Photoinduced Paramagnetism of Group III
Impurities in AIVBVi Narrow-GapSemiconductors.
A. N. Vasil'ev", T. N. Voloshok*, J. K. Warchulskaf and H. Kageyama*
Low Temperature Physics Department, Moscow State University, Moscow 119899, Russia
1nternational Laboratory of High Magnetic Fields and Low Temperature, Wroclaw 53-421, Poland^Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

1Abstract. At low temperatures some ionic, covalent and mixed bonding compounds, as well as
semiconducting heterostructures and quantum wells exhibit persistent photoconductivity. This
term is used to describe the striking observation that the conductivity of these compounds and/orstructures is greatly enhanced by visible or infrared illumination and that the low resistance stateis maintained for a long time after switching off the illumination. To describe this effect invariously doped ionic-covalent semiconductors the models of repulsive barrier for both electron
emission and capture were introduced based primarily on the assumption of dopant'sdisplacement in the host's crystal lattice. Here we report on the magnetic counterpart of thisphenomenon, which however does not exactly meet the expectations based on transportmeasurements. It was found that the magnetic response of AIVBvi narrow-gap semiconductors
doped with Cm impurities possesses features of both relaxation phenomena and light-induced
phase transition. Exposure of PbTe:Ga, PbTeJn and PbojsSno^sTeiln single crystals to whitelight illumination at low temperatures resulted initially in an increase of the diamagnetic
response and then in the appearance of a sharp paramagnetic peak at heating.

The objects of the present study belong to the unique family of AiyBVi
semiconductors constituted by nine binary compounds (Ge, Sn, Pb) - (S, Se, Te) andtheir solid solutions. All lead chalcogenides as well as high-temperature phases ofgermanium telluride and tin telluride possess the rocksalt structure [1]. On lowering
the temperature GeTe at 670 K and SnTe at T < 100 K transform into therhombohedral phase through the softening of transverse optical mode in [111]direction, while lead telluride remains a virtual ferroelectric with anomalously highdielectric constants [2]. While doped with Cm elements the inherently unstable AjyBvi
semiconductors show persistent photoconductivity, which is most pronounced if theimpurity levels fall into the forbidden gap [3, 4]. In contemporary understanding [5],Cm impurities in AiyBVi matrix dissociate, 2Cm0 = Cm" + Cm+, into negatively
charged Cm" and positively charged Cm+ centers, creating therefore deep and shallow
levels in the energy spectrum of the semiconductor. The transitions between theselevels lead to the appearance of a resonant peak at the edge of fundamental adsorption
in the AiyByiiCm spectra of photoconductivity. However, the microscopic picture forCP554, Physics in Local Lattice Distortions, edited by H. Oyanagi and A. Bianconi©2001 American Institute of Physics l-56396-984-X/01/$18.00

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this deep-shallow transformation is far from being clear. Both Cm and Cm+ being insinglet states are diamagnetic, but the intermediate metastable Cm° state may carry amagnetic moment. To get additional insight into the processes of photoexcitation of
Cm dopants in AryBvi semiconductors the magnetic properties of PbTeiGa, PbTeiln
and Pbo.75Sn0.25Te:In single crystals were studied under various regimes of white-lightillumination along with the measurements of transport properties.
The resistivity of the bulk samples ( 1 x 1 x 5 mm3) was measured with the help of
the four-probe technique in a brass chamber equipped with a miniature incandescencelamp.
Its radiation parameters were chosen to fit the resonant peak ofphotoconductivity in PbTe:Ga.
At cooling in darkness the resistivity ofPbo.75Sn0.25Te:0.5at.%In and PbTe:0.3at%Ga show the semiconducting behaviour as
expected for the activation of electrons from deep levels into the conduction band.
Illumination within a few seconds (~ 20 mW/cm2) at low temperatures resulted in adecrease of resistivity by a few orders of magnitude persisting for many hours. At
heating, as shown in Fig. 1, the resistivity of the samples increased reaching
maximums at T ~ 20 K in Pbo.75Sn0.25Te:0.5at.%In and at T ~ 70 K inPbTe:0.3at%Ga. The heating of the samples under permanent illumination (~ 2mW/cm2) resulted in a decrease of the resistivity peaks, but their positions remain
practically unchanged.
The effect of persistent photoconductivity was lesspronounced in PbTe: lat.%In, since the In level in PbTe falls into the conduction band.

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Temperature (K)125150acs2 4FIGURE 1. Temperature dependencies of resistivity at heating after brief illumination at lowtemperature (solid circles) and under permanent illumination (open circles), a, Pbo.75Sno.25Te:0.5at.%rn;b, PbTe:0.3at.%Ga.

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The influence of illumination on the magnetic properties of ArvBviiCm compounds
was studied mostly by a Faraday balance technique. The construction of thisapparatus did not allow for the complete screening of the sample from the daylight.
Additional illumination of the sample was provided by the incandescense lamp
through the channel of measuring device. The sample mounted on Kevlar thread wascooled down to liquid helium temperature in the absence of a magnetic field and thenit was briefly illuminated (~ 20 mW/cm2). After that the source of additional
illumination was switched off and measurements were performed at heating with therate of about IK/min at H = 4.28 kOe. Surprisingly, the results of magnetic
measurements did not meet the expectations proceeding from the transportmeasurements.
The temperature dependencies of magnetic susceptibility of PbTe:lat.%In,Pbo.75Sn0.25Te:0.5at.%In and Pbo.75Sn0.25Te:2at%In are shown in Fig. 2. At low
temperatures the illumination of samples resulted in an increase of diamagnetic
response compared with the reference data of undoped sample of PbTe (see the insetin Fig. 2a). At heating an additional diamagnetism was lifted off and at approximately
50 K strong paramagnetic peak with sharp high temperature edge appeared. As can beseen from the comparison of curves for Pbo.75Sn0.25Te:0.5at.%In andPbo.75Sn0.25Te:2at.%In (Fig. 2b), an increase of impurity content resulted in anincrease of both paramagnetic background and paramagnetic peak.

