Journal of Crystal Growth 57 (1982) 563-569 North-Holland Publishing Company
GROWTH AND PHYSICAL A.I. MERKULOV, Institute Received
15 May 1981; manuscript
DEFECT SINGLE CRYSTALS
and V.E. TEZLEVAN
of Sciences of the Moldmicm SSR, 277028 Kishinev, USSR received in final form 15 August
Results for Cd,_,Me,Cr,Se, (Me=In, Ag; O
single crystals grown by chemical transport reaction technique are from stoichiometry on electrical and magnetic characteristics have been the variable valence of chromium ions.
1. Introduction CdCr,Se, is a typical case of variable composition within a simple cubic lattice (normal spinal type) where the influences of doping level and of deviation from stoichiometric composition (two types of defect) on physical properties are comparable. Wide variations of dopant concentration strongly affect both electrical characteristics [ 1,2] and magnetic exchange character . In this paper we present the results of growth and of investigations of some physical properties of cadmium selenochromite defect single crystals.
lattice parameters depending upon the doping level (fig. 1). We have also synthesized CdCr,Se, with a deficiency and surplus of cadmium. The lattice parameter of the obtained single-phase material obtained was the same as that of the stoichiometric undoped crystals (a = 10.748 A). For the Cd , _x Me& Se, single crystals, growth was done using the chemical transport reaction technique. The polycrystalline powder was mixed with CdCl, in the appropriate mole ratio (see table 1) and placed in a silicon tube which was then evacuated and sealed off. Cadmium selenochromitk’is decomposed at high temperatures, and as a result of the interaction
2. Crystal growth Cd, _XMeXCr,Se, (Me = In, Al, Ag, Cu; 0 G x G 0.1) polycrystalline powders were synthesized from elementary components by the solid state chemical reaction technique at temperatures close to the peritectic temperature (- 1170 K) . The compounds with deviations of selenium from stoichiometry were synthesized at 1208- 12 18 K, since excess of Se pressure increases the decomposition temperature according to Kiyosawa and Masumoto [S]. Cadmium selenochromite synthesized according to the stoichiometric formula shows the presence of a second phase (3-5 CdSe), whereas the presence of excess selenium (0.02-0.05 mg/ml) allows one to obtain the single phase material with 0022-0248/82/0000-0000/$02.75
0 1982 North-Holland
Fig. 1. Dependence of the lattice parameter doping for Cd, _xMe,Cr,Se, single crystals.
ht z ) a on the level of
A. I. Merkuloo
Tabel 1 Growth conditions
for Cd, _xMe,Cr,Se,
et al. / Growth and properties
defect single ctystuls
Mole ratio of CdCl, and polycrystalline powder
Range of temperature
I : 19
Cd, _xAg,Cr,Se, x~O.005
I : 15
Cd, _ ,AI,Cr,Se, x~O.01
Cd, _xIn,Cr,Se., x~O.01
with chlorine, volatile substances of various kinds are formed. The presence of dopant metals increases the variety of materials in the gas phase. Probable chemical reactions which are supposed to take place are: Cd&(l)
+ Cd(g) --) Cd&l,(g),
+ 2 CrCl,(g)
(1) + Me,Cl,(g)
+ 2 Se,(g) + Cd, _xMe,Cr,Se,(s) +Cd,-&l,(l)
+ 4 Cl,(g),
where (1) stands for liquid, (g) for gas, and (s) for solid. For undoped materials, the transport rate was about 2 X lo-’ mole/h.
Fig. 2. Dependence of the chemical transport rate on the composition of the charge: (1) more than I at% Al, (2) more than 1 at% In, (3) undoped composition, (4) more than 0.5 at% Ag.
