RARE-EARTH SEMICONDUCTORS STUDIES IN THE SOVIET UNION

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RARE-EARTH SEMICONDUCTORS STUDIES IN THE SOVIET UNION

I. Smirnov

To cite this version:

I. Smirnov. RARE-EARTH SEMICONDUCTORS STUDIES IN THE SOVIET UNION. Journal de

Physique Colloques, 1980, 41 (C5), pp.C5-143-C5-154. �10.1051/jphyscol:1980525�. �jpa-00219960�

JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 6, Tome 41, juin 1980, page C5-143

RARE-EARTH SEMICONDUCTORS STUDIES IN THE SOVIET UNION

I.A. Smirnov

A.F. Ioffe Physico-Technical Institute Academy of Sciences of the USSR, 194021, Leningrad, U.R.S.S.

«ésumé.- Les résultats des recherches dans le domaine de physique des Semiconducteurs des terres rares en URSS sont examinés dans un exposé. On discute les résultats ex- périmentaux des propriétés physiques et physico-chimiques dans le cas de ces subs- tances. L'attention principale est faite sur la discution des résultats expérimen- taux nouveaux pour SmS, EuO et la solution solide sur la base de celles-ci.

Abstract.- The investigations in the field of the physics of Rare-Earth Semiconduc- tors in the USSR are considered in this report. The experimental data on physical and physico-chemical properties of these materials are discussed. The main attention in the report is paid to the discussion of the new experimental results for SmS, EuO and some of their solid solutions.

During the past decade the study of Rare-Earth semiconductors (RES) has received increasing attention in the Soviet Union /l/.

The RES research has a wide geographic dis- tribution being performed in Moscow, Leningrad, Kiev, Kharkov, Sverdlovsk, Baku, Erevan, Tbilisi, Dushanbe, Makhachkala. The follo- wing RES are investigated : EuX, SmX,

E u

I-x

L n

x

X

'

S m

l - x

L n +

x

3 ( + 2 ) s

'

L n

2

X

3 '

L n

3

X

4 ' Ln„X,-Ln,X. (where X = S, Se, Te, Ln = Rare Earth Metal (REM)), LnB

c

, (Ln = Sm, Eu, Yb), polysulfides (LnS,, Ln.S

7

, Ln

3

S_), pnictides

(LnZ , Ln._Z,, Ln .Z ., where Z = P, Sb, Bi) , and complex ternary and quaternary compounds containing RE and transition metals. Inves- tigations are made on powders, poly- and single crystals and thin films. The studies comprise optical, magneto-optical, magnetic, galvanomagnetic, electrical, thermal,

thermo-electrical, acoustic, resonance, mechanical, crystallo-chemical and physico- chemical properties. The different groups study the kinetics and thermo-dynamics of crystal synthesis and growth as well as thin film preparation. They develop also the theory of RES. The two methods most widely used in the Soviet Union to determine the valence state of RE-ions (primarily Ce, Sm, Eu, Yb) in compounds are the following : I) measurement of the X-ray K-line shift / 2 / and 2) X-ray L... absorption spectroscopy /3/. Both methods give the possibility to obtain information on the valence state of

RE-ion found in the bulk of a substance and permit to use the samples in the form of powders and thin films.

Work on RES is coordinated by the section conducted by the Academy of Sciences of the USSR. T he RES Information Center attached to the A .F .Ioffe Physico-T echnical Institute issues current RES bibliography /4/ and reviews /5,6/.

The present paper reviews only a small portion of recent results obtained on RES at a number of laboratories in the Soviet Union during the years of 1978 and 1979. The "magneto-exciton" (SmS and SmS- based solid solutions) and magnetic (EuX, Ln.X„, Ln-X., LnB

fi

) semiconductors will be discussed in detail . A brief review of the recent results obtained on certain sulfides and complex RES will also be given below.

