Journal of the Physical Society of Japan
Vol. 70, No. 8, August, 2001, pp. 2242{2244LETTERS

Magnetic Counterpart of Persistent Photoconductivity
in Narrow-Gap Semiconductors
Alexander N. VASIL’EV*, Tatyana N. VOLOSHOK, Jolanta K. W ARCHULSKA1 and Hiroshi K AGEYAMA2
1Low Temperature Physics Department, Moscow State University, Moscow 119899, Russia
International Laboratory of High Magnetic Fields and Low Temperature, Wroclaw 53-421, Poland2
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581(Received April 19, 2001)

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 phenomenon in which the conductivity of these compounds
and/or structures is observed to be greatly enhanced by visible or infrared illumination and thelow resistance state is maintained for a long time after switching o7 the illumination. To describe
this e7ect in variously doped ionic-covalent semiconductors, models of repulsive barriers for bothelectron emission and capture were introduced based primarily on the assumption of dopants
displacement 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 AIV BVI narrow-gap semiconductors
doped with CIII impurities possesses features of both relaxation phenomena and light-induced
phase transition. Exposure of PbTe:Ga, PbTe:In and Pb0:75 Sn0:25 Te:In 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 upon heating.
KEYWORDS: narrow-gap semiconductors, persistent photoconductivity, magnetic susceptibility

The materials in the present study belong to theunique family of AIV BVI semiconductors consisting ofnine binary compounds (Ge,Sn,Pb){(S,Se,Te) and theirsolid solutions. The appearance of a narrow gap in theirenergy spectrum stems genetically from the Peierls
splitting of quasi-one-dimensional zones formed by porbitals of metals and chalcogenes.1) All lead chalcogenides as well as high-temperature phases of germanium
telluride and tin telluride possess a rocksalt structure.2)
On lowering the temperature, GeTe at 670 K and SnTe
at T  100 K transform into the rhombohedral phasethrough the softening of the transverse optical mode in
the [111] direction, while lead telluride remains a virtual
ferroelectric with anomalously high dielectric constants.3) When doped with CIII elements, the inherently
unstable AIV BVI semiconductors show persistent photoconductivity, which is most pronounced if the impurity
levels fall into the forbidden gap.4,5) In the contemporary
understanding,6) CIII impurities in the AIV BVI matrix
dissociate, 2CIII 0 ¼ CIII  þ CIII þ , into negatively charged
CIII  and positively charged CIII þ centers, creating deepand shallow levels in the energy spectrum of thesemiconductor. The transitions between these levels leadto the appearance of a resonant peak at the edge offundamental adsorption in the AIV BVI :CIII spectra ofphotoconductivity. However, the microscopic picture forthis deep-shallow transformation is far from clear. BothCIII  and CIII þ being in singlet states are diamagnetic,
but the intermediate metastable CIII 0 state may carry amagnetic moment. To gain additional insight into theprocesses of photoexcitation of CIII dopants in AIV BVI*Correspondence and requests for materials should be addressed toA.N.V. E-mail: vasil@lt.phys.msu.su

semiconductors, the magnetic properties of PbTe:Ga,
PbTe:In and Pb0:75 Sn0:25 Te:In single crystals werestudied under various regimes of white-light illumination
along with the measurements of transport properties.
The resistivity of the bulk samples (1  1  5 mm3 )was measured using the four-probe technique in a brasschamber equipped with a miniature incandescence lamp.
Its radiation parameters were chosen to Et the resonantpeak of photoconductivity in PbTe:Ga.7) On cooling in
darkness, the resistivity of Pb0:75 Sn0:25 Te:0.5 at.%In andPbTe:0.3 at.%Ga show 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 ina decrease of resistivity by a few orders of magnitude
persisting for many hours. Upon heating, as shown inFig. 1, the resistivity of the samples increased, reachingmaximums at T  20 K in Pb0:75 Sn0:25 Te:0.5 at.%In andat T  70 K in PbTe:0.3 at.%Ga. The heating of thesamples under permanent illumination (2 mW/cm2 )resulted in a decrease of the resistivity peaks, but theirpositions remain practically unchanged. The e7ect ofpersistent photoconductivity was less pronounced in
PbTe:1 at.%In, since the In level in PbTe falls into theconduction band.8)
The inIuence of illumination on the magnetic properties of AIV BVI :CIII compounds was mainly studied by aFaraday balance technique. The construction of thisapparatus did not allow for the complete screening of thesample from daylight. Additional illumination of thesample was provided by the incandescense lamp throughthe channel of the measuring device. The sample,mounted on Kevlar thread, was cooled to liquid helium2242

201015202530

3540

6 b4

a-0.400.4

b2-0.42
00.2

50

1100

0.0

0-0.2-1-0.4-2-0.6-320100 200 300

020406080

Temperature (K)255075100 125
Temperature (K)150Fig. 1. Temperature dependencies of resistivity upon heating after
brief illumination at low temperature (solid circles ) and underpermanent illumination (open squares ). (a) Pb0:75 Sn0:25 Te:0.5 at.%In; (b) PbTe:0.3 at.%Ga.

