Nature of deep defects in bulk VHF-GD a-Si:H

JOURNA L OF

Journal of Non-Crystalline Solids 137&138 (1991) 335-338 North-Holland

NON-CRYSTALLINE SOLIDS

NATURE OF DEEP DEFECTS IN BULK VHF-GD A-SI:H M. FAVRE, A. SHAH, J. HUBIN, Institute of Microtechnology, University of Neuchfitel, Breguet 2, CH-2000 Neuchfitel E. BUSTARRET, M.A. HACHICHA, S. BASROUR BP 166, LEPES, CNRS, 38042 F-Grenoble

The authors report on the comparison of defect densities measured by the two techniques, ESR and PDS, on a large range of thicknesses, 500,~_
can be considered at best only equal for thicknesses d<2~.m. The difference between the curves N~DS(d)~ and NESR(d) indicate that a significant proportion of charged defects must be present in bulk a-Si:H. They are considered to be in the D- state. Charge neutrality can be maintained by assuming positive charges, these being either trapped holes in the deep part of the valence bandtail or positively charged impurities. The dark conductivity activation energy Ea and the Rose exponent y were also measured, and the results obtained confirm the interpretation that in thin layers most defects are in the D ° state, but show that in thick layers there are in the D- state. 2. EXPERIMENTAL All films used in this study have been prepared on Dow Coming 7059 glass substrates by the VHF-GD technique described elsewhere 3 at a plasma excitation frequency of 70 MHz. The thickness of the a-Si:H films has been varied from 500 A to 80 ~tm. The deposition parameters are: substrate temperature Ts = 160 oC, silane pressure p=0.28 mbar, power density P=0.1 W/cm 3, and a silane gas flow of 18.5 sccm yielding a deposition rate of 16 to 17/~lsec. For all thicknesses the relative values of NPdDS(d) were determined using the integrated excess absorption method. In the case of ESR, the signal of the substrate is deducted from the ESR total defect density. To be able to adjust the defect density curves of ESR and PDS with respect to each other, we assume that for thin layers (d<2btm): NPDS _ 1,4ESR Photoconductivity tSph and dark conductivity d -*'d " tSd were measured in a gap configuration. For all samples with thicknesses exceeding 0.1 gm, we determined E a from the slope ~d(1/T); (T=temperature); and y from the slope of Crph(G) (G=generation rate) in a log-log plot.

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M. Favre et aL / Nature of deep defects in bulk VHF-GD a-Si:H

336 3. RESULTS

~.PDS

Fig. 1 shows the variation of " ' d and NES R a s function of thickness d. For thin films, d<0.5 p.m, both values are equal. For thicker films, NPdDs > "IN ESR d NIPDS

.

Fig. 2 shows the dark conductivity activation energy Ea and the Rose exponent Y as a function of thickness. 10 4 !z~' 7 •

'~'"1 /x ~

' I

' ''""1

'

' """1

'

''""

,.~ 10 3 A

10 2

. Azxzx~ PDS 4, A

ESR

~_~101

.

.

ESR

4.1 Determination of the curves for t'~d ano 1"4d PDS measures absorption c~(E) resulting from all transitions between occupied and empty states, separated by an energy E. A typical PDS absorption spectrum 6 can be divided into three distinct regions, as a function of energy: (ZA: absorption mainly due to band to band transitions, for hV>Egap , the gap energy; 0q3: exponential region, C~B = c%exp(-E/Eo) , mainly due to transition i n v o l v i n g the bandtails; o~C: for hv = Egap /2, we essentially have absorption due to transitions associated with deep gap states, i.e. with dangling bonds (DB's). W e can write for the absorption7: Ef

A

or(E) = cte/E

*

[ Ni(e)F i Nf ( e - E)(1-Ff)de Ef - ' E

(1)

a 10 0

i

10 -2

i,,...I

t ,ll,...I

10 -1

. . ..iJ,,I

10 0

101

10 2

t h i c k n e s s d [p-m]

FIGURE 1 Defect density in arbitrary units measured by PDS and ESR as a function of sample thickness d 0.9

O

1

Ea 7

o

0.9 0.8 O 0.8 LU= 0.7

0 C,

0.6

........

10 -2

I

.......

