Brian Wagner

A Four-Terminal, Inline, Chalcogenide Phase-Change RF Switch Using an Independent Resistive Heater for Thermal Actuation

An inline chalcogenide phase-change radio-frequency (RF) switch using germanium telluride and driven by an integrated, electrically isolated thin-film heater for thermal actuation has been fabricated. A voltage pulse applied to the heater terminals was used to transition the phase-change material between the crystalline and amorphous states. An ON-state resistance of 4.5 Ω (0.08 Ω-mm) with an OFF-state capacitance and resistance of 35 fF and 0.5 MΩ, respectively, were measured resulting in an RF switch cutoff frequency (Fco) of 1.0 THz and an OFF/ON resistance ratio of 105. The output third-order intercept point measured , with zero power consumption during steady-state operation, making it a nonvolatile RF switch. To the best of our knowledge, this is the first reported implementation of an RF phase change switch in a four-terminal, inline configuration.

Low-loss latching microwave switch using thermally pulsed non-volatile chalcogenide phase change materials

A high performance RF (radio-frequency) switch based on the phase change effect in germanium-telluride (GeTe) is described. Thermal pulses applied to a separate independent thin film heating element for 0.1–1.5 μs toggles the switch in a latching fashion. Being non-volatile, no power is required to hold the switch in the on- or off-state. State-of-the-art solid-state RF switches currently in use have an on-state loss of 1 dB; here, we demonstrate an inline phase change switch with a low on-state resistance showing over a frequency range of 0-40 GHz an insertion loss of just 0.1–0.24 dB.

Crystal growth, fabrication, and design of mercurous bromide acousto-optic tunable filters

Device-quality single crystals of mercurous bromide were grown by the physical vapor transport method. Crystals transmitted light wavelengths up to 30 µm and did not show any absorption bands. Detailed x-ray Laue and x-ray diffraction studies were used to characterize and orient the crystals. Optical evaluation was performed by fabricating slabs of crystals. A design was developed to fabricate acousto-optic tunable filters with 10-deg off-axis orientation operating in the mid- and long-wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 16-mm optical aperture for the 10-deg design. A theoretical tuning curve for a mercurous bromide crystal-based AOTF using this design was also computed for the first time. Experimentally measured data on frequency matching agreed well with the theoretical predictions, and the transducer thickness was suitable for filtering 7.58 µm with the fabricated AOTF.

12.5 THz Fco GeTe Inline Phase-Change Switch Technology for Reconfigurable RF and Switching Applications

Improvements to the GeTe inline phase-change switch (IPCS) technology have resulted in a record-performing radio-frequency (RF) switch. An ON-state resistance of 0.9 Ω (0.027 Ω·mm) with an OFF-state capacitance and resistance of 14.1 fF and 30 kΩ, respectively, were measured, resulting in a calculated switch cutoff frequency (Fco) of 12.5 THz. This represents the highest reported Fco achieved with chalcogenide switches to date. The threshold voltage (Vth) for these devices was measured at 3V and the measured third-order intercept point (TOI) was 72 dBm. Single-pole, single-throw (SPST) switches were fabricated, with a measured insertion loss less than 0.15 dB in the ON-state, and 15dB isolation in the OFF-state at 18 GHz. Single-pole, double-throw (SPDT) switches were fabricated using a complete backside process with through-substrate vias, with a measured insertion loss 0.25 dB, and 35dB isolation.

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.

Operational characteristics of a long-wavelength IR multispectral imager based on an acousto-optic tunable filter

We have experimentally demonstrated and report on the results of crystal growth, fabrication, design, development, and performance for the long-wavelength infrared (LWIR) hyperspectral imager based on an acousto-optic tunable filter (AOTF) utilizing an efficient crystal, thallium arsenic selenide (Ti3AsSe3 TAS). Results on the growth of 40-mm-diameter, 15-cm-long crystal boules, to fabricate 4.0-cm-long AOTF devices, and on the system design and performance are presented. To achieve an 8-cm−1-resolution AOTF, we developed a design utilizing growth at 10.6 deg off from the c axis of the crystal and achieved >37% efficiency. A system concept was developed with high efficiency, resolution, and throughput utilizing this TAS AOTF. The test setup consisted of an LWIR camera (microbolometer), the AOTF, and a blackbody radiative source (hot filament), and represents the first time AOTF imaging has been achieved with a microbolometer camera. The filament was placed 25 cm in front of the AOTF, and the camera was aligned to the first-order diffracted beam of the AOTF. The AOTF was tuned to 10.6-µm wavelength by applying a 13.9-MHz rf signal to the transducer. Preliminary experimental results obtained for SF6 gas utilizing this system are reported.

Thermal analysis of an indirectly heat pulsed non-volatile phase change material microwave switch

We show the finite element simulation of the melt/quench process in a phase change material (GeTe, germanium telluride) used for a radio frequency switch. The device is thermally activated by an independent NiCrSi (nickel chrome silicon) thin film heating element beneath a dielectric separating it electrically from the phase change layer. A comparison is made between the predicted and experimental minimum power to amorphize (MPA) for various thermal pulse powers and pulse time lengths. By including both the specific heat and latent heat of fusion for GeTe, we find that the MPA and the minimum power to crystallize follow the form of a hyperbola on the power time effect plot. We also find that the simulated time at which the entire center GeTe layer achieves melting accurately matches the MPA curve for pulse durations ranging from 75–1500 ns and pulse powers from 1.6–4 W.

Design and fabrication of mercurous bromide acousto-optic tunable filters

Mercurous bromide crystals with very good optical transparency were grown by the physical vapor transport method. A design was developed to fabricate 10-degree orientation acousto-optic tunable filters operating in the mid and long wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 13-15 mm diameter. A theoretical tuning curve for a mercurous bromide crystal based AOTF using this design was also computed for the first time.

Morphological analysis of GeTe in inline phase change switches

Crystallization and amorphization phenomena in indirectly heated phase change material-based devices were investigated. Scanning transmission electron microscopy was utilized to explore GeTe phase transition processes in the context of the unique inline phase change switch (IPCS) architecture. A monolithically integrated thin film heating element successfully converted GeTe to ON and OFF states. Device cycling prompted the formation of an active area which sustains the majority of structural changes during pulsing. A transition region on both sides of the active area consisting of polycrystalline GeTe and small nuclei (<15 nm) in an amorphous matrix was also observed. The switching mechanism, determined by variations in pulsing parameters, was shown to be predominantly growth-driven. A preliminary model for crystallization and amorphization in IPCS devices is presented.

Nanowire-based photodetectors: growth and development of chalcogenide nanostructured detectors

This work showcases developments in growth and performance of nanowire (NW) based photodetectors. Specifically the ability to transition from single NW devices to device arrays will be discussed. We have demonstrated the growth of semiconducting nanowires (NWs) using the physical vapor transport (PVT) method. CdSe and ZnSe NWs were grown and showed promising optical properties, including high transparency and a high ratio of band edge/deep level defect emission in photoluminescence (PL) measurements. Metal-semiconductor-metal (MSM) structures were fabricated from an array of ZnSe NWs, which showed an average increase of 10x in photocurrent and up to 720x for an individual device.