Lateral photovoltaic effect on amorphous silicon junctions

Journal of Non-Crystalline Solids 97&98 (1987) 1363-1366 North-Holland, Amsterdam

1363

LATERAL PHOTOVOLTAIC EFFECT ON AMORPHOUS SILICON JUNCTIONS

SU ZIMIN, LIU Jingxi, PENG Shaoqi and FEI Qingyu Department of Physics, Zhongshan University, Guangzhou, China

Lateral photovoltaic effect (LPE) in some amorphous silicon junctions was investigated. It is demonstrated in both theoretical analysis and experimental results that the lateral photovoltage (LPV) is proportional to the light intensity and decays exponentially with the distance to the light spot position for the small signal case, but is related to the inversion of the light intensity and to the light spot position in a logarithmical function for the large signal case.

i. INTRODUCTION Lateral photovoltaic effect was first observed by Schottky I in 1930, and rediscovered by Wallmark 2 in 1957. device applications, previous LPE ductors. ctures,

Since then, this effect, as well as its

has been extensively investigated by many authors 3.

studies,

The

however, were mostly confined to crystalline semicon-

In this work, we will investigate the LPE in amorphous in view of the advantages of the application of amorphous

silicon strusilicon to

LPE device, compared to that of the conventional crystalline silicon,such as low cost and ease of fabrication in large area.

2. EXPERIMENTAL Each of the samples used in this study

is constructed by an undoped a-Si:H

film (week n-type) deposited by the RF glow discharge decomposition of silane on a substrate of either doped a-Si:H or c-Si.

In particular,

detailed mea-

surement of LPV as a function of light spot position and light intensity is performed on an a-Si:H/p-c-Si sample of

rectangular shape (3Omm long by 1.3mm

wide), where the conductivity activation energy of the a-Si:H film is about 0.5ev and the resistivity of the p-c-$i substrate is 1 ~cm.

The non-uniform

illumination on the a-Si:H film is a He-Ne laser beam with wavelength of 6328~ and power variable from l hw to 2mW.

3. RESULTS AND DISCUSSION The LPE is found in all of the samples described above except the one where the substrate is of n-type c-Si.

Fig.l shows the result of lateral photovol-

tage as a function of light spot position for two incident powers measured from two A1 contacts (separated by 16mm) on the a-Si:H film of the a-Si:H/p-c-Si ~Present address: CEPREI, P.O. Box. 1252, Guangzhou, China.

0022-3093/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Z. Su et al. / Lateral photovoltaic effect

1364

sample.

The results in other samples are basically the same except for the

sign of the LPV signal, which depends on the type of substrate.

That is, for

a sample with a p-type substrate such as p-c-Si or P+-a-Si:H, the sign is such that the contact nearer to the light spot position is at a lower potential, while it is at a higher potential for an a-Si:H/n+-a-Si:H sample.

This dif-

ference in the sign of LPV signal can be easily understood from the following model.

The hole-electron pairs generated in the a-Si:H film by the incident

photons are separated by the build-in field of the junction,

with carriers of

one type remaining in the a-Si:H film and those of the other type being swept into the substrate.

This separation of carriers sets up a transversal photo-

voltage of which the magnitude varies along junction due to the non-uniform concentration of the separated charges.

Meanwhile, under the influence of the

potential difference, the majority carriers in the a-Si:H film flow away from the irradiated region until they disappear by recombination with the carriers of the opposite type through the reverse leakage current.

Since the carriers

remaining in the a-Si:H film for a sample with a p-type substrate are electrons, the potential of the a-Si:H layer is lowered by the charge separation, with the lowest point at the light spot position.

The result that no LPE is found in

a sample with an n-c-Si substrate is attributed to no efficient built-in field existing on the a-Si:H/n-c-Si junction. For the case that the sheet conductivity of the a-Si:H film satisfies os ~DC A ( ~i0-i°~ -I

),

where D is the thermodiffusivity of the carriers in a-Si:H

film and C A the capacitance of the junction, V(x), for an infinite one-dimensional

the steady state LPV distribution,

sample was described by Lucovsky's

equation 3 d2V

q

-dx~ +

Os

with the boundary

qlo6 (x)= Js

eqv/kT

i)

(I)

~(

conditions as

when x ÷ ± ~ ,

V+O

and

dV ~x÷O

(2)

where q is the internal quantum efficiency; Io the total flux of incident photons; and the origin of x is selected at the light spot position.