I. SmS .- The recent work on SmS has been devoted mostly to a) the more accurate definition of its energy band structure

(the determinations of the mutual location of d and s-subbands in the conduction band and the energy gap Eg between 4f-levels and conduction band), b) to the investigation of the peculiarity of semiconductor-metal phase transition under hydrostatic pressure, and finally c) to the determination of certain physical parameters of both metal -SmS (M) and semiconductor -SmS(S) modifi- cations .

la. As it was shown earlier /7/, the s-band

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980525

JOURNAL DE PHYSIQUE

of SmS(S) is located lower on the energy scale than the d-band. This conclusion was confirmed in /8/, where Hall coefficient

% and electrical conductivity a were.

measured under uniaxial pressure (fig .l) .

If the conduction band of SmS is formed by 5d (t ) orbitals of S~I '(and some orbitals of g2 ^2, /8/, then each of the three 5d(t2g)- subbands should be characterized by an anisotropic effective mass. If the conduction band has s-type, then the effec- tive mass should be isotropic. In the kinetic equation validity region the approximation of relaxation time gives :

where o,, and oL are values of electrical conductivity parallel and perpendicular to the pressure, respectively. From figure 1 it follows that A=1. Thus we are led to the conclusion that withip the experimental error the effective mass of the electrons in the conduction band should be isotropic.

Fig. 1 : Variation of electrical conducti- vity u with uniaxial pressure applied in

[loo] direction of SmS /8/ 1,2 - ^{o at}

current // P, 3,4- a at current 1 P . ^1,3-

direct, 2,4- reverse run.

2a. Few it should be consideredsthat it has been proved, that the Eg in SmS (S) is 0.2 eV (1) . Recently this fact was confir- med from measurements of X-ray K-line shift

/12,13/, photoconductivity /14/ and optical absorption /15/. The temperature dependence for X-ray K-line shift shown in figure 2 a, b reveales a continuous increase in the Sm valence value from 2 to 2.18

-1

0.2 as temperature increases from 300 to 1000 K.

Fig. 2 : a) Dependence of X-ray shift on a type of line /13/ 1- calibration curve for SM 3F3 - ~ m + 2 ~ 1 2 ; 2 - ~ m ~ at lOOOK (the shifts are given with respect to SmS at 300K);

3 - control experiment for SmS at 300K.

b) Temperature dependence of Sm K B line shift in SmS /13/.

On the assumtion of thermal excitation of 4f-electrons into the conduction band the energy for the ~ m + to ~ m transition + ~ appeared to be 0.18 + 0.01 eV /13/.

lb. An investigation of the volume variation ratio ( 'v/vo) at 300 K for SmS single crystals has been made in /16/ using a piezometer with lead as the medium for the pressure transfer. W fracture of the sample resulted from repeated direct and reverse semiconductor ( E l ) - metal (M) transition runs. Following the fourth of fifth run the ratio *V/VO (p) reached a constant value and remained unaltered in subsequent runs. The values of pressure equal to 4 -7 kbar (in the first run the pressure = 6.5 kbar) and 1.7 kbar for direct and reverse BZM phase transition, respectively, appeared to be characteristic for such a "trained" sample, which additio- nally exhibited "fine structure" in its

(1) The value of Eg - 0.22 eV was earlier obtained from data on electrical, galvano- magnetic/9,10/, magnetic/ll/ and thermal/7/

properties of SmS (S) .

phase transition (Fig -3) .

Fig .3 : Dependence of *V/VO on P for SmS single crystal (following the fifth run)/l6/.

2b. T-P phase diagram for SmS has been examined in /17/ (method of the inves- tiqation : ~iezometrv and dilatometrv up to 32 kbar, T = 77-5503). A curve of the isostructural B

+

MI phase transition has a S-shape (Fig.4) .

Fig .4 : T-P phase diagram of SmS/17/.

K - critical point, 1-equilibrium line for B+M1 transition; 2- position of PC? marking the onest of constant compressibility.

