Mikhail Patrashin

At the Dawn of a New Era in Terahertz Technology

The National Institute of Information and Communications Technology (NICT, Japan) started the Terahertz Project in April 2006. Its fundamental purpose in the next five years is to enable a nationwide technical infrastructure to be created for diverse applications of terahertz technology. The technical infrastructure includes the development of semiconductor devices such as terahertz quantum cascade lasers, terahertz-range quantum well photodetectors, and high-precision tunable continuous wave sources. It also includes pulsed terahertz measurement systems, modeling and measurement of atmospheric propagation, and the establishment of a framework to construct a materials database in the terahertz range including standardization of the measurement protocol. These are common technical infrastructure even in any terahertz systems. In this article, we report the current status of developments in these fields such as terahertz quantum cascade lasers (THz-QCLs) (with peak power of 30 mW, 3.1 THz), terahertz-range quantum well photodetectors (THz-QWPs) (tuned at 3 THz) an ultrawideband terahertz time domain spectroscopy (THz-TDS) system (with measurement range of from 0.1 to 15 THz), an example of a database for materials of fine art, and results obtained from measuring atmospheric propagation.

Charge sensitive infrared phototransistor for 45 μm wavelength

The detection wavelength of charge-sensitive infrared phototransistors (CSIPs), originally developed for 15 μm wavelength radiation, is expanded to longer wavelengths of ∼45 μm. The CSIPs are fabricated on GaAs/AlGaAs crystals with bilayer two-dimensional electron gas. Photoresponse at targeted wavelengths is confirmed. Low quantum efficiency of photoresponse, on the order of 10−4, has been ascribed to electron traps (Al–O complex) contained in an AlGaAs barrier layer. Several possible approaches for improving the detector performance are suggested.

GaAsSb/InAlAs/InGaAs Tunnel Diodes for Millimeter Wave Detection in 220–330-GHz Band

We report on high-frequency performance and temperature stability of zero-bias GaAsSb/InAlAs/InGaAs tunnel diodes for millimeter-wave detection in 220-330-GHz band. The average voltage sensitivity of 1400 V/W has been achieved in 0.8 × 0.8μm2 mesa devices at room temperature. Measured current-voltage characteristics revealed a superior temperature stability of the devices compared with Schottky barrier diodes. The expected sensitivity variations over a temperature range from T=17-300 K are 1.7 dB.

Type-II InAs/GaInSb superlattices for terahertz range photodetectors

We designed InAs/Ga0.6In0.4Sb superlattice (SL) material for terahertz-range photodetectors. Depending on the thicknesses of the InAs and Ga0.6In0.4Sb layers, the SL energy gap Eg can be adjusted to be between 8-25 meV, which corresponds to a cut-off frequency from 2 to 6 THz. Different designs were numerically evaluated by using the eightband k•p model. The calculations show that the SL energy gap is sensitive to monolayer (ML) scale variations in layer thickness, and that realization of the design parameters requires better than 1ML accuracy of epitaxial growth. A 40-period strained Ga0.6In0.4Sb SL with alternating InSb (1ML) and GaAs (1ML) interfaces was grown by a molecular beam epitaxy on a GaSb substrate; the target energy gap Eg was 9 meV. The SL samples were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence and absorption spectroscopy measurements. Despite the large lattice mismatch between InAs and Ga0.6In0.4Sb, the XRD and AFM measurements showed that the SL had good structural and surface quality and an accurate layer structure. The surface roughness was 0.22 nm.

Heavy and light hole transport in nominally undoped GaSb substrates

In this work, we report results of a study of electronic transport in nominally undoped p-type GaSb wafers typically employed as substrate material for the epitaxial growth of InAs/GaInSb type-II superlattices. Magnetic field dependent Hall-effect measurements and high-resolution mobility spectrum analysis clearly indicate p-type conductivity due to carriers in both the heavy and light hole bands. The extracted hole concentrations indicate a thermal activation energy of 17.8 meV for the dominant native acceptor-like defects. A temperature-independent effective mass ratio of 9.0 ± 0.8 was determined from the ratio of measured heavy and light hole concentrations. Over the 56 K–300 K temperature range, the light hole mobility was found to be 4.7 ± 0.7 times higher than the heavy hole mobility. The measured room temperature mobilities for the light and heavy holes were 2550 cm2/Vs and 520 cm2/Vs, respectively.

