Pouya Dianat

A highly tunable heterostructure metal-semiconductor-metal capacitor utilizing embedded 2-dimensional charge

We report on a variable capacitor that is formed between Schottky contacts and the two dimensional electron gas (2DEG) in a planar metal-semiconductor-metal structure. Device capacitance at low bias is twice the series capacitance of anode and cathode, enhancing to a maximum value, Cmax, at a threshold voltage, before reaching a minimum, Cmin, lower than the geometric capacitance of the coplanar contacts, thus resulting in ultra high Cmax/Cmin tuning ratio. Sensitivity, the normalized change of capacitance with voltage, is also very large. The dense reservoir of the 2DEG charge maintained between contacts is shown to be responsible for this remarkable performance.

An Unconventional Hybrid Variable Capacitor With a 2-D Electron Gas

Moderation of internal quantum mechanical energies, such as exchange energy of an unconventional contact, comprised of a system of 2-D charge carriers, improves performance merits of variable capacitors, varactors, mainly in tuning ratio (TR), and sensitivity, S. Energy transfer from the unconventional contact to the dielectric increases the energy density and enhances the capacitance of the varactor. Here, we analyze the performance of an unconventional varactor based on a planar metal-semiconductor-metal (MSM) structure with an embedded layer of high-density 2-D electron gas (2DEG). Through localized field-assisted manipulation of the 2DEG density, a twice larger equilibrium capacitance and a minimum capacitance, less than the geometric capacitance of a conventional MSM, are achieved. Moreover, the maximum capacitance increases through a Batman-shaped capacitance enhancement at a threshold voltage. Therefore, giant is attained while maintaining quality factors of up to 30. Capacitance-voltage characteristics exhibit a switched-capacitor behavior with S as high as 350 that is due to localized transitions from a dense 2DEG to a complete depletion. This MSM 2-D varactor combines the unconventional features of 2DEG with superior electrical properties of MSMs.

Anomalous Capacitance Enhancement Triggered by Light

Capacitance of capacitors in which one or both plates are made of a two-dimensional charge system (2DCS) can be increased beyond their geometric structural value. This anomalous capacitance enhancement (CE) is a consequence of manipulation of quantum mechanical exchange and correlation energies in the ground state energy of the 2DCS. Macroscopically, it occurs at critical charge densities corresponding to transition from an interacting “metallic” to a noninteracting “insulator” mode in the 2-D system. Here, we apply this concept to a metal-semiconductor-metal capacitor with an embedded two-dimensional hole system (2DHS) underneath the plates for realization of a capacitance-based photodetector. Under sufficient illumination, and at critical voltages the device shows a giant CE of 200% and a peak-to-valley ratio of over 4 at probe frequencies larger than 10 kHz. Remarkably, the light-to-dark capacitance ratio due to CE at this critical voltage is well over 40. Transition of the 2DHS from insulator to metallic, enforced by charge density manipulation due to light-generated carriers, accounts for this behavior, which may be used in optical sensing, photo capacitors, and photo transistors.

Performance Enhancement of a GaAs Detector with a Vertical Field and an Embedded Thin Low-Temperature Grown Layer

Low temperature growth of GaAs (LT-GaAs) near 200 °C results in a recombination lifetime of nearly 1 ps, compared with approximately 1 ns for regular temperature ∼600 °C grown GaAs (RT-GaAs), making it suitable for ultra high speed detection applications. However, LT-GaAs detectors usually suffer from low responsivity due to low carrier mobility. Here we report electro-optic sampling time response measurements of a detector that employs an AlGaAs heterojunction, a thin layer of LT-GaAs, a channel of RT-GaAs, and a vertical electric field that together facilitate collection of optically generated electrons while suppressing collection of lower mobility holes. Consequently, these devices have detection efficiency near that of RT-GaAs yet provide pulse widths nearly an order of magnitude faster—∼6 ps for a cathode-anode separation of 1.3 μm and ∼12 ps for distances more than 3 μm.

A Planar Switchable Capacitor with Embedded Two-Dimensional Electron System for Higher Integrations in VLSI and RFIC

A metal-semiconductor-metal capacitor with embedded two-dimensional charge is designed and fabricated. Capacitance-Voltage characteristics exhibit switchability with a large voltage sensitivity. Maximum and minimum capacitances outperform previous predictions with potential applicability in RFICs and VLSI for reducing the cross-talk among transmission lines and achievement of higher integrations. The device can replace bulky conductors with its negative capacitance feature. The large light sensitivity in the C-V makes this capacitor an ideal candidate for monolithic microwave-photonic integrated circuits.

Opto-plasmonic devices: controlling light with electrons

We investigate mechanisms by which interaction of light and matter may be affected by electrons, and show how this can lead to optoelectronic devices with superior properties. In particular, confined cloud of electron gas allows sculpting a wave function that affects both emission and absorption of radiation, while its collective, plasmonic, excitation may be used for optical wave guiding, coupling and radiation. Such processes require much less energy and are much faster than classical kinetic energy-based charge transport in traditional electronics. Here we present thin-film photodetectors in which 2D electron and hole charges allow operation in hundreds of GHz, without applied bias, requiring a fraction of microwatt of optical power. The 2D channel can also be structured to provide the momentum change that is required for coupling to excitation at THz range. The confined charge is then used as a plate of (an unconventional) capacitor which changes states by a factor of >1000, in tens of fs, requiring atto-joules of energy which is also switchable by light. This opto-plasmonic capacitor finds application in threshold logic based neuromorphic systems. These thin-film devices are produced in bottom-up core-shell nanowire (CSNW) technology, resulting in resonant optical cavities whose properties are controlled by 2D and 1D charge plasma, with orders of magnitude increase in absorption and emission of light that leads to lasing at room temperature even without vertical structure. Since CSNWs can be grown on Si, they can be good candidate platforms for Photonic Integrated Circuits (PIC) and Silicon Photonics.

High-speed high-sensitivity low power photodetector with electron and hole charge plasma

High speed photodetectors’ intrinsic response is limited by the transit time of the carriers that light generates to the contacts that collect these carriers generating an electric response in the external circuitry. By contrast, charge plasma confined in a semiconductor can transfer energy, hence respond much faster, than the field-induced carrier drift current. The analogy is to a drop exciting a wave in a reservoir, which is detected more rapidly than the drop’s transport by current flow. Here we construct a photodetector device in which charge reservoirs of confined two dimensional electron and hole gasses (2DEG, 2DHG) mediate the photodetector response circumventing charge transport limitations in both expended energy and required velocity. In response to short optical pulses, this device produces electrical pulses which are almost two orders-of-magnitude shorter than the same device without the charge reservoirs. In addition to speed, the sensitivity of this process allows us to measure, at room temperature, as low as 11,000 photons. The device is shown to operate without applied bias, with high responsivity, at hundreds of gigahertz. These micro plasma devices can have a range of applications such as optical communication with fraction of a microwatt power compared to the present tens of milliwatts, ultrasensitive detection of light without need for cryogenic cooling, photovoltaic devices that are capable of harvesting dim light, detectors of THz radiation, and in detection of charged particles.

Unconventional photo capacitor with giant light induced capacitance enhancement

Abstract This chapter explains a new concept for photodetection through management of internal energies in a two-dimensional charge system (2DCS). The exchange and correlation energy terms, that are associated to quantum mechanical many body interactions of a 2DCS, are manipulated through light generated carriers. This is then manifested as a giant enhancement in capacitance of a metal-semiconductor-metal (MSM) variable capacitor with an embedded 2DCS. This feature is controllable by incident light intensity, making the MSM-2DCS device suitable for non-transport based photodetection.