Garret Moddel

Macroelectronics: Perspectives on Technology and Applications

Flexible, large area electronics - macroelectronics - using amorphous silicon, low-temperature polysilicon, or various organic and inorganic nanocrystalline semiconductor materials is beginning to show great promise. While much of the activity in macroelectronics has been display-centric, a number of applications where macroelectronics is needed to enable solutions that are otherwise not feasible are beginning to attract technical and/or commercial interest. In this paper, we discuss the application drivers and the technology needs and device performance requirements to enable high performance applications to include RF systems.

Applicability of Metal/Insulator/Metal (MIM) Diodes to Solar Rectennas

The current-voltage (I-V) characteristics of metal/insulator/metal (MIM) diodes illuminated at optical frequencies are modeled using a semiclassical approach that accounts for the photon energy of the radiation. Instead of classical small-signal rectification, in which a continuous span of the dc I-V curve is sampled during rectification, at optical frequencies, the radiation samples the dc I-V curve at discrete voltage steps separated by the photon energy (divided by the electronic charge). As a result, the diode resistance and responsivity differ from their classical values. At optical frequencies, a diode with even a moderate forward-to-reverse current asymmetry exhibits high quantum efficiency. An analysis is carried out to determine the requirements imposed by the operating frequency on the circuit parameters of antenna-coupled diode rectifiers, which are also called rectennas. Diodes with low resistance and capacitance are required for the RC time constant of the rectenna to be smaller than the reciprocal of the operating frequency and to couple energy efficiently from the antenna. Existing MIM diodes do not meet the requirements to operate efficiently at visible-to-near-infrared wavelengths.

High-speed binary optically addressed spatial light modulator

We describe the structure and operating characteristics of a high‐speed optically addressed spatial light modulator (OASLM) with a hydrogenated amorphous silicon (a‐Si:H) photosensor and a ferroelectric liquid‐crystal modulator. The photosensor is a p‐i‐n photodiode, which switches the liquid crystal into one of two stable states. Under a write‐light intensity of 6 mW/cm2, the OASLM exhibits a response time of 155 μs, a contrast ratio of 20:1, and a resolution of 40 lp/mm. The writing sensitivity per pixel is 0.1 pJ.

Interpretation of the conductance and capacitance frequency dependence of hydrogenated amorphous silicon Schottky barrier diodes

We present a general model of the frequency dependence of conductance and capacitance in a‐Si:H Schottky diodes. In order to circumvent several questionable assumptions required in the analysis of capacitance voltage characteristics, the frequency dependence of sputtered a‐Si:H devices is measured with no applied dc voltage. We obtain independent, consistent values of the depletion width and of the density of states at the Fermi level and below from both conductance and capacitance at both low and high modulation frequencies. We show that the linear frequency dependence of conductance cannot be attributed to hopping conductance, but rather to the interaction of gap states with free carriers. Our study shows that the interaction kinetics of the states around the Fermi level with the conduction‐band carriers is so fast that the response of the diode is limited by the band transport of these carriers, which rapidly thermalize and distribute themselves through the continuum of states from the conduction band ...

Traveling-Wave Metal/Insulator/Metal Diodes for Improved Infrared Bandwidth and Efficiency of Antenna-Coupled Rectifiers

We evaluate a technique to improve the performance of antenna-coupled diode rectifiers working in the IR. Efficient operation of conventional, lumped-element rectifiers is limited to the low terahertz. By using femtosecond-fast MIM diodes in a traveling-wave (TW) configuration, we obtain a distributed rectifier with improved bandwidth. This design gives higher detection efficiency due to a good match between the antenna impedance and the geometry-controlled impedance of the TW structure. We have developed a method for calculating the responsivity of the antenna-coupled TW detector. Three TW devices, made from different materials, are simulated to obtain their impedance and responsivity at 1.5, 3, 5, and 10 μm wavelengths. The characteristic impedance of a 100-nm-wide TW is in the range of 50 Ω and has a small variation with frequency. A peak responsivity of 0.086 A/W is obtained for the Nb-Nb2 O5 -Nb TW diode at 3-μm wavelength. This corresponds to a quantum efficiency of 3.6% and is a significant improvement over the antenna-coupled lumped-element diode rectifiers. For IR imaging, this results in a normalized detectivity of 4 × 106 Jones at 3 μm. We have identified several ways for improving the detectivity of the TW detector. Possible methods include decreasing the diode resistance, reducing the noise, and increasing the effective antenna area.

