Magnetic semiconductors have received a great deal of interest due to their potential application in spintronics devices, which could have control over both spin and electronic degrees of freedom. These systems also reveal fascinating physics due to the interplay between electronic, magnetic and structural properties. Extensive studies of the electronic structure and carrier dynamics in III-V ferromagnetic semiconductors have been carried out in our group in collaboration with the group of David Awschalom (UCSB). Current and future projects are aimed at the effects of tuning carrier density and disorder in these systems. This work is supported by ONR.
Fig 1. (From Burch et. al 2006) (Left panel) Doping dependent studies of Ga1-xMnxAs have revealed increased conductivity as well as a red shift of the mid-infrared peak as carrier concentration is increased in heavier doped samples. (Right panel) By separating free carrier contributions from that of interband transitions, and taking advantage of optical sum rules, the free carrier mass can be extracted. The heavy masses found, as well as the red shift of the mid-IR resonance with increased carriers have clearly demonstrated conduction in a Mn induced impurity band.
Fig. 2 (From Chapler et. al 2011) Systematic studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic (Be) acceptors. We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga1-xMnxAs (a), which persists across the IMT (b), extending at least an order of magnitude beyond the IMT boundary (c). Additionally, we observe the coexistence of “metallic” and “insulating” trends over a broad range of Mn concentrations (d), underscoring the unconventional nature of Mn-doped GaAs past the onset of conduction. Ga1-xBexAs samples, however, display conventional metallicity just above the critical IMT concentration (x=2.7*10 -4), with features indicative of transport within the host valance band (e). Therefore, our results establish that the onset of conduction in magnetically doped GaAs is distinct from genuine metallic behavior due to extended states in the host VB. The * denotes the sample has been subjected to post growth annealing.
B. C. Chapler, S. Mack, R. C. Myers, A. Frenzel, B. C. Pursley, K. S. Burch, A. M. Dattelbaum, N. Samarth, D. D. Awschalom, and D. N. Basov. “Ferromagnetism and infrared electrodynamics of Ga1-xMnxAs” Phys. Rev. B 87, 205314 (2013). LINK
B. C. Chapler, S. Mack, L. Ju, T. W. Elson, B. W. Boudouris, E. Namdas, J. D. Yuen, A. J. Heeger, N. Samarth, M. Di Ventra, R. A. Segalman, D. D. Awschalom, F. Wang, and D. N. Basov. “Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices” Phys. Rev. B 86, 165302 (2012). LINK
B. C. Chapler, R. C. Myers, S. Mack, A. Frenzel, B. C. Pursley, K. S. Burch, E. J. Singley, A. M. Dattelbaum, N. Samarth, D. D. Awschalom, and D. N. Basov “Infrared probe of the insulator-to-metal transition in Ga1-xMnx>As and Ga1-xBexAs” Phys. Rev. B 84, 081203(R) (2011). PDF
K. S. Burch, D. D. Awschalom, D. N. Basov, “Optical properties of III-Mn-V ferromagnetic semiconductors”, Journal of Magnetism and Magnetic Materials 320, 3207 (2008) PDF
K.S. Burch, D. B. Shrekenhamer, E. J. Singley, J. Stephens, B. L. Sheu, R. K. Kawakami, P. Schiffer, N. Samarth, D. D. Awschalom, and D. N. Basov, “Impurity band conduction in a high temperature ferromagnetic semiconductor”, Phys. Rev. Lett. 97, 087208 (2006) PDF
K. S. Burch, E. J. Singley, J. Stephens, R. K. Kawakami, D. D. Awschalom, and D. N. Basov, “Infrared survey of the carrier dynamics in III-V digital ferromagnetic heterostructures”, Phys. Rev. B 71, 125346 (2005) PDF
K. S. Burch, J. Stephens, R. K. Kawakami, D. D. Awschalom, and D. N. Basov “Ellipsometric study of the electronic structure of Ga1-xMnxAs and low-temperature GaAs”, Phys. Rev. B 70, 205208 (2004) PDF
E.J.Singley, K.S. Burch, R.Kawakami, J.Stephens, D.D.Awschalom, D.N.Basov, “Electronic structure and carrier dynamics of Ga1-xMnxAs thin films”, Physical Review B 68, 165204 (2003). PDF