Cr-doped ZnTe Research Articles

Chemically induced ferromagnetism near room temperature in single crystal (Zn1-x Cr x )Te half-metal.

Magnetic semiconductors are at the core of recent spintronics research endeavors. Chemically doped II-VI diluted magnetic semiconductors, such as (Zn1-x Cr x )Te, provide a promising platform in this quest. However, a detailed knowledge of the microscopic nature of magnetic ground state is necessary for any practical application. Here, we report on the synergistic study of (Zn1-x Cr x )Te single crystals using elastic neutron scattering measurements and density functional calculations. For the first time, our research unveils the intrinsic properties of ferromagnetic state in a macroscopic specimen of (Zn0.8Cr0.2)Te. The ferromagnetism is onset at T C ∼ 290 K and remains somewhat independent to modest change in the substitution coefficient x. We show that magnetic moments on Zn/Cr sites develop ferromagnetic correlation in the a-c plane with a large ordered moment of μ = 3.08 μ B. Magnetic moment across the lattice is induced via the mediation of Te sites, uncoupled to the number of dopant carriers as inferred from the density functional calculation. Additionally, the ab initio calculations also reveal half-metallicity in x = 0.2 composition. These properties are highly desirable for future spintronic applications.

Transparency and room temperature ferromagnetism in diluted magnetic polycrystalline Zn1−xCrxTe non-oxide II-VI semiconductor compounds

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.

High performance intermediate-band solar cells based on ZnTe:Cr with ZnO:Al electron transport layer

An intermediated-band in Cr-doped ZnTe (ZnTe:Cr) thin films was analyzed and high performance intermediate-band solar cells (IBSCs) were fabricated with a high-quality ZnO:Al electron transport layer (ETL). The energy band structure and defect states of the ZnTe:Cr absorber layer by means of ultraviolet photoelectron spectroscopy and deep level transient spectroscopy were investigated, and then two defect levels, L2 and L4, are positioned 0.3 and 1.14 eV above the valence band. Especially, it was confirmed that the 1.14 eV activation energy of the L4 defects corresponds to the energy required to reduce Cr+ to Cr2+, which these defects are responsible for the intermediate band observed in the ZnTe:Cr thin film. From the optical and electrical measurements, the ZnO:Al thin film grown under the partial oxygen pressure (p(O2)) of 1 mTorr show high mobility, low resistivity, and few antisite oxygens defects. Finally, the ZnTe:Cr IBSC with ETL of ZnO:Al grown under 1 mTorr p(O2) showed photoelectric power conversion efficiency of 8.13%.

High power conversion efficiency of intermediate band photovoltaic solar cell based on Cr-doped ZnTe

We report on a high-performance intermediate band solar cell (IBSC) based on Cr-doped ZnTe (ZnTe:Cr) fabricated using a pulsed laser deposition (PLD) method. Chromium (Cr) was uniformly distributed in the ZnTe:Cr thin film with an atomic concentration of about 3.5%, and the ZnTe:Cr thin film showed p-type electrical conductivity. The ZnTe:Cr thin film had higher absorption coefficients than those of undoped ZnTe films in the photon energy range below band gap of ZnTe (2.2eV). The enhanced absorption coefficients of the ZnTe:Cr thin film were attributed to the photoionization energy between Cr2+ and Cr+ (Cr2+⇌ Cr+), which acted as the IB to absorb photons below the bandgap of ZnTe (2.2eV). Illumination with an AM 1.5G solar spectrum on the ZnTe:Cr IBSC generated a large short circuit current of 21.18mA/cm2, an open circuit voltage of 0.48V, and a fill factor of 0.58, yielding a power conversion efficiency (PCE) of 5.9%, the highest reported PCE in an IBSC based on impurity-doped ZnTe.

