Gallium nitride (GaN) is a promising wide-bandgap material for high-power electronics, where GaN-on-GaN homoepitaxy is being developed for fabrication of compact high-voltage vertical devices. However, variation in GaN substrate quality strongly influences the properties of epitaxial layers grown on top, which in turn affects device performance and reliability. Hence, better knowledge of the surface electronic properties is needed, especially after wafer processing steps that can introduce surface contaminants and oxide layers. Photoemission-based techniques provide chemical and electronic information but are surface-sensitive; therefore, the formation of native oxides or contamination from ambient conditions can affect findings. Here, we present the initial results of various surface treatment methods on the electronic properties of p-type GaN epitaxial layers grown via metal-organic chemical vapor deposition (MOCVD) in preparation for photoemission electron spectroscopy and microscopy characterization. We use X-ray photoelectron spectroscopy (XPS) to evaluate changes in residual contamination after treatment. We find that piranha-based cleaning methods have large reductions in surface carbon contamination, while NH4OH and HCl-based treatments remove surface oxide. The elemental core levels and valence band correspondingly exhibit binding energy shifts with the different treatment methods, indicating reduced surface band-bending. Both XPS and initial photoemission electron microscopy results of the photoelectron yield suggest a deeper valence band edge location with respect to the Fermi energy measured for the forming gas plasma-cleaned sample. These results demonstrate that combined ex-situ treatments for carbon and oxygen removal are more effective, yet further in-situ cleaning is necessary for more complete contaminant removal.
Ga2O3 is the only ultra-wide bandgap semiconductor with melt-growth substrate technology similar to that of Si, heterostructure device technology similar to that of the III-Nitride family, and high growth rate (GR) epitaxial technologies such as MOCVD and HVPE to support the development of ultra-high-breakdown voltage devices competitive with SiC technology. We report a Ga2O3 transistor device based on a high-GR MOCVD technology (Agnitron Technology’s Agilis 100 reactor). We have demonstrated for the first time a β-Ga2O3 MOSFET grown by high-GR MOCVD resulting in significantly improved epilayer quality. The high GR demonstrated via this method paves the road for demonstration of high breakdown voltage devices on a thick Ga2O3 buffer layer.
Ga2O3 is the only ultra-wide bandgap semiconductor with melt-growth substrate technology similar to that of Si, heterostructure device technology similar to that of the III-Nitride family, and high growth rate (GR) epitaxial technologies such as MOCVD and HVPE to support the development of ultra-high-breakdown voltage devices competitive with SiC technology. We have demonstrated for the first time a β-Ga2O3 MOSFET grown by high-GR MOCVD (Agnitron Technology’s Agilis 100 reactor) with record high mobility of 170 cm2/Vs, despite increased carrier scattering rate in the doped channel, facilitated by a significant improvement in epilayer quality. The high GR demonstrated via this method paves the road for demonstration of high breakdown voltage devices on a thick Ga2O3 buffer layer. [1] M.J. Tadjer et al., J. Phys. D: Appl. Phys. 54 (2021) 034005.
The realization of next-generation vertical GaN devices relies heavily on advances in both epitaxial growth of GaN drift layers on commercially available GaN substrates and selective area n-type and p-type doping in a planar process. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates span defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. . In many other semiconductor materials such as Si and SiC, ion implantation is a routine step in most processing sequences for selective area doping and greatly facilitates manufacturing by avoiding the complicated etch/regrowth process. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge though. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been shown to activate up to ~10% of the implanted Mg dopant atoms using a combination of a temporary thermally stable capping layer, annealing in a nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. This paper will present an assessment of substrate-dependent effects on the quality of homoepitaxial GaN films, evaluate ion implantation processing for selective area doping, address basic vertical devices to identify process module development toward practical MOSFET devices.
Soliton-like pulses with a 1984-nm center wavelength are produced from a Tm-doped mode-locked fiber laser. The linear cavity has a graphene saturable absorber mirror at one end and a fiber Bragg grating as the output coupler. The laser operates without dispersion compensation, and the repetition rate was tuned from 20 to 5 MHz by the addition of SMF-28 fiber. The dry transfer process used to place the graphene on a mirror could be extended to any optical substrate. This enables integration of graphene with optics such as an optical window coated with a graphene filter or a graphene-saturable absorber placed directly on a semiconductor laser facet.
Joshua Caldwell, Travis Anderson, Karl Hobart, Glenn Jernigan, James Culbertson, Marko Tadjer, Fritz Kub, Joseph Tedesco, Jennifer Hite, Michael Mastro, Rachael Myers-Ward, Charles Eddy, Paul Campbell, D. Kurt Gaskill
Epitaxial graphene (EG) grown on the carbon-face of SiC has been shown to exhibit higher carrier mobilities
in comparison graphene grown via most other methods, while also remaining amenable to wafer-scale growth
and fabrication. The ability to transfer large area (>mm2), continuous graphene films to arbitrary substrates
while maintaining the electrical and structural integrity of the film is desirable. We demonstrate the dry
transfer of EG from the C-face of 4H-SiC onto SiO2, GaN and Al2O3 substrates using thermal release tape
both with and without a PMMA backing layer. Van der Pauw devices fabricated from C-face EG that were
transferred to SiO2 and sapphire exhibited similar Hall effect mobilities, with an approximate three-fold
reduction in carrier density when compared to devices fabricated on as-grown material. Raman spectroscopy illustrated that the optimized transfer process did not adversely affect the material quality, while XPS was used to both determine the transfer efficiency as well as to observe the presence of atomic silicon within the as-grown EG films. This latter observation may provide insight into the identity of the native dopant within these materials and/or a point defect that could be limiting carrier mobility.
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