Origin of lateral photovoltage in hydrogenatedamorphous silicon and silicon germanium thin films

Alok Srivastava a, P. Agarwal b, S.C. Agarwal a,*

a Department of Physics, Indian Institute of Technology, Kanpur 208 016, Indiab Department of Physics, Indian Institute of Technology, Guwahati 781 039, India

Abstract

Large lateral photovoltages (LPVs) have been measured in hydrogenated amorphous silicon and silicon germanium

thin films. We find that LPV increases upon light soaking (LS) the samples. The diffusion lengths of photocarriers have

been measured by the steady state photocarrier grating (SSPG) technique in annealed and light soaked states. Our study

shows that the knowledge of carrier diffusion lengths and photoconductivity is not sufficient in understanding LPV and

that it can be explained in terms of potential fluctuations present in the sample. � 2002 Elsevier Science B.V. All rights

reserved.

PACS: 72.20.Jv; 72.40.+w; 73.50.Pz; 73.61.)Jc

1. Introduction

Lateral photovoltage (LPV) appears as an opencircuit voltage between two coplanar electrodes onhydrogenated amorphous silicon (a-Si:H) filmsupon illumination by a laser between the elec-trodes. A large LPV, observed in a-Si:H, is oftechnological interest as it indicates the possibilityof using a-Si:H films as large area position sensi-tive detectors. There have been several reports onthe measurement of LPV in a-Si:H p–i–n devices[1–3]. LPV has also been studied earlier on othersystems and device structures [4,5].Our observation of LPV between two coplanar

electrodes on a-Si:H films [6,7] is interesting since

we do not require a p–i–n device structure andLPV is as high as 100 mV in some cases. One maytry to understand LPV as follows: when the sampleis illuminated, a diffusion of photocarriers occursfrom the illuminated to the dark region. Electronsand holes having different diffusion lengths setupdifferent charge profiles and if the illuminated re-gion is closer to one electrode than the other, LPVwill be observed. LPV calculated by assumingpossible charge profiles for electrons and holesusing diffusion lengths obtained from the steadystate photocarrier grating (SSPG) [8] technique,turns out to be much smaller than the experi-mental values.Although the diffusion length decreases after

light soaking (LS) as expected [8–10], we havefound that LPV increases which is counterintu-itive. Thus LPV is an interesting phenomenon tostudy.

Journal of Non-Crystalline Solids 299–302 (2002) 430–433

www.elsevier.com/locate/jnoncrysol

* Corresponding author.

E-mail address: sca@iitk.ac.in (S.C. Agarwal).

0022-3093/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0022 -3093 (01 )01176 -0

2. Experimental

Hydrogenated amorphous silicon and silicongermanium films were deposited on Corning 7059substrates by rf glow discharge decomposition.The density of states was obtained by con-stant photocurrent method (CPM). LS was doneby exposing the samples to heat filtered light(� 100 mW=cm2) for 24 h in vacuum. Two copla-nar nichrome electrodes, 5 mm long and 20 mmapart, were used for LPV measurements whereasfor the diffusion length measurement by SSPG, theelectrode separation was about 1 mm.The schematic setup for LPV measurement is

shown in Fig. 1. A small area of nearly 3 mm di-ameter is illuminated using a diode laser ðk ¼637 nmÞ of about 3 mW power. LPV is measuredbetween the electrodes A and B using an elec-trometer. The illumination of electrodes is avoidedby covering them with insulated metal strips.Measurements are done in vacuum chamberequipped with a quartz window and heating ar-rangement.

3. Results and discussion

The ambipolar (hole) diffusion length L, in theannealed state, obtained from SSPG [8] for the a-Si:H and a-SiGe:H samples are 0:164� 0:002 and0:074� 0:009 lm, respectively. Upon LS, thesechange to 0:121� 0:009 and 0:033� 0:044 lm,respectively. 1 This decrease in L after LS is as

expected and is attributed to the increase in thedensity of localized states upon LS [8–10].Fig. 2 shows LPV as a function of position ðxÞ

of the laser on the a-Si:H sample. LPV increasesfrom its value 25 mV at x ¼ 0:5 cm to about 75mV after LS. Similar trend is seen in LPV on the a-SiGe:H sample shown in Fig. 3. LPV increasesupon LS and it goes from 7.5 mV to about 15 mVat x ¼ 0:5 cm 2. Moreover, the electrode closer tothe light spot is always negative.Photocarriers diffuse from the illuminated to

the dark region and produce charge profiles whichdepend on their diffusion lengths. It is, therefore,possible to estimate LPV which is the potentialdifference between the electrodes situated at un-equal distances from the illuminated spot. As-suming exponential charge profiles with diffusionlengths Le and Lh for electrons and holes, respec-tively, one can find the difference in potentialVB � VA, which is the measured LPV. In one di-mension we obtain

