This project focused on developing a novel, scalable, and economic growth technique for bulk gallium nitride (GaN), a critical material for next-generation high-temperature power electronics. Large area, high-quality bulk GaN is required as a substrate material in order to grow highly efficient bipolar transistors for inverters and power conditioning. Attempting to grow GaN in bulk by traditional precipitation methods forces extreme thermodynamic and kinetic conditions, putting these techniques at the extremes of experimental science, which is unsuitable for large-area, cost-effective substrate growth. The Electrochemical Solution Growth (ESG) technique is a novel concept that addresses these issues in a unique way, and was developed at Sandia National Laboratories (SNL), in part under this program. The crucial step in demonstrating the technique’s feasibility was to deposit high-quality GaN on a seed crystal. The bulk of SNL’s activities were focused on developing conditions for deposition of GaN on a seed crystal (a thin film of GaN grown by metal organic chemical vapor phase deposition (MOCVD) on c-axis oriented sapphire) in a molten salt electrolyte solution using a rotating disk reactor (RDR) ESG apparatus. This project was actively funded from FY08 to FY12 by the Energy Storage Program and GaN Initiative for Grid Applications (GIGA) program of the Office of Electricity Delivery and Energy Reliability (OE) in the U.S. Department of Energy (DOE). Some activities focused on silicon doping of GaN occurred in FY13 but only through the use of carryover funds.
Magnetic nitrides, if manufactured in bulk form, would provide designers of transformers and inductors with a new class of better performing and affordable soft magnetic materials. According to experimental results from thin films and/or theoretical calculations, magnetic nitrides would have magnetic moments well in excess of current state of the art soft magnets. Furthermore, magnetic nitrides would have higher resistivities than current transformer core materials and therefore not require the use of laminates of inactive material to limit eddy current losses. However, almost all of the magnetic nitrides have been elusive except in difficult to reproduce thin films or as inclusions in another material. Now, through its ability to reduce atmospheric nitrogen, the electrochemical solution growth (ESG) technique can bring highly sought after (and previously inaccessible) new magnetic nitrides into existence in bulk form. This method utilizes a molten salt as a solvent to solubilize metal cations and nitrogen ions produced electrochemically and form nitrogen compounds. Unlike other growth methods, the scalable ESG process can sustain high growth rates (~mm/hr) even under reasonable operating conditions (atmospheric pressure and 500 °C). Ultimately, this translates into a high throughput, low cost, manufacturing process. The ESG process has already been used successfully to grow high quality GaN. Below, the experimental results of an exploratory express LDRD project to access the viability of the ESG technique to grow magnetic nitrides will be presented.
This work demonstrates the use of FIB implanted Ga as a lithographic mask for plasma etching of Nb films. Using a highly collimated Ga beam of a FIB, Nb is implanted 12 nm deep with a 14 nm thick Ga layer providing etch selectivity better than 15:1 with fluorine based etch chemistry. Implanted square test patterns, both 10 um by and 10 um and 100 um by 100 um, demonstrate that doses above than 7.5 x 1015 cm-2 at 30 kV provide adequate mask protection for a 205 nm thick, sputtered Nb film. The resolution of this dry lithographic technique is demonstrated by fabrication of nanowires 75 nm wide by 10 um long connected to 50 um wide contact pads. The residual resistance ratio of patterned Nb films was 3. The superconducting transition temperature, Tc =7.7 K, was measured using MPMS. This nanoscale, dry lithographic technique was extended to sputtered TiN and Ta here and could be used on other fluorine etched superconductors such as NbN, NbSi, and NbTi.