PHOTOCONDUCTIVITY IN LIQUID SELENIUM

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PHOTOCONDUCTIVITY IN LIQUID SELENIUM

J. Rabit, J. Perron

To cite this version:

J. Rabit, J. Perron. PHOTOCONDUCTIVITY IN LIQUID SELENIUM. Journal de Physique Collo-

ques, 1981, 42 (C4), pp.C5-1047-C5-1050. �10.1051/jphyscol:19814229�. �jpa-00220859�

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CoZZoque C4, suppZ6ment au nolO, Tome 42, octobre 1981 page C4-l047

PHOTOCONDUCTIVITY IN LIQUID SELENIUM

J. Rabitand J.C. Perron

L. G. E. P. - E. S. E. Plateau du MouZon, 91 190 Cif-SUP-Yvette, France

Abstract.- The steady state photoconductivity is studied between 230°C and 350°C.

The electrical field dependence and the variation with the incident light flux are found to be linear. The activation energy of the reciprocal of the photocurrent varies with the photon energy only. A photoconductivity gap is linearly extrapolated and is closed to 2 eV. Transient photoconductivity measurements give a thermally activated hole lifetime. These results are analyzed by means of Onsager theory for the quantum efficiency and the other are interpreted by the model of defect,states in disordered semiconductors and compared to the data from measurements of E.S.R.

and viscosity.

Introduction.- The semiconducting properties of selenium in liquid state are now widely known. In the literature some experimental results on electronic transport or optical properties may be found. So, measurements o f the optical absorption 111, (21, electrical conductivity 131,141,(51 or electron-spin-resonance (61 are reported. RFter a long controversy about the existence of rings, the linear chain like structure of liquid selenium is now well established by experiments t7),(8iand theoretical calcu- lations 19 1, IlOl,~ll1.

In order to determine the properties of some peculiar electronic states the photoconductivity studies make up an efficient tool. The occurence of useful results was supported by the pionnering work of VENGRIS S.A. (121on transient photocort ductivity, in amorphous and liquid selenium. So, we have performed some measurements of photoconductivity both in steady state and transient experiments.

Finally a coherent model is given in order to explain the various data.

The spectral dependence is mainly controlled by the quantum efficiency, which is a n a lyzed bv means of the Onsager theory in the same way others workers explain the photogeneration in amorphous selenium 113) (141. Then the concept of defect centers in disordered materials is used to account for the temperature dependence of the steady state photocurrent and of the hole lifetime. The holes are assumed to be the majori- ty carriers. Endly, the density and the capture cross section of these defects are calculated from our results and from the existing measurements of viscosity and electron-spin-resonance in liquid selenium.

Results.- The measurements of steady-state photoconductivity have been performed in transverse and sandwich structures. The used techniques and apparatus were described earlier and elsewhere (151. Yet, the experimental difficulties have restricted our studies in the temperature range from 220°C to 350°C. Moreover, the non linear switching phenomena, occuring for an electric field value near 3 . 1 0 ~ V/cm set a limit to the usable applied voltage. Then the following results are obtained.

- ~ ~ e c t r ; c _ f ~ g ~ d - d e p e p ~ ~ ~ g g - The dark current and the photocurrent display a linear dependence [Fig. 1 ) on the electric field at all the obtainable temperatures. light fluxes and wavelengths.

- L;ght_fisx-dependence - For the used wavelengths the absorption coefficient is greater than 10~-cm'~;-thereafter all the incident light is absorbed by the sample [thicker than 3 0 pm). The measured variations are linear (Fig. 21.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814229

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C4- 1048 JOURNAL DE PHYSIQUE

- Photon . . . energy dependence - In these experiments the photocurrent is always much smaller than the dark current and can be written as :

where IA i s the photon flux,

q

the quantum efficiency, p p the hole mobility and K a constant dependent on the applied voltage and the geometrical constants only.

So, iph is independent of the optical absorption and the curves (Fig. 31 essential- ly display the behaviour o f 7

.

From these data a photoconductivity gap is linearly extrapolated (Fig. 41.

-

leryera&"'"_iepenrdence

- The reciprocal of iph is found tn be thermally activated Yet, the activation energy is dependent on the wavelength. The mobility and the lifetime are assumed independent on the photon energy. S o , q is thermally activated with a wavelength dependent activation energy. Using the values given for amorphous selenium 1 1 6 I the quantity

pp.T

p is found to be thermally activated. The activation energy is independent on the wavelength (Fig. 31 and near 0.57 eV. Endly, the photoconductivity gap is decreasing with the temperature [Fig. 41 ; the coefficient is about 9 . 1 0 - ~ eV/K a bit larger than in solids but rather normal in liquids.

The measurement of the drift mobility was the very goal of the transient experiments in sandwich structure. So, the decay of the photocurrent had to be ana-

lyzed with a known time-dependence. Non-dispersive and dispersive models have been tried but they all have failed. In fact, an exponential decay give the best fit.

Yet, the calculated time constant is mtonly dependent on temperature but also on the electric field [Fig. 151, showing a limit value as the electric field increa- ses. So, its temperature dependence has been carried out at high field. In this case the reciprocal of the hole-lifetime is thermally activated and the value of the activation energy is closed to 0.5 eV. These experiments can be performed only with a negative polarization on the illuminated electrode. Therefore we assume the electrical conductivity is mainly p-type.

Fig.1 Dark currentC1 I and photocurrent(1

0 F i g . 2 Light intensity dependence of the

dependence on electric field(F1. photocurrent I1 l.

D h

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inset: photon energy dependence of the conductivity gap.

activation energy : quantum effi- Inset: gap vs temperature.

ciency (l) ,photocurrent (21, hole life- time (3).

