Azerbaijnn Academy of Sciences, Institute of Physics, Narimanov Prospekt 33, Baku 370143 (U.S.S.R.). (Received June 8,1983; accepted May 21,1984).
Thin Solid Films, 117(1984)181-85
81
ELECTRONICS AND OPTICS
PHOTOSTIMULATED METAL-GaSe-METAL S. N. MUSTAFAEVA
POLARIZATION AND DEPOLARIZATION THIN FILM STRUCTURES
IN
AND G. B. ABDULLAEV
Azerbaijnn Academy of Sciences, Institute of Physics, Narimanov Prospekt 33, Baku 370143 (U.S.S.R.) (Received June 8,1983;
accepted
May 21,1984)
The processes of photostimulated polarization and photostimulated depolarization were discovered in metal-GaSe-metal, thin film systems and investigated in detail. The peaks corresponding to the levels of 0.20,0.40 and 0.62 eV measured from the valence band of GaSe films were observed in the spectral curves of the photodepolarization currents. The value of the forbidden gap is found to be 2.02 eV. The experimental dependence of polarization on an external electric field and on the polarization time were studied.
The investigation of photostimulated polarization and depolarization processes in M-GaSe-M (M = metal) thin film structures is of interest from the point of view of gaining information on the energy band and the local centres in the forbidden gap as well as on the non-uniformities in the close-to-contact region. From an investigation of the electrical properties of amorphous GaSe films it has been shown that charges accumulate in the close-to-contact region of M-GaSe-M systems as a result of the applied electric field. These charges create an internal field (dark polarization) which exists after the external electric field has been turned off ‘. Heating of the polarized sample leads to leakage of the accumulated charge; this causes a thermostimulated depolarization current to flow in the circuit. The development of photosensitive GaSe films’ allows the samples to be depolarized with light (photostimulated depolarization). The high dark resistance and the photosensitivity of amorphous GaSe films led us to suggest that photostimulated polarization may take place in M-GaSe-M thin film systems. Investigations have verified this suggestion. It has been found that photostimulated polarization predominates over dark polarization for a fixed electric field. In addition, it has been shown that photostimulated polarization arises as a result of the space separation of non-equilibrium current carriers and their localization at the deep trapping levels through the influence of an electric field3. The investigations of photostimulated polarization allow us to obtain information about the parameters of trapping levels in semiconductors. The spectral curve of the photostimulated depolarization current must have several maxima corresponding to the transitions of the charge carriers from the trapping levels. The results of an investigation of photostimulated polarization and depolariza0040-6090/84/$3.00
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MUSTAFAEVA,
G. B. ABDULLMV
tion processes in M-GaSe-M systems are reported in the present paper. The samples were prepared as a sandwich structure with semitransparent aluminium electrodes. Photosensitive GaSe amorphous films obtained by the flash evaporation of p-GaSe crystals’ were used as the active elements. The specific resistance of GaSe films measured at room temperature was found to be between 5 x 10” and 1Ol3 R cm. The thickness of the GaSe films was about 1 urn. The experiments were carried out as follows. A fixed electric field was applied to the specimen in the dark for 15 min; the electric field was then turned off. The sample was subsequently short circuited and illuminated; a residual photocurrent whose direction was opposite to that due to the external electric field was found. Figure 1 shows the spectral curves of the photostimulated depolarization current in an M-GaSe-M thin film system polarized in various electric fields. Two peaks in the curves can be observed corresponding to energies of 1.62 and 1.82 eV; two steps can also be observed, one of which is responsible for the band-to-band transition (2.02 eV), whereas the other is situated at 1.40 eV. The same peaks were observed in the photocurrent spectra of amorphous GaSe films2. As is evident from Fig. 1, the step corresponding to the band-to-band transition (2.02 eV) has the lowest amplitude. This is explained by the fact that as the photon energy is increased the internal field of the Al-GaSe-Al system is decreased. The dependence of the initial photostimulated depolarization current on the polarization field at various incident photon energies is illustrated in Fig. 2. As is evident from Fig. 2, the photostimulated depolarization current increases linearly at first until it reaches the saturation point. During illumination of the polarized sample the observed photostimulated depolarization current in the system decreased slowly with time to zero. Figure 3 shows the dependence of the photostimulated depolarization current J PSDon illumination time for various polarization fields and hv = 1.68 eV. The I.0
1.5
2.0
2.5 hv,eV
4
0
4
8
12
16 I&, 10’ V/cm
Fig. 1. Spectral curves of the photostimulated depolarization current in an Al-GaSe-Al polarized at various electric fields F,, (fpD1= 15 min): curve 1, Fp., = 1.4x 10” V cm-‘; F,, = 2.8 x 104Vcm-‘;curve3, Fpo, = 7 x 104Vcm-‘;curve4,F,, = 1O’V cn-‘. Fig. 2. Dependence photon energies.
of the photostimulated
depolarization
current on the polarization
system
curve 2,
field for various
PHOTOSTIMULATEDPOLARIZATION IN METAL-G&Z-METAL
THIN FILMS
values of the charges released as a result of photostimulated determined by calculating
depolarization
83
were
With increasing polarization field F’, the charge increases linearly and reaches saturation at high values of the electric field. The maximum value of the charge density was about 6 x lo-’ C cmP2. The dependence of the charge released on illumination time is shown in Fig. 4. If the geometrical dimensions of the region of concentrated charge and the amount of charge Q (C) are known, it is possible to evaluate the trap concentration responsible for the processes of charge accumulation4: Q N, = ed, S
where e is the electron charge, d, is the region of concentrated charges (according to ref. 1, d, = low5 cm) and S is the contact area. A value of 4 x 10” cm-’ was found for N,.