At T>300K the ratio a/d~ is positive and at T<300K - negative. It was found that the critical point occurred at 700K and

%

8 kbar. The compressibility was measured within 3 2 kbar and 298-5 23K. The boundary between M1 and Mi phases (Fig.4) was derived from a sharp change of the compressibility at Pcr . ^{(for Pcr} + 15-20 kbar the compres-

sibility became constant) . Thus the T-P phase diagram for SmS has three regions :

B - the semiconductor phase, MI - the mixed- valence metallic phase (valence of Sm ions m = 2.7), and M 2 - metallic phase with m=3.

lc. When a solid is irradiated by a short laser pulse, the resulting thermal expansion, generates ultrasonic vibrations at frequencies corresponding to the Fourier- spectrum of the laser pulse. In /18/ for the amplification of the ultrasound power the SmS metallic film has been used, which under heating had a metal- semiconductor phase transition involving a large volume change /1/. In the case that the intensity of the laser beam is sufficient to heat the film to the temperature of the phase transition there is a sharp increase in the value of ultrasound amplification, which is shown in figure 5, to be an order of magnitude greater than that for a metallic Cr film treated under the same conditions.

Fig. 5:. Dependence of ultrasound power on laser beam intensity for SmS and Cr films

(duration of laser pulse - 40 nsec, elastic wave frequency - 30 MHz) .

2c. A measurement of the linear expansion coefficient 6(T= 80-700 K) for the SmS(S) has been carried out in /19/.

Within 120-140 K there is "a step" in the temperature dependence 6 which might'be attributed to the excitation of ~ m + multi- plet levels of higher order U=1. J =2) . ^/20/

reported new data on the homogeneity range

SmS (S) and /21/ on the anomalies found in

the absolute value of SmS(S) magnetic suscep

tibility. The normal reflection spectrum of

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SmS(S) has been measured in /22/ at 300 K in the range of 0.05 - 22 eV. The result obtained for the range of 0.05 - ^{12 eV}

appeared to be in good agreement with those obtained in /23/. An absorption maximum which has been observed to occur at 14.5 eV /22/ is associated with the transfer of electrons from the deep 3s2 sheil of the sulfur to the conduction band.

The value of the plasma frequency appeared to be 2.4 eV as a result of the investigation of the thermal reflectivity of SmS metallic films, performed in / 2 4 / for the range of 1.2 - ^{3.8 eV}

3c. The rate of the metal-semiconduc- tor phase transition in thin SmS metallic films (by heating) was estimated in /25/

and appeared to be 10-~s. The threshold recording energy density (Erec ) appeared to be 0 -046 J /cm 2 . The influende of various factors on E was assessed : time and

rec .

wavelength of the recording laser pulse, film thickness and substrate materials, presence of the atmosphere, and doping. A

recording was made with a density of 3x10 ⁷ bit/cm 2 / ^{25/. In} / 26/ the data on holographic recording in SmS inetallic films is reported.

It was obtained : diffraction efficiency

- ^1-4%, resolution - 1500 line/mrn.- 2. SmS based solid solutions .- ^The

study of T-X and P-T-X phase diagrams and the determination of high and low tempera- ture critical points for Sm Gd S /27+31/

1-X X

as the valence state of Sm and Yb ions in Sml-xYbxS have been the main points of interests in the recent years.

AT-P-X phase diagram has been obtai- ned in /27/ for the Sml-xGdxS (Figs - 6 , 7) .

The line of the equilibrium : semiconductor- metal exhibited the S-shape for all compo- sitions. The line of high temperature criti- cal points was determined /27,30/. The absolute value of the critical points

decrease with the increase of the concentra- tion of Gd. At x = 0.25 and T = 100-150K there is a second low temperature critical point (K2 ) on the line of isostructural phase transition / 28/ (Fig - 8 ) .

Fig. 6 : T -P phase diagrams for Sml-xGd,S.

Solid lines - ^{B ~ M} transitions; dash-and- dot line - equilibrium line. 1 - ^{from /30/}

2 - ^{from / 27/.}

Fig. 7 : P q - X volume phase diagram for Sml-xGdxS. 1 - ^{B+M and 2} - ^M-tB transitions, 3 - line of critical points of the liquid- vapor type, 4 - isotermal sections at 77 and 300K / 27/ .

Pig. 8 : Isostructural equilibrium diagram for Sml-,GdxS /28/. K1 and K2 - ^{high and}

low temperature critical points. Pointed

lines denote direct and reverse transitions,

solid line - phase equilibrium.