Integrating

eq.(1) subject to the boundary of eq.(2) leads to dV - -(2kTJs)~( T i eqv/kT- i - ~ov )~ dx q os

(for x > O)

(3)

Then, i n t e g r a t i n g eq.(1) with respect to x from - ~ to +m and using eq.(3) gives

_~ -~~Io = f+oo Js ( eqv/kT - 1 ) dx = 2 ~ so that

( e qv°/kT- 1 --q~-~ )½ BIo kT = '

.~,dV~-l~, ~Js - k,e qv/kT -i)~-~x J uv B = q~

/ (SJsos kT) ½

where V o is the photovoltage at the position of the light spot. mate solution of eqs. (3) and (5) for two limits is

(4) (5)

The approxi-

1365

Z. Suet al. / Lateral photovoltaic effect

/ZBq

foe -x/x° KT 2kT in ( i x - qBIo + ~

V(lo,x) = { where

Xo

(for V K ~ K T / 2 q ) (6) )

(for V > ~ 3 k T / q )

" q Js "

(7)

In order to compare with the above theoretical results more directly, the LPV signal is taken between one contact on the a-Si film and the substrate. Fig.2 shows (in symbols) the results of the light spot position dependence of LPV at various light intensities for the a-Si:H/p-c-Si sample. If the results -my/2 are plotted in terms of e vs. x and InV vs. x, as shown in fig.3, one finds that e -my/2 is linear with x for large signal case while for small signal case, InV is in good linearity with x, as predicted in eq.(6). ferent light intensities,

the straight lines of inV ~ x and e -my/2 ~ x shift in

parallel, with the same slopes of 2.4cm -l and 1.7cm -l , respectively. the ratio of the slopes is equal to ~ ¢ ~ retical prediction.

For dif-

Hence,

also in good agreement with the theo-

(m = q/kT)

Fig. 4(a) and (b) show the dependence of LPV on light intensity at various light spot positions for weak and strong illuminations respectively. illumination,

For weak

it is shown in fig. 4(a) that the LPV signal is proportional to the

incident light intensity. But for strong illumination,

it is shown in fig.4(b)

that the function e -my/2 is linear with the inversion of the light intensity for different light spot position with the same slope of

0.O32mW.

Therefore,

the relationship between LPV and light intensity as shown in eq. (6) is also verified by the experiments. From the slopes of the straight lines in fig.3 and fig.4, and if the reasonable value of N ~ I O -2 is used 4, the physical quantities of o s and Js are _7

determined to be 4.6xi0

/~ and 7.3 lO-SA/cm ~, respectively.

Thus, the barrier

of the junction for reverse leakage current, ~ , is about 0.87ev, equal to the energy from the Fermi level to the conduction band edge of the p-c-Si substrate. Substituting these values of o s and Js into eqs.(3) and (5), the LPV distribution in the all region for various light intensities can be calculated numerically, as shown by the solid

lines in fig.l and fig.2.

In fig.l and fig.2, it is shown that the LPV signals on an amorphous con junction are large compared to those on a c-Si junction 2'4. this, we first define the light sensitivity,

sili-

For explaining

S, of a LPE device to be the ratio

of the photovoltage at the illuminated spot to the flux of incident light for weak illumination. S =

From eq.(5), we have

qq ! 2 (mJ s Os) 2

(8)

Thus, the high measured values of LPV on an a-Si:H junction can be understood theoretically as being due to the low conductivity of a-Si:H film and the high

1366

Z. S u e t al. / Lateral photovoltaic effect

barrier of an a-Si:H junction. For pratical applications,

it is usually required that LPE devices be in

high linearity with the light spot position, meter, xo, should be large enough.

that is, the spatial decay para-

This can be achieved by appropriately

creasing the junction barrier and the conductivity to

eq.(7).

However,

in the case that Xo is not much smaller than the length

of the sample, the influence ed in theoretical

in-

of the a-Si:H film, according

of the finite boundary effect should be consider-

treatment.

REFERENCES I) W.Schottky,

Phys. Z., Leipzig,

2) J. T. Wallmark,

31(1930) 913.

Proc. Inst. Radio Eng., 45 (1951) 474.

3) e.g., G. Lucovsky,

J. Appl. Phys., 31 (1960) 1088.

4) B. F. Levine, et al., Appl. Phys. Lett., 49 (1986) 1608. 0.7

,,A'~"D

o-2.0mW • -. 44row

•" .~'" 0.5

¢°

~F

///.

~0

13~

~D

I.d"

2

x (mm)

¢"

o -

-~0

--

0.1

2.0mW 98~W

i

0

I L\"

. 13.w ---

I ? ~

Theory

>

1

i

i

2

3

t

i

~

,.,': "'.

o6

j

Io (~4) (a)

FIGL~E 2 I ~ in an a-Si :H/p-c-Si ssmple, taken frcm one contact on a-Si and substrate.

i

i

7

8

0 9

~

......

-'"

>1o~o~~"''~o 6b so ~'o. x (ram)

,

4 5 6 x (mm)

FIOJRE 3 exp(-mV/2) vs. x and inV vs. x for various light intensities.

FI@jRE i LPV distribution in an a-Si:H/p-c-Si s~le, taken from t ~ contacts on a-Si:H.

80

38~W

.-

"~ 02

_~o t / y •

"-

.-,

°i!s i/Io (.A4-) (b)

FI(INE 4 Relationships between IPV and light intensity for week (a) and strong (b) illt~ninations at x = Imm (o), 2nm(.), and ~(-), respectively.