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Spin dependent and nonlinear effects in ZnCrSe and ZnCrTe
Abstract
Magneto-optical investigations suggest that spin-dependent scattering of free carriers and/or spin selectivity in carrier trapping via a midgap level of chromium result in a strong polarization of free-to-bound photoluminescence in wide band gap ZnCrSe and ZnCrTe. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Polycrystalline Zn1−xCrxTe samples for x = 0.00, 0.05, 0.10, 0.15, 0.18, and 0.20 were synthesized by solid-state reaction, and their physical properties were studied for the purpose of identifying a novel non-oxide transparent diluted magnetic semiconductor (DMS). Their transport properties exhibit semiconducting behavior for low Cr concentrations and become conductors for higher Cr concentration concentrations. Ferromagnetic behavior was observed in Cr-doped ZnTe with Curie temperature Tc increasing as Cr concentration increases. At x = 0.20 Cr concentration, the system has Tc above room temperature, indicating an interesting II-VI ferromagnetic semiconductor. Furthermore, optical transparency in the visible light range was found to be 30% - 85% for different Cr concentration concentrations. Also, according to the electrical transport measurement data in this study, the resistivity doesn’t exhibit any significant change as the temperature increases for x=0.20. This consistent resistivity exhibits a possible candidacy for further studies of the sample Zn0.80Cr0.20Te for half-metallic ferromagnetic properties.
Role of deep impurity levels in carrier generation, transport, and recombination were investigated in bulk ZnSe:Cr and ZnTe:V:Al crystals by four-wave mixing technique. The temporal and exposure dependencies of optical nonlinearities in ZnSe:Cr evidenced an influence of Cr(1+)/Cr(2+) states in carrier generation, exhibited very fast carrier relaxation, and revealed the presence of competing recombination mechanisms. Similar investigations in ZnTe:V:Al showed an effective carrier generation from Al-induced defect complexes as well as very fast carrier capture by Zn-vacancies.
Optical and magneto-optical properties of ZnCoO films grown at low
temperature by Atomic Layer Deposition are discussed. Strong wide band
absorption, with onset at about 2.4 eV, is observed in ZnCoO in addition to
Co-related intra-shell transitions. This absorption band is related to Co 2+ to
3+ photo-ionization transition. A strong photoluminescence (PL) quenching is
observed, which we relate to Co recharging in ZnO lattice. Mechanisms of PL
quenching are discussed.
Optical and magnetic properties of ZnMnO films are discussed based on the results of cathodoluminescence, photoluminescence, and magneto-photoluminescence investigations. We show that photoluminescence/cathodoluminescence emissions are strongly quenched and become in-plane inhomogeneous in samples with increased Mn fractions. Strong polarization of photoluminescence is observed, even though excitonic lines do not shift and are not split at magnetic fields up to 6 T.
Optical and magneto-optical properties of ZnMnO films grown at low
temperature by Atomic Layer Deposition are discussed. A strong polarization of
excitonic photoluminescence is reported, surprisingly observed without
splitting or spectral shift of excitonic transitions. Present results suggest
possibility of Mn recharging in ZnO lattice. Strong absorption, with onset at
about 2.1 eV, is related to Mn 2+ to 3+ photo-ionization. We propose that the
observed strong circular polarization of excitonic emission is of a similar
character as the one observed by us for ZnSe:Cr.
An efficient energy-transfer channel from photocarriers to the Mn spin system via a two-dimensional electron gas (2DEG) in n-type modulation-doped Cd0.99Mn0.01Te/Cd0.76Mg0.24Te quantum wells has been found. The energy relaxation of photoexcited carriers is assumed to cause heating of the electron gas, which subsequently leads to an increase of the temperature of the Mn spin system. The mechanism of the energy transfer from the 2DEG to the Mn system involves a spin-flip scattering process originating in a strong electron-Mn exchange interaction. We have observed a suppression of the Mn heating with an increasing magnetic field which results in unusual energy shifts of the exciton, and trion features seen both in the photoluminescence and in reflectivity spectra. A theoretical model has been developed which is in a good agreement with experimental results. In the framework of this model we also analyze the details of the dependence of Mn-ion heating on the electron concentration and on the magnetic ion content.
Single crystals of intrinsic ZnSe were grown by the seeded physical vapor transport method and the diffusion doping was utilized to incorporate Cr in these crystals. The radiative recombinations in these samples with Cr concentration in the range 1.0–10.2×1019 cm−3 were studied by the steady state photoluminescence technique. It was found that the Cr deep centers inhibit the band-to-band emission in Cr-doped ZnSe. Except in undoped single crystals, no emission corresponding to the band-to-band transition was observed from any of the doped samples. Instead, the higher wavelength emissions associated with Cr deep levels were obtained. This capture of photoexcited carriers by deep centers was verified using different excitation wavelengths. The role of chromium impurities in nonradiative recombination processes was also evidenced from the sharp decreases in the deep level emission intensity with increasing Cr concentration. © 1999 American Institute of Physics.
