Magnetic anisotropy of singly Mn-doped InAs/GaAs quantum dots

• Published in: Physical review. B, Condensed matter and materials physics Vol. 80; no. 16
• Main Authors: Krebs, Olivier, Benjamin, Emile, Lemaître, Aristide
• Format: Journal Article
• Language: English
• Published: American Physical Society 09-10-2009
• Subjects: Condensed Matter; Other; Physics; 7135Pq; number͑s͒: 7225Fe; 7867Hc; 7225Rb
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.80.165315

We report on the microphotoluminescence spectroscopy of InAs/GaAs quantum dots ͑QDs͒ doped by a single Mn atom in a magnetic field either longitudinal or perpendicular to the optical axis. In both cases the spectral features of positive trion ͑X + ͒ are found to split into strongly circularly polarized components, an effect very surprising in a perpendicular magnetic field. The field-induced splitting is ascribed to the transverse Zeeman splitting of the neutral acceptor complex A 0 issued by the Mn impurity, whereas the circular optical selection rules result from the p-d exchange which acts as a very strong longitudinal magnetic field inhibiting the spin mixing by the transverse field of the QD heavy-hole ground state. A theoretical model of the spin interactions which includes ͑i͒ the local strain anisotropy experienced by the acceptor level and ͑ii͒ the anisotropic exchange due to the out-of-center Mn position provides a very good agreement with our observations. Doping a semiconductor quantum dot ͑QD͒ with a single Mn atom brings up remarkable spin-related properties due to the sp-d exchange interactions between the confined carriers ͑electron and hole͒ and the magnetic impurity. In the last few years Mn-doped CdTe QD's have been extensively studied by microphotoluminescence ͑-PL͒ spectroscopy in an external magnetic field. 1-4 Most of the observations were very well interpreted by assuming a 5/2 spin for the Mn ion acting on the carriers confined in a quantum dot through Heisenberg Hamiltonians. Yet, the strong vertical confinement of QD's along their growth axis, as well as their in-plane biaxial strain were shown to modify significantly the spectral features because of the resulting heavy-hole nature of the valence-band ground state. The quite specific signature of InAs/GaAs quantum dots doped with a single Mn atom has been recently uncovered in-PL spectroscopy. 5 In this system, the Mn impurity acts as an effective J = 1 spin with a noticeable fine-structure splitting in zero magnetic field. This results from the neutral ac-ceptor ͑A 0 ͒ complex formed by Mn in a III-V matrix, namely, a negatively charged center A − and a bound hole h 1. 6-9 The J = 1 spin corresponds to the ground state of the 3d 5 Mn spin S =5/ 2 and the bound hole total angular momentum J h 1 =3/ 2 which interact via the antiferromagnetic p-d exchange. Its zero-field splitting results from some local anisotropy of the potential experienced by the bound hole. 5,10,11 Within this interpretation, the anisotropy of the A 0 complex does not affect the optical selection rules of the QD interband transitions which still involve a conduction electron ͑e͒ and a valence-band hole ͑h 2 ͒ essentially of heavy-hole character both with S-like orbital. This was shown in Ref. 5 where a longitudinal magnetic field split all the optical transitions into their circularly polarized ͑ Ϯ ͒ components. In this paper, we show that the optical selection rules in a transverse magnetic field are in contrast deeply affected by the anisotropy of the A 0 effective spin, besides in a rather nonintuitive way. Indeed, in a magnetic field perpendicular to the optical axis, the optical transitions which are expected to be linearly polarized as usually encountered in undoped InAs QDs, 12-14 exhibit for Mn-doped InAs QDs a strong circular polarization ͑ + or − ͒. We show that this effect results from the A 0 spin anisotropy which enables to split by Zeeman effect the ferromagnetic ͑FM͒ and antiferromagnetic ͑AFM͒ configurations of the h 2-A 0 complex, while the heavy-hole h 2 keeps a well-defined pseudospin ⇑͑J h 2 ,z =+3/ 2͒ or ⇓͑J h 2 ,z =−3/ 2͒. We studied a sample grown by molecular-beam epitaxy on a semi-insulating GaAs ͓001͔ substrate which consists of a single layer of InAs/GaAs QD's randomly doped by a single Mn atom ͑see Ref. 5 for details͒. We estimate that ϳ0.1-1% of the quantum dots are effectively doped by a single Mn atom. The-PL spectroscopy of individual Mn-doped InAs QDs was carried out with a split-coil magneto-optic cryostat. A 2 mm focal-length aspheric lens ͑NA 0.5͒ actuated by piezo motors was used to focus the He-Ne exci-tation laser and to collect the PL from the sample. This compact microscope, which integrates both the sample and the optical lens, can be rotated about the vertical axis of the cryostat in order to change the magnetic field direction with respect to the optical axis from parallel ͑Faraday configura-tion͒ to perpendicular ͑Voigt configuration͒. Relying on in situ sample imaging we could therefore study the same quantum dot in both configurations. All measurements presented here were performed at low temperature ͑T =2 K͒. The collected PL was dispersed by a 0.6 m focal-length double spectrometer and detected by a nitrogen-cooled CCD array camera. Figures 1 and 2 report on the optical spectroscopy of a charged exciton X + in the same Mn-doped InAs QD ͑QD1͒ measured, respectively, in Faraday and Voigt configuration. Let us first comment on the results shown in Fig. 1. As evidenced in Ref. 5, the Mn-doped InAs QD is identified by its spectral features in zero magnetic field shown in Fig. 1͑b͒. It consists of two doublets separated by the exchange energy ⌬ between the FM and AFM configurations of h 2-A 0 , plus a weaker line denoted O which corresponds to the transition involving the A 0 state J z = 0. Another specific feature is the equal splitting ␦ of both FM and AFM doublets which is ascribed to the fine structure of A 0 in its anisotropic environment .