II-VI Diluted Magnetic Semiconductors Research Articles

Upper bound for thes−dexchange integral inn-(Ga,Mn)N:Si from magnetotransport studies

A series of recent magneto-optical studies pointed to contradicting values of the $s\text{\ensuremath{-}}d$ exchange energy ${N}_{0}\ensuremath{\alpha}$ in Mn-doped GaAs and GaN as well as in Fe-doped GaN. Here, a strong sensitivity of weak-localization phenomena to symmetry-breaking perturbations (such as spin-splitting and spin-disorder scattering) is exploited to evaluate the magnitude of ${N}_{0}\ensuremath{\alpha}$ for $n$-type wurtzite (Ga,Mn)N:Si films grown by metalorganic vapor phase epitaxy. Millikelvin magnetoresistance studies and their quantitative interpretation point to ${N}_{0}\ensuremath{\alpha}<40$ meV, a value at least 5 times smaller than the one found with similar measurements on, e.g., $n$-(Zn,Mn)O. It is shown that this striking difference in the values of the $s\text{\ensuremath{-}}d$ coupling between $n$-type III-V and II-VI dilute magnetic semiconductors can be explained by a theory that takes into account the acceptor character of Mn in III-V compounds.

Exchange integrals in Mn- and Co-doped II-VI semiconductors

Exchange integrals between nearest-neighbor (NN) transition metal ions in II-VI diluted magnetic semiconductors (DMSs) are calculated within a local superexchange model, which includes orbital-dependent transfer, on-site Coulomb repulsion and Hund's exchange between $3d$ electrons, and ligand field effects. This extended model gives a quantitative account for the available experimental data on the NN exchange constants in all II-VI DMS family (wurtzite and zinc-blende) doped by cobalt or manganese. As expected, all obtained exchange integrals are antiferromagnetic. Remarkably, the model input parameters are taken directly from the photoemission spectroscopy. We show that in the case of Co-doped compounds, as compared to Mn-doped ones, the exchange process has at least two salient features. The first one is that the electron transfer between NN $\mathrm{Co}{}^{2+} 3d$ orbitals strongly depends on their symmetry positions in the crystal lattice. The second one is related to a peculiar virtual process, involving empty and occupied $\mathrm{Co}{}^{2+} 3d$ orbitals, which leads to an additional ferromagnetic contribution to the exchange constant. We argue that our systematic study of the superexchange opens a pathway toward an understanding of other exchange mechanisms occurring in DMSs.

Homogeneous and heterogeneous magnetism in (Zn,Co)O: From a random antiferromagnet to a dipolar superferromagnet by changing the growth temperature

A series of (Zn,Co)O layers with Co contents $x$ up to 40$%$ grown by atomic layer deposition have been investigated. All structures deposited at 160${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C show magnetic properties specific to II-VI dilute magnetic semiconductors with localized spins $S=3/2$ coupled by strong but short-range antiferromagnetic interactions resulting in low-temperature spin-glass freezing for $x=0.16$ and 0.4. At higher growth temperature (200${}^{\ensuremath{\circ}}$C) metallic Co nanocrystals precipitate in two locations giving rise to two different magnetic responses: (i) a superparamagnetic contribution coming from volume disperse nanocrystals; (ii) a ferromagneticlike behavior brought about by nanocrystals residing at the (Zn,Co)O/substrate interface. It is shown that the dipolar coupling within the interfacial two-dimensional dense dispersion of nanocrystals is responsible for the ferromagneticlike behavior.

Effect of a magnetic field on energy transfer of band states to the Mn2+ 3d shell in the CdMgTe matrix with ultrathin CdMnTe layers

The effect of external magnetic fields on two radiative (band-to-band and on-site) recombination channels in II-VI dilute magnetic semiconductors and related nanostructures has been considered. The 3d on-site emission of manganese ions in CdMgTe matrices containing periodic inclusions of CdMnTe narrow-band-gap layers with thicknesses of 0.5, 1.5, and 3.0 monolayers has been investigated in magnetic fields of up to 6 T. It has been shown that, in a magnetic field, luminescence of manganese ions weakens because of the decrease in the rate of spin-dependent excitation transfer from band states to the Mn2+ 3d shell. The maximum suppression of 3d luminescence has been observed in the matrix with a CdMnTe layer 3.0 monolayers thick. This indicates that the main factor responsible for the energy transfer is the internal field near the CdMnTe layers, which determines the magnetic splitting and spin polarization of band states.

