Studies of size effects on thermal conductivity typically necessitate the fabrication of a comprehensive film thickness series. In this Letter, we demonstrate how material fabricated in a wedged geometry can enable similar, yet higher-throughput measurements to accelerate experimental analysis. Frequency domain thermoreflectance (FDTR) is used to simultaneously determine the thermal conductivity and thickness of a wedged silicon film for thicknesses between 100 nm and 17 μm by considering these features as fitting parameters in a thermal model. FDTR-deduced thicknesses are compared to values obtained from cross-sectional scanning electron microscopy, and corresponding thermal conductivity measurements are compared against several thickness-dependent analytical models based upon solutions to the Boltzmann transport equation. Our results demonstrate how the insight gained from a series of thin films can be obtained via fabrication of a single sample.

Maximum power handling, spike leakage, and failure mechanisms have been characterized for limiters based on the thermally triggered metal-insulator transition of vanadium dioxide. These attributes are determined by properties of the metal-insulator material such as on/off resistance ratio, geometric properties that determine the film resistance and the currentcarrying capability of the device, and thermal properties such as heatsinking and thermal coupling. A limiter with greater than 10 GHz of bandwidth demonstrated 0.5 dB loss, 27 dBm threshold power, 8 Watts blocking power, and 0.4 mJ spike leakage at frequencies near 2 GHz. A separate limiter optimized for high power blocked over 60 Watts of incident power with leakage less than 25 dBm after triggering. The power handling demonstrates promise for these limiter devices, and device optimization presents opportunities for additional improvement in spike leakage, response speed, and reliability.

The concept for a new, frequency-selective limiting filter is presented. This is accomplished by placing a phase change vanadium dioxide (VO2) film at the proper node of the filter. When the high-powered microwave signal reaches a certain threshold, the VO2 undergoes a phase transition from the monoclinic “insulator state” to the tetragonal “metallic state”. This crystallographic change is accompanied by a 3 order of magnitude drop in the film's resistivity, and creates a short circuit at a section of the filter, changing a pole to a zero, and rejecting further undesirable high-powered signals from damaging sensitive receiver components. This paper details the design and simulation of the filter, along with measurement results from VO2 films and the filter element. This filter element begins rejecting at about 2 W input power, with isolation of over 16 dB to over 23 W input power, and is unaffected by an out-of band interferer of over 25 W. The architecture presented allows for filter banks capable of automatically-rejecting interferers, yet allowing signals of interest to pass.

Conversion of plane waves to surface waves prior to detection allows key advantages in changes to the architecture of the detector pixels in a focal plane array. We have integrated subwavelength patterned metal nanoantennas with various detector materials to incorporate these advantages: midwave infrared indium gallium arsenide antimonide detectors and longwave infrared graphene detectors.Nanoantennas offer a means to make infrared detectors much thinner by converting incoming plane waves to more tightly bound and concentrated surface waves. Thinner architectures reduce both dark current and crosstalk for improved performance. For graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass plane wave absorption is insufficient. Using III-V detector material, we reduced thickness by over an order of magnitude compared to traditional devices.We will discuss Sandia's motivation for these devices, which go beyond simple improvement in traditional performance metrics. The simulation methodology and design rules will be discussed in detail. We will also offer an overview of the fabrication processes required to make these subwavelength structures on at times complex underlying devices based on III-V detector material or graphene on silicon or silicon carbide. Finally, we will present our latest infrared detector characterization results for both III-V and graphene structures.

Carrier lifetime and dark current measurements are reported for a mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetector. Minority carrier lifetimes are measured using a non-contact time-resolved microwave technique on unprocessed portions of the nBn wafer and the Auger recombination Bloch function parameter is determined to be vertical bar F1F2 vertical bar = 0.292. The measured lifetimes are also used to calculate the expected diffusion dark current of the nBn devices and are compared with the experimental dark current measured in processed photodetector pixels from the same wafer. Excellent agreement is found between the two, highlighting the important relationship between lifetimes and diffusion currents in nBn photodetectors. (C) 2015 AIP Publishing LLC.

