Heterojunction Engineering of Semiconductor Ferromagnetism*

While bandgap engineering and wavefunction engineering are established in \textit{nonmagnetic} semiconductor heterostructures, we aim to broaden their field to \textit{magnetic} heterostructures and to extend the degree of freedom in designing spin-related properties in semiconductors. In this study, we introduced delta-doping of magnetic (Mn) impurities in the quasi two-dimensional hole gas at the interface of GaAs/$p$-AlGaAs heterostructures, and successfully maximized the ferromagnetic order among the Mn spins by overlapping the hole wavefunction with the Mn delta-doping profile. Selectively $p$-doped heterostructures (Mn-delta-doped GaAs / Be-doped AlGaAs) were prepared by molecular beam epitaxy, in which holes are supplied from the Be-doped $p$-AlGaAs layer to the Mn-delta-doped channel, and ferromagnetic ordering was clearly observed in magnetotransport measurements [1]. In the heterostructure prepared with proper conditions, its ferromagnetic transition temperature ($T_{c})$ was 172 K, higher than the $T_{c}$ of InAs- or GaAs-based random-alloy magnetic semiconductors [2]. It was also found that in more suitably designed heterostructures with low-temperature annealing, $T_{c}$ can be higher than 200 K [3]. Furthermore, we show the control of ferromagnetism in the heterostructures by using gate electric field and light irradiation at relatively high temperatures ($\sim $100 K) [4]. [1] A. M. Nazmul, S. Sugahara, and M. Tanaka, Appl. Phys. Lett. \textbf{80}, 3120 (2002). [2] A. M. Nazmul, S. Sugahara, and M. Tanaka, Phys. Rev. \textbf{B67}, 241308(R) (2003). [3] A. M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, submitted. [4] A. M. Nazmul, S. Kobayashi, S. Sugahara, and M. Tanaka, Jpn. J. Appl. Phys. \textbf{43}, L233 (2004).