Electronic structures andp−dexchange interaction of Mn-doped diluted magnetic semiconductors

Electronic structures of Mn-doped II-VI diluted magnetic semiconductors (DMSs) were studied by $\text{Mn}\text{ }2p$ core absorption (XAS), $\text{Mn}\text{ }2p$ core-level photoelectron (XPS), and $\text{Mn}\text{ }2p\text{\ensuremath{-}}3d$ resonance photoelectron spectroscopy (RPES). The $\text{Mn}\text{ }2p$ XAS, $2p$ XPS, and $2p\text{\ensuremath{-}}3d$ RPES spectra were analyzed by configuration-interaction theory based on the cluster model. We then obtained a strong negative $p\text{\ensuremath{-}}d$ exchange constant between anion $p$ bands and $\text{Mn}\text{ }3d$ orbitals for the wide gap DMSs ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{Se}$ and ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{S}$. In contrast, for the narrow gap DMS ${\text{Hg}}_{1\ensuremath{-}x\ensuremath{-}y}{\text{Cd}}_{x}{\text{Mn}}_{y}\text{Te}$, we found a weak negative $p\text{\ensuremath{-}}d$ exchange constant. By comparing these results with the results of the previous work, we found that the $p\text{\ensuremath{-}}d$ exchange constant in wide and narrow gap II-VI DMSs is proportional to the absolute band-gap energy. This phenomenon can be interpreted by the change in the density of states of anion $p$ bands, which hybridized with the $\text{Mn}\text{ }3d$ orbitals, at the valence-band maximum. These results suggest that one can control the $p\text{\ensuremath{-}}d$ exchange interaction in DMSs by changing the band-gap energy.

Effects of alloying, ordering, and strain on the exchange parameters of II-VI dilute magnetic semiconductors

Using the band-structure method, we have studied the coupling between the magnetic ion $d$ states and the itinerant valence and conduction states in ${\mathrm{Be}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$, ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}$, ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$, and ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ semiconductor alloys. We show that alloying, atomic ordering, and interfaces in semiconductor heterostructures have large effects on the ${N}_{0}\ensuremath{\alpha}$ and ${N}_{0}\ensuremath{\beta}$ exchange parameters. General chemical trends on the variation of the exchange parameters as a function of the alloy concentration are revealed. Our results successfully explain recent puzzling experimental results, which show that ${N}_{0}\ensuremath{\beta}$ in ${\mathrm{Be}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$, measured in the ${\mathrm{Be}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}∕\mathrm{Zn}\mathrm{Se}$ superlattice, is much smaller than one would expect for bulk ${\mathrm{Be}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$.

Magnetic Impurity Band and Photoinduced Magnetic Phase Transition in Diluted Magnetic Semiconductors

Applying the dynamical coherent potential approximation (dynamical CPA) to a model of diluted magnetic semiconductors (DMSs), in which both random impurity distribution and thermal fluctuation of localized spins are taken into account, the spin-polarized band and the carrier spin polarization are calculated for various magnetizations. In order to clarify the role of impurity depth on the occurrence of ferromagnetism, three typical cases are investigated: (a) II-VI DMS, (b) deep impurity level, and (c) strong exchange interaction. The present study reveals that the impurity depth of magnetic ions strongly enhances the carrier spin polarization (CSP) and accordingly, leads to a high Curie temperature. This means that photoinduced ferromagnetism with high Curie temperature can be expected in a DMS with a deep impurity depth and strong exchange interaction.

Picosecond magnetic spectroscopy with integrated DC SQUIDs

Advanced VLSI (very large scale integration) technology was used to fabricate ultraminiature integrated SQUID (superconducting quantum interference device) susceptometers. With the appropriate design parameters, the sensitivity of the devices approaches the quantum limit. The use of integrated circuits in conjunction with pulsed optical techniques allows magnetic systems to be probed with a picosecond time resolution. The response can be mapped out as a function of the energy of the optical excitation, providing detailed spectroscopic information. Applying these techniques to the study of II-VI dilute magnetic semiconductors has yielded new insight into the mechanics of magnetic polaron formation and the dynamics of the magnetic spins. First experiments were carried out on a small approximately 10*10*1- mu m/sup 3/ single crystal platelet of Cd/sub 0.8/Mn/sub 0.2/Te. The results of the time-averaged magnetic spectroscopy at two different temperatures are presented, displaying the magnetic response to optical excitation at constant intensity from a photon energy of 1.83 to 2.0 eV.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic properties of Cu2Zn1-xMnxGeS4: Antiferromagnetic interactions in the wurtz-stannite structure.