temperature in the absence of a magnetic Eeld and thenit was brieIy illuminated (20 mW/cm2 ). After that,the source of additional illumination was switched o7and measurements were performed upon heating at therate of about 1 K/min at H ¼ 4:28 kOe. Surprisingly,
the results of magnetic measurements did not meet theexpectations suggested from the transport measurements.
The temperature dependencies of magnetic susceptibility of PbTe:1 at.%In, Pb0:75 Sn0:25 Te:0.5 at.%In andPb0:75 Sn0:25 Te:2 at.%In are shown in Fig. 2. At low
temperatures, the illumination of samples resulted in an
increase of diamagnetic response compared with thereference data of an undoped sample of PbTe [see theinset in Fig. 2(a)]. Upon heating an additional diamagnetism was removed and at approximately 50 K, astrong paramagnetic peak with a sharp high temperatureedge appeared. As can be seen from the comparison ofcurves for Pb0:75 Sn0:25 Te:0.5 at.%In and Pb0:75 Sn0:25 Te:2 at.%In Fig. 2(b), an increase in the impurity
content resulted in an increase in both paramagnetic
background and paramagnetic peak.
The magnetic properties of PbTe:0.3 at.%Ga werestudied by both Faraday balance and SQUID magnetometer techniques. In Faraday balance measurements,
this sample shows mainly the same behavior as other
AIV BVI :CIII compounds. The SQUID measurements
revealed another unexpected feature of paramagnetic
singularity. In this case no illumination of the sample byan incandescence lamp was employed, but prior to themeasurements the sample was exposed to daylight atroom temperature and then was cooled to liquid heliumtemperature. Once again the behavior similar to that
described above was present, however upon subsequentFig. 2. Temperature dependencies of magnetic susceptibility uponheating after brief illumination at low temperature. (a) PbTe:1 at.%In; (b) Pb0:75 Sn0:25 Te:0.5 at.%In (solid circles ) andPb0:75 Sn0:25 Te:2 at.%In (open circles ). The inset in panel (a) shows
the temperature dependence of the magnetic susceptibility of theundoped sample of PbTe.

cooling in a dark chamber of the SQUID magnetometer,
the paramagnetic peak was not observed at all and thediamagnetic downturn at low temperatures was replaced
by a paramagnetic upturn. The increase of the exposure
to white-light illumination at room temperature resulted
Enally in the appearance of both diamagnetic downturnand a paramagnetic peak, as shown in Fig. 3. In general,the magnetic response of a PbTe:0.3 at.%Ga single

-0.39

-603

0.6

-64

Magnetic susceptibility (10 emu/g)

6 a2243

Magnetic susceptibility (10 emu/g)

Resistivity (10-2cm)

Resistivity (10 -5cm)

LETTERSa-0.39

-0.40

-0.40

-0.41

-0.41

-0.42

-0.42

b-0.43
-0.38
-0.39
-0.40
-0.41

c-0.25 d-0.30
-0.35
-0.40
-0.45

-0.42

-0.50

-0.43-0.55
0 20 40 60 80100
0 20 40 60 80100
Temperature (K)Fig. 3. Temperature dependencies of magnetic susceptibility of
PbTe:0.3 at.%Ga both upon heating (open circles ) and cooling(solid circles ) measured in the dark chamber of SQUID
magnetometer after di7erent exposures to white-light illumination(texp ) at room temperature. (a) texp ¼ 5  102 sec; (b) texp ¼
5  103 sec; (c) texp ¼ 1:5  104 sec; (d) texp 105 sec.

Magnetization (10 -3emu/g)

2244

LETTERS

0-10-2042-30

0-2-40 -4-6 -4 -2 0 2 4 6-100102030
Magnetic field (kOe)

40Fig. 4. Field dependence of PbTe:0.3 at.%Ga magnetization at
T ¼ 5 K. The inset shows the hysteresis in a low magnetic Eeld.

crystal resembles the behavior of a spin glass system,
showing a paramagnetic peak in the zero-Eeld-cooled
regime and a smooth paramagnetic upturn in the Eeldcooled regime. At any temperature, the magnetic
response of the sample consisted of Eeld-dependent
paramagnetic and Eeld-independent diamagnetic parts,
but the pronounced hysteresis of the magnetization
could be observed (Fig. 4) only below the paramagnetic
peak temperature.
In our opinion, the di7erence in critical temperatures
for singularities in transport and magnetic properties
reIects a previously hidden aspect of the interaction oflight with AIV BVI :CIII compounds. Upon cooling indarkness, the electrons leaving the conduction bandoccupy dopant centers, CIII  being a substitute for metalin the octahedral surrounding of chalcogene ions andCIII þ being shifted probably in the [111] direction.
Illumination of the sample resulted in the recharging of
CIII  and CIII þ centers, leading to a metastable situation
where the charges of some impurities do not correspondto their local surroundings. Di7erent light-induced
displacements of dopants could result in the formationof an energy level either in the conduction band or in thegap. In the former case, it would result in persistent
photoconductivity, and in the latter case, it would leadto variations in magnetic properties. The recharging ofCIII  and CIII þ centers always requires redistribution of

two electrons, otherwise the illumination of a sample at
low temperatures would be accompanied by a paramagnetic upturn and not by a diamagnetic 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. Upon heating, the probability of overcoming the potential barrier separating stable andmetastable states of impurities increases. The relaxationof elastic stresses at any center overcoming the potential
barrier is followed at the paramagnetic peak temperatureby an avalanche-like process of recharging due to theproximity of the system to a phase transition. This
simulation of a phase transition is accompanied by asharp increase in paramagnetic susceptibility in the same
way as one occurs during phase transitions in AIV BVI
semiconductors.9{11) A remarkable feature of the obtained results is the spin glass behaviour of a system
nominally not containing any magnetic ingredients. Thismeans that in some crystal environments, atoms ofnontransition metals may carry a magnetic moment as
was predicted in refs. 12 and 13 and, moreover, mayeven interact with each other, a small number ofelectrons being present in the conduction band.14)
We thank B. A. Volkov and D. I. Khomskii fordiscussions on theoretical aspects of the observed
phenomena.
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Vol. 23, p. 184.
14) G. Bastard and C. Lewiner: Phys. Rev. B 20 (1979) 4256.