The X-ray investigation of a Smo.85 GdO-15S, previously subjected to hydrosta-

- . -

tic pressure, showed that the metallic phase exists in the temperature range 160-

4 26 K / 29/. (Fig .9) .

sat

Pig. 9 : Temperature dependence for lattice constant a in Sm0.85Gdo.15S subjected to hydrostatic pressure / 29/.

The metal-semiconductor phase transition occurs at 160 and 426 K and besides the volume changes by 10 and 8%, respectively.

Some interesting data have been obtained in /12/ on the valence state of Sm and Yb ions in Sml-xYbxS using the X- ray K-line shift method. The Sm valence in this solid solution increases with x (Fig.

10) inspite of the fact that the lattice constant was proved to be almost linearly decreasing from SmS to YbS. Similar effect has been observed earlier in Sm Eu S

1-x x /32,33/. Also, a change of Yb valence

(ybf 2, 4f14

+

ybf 3 1 4f 13) has been observed at high temperatures (1000K) .

3. EuX and EuX-based solid solutions.- One of the principal tasks facing the experi-

mentalist has been to raise the Curie temperature ( TCL of EuO without changing its semiconductor properties.

Fig -10 : Variation of Sm valency (m = 2t- n )

in 3) S with x . ^1,2,3 - ^Sml-,YbxS

at 77,300 and lOOOK respectively 13, 4- Sml-xNdxS at 300K /39/; 5- Sml-xEuxS at 300K /3 2/

This problem has been discussed in a number of recent papers /34-37/. A significantly higher T C (130-150 K) has been obtained in

Sm 0 (X 6 0.08) /34-36/ and non-stoi- EU1-x x

chiometric EUO films (with exdess of Eu) /37/ without changing its semiconductor properties (Fig - 1 1) .

EUO ~ ~ - ! ~ ' u ~ r n r : p _

r.. ..s..o c 9-m'rrcn zLooa

-

iu,..Gd.O D ?-id'* m i:0.04-

- 11 ^- I....,

Tc K

P",

.

^{Y4.O a}

Fig.11 : Dependence of T c in Eul-,Ln,O on lattice constant a. A - stoichiometrlc EuO,

p %

10~0hm.cm;Eu -xSmx /34/, at point C :

x = 0.08. p

%

10j0hm .cm; Eul-XYbXO /38/.

at point B : x

=

0.053; E U ~ - ~ G ~ ~ O /6/, at point D : x = 0.04,

p

%lo-30hm.cm.

In both cases an increasedTC should be

attributed to the formation of the magnetic

JOURNAL

DE PHYSIQUE

impurity states (MIS). They occur around the Sm ions of 4f55d1- electronic configu- ration in (2) and around oxygen vacancies in the films.

In /35/ and E u ~ - ~ G ~ ~ O /40/

the presence of two Curie points : T and c 1 T (Fig .12,13) has been discovered.

c 2

c,,

^{J/,de K}

~T(H,T),K

Fig. 12 : Dependence of heat capacity (1,2) and magnetocaloric effect (3) on T in Eul-xSmxO /35/ 1-EuO, 2,3- composition with x = 0.084.

Fig. 13 : Dependence of a, T r and parama- gnetic curie- temperature e iz Eul-,~rn,0 on x /35/.

T is very close to T C for pure EuO and c1

is slightly rising with x. T shows a

c 2

( 2 ) This configuration has been proved in

/35,36/ from the data on the X-ray Llll absorption spectra.

strong dependence on the composition of a solid solution (Fig.13) . ^A following quali- tative explanation of this phenomenon could be given. For example, in the crystals of E u ~ - ~ S ~ ~ O with x < 0.08 there are the regions, which are free from the Sm impuri- ty ions - the "free phase" of EuO (3) and the regions which have magnetic quasi

5 1 molecules - MIS (Sm ions of the 4f 5d

+ 2) .

configuration and 12 nearest ions of Eu The "free phase" of EuO must be responsible

for T , while MIS must be responsible for

rn

c 1

Ic2 -

4. Ln X and Ln3X_4 .- The progress made by - 2 -3

the Soviet investigators in the study of these materials is based on the important advances made in the development of a technique which proves efficiency in prepa- ration of the large transparent Ln2X3 single crystals /41/; perfect Ln3X4 and Ln2X3 -

Ln3X4 samples.