A midinfrared broadly tunable around 2.5 mum room-temperature continuous-wave diode-pumped Cr:ZnSe laser is reported. The laser is tunable over 350 nm and delivers up to 70 mW of the output radiation at 460 mW of the absorbed pump power and 17.5% slope efficiency. We observe the analog of the optical switching process, where the laser output is strongly modulated by only a few milliwatts of the visible radiation. (C) 2002 American Institute of Physics.
We measured free exciton splitting and Faraday rotation of the diluted magnetic semiconductor Zn1-xCrxSe (x
The EPR spectra of Cr2+ in cubic ZnS, ZnSe, ZnTe, and CdTe,
and in hexagonal ZnS and CdS are reported. For each, ΔM=+/-4,
+/-2, +/-1 transitions are observed allowing complete analysis of the
S=2 spin Hamiltonian. A static tetragonal Jahn-Teller distortion is
observed in all cases. Stress-alignment studies are described which
allow a direct estimate of the Jahn-Teller coupling coefficients. From
these, the magnitudes of the Jahn-Teller energies are estimated to be
~500 cm-1. A ligand-field model is developed which
satisfactorily accounts for the fine-structure parameters and their
large variation from one crystal to another. In this treatment, ligand
contributions to the spin-orbit interaction with excited d4
terms are also included and found to be important. Strain-coupling
coefficients, describing the changes in the fine-structure terms under
externally applied stress, have been experimentally determined for CdTe
(Cr2+). A simple theory for this effect is developed which
includes ligand contributions to the spin-orbit interactions but relies
on point-ion estimates for the strain matrix elements. Agreement with
experiment is reasonable for the tetragonal coefficients, but the wrong
sign is predicted for the trigonal coefficients. The calculated trigonal
coefficients for CdS do, however, satisfactorily account for the small
departures from tetragonal symmetry observed in the spin Hamiltonian for
Cr2+ in this hexagonal wurtzite lattice.
We report the observation and analyze the mechanism of efficient blue anti-Stokes emission in ZnSe:Cr. This emission and the intra-shell transition of chromium Cr2+ at 2.4μm are induced by 2+→1+ ionization transition of chromium ions in ZnSe:Cr. The efficiency of the anti-Stokes photoluminescence anticorrelates with the efficiency of the infrared emission. The latter emission dominates at increased temperatures. We conclude that the photoionization excitation mechanism can be applied for optical pumping of 2.4μm laser action in ZnSe:Cr.
The origin of a fast component of Mn2+ intra-shell photoluminescence (PL) decay is discussed. We demonstrate that efficient spin-dependent interactions between localised spins of Mn ions and spins of free carriers result in a variation of formation and decay rates of PL emissions in diluted magnetic semiconductors and affect spin selection rules for radiative recombination transitions.
We review the physical properties of diluted magnetic semiconductors (DMS) of the type A<sup>II</sup> 1-x Mn x B<sup>VI</sup> (e.g., Cd 1-x Mn x Se, Hg 1-x Mn x Te). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic A<sup>II</sup>B<sup>VI</sup> ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d<sup>5</sup> shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys. In particular, we discuss the mechanism of the Mn<sup>+</sup><sup>+</sup>‐Mn<sup>+</sup><sup>+</sup> exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susceptibility of DMS in the paramagnetic range; we describe a somewhat empirical picture of the spin‐glasslike freezing in the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys, and its relationship to the short range antiferromagnetic order revealed by neutron scattering; and we point out some not yet fully understood questions concerning spin dynamics in DMS revealed by electron paramagnetic resonance. We then discuss the sp‐d exchange interaction between the sp band electrons of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloy and the 3d<sup>5</sup> electrons associated with the Mn atoms. Here we present a general formulati-
on of the exchange problem, followed by the most representative examples of its physical consequences, such as the giant Faraday rotation, the magnetic‐field‐induced metal‐to‐insulator transition in DMS, and the properties of the bound magnetic polaron. Next, we give considerable attention to the extremely exciting physics of quantum wells and superlattices involving DMS. We begin by describing the properties of the two‐dimensional gas existing at a DMS interface. We then briefly describe the current status of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> layers and superlattices (systems already successfully grown; methods of preparation; and basic nonmagnetic properties of the layered structures). We then describe new features observed in the magnetic behavior of the quasi‐two‐dimensional ultrathin DMS layers; and we discuss the exciting possibilities which the sp‐d exchange interaction offers in the quantum‐well situation. Finally, we list a number of topics which involve DMS but which have not been explicitly covered in this review such as elastic properties of DMS, DMS‐based devices, and the emerging work on diluted magnetic semiconductors other than the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys—and we provide relevant literature references to these omitted topics.