Fig. 2 shows the results for the dependence of transport rate on composition for various dopant metals. The presence of such dopants as indium or aluminium at a level of more than 1 at% increases the transport rate (curves 1 and 2 in fig. 2) in comparison with undoped materials (curve 3 in fig. 2). It may be that the presence of indium (or aluminium) leads to the formation of metal complexes, and these complexes may serve as catalysts for chloride formation which accelerate the chemical reaction. The presence of such dopants as silver (or copper) at a level of more than 0.5 at% reduces the transport rate, and the crystal growth eventually ceases (curve 4 in fig. 2). This phenomenon may be connected with the formation of stable silver (or copper) chlorides which do not decompose in the crystallization zone. As a result, the amount of the free transport agent in the gas decreases. Investigations of the presence of chlorine on CdCr,Se, single crystals doped with silver  support our assumption. The growth conditions for Cd, _xMe,Cr,Se, single crystals (table 1) were determined from the dependence of the material transport rate on chemical composition, ampoule cross - section, amount of transport agent, and selenium partial pressure. The length of the ampoules in all the investigations was constant (100 mm). The growth conditions for crystals with a deficiency or surplus of cadmium were analogous to those used for the crystal growth of stoichiometric single crystals. The single crystals grown by chemical transport reactions had the form of platelets with thick-
et nl. / Growth und properties of CdCr,Se,
nesses of the order of 1 mm and with a developed [ 11 l] surface. The maximum crystal size was 5-7 mm (average size was 2-3 mm). Besides the deliberate formation of nonstoichiometric crystals by growth, structural defects ‘were also formed by the annealing of undoped crystals in vacuum or in an excess of selenium at a temperature of about 800 K.
3. Electrical properties
defect single crystals
dence of the electrical conductivity on the dopant concentration (fig. 3). Such a behaviour may be connected with a limitation of the replacement of Cd by In with, at x = 0.05, the In beginning to replace chromium in the octahedral sublattice. The dependence of the lattice parameter a on dopant concentration (fig. 1) may be considered to be indirect confirmation of such a supposition. The chemical formula for such crystals may be written as: Cd , _XInXCr2(, _Z,In,,Se,,
Electrical properties of defect CdCr,Se, single crystals were investigated using the two and four probe method. Ohmic contacts were formed with a microwire, in a glass insulation, by the spark method . The microwire diameter was 20-50 pm. The characteristics of the contacts were examined by looking at the potential distribution along the sample. Undoped single crystals were of p-type conductivity and had a resistivity of - 106-10’ L? cm at 300 K. A change of conductivity type (from p- to n-type) was observed in the case of indium doping with concentrations of more than 1 at%. The electrical conductivity u of the Cd,_,In,Cr,Se, (0.01 c x Q 0.1) single crystals was characterized by a minimum in the region of the phase transition temperature (T, = 130 K) (81. The magnitude of the electrical conductivity at this minimum essentially depends on the doping level. So, for example, with an increase of the doping concentration up to x = 0.05. A further increase of x (up to x = 0.1) was accompanied by an abnormal depen-
where x 5 0.05.
Carrier mobilities of 10-2-10-3 cm2 V-’ s-’ for Cd,_,In,Cr,Se, single crystals with x = 0.01, 0.03 and 0.05 were determined from measurements of the Hall effect in the temperature region 77300 K. Pinch and Berger  have proposed that there is a connection between the phenomena of a transition of some of the Cr3+ ions to Cr2+ ions and the substitution of Cd by In in the condition of electrical neutrality in the lattice. A hoppingtype conductivity (exchange of valence) in such crystals could then arise from the presence of Cr ions with variable valence and small values of carrier mobility. The activation energy of the conductivity will then be determined by the electron exchange process Cr3+ +Cr2+ --f Cr2+ +Cr3+. The temperature dependence of the magnetoresistance Ap/p in Cd, _XInXCr2Se, single crystals (x = 0.01-0.05) in an external magnetic field of
Fig. 3. Dependence of the electrical concentration in the Cd, ox In,Cr,Se,
conductivity on dopant single crystals.
Fig. 4. Temperature dependence of magnetoresistance Cd,_,In,Cr,Se, single crystals: (I) x=0.05, (2) x=0.03, x=0.01.
A.I. Merkulov et cd. / Growth and properties of CdCr,Se,
intensity 7.5 kOe is shown in fig. 4. We found that the position of the magnetoresistance minimum depends on the doping level and shifts to lower temperatures with increasing degree of doping. The shift of the Ap/p minimum which was observed at the phase transition temperature may be understood by taking into account the variable valence of the chromium ions. The presence of Cr’+ ions in the CdCr,Se, single crystals doped with indium must decrease the phase transition temperature according to Minakof et al. . The crystallochemical formula of such crystals may be written as
and an increase in concentration of indium dopant will increase the concentration of Cr2’ ions and consequently change the character of the s-d exchange. The temperature dependence of the electrical conductivity of the Cd, PXAg,Cr2Se, single crystals for 0 G x G 0.07 was investigated [lo]. As the silver concentration increases, the absolute conductivity also increases, but the relative change of u decreases with higher temperatures. The conductivity curves have kinks within the temperature range of the phase transition. These kinks shift to higher temperatures as the silver concentration is increased [lo]. The absolute value of the activation energy E,,, for the given crystals at a temperature T> T, is greater than its value below the critical temperature. 1
Fig. 5. Temperature dependence of electrical conductivity nonstoichiometric CdCr,Se, single crystals: (I) vacuum nealling, (2) crystal with excess cadmium, (3) annealing excess selenium pressure.