Effect of Cr/In-doping on the crystalline quality of bulk ZnTe crystals grown from Te solution by temperature gradient solution growth (TGSG) method**Project supported by the National Basic Research Program of China (No. 2011CB610406), the National Natural Science Foundation of China (No. 51372205), the 111 Project of China (No. B08040), the Specialized Research Fund for the Doctoral Program of Higher Education

The properties of undoped, Cr-doped, and In-doped bulk ZnTe crystals grown by the TGSG method were compared. Cr/In-doping leads to a slight red-shift of the absorption edge. Cr-doping also creates two characteristic absorption bands, centered at about 1750 nm and beneath the fundamental absorption edge. However, the fundamental reflectance spectra are not sensitive to the dopants. The resistivity of undoped, Cr-doped, and In-doped ZnTe is about 102 Ω·cm, 103 Ω·cm, and 108 Ω·cm, respectively. Only In-doped ZnTe has an IR transmittance higher than 60% in the range of 500 to 4000 cm−1. However, the IR transmittance of Cr-doped ZnTe is very low and decreases greatly as the wavenumber increases, which is mainly attributed to the scattering effects caused by some defects generated by Cr-doping.

Characterization of Cr-doped ZnTe Crystals Grown by Temperature Gradient Solution Growth (TGSG)

Cr-doped ZnTe ingot was grown by temperature gradient solution growth (TGSG), using CrTe and Te as dopant source and solvent, respectively. Single crystals in the head of the ingot have larger size of more than 10 mm× 10 mm and less Te inclusions. The size, shape and distribution of Te inclusions well reflect the thermal field distribu- tion of the ingot. Radial asymmetric thermal field of the ingot leads to radial asymmetric distribution of Te inclusions. The thermal migration of Te inclusions under thermal gradient results in their merging, enlargement and enlongation. Defects such as cracks and voids can be introduced into ZnTe matrix by Te inclusions. Some excess amount of CrTe is rejected by the advancing interface, which indicates that the present growth method has self purification effect. The re- sistivity of Cr-doped ZnTe ( about 1000 Ω·cm ) is slightly higher than that of ZnTe (about 300 Ω·cm). Cr-doped ZnTe exhibits an absorption centered at about 1750 nm, which means Cr 2+ ions are successfully incorporated into ZnTe. IR

Preparation and Characterization of The Chromium Doped ZnTe Thin Films

The chromium doped zinc telluride (ZnTe:Cr) as well as ZnTe thin films and their sandwich structures were prepared onto glass substrate by thermal evaporation method under the vacuum of 10-5 Torr. We have studied the structural, optical and electrical properties of thermally evaporated Cr-doped ZnTe thin films as a function of Cr concentration. XRD measurements show that Cr-doped ZnTe films possess the mix phase of cubical and hexagonal structure of ZnTe thin film. The optical energy band gap (Eg) calculated from the optical absorption spectra which was observed around 2.57 eV for undoped ZnTe, and reduced to 1.47 eV for the Cr-doped thin films. The result of I-V characteristics is also presented in this paper.

Evolution of magnetic circular dichroism of pure ZnTe in magnetic field: Spectral similarity between undoped and Cr-doped ZnTe

Magnetic circular dichroism (MCD) spectra of pure ZnTe in various external magnetic field have been studied with first-principles calculations. By applying magnetic field on undoped semiconductor, the effects of spin splitting in host's bands due to $s,p\text{\ensuremath{-}}d$ exchange interactions in diluted magnetic semiconductors (DMSs) have been largely simulated. At the same time, the complexity of band structure in DMS is avoided since no magnetic ion is doped, which facilitates the band-to-band analysis of MCD spectra. The obtained MCD spectra of undoped ZnTe within low magnetic field are found to be proportional to the field strength and comparable with the experimental measurements. Within high magnetic field, where the Zeeman splitting is comparable with spin-orbit coupling strength, the MCD spectra become as similar as those of Cr-doped ZnTe around the $L$ critical points. Thus, the MCD signal contributed by the spin-splitted bands of host semiconductor is identified and separated from those due to the introduction of $\mathrm{Cr}\phantom{\rule{0.2em}{0ex}}3d$ or $4s$ impurity bands. These results indicate that the experimentally observed MCD spectra can be well reproduced and understood by band structure calculations.

