A. Amore Bonapasta

How much room for BiGa heteroantisites in GaAs1−xBix?

We addressed the issue of bismuth heteroantisite defects (BiGa) in GaAs1−xBix/GaAs epilayers by coupling x-ray absorption spectroscopy at the bismuth edge with density functional theory calculations of the defect structure. Calculations predict a large relaxation of the Bi-As interatomic distances when Bi atoms substitute Ga, however we found no experimental evidence of it. Quantitative analysis of the x-ray absorption spectra allows us to establish a maximum concentration limit for BiGa, which corresponds to about 5% of the total Bi atoms. BiGa do not account for the modifications in the spectra previously attributed to short range ordering.

Ferromagnetism and Conductivity in Hydrogen Irradiated Co-Doped ZnO Thin Films.

Impressive changes in the transport and ferromagnetic properties of Co-doped ZnO thin films have been obtained by postgrowth hydrogen irradiation at temperatures of 400 °C. Hydrogen incorporation increases the saturation magnetization by one order of magnitude (up to ∼1.50 μB/Co) and increases the carrier density and mobility by about a factor of two. In addition to the magnetic characterization, the transport and structural properties of hydrogenated ZnO:Co have been investigated by Hall effect, local probe conductivity measurements, micro-Raman, and X-ray absorption spectroscopy. Particular care has been given to the detection of Co oxides and metal Co nanophases, whose influence on the increase in the transport and ferromagnetic properties can be excluded on the ground of the achieved results. The enhancement in ferromagnetism is directly related to the dose of H introduced in the samples. On the contrary, despite the shallow donor character of H atoms, the increase in carrier density n is not related to the H dose. These apparently contradictory effects of H are fully accounted for by a mechanism based on a theoretical model involving Co-VO (Co-O vacancy) pairs.

Effects of n-type doping on active Fe sites in ion implanted Fe in InP

An n-type doping significantly increases the number of active Fe atoms substituting In (FeIn) in Fe-implanted InP. We address the origin of this doping effect by investigating the relative stability of neutral and charged FeIn states with first-principles theoretical methods. The present results show that donor-acceptor pairs have direct stabilizing effects on FeIn related to charge rearrangements strongly localized at the Fe site and involving d orbitals. The resulting microscopic description of the doping effects accounts for the experimental findings and provides a significant guideline for tuning electronic and optical properties of Fe implanted InP and InP-based compounds.

Photoluminescence and infrared absorption study of isoelectronic impurity passivation by hydrogen

The effects of H irradiation and thermal annealing on the optical properties of (InGa)(AsN) heterostructures have been investigated by photoluminescence (PL) and infrared absorption, as well as by theoretical methods. It has been found that different N clusters contribute to the band gap red-shift reported for (InGa)(AsN) alloys, with a sizable localization of the carrier wavefunctions around N atoms. Infrared absorption measurements indicate that two different NH complexes are formed, whose vibrational frequencies are in good agreement with theoretical estimates. The ability of hydrogen to passivate different isoelectronic impurities is confirmed by PL results in H irradiated Zn(STe).

Defect-induced Magnetism in Cobalt-doped ZnO Epilayers

We used a synergic Co-edge X-ray absorption spectroscopy (XAS) and density functional theory calculations approach to perform a study of defects which could account for the room temperature ferromagnetism of ZnCoO, an oxide of great potential interest in semiconductor spintronics. Our results suggest that a key role is played by specific defect complexes in which O vacancies are located close to the Co atoms. Extended defects such as Co clusters have a marginal function, although we observe their formation at the epilayer surface under certain growth conditions. We also show preliminary results of the study of hydrogen-induced defects in ZnCoO epilayers deliberately hydrogen irradiated via a Kaufman source. Hydrogen was in fact predicted to mediate a ferromagnetic spin-spin interaction between neighboring magnetic impurities.

Influence of the surface structure on the magnetic properties of Zn1−xCoxO

The surface of ferromagnetic Zn1−xCoxO wurtzite epilayers has been studied by coupling atomic force microscopy and advanced x-ray spectroscopy. We found that, even in high-quality epilayers, the formation of Co clusters and iso-space-group Co-rich regions can take place at the sample surface while the bulk maintains random Co distribution. Comparing structural characterization with magnetometry, we show that these surface modifications are not at the origin of the magnetic properties of the material. Quite the reverse, ferromagnetic behavior is enhanced in the sample characterized by the less defective surface.

Magnetic and X-ray absorption investigations of Co-doped ZnO films

We present an investigation of magnetic and structural properties of Co-doped ZnO (ZCO) film grown by pulsed laser deposition at different dopant concentrations (cCo). X-ray diffraction patterns show that the films are single phase and exhibit ferromagnetism (FM) above room temperature (RT) with coercive fields up to 700 Oe. X-ray absorption fine-structure spectroscopy (XAFS) at the Co edge suggests that in films grown below 600°C dopant clustering involve less than 10% of the Co atoms in the alloy. In samples grown at higher temperature a larger fraction of Co atoms is involved in the formation of small metallic clusters. The experimental work has been accompanied by preliminary first-principles Density Functional Theory calculations.

Theory of Hydrogen Complexes in MnxGa1-xAs Dilute Magnetic Semiconductors

Atomic hydrogen diffuses in semiconductor lattices and binds to impurities by forming complexes that can lead to a full neutralization of the impurity effects. In the present paper, the structural, vibrational, electronic and magnetic properties of complexes formed by H in the MnxGa1-xAs (x=0.03) dilute magnetic semiconductor have been investigated by using first-principles DFT-LSD and LDA+U theoretical methods. The results account for recent experimental findings showing a H passivation of the electronic and magnetic properties of Mn in GaAs. Moreover, they show that electron correlation has crucial effects on the properties of H-Mn complexes.

Defect engineering in III–V ternary alloys: effects of strain and local charge on the formation of substitutional and interstitial native defects

Abstract The effects of external and internal strains and of defect charges on the formation of vacancies, antisites and interstitials in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that strain and doping permit a defect engineering of III–V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while vacancies may be favored only by extreme conditions of compressive strain. Interstitials may be moderately favored by doping and tensile strain.

Defect engineering in III–V ternary alloys: Effects of strain and local charge on the formation of native deep defects

Abstract The effects of external and internal strains, and of defect charges on the formation of vacancies and antisites in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that a proper use of strain and defect charges permits the development of a defect engineering of III–V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while the formation of vacancies may be favored only by extreme conditions of compressive strain.