Conductivity stimulated by temperature oscillations in dissociated cadmium telluride

and cadmium sulfide solid solutions

A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin

St. Petersburg Technological Institute, 198013 St. Petersburg, Russia~Submitted October 7, 1996; accepted for publication January 28, 1997!Fiz. Tekh. Poluprovodn.31, 966–968~August 1997!

The relaxation properties of films of dissociated cadmium sulfide and cadmium telluride solidsolutions have been investigated. Conductivity stimulated by temperature oscillationswas observed. The relaxations caused by a change in the external electric field and temperaturewere studied. It was determined that residual conductivity and increasing current relaxationsare characteristic of the experimental samples. The results are interpreted in a model of aninhomogeneous semiconductor. ©1997 American Institute of Physics.@S1063-7826~97!02108-X#

In Ref. 1 , relaxational processes in CdSxTe12x layers at seemingly ‘‘switched off.’’ When the equality betweent1

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low temperatures were reported. In the present study wevestigated of the characteristic features of processes ocring at higher temperatures — above 300 K. The long-tirelaxation processes initiated in CdSxTe12x layers, subjectedto thermally activated dissociation, by a change in tempeture, illumination, and external voltage were investigated

A sharp increase in voltage produced a rapid increasthe current to a maximum value, after which the currentcreased monotonically to a minimum value and then agincreased to a steady-state value. The characteristic curise time~of the order of 1 min at 400 K! was many timeslonger than the decay time and depended exponentiallythe temperature. Short-circuiting the current after a stationcurrent was established gave rise to a monotonic decreathe current. The relaxation time of the ‘‘residual conductity’’ depended on the temperature. It could be decreasedstantially by light with photon energy\v.1.1 eV. Currentrelaxation increased sharply as a result of irradiation.

Figure 1 shows the results of an investigation of trelaxation properties arising as a result of temperature vations. Curves1 and2, obtained by heating at a constant racontain sections of a rapid increase in conductivity. Increing the heating rate shifted the onset of rapid growthhigher temperatures.

A nontrivial phenomenon was also found — conductity stimulated by temperature oscillations~CSTO!, i.e., tem-perature oscillations gave rise to a substantial increase inconductivity of the samples. The magnitude of the additioconductivity ~CSTO amplitude! was temperature-depende~curves3 and4!. Holding a sample at a constant temperatueven for a much longer period of time, had virtually no effeon its conductivity. The effect was reversible.

In order for electronic equilibrium to be established ininhomogeneous semiconductor, potential barriers musovercome. This factor accounts for the long equilibratitime and its exponential temperature dependence.2–4 When asystem in equilibrium is heated continuously, its conductity will assume a new equilibrium value only when the chaacteristic heating timet1 equals the characteristic equilibration time t between the impurity levels and the conductiband.4 At lower temperatures the effect of impurities

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andt is reached, the impurities are ‘‘switched on’’ and thfree current through the system increases sharply. Ancrease in the rate of heating decreasest1 ; a higher tempera-ture is therefore required to switch on the impurities, whiaccounts for the shift in the region of sharp current grow

Proceeding fromt15t and using the results of Ref. 4

t5t0 exp~Er /kT!,

t15Dr/u~dr0 /dT!~dT/dt!u, ~1!

we obtain the approximate equality

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~dT/dt!1T22J ~2!

whereDr is the sensitivity of the apparatus, andr0 is thequantity measured by the apparatus.

To determineEr it is sufficient to know the temperatureat which the impurities are activated,Ti , for different rates ofheating (dT/dt) i . The valueEr'0.4 eV was obtained fromEq. ~2! for the sample corresponding to Fig. 1. AccordingRef. 3 , this quantity determines the energy required for etrons to be transferred from high-resistance regions~HRR! ofan inhomogeneous system into low-resistance regi~LRR!. In the case of the experimental samples, the Hand LRR evidently correspond to switching on of a dissoated solid solution with a large and small quasigap, resptively. Surface states~SSs! with energy«s , which contributeto the formation of the potential well of the bands, apresent directly at the boundaries of these regions. Theergy diagram of the system can therefore be representeshown in Fig. 2. An external field applied to such a syst~Fig. 2b!, because of the high inhomogeneity of the systedistorts the potential well.1,5 An external field is rapidlyscreened in LRR and vice versa in HRR. As a result, eqlibrium between the electrons in the conduction band andimpurity levels« i in the bulk and the surface states«s breaksdown. Near the boundaries of the LRR, oriented towardpositive pole, the free-carrier density increases and cotions are created for trapping of carriers in«s , which givesrise to an increase in the intercrystallite barriers and there

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a decrease of the through current. On the other side ofLRR the electron density decreases. Conditions arise forization of«s . This process decreases the intercrystallite briers and increases the conductivity. If«s is greater than theamplitude of the potential well of the conduction band, ththe rate of the second process will be lower. Equilibratafter an external voltage is switched on abruptly will thencharacterized by a current which initially decreases and tincreases, consistent with our experiment. On this basis,energy calculated from the slope angle of the tempera

FIG. 1. Temperature dependences of the current densityj with monotonicheating of the sample at a constant ratedT/dt50.2 ~1! and 0.07 K/s~2! andin the presence of temperature oscillations~3! as well as the amplitudeD j / jof the conductivity stimulated by temperature oscillations~4!.