The effect of defects on the dark current characteristics of MWIR, III-V nBn detectors has been studied. Two different types of defects are compared, those produced by lattice mismatch and by proton irradiation. It is shown that the introduction of defects always elevates dark currents; however the effect on dark current is different for nBn detectors and conventional photodiodes. The dark currents of nBn detectors are found to be more tolerant of defects compared to pn-junction based devices. Defects more weakly increase dark currents, and cooling reduces the defect-produced dark currents more rapidly in nBn detectors than in conventional photodiodes.

This paper investigates and models the dc behavior of thin-film-based switching devices. The devices are based on sputtered vanadium dioxide thin films that transition from 200 kΩ/□ at room temperature to 390 Ω/□ at temperatures above 68°C, with the transition occurring over a narrow temperature range. The device resistance is characterized over temperature and under current- and voltage-sourced electrical bias. The finite-element model predicts the device's nonuniform switching behavior. Electrothermally heated devices show the same transition ratio and switching behavior as externally heated devices suggesting a purely electrothermal switching mechanism.

We report the electrical characteristics of Schottky contacts and high-hole-mobility, enhancement-mode, p-channel metal semiconductor field effect transistors (MESFETs) fabricated on Ge epitaxially grown on Si substrates. The Ge film covers the entire underlying Si substrate at the wafer scale without mesas or limited-area growth. The device performance is characterized primarily as a function of threading dislocation density in the epitaxial Ge film (2 × 107, 5 × 107, 7 × 107, and 2 × 108 cm−2) and dielectric layers (SiO2, Al2O3, and HfO2) inserted between gate metal and Ge. The thin dielectric layers (∼1.3 nm) are used to unpin the Fermi level. The device performance improves with decreasing threading dislocation density and the use of HfO2. The hole mobility in the Ge film with 2 × 107 cm−2 dislocation density, obtained from Hall measurements, is 1020 cm2/V-s. Capacitance-voltage measurements on Schottky contacts provide the energy-dependent interfacial trap density of 6 × 1011 cm−2 eV−1, while current-voltage measurements provide an ON/OFF current ratio of 250. Based on the current-voltage characteristics of p-MESFETs, we have obtained an external transconductance of 7 mS/mm and low-field, effective hole-mobility of 307 cm2/V-s under 0.1 MV/cm at room temperature. The cut-off frequency of MESFETs is 10 GHz at 200 K and 2 GHz at 300 K. These results compare well with other reported transistor performance.

This paper investigates and models the dc behavior of thin-film-based switching devices. The devices are based on sputtered vanadium dioxide thin films that transition from 200 kΩ/□ at room temperature to 390 Ω/□ at temperatures above 68°C, with the transition occurring over a narrow temperature range. The device resistance is characterized over temperature and under current- and voltage-sourced electrical bias. The finite-element model predicts the device's nonuniform switching behavior. Electrothermally heated devices show the same transition ratio and switching behavior as externally heated devices suggesting a purely electrothermal switching mechanism.

Nanoantennas are an enabling technology for visible to terahertz components and may be used with a variety of detector materials. We have integrated subwavelength patterned metal nanoantennas with various detector materials for infrared detection: midwave infrared indium gallium arsenide antimonide detectors, longwave infrared graphene detectors, and shortwave infrared germanium detectors.Nanoantennas offer a means to make infrared detectors much thinner, thus lowering the dark current and improving performance. The nanoantenna converts incoming plane waves to more tightly bound and concentrated surface waves. The active material only needs to extend as far as these bound fields. In the case of graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass absorption is insufficient. The nanoantenna is thus the enabling component of these thin devices. However nanoantenna integration and fabrication vary considerably across these platforms as do the considerations taken into account during design.Here we discuss the motivation for these devices and show examples for the three material systems. Characterization results are included for the midwave infrared detector.