A detailed light-induced electron-spin-resonance study of iron and chromium impurities in ZnSe is presented. The position of the Fe 2+/3+ energy level in the band gap of ZnSe is determined. The role of iron and chromium impurities in nonradiative recombination processes in ZnSe is discussed.
A theory of superexchange in Cr-based ${\mathit{A}}_{\mathrm{II}}$${\mathit{B}}_{\mathrm{VI}}$ diluted magnetic semiconductors (DMS's) is presented. The spin interaction between ${\mathrm{Cr}}^{2+}$ ions, calculated numerically for zinc chalcogenides, exhibits properties unique in DMS's, it is ferromagnetic, and depends on the relative orientations of the ionic Jahn-Teller distortions. We explain the origin of this unusual behavior and predict a ferromagnetic character of the superexchange in other Cr-based ${\mathit{A}}_{\mathrm{II}}$${\mathit{B}}_{\mathrm{VI}}$ DMS's. \textcopyright{} 1996 The American Physical Society.
The results of comprehensive studies of photoluminescence, photoluminescence excitation, electron spin resonance, and photoconductivity of chromium-doped ZnSe are discussed. It is shown that the blue anti-Stokes luminescence of ZnSe:Cr is induced by the two-step ionization transitions of Cr in which the charge state of chromium is changed from 2+ to 1+ and from 1+ to 2+. We also propose that some cooperative processes can contribute to excitation of the anti-Stokes luminescence. The Auger-type energy transfer mechanism of the photoluminescence excitation is discussed. © 1996 The American Physical Society.
The s,p-d exchange interaction in Cr-based diluted magnetic semiconductors was studied by means of polarized magnetoreflectance and magnetization at T=2 K. The exchange constant N0alpha-N0beta for four different zinc chalcogenides (Zn1-xCrxSe, Zn1-xCrxSe0.95S0.05, Zn1-xCrxS, and Zn1-xCrxTe) was determined. Assuming constant N0alpha a chemical trend in N0beta variation is discussed.
Some interesting physical effects are predicted for diluted magnetic semiconductors. It is shown that the spin properties of zinc-blende-type ${\mathit{A}}^{\mathrm{II}}$${\mathit{B}}^{\mathrm{VI}}$ diluted magnetic semiconductors depend crucially on the electronic configuration of the transition-metal ions. Depending on the filling of the ionic d shell, the kinetic exchange Hamiltonian evolves from antiferromagnetic to ferromagnetic Kondo-like form via more complicated forms sensitive to Jahn-Teller distortions.
Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena
in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic
interactions that lead to a Curie temperature (T
C) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition
metals in 1950, can explain T
C of Ga1−
xMnxAs and that of its II-VI counterpart Zn1−
xMnxTe and is used to predict materials with T
Cexceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.
The absorption and emission properties of transition metal
(TM)-doped zinc chalcogenides have been investigated to understand their
potential application as room-temperature, mid-infrared tunable laser
media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with
Cr<sup>2+</sup>, Co<sup>2+</sup>, Ni<sup>2+</sup>, or Fe<sup>2+</sup>
have been evaluated. The absorption and emission properties are
presented and discussed in terms of the energy levels from which they
arise. The absorption spectra of the crystals studied exhibit strong
bands between 1.4 and 2.0 μm which overlap with the output of
strained-layer InGaAs diodes. The room-temperature emission spectra
reveal wide-band emissions from 2-3 μm for Cr and from 2.8-4.0 μm
for Co, Cr luminesces strongly at room temperature; Co exhibits
significant losses from nonradiative decay at temperatures above 200 K,
and Ni and Fe only luminesce at low temperatures, Cr<sup>2+</sup> is
estimated to have the highest quantum yield at room temperature among
the media investigated with values of ~75-100%. Laser demonstrations of
Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity
with a Co:MgF<sub>2</sub> pump laser. The output of both lasers were
determined to peak at wavelengths near 2.35 μm, and both lasers
demonstrated a maximum slope efficiency of approximately 20%. Based on
these initial results, the Cr<sup>2+</sup> ion is predicted to be a
highly favorable laser ion for the mid-IR when doped into the zinc
chalcogenides; Co<sup>2+</sup> may also serve usefully, but laser
demonstrations yet remain to be performed