Importance of argon pressure in the preparation of rf‐sputtered amorphous silicon–hydrogen alloys

We report the influence of argon partial pressure pAr on the photoconductivity, four‐probe conductivity and photoluminescence of rf‐sputtered a‐Si–H alloys. As pAr is increased from 5 to 30 mTorr, for fixed hydrogen partial pressure pH, photoconductivity increases by as much as three orders of magnitude and then saturates. Over this range the dark conductivity activation energy decreases, a plot of log conductivity versus inverse temperature becomes increasingly less linear and the photoluminescence intensity increases slightly and then drops back. We note that high pAr films become slowly contaminated upon exposure to air. These results are interpreted in terms of a reduction in energetic silicon atom bombardment of the growing film with increased pAr. For our sputtering arrangement, we show that silicon atoms ejected from the target become significantly more thermalized at pAr=10 mTorr than at 5 mTorr. Based primarily on the fact that the conductivity activation energy is more strongly dependent on pH f...

Graphene geometric diodes for terahertz rectennas

We demonstrate a new thin-film graphene diode called a geometric diode that relies on geometric asymmetry to provide rectification at 28 THz. The geometric diode is coupled to an optical antenna to form a rectenna that rectifies incoming radiation. This is the first reported graphene-based antenna-coupled diode working at 28 THz, and potentially at optical frequencies. The planar structure of the geometric diode provides a low RC time constant, on the order of 10−15 s, required for operation at optical frequencies, and a low impedance for efficient power transfer from the antenna. Fabricated geometric diodes show asymmetric current–voltage characteristics consistent with Monte Carlo simulations for the devices. Rectennas employing the geometric diode coupled to metal and graphene antennas rectify 10.6 µm radiation, corresponding to an operating frequency of 28 THz. The graphene bowtie antenna is the first demonstrated functional antenna made using graphene. Its response indicates that graphene is a suitable terahertz resonator material. Applications for this terahertz diode include terahertz-wave and optical detection, ultra-high-speed electronics and optical power conversion.

Motivations for using ferroelectric liquid crystal spatial light modulators in neurocomputing

Spatial light modulators can be used in neurocomputing as input and output display devices and storage media for the synaptic weights. We discuss the operating characteristics of a new class of spatial light modulator that utilize ferroelectric liquid crystals and their application to building optical neural network architectures.

Electrically and Optically Controlled Light Modulation and Color Switching Using Helix Distortion of Ferroelectric Liquid Crystals

Abstract The electro-optic effect associated with the helix distortion of chiral smectic C (SmC*) liquid crystals and its application to light modulation are investigated. Optically and electrically addressed spatial light modulators based on this effect are demonstrated. They exhibit an analog response, response times as short as 100 μs, good contrast ratio and resolution, and color switching capability. The analog response is shown to be a result of the gradual distortion of the helix at low voltages. Optically this corresponds to a linear rotation of the average optic axis and change of the average effective birefringence. From measurements of the wavelength of the maximum transmission the birefringence has been found to change as a function of dc voltage and frequency of ac voltage and magnitude. The agreement between theory and experiment suggests that color changes are due solely to birefringence changes associated with changes in the geometry of the helix. We derive expressions for the angle of rot...

Degradation of liquid crystal device performance due to selective adsorption of ions

Performance degradation in liquid crystal cells was studied by analyzing the time evolution of optical and integrated current vs voltage hysteresis curves as test cells were subjected to a dc bias. We find evidence for permanent increases in liquid crystal mobile ion populations, the primary cause of device performance degradation, and suggest this permanent increase is due to selective adsorption by the alignment layers of ions of a single charge sign combined with the presence of a neutral ionizable species in the liquid crystal.