Extensive research of the physico- chemical properties of Ln2X3 and Ln3X4 has been under way at the various laboratories of the Sovjet Union in recent years.

The X-ray investigation of Ln2S3 carried out in /42/ has yielded diffraction patterns of two types suggesting the

possibility of two isostructural series for these compounds, namely f3 and f3' (Fig -14) .

Fig. 14 : f3 and 6' Sm2S3 (from X-ray measme- ments) /42/.

In /43/ an experiment has been made with the aim of determining the homogeneity range of the y - phase of Ln2S3 - ^Ln3S4

(Ln - ^{La, Pr, Sm, Gd, Dy.)} .

+ ²

(3) In this regions prevails EZ2+Eu

interaction.

The phase diagrams for the several systems have been received in /44,46/ from the data of differential thermal and X-ray analyses. Figures 15-17 demonstrate a phase diagram of a more thoroughly studied Yb-S systems.

Pig. 15 : Phase diagram for Yb-S (0-62.5 at

% S, p = 4.5 atm) /46/.

It exhibits two features common for the Ln- chalchogen systems : an absence of compounds in the region rich in the rare earth metals and a tendency to form a large number of phases in the region of 50 - ^{75 at % of}

chalchogen. A brief discussion of the diaqram seems to be relevant.

E

- ^{Yb2S3 and}

9 - - yb3S5 (Fiq.15) have been obtained as a result of a peretechtical reaction at 1470 and 910°C, respectively; the homogeneity range for Y ~ S is : ybSOsg6 - ^YbS1 .08; Yb5S7

(Fig.16) has been obtained at 7 kbar and 1 0 0 0 ~ ~ ~ and Yb2S3 - only from a solution in molten KI . ^y - ^Yb2S3, - ^Yb2S3 - ^{(Fig .17)} ,

Yb2S3 - rhombohedral, YbS1 .7 and YbS2-x have been obtained under the high pressure

(30- 90 Kbar) and T = 1000-2000 OC.

Magnetic and electrical properties of a series of Gd2S3 samples (stoichiometric and having an excess of Gd) have been studied in a wide range of temperatures and magnetic fields /47/.

A number of recent papers have been devoted to the investigation of electrical and thermoelectrical /48/ (Ln2S3 : Ln - ^Gd,

Ho, Er, Yb, Lu, Sc), optical /49,50/

(Ln2S3 : Ln - Y , La,Tb, Dy, Ho, Er,Tm, Yb and Lu) , magnetic /51/ (Sm2S3 - Sm3S4) and thermal /52/ (La2Te3 - La3Te4, P r 2 e 3 -

Pr 3 T e 4 ) properties.

Fig. 16 : "Hypothetical" Yb-S phase diagram /46/.

Fig. 17 : P-T phase diagram for c-Yb2S3 (60 at .%S)/46/ (y - ^and

^y7

- Yb2S3 have been obtained from

E

-Yb2S3 under the conditions of high pressure and temperature)

For the first time the investigation of the optical properties of the perfect La2S3 single crystals was carried out in /49/. A window of transparency (0 -6 - ^{1 5 ~ )}

(Fig .l8) was discovered.

5.Rare Earth Hexaborides.- During the past

few years attention has been focused on Eu

hexaboride and, specifically, on its band

structure. In the model proposed by the

French school /53/ the bottom of the conduc-

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tion band of EuB is formed by 5d-6s states of Eu, while 4f76- levels of EU+ are located in the forbidden gap at a distance of about 0.3 eV from the conduction band edge.

Fig.18 : Transmission spectrum of single crystal La2S3 (thickness of plate is 315y) It appears, therefore, that the band scheme for EuB6 is the same as the one for SmS.