10 -1

ii

10 0

i

0.7

Ni,f are densities of initial and final states, Fi,f are the occupation functions of these states. Equation (1) is based on several assumptions, i. a. the one that the momentum matrix elements p2(E) are constant in the energy interval considered. We assume in the following that the function ctc(E) can be taken as a correct relative measure for the density of deep defects, even if we are not able to determine the proportionality factor precisely. We have used three different methods in evaluating o~c(E) for purposes of obtaining *rcPDs first, we have used 'd : the integrated excess absorption according to 1, but without employing the calibration factor given inl; secondly we have performed a deconvolution of the absorption spectra co(E) of equation I, according to Curtins et al 8 and have PDS .

taken the resulting peak in the DOS as measure for 1"% ;

© ,3 0

.......

I

101

i

0.6

iiiiJ

02

thickness [p.m] FIGURE 2 Activation energy Ea and Rose exponent ? as a function of thickness 4. DISCUSSION Let us first examine the confidence level we may attribute to the relative difference between the two curves of Fig. 1, as it is on this relative difference on which we base the whole interpretation of the present results. (We will refrain here from giving detailed comments on the ~TPDS - ~.ESR absolute values to be attributed to t'~d ano t'~a , as the respective calibration factors are a matter of controversy at this moment4,5).

third, we have simply taken 0c(1.2 eV), according to Wyrsch et al 4. These three procedures all give similar results within a tolerance band of + 20%. We therefore conclude that the relative variation of NPdDS with thickness is due to the difference between surface/interface and bulk defect densities 8 and not to the evaluation procedure utilized. As f o r . ~ESR , d ' it is here basically possible to obtain absolute values of defect densities, but even so this is difficult, because of the cavity fill factor 4. Therefore, even in the case o f . N, I EdS R , we concentrate only on the relative behavior as a function of thickness. Now we m u s t note that ESR only "sees" the paramagnetic centres, i.e. the neutral DB's, D °. Therefore NESR d = NDO the density of neutral DB's. On the other hand, PDS "sees" all transitions, and therefore all DB's, Thus a correct relative calibration will always have to yield

337

M. Favre et al. / Nature o f deep defects in bulk VHF-GD a-Si:H

r% we N P _eD..ESR S _. ..T

.nave somewhat arbitrarily chosen to set r~ d = ix d yor t -m. n samples, as we believe that it is reasonable to assume that almost all defects are in the D ° state as long as Ea > 0.8 eV, (see Fig. 2) This choice will be confirmed later, by separate measurements (sections 4.3 PDS

~.ESR

1

.,,,,

i,

~".,i

....

J, , ~ - ~

....

r

and 4.4). 4.2 Absolute calibration of defect densities 4,5 For information, we give the absolute ESR defect density values obtained by calibration of the sample d=2.03 gm at two different laboratories and subsequent averaging: NESR 1.5x1016 cm-3.With this reference point, the reader d = can See that absolute defect densities are obtained from Fig.l, if the scale of arbitrary units is multiplied by 1014 cm -3. With such a scale, the absolute defect densities obtained are very near to those obtained by using the calibration factor in 1. 4.3 Defect charge state, activation energy For the thicker samples (d>10 btm) of this series, we obtain from Fig. 1 (assuming that all charged DB's are D-) the ratio of the density of charged over neutral DB's: ND- 2 NDo- 1 (2) Assume now a position of the D ° level at 0.9 eV below the conduction band edge Ec, as supported by strong evidence from capacitance measurements 9. With a correlation energy EU= 0.15 eV, the theoretical defect state occupation w o u l d have, according to statistical c o n s i d e r a t i o n s 10, the form indicated in Fig.3 by the continuous line. When determining the position of the Fermi level Ef from Ea, we have to take into account the statistical shift (SS). Based on curves given by Overhof and Thomas 11, the correction due to SS is less than 0.02 eV and will therefore be neglected, by setting Ea=Ec-Ef. We are thus able to deduce the experimental occupation function from Fig. 1, and utilizing our measured values of Ea(d), plot them as experimental points in Fig.3. The correspondence between theoretical and experimental values in Fig.3 is seen to be remarkable. The resulting correlation energy EU of 0.15 eV is quite low, but such low values of EU have already been calculated 12 or measured 13 before. If a significant proportion of defects are negatively charged (and hardly any defects are positively charged), there has to be other positive charges in order to maintain local neutrality: in our view either holes trapped in the lower part of the valence bandtail, or positively charged impurities (or a combination of both). At this stage we are unable to differentiate between these two possibilities. Just