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S*-0.60020406080Temperature (K)FIGURE 2. Temperature dependencies of magnetic susceptibility at heating after brief illumination at
low temperature, a, PbTe: lat.%In; b, Pbo.75Sn0.25Te:0.5at.%In (solid circles) and Pbo.75Srio.25Te:2at.%In(open circles). The inset to panel a shows the temperature dependence of the magnetic susceptibility ofthe undoped sample of PbTe.

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The magnetic properties of PbTe:0.3at.%Ga were studied by both Faraday balanceand SQUID magnetometer techniques. In Faraday balance measurements this sample
shows mainly the same behaviour as other ArvByiiCm compounds. The SQUID
measurements revealed another unexpected feature of paramagnetic singularity. In
this case no illumination of the sample by incandescence lamp was employed, but
prior to measurements the sample was exposed to daylight at room temperature andthen was cooled to liquid helium temperature. Once again the behaviour similar tothat described above was present, however at subsequent cooling in dark chamber of
SQUID magnetometer the paramagnetic peak was not observed at all and thediamagnetic downturn at low temperatures was substituted by a paramagnetic upturn.
The increase of the exposure to white-light illumination at room temperature resulted
finally in the appearance of both diamagnetic downturn and paramagnetic peak, asshown in Fig. 3. In general, the magnetic response of a PbTe:0.3at.%Ga single crystal
resembles the behaviour of a spin glass system, showing a paramagnetic peak in thezero field cooled regime and a smooth paramagnetic upturn in the field cooled regime.
At any temperature the magnetic response of the sample consisted of field dependent
paramagnetic and field independent diamagnetic parts, but the pronounced hysteresisof the magnetization could be observed (Fig. 4) only below the paramagnetic peaktemperature.

to-0.39

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Temperature (K)
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FIGURE 3. Temperature dependencies of magnetic susceptibility of PbTe:0.3at.%Ga both at heating(open circles) and cooling (solid circles) measured in the dark chamber of SQUID magnetometer after
different exposures to white-light illumination (texp) at room temperature, a, texp = 5 x 102 sec; b, texp = 5x 103 sec; c, texp = 1.5 x 104 sec; d, texp > 105 sec.

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In our opinion, the difference in critical temperatures for singularities in transportand magnetic properties reflects a previously hidden aspect of the interaction of lightwith AiyBvi:Cni compounds. At cooling in darkness the electrons leaving theconduction band occupy dopant centers, Cm being a substitute for metal in octahedralsurrounding of chalcogene ions and Cm+ being shifted probably in the [111] direction.
Illumination of the sample resulted in the recharging of Cm" and Cm+ centers leading
to a metastable situation where the charges of some impurities do not correspond totheir local surroundings. Different light-induced displacements of dopants could resultin the formation of an energy level either in the conduction band or in the gap. In theformer case it would result in persistent photoconductivity, in the latter case it would
lead to variations in magnetic properties. The recharging of Cm and Cm+ centers
always requires redistribution of two electrons, otherwise the illumination of sample at
low temperatures would be accompanied by a paramagnetic upturn and not by adiamagnetic downturn. The electrons at shallow impurity levels in the gap
presumably possess higher radii of localization, providing therefore the increase of
diamagnetic response at low temperatures. At heating, the probability of overcoming
the potential barrier separating stable and metastable states of impurities increases.
The relaxation of elastic stresses at any center overcoming the potential barrier isfollowed at the paramagnetic peak temperature by an avalanche-like process ofrecharging due to the proximity of the system to a phase transition. This simulation ofa phase transition is accompanied by a sharp increase of paramagnetic susceptibility inf

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FIGURE 4. Field dependence of PbTe:0.3at.%Ga magnetization at T = 5K. The inset shows thehysteresis in low magnetic field.

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the same way as it occurs at phase transitions in AiyBvi semiconductors [6]. A
remarkable feature of the obtained results is the spin glass behaviour of a system,
nominally not containing any magnetic ingredients. It means that in some crystal
environments atoms of non-transition metals may carry a magnetic moment as waspredicted in [7, 8] and, moreover, may even interact, a small number of electrons
being present in the conduction band [9].

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4. Martinez, A., Abbundi, R.J., Houston, B., Davis, J.L., and Allgaier, R.S., /. Appl Phys. 57, 11651170(1985).
5. Drabkin, LA., and Moizhes, B.Ya., "The spontaneous dissociation of impurity neutral states onpositevely and negatively charged states" in Sov. Phys.: Semiconductors 15, 625-630 (1981).
6. Lirvinov, V.I., and Dugaev, V.K., Sov. Phys.: JETP, 77, 335-343 (1979).
7. Anderson, P.W., Phys. Rev. 124, 41-53 (1961).
8. Kondo, J., Solid State Physics 23, New York: Academic Press, 1969, pp. 184-281.
9. Bastard, G., and Lewiner, C., Phys. Rev. B 20, 4256-4265 (1979).

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