Fundamental Defect Complexes and Nanostructuring of Silicon by Ion Beams

Silicon technology has become a cornerstone for the technological advances in our society for the last five decades. For carrying on with minimization of electronic devices a huge effort has been directed toward the technological development and fundamental understanding of physical processes associated with the ion implantation into silicon. Indeed, in ion implantation the impurities are intentionally introduced into a matrix lattice with the help of accelerated ion beams selectively modifying the properties of the implanted area. In addition to the introduction of the doping impurity the penetrating ions create defects that can be electrically active potentially affecting the device performance. In spite of a long research activity in the field there are still several open fundamental questions remaining, and this thesis contributes to the understanding of ion implantation induced defect complexes in silicon. Firstly, we have studied the electrical properties of vacancy type point defect complexes generated in single collision cascades during heavy ion bombardment of silicon. Because of a high generation rate of defects within the “ion track” regions, a characteristic pattern of nanochannels having modified Fermi levels due to the local compensation around each ion trajectory is formed in n-type Si. The phenomenon has been studied using spectroscopic and imaging techniques, specifically deep level transient spectroscopy (DLTS) and scanning capacitance microscopy (SCM). The SCM measurements show a characteristic random pattern of reduced SCM signal correlated with the density of the ion impacts. Moreover, a strong correlation is detected between the probing frequency and the emission rate of the single negative acceptor level of the divacancy V2(-/0) in Si. Further, DLTS reveals a significant filling time increase for all electronic levels originated from vacancy complexes with increasing ion mass as probed within the ion track regions. The results of isochronal annealing studies of vacancy complexes generated by heavy ion implants are also explained in terms of the revisited local compensation model. An improvement of the model is proposed, where the divacancy is considered to be available in two fractions; (1) highly localized centers along the core track regions V2dense and (2) centers located outside ion tracks V2dilute. The relative abundance of V2dense/V2dilute is ion mass dependent. In this model the V2dense fraction does not contribute to the doubly negative divacancy V2(=/-) signal due to local carrier compensation, and the DLTS amplitude of V2(=/-) is determined only by the V2dilute fraction. Our finding clarifies a long lasting discussion in literature on the DLTS amplitude difference between V2(-/0) and V2(=/-) in ion implanted n-type Si. Secondly, the thesis contains an investigation of the dominant electron trap in p-type Si (Ec -0.25eV), where Ec is the conduction band edge. The Ec - 0.25eV trap has previously been ascribed to the boron interstitial-oxygen interstitial (BiOi) complex, but our study shows no oxygen and only a weak boron dependence on the intensity of the level, challenging the BiOi identification. Finally, the thesis explores the use of defect engineering by introducing nanosized vacancy clusters (cavities) when synthesizing buried SiO2 by ion implantation (SIMOX). Scanning spreading resistance microscopy measurements show that oxide nucleation can be enhanced by introducing cavities, potentially reducing the required oxygen dose during the SIMOX processing

Conversion pathways of primary defects by annealing in proton-irradiated n-type 4H-SiC

The development of defect populations after proton irradiation of n-type 4H-SiC and subsequent annealing experiments is studied by means of deep level transient (DLTS) and photoluminescence (PL) spectroscopy. A comprehensive model is suggested describing the evolution and interconversion of irradiation-induced point defects during annealing below 1000{\deg}C. The model proposes the EH4 and EH5 traps frequently found by DLTS to originate from the (+/0) charge transition level belonging to different configurations of the carbon antisite-carbon vacancy (CAV) complex. Furthermore, we show that the transformation channel between the silicon vacancy (VSi) and CAV is effectively blocked under n-type conditions, but becomes available in samples where the Fermi level has moved towards the center of the band gap due to irradiation-induced donor compensation. The annealing of VSi and the carbon vacancy (VC) is shown to be dominated by recombination with residual self-interstitials at temperatures of up to 400{\deg}C. Going to higher temperatures, a decay of the CAV pair density is reported which is closely correlated to a renewed increase of VC concentration. A conceivable explanation for this process is the dissociation of the CAV pair into separate carbon anitisites and VC defects. Lastly, the presented data supports the claim that the removal of free carriers in irradiated SiC is due to introduced compensating defects and not passivation of shallow nitrogen donors

Composition and structure of Pd nanoclusters in SiOx_x thin film

The nucleation, distribution, composition and structure of Pd nanocrystals in SiO$_2$ multilayers containing Ge, Si, and Pd are studied using High Resolution Transmission Electron Microscopy (HRTEM) and X-ray Photoelectron Spectroscopy (XPS), before and after heat treatment. The Pd nanocrystals in the as deposited sample seem to be capped by a layer of PdO$_x$. A 1-2 eV shift in binding energy was found for the Pd-3d XPS peak, due to initial state Pd to O charge transfer in this layer. The heat treatment results in a decomposition of PdO and Pd into pure Pd nanocrystals and SiO$_2$

Cross-Sectional Carrier Lifetime Profiling and Deep Level Monitoring in Silicon Carbide Films Exhibiting Variable Carbon Vacancy Concentrations

