Matthew R. King

Development of cap-free sputtered GeTe films for inline phase change switch based RF circuits

Germanium telluride (GeTe) films have been recently demonstrated as the active element in low-loss RF switches where a 7.3 THz cut-off frequency (Fco) was achieved. In order to simultaneously realize the low ON-state transmission loss and large OFF-state isolation required for this application, significant optimization of the GeTe films was required. In particular, minimizing contact resistance (Rc) and sheet resistivity (Rsheet) without the use of a capping layer is a necessity. Varying the GeTe deposition conditions led to a wide range of structural, chemical, and electrical properties, which ultimately enabled the demonstration of a capless GeTe inline phase change switch (IPCS) structure. Conversely, improper deposition conditions led to extensive oxidation which would push Rc and Rsheet to unacceptable levels. In addition to its relevance for IPCS devices, this work has implications for the environmental stability of GeTe as a function of its physical morphology.

Top-down fabrication of large-area GaN micro- and nanopillars

Large-area gallium nitride (GaN) micro- and nanopillar (NP) arrays were fabricated by plasma etching of lithographically patterned GaN thin-film grown on Si substrate. Deep-ultraviolet lithography, inductively coupled plasma (ICP) etching, and subsequent chemical treatments were effectively utilized to fabricate GaN pillars with diameters ranging from 250 nm to 10 μm. The impact of various plasma etching process parameters and chemical etchants on the morphology, strain, and surface defects of these NPs were studied using scanning-electron microscopy, photoluminescence (PL), and Raman spectroscopy. It was found that the shape of the NPs can be controlled by the substrate temperature during the plasma etch and by using different gas chemistries. Room-temperature PL and Raman spectroscopy measurements revealed significant strain relaxation in 250 nm diameter pillars as compared to 10 μm diameter pillars. PL measurement also indicated that the surface damage from the plasma etch can be removed by etching in KOH-ethylene glycol solution. Post-ICP selective wet chemical etch enabled us to fabricate functional structures such as micro- and nanodisks of GaN, which potentially could be utilized in nitride-based resonators and lasers.Large-area gallium nitride (GaN) micro- and nanopillar (NP) arrays were fabricated by plasma etching of lithographically patterned GaN thin-film grown on Si substrate. Deep-ultraviolet lithography, inductively coupled plasma (ICP) etching, and subsequent chemical treatments were effectively utilized to fabricate GaN pillars with diameters ranging from 250 nm to 10 μm. The impact of various plasma etching process parameters and chemical etchants on the morphology, strain, and surface defects of these NPs were studied using scanning-electron microscopy, photoluminescence (PL), and Raman spectroscopy. It was found that the shape of the NPs can be controlled by the substrate temperature during the plasma etch and by using different gas chemistries. Room-temperature PL and Raman spectroscopy measurements revealed significant strain relaxation in 250 nm diameter pillars as compared to 10 μm diameter pillars. PL measurement also indicated that the surface damage from the plasma etch can be removed by etching in ...

Substrate agnostic monolithic integration of the inline phase-change switch technology

Omni-directional GeTe inline phase-change switches (IPCS) have been fabricated and heterogeneously integrated with commercial SiGe BiCMOS technology to create a reconfigurable receiver. The reconfigurable receiver required integrating thirteen (13) 8-port and two (2) 4-port omni-directional switch circuits with a commercial SiGe IC, requiring very stable and repeatable performance from the 112 integrated GeTe IPCS devices. Insertion loss, isolation, and cycling data will be presented, as well as performance issues encountered during the heterogeneous integration process. A new monolithic integration scheme is briefly discussed that is independent of the substrate and semiconductor technology used. This integration plan enables the monolithic fabrication of GeTe IPCS devices on any semiconductor technology, allowing low-loss, low-power, broadband reconfigurable RF systems and SoCs (system-on-chip) to be realized in any technology.

Structural and optical nanoscale analysis of GaN core–shell microrod arrays fabricated by combined top-down and bottom-up process on Si(111)

Large arrays of GaN core–shell microrods were fabricated on Si(111) substrates applying a combined bottom-up and top-down approach which includes inductively coupled plasma (ICP) etching of patterned GaN films grown by metal–organic vapor phase epitaxy (MOVPE) and selective overgrowth of obtained GaN/Si pillars using hydride vapor phase epitaxy (HVPE). The structural and optical properties of individual core–shell microrods have been studied with a nanometer scale spatial resolution using low-temperature cathodoluminescence spectroscopy (CL) directly performed in a scanning electron microscope (SEM) and in a scanning transmission electron microscope (STEM). SEM, TEM, and CL measurements reveal the formation of distinct growth domains during the HVPE overgrowth. A high free-carrier concentration observed in the non-polar HVPE shells is assigned to in-diffusion of silicon atoms from the substrate. In contrast, the HVPE shells directly grown on top of the c-plane of the GaN pillars reveal a lower free-carrier concentration.

Faceting control in core-shell GaN micropillars using selective epitaxy

We report on the fabrication of large-area, vertically aligned GaN epitaxial core-shell micropillar arrays. The two-step process consists of inductively coupled plasma (ICP) etching of lithographically patterned GaN-on-Si substrate to produce an array of micropillars followed by selective growth of GaN shells over these pillars using Hydride Vapor Phase Epitaxy (HVPE). The most significant aspect of the study is the demonstration of the sidewall facet control in the shells, ranging from {1101} semi-polar to {1100} non-polar planes, by employing a post-ICP chemical etch and by tuning the HVPE growth temperature. Room-temperature photoluminescence, cathodoluminescence, and Raman scattering measurements reveal substantial reduction of parasitic yellow luminescence as well as strain-relaxation in the core-shell structures. In addition, X-ray diffraction indicates improved crystal quality after the shell formation. This study demonstrates the feasibility of selective epitaxy on micro-/nano- engineered templa...

Recent advances in fabrication and characterization of GeTe-based phase-change RF switches and MMICs

Recent progress in germanium telluride (GeTe) based phase-change RF switch technology development has resulted in switches with tens of thousands of switching cycles and μs-level switching times. A highly compact (0.33 × 0.61mm) series-shunt, single-pole double-throw (SPDT) switch based on 3rd generation inline phase change switch (IPCS) devices was built and characterized. The SPDT switch exhibited less than 1.1dB insertion loss and greater than 39dB isolation in the DC-65GHz bandwidth. It achieved <2μs settling time for RF switching between the throws. In addition, 30,000 switching operations were demonstrated with <0.05dB variation in insertion loss and < 2dB variation in isolation.