Branko Pivac

Vacancy-related complexes in neutron-irradiated silicon

Electrically active defects induced by neutron irradiation in n-type Czochralski-grown (Cz) Si crystals have been studied by means of capacitance transient techniques. These neutron-induced defects are compared with those created by electron irradiation and self-ion implantation. Four electron traps with the activation energies for electron emission of 0.12, 0.16, 0.24 and 0.42 eV were observed after neutron irradiation in phosphorous-doped Cz Si crystals. It is inferred that the E(0.12) and E(0.16) traps are related to the single-acceptor states of the silicon self-interstitial-oxygen dimer complex (IO 2i ) and the vacancy-oxygen pair (VO), respectively. The E(0.24) trap is associated with the electron emission from the double-acceptor state of the divacancy (V 2 ). However, an asymmetric peak with its maximum at around 220 K and an activation energy for electron emission of 0.42 eV dominated the spectra. We used high resolution Laplace DLTS to investigate the structure of E(0.42) and found that this signal is complex, consisting of contributions from several defects. From the annealing behaviour, it was revealed that as some of these defects anneal out they are sources of vacancies evidenced by an increase in the concentration of VO and V 2 . It is suggested that some of the defects contributing to the E(0.42) peak are related to small vacancy clusters.

Transient response times of a-Si : H p-i-n color detector

Transients and relevant response times of a-Si : H p-i-n photodetector under various illuminations (in the visible range) and bias voltage conditions were studied. The model/method for possible color detection using on and off transient response times is proposed. Depending on illumination and bias pulse types, one or two processes are found to be involved in the conduction mechanism, including transition and trapping of both charge carrier types. Characteristic photocurrent transients and response times under modulated monochromatic and chromatic visible light illumination enable color recognition. The peculiar behavior of the blue light transients has been also plausibly explained by means of the proposed model

Understanding ion implantation defects in germanium

The recent interest in germanium as an alternative channel material for PMOS has revealed major differences from silicon in relation to ion implantation. In this paper we describe some initial results of a fundamental study into defect creation and removal in ion implanted germanium. In this stage of the work we have used silicon and germanium implants into germanium and into germanium rich silicon-germanium. The defect evolution in these samples is compared with electron and neutron irradiated material using annealing studies in conjunction with deep level transient spectroscopy, positron annihilation and Rutherford back scattering. It is proposed that both vacancy and interstitial clustering are important mechanisms in implanted germanium and the likely significance of this is discussed. copyright The Electrochemical Society.

Defects in polycrystalline silicon studied by IBICC

In the research of semiconducting materials, ion beam-induced charge collection (IBICC) technique can provide interesting and straightforward information about the different electronic device characteristics. This nuclear microprobe technique was used for the qualitative analysis of charge collection efficiency spatial distribution in three different types of EFG silicon material. Using IBICC technique, we studied the influence of present light impurities (oxygen, carbon) on electrical activity of extended defects. It is shown that oxygen segregating close to structural defects influences their electrical activity, while for carbon we did not observe the same effect. We demonstrated that IBICC technique could be applied to provide spatial information about the position of electrically active defects, and/or their activation or deactivation during subsequent processing.

Structure and transport properties of Ge quantum dots in a SiO2 matrix

Germanium (Ge) nanoparticles or quantum dots (QDs) embedded in a transparent dielectric matrix have properties radically different from the bulk semiconductor and present a great potential for application in electronic and optoelectronic devices. Due to quantum confinement properties, the optical bandgap of QD-based materials can be tuned by varying the nanoparticle size. These properties may be exploited for the fabrication of nanoscale electronic devices or advanced solar cells.In this work we explored structural and transport properties of QD based superstructures for advanced solar cells. Magnetron cosputtering was used for deposition and upon suitable thermal treatment a superstructure of QDs was formed. Transport properties were explored by I–V measurement in the dark together with a C–V characterization. The obtained results were modeled with the known transport mechanisms for QDs containing materials. A special emphasis is given to trap controlled space charge limited current and hopping conductivity mechanism.We have shown that in our samples a significant charge is stored in the SiO2 layers with embedded Ge QDs. That charge is predominantly stored into traps at or close to the Ge(QDs)/SiO2 interface.

Defects in carbon and oxygen implanted p-type silicon

Abstract Both oxygen and carbon ion implantation are frequently used to form either insulating buried SiO2 or SiC layer for various purposes. This creates a renewal of the interest in defects produced during such implantation processes. In the present paper we report on deep level transient spectroscopy studies of defect states occurring in boron-doped p-type silicon after high dose C+ and CO+ ion implantation and subsequent thermal annealing. It is shown that the predominant defect created during the implantation is in both cases related to silicon selfinterstitial clusters, which upon annealing at higher temperatures evolve to extended structural defects.

