On the Anomalies in the Temperature Dependence of Conductivity in Hg_1−xCd_xTe

Abstract

The temperature (1.8 K T 5 77 K) and magnetic field (0 5 H 22 kOe) dependence of the conductivity (r, longitudinal magnetoconductivity a , , , and of the Hall coefficient R for gapless Hgl-,Cd,Te crystals are measured. At temperatures Tmin = 6 to 11 K there occur deep minima of conductivity (r(T). It is shown that the origin of these minima cannot be accounted for by the resonance scattering of electrons by acceptors. The miiiima in o(T) arise from the decrease in electron mobility caused by the increa% of the number of charged centres with increasing temperature when T < T m i n and by the increase of the concentration of electron-hole pairs when T > > Tmin. Thc anomalies in the conductivity (plateau) and the Hall effect (double inversion of sign) observed in the temperature range 20 K < T < 40 K are due to the contribution of three types of carriers, viz. light conduction band electrons, heavy acceptor band electrons, and valence band holes.Die Teniperntclr (1,s K 5 T 5 77 R)-und Magnetfeld (0 H 5 22 k0e)-Abhiingigkeit der Leita fahigkeit (r, die longitudinalc Magnetoleitfahigkeit uzzt und der Hallkoeffizient R werden fur gaplose Hgi -ZCd,Te-Kristalle gemessen. Bei Temperaturen Tmjn = 6 bis 11 I ( treten tiefe Minima der LeitfShigkeit a( T) auf. Es wird gezeigt, daB der Ursprung dieser Minima nicht mit der Resonanzstrenung voii Elektronen durch Akzeptoren erkliirt werden kann. Die Minima in a( 2') rhhren von dem Abfall der Elektronenbeweglichkeiten her,. der durch das Anwachsen der Zahl der geladenen Zentren mit zunehmender Temperatur, wenn T < Tmin, und durch den Anstieg der Konzentration der Elektron-Loch-Paare, wenn T > Tmin ist, hervorgerufen wird Die Anomalien dcr Leitfahigkeit (Plateau) und des Hall-Effekts (doppelte Inversion des Vorzeichens), die im Temperaturbereich 20 K < T < 40 K beobachtet werden, werden durch den Beitrag vondrei'A t e n Ton Lidungstragern verursacht : Ieichte Leitungsbandelektronen, schwere Akzeptorbandelektronen und Valenzbandlocher. InirodiictioiiGapless p-type semiconductors HgTe and Hgl-,Cd,Te with a cadmium content of x < 0.16 have repeatedly exhibited two features peculiar to the temperature dependence of conductivity G, viz. a minimum at T == 9 K [l to 61 and a plateau a t T = 20 to 40 B [l, 2, 7 to 151. Since the problem concerning these peculiarities remains yet to be ascertained we have undertaken a new attempt to examine the reaaons for the

Impurity states and electron transport in gapless semiconductors

“…If these charge carriers are assumed to assure the negative sign of the Hall component axy it becomes qualitatively clear why the Hall coefficient undergoes double reversal of sign of R0(T) for narrowgap semiconductors or of the anomalous dependence R~(T) for GS. This point of view concerning the nature of 'heavy' electrons is discussed by Zakrszewski and Dziuba (1972), Dziuba (1974), Szlenk et al (1979, Mauger et al (1974Mauger et al ( ), V6ri6 et al (1978, Arapov et al (1977Arapov et al ( , 1979bArapov et al ( , 1982b, Gluzman et al (1981). Some authors directly associate the concentration of'heavy' electrons with that of ionized acceptors, assuming ne2 = NA (V6ri6 et al, Arapov et al).…”

“…At (1) 0.01kOe, (2) 0.5kOe, (3) 1.5kOe, (8) 22kOe (Arapov et al 1982b). With increasing magnetic field the electron contribution is suppressed and this peculiarity should enhance.…”

“…Temperature minimum of conductivity in gapless semiconductors GS of the p-type repeatedly exhibited the following peculiarities of the temperature dependence of conductivity: a minimum of a(T) (figure 45) in the 5-10 K range (Otmezguine et al 1970, Stankiewicz et al 1971, Finck et al 1972, Brandt et al 1974, Kozacki et al 1976, Arapov et al 1977, 1982a and a plateau (figure 46) in the 20-40 K range (Ivanov-Omskii et al 1967, Finck et al 1972, Stankiewicz and Giriat 1976, Dziuba 1974, V6ri6 et al 1978, Szymanska 1977, Dietl et al 1978, Szlenk 1979, Arapov et al 1982b.…”

“…This means that the Fermi level, too, does not vary with temperature and, consequently, there is no reason to believe that the occurrence of a tr(T) minimum is due to the resonance scattering of band electrons at eF(T ) = egl. Arapov et al (1977Arapov et al ( , 1982b ascribe the formation of a minimum on the tr versus T curve to the competition of two factors; a decrease in electron mobility on account of the increasing number of charged acceptors NA in the region T < Tmi, and an increase in electron concentration at T > Tmi,. A natural cause of the growth of N£ with temperature is the thermal excitation of electrons from the valence band onto the acceptor level.…”

Peculiarities of the Magnetoresistance of Hg1−xMnxTe

“…However, we did not manage to find any peculiarities in the eSx( T) and eZz( T ) curves at 1. 8 Since the amplitudes of SdH oscillations decrease drastically at T > 4 K in our samples it is hardly to be expected that thermooscillation peaks may manifest themselves a t higher temperatures, T > 10 K. The way the authors of [2] interpret the maximum g,,(T) a t temperatures 25 K 5 T 5 40 K as a thermooscillation peak is, to our mind, also not convincing, because a t such high temperatures the condition of electron gas degeneracy required to observe SdH oscillations works badly ( E~/~T = = 1.5). I n addition to this, in the temperature interval 25 K 5 T …”