Ultrasonic investigation of the Jahn-Teller effect in GaAs semiconductors doped by transition metals

Abstract

Transition-metal-doped semiconductor GaAs crystals are used as model systems for spintronic research, as well as in other applications. To explore the structure and properties of such impurities, we extended the methodology of ultrasonic investigation of the Jahn-Teller effect in dielectric impurity-centers to the study of significantly different semiconductor impurities using the GaAs:Cu crystal as an example. Phase velocity and attenuation of ultrasound in this system were measured in the temperature interval of 1.9–80 K at 52 MHz and 156 MHz. The anomaly in the velocity and a peak of attenuation found for the longitudinal and slow shear waves indicate the presence of the Jahn-Teller effect with the e-type local distortions of the CuGa4As impurity complex. The temperature dependence of the elastic modulus and relaxation time shows that below 5 K, the thermal activation mechanism of relaxation is possibly replaced by resonance type transitions. The main parameters of the Jahn-Teller effect, stabilization energy, minima positions and the barrier between them, frequency of pseudorotation of the distortions, and the tunneling splitting of the ground state energy, as well as the constant of exchange interaction between the two holes in Cu2+ centers and the concentration of the centers were estimated.

Jahn–Teller and Pseudo-Jahn–Teller Effects: From Particular Features to General Tools in Exploring Molecular and Solid State Properties

“…The novel approach to the study local JTE in crystals by means of ultrasonic experiments, developed during the past decade, demonstrated high efficiency in obtaining important information about the structure and properties of the system. ,− Distinguished from ESR, optical, and other similar experimental studies, in which the measurement perturbation primarily engages with the electronic subsystem, ultrasound interacts directly with the nuclear arrangement, thus allowing to reveal the local adiabatic potential energy surface (APES) of the JTE center in the crystal. When propagating through the crystal containing JTE centers, and dependent on direction of propagation and polarization, the ultrasonic wave unequally shifts the minima of the multiminimum APES, making them nonequivalent, thus creating a nonequilibrium distribution of the JT complexes over the energy levels (Figure ).…”

Interplay Between Relaxation and Resonance in Ultrasound Attenuation by the Cubic Crystal ZnSe:Cr

“…Most of these materials demonstrate structural instability induced by the Jahn–Teller effect (JTE). Acoustic studies proved to be instrumental in studies of structure and properties of impurities in crystals, especially when attributed to the JTE; practically all the impurities have degenerate local electronic states (ground or excited) which effectively couple to local lattice distortions…”

“…In most cases the mechanisms of ultrasound attenuation are of relaxation origin. Jahn–Teller (JT) impurity or vacancy centers in semiconductor crystals usually have intrinsic relaxation times of the order of 10 −7 –10 −9 s, which allows for strong coupling with ultrasonic waves with the typical frequency range of 10 MHz–1 GHz. Another property of these centers is the presence of small splitting of the energy levels (e.g., due to the tunneling effect) .…”

Numerical adiabatic potentials of orthorhombic Jahn-Teller effects retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr crystal

“…Distinguished from ESR and other similar experimental studies, in which the ex-ternal perturbation affects the electronic structure (indirectly influenced by the averaged nuclear dynamics), ultrasound interacts directly with the nuclear arrangement, thus allowing to reveal their adiabatic potential energy surface (APES). It was shown that ultrasound attenuation by JT centers with tetragonal or trigonal distortions, subject to E ⊗ e, T ⊗ e, or T ⊗ t 2 problems of the JTE 9,10 , allows for estimations of several parameters of the APES, including the JT active modes, linear vibronic coupling constants, minima positions and energy barriers between them [5][6][7][8] .…”

“…Along with traditional methods of investigation of electronic structure, dynamics and local symmetry of crystal dopants and other active centers in polyatomic systems by means of experimental electron spin resonance (ESR) and pulse techniques using electron spin echo (ESE, ES-EEM spectroscopy) [1][2][3][4] , recent studies of II-VI:3d and III-V:3d crystals by means of ultrasonic experiments demonstrated the efficiency of this novel approach in obtaining important information about the structure and properties of Jahn-Teller effect (JTE) centers in such crystals [5][6][7][8] . Ultrasonic experiments are uniquely suited for investigation of JTE problems because the interaction of the ultrasound wave of strain produces characteristic anomalies in speed, relaxation, and attenuation when its direction of propagation and polarization coincides with the direction of JTE distortions.…”