H.P. Gislason

Paramagnetism in Mn/Fe implanted ZnO

Prompted by the generally poor understanding of the nature of magnetic phenomena in 3d-metal doped ZnO, we have undertaken on-line F57e Mossbauer spectroscopy on ZnO single crystals in an external magnetic field of 0.6 T, following the implantation of radioactive M57n ions at room temperature. The Mossbauer spectra of the dilute Fe impurities are dominated by sextets whose angular dependence rules out an ordered magnetic state (which had been previously proposed) but are well accounted for on the basis of Fe3+ paramagnetic centers on substitutional Zn sites with unusually long relaxation times (>20 ns).

Lattice locations and properties of Fe in Co/Fe co-implanted ZnO

The lattice locations and electronic configurations of Fe in 57Co/57Fe implanted ZnO (to (5‐6) × 1014 Fe/cm−2) have been studied by 57Fe Mossbauer emission spectroscopy. The spectra acquired upon room temperature implantation show ∼20% of the probe atoms as Fe2+ on perturbed Zn sites and the remaining fraction as Fe2+ in damage sites of interstitial character. After annealing at 773 K, ∼20% remain on crystalline sites, while the damage fraction has partly disappeared and instead a ∼30% fraction occurs as high‐spin Fe3+, presumably in precipitates. This suggests that precipitation of Co/Fe in ZnO likely takes place at relatively low temperatures, thus explaining some of the discrepancies in the literature regarding magnetic properties of 3d metal‐doped ZnO.

Characterization of photoconductivity in AlxGa1?xN materials

We report the observation of optical quenching of photoconductivity at room temperature in AlGaN materials grown by metalorganic chemical vapour deposition. Multiple hole traps located in the range between Ev and Ev + 1.25 eV are believed to be responsible for the optical quenching. Also, investigation of the photocurrent allows the identification of a deep centre that controls the slow buildup kinetic of the photocurrent when the samples are illuminated with sub-bandgap photon energies.

Fe charge state adjustment in ZnO upon ion implantation

The influence of the ion implantation process on the charge state of dilute (57)Fe impurities implanted as radioactive (57)Mn in ZnO is investigated by (57)Fe emission Mössbauer spectroscopy. One sample is additionally implanted with stable (23)Na impurities. Both Fe(2+) and Fe(3+) charge states are observed, and the Fe(3+)/Fe(2+) ratio is found to increase with the fluence of both (57)Mn/(57)Fe and (23)Na ions, demonstrating that the build-up of Fe(3+) is not related to the chemical nature of the implanted ions. The results are interpreted in terms of radiation damage induced changes of the Fermi level, and illustrate that the Fe(3+)/Fe(2+) ratio can be adjusted by ion implantation. The spin-lattice relaxation time for Fe(3+) in ZnO is found to be independent of the implantation fluence, and is evidently an intrinsic property of the system.

Spin–lattice relaxations of paramagnetic Fe3+ in ZnO

The spin–lattice relaxation rate of paramagnetic Fe3+ in single-crystalline ZnO has been determined following low-fluence (Φ<1012 cm−2) 60 keV implantation of 57Mn+ (T1/2=1.5 min) and emission Mossbauer spectroscopy on the 57Fe daughter nucleus in the temperature range from 300 to 664 K. The spin–lattice relaxation of Fe3+ is found to follow a T9 temperature dependence, in contrast to the T2 dependence expected for a two-phonon Raman process determined in both single-crystal MgO and α-Al2O3 using the same analysis method of the Mossbauer spectra measured without an applied external magnetic field. This is an unexpected result since ZnO has a lower Debye temperature than both MgO and α-Al2O3.

Defect annealing in Mn/Fe-implanted TiO2 (rutile)

A study of the annealing processes and charge state of dilute Fe in rutile TiO2 single crystals was performed in the temperature range 143?662?K, utilizing online 57Fe emission M?ssbauer spectroscopy following low concentrations ( ?350?K.

Ferromagnetism and point defects in ZnCoO

We report on optical and electrical properties of oxygen-vacancy (VO) related defects in cobalt-doped ZnO materials grown using molecular beam epitaxy (MBE). We probe the concentration of the VO in samples with various cobalt contents, which were annealed under different conditions, and correlate it to the magnetic properties of the samples. Our results suggest that the oxygen vacancy is a major player in the observed ferromagnetism of Co-doped ZnO.

The observation of persistent photoconductivity in N-doped p-type ZnSe/GaAs heterojunctions

We report on the persistent photocurrent (PPC) in p-type nitrogen-doped ZnSe epilayers grown by molecular beam epitaxy on GaAs. Its time-evolution scale ranges from several minutes to hours. The PPC is observed with a non-exponential decay up to room temperature in some samples. A typical decay is composed of an initial transient, which is well described by a stretched exponential, and a subsequent slower transient. The time constant of the first transient has a thermal activation energy of about 0.35 eV. Annealing samples at C decreases both the magnitude of the initial transient of the PPC and the height of a deep-level transient spectroscopy (DLTS) signal from the interface states between ZnSe and GaAs, suggesting that there is a correlation between the persistent photocurrent and the interface states. Also the dependence of the PPC on the wavelength of the illumination suggests that the initial transient originates from the hetero-interface. From current-temperature measurements, we estimate the barrier at the heterojunction to be 0.8 eV. This large value indicates that holes are trapped in a two-dimensional quantum well at the heterojunction to the GaAs substrate. We conclude that the PPC has two origins; namely the presence of metastable centres in the ZnSe layer, close to the hetero-interface, which are similar to DX centres in GaAlAs, and tunnelling of trapped holes from the two-dimensional quantum well through the barrier.

Charge states and lattice sites of dilute implanted Sn in ZnO

The common charge states of Sn are 2+  and 4+. While charge neutrality considerations favour 2+  to be the natural charge state of Sn in ZnO, there are several reports suggesting the 4+  state instead. In order to investigate the charge states, lattice sites, and the effect of the ion implantation process of dilute Sn atoms in ZnO, we have performed 119Sn emission Mössbauer spectroscopy on ZnO single crystal samples following ion implantation of radioactive 119In (T ½  =  2.4 min) at temperatures between 96 K and 762 K. Complementary perturbed angular correlation measurements on 111mCd implanted ZnO were also conducted. Our results show that the 2+  state is the natural charge state for Sn in defect free ZnO and that the 4+  charge state is stabilized by acceptor defects created in the implantation process.

Passivation of impurities in semiconductors by hydrogen and light metal ions

Books as well as numerous articles have been written about hydrogen passivation in classical semiconductors such as Si and GaAs. The subject has gained a renewed interest recently since hydrogen is widely considered to saturate the hole conductivity of the wide bandgap semiconductors GaN and ZnSe which are currently most promising for blue light emitting devices. Other group-I impurities are capable of compensating the electrical conductivity of semiconductors both through directly neutralising (passivating) the impurity or providing space charge of polarity opposite to that of the dominating one. The paper reviews the similarities and differences between hydrogen and its light metallic neighbour in the periodic table, lithium. Also we provide a comparison with the heavier interstitial copper which is known for its ability to passivate shallow acceptors. Finally fundamental differences between shallow-level and deep level passivation will be addressed.