FIG. 2. Schematic representation of a band diagram of a decomposedsolution of cadmium sulfide and telluride without~a! and with~b! the actionof an external electric field.

824 Semiconductors 31 (8), August 1997

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rent increase can be interpreted as an ionization ene«s;0.95 eV. This result agrees with Ref. 6 .

The investigations of the relaxations of the ‘‘residuconductivity’’ confirm the model. Removing the externfield again changes the potential well and again the equrium between the band and the levels is disrupted. Thecalized charges which are generated by an external fieldwhich screen the quasidipoles set up the potential differeat the electrodes. We thus obtain the relaxation current wcharacteristic time determined by emptying of the traps.

The observed acceleration of the relaxation of residconductivity by light with \v.1.1 eV, i.e., with energiesclose to the experimental valueEr'0.95 eV ~ionization ofthe BS level«s), provides additional support for the mode

Let us examine the processes accompanying the tperature oscillations. A temperature change destroysLRR–HRR equilibrium. Restoration of equilibrium requirean anomalously long time. Extrapolation of the experimentemperature dependence of the characteristic time of thecreasing relaxations shows that for the experimental samat T5300 K the characteristic timet will equal tens ofhours. The large value oft explains the apparent stability othe current at relatively low temperatures. However, a lotime is required for the nonequilibrium carriers to be tranferred from HRR to LRR via an activational path, butdifferent, faster path is possible by hopping along impurlevels. The hopping relaxation mechanism is all the mlikely because II–VI layers synthesized by vacuum condsation characteristically have a high density of states inquasigap.7

On the strength of the large magnitude of the potenwell of the system, the electron hops must occur as mtiphonon processes. A necessary condition for such hopsthermal fluctuations which level the wells between whihops occur.8 Temperature oscillations promote fluctuationEvidently, this is what causes the acceleration of the relation processes, which is manifested in the form ofCSTO.

The tunneling CSTO mechanism agrees with the expmental temperature dependence of the CSTO amplitude~Fig.1, curve4!. Indeed, the experimentally recorded conductivcan be conventionally divided into two components: 1! con-ductivity after establishment of quasiequilibrium in the ba~established rapidly! and 2! additional conductivity arisingafter equilibrium is established between the conduction band the levels~this equilibrium is established slowly!. Sincethe position of the Fermi level is a nonlinear function of ttemperature, the contribution of the additional conductivincreases with temperature. Therefore, the CSTO amplitshould also increase. However, while the contribution ofadditional conductivity increases, its rise time decreasesponentially. Hence, even at the first oscillation, the expementally recorded conductivity will contain a part in the aditional conductivity that increases with temperature. Tgives rise to a corresponding decrease in the CSTO amtude. The competition between the processes is responfor the maximum in the temperature curve of the CSTO aplitude.

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824Belyaev et al.

In closing, we wish to make two remarks. The first oneosysiego

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This work was sponsored by the All-Russia Fund for

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concerns the increasing-relaxation processes. The propmodel is close to the one obtained from a numerical analof transient currents in a semiconductor with barrcontacts.9 The only difference is in the barrier-lowerinmechanism. In Ref. 9 it is assumed that only a field actsthe barriers, which is amplified by charge redistribution; tboundary states are neglected. In the model consideredthe field action is followed by a change in the barriers aresult of a change in the charge occupying the BSs. It seto us that in the systems investigated the mechanism of9 could have occurred on each reverse-biased heterojuncbut since BSs are undoubtedly present in polycrystals,field effect on the barriers should, as a rule, give rise tomechanism described above.

The second remark concerns the experimental obsetion of CSTO. As shown above, samples not only withunique potential well, but also with a high density of statesthe quasigap are required for experimental observation.dently, this is why the phenomenon described above is raobserved.

825 Semiconductors 31 (8), August 1997

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Fundamental Research under contract No. 96-02-19138.

1A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin, Fiz. Tekh. Poluprovod31, 286 ~1997! @Semiconductors31, 177 ~1997!#.

2A. Ya. Shik, Zh. Eksp. Teor. Fiz.71, 1159~1976! @Sov. Phys. JETP44,606 ~1976!#.

3A. Ya. Shik and A. Ya. Vul’, Fiz. Tekh. Poluprovodn.8, 1675 ~1974!@Sov. Phys. Semicond.8, 1085~1974!#.

4A. Ya. Vul’, Sh. I. Nabiev, and P. ya. Shik, Fiz. Tekh. Poluprovodn.11,506 ~1977! @Sov. Phys. Semicond.11, 292 ~1977!#.

5B. I. Shklovski�, Fiz. Tekh. Poluprovodn.13, 93 ~1979! @Sov. Phys. Semi-cond.13, 53 ~1979!#.

6V. A. Smentyna,Author’s Abstract of Doctoral Dissertation@in Russian#,Semiconductors Institute of the Ukrainian Academy of Sciences, K1988.

7A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin, Thin Sol. Films,158, 25~1988!.

8N. Mott and E. Davis,Electronic Processes in Non-Crystalline Material,Oxford University Press, N. Y., 1979@Russian trans., Mir, Moscow,1982#.

9B. A. Bobylev and E´ . G. Kostsov, Fiz. Tekh. Poluprovodn.23, 224~1989!@Sov. Phys. Semicond.23, 139 ~1989!#.

Translated by M. E. Alferieff

825Belyaev et al.