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이상훈,Furdyna J.K.,Liu X.,정선재 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.2
We present a systematic study on the properties of the ferromagnetic (FM) III-V-based semiconductor GaMnAs, including extrinsically doped GaMnAs : Be and its heterostructures in the form of superlattices. In an attempt to further improve the Curie temperature Tc of GaMnAs, we have undertaken a systematic program of extrinsic p-doping of this material. In GaMnAs with low x (x = 0.03), Tc is, indeed, seen to increase monotonically with increasing Be doping. We also studied the effect of p-doping of non-magnetic spacer layers on the magnetic properties of GaMnAs/GaAs superlattices (SLs). While Be-doped SLs exhibited relatively robust remanent magnetization and a larger coercivity over a broad temperature range, undoped SLs showed a fast decrease in the remanent magnetization with temperature, and a rather small coercivity. We propose that the observed hardness of the magnetization in SLs with Be-doped GaAs layers is related to the interlayer coupling introduced by the doping of the non-magnetic layers.
Inter-dot spin exchange interaction in coupled II–VI semiconductor quantum dots
Lee, S.,Dobrowolska, M.,Furdyna, J. K. WILEY-VCH Verlag 2006 Physica status solidi. PSS. B, Basic solid state p Vol.243 No.4
<P>We have performed polarization selective photoluminescence (PL) experiment on a series of self-assembled quantum dots (QDs) in the form of single- and double-QD layer systems. The double layers were formed from diluted magnetic semiconductor (DMS) (either CdMnSe or CdZnMnSe) and non-DMS (either CdSe or CdZnSe) layers, separated by non-DMS ZnSe barriers. The peaks from DMS and non-DMS QD layers are clearly resolved in the PL spectra taken at zero magnetic field. When a magnetic field is applied to the double layer QD (DLQD) system, the intensities of the circularly polarized PL peaks corresponding to the non-DMS (i.e., CdSe and CdZnSe) layers exhibit significant changes, reflecting correspondingly large changes in the degrees of spin polarization of the non-DMS QDs. This enhancement of spin polarization observed in the double-layer QD structures is interpreted in terms of anti-parallel spin interaction between carriers localized in the coupled QD pairs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</P>
Carrier relaxation processes in magnetic semiconductor quantum dot systems
이상훈,J. K. Furdyna,M. Dobrowolska 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.4
The energy relaxation process of dSe/ZnMnSe quantum dots (QDs) was investigated using polarization-selective magneto-photoluminescence (PL). Peaks from the Mn2+ internal transition, the CdSe QDs, and the ZnMnSe barrier were observed in the system. The CdSe QD peak was relatively weak while the intensity of the Mn2+ transition was strong in the spectrum taken at zero magnetic field. However, the peaks intensities changed significantly when a magnetic field was applied, and the situation became reversed (i.e., the CdSe QD peak become stronger than the peak due to the Mn2+ internal transition, specifically for + polarization). The opposite intensity behavior was observed in the peak from the Mn2+ ions and in the PL of the CdSe QDs and revealed that these two carrier capture centers competed with each other for carriers excited in the ZnMnSe barrier. The variation in the Mn2+ internal transition with the magnetic field was well described by using a model developed under an angular momentum selection rule. This suggests that competition between the two energy transfer processes in the CdSe/ZnMnSe QD system is responsible for the observed PL intensity behavior.
Spin Relaxation Time of CdZnSe/ZnSe Self-Assembled Quantum Dots in a Magnetic Field
이상훈,J. K. Furdyna,M. Dobrowolska 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.1
We have performed polarization-resolved magneto-photoluminescence (PL) experiments on a selfassembled CdZnSe quantum dot (QD) system to investigate the influence of an external magnetic field on the spin relaxation time of excitons in QDs. When the QDs are excited with circularly polarized light, the PL emissions at phonon resonance energies exhibit a noticeable degree of circular polarization (33 % and 6 % for 1-LO and 2-LO resonances, respectively) even at zero magnetic field while the PL at non-resonant positions shows zero circular polarization. The degree of polarization increases significantly when an external magnetic field is applied. Based on a simple two-level rate equation model, including a spin-flip time s and an exciton recombination time r, we are able to extract the magnetic field dependence of the ratio s/r. This ratio increases monotonically with the field and eventually becomes larger than 1, indicating that at high magnetic fields, spin relaxation times can exceed exciton recombination times.
이상훈,J. K. Furdyna,M. Dobrowolska 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.50 No.3
The magneto-optical properties of quantum-dot (QD) and quantum-well (QW) coupled structures are investigated using magneto-photoluminescence (PL). The coupled system consists of a self-assembled non-magnetic CdSe quantum-dot layer and a 7-nm ZnCdMnSe diluted- magnetic-semiconductor (DMS) quantum well (QW) layer. We have observed two well-separated PL peaks, one corresponding to emission from the CdSe QD layer and the other to the emission for ZnCdMnSe QW layer. The temperature dependence of the PL intensity clearly demonstrates the characteristics of zero- and two-dimensional structures for the CdSe QDs and the ZnCdMnSe QW, respectively. The significant PL energy shift and intensity variation observed for the DMS QW in a magnetic field can be understood based on the giant Zeeman splitting of the band edges of the DMSs. Furthermore, we have observed a type-II transition between the conduction band of the DMS QW and the valence band of non-DMS QD, which enables us to determine the band alignment of the coupled systems.