The carrier lifetime control over 150 μm thick 4H-SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high- and moderate-temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth-profiling, capacitance–voltage (CV) and deep-level transient spectroscopy (DLTS), with a novel all-optical approach of cross-sectional carrier lifetime profiling across 4H-SiC epilayer/substrate based on imaging time-resolved photoluminescence (TRPL) spectroscopy in orthogonal pump-probe geometry, which readily exposes in-depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high-temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate-temperature (1500 °C) post-annealing of variable duration under C-rich thermodynamic equilibrium conditions. The post-annealing carried out for 5 h in effect eliminates VC throughout the entire ultra-thick epilayer. The reduction of VC-related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed.publishedVersio

Dynamic Impurity Redistributions in Kesterite Absorbers

Cu2ZnSn(S,Se)4 is a promising nontoxic earth-abundant solar cell absorber. To optimize the thin films for solar cell device performance, postdeposition treatments at temperatures below the crystallization temperature are normally performed, which alter the surface and bulk properties. The polycrystalline thin films contain relatively high concentrations of impurities, such as sodium, oxygen and hydrogen. During the treatments, these impurities migrate and likely agglomerate at lattice defects or interfaces. Herein, impurity redistribution after air annealing for temperatures up to 200 \ub0C and short heavy water treatments are studied. In addition, nonuniformities of the sodium distribution on a nanometer and micrometer scale are characterized by atom probe tomography and secondary ion mass spectrometry, respectively. Sodium and oxygen correlate to a greater extent after heat treatments, supporting strong binding between the two impurities. Redistributions of these impurities occur even at room temperature over longer time periods. Heavy water treatments confirm out-diffusion of sodium with more incorporation of oxygen and hydrogen. It is observed that the increased hydrogen content does not originate from the heavy water. The existence of an “ice-like” layer on top of the Cu2ZnSnS4 layer is proposed

Equilibrium shape of nano-cavities in H implanted ZnO

Thermally equilibrated nano-cavities are formed in ZnO by H implantation and subsequent high temperature annealing to determine the relative surface formation energies and step energies of ZnO from reverse Wulff construction and related analysis. H adsorption, vicinal surfaces, and surface polarity are found to play an important role in determining the final thermal equilibrium shape of the nano-cavities. Under H coverage, the O-terminated surface shows a significantly lower surface formation energy than the Zn-terminated surface

Isolation of Single Donors in ZnO

The shallow donor in zinc oxide (ZnO) is a promising semiconductor spin qubit with optical access. Single indium donors are isolated in a commercial ZnO substrate using plasma focused ion beam (PFIB) milling. Quantum emitters are identified optically by spatial and frequency filtering. The indium donor assignment is based on the optical bound exciton transition energy and magnetic dependence. The single donor emission is intensity and frequency stable with a transition linewidth less than twice the lifetime limit. The isolation of optically stable single donors post-FIB fabrication is promising for optical device integration required for scalable quantum technologies based on single donors in direct band gap semiconductors.Comment: E. R. Hansen and V. Niaouris contributed equally to this work. 13 pages, 11 figure

Properties of donor qubits in ZnO formed by indium ion implantation

Shallow neutral donors (D$^\mathrm{0}$) in ZnO have emerged as a promising candidate for solid-state spin qubits. Here, we report on the formation of D$^\mathrm{0}$ in ZnO via implantation of In and subsequent annealing. The implanted In donors exhibit optical and spin properties on par with $\textit{in situ}$ doped donors. The inhomogeneous linewidth of the donor-bound exciton transition is less than 10 GHz, comparable to the optical linewidth of $\textit{in situ}$ In. Longitudinal spin relaxation times ($T_1$) exceed reported values for $\textit{in situ}$ Ga donors, indicating that residual In implantation damage does not degrade $T_1$. Two laser Raman spectroscopy on the donor spin reveals the hyperfine interaction of the donor electron with the spin-9/2 In nuclei. This work is an important step toward the deterministic formation of In donor qubits in ZnO with optical access to a long-lived nuclear spin memory

Universal radiation tolerant semiconductor

Radiation tolerance is determined as the ability of crystalline materials to withstand the accumulation of the radiation induced disorder. Nevertheless, for sufficiently high fluences, in all by far known semiconductors it ends up with either very high disorder levels or amorphization. Here we show that gamma/beta double polymorph Ga2O3 structures exhibit remarkably high radiation tolerance. Specifically, for room temperature experiments, they tolerate a disorder equivalent to hundreds of displacements per atom, without severe degradations of crystallinity; in comparison with, e.g., Si amorphizable already with the lattice atoms displaced just once. We explain this behavior by an interesting combination of the Ga- and O- sublattice properties in gamma-Ga2O3. In particular, O-sublattice exhibits a strong recrystallization trend to recover the face-centered-cubic stacking despite the stronger displacement of O atoms compared to Ga during the active periods of cascades. Notably, we also explained the origin of the beta-to-gamma Ga2O3 transformation, as a function of the increased disorder in beta-Ga2O3 and studied the phenomena as a function of the chemical nature of the implanted atoms. As a result, we conclude that gamma/beta double polymorph Ga2O3 structures, in terms of their radiation tolerance properties, benchmark a class of universal radiation tolerant semiconductors