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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M. Favre et al. / Surface~interface and bulk defect density

account for the finite thickness do we introduce a slightly modified function: -z/d O riD(Z) = N O e + Nb (3) The PDS defect density based on this distribution will be obtained as: t~ d l J_ ND(d) = ~- nD(Z)dz= Nodo ~ (l _ e-d/do) + NI,

(4)

[ In the limit case do-d, (4) is equivalent to (2), if we set Ns=Nod o. Experimentally one can obtain do, ifN o is known, by fitting (4) to the measured data. Good quality a-Si:H films show typical values of Ns=l-5xl012/cm2 and Nb=l-5xl015/cm 3 2-4. NO is reasonably well estimated with -1018/cm 3 so that do= Ns/No=I00-500A. These data illustrate an important (and often ignored) fact: Defect densities (calculated via PDS spectra) are only meaningful if values for N s (or No, do) and N b are specified along with N D. For d<5 Ixm the first term in (2) and (4) may be predominant and therefore the bulk fraction of the defects is masked by the high surface/interface fraction (Note: The first term in (2) and (4) accounts for both, the defect density at the substrate/a-Si:H and a-Si:H/air-vacuum interfaces. The latter contributes negligeably in this study 12). 2. EXPERIMENTAL All samples discussed in this study were prepared by a new, high-rate VHF-GD (Very High Frequency Glow-Discharge) method described elsewhere 4,6,7 at a plasma excitation frequency of 70 MHz. Thanks to this deposition technique the thickness of the a-Si:H films could be varied over 4 decades from -43.01 to ~100 I.tm. Coming 7059 glass (0.8 mm) were used as substrate. The deposition parameters are: substrate temperature Ts=300°C, silane pressure p=0.28 mbar, power density P--0.1 W/cm3 and a silane gas flow of 18.5 sccm yielding a deposition rate of -16-17/~/sec. d was determined gravimetrically with a Mettler AEI63 balance and assuming a constant density of 2.2 g/cm3.The PDS spectra were evaluated and corrected for very thin samples according to the procedure described in Jackson 2. N D has been calculated using the model described by Jackson 8. The mobility-lifetime product ~rlg'~(d) was deduced from photoconductivity measurements AGph (HLX lamp white light, approximately AM1.5 spectrum) with illumination of the film from the top side and using A1 electrodes in gap arrangement at an electric field of 300 V/cm. AGph could not reliably be measured for d smaller than ~0.1 I~m (due to band-bending, etc.). The relationship used is: ~TiW~(d) =

AGPhd q ~ (1 -e"a't)

(5)

where q=l.6xl0 -t9 Clb, t~=1017/cm2sec denotes the photon flux with energy higher than the bandgap Eg=l.70 eV. ~ is the absorption coefficient at 600 nm, the wavelength which approximately coincides with the maximum intensity of the lamp (ct = average absorption coefficient). Additional properties of similar a-Si:H films were reported in 4,6,7 4. RESULTS AND DISCUSSION Fig.1 shows the (expected) decrease of N D from ~1018/cm 3 at d=100 ]k down to 2-3x1015/cm 3 at d=100 gin. An estimate of the thickness do can be obtained by fitting (4) to the experimental data. The best fit is obtained with No=l.0xl018/cm 3, Nb=2.5xl015/cm 3 and (Io=200 A (curve B). The dotted line in Fig.1 represents a fit according to (2) with Ns= 2.0xl012/cm 2. For d>500 A the two models are in agreement. For comparison purposes the curve C with do=20 A and curve A with do=2000 A are also added in Fig.1.

M. Favre et al. / Surface/interface and bulk defect density

733

The above values for N s, N b are typical for good quality material 1-4. We now have the important result that d o is of the order of several hundreds of .~; similar values have also been deduced from photoluminescence studies by Tiedje 9. Next let us consider the variation of£'qp.x(d). Several studies show that ZrllX'[(z)varies approximately linearly with the reciprocal defect density nD(z) 10,11. The measured Zrll.t't(d) is a weighted function of E~Wc(z) over d and can be written as: d

Z~p.'~(d) =

d

w(z) £11WC(z)dz =

no(z----3 dz ; w(z) =

ec~z

(6)

1 - e ~a'~

where A is a proportionality factor. The weighting function w(z) simply accounts for the fact that the absorption of light and consequently the cartier generation decreases exponentially in -z direction

',,~D

10 is

~

10-6

FT"" 1017

~---~10-7

1016

I ~ 10-~

u~

X

a-

2 i0 Is

I I1111H1

0.01

0.1

I I I~111]

I ~ II1+11[

1

10

10 -9

I t Illll

100

d [.urn]

~ i ill=ill

).01

0.1

i l~ttllll

~ Itlll.]

1 10 d [,urn]

i =l*~lt

100

Fig.1 PDS defect density N D as function of sample thickness d. The full lines are theoretical curves calculated from (4) with No=101S/cm 3 and Nb=2.5x1015/cm 3.