Susceptibility and magnetization measurements were carried out on ${\mathrm{Cu}}_{2}$${\mathrm{MnGeS}}_{4}$, and on ${\mathrm{Cu}}_{2}$${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$${\mathrm{GeS}}_{4}$ with 0\ensuremath{\le}x\ensuremath{\le}0.1. These magnetic (or semimagnetic) semiconductors have the wurtz-stannite structure. The data indicate that ${\mathrm{Cu}}_{2}$${\mathrm{MnGeS}}_{4}$ is a low-anisotropy antiferromagnet with a N\'eel temperature ${T}_{N}$=8.25\ifmmode\pm\else\textpm\fi{}0.3 K. The high-field transition from the canted phase to the paramagnetic phase of ${\mathrm{Cu}}_{2}$${\mathrm{MnGeS}}_{4}$ was observed at low temperatures. The transition field gives ${H}_{E}$\ensuremath{\simeq}90 kOe for the intersublattice exchange field at T=0. The anisotropy field ${H}_{A}$ at T=0 is of order 3 kOe. Differences in the magnetic behavior of several ${\mathrm{Cu}}_{2}$${\mathrm{MnGeS}}_{4}$ samples are attributed to differences in the carrier concentration. Values of the Curie constant C for the samples with x\ensuremath{\le}0.1, and also for x=1, are close to those expected theoretically. For x\ensuremath{\le}0.1, the Curie-Weiss temperature FTHETA is approximately equal to ${\mathrm{FTHETA}}_{0}$x, where ${\mathrm{FTHETA}}_{0}$=-32 K. This is consistent with a random Mn distribution over the group-II cation sites. The value of FTHETA for ${\mathrm{Cu}}_{2}$${\mathrm{MnGeS}}_{4}$ (x=1), however, is well below ${\mathrm{FTHETA}}_{0}$, which suggests that the Mn-Mn exchange interactions decrease with increasing x. In all samples, the field required to saturate the magnetization at low temperatures is below 200 kOe. The relatively low saturation field, and the values for the Curie-Weiss temperatures, show that the Mn-Mn antiferromagnetic interactions are much weaker than in the analogous II-VI dilute magnetic semiconductors. This confirms the prediction of Wolff et al. and Heiman et al.

Wide-gap II-VI superlattices

At this time, there is evident a sharp increase of interest in the II-VI class of semiconducting compounds largely due to recent success in the growth of these materials by molecular beam epitaxy (MBE). One of the more important areas for application of high-quality II-VI films is infrared imaging where CdTe deposited by MBE onto GaAs substrates is proposed as the substrate for subsequent HgCdTe and HgTe/CdTe superlattice deposition. Moreover, interest in the wide-gap II-VI compounds is stimulated by the need for electronically addressable flat panel display devices, and for the development of wide-gap (blue) LED and injection laser devices. For applications in the blue portion of the visible spectrum, ZnSe and ZnS have long been favored candidates. Very high quality ZnSe has recently been grown by MBE. The photoluminescence spectra of the MBE-grown ZnSe samples grown at Purdue University and elsewhere strongly suggests that, in many cases, the film quality exceeds that obtainable in bulk form. In addition to the Conventional II-VI materials, a new class of materials called diluted magnetic semiconductors (DMS) are currently receiving considerable attention. DMS are II-VI semiconductors such as CdTe or ZnSe with a fraction of the group II element substituted by a magnetic transition element such as Mn. The incorporation of Mn leads to very large magnetooptic effects, on the Order of several hundred times that exhibited by conventional semiconductors of a comparable bandgap. An especially significant feature of II-VI DMS materials is the increase in bandgap resulting from Mn incorporation. The band-gap increases with Mn mole fraction in a manner similar to the effect of Al in the (Ga, Al)As system, and with similar implications to the creation of quantum well structures and superlattices. The first part of this paper reviews our recent work involving quantum well structures in the (Zn, Mn)Se system, with the second part emphasizing (Cd, Mn)Te. Included in the discussion of superlattice structures in (Cd, Mn)Te is a description of recent results concerning the heteroepitaxial growth of CdTe on GaAs-materials having a lattice constant mismatch of 14.6 percent.

Electronic Theory of Mn - Alloyed Diluted Magnetic Semiconductors

AbstractRecent calculations of the electronic structure and magnetic interactions in Mn - alloyed II-VI diluted magnetic semiconductors (DMS) are summarized. Detailed band structure results are obtained using an empirical tight-binding, coherent potential approximation approach with input from experiment and local spin density band calculations. The dominant magnetic interactions in these systems result from hybridization between spin-split Mn d states and sp valence bands. Superexchange between Mn moments is well described by a simple three-level model which yields accurate Mn - Mn exchange constants for a variety of II-VI DMS as well as the rocksalt insulators MnO and α-MnS.

Raman Scattering by Magnetic Excitations in Diluted Magnetic Semiconductors

AbstractRaman scattering provides significant information on the nature of the magnetic excitations of diluted magnetic semiconductors (DMS). Transitions involving an exchange of a quantum of angular momentum between the system and the radiation field result in Raman lines with shifts equal to the energy of the magnetic excitations in which the total spin of the crystal changes by ħ. Such transitions have a one-to-one correspondence with those seen in magnetic resonance but observed in the optical rather than in the microwave region of the electromagnetic spectrum. In the Mn-based II-VI DMS a variety of magnetic excitations have been observed in their Raman spectra. Microscopic models underlying these phenomena provide powerful insights into the nature of the magnetism of DMS.