defect single ctystals
As the silver replaces cadmium, the existence of acceptor states (with the lattice remaining electrically neutral) seems to be associated with the re-charging of some Cr3+ ions to Cr4+ ions [ 1 I]. The conductivity at high temperatures is due to motion of holes which appear within the valence band as a result of electrons filling the acceptor levels. These holes have a high mobility (of the order of lo2 cm2 V -’ sP ‘) and are the principal factor in the conductivity. The carrier mobility drops sharply within the ferromagnetic-state range of temperatures, and conductivity here may be due to the motion of holes within a narrow acceptor d-band. The activation energy in this case will be determined by the electron exchange processes symbolically represented by the reaction Cr4+ +Cr3+ + Cr3+ +Cr4+ . The crystallochemical formula of such crystals may be written as
The magnetoresistivity for Cd, _XAgXCr,Se4 single crystals with x = 0.03, 0.05 and 0.07 in an external magnetic field of intensity H = 7.5 kOe show a change of sign at the phase transition temperature [lo]. Such a temperature behaviour of Ap/p was not observed for the Cd,-,Ag,Cr,Se, single crystals with x < 0.03. The temperature dependence of the electrical conductivity of non-stoichiometric CdCr,Se, single crystals is shown in fig. 5. The annealing of crystals in vacuum changes the crystal conductivity from p- to n-type, as has also been shown by the measurements of the thermoelectrical characteristics . This may be explained by the fact that within the annealing temperature range used the selenium vapour partial pressure was much greater than that of the cadmium vapour [ 131, and the predominant defects in the annealed crystals must be the selenium vacancies which are responsible for the n-type conductivity (curve 1 in fig. 5). The formation of defects as a result of annealing is characterized by re-charging of some of the Cr3+ ions and the crystallochemical formula may then be written as
et al. / Growth and properties
where y > x and Cl = vacancy. The temperature dependence of electrical conductivity for the single crystals with excess of cadmium shows a break at the phase transition temperature, which is characteristic for the Indoped CdCr,Se, (curve 2 in fig. 5). And the excess of cadmium in CdCr,Se, may be considered to be donor states which also re-charge some of the Cr3+ ions as in the case of In-doped crystals. An increase of electrical conductivity (in comparison with the undoped single crystals [lo]) and a decrease of activation energy in the low temperature region (curve 3 in fig. 5) was observed when crystal annealing was done at an excess selenium pressure. Such a behaviour of conductivity may be the result of vacancy formation in the cadmium sublattice. The results of investigations of the electrical conductivity of Cd0,991n0,,,Cr,Se, single crystals annealed under similar conditions [ 141 may be considered a confirmation of such a proposal. The crystallochemical formula of the CdCr,Se, single crystals annealed at excess selenium pressure may be written as
4. Magnetic properties According to Baltzer et al. , strong ferromagnetic and weak antiferromagnetic exchange interaction takes place in the magnetic semiconductor CdCr,Se,. The influence of anionic substitution (Se --* S) and cationic substitution (Cd + Hg ---)Zn) on the nature of the exchange interaction in the spine1 CdCr,Se, has been investigated in refs. [16,17]. But published papers give little information on the influence of substitution of Cd by Ag upon the exchange interaction. The temperature dependence of the specific magnetization m, and inverse susceptibility l/x(T) for CdCr,Se, single crystals with different contents of acceptor dopant has been investigated [ 181. The linearity of l/x =f( T) at high temperatures and the positive Curie-Weiss constant indicate a ferromagnetism in the investigated crystals. The Curie-Weiss temperature 0 of 149 K determined for the undoped crystals is significantly
defect single crystals