Fig.2 Mobility-lifetime product as function of sample thickness d. The full curve E was calculated from (8) with Eo=9.0x10-7 cm2/V,

For A: do=2000A, B: do=200,~ and for C: do=20A. The dotted line D has been calculated from (2) with Ns=2.0xl012/cm2 and Nb=2.5xl015/cm3

a--0.997, k-t =3.0 tam, (~-t =0.5 tam. Curve F was calculated using (3) and (6) and taking N0=1018/cm3, Nb=2.5xl0tS/cm 3 and clo=200A.

(illumination from top surface). The photoconductivity method therefore probes the top film surface with a thickness of approximately 1/o,=0.5 p.m (o~=2xl04/cm). In Fig.2, (6) is represented by the dotted curve F, using (3) for nD(z). A first surprising observation is that the experimental data for £'qtax(d) are shifted by more than one order of magnitude below the calculated ones in the thickness range ---0.2-1 Ixm. In fact, ~p:~(d) gets thickness independent only for d larger than approximately -3 tam. This is by more than two orders of magnitude larger than the the thickness of the high defect density layer do=200 A. Spear 5 t-rods a Zrltax(d) varying parabolicaUy with d in the range -0.1 to -6 Ixm and a flattening off of the curve for larger d values. The parabolic dependence found in 5 is attributed to non-uniformity of the internal field (sandwich structure). In the present case we find a linear dependence of r-rlta'~(d) for d between 4).1 and ~3 Ixm and constant values for d>5 tam (electrodes in gap arrangement). The Z'ql.t'c(z) *- nD(z)-t law does not hold. We are therefore raising the hypothesis that other factors (which do not influence nD(z) ) are

734

M. Faore et al. / Surface~interface and bulk defect density

involved, e.g a gradual variation of Zrlp.X(z) accross the sample. Such a variation of Zrlp.X(z) could originate from initial nucleation mechanisms, structural and/or stress relaxation phenomena, impurity incorporation profiles, etc. Changes could either occur to the mobility p. or/and to the lifetime x of the majority and minority carders. In order to explain the experimental data given in Fig.2 we assume that Y~p.x(z) varies exponentially with z according to: Zrlp.X(z) = Z° (1 - a e-kz)

(7)

The measured Z'qp.x(d) will also have to be weighted over the thickness d: .d

ZrIW~(d) =

a k Z ° e~Xd e-kd w(z) E'qwc(z) dz = E o + - -

k-ct

(8)

1 - e -ad

From Fig.2 one can see that a good fit to the experimental values is obtained for a characteristic decay length of k-1=3 p.m (all other fitting parameters are listed in the figure caption). This length is considerably larger than the thickness do of the high defect density layer (see Fig.l), it is, however, comparable with values found in 5. The agreement between theoretical and experimental data consequently supports the hypothesis of a gradual variation of material properties along z. 4.CONCLUSIONS From comparison of PDS spectra with photoconductivity data as function of film thickness we find experimental evidence for the presence of material inhomogeneities along the film growth axis z. The mobility-lifetime product measurement has revealed a characteristic decay length for this variation of the order of - 3 p.m. An important observation is that these inhomogeneities cannot be 'seen' with PDS, in fact the characteristic decay length do for the defect density distribution is of the order of -200 A, i.e. a factor 100 to 200 smaller. At present there is no clear understanding as to the origin of these inhomogeneities. They may, however, be related to problems of initial nucleation mechanisms, stress and/or structural relaxation, impurity profiles, etc.. Also, it is not known here, whether the changes in the mobility.lifetime product are due to changes in the mobility and/or the lifetime. Additional, complementary, measurements (of transport, structural and compositional parameters) on the samples discussed in this study will be needed to clarify this question. REFERENCES 1) Z E.Smith, et.al. Appl. Phys. Lett. 50(1987)1521 2) W.B.Jackson, et. al. Appl. Phys. Lett. 42(1983)105 3) F.Boulitrop, et. al. J. Appl. Phys. 58(1985)3494 4) H.Curtins, et.al. Proc. IEEE Photovoltaic Specialists Conf. New Orleans, May 4-8, 1987 5) W.E.Spear, J. of Non-Cryst. Solids 59(1983)1 6) H.Curtins, et al. Electronics Lett. 23(1986)228 7) H.Curtins, et. al. Proc. MRS Conf., Anaheim, CA, April 21-23, 1987 8) W.B.Jackson et. al. Phys. Rev. B 25(1982)5559 9) T.Tiedje, et al. J. of Non-Cryst. Solids 77&78(1985)I031 10) M.Stutzmann, et. al. Rev. B 32(1985)23 11) C.R.Wronski, et al. Intl. Conf. on Stab. of Amor. Si Alloy Mat. and Dev. Paolo Alto, CA, 1987 12) Y.Ziegler, University Neueh~tel, IMT, 1987 (unpublished) This work was supported by Swiss Federal Research grant OFEN(REN)85/16.