Electronic structure, lattice location and stability of dopants in wide band gap semiconductors

The importance of functional nanomaterials, such as nanowires (NW) and thin films, in science and technology has been increasing exponentially over the years. Due to their small size and increasing surface-to-volume ratio, new (predictable or unsuspected) properties arise with potential applications in industry. However, the smaller the de- vices get, the more their properties depend on the scale factor, geometry, symmetry and localized interactions of defects and impurities, which are no longer diluted in the constituent materials[1,2]. Moreover, it is known that the atomic positions and charge valences severely influence the properties of nanomaterials, since they depend on the local character of the electron density. In particular for NW, their integration in de- vices requires the control of electrical properties and optical homogeneity, which is not systemically achieved[3,4]. Since doping during growth is mainly ruled by equilibrium thermodynamic reactions, a dramatical change of morphology, density, alignment and homogeneity can occur during the process (e.g., the dopant concentrations are generally higher in near-surface regions[4,5]). Alternatively, ion implantation proves to be a versatile doping technique. However, it has some inherent and undesirable features regarding the effects of the implantation damage, which need to be removed, generally by thermal annealing, in order to activate the dopants. The Perturbed Angular Correlation (PAC) technique is particularly suitable for the follow-up of these processes at the atomic scale while studying the interactions of the implanted probe with point-like defects and impurities / dopants introduced by ion implantation, diffusion or when de- posited by evaporation (soft-landing) on surfaces and build-up interfaces of thin films. After achieving proper annealing, the combination of the e− − γ and γ − γ PAC tech- niques can be used to provide a dynamic picture of locally induced electronic excitations and subsequent recombination phenomena into the probe’s neighborhood. In this work, this methodology was used to investigate the electronic structure, stability and lattice location of single dopant atoms in samples of powder pellets, thin films and nanowires (NW) of the wide band gap semiconductors Ga2O3, GaN and ZnO, for which growing thin-films and NWs is well achievable, the latter with good control of lengths, diameters and “aspect ratio”. These materials have great potential for applications in nano- and microelectronics, optoelectronics and nano-sensors[1,3–6]. However, before this work there was no available tool to analyze experimental data in cases where the hyperfine field around the probes is time-dependent, such as in the case of probes which decay by electron-capture (e.g., 111In/111Cd) or by conversion electron emission (e.g., 181Hf/181Ta). Therefore, the first part of this work was the development of PACme, a C++ program that allows the simulation, analysis and visualization of the experimental PAC observable for static and dynamic (time-dependent) hyperfine fields considering a stochastic set of different possible states and the transitions between them. It is completely generalized in terms of the number of different fractions (probe environments) and different states of each fraction, nuclear spin of the probes and crystal structure (single crystal or polycrystal) of the material under study. Moreover, it is the first soft- ware, to our knowledge, to analytically integrate a static frequency distribution for each individual state in dynamic cases, where the frequency characterizing a single state is no longer a sharp value but is instead represented by a normalized distribution curve centered at that value. Static distributions are crucial in the analysis of PAC spectra since small deviations in the electric field gradient and/or magnetic field around probes in equivalent lattice sites (e.g. due to distant defects from the probes) are very frequent and change the spectra in a very strong way. The first PAC measurements were per- formed as a function of annealing temperature after implantation of 111mCd probes in samples of ZnO/CdxZn1–xO and Ga2O3. For ZnO, the substitution of Cd probes at Zn sites was observed, as well as the formation of a probe-defect complex. The ternary CdxZn1–xO (x = 0.16) showed good macroscopic crystal quality but revealed some clustering of local defects around the probe Cd atoms, which could not be annealed. In the Ga2O3 samples, the substitution of the Cd probes in the octahedral Ga-site was observed, demonstrating the potential of ion-implantation for the doping of nanowires[2]. Using the knowledge gained from the first experiments, new PAC measurements were performed after implantation of 111mCd probes and diffusion of 111In probes, both decaying to stable 111Cd, in samples of Ga2O3. The location and charge state of the Cd probe atoms were determined by comparing the PAC results to density functional theory (DFT) simulations. A comprehensive analysis regarding the impact of introducing Cd dopant atoms into the Ga2O3 lattice was performed; the density of states, band structure and the electron density around the Cd atoms and their first neighbors were studied. It was shown that Cd induces an impurity band located 0.42 eV above the valence band which is completely filled by an extra electron in its most stable state, confirming its acceptor character. An activation energy of 0.54(1) eV related with the electronic charge carrier mobility in the material was obtained. It was therefore possible to confirm that Cd becomes an acceptor dopant in the octahedral Ga-site of Ga2O3, thus being a very good candidate for p-type doping in this material. Finally, PAC measurements were performed after implantation of 181Hf/181Ta probes into Si-doped (n-type dopant) and Zn-doped (p-type dopant) samples of GaN in order to study the lattice site of the im- planted ions and the combination of the e− − γ and γ − γ PAC techniques was used to study the recombination of ionized and excited electronic states of the impurity/dopant as a function of temperature and dopant. This demonstrates the potential use of PAC for the study of electrical properties of materials where, for instance, Hall effect measurements are difficult / impossible to perform. All the results were later compared to DFT simulations via the implementation of different atomic local models, where the electric field gradient and density of states of each configuration was calculated.