Stefan Decoster

Tensile strained GeSn on Si by solid phase epitaxy

We demonstrate single crystalline GeSn with tensile strain on silicon substrates. Amorphous GeSn layers are obtained by limiting the adatom surface mobility during deposition. Subsequent annealing transforms the amorphous layer into single crystalline GeSn by solid phase epitaxy. Excellent structural quality is demonstrated for layers with up to 6.1% of Sn. The GeSn layers show tensile strain (up to +0.34%), which lowers the difference between direct and indirect band transition and makes this method promising for obtaining direct band gap group IV layers. GeSn with 4.5% Sn shows increased optical absorption compared to Ge and an optical band gap of 0.52 eV.

Direct evidence for Sb as a Zn site impurity in ZnO

The lattice location of ion implanted antimony in zinc oxide has been determined by means of β− emission channeling from the radioactive S124b isotope. Following 30 keV implantation of S124b into a single-crystalline ZnO sample to a fluence of 1×1014 cm−2, the angular-dependent emission rate of β− particles around several crystallographic directions was measured with a position-sensitive Si detector. The majority of Sb was found to occupy Zn sites, with the possible fraction on O sites being at maximum 5%–6%.

Implantation-induced damage in Ge: strain and disorder profiles during defect accumulation and recovery

We present an experimental study of structural lattice damage in Ge induced by ion implantation. From the strain and disorder profiles, calculated from x-ray diffraction and ion channelling experiments we have investigated the defect accumulation as a function of ion fluence, mass, energy and current density as well as the damage recovery and recrystallization of the implanted region upon annealing. The damage accumulation process can be divided into three different regimes, based on the ion fluence. In the lowest fluence regime, the strain and the defect fraction are linearly proportional to the ion fluence, and the number of defects in the implanted layer is directly related to the deposited energy that is converted into the creation of vacancies. In the second regime, the damage accumulation process is more efficient, due to the increased defect density in the implanted layer. The third fluence regime starts at the critical fluence for amorphization, and this value has been determined for a wide range of ion masses and energies. The recovery study of the implantation-induced damage has revealed two distinct annealing steps. Rapid thermal annealing at temperatures as low as 100 °C results in the removal of isolated defects, which are present in the low-fluence implanted samples, as well as in the tail of the implantation profile of heavily damaged samples. Annealing at 350 °C results in the recrystallization of amorphous Ge at the amorphous–crystalline interface at a rate of 14 ± 3 nm min−1. Although Ge amorphizes at much lower fluences than Si, the influence of the studied implantation parameters on the damage accumulation process is comparable for both group IV semiconductors. This extended experimental overview of implantation-induced structural damage partly fills the large knowledge gap on implantation-related issues in Ge, and provides relevant and complementary information for defect studies in Ge and, in general, for any study using implanted Ge.

Lattice location study of implanted In in Ge

We report on emission channeling experiments to determine the lattice location and the thermal stability of implanted I111n atoms in Ge. The majority of the In atoms was found on the substitutional site, which is a thermally stable site at least up to 500 °C. We also found strong indication that directly after implantation, a fraction of the implanted I111n atoms occupies the bond-centered (BC) site. This fraction disappears after annealing at 300 °C. From comparison with ab initio calculations, electrical studies, and perturbed angular correlation experiments, the In atoms on the BC site can be related to In-vacancy and In-self-interstitial defect complexes. The activation energy for dissociation of this BC related defect was found to be below 1.6 eV.

Searching for room temperature ferromagnetism in transition metal implanted ZnO and GaN

Significant progress in the field of wide-gap dilute magnetic semiconductors (DMS) depends on the discovery of a material system which not only shows high-temperature ferromagnetism but is also simple to prepare and thus easy to reproduce. In this context, ion implantation is an attractive doping method, being both relatively simple and highly reproducible. Here, we report on the search for high-temperature ferromagnetism in ZnO and GaN implanted with Mn, Fe, and Co, prepared under a wide range of implantation and post-processing conditions. We focused on the low concentration regime (∼0.3−4%) in order to avoid phase segregation and applied strict experimental procedures to avoid ferromagnetic contamination. Despite the wide range of materials, implantation and post-processing conditions, none of the DMS systems showed room-temperature ferromagnetism. These results support the view that dilute transition-metal moments do not order ferromagnetically in ZnO and GaN.

Direct identification of interstitial Mn in heavily p-type doped GaAs and evidence of its high thermal stability

We report on the lattice location of Mn in heavily p-type doped GaAs by means of β− emission channeling from the decay of M56n. The majority of the Mn atoms substitute for Ga and up to 31% occupy the tetrahedral interstitial site with As nearest neighbors. Contrary to the general belief, we find that interstitial Mn is immobile up to 400 °C, with an activation energy for diffusion of 1.7–2.3 eV. Such high thermal stability of interstitial Mn has significant implications on the strategies and prospects for achieving room temperature ferromagnetism in Ga1−xMnxAs.

Diluted manganese on the bond-centered site in germanium

The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes.

Electrical characterization of defects introduced in n-type Ge during indium implantation

The authors have employed deep level transient spectroscopy to investigate the defects introduced in n-type Ge during 160keV indium (In) ion implantation. Our results show that In implantation introduces three prominent electron traps with energy levels at EC−0.09eV, EC−0.15eV, and EC−0.30eV, respectively. The authors have found that these defects are different from the point defects introduced by electron irradiation but that they do not involve In. Annealing at 600°C removed all the defects introduced during In implantation but results in a single prominent defect with a level at EC−0.35eV.

Mixed Zn and O substitution of Co and Mn in ZnO

The physical properties of an impurity atom in a semiconductor are primarily determined by the lattice site it occupies. In general, this occupancy can be correctly predicted based on chemical intuition, but not always. We report on one such exception in the dilute magnetic semiconductors Co- and Mn-doped ZnO, experimentally determining the lattice location of Co and Mn using {beta}{sup -}-emission channeling from the decay of radioactive {sup 61}Co and {sup 56}Mn implanted at the ISOLDE facility at CERN. Surprisingly, in addition to the majority substituting for Zn, we find up to 18% (27%) of the Co (Mn) atoms in O sites, which is virtually unaffected by thermal annealing up to 900 deg. C. We discuss how this anion site configuration, which had never been considered before for any transition metal in any metal oxide material, may in fact have a low formation energy. This suggests a change in paradigm regarding transition-metal incorporation in ZnO and possibly other oxides and wide-gap semiconductors.

Experimental evidence of tetrahedral interstitial and bond-centered Er in Ge

We report on an emission channeling study of the lattice site location of implanted Er in Ge together with its thermal stability. We found direct experimental evidence of Er atoms located on the tetrahedral (T) interstitial site and on the bond-centered (BC) site, with a maximum total occupancy after annealing at 400°C. Whereas Er is expected to occupy the T site in a diamond crystal structure, the observation of BC Er in Ge is more surprising and believed to be related to the Er-vacancy defect in the split-vacancy complex configuration.