But the experiment showed /54/, that while

p strongly decreased in EuB6, when the pressure was applied, the semiconductor- metal phase transition did not occur up to

16 kbar (Fig.19).

change of the electronic band structure of EuB6. A measurements of X-ray K-line shift made for pressures up to 13 kbar revealed no Eu+ in EuB6 within the expe- nental error, thus we must imply that the decrease of

p

with P should be due to the altered parameters of the band structure. This conclusion was supported by the data obtai- ned in /55/ on p measured in EuB6 under the pressure up to 100 kbar. No semiconductor- metal phase transition was observed within this range of pressure. At

Q

40 kbar p was found to reach saturation (ps ) . ^{ps for EuB6}

is 1100 ykf2.cm as compared with 15pkO.cm for metallic LaB6. This fact led the authors of /55/ to a conclusion that once the pressure reaches the value of 40 Kbar, the conduction band absorbs the impurity level, which must be at 0.03 2 eV from the bottom of the conduction band. A more detailed investigation of electrical and galvano-magnetic properties of EuB6 under the hydrostatic pressure up to 7 kbar and in the temperature range of 4.2 - ^{300 K has}

been made in /56/. The investigation of magnetic and structural properties of EuB6, E u ~ , ~ C ~ ~ B ~ and E u ~ - ~ N ~ ~ B ~ has been reported in /57,58/ (Fig. 20 a, b) .

- - -

- -

-

-

- -80

-

,

_{\ -} ^{I ,}

300K-

₀ ^I ₀₂ ^{I ,}₀₄ ^{I ,}_0.6 ^I ₀₈ ^{, ~} _I0 ^,

I I 1 t I \ l ' , I , I I I I I I , , X

o

2 4 6

a

10 ~2 i4 ?.bz

Fig. 20 a) Dependence of 0 on x for Eu

Fig -19 : P/ (P) for EuB6

(

2-5) , ^{LaB6 ( 1) and} LnxB6 : E ~ ~ - ~ C e ~ B ~ / 5 8 a / , ~u~-,~d~r;~/58l!~~?

SmS

p o

- m88sured in absence of pressure; EuB6,,Cx/61/. E U ~ - ~ L ~ , B ~ / ~ ~ / . E U ~ - ~ G ~ ~ B ~ / G ~ / data for SmS taken from /59/; 1-5 - ^numbers

of samples (first symbol denotes the direct 6.The Complex Rare Earth Semiconductors.- second the reverse run) /54/. In the Soviet Union the physical and chemi- The variation of p with P should be ascribed cal properties of this class of compouads

to the appearance of ~ u + ~ i o n s or to the are mainly studied by the Baku school. Their

investigation is focused around the phase diagrams, physico-chemical, magnetic, and thermoelectric properties.

Fig. 20 b) Dependence of a on X for Eu LnxB6 1- E L ~ ~ - ~ L ~ ~ B ~ / ~ O / , . 2- rUl- hgaj

3- E u ~ - ~ M ~ B ~ /58b/; 4- E~B~-XC,AO/. 5- EuB6 .

The materials under study may be classified into four groups

1) The first group includes a) LnX - Me2X3 (Ln - ^{Eu, Yb; Me} - ^{Ga, In; X} - S, Se, T e .)

which exhibit two congruently melting

compounds of LnMe2X4 and LnMe4X7 and b) LnX

- MeX (Ln-Sn, Yb) /63-66/.

2) The second group comprises a) Ln2X3 -

B2X3 (B - Sb, Bi) with one incongruently melting compound LnBX3 and b) LnX - ^B2X3

within which two compounds LnB2X4 and LnB4X7 are formed /67-73/.

3) The third group consists of compounds of the type LnCrX3, CrLn2X4, CrLn4X7 (X -

S, Se) and LnCr 2X4 (X - S, Se, Te; Ln -

extends from Gd to Lu) /74-76/.

4) The fourth group contains the systems of the Ln203 - ^Gd2(In2) S3 type /77-78/.

Acknowledgments.- The author would like to thank I .L . Aptekar, E .Yu . ^{Tonkov, P .G} .

Rustamov, V .A. Obolonchik, A .A. Eliseev, M.H. Abdusalamova, A . A . Samochvalov, 0 .A.

Sadovskj 'a, A .A. Kaamarsin, M .F. Mboka,

L .D. Finkelstein, V.A. Shaburov for the

reprints and for the help in the preparation

of this paper.

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PHYSIQUE

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