Fig. 3. Tie dependence of Jme at various polarization fields F,, (hv = 1.68 eV): curve 1, F,_, = 1.4 x lo4 Vcm-i;curve2,F,,, = 2.8 x lo4 V cm-‘; curve 3, FpO,= 5.7x 104Vcm-‘;curve4,F,, = lOsVcm_i. Fig. 4. Dependence of the charge released on the time of illumination (hv = 1.94eV) for a sample polarized in various electric fields F,,: curvel,F,,=7x103Vcm-1;curve2,F,,=l.4x104Vcm-1; curve 3, FpO,=2x104Vcm-‘;curve4,F,,=2.8x1~Vcm-1.
The experimental results described are for a polarization time of 15 min. To investigate the dependence of polarization on the polarization time, an external electric field was applied to the specimen for various lengths of time. The polarized samples were illuminated with white and monochromatic light. Typical results of these studies are shown graphically in Fig. 5. The JPSD(tpol)curves are characterized by the region of current saturation in a certain time interval. The experimental data obtained can be explained in terms of the relay mechanism of charge transports. As has been shown in ref. 1, when an electric field is applied to an M-GaSe-M thin film system, the charges injected from the contact are captured by deep traps and accumulate there. From current-voltage measurements of amorphous GaSe films it
84
S. N.
MUSTAFAEVA.
G. B. ABDULLAEV
4
3
t&M 3
6
9
12
(4
,min.
0
15
I
I
IO
20
tpol. ,min
30
(W
Fig. 5. Dependence of the polarization on the time for which the fields were applied: (a) Fpl = 7 x lo4 V cm-’ (hv = 1.85 eV); (b) Fpo, = 1.4 x lo4 V cm-’ (white light).
was detected that, on the application of a constant electric field, the current through the system falls with time. The number of charges captured by the traps depends on the number of non-equilibrium current carriers and the density of the trapping levels. After the external electric field has been turned off, the charges captured by the deep traps create an inner electric field of opposite polarity’. In Al-GaSe-Al systems the charges were released by light. Levels with activation energies of 0.20, 0.40 and 0.62 eV which were observed in the spectra of photostimulated depolarization currents were found also in the temperature dependence of the conductivity of amorphous GaSe films and in the photocurrent spectra of these samples2. The experimental data described are for Al-GaSe-Al samples polarized in the dark. As has been noted, photostimulated polarization was found in the indicated systems. Figure 6(a) shows spectral curves of the photostimulated depolarization currents for the sample polarized in the dark (curve 1) and in monochromatic light (curves 2-4). It must be noted that with increasing polarizing electric field the difference between the values of the dark and photostimulated polarizations decreases. At FPo, > 5 x lo4 V cm -i the amount of dark polarization was greater
(4
(b)
Fig. 6. Spectral curves of photostimulated depolarization currents for the sample polarized at two (curve 1, in the dark; curve 2, hv = 1.68eV; curve 3, electric fields: (a) F,, = 1.4x lo4 Vcm-’ hv = 1.88eV; curve 4, hv = 2.02eV); (b) F,,,,, = 5.7x lo4 Vcm-’ (curve 5, in the dark; curve 6, hv = 1.68 eV).
PHOTOSTIMULATED
POLARIZATION
IN METAL--G&k-METAL
THIN FILMS
85
than that of photopolarization (Fig. 6(b)). As is seen from Fig. 6, the same peaks are revealed from the spectra of photostimulated polarization currents for samples polarized in the dark and with light. The experimental dependence of polarization on an external electric field and on the polarization time is in good agreement with the previous resultsl. Thus, the same levels with energies of 0.20,0.40 and 0.62 eV measured from the valence band are responsible for the processes of conductivity, charge accumulation (polarization), photoconductivity, photostimulated polarization and photostimulated depolarization observed in Al-GaSe-Al thin film systems. REFERENCES
1 2 3 4 5
G. B. Abdullaev, B. G. Tagiev, S. N. Mustafaeva, I. A. Gasanov and E. N. Ibragimova, Thin Solid Films, 76(1981) 163. G. B. Abdullaev, B. G. Tagiev, S. N. Mustafaeva and G. M. Mamedov, Izo. Akad. Nauk AZ. S.S.R., I (1978) 36. V. M. Fridkin, Physics of Eiectrophotographic Processes, Moscow, 1966: G. S. Nadkarni and J. G. Simmons, Phys. Rev. B, 7(8) (1973) 3719. B. L. Timan, Fiz. Tekh. Poluprovodn., 7 (1973) 225.