MBE growth and properties of Cr-doped ZnTe on GaAs(001)

Cr-doped ZnTe diluted magnetic semiconductor thin films were grown on semi-insulating GaAs [001] substrates at low temperature by solid-source molecular-beam epitaxy. Zn 1− x Cr x Te samples with Cr concentrations x = 0.026 and x = 0.141 were prepared. The magnetization versus magnetic field ( M– H) measurement showed a clear hysteresis loop at low temperature for these samples. For higher Cr doping with nominal Cr concentration of x > 0.18, we obtained a Curie temperature of 365 K; the highest reported so far for thin film sample. However, this strong ferromagnetic momentum could possibly be due to Cr 1− δ Te precipitate in the Cr-doped ZnTe system.

First-principles study of ferromagnetic coupling in Zn1−xCrxTe thin film

Using gradient-corrected density functional theory and supercell technique, we have calculated total energies, electronic structure, and magnetic properties of Cr-doped ZnTe in both bulk and thin-film configurations. Calculations with full geometry optimization for a Zn1−xCrxTe supercell were carried out for different Cr concentrations (x=0.095, 0.143, and 0.19) and by varying the sites Cr atoms occupy. We show that the ferromagnetic phase of Zn1−xCrxTe in both bulk and thin film is energetically the most preferable state irrespective of the concentration and∕or site occupation of the Cr atom. The strong hybridization between Cr3d and Te5p states is found to be responsible for the ferromagnetic coupling, in agreement with recent experiments.

Photoluminescence of ZnTe and ZnTe:Cr grown by molecular-beam epitaxy

Photoluminescence (PL) from undoped and chromium-doped ZnTe epilayers was studied in the spectral range from 0.3to2.4eV over the temperature range from 5K to room temperature. Films approximately 2μm thick were grown by molecular-beam epitaxy on GaAs substrates. For undoped ZnTe, spectra are dominated by a sharp acceptor-bound-exciton line related to arsenic, with only weak emission observed due to substitutional oxygen (OTe). An analysis of strain-induced shifts in excitonic energies was performed to clarify transition assignments for two PL bands near 2.375 and 2.379eV. For ZnTe:Cr epilayers, the infrared Cr2+ emission near 2.5μm (0.5–0.6eV) was found to be a function of both doping concentration and Zn∕Te beam-equivalent-pressure ratio. Direct excitation of the Cr2+ ions with 1.89-μm light produced emission up to room temperature. Indirect excitation with 514.5-nm light produced emission that could be detected up to 180K. Luminescence decay curves measured from a ZnTe:Cr film ([Cr]∼1×1018cm−3) using pulsed excitation at 355 and at 532nm indicated two lifetimes: τ1=3(±1)μs and τ2⩾10μs. The shorter lifetime agrees with values reported previously for Cr-doped ZnTe bulk crystals and thin films under 1.9-μm pulsed excitation.

Electronic structure and magnetism of diluted magnetic semiconductors

The electronic structure and magnetism of selected diluted magnetic semiconductors(DMS) is reviewed. It is argued that the effect of antisite defects plays an important role inthe magnetism of DMS materials and that these defects lower the saturation moment andordering temperature. We also show that the interatomic exchange of these materials isshort ranged. By combining first principles calculations of interatomic exchangeinteractions with a classical Heisenberg model and Monte Carlo simulations, we show thatthe observed critical temperatures of a broad range of diluted magnetic semiconductors,involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, are reproducedwith good accuracy. We show that agreement between theory and experiment isobtained only when the magnetic atoms are randomly positioned on the Ga (or Zn)sites. This suggests that the ordering of DMS materials is heavily influenced bymagnetic percolation and that the measured critical temperatures should be verysensitive to details in the sample preparation, in agreement with observations.