Development of components for cost effective terahertz measurement system: terahertz quantum cascade laser and terahertz quantum well infrared photo-detector

Terahertz imaging and spectroscopy have attracted a lot of attention in recent years, because monocycle terahertz radiation can be generated using an ultra-short pulse laser and semiconductor device technologies. The availability of monocycle terahertz radiation sources has encouraged innovative research and development activities worldwide in an extremely wide range of applications, from security to medical systems. However, the fundamental device technology, namely the semiconductor emitter, amplifier, modulator, focal plane array detector, and optical thin film among others, in the terahertz frequencies has not yet been fully established. Therefore, a measurement system in the terahertz range remains a costly alternative. We report in this paper our recent developments of a terahertz quantum cascade laser (THz-QCL) and a terahertz quantum well photo-detector (THz-QWIP). We believe that the combination of a semiconductor emitter (THz-QCL) and a semiconductor detector array (THz-QWIP) is a good choice for developing a cost-effective measurement system for a given terahertz range (from 1.5 THz to 5.0 THz), because both of these items are based on mass-production semiconductor fabrication techniques. We fabricated the THz-QCLs using a resonant longitudinal-optical phonon depopulation (RPD) scheme, which is made up of both a GaAs/AlGaAs material system and a GaSb/AlGaSb material system. The GaAs/AlGaAs THz-QCL has already successfully demonstrated a high peak power (about 30 milliwatts in pulsed operation) operation at 3.1 THz and a high operating temperature (123K). On the other hand, we have fabricated a THz-QWIP structure consisting of 20 periods of GaAs/Al0.02Ga0.98As quantum wells with a grating coupler on the top of detector devices, and successfully operated it at 3 THz with a responsivity of 13mA/W. We now believe we are ready to make a cost-effective measurement system, although both of the devices still require cryogenic coolers.

CSIP – A Novel Photon-Counting Detector Applicable for the SPICA Far-Infrared Instrument

We describe a novel GaAs/AlGaAs double-quantumwell device for the infrared photon detection, called ChargeSensitive Infrared Phototransistor (CSIP). The principle of CSIP detector is the photo-excitation of an intersubband transition in a QW as an charge integrating gate and the signal amplification by another QW as a channel with very high gain, which provides us with extremely high responsivity (10 4 – 10 6 A/W). It has been demonstrated that the CSIP designed for the mid-infrared wavelength (14.7 μm) has an excellent sensitivity; the noise equivalent power (NEP) of 7 × 10 -19 W/ with the quantum effciency of ~ 2%. Advantages of the CSIP against the other highly sensitive detectors are, huge dynamic range of > 10 6 , low output impedance of 10 3 – 10 4 Ohms, and relatively high operation temperature (> 2 K). We discuss possible applications of the CSIP to FIR photon detection covering 35 – 60 μm waveband, which is a gap uncovered with presently available photoconductors.

Terahertz range GaAs/AlGaAs quantum well photodetector

We have designed a GaAs/AlGaAs detector with a targeted peak frequency of 3 THz (100 mum) in an effort to extend the successful implementation of QWIP (Quantum Well Infrared Photodetector) arrays to the terahertz range. A simple multilayer structure has 18-nm GaAs QWs sandwiched between AlGaAs barriers with an Al alloy fraction of 2%. Despite the low Al content (2%), we obtained consistent results with MBE growth and could control the level of dark current and the impedance of fabricated devices within design-specific requirements. The level of dark current in optimized samples was a few muA/cm-2 and the detector observed response close to the designed detection wavelength. The responsivity of the detector was measured with a calibrated blackbody source. A responsivity of 13 mA/W at an electric bias of 40 mV and an operating temperature of 4 K was obtained.

Terahertz-range quantum well photodetector

We have designed and fabricated GaAs/AlGaAs QWIP photodetector for THz range of spectrum (3THz, 100 μm). To evaluate suitability of this type of detector for real-time THz imaging, a prototype of a small array have been built by integrating detector elements with cryogenic readout electronics. Up to 32 individual channels can be measured with this system at temperatures down to 4K. In this paper we present the design and expected performance of GaAs/AlGaAs THz QWIP integrated with cryogenic readout electronics (CRE), and discuss key development issues related to the design.

Zero-bias GaAsSb/InAlAs/InGaAs tunnel diode detectors for 220–330 GHz range

This presentation describes on-wafer characterization of GaAsSb/InAlAs/InGaAs tunnel diodes for direct detection in 220-330GHz band. Voltage sensitivity above 1000V/W was measured in 0.8μm×0.8μm mesa device at room temperature. The detectors demonstrated enhanced temperature stability of the characteristics compared to zero-bias Schottky barrier diodes. The estimated variations of the zero-bias sensitivity at temperatures from 17K to 300K were less than 2dB.