VB � VA ¼ LPV

¼ NeðL2h � L2eÞ4p�0�

1

ðsþ xÞ3

(� 1

ðs� xÞ3

);

ð1Þwhere 2s is the electrode separation and x is thedistance of light spot from the center of the

Fig. 1. Schematic setup for the LPV measurement. The vertical

arrow is the laser beam incident upon the a-Si:H film. The black

bands A and B are coplanar electrodes covered by insulated

metal strips to avoid contact illumination.

1 The L� DL is estimated by the least square fitting of theSSPG data to the theoretical expression of [8].

Fig. 2. LPV measured at a 371 K before and after LS on a-Si:H

showing an increase upon LS. Lines drawn are guide to the eye.

2 The error bars in LPV data are smaller than the symbol

size.

A. Srivastava et al. / Journal of Non-Crystalline Solids 299–302 (2002) 430–433 431

film [11]. Further, N is the total number of photo-generated carriers, responsible for the electrostaticpotential. The 1=x3 dependence can be easily un-derstood if we note that the total photogeneratedcharge is zero and produces a symmetric profileafter diffusion, thus making the quadrupole termas the most dominant in the multipole expan-sion. In a-Si:H, Le � 10Lh ¼ 10L (L obtained fromSSPG). For a typical calculation we take L ¼0:1 lm; s ¼ 9 mm and dielectric constant of a-Si:H� ¼ 12. From the photoconductivity of our sam-ples we estimate N 6 4� 1011 including the lo-calized charge. The LPV expected from thiscalculation is shown in Fig. 4.A comparison between Figs. 2 and 4 shows that

the calculated LPV is much smaller than the ex-

perimental values. Another issue that this analysiscannot resolve is the effect of LS on LPV. Since thediffusion length of carriers (measured by SSPG)and photoconductivity decrease upon LS (Stae-bler–Wronski effect), we expect LPV to decrease.The observed opposite trend thus indicates thatthe present considerations are inadequate to ex-plain our results.Although we have tried to prevent illumination

of electrodes by carefully covering them, we can-not completely rule out the possibility of a smallintensity reaching there. If so this needs to be in-vestigated further. Here we take the view that thisdoes not contribute significantly to LPV and sug-gest that the potential fluctuations arising from theheterogeneities in a-Si:H are responsible for thenature of LPV as observed. The potential fluctu-ations help in (i) separating the charges thusmaking it difficult for them to recombine and (ii) inaccumulation of charges in the localized stateswhich may coexist above the percolation edgealong with the extended states [12]. The charges inthe localized states above the percolation cannotbe taken into account by photoconductivity as itsees only carriers in the channel states [12]. Thuspotential fluctuations may lead to larger diffusionlength (L) [12] and larger N. The L measured bySSPG is in the presence of a strong bias light [8]which reduces the effect of potential fluctuations.Since LS is known to increase potential fluctu-

ations [13,14], the resulting increase in the LPVafter LS is understandable. Further, we obtainedmuch smaller LPV when measured in the presenceof bias light. This is as expected on the basis of ourconjecture.

4. Conclusions

Hole diffusion length (L) and LPV have beenmeasured in a-Si:H films in annealed as well as lightsoaked states. L decreases upon LS whereas LPVincreases. The magnitude of LPV and effect of LScan not be explained by simple considerations ofdiffusion lengths of carriers and their number asobtained from photoconductivity. We suggest thatour results can be explained by considering po-tential fluctuations present in this material.

Fig. 3. LPV measured at 366 K before and later LS a-SiGe:H,

LPV increases after LS. Lines drawn are guide to the eye.

Fig. 4. LPV as calculated using the Eq. (1).

432 A. Srivastava et al. / Journal of Non-Crystalline Solids 299–302 (2002) 430–433

Acknowledgements

This work is supported by Department of Sci-ence and Technology and Council of ScientificResearch, New Delhi, India.

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