lower (by 40 to 60 K) than the values given in the literature [ 15,161. This fact also indicates the level of perfection and purity of the undoped single crystals used. The results of measurements of the Curie temperatures T, and the Curie-Weiss temperature 8 for the Cd, _xAg,Cr,Se, single crystals, and the dependence upon x, have been given in ref. . (The Curie temperature T, was determined from the magnetization isotherms m,(H) and from the dependence of H/m, upon mf.) The generally used theoretical model for the estimation of the exchange interaction constants for spinels of the ACr,X, type suggested by Baltzer et al. [ 151 is not applicable in our case because it is limited to e/T, s-- 1.5. The calculation of the ferromagnetic exchange constant (J/k) and the antiferromagnetic exchange constant (K/k, where k is the Boltzmann constant) was carried out by the method described in ref. . Fig. 6 shows the calculated dependence of the J/k and K/k constants on the concentration of Ag dopant. In contrast to the wide class of magnetic oxide materials in which the primary interaction appears to be an antiferromagnetic exchange interaction, in CdCr,Se, the main interaction shows the superposition of ferro- and antiferromagnetic interactions. For the Cd,_.Ag,Cr,Se, single crystals, while the ferromagnetic exchange constant J/k increases over the concentration range investigated, the antiferromagnetic exchange constant K/k practically does not change with increasing
Fig. 6. Dependences of the Curie temperature T,, the CurieWeiss temperature 8, and the exchange constants J/k and K/k on Ag dopant concentration.
et 01. / Growth crnd properties of CdCr,Se,
Ag concentration. The comparison of the dependence of T, on the structure parameter of the crystals given in ref.  with the dependence of the parameter a on the Ag content (fig. 1) allows us to postulate that in addition to the 90’ superexchange between the nearest neighbours of the type Cr3+-Se*-_Cr3+ in the crystals studied, a definite type of ferromagnetic exchange with participation of Cr 4+ ions takes place, and this leads to an increase of the magnetic transition temperature. The model of the variable valence of chromium ions may be used for the interpretation of magnetic characteristics of nonstoichiometric crystals. Annealing of the crystals in vacuum increased the temperature 19(up to 220 K), while T,stayed constant at 130 K and the magnetization of such crystals decreased (down to 37 G cm3 g g ‘). This is explained by the existence in the crystals of Cr*+ ions antiferromagnetically connected with Cr3+ ions. Finally, we consider the structure-sensitive parameter of the undoped single crystals. Fig. 7 shows the temperature dependence of the FMR line width (2AHa) for various crystallographic axes. The line width for the [loo] axis is constant and equal to 1.6 Oe in the temperature range of 7-80 K. At present this is the lowest FMR line width which has been reported for chromium chalcogenide spinels. The ionic contribution to 2 AH, for the [loo] axis is practically absent (except for the lowest temperatures). The constant
defect single cqatuls
line width for this axis over the wide temperature range is caused by a compensation of the contribution from the intrinsic processes (which increase with temperature) by the contribution of scattering on both surface and bulk properties.
5. Conclusion The growth of cadmium selenochromite single crystals by the technique of chemical transport has been developed. In this technique the starting materials were either the elementary components or compositions synthesized by solid state reactions. Undoped, In(Ag) doped, and nonstoichiometric single crystals have been grown using the technique. It was found that a dopant such as indium (or aluminium) accelerates while a dopant such as silver (or copper) retards the chemical transport rate as compared with undoped single crystals. The electrical characteristics have been investigated and the main parameters of doped crystals determined. It has been shown that there is a possibility of hopping-type conductivity in the doped single crystals. The influence of deviations from stoichiometry on the electrical conductivity has been investigated and crystallochemical formulae which allow one to explain both the electrical and magnetic characteristics of crystals have been given. The ferromagnetic Curie temperature (T,)for the cadmium selenochromite single crystals was found to increase (by 25-30 K) as the Cd is replaced by Ag. This has led to the conclusion that there is a participation of Cr4+ ions in 90” superexchange. Finally, it has been shown that the undoped single crystals over a wide temperature range are characterized by a FMR line width which is lower than any reported for other magnetic semiconductors.
Fig. 7. Temperature dependence various crystallographic axes.
of the FMR
The authors wish to thank Professors A.G. Gurevich and Yu.M. Yakovlev from the Physico-
et ul. / Growth und properties of CdCr,Se,
Technical Institute of the Academy of Sciences of the USSR for helpful discussions.
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