0.8 tO

0.6

"1

8 0.4

0 0.2 0

-1

-0.9

-0.8 -0.7 Ec-E f [eV]

-0.6

-0.5

FIGURE 3 Occupation function for the two charge states of Dangling Bonds (o,-) as a function of Fermi Energy, and experimental occupation (.) of D o state as an example, let us look at the case of positively charged impurities, having a density equal to the D- density, and distributed homogeniously throughout the sample. In the zone of bandbending, local neutrality is not maintained and the impurities have the role of shielding the negatively charged interface or surface. Thus Ec-Ef is large and almost all defects are neutral (D°). Then, when by increasing thickness, we approach a bulk-dominated situation, we have to populate the D- defects to compensate the charge of the impurities. 4.4 Rose exponent 7 of photoconductivity If one looks at that part of the recombination flow that commutes over the DB's, one can see that there is a direct link between "¢ and the occupation of charged DB's.To illustrate this link, let us write the photoconductivity in a classical manner 14 as: O

0

Oph= q ~nnf +q btp pf,

(3) O

O

where q is the charge of the electron, btn and btp are the band mobilities, and nf, pf the number of free electrons and holes; they can be calculated from the steady-state condition G=R (G=generation rate, R=recombination rate); for R, we shall now use an expression that contains only flow over DB's (R=RdB); this expression is obtained by taking occupation functions for DB's published recently 15 and by fomulating the capture probabilities in a classical manner (details will be published elsewherel6). We obtain 2 2 pfnf+ pfnf RdB-

2 2 (Y-+vthNdB' (4) pf + (o+/O0)nf pf + nf

338

M. Favre et aL /Nature of deep defects in bulk VHF-GD a-Si:H

pf being the density of free holes, and ~_+,~o the capture cross sections of charged and neutral DB's, respectively, vth the thermal velocity and NclBthe total number of DB's. If we now assume nf>>pf, we obtain: RdB= (~-+vthNdBnf)/(~ + p?

(5)

Thus, the photoconductivity becomes: (I -+ --"1-

classical assumption that "dangling bonds are mainly neutral" can not be maintained as a general statement. We are at this stage unable to decide whether the presence of Din samples is linked with a concentration of positively charged impurities, or not. Further study is needed on this point. Further investigation is also needed to ascertain how the assumption of a distribution of DB energies would influence our interpretation.

n- - f

Oph= go G ° ° Pf ~J+vthNdB (6) Two limit cases (a) and (b) should now be distinguished depending upon the relative importance of the two terms in the numerator of equation (6) (a) nf/pf 100). Thus, the term ~_+/~o dominates and as this term is constant, (lph is proportional to G, i.e. y = 1. (b) nf/pf is much larger than cr-+/o-°. Here, the term nf/pf dominates and 7 ~ 1. In fact, by looking at the neutrality condition, and taking into account all negative and positive charges, according to a DOS model and algebraic expressions given in 15, we can deduce that, in this case, nf/pf decreases with increasing G, according to a power law, and in such a way that y=E~ctEo= 0.6 i.e. 7 is equal to the ratio of the conduction over the valence bandtail parameters. Based on the occupation function15, one is able to associate the condition NDo >> ND- with case (a) and the condition ND>>ND ° with case (b); thus, the measured values of y shown in Fig. 2 are in accordance with the interpretation given in section 4.3 for the occupation of DB's. Note that similar results linking DB occupation with the Rose exponent 7 are found numerically by Jousse and Vaillant18 and Morgado 19. 5. CONCLUSIONS We have investigated an extensive series of a-Si:H samples, deposited by VHF in our laboratory. We have shown that in the thicker (d>10gm) layers, most of the bulk dangling bonds (DB's) are in a diamagnetic, i.e. charged state. These DB's are found to be negatively charged (D-). The presence of a substantial proportion of D- is linked to low values of the dark conductivity activation energy E a and the Rose coefficient y of photoconductivity. These results are obtained on layers produced by VHF deposition, and the conclusions may be different for other deposition methods. At any rate, the

ACKNOWLEDGEMENTS This work is funded by the Swiss Federal Research Grant OFEN EF-REN 90 (045) REFERENCES 1. 2.

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