Discussion.- The quantum efficiency in a-Se has been extensively studied 1131,(14),(151.

In L.Se the short mean free path of the carriers (some interatomic distances1 governs the properties of 7 . In the classical picture an electron-hole shares the excess photon energy (on the bandgapl. If this excess is great enough the two carriers may be separated, if not a geminate recombinaison occurs. The Onsager theory (161. offers a good fit for the experimental results. In our case the influence of the electric field is always weak and, therefore, neglected. To perform the calculation the un- known hole mobility and phonon frequency have to be evaluated. Using likely values a fine fit of the experimental curves has been obtained. So, the physical process of photogeneration is probably well described. The photoconductivity gap found here is a little larger than the optical gap deduced from absorption measurements. The diffe- rence (0.2. 0.3 eV1 is closed to the obtained value for a.Se and must be analyzed in the same way the quantum efficiency is.

The analysis of dependence of the hole life time on the electric field was not obvious because in these experiments no transit occurs. The observed phenomenon is merely a transient photoconductivity one. So a numerical simulation of the pro- blem has been done involving a lot of parameters : the recombinaison rates, the existence of various traps, the diffusion coefficient etc... Finally, the variations of this time constant may result from the existence of a space charge and of the diffusion phenomenon which cannot be neglected. As a matter of fact, when the electric field is weak, an important part of the excess holes is diffusing backward the electric field and disappears at the electrode, decreasing the photocurrent value. When the electric field is strong the drift velocity is important enough to keep the holes out of the semi-transparent electrode. So, the actual hole life time is only observed at high values of the electric field. Then, -cp-l is thermally activated with an energy of 0.57 eV. One type of recombinaison centers being assumed 'tp-l is propor- tionnal to their density nr, which is, therefore thermally activated. This activation energy must be compared to the data given by some other techniques. By viscosity RIALLAND [8]gives 0.5 eV and by E.S.R. KONINGSBERGER [6]gives 0.63 eV. The viscosity is dependent on the mean chain levgth i.e the number of chain ends and the E.S.R.

is dependent on the non-paired electrons which may exist only on the neutral defects C; or C; [with the KASTNER1s notationl. Following VANDERBILT 0. [17]. the occurence of C; centers is greater than the C S ones. The neutral defects are unstable, so in L.Se we assume an equilibrium between the C?. Cl- et C3+ centers. The negative de-

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C4- 1050 JOURNAL DE PHYSIQUE

I ^I l I I

1 2 3 F[kV/cml

Fig.5 Electric field depennence o f the lifetime

T

-

Î Î Î

-

fects Cl- are identified as the recombinaison

Conclusion.- The originai results on photoconductivity in liquid selenium described here and the used theoretical models show clearly the great similarity between the electronic transport properties of amorphous and liquid selenium. The coherence of our results with the measurements of viscosity and E.S.R. is an evidence of the efficiency of such a description. Yet a lot of more accurate results have to be obtained in particular the impurity influence, must be checked in order to clarify the transport properties in liquid selenium.

P S

References.-

( 1 1 SIEMSEN K.J., FENTON E.W., Phys. Rev.

=,

3 (19671 632.

(21 RABIT J., PERRON J.C., Phys. St. Sol.(bI

65

(19741 255.

131 MAHOJURI F., J. Phvs. C (Sol. St Phvs.1 8 (19751 2248. - (41 BAKER E.H., J. ~ h e k Soc. A (19671 1558.

-

surements (i.e 1018 an effective capture cross-section is obtained ( I O - ~ ~ cm2)

This value is weak for an attractive recombinaison center but assuming a negative

5 correlation energy on Cq [18], the negative

defects Cl- may be much like repulsive than attractive centers for the holes.

L.Se 230'C

(51 ABOINOV 0. Sh., NAMAZOV V.R., ALLIEV G.M. zr. Akad. Nauk SSSR Neorganicheskice materialy

10,

1 1 (19741 1960.

(61 KONINGSBERGER O.C., VAN WOLPUT J.H.M.C., RIETER P.C.U., Chem. Phys. Lett.

2,

¹

(19711 145.

(71 MISAWA N., SUZUKI K., Trans. J.T.M.

18

c19771 427.

(81 RIALLANO J.F., PERRON J.C., Proc. 6th Int. Conf. Am. Liq. Semiconductors (19761 371.

(91 LONG N., GALLISON P., ALBEN R., CONNELL G.A.M., Phys. Rev. B,

13,

4 (19761 1821.

(101 MISAWA. M. SUZUKI K., J. Phys. Soc. Japan 445 (19781 1612.

(111 MALAURENT J.C., OIXMIER J., Proc. 8th Int. Conf. Am. Liq. semiconductors (19791 1227.

[l21 VENGRIS S.A., VISCHAKAS Yu. K., SAKALAS A.P., YUSJKA G.B., Sov. Phys. Semicond., 6, 6 (19721 903.

(131 PAI D.M., ENCK R.G., Phys. Rev. - B

11,

12 (19751 5163.

(141 ENCK R.G., PFISTER G., In photoconductivity and related phenomena - Ed. by MORT J. and PAI O.M. - Elsevier (19761 216.

(151 RABIT J. Thesis. Paris 1981.

(161 KNIGHTS J.C., OAVIS E.A., J. Phys. Chem. Solids, 35, (19741 543.

(171 VANOERBILT 0.. JOANNOPOULOS J.O., Phys. Rev. B 22,(61 (19801 2927.

(181 ANOERSON P.W., Phys. Rev. Lett.

34,

15 (19751 953.

centers for the excess hole and using the evaluated density from viscosity and E.S.R. mea-