Project leader: Nikhilendu Tiwary   Personnel

Deep color centers in wide-bandgap semiconductors, that is point defects with highly localized electronic bound states, have emerged in the recent decade as a viable platform for quantum technologies including ultrasensitive magnetometers, spin qubits, single-photon emitters, and even biosensors. They are formed by vacancies, misplaced atoms, foreign species atoms, or atom-vacancy complexes in the semiconductor lattice. Importantly, deep color centers can operate at room temperature (RT), in contrast to most other available QT platforms that require millikelvin temperatures to function.

AlN has attracted attention as a host of intrinsic point defects that act as single-photon sources. Single-photon emitters in AlN were observed in as-grown samples fabricated by metal organic vapor phase epitaxy (MOVPE). The formation of quantum emitters has also been observed in AlN films after implantation with helium (He) ions and thermal annealing, and recently, with Zr ions. All these experimental reports as well as the theoretical predictions point towards both the Al and N sublattice vacancies together with vacancy-impurity complexes providing promising avenues for development of efficient quantum emitters. Inspired by the hot-off-the-press report from Cam-bridge on AlN-on-Si acting as a potential host for single photon emitters, we follow this lead as it fits directly into our expertise.

Engineering point defects (source of single photon emitters) and understanding their formation and migration in AlN requires a detailed investigation of its relation to the growth process parame-ters. We will employ MOVPE (metal organic vapor phase epitaxy) which is a high temperature (~1200 °C) epitaxial process for the growth of single crystal AlN on Si substrates. Specifically, the epitaxial growth, lower contamination levels, reduced surface roughness, and defect control by tuning process parameters are the key benefits of the MOVPE process. The crystal quality of AlN depends strongly on the substrates and the corresponding lattice mismatch. The lattice mismatch along with a large difference in thermal expansion coefficients at high growth temperatures are the source of various defects such as dislocations (misfit, threading), point defects, vacancies, and strain. High growth temperatures also result in undesirable effects such as doping of Al in Si, and activation of the thermal donors and acceptors. This in turn also results in high conductivity at the AlN-Si interface known as parasitic surface conductivity (PSC). Suppressing PSC at the interface will be carried out by investigating various barrier layer incorporation such as nitridation and for-mation of SiCxNy at the interface. The barrier layer incorporation also results in stress relaxation of the epi layers.

The MOVPE growth process parameters will be optimized to obtain a high crystal quality AlN layer on 150 mm Si (111) wafers with low unwanted defects and low inherent contaminations. The as grown AlN films will be characterized for its crystal quality, strain, morphology using advance techniques, such as HRXRD, AFM, SEM, and TEM. The point defect states (vacancies, antisites), im-pact of impurities (e.g., carbon) and inherent impurities in silicon wafers (e.g., O, N) and their complexes on bulk AlN and the AlN-Si interface will be investigated using advance characterization techniques such as positron annihilation spectroscopy (PAS), ion beam analysis (ERDA, RBS, SIMS), SEM, and TEM to understand the nature of defects and role of impurities in crystal quality, atomic interdiffusion at the AlN-Si interface, and their correlation with MOVPE growth parameters. For quantifying the traps and interfacial PSC at AlN-Si interface, electrical measurements will be car-ried out by designing test vehicles such as metal-insulator-semiconductor (MIS) capacitors and co-planar wave guides (CPW) on as-grown AlN samples. After MOVPE process optimization, initial mi-crostructural and electrical characterization, samples with various AlN thicknesses ranging from 100’s of nm to a few microns, will be prepared for further investigations and processing. Optical characterization (continuous wave and pulsed autocorrelation measurements) of the single pho-ton emission capabilities of the defects incorporated during growth will be carried out at University of Helsinki.

Subsequently, ion implantation processing (at the Accelerator laboratory, University of Helsinki) with various dopants in optimized AlN layers will be carried out with the objective of engineering and generating specific defect formations and investigate those as single photon emission sources. A wide variety of ion species, implantation energies, and fluences will be investigated for finding the optimal processing conditions. Optical characterization of the single photon emitters incorpo-rated during growth and ion implantation processing will reveal the usefulness of the identified defects. These efforts will be used to further tune the growth conditions and ion implantation processing parameters to optimize the production of single photon emission centers.

Collaboration

There will be active collaboration between Electronics integration and reliability group (EILB), Aalto University and Accelerator laboratory, University of Helsinki. The project will extensively utilize facilities in Otaniemi micro- and nano-technology research infrastructure (OtaNano): Micronova and Nanomicroscopy center for the growth of AlN, microstructural and electrical characterization, and facilities in Helsinki Accelerator Laboratory for in depth material characterization, ion implantation and optical characterization.