Le Van Khoi

Ferromagnetism in II–VI Compounds

The current status and prospects of research on ferromagnetism in II-VI diluted magnetic semiconductors and their nanostructures are reviewed. So far, in agreement with theoretical predictions, the ferromagnetic ordering has been observed above 1 K for p-type modulation-doped Cd 1-x Mn x Te/Cd 1-y-z Mg y Zn z Te:N heterostructures as well as for Zn 1-x Mn x Te:N and Be 1-x Mn x Te:N epilayers and Bridgman grown Zn 1-x Mn x Te:P but not for n-type films of Zn 1-x Mn x O:Al. Diverse aspects of theoretical models as well as hopes associated with the materials containing magnetic ions other than Mn and encouraging results for II-VI/III-V systems are described.

Near band-gap optical nonlinearities and bistability in Cd1−xMnxTe

Abstract Strong thermally induced self-focusing of laser beam near the band-edge in Cd1−xMnxTe with 0

Annealing-induced changes in electrical, optical, and magnetic properties of phosphorus doped bulk Zn1-xMnxTe

We report on successful growth of heavily p-type doped bulk Zn 1-x Mn x Te crystals. Free hole density as high as 10 19 cm -3 has been achieved by use of Zn 3 P 2 as a source of phosphorus acceptors and by post-growth high pressure annealing. The latter proved to be of a vital importance, removing a strong compensation present in as-grown materials. As a result, the annealed crystals exhibit a very bright green photoluminescence (λ 520 nm) at temperatures as high as 160 K. The annealed samples with hole concentration p > 2 × 10 18 cm -3 show a positive Curie-Weiss temperature and open hysteresis curves, characteristic of a system with ferromagnetic correlations.

Indication of ferromagnetic ordering in p-Zn1−xMnxTe

Abstract P-type doped Zn 1− x Mn x Te has been grown by molecular beam epitaxy and by the Bridgman method. Our SQUID measurements indicate ferromagnetic ordering above 1 K, for p⩾10 19 cm −3 . It is argued that weakly localized holes are responsible for the ferromagnetic correlation in the insulating phase.

Magnetic characterisation of highly doped MBE grown Be1-xMnxTe and bulk Zn1-xMnxTe

Magnetic properties of two new classes of highly p-type doped II-VI diluted magnetic semiconductors (DMS): thin layers of Be 1-x Mn x Te:N and bulk Zn 1-x Mn x Te:P are presented. Despite the fact that the samples are insulating, we observe clear indications of the low temperature ferromagnetic correlation, namely: (i) positive Curie-Weiss temperature (T CW 2 K), and (ii) open hysteresis loops at temperatures below T CW . A qualitative analysis of the results favours those models of the carrier-mediated ferromagnetism in DMS which allow for the coupling of only a part of the available spins.

High pressure study of ZnSe:Cr2+ crystals: the origin of the 1.25 eV luminescence

High pressure studies of Cr2+ luminescence in ZnSe crystals were performed at low temperatures. In addition to the mid-infrared luminescence transitions from the first excited 5E level, higher energy luminescence was also observed. These two luminescence bands exhibit similar pressure coefficients but of opposite signs, which identify the origin of the higher energy band as the transitions from the second excited 3T1 level. At higher pressures, increased covalence effects are observed, which result in the appearance of an additional peak in the luminescence due to changes in the spin–orbit coupling strength.

Carrier-Concentration and Magnetic-Field Effect on Mn2+ Luminescence in Bulk Zn1−x Mn x Te Crystals

Measurements of photoluminescence, reflectance, and Hall effect as functions of the phosphorus doping level, temperature, and magnetic field in p-Zn1−xMnxTe crystals are reported. The photoluminescence reveals two competing intra-Mn recombination channels. The first channel originates in the energy transfer from the photogenerated carriers to the internal Mn2+(3d5) states leading to the intra-Mn radiative recombination. The second one originates in the energy transfer from Mn ions to thermally activated carriers leading to intra-Mn nonradiative recombination. The Mn emission can be controlled either by changing the doping level with the phosphorus impurities or by varying the temperature and/or an external magnetic field.

Electroluminescence and positive magnetoresistance near the Curie–Weiss temperature in the Zn1−xMnxTe light emitting devices

The light emitting devices (LEDs) based on the p-Zn1−xMnxTe bicrystals have been fabricated. The Zn1−xMnxTe LEDs produce red and green emission originated from the internal d-shell transitions of the Mn2+ ions and the donor-acceptor-pairs recombination, respectively. The presence of the acceptor bound magnetic polarons and their clusters in the Zn1−xMnxTe crystals near the Curie–Weiss temperature was observed. It induces a critical behavior of the magnetic field dependence of the green emission intensity and a positive magnetoresistance in the Zn1−xMnxTe LEDs.

Lattice Sites of Phosphorus in ZnMnTe – A Compensation Study

In this paper we investigate phosphorus doped ZnMnTe by means of infrared absorption. We report pronounced changes in the absorption spectra upon increasing phosphorus doping and upon annealing at 850 °C in nitrogen atmosphere at high pressure. From the far-infrared local vibrational spectra we identify the lattice sites of phosphorus in ZnMnTe and interpret the strong compensation of the as-grown material in terms of at least partial passivation by hydrogen.

Magnetoresistive switching and highly polarized electroluminescence from semimagnetic semiconductor bicrystals Zn1−xMnxTe

The Zn1−xMnxTe bicrystals grown by the high-pressure Bridgman method exhibit magnetic-field-sensitive current-voltage characteristics. The threshold voltage of avalanche breakdown decreases by 40% as the magnetic field increases from 0 to 14 T. In the postbreakdown regime, the Zn1−xMnxTe bicrystals emit an intense red and green electroluminescence. Under magnetic field, the green emission exhibits a high degree of circular polarization which reaches 0.95 and 0.78 at 1.8 K and 4.2 K, respectively. The relation between the photon energy of the green emission and the threshold voltage has been studied.