Characterization of Laser Deposited Ti-6Al-4V to Nb Gradient Alloys

An alloy was fabricated with Ti-6Al-4V and Nb powder using laser deposition (LD) to form a compositional gradient. The gradient was deposited, starting with Ti-6Al-4V powder, onto a forged Ti-6Al-4V substrate in an Argon environment. Niobium (Nb) composition increased by 4-at.% with each layer deposited until the composition reached 100-at.% Nb. This process yielded steep thermal gradients and affected the microstructure and mechanical properties across the compositional gradient. To observe the microstructural changes in the alloy, an etched gradient was viewed with optical microscopy at 1000x, where the grain structure was observed to be an acicular α phase at 100-at.% Ti-6Al-4V. As the at.% of Nb increased, the amount of β- Ti increased, the grain size decreased and the porosity increased. The 100-at.% Nb layers also exhibited porosity between layers. XRD scans of the LD Ti-6Al-4V indicated much less β-Ti present compared to the forged substrate. A peak shift in BCC with increasing Nb represented a gradual transition from β-Ti to Nb. A decrease in HCP peak intensities and increase in BCC indicated the decreasing amount of α-Ti and increasing amount of Nb along the gradient, where at 55-60 at.%, α-Ti was observed to be negligible. Microhardness was also used to probe the uniformity within layers and the change in strength across layers. A large variance was present in the layers of 50-90 at.%, while a general decrease in hardness was observed with the addition of Nb

Demonstration of a monocrystalline GaAs-β\beta-Ga2_2O3_3 p-n heterojunction

In this work, we report the fabrication and characterizations of a monocrystalline GaAs/$\beta$-Ga$_2$O$_3$ p-n heterojunction by employing semiconductor grafting technology. The heterojunction was created by lifting off and transfer printing a p-type GaAs single crystal nanomembrane to an Al$_2$O$_3$-coated n-type$\beta$-Ga$_2$O$_3$ epitaxial substrate. The resultant heterojunction diodes exhibit remarkable performance metrics, including an ideality factor of 1.23, a high rectification ratio of 8.04E9 at +/- 4V, and a turn on voltage of 2.35 V. Furthermore, at +5 V, the diode displays a large current density of 2500 A/cm$^2$ along with a low ON resistance of 2 m$\Omega\cdot$cm$^2$.Comment: 14 pages, 5 figure

Monocrystalline Si/β\beta-Ga2_2O3_3 p-n heterojunction diodes fabricated via grafting

The $\beta$-Ga$_2$O$_3$ has exceptional electronic properties with vast potential in power and RF electronics. Despite the excellent demonstrations of high-performance unipolar devices, the lack of p-type doping in $\beta$-Ga$_2$O$_3$ has hindered the development of Ga$_2$O$_3$-based bipolar devices. The approach of p-n diodes formed by polycrystalline p-type oxides with n-type $\beta$-Ga$_2$O$_3$ can face severe challenges in further advancing the $\beta$-Ga$_2$O$_3$ bipolar devices due to their unfavorable band alignment and the poor p-type oxide crystal quality. In this work, we applied the semiconductor grafting approach to fabricate monocrystalline Si/$\beta$-Ga$_2$O$_3$ p-n diodes for the first time. With enhanced concentration of oxygen atoms at the interface of Si/$\beta$-Ga$_2$O$_3$, double side surface passivation was achieved for both Si and $\beta$-Ga$_2$O$_3$ with an interface Dit value of 1-3 x 1012 /cm2 eV. A Si/$\beta$-Ga$_2$O$_3$ p-n diode array with high fabrication yield was demonstrated along with a diode rectification of 1.3 x 107 at +/- 2 V, a diode ideality factor of 1.13 and avalanche reverse breakdown characteristics. The diodes C-V shows frequency dispersion-free characteristics from 10 kHz to 2 MHz. Our work has set the foundation toward future development of $\beta$-Ga$_2$O$_3$-based transistors.Comment: 32 pages, 10 figures. The preliminary data were presented as a poster in the 5th US Gallium Oxide Workshop, Washington, DC. August 07-10, 202

Demonstration and Modeling of a Nitride-based Heterojunction Bipolar Transistor Using Nanomembrane Transfer

Heterojunction bipolar transistors (HBT) are sought as a building block for implementation of modern broadband electronics, defense applications, and other mm-wave electronic systems demanding in high-speed and high-power performance. The push for greater frequency performance, higher power densities and reliability have pushed research towards wide-bandgap materials such as Gallium Nitride (GaN) given its superior intrinsic material properties, providing for higher breakdown voltage, and enabling higher power performance. However, GaN faces a fundamental limitation with p-type doping, limiting its adoption in RF power electronics. Attempts at a wide bandgap HBT to-date has exclusively relied on epitaxial growth using metal-organic chemical vapor deposition or molecular beam epitaxy (MBE) to fabricate a GaN-based transistor; however, none have yielded a device with both sufficient current gain and transition frequency - both key measures of performance in a bipolar transistor. Alternative materials for the different HBT layers have been considered but are limited by significant lattice mismatch. To bypass p-type GaN limitations and improve the base-collector junction with minimal interface trap density, a nanomembrane interlayer device transfer is proposed as an alternative for fabrication of an HBT. There are two aims in this dissertation: to demonstrate an experimental HBT with a sufficient current gain above 20 and demonstrate using computer-aided modeling that sufficient frequency performance can be achieved – both to demonstrate that a wide-bandgap HBT is both possible and worth further exploration for RF electronics. In Chapter 1, the landscape of research into wide-bandgap bipolar transistors is presented. In Chapter 2, multiple methods of integration for different diode pairs within the HBT are evaluated for probability of success in the overall device, where a GaAs-GaN base-collector diode is demonstrated to have the best performance using nanomembrane layer transfer. Additionally, an MBE-grown AlGaAs-GaAs film stack was transferred and demonstrated an emitter-base structure can be transferred with no degradation in performance. In Chapter 3, fabrication of the HBT is demonstrated using the best methods selected from Chapter 2, where a AlGaAs-GaAs-GaN HBT was demonstrated to have a current gain greater than 70. In Chapter 4, technology computer-aided design simulations were developed to validate the DC results shown in Chapter 2 and 3, and simulated transition frequencies of at least 60 GHz for the experimental structure fabricated in Chapter 3. Finally, in Chapter 5, next steps are outlined for exploration beyond this initial proof-of-concept device. Taken altogether, this dissertation serves to demonstrate a device structure with the potential to vastly exceed existing solutions that can be applied to a vast array of wide-bandgap materials without being limited by its p-type analogues, enabling performance for a wide array of applications in the next generations of electronics