Influence of stress on the properties of Ge nanocrystals in an SiO2 matrix

In this work, self-assembled Ge quantum dot (QD) formation in a dielectric matrix is explored. Of particular interest were their structural and optical properties, in order to understand the stress build-up in such a process and its impact on the material properties during processing. To this end, thin films consisting of (Ge + SiO2)/SiO2 multilayers grown by RF magnetron sputtering were deposited at room temperature. Annealing of such films at 873 K in inert N2 atmosphere produced, at the position of the Ge-rich SiO2 layers, a high lateral density (about 1012 cm x022) of Ge QDs with a good crystallinity. SiO2 spacer layers separated the adjacent Ge-rich layers, where the Ge QDs were formed with a diameter of about the size of the (Ge + SiO2) as-deposited layer thickness, and created a good vertical repeatability, confirmed by the appearance of a Bragg sheet in two-dimensional small-angle X-ray scattering patterns. The structural analysis, by wide-angle X-ray diffraction, grazingincidence small-angle X-ray scattering and transmission electron microscopy, has shown that the described processing of the films induced large compressive stress on the formed QDs. Optical analysis by time-resolved photoluminescence (PL) revealed that the high density of crystalline Ge QDs embedded in the amorphous SiO2 matrix produced a strong luminescence in the visible part of the spectrum at 2–2.5 eV photon energy. It is shown that the decay dynamics in this energy range are very fast, and therefore the transitions that create such PL are attributed to matrix defects present in the shell surrounding the Ge QD surface (interface region with the matrix). The measured PL peak, though wide at its half-width, when analysed in consecutive short spectral segments showed the same decay dynamics, suggesting the same mechanism of relaxation.

Vacancy Clusters in Germanium

Fast neutron irradiation of germanium has been used to study vacancy reactions and vacancy clustering in germanium as a model system to understand ion implantation and the vacancy reactions which are responsible for the apparently low n-type doping ceiling in implanted germanium. It is found that at low neutron doses (~1011cm-2) the damage produced is very similar to that resulting from electron or gamma irradiation whereas at higher doses (> 1013cm-2) the damage is similar to that resulting from ion implantation as observed in the region near the peak of a doping implant. Electrical measurements including CV profiling, spreading resistance, Deep- Level Transient-Spectroscopy and high resolution Laplace Deep-Level Transient-Spectroscopy have been used in conjunction with positron annihilation and annealing studies. In germanium most radiation and implantation defects are acceptor like and in n-type material the vacancy is negatively charged. In consequence the coulombic repulsion between two vacancies and between vacancies and other radiation-induced defects mitigates against the formation of complexes so that simple defects such as the vacancy donor pair predominate. However in the case of ion implantation and neutron irradiation it is postulated that localized high concentrations of acceptor like defects produce regions of type inversion in which the vacancy is neutral and can combine with itself or with other radiation induced acceptor like defects. In this paper the progression from simple damage to complex damage with increasing neutron dose is examined.

UV light induced defects in amorphous silicon thin films

Abstract The effect of light soaking on a-Si:H films is well known as the Staebler–Wronski effect, though its complete mechanism is not yet clear. We have studied the effect of light soaking with UV light on intrinsic a-Si:H films, as well as the effect of thermal annealing in the dark. It is shown that the light soaking of the films in the air did not affect hydrogen concentration from Si–H bonds and at same time oxidation of the films is observed. It means that oxygen incorporation was due to broken back-bonds to Si–H which are very likely weak bonds. Moreover it is found that UV irradiation produced oxidation and caused even minor Si–H bond-breaking.

Closely packed Ge quantum dots in ITO matrix: influence of Ge crystallization on optical and electrical properties

In the present work, a method for the low-temperature production of the material consisting of closely packed Ge QDs embedded in ITO matrix is described. The films are produced by magnetron sputtering deposition followed by thermal annealing. It is shown that the conductivity and optical properties of the films depend on the structure, Ge content in the ITO matrix as well as on the annealing conditions. The conductivity of the films changes up to seven orders of magnitude in dependence on the annealing conditions, and it shows transformation from semiconductor to metallic behavior. The optical properties are also strongly affected by the preparation and annealing conditions, so both conductivity and optical properties can be controllably manipulated. In addition, the crystallization of Ge is found to occur already at 300 °C, which is significantly lower than the crystallization temperature of Ge produced by the same method in silica and alumina matrices.