We show simulation results of the integration of a nanoantenna in close proximity to the active material of a photodetector. The nanoantenna allows a much thinner active layer to be used for the same amount of incident light absorption. This is accomplished through the nanoantenna coupling incoming radiation to surface plasmon modes bound to the metal surface. These modes are tightly bound and only require a thin layer of active material to allow complete absorption. Moreover, the nanoantenna impedance matches the incoming radiation to the surface waves without the need for an antireflection coating. While the nanoantenna concept may be applied to any active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The addition of the nanoantenna to the photodiode requires changes to the geometry of the stack beyond the simple addition of the nanoantenna and thinning the active layer. We will show simulations of the electric fields in the nanoantenna and the active region and optimized designs to maximize absorption in the active layer as opposed to absorption in the metal of the nanoantenna. We will review the fabrication processes.

The fidelity of a Tersoff-based empirical potential model for the Ge–Si–O ternary atomic system is studied in detail, with the ultimate aim of validating the potential for later use in large-scale simulations of Ge selective epitaxial growth. Several comparisons are presented between the predictions of atomistic simulations based on the empirical potential studied here and prior experimental measurements and electronic structure calculations. The points of comparison include the structure and thermodynamics of bulk amorphous silica (a-SiO2), the a-SiO2 free surface, the c-Si/a-SiO2 interface, the c-Ge/a-SiO2 interface, and the desorption, wetting, and diffusion behavior of Ge atoms on a-SiO2 surfaces. A single fitting parameter, which describes the strength of the Ge–O interaction, is used to establish good agreement between the empirical potential predictions and experimental measurements across all points of comparison. We conclude that a Tersoff-based empirical potential, while it neglects explicit Coulombic interactions and is highly simplified, is a reasonable basis for probing certain features of the Ge/SiO2/Si material system.

High-quality Ge-on-Si heterostructures have been explored for many applications, including near infrared photodetectors and integration with III–V films for multijunction photovoltaics. However, the lattice mismatch between Ge and Si often leads to a high density of defects. Introducing annealing steps prior to and after full Ge island coalescence is found to reduce the defect density. The defect density in Ge is also found to decrease with increasing dopant density in Si substrates, likely due to the defect pinning near the Ge-Si interface by dopants. The authors establish an empirical correlation between the minority carrier lifetime (τG) and the defect density in the Ge film (ρD) as a function of distance from the Ge-Si interface: τGe = C/ρD, where C is a proportionality constant and a fitting parameter which is determined to be 0.17 and 0.22 s/cm2 for Ge films grown on low-doped, high-resistivity Si substrates and high-doped, low-resistivity Si substrates, respectively. The effective minority carrier lifetime measured as a function of Ge film thickness is then related to the recombination velocity on Ge film surface, average minority carrier lifetime within Ge film, and recombination velocity at the Ge-Si interface. Using this relation, the authors estimate the Ge-Si interface recombination velocity for Ge films grown on low-doped, high-resistivity and high-doped, low-resistivity Si substrates to be 220 and 100 cm/s, respectively.

Detectors that take full advantage of the energy confinement offered by surface waves could have significant performance advantages in dark current and optical functionality. We use a subwavelength patterned metal nanoantenna structure to convert incoming plane waves to these surface waves.

We have fabricated low-dark-current InGaAs photodetectors utilizing an epitaxial structure incorporating an InAlGaAs passivation layer and a simple mesa isolation process, and requiring no implant or diffusion steps. At 295 K, areal and perimeter dark current contributions are 15 nA/cm(2) and 9 pA/cm, respectively, in devices with large aspect ratios biased at -0.1 V. High responsivity was achieved even at zero bias in these devices. Devices were modeled using a commercial drift-diffusion simulator. Good fits to reverse dark current-voltage measurements were obtained using a model that included both bulk and interfacial generation mechanisms. Assuming similar electron and hole Shockley-Read-Hall lifetimes, dark current under small reverse bias are consistent with generation at the interface between the absorber and underlying layers. With increasing negative bias a large increase in dark current is associated with depletion near the InAlGaAs/absorber interface, while small increases in current at large reverse bias suggest long Shockley-Read-Hall lifetimes in the absorber. Forward biasing of these devices results in efficient injection of minority carrier holes into the absorber region, mimicking photogeneration and providing a method to predict the performance of illuminated detector arrays.