Heterostructure Backward Diodes Research Articles

Polarization-Resolved THz Imaging With Orthogonal Heterostructure Backward Diode Detectors

A fully integrated lens-coupled dual-polarization detector for imaging was designed, fabricated, and characterized in the terahertz (THz) region. Two zero-bias heterostructure backward diodes (HBDs) were monolithically integrated into a planar dual-polarization annular-slot antenna. An impedance matching network consisting of an interdigitated DC block and shorted stubs was implemented for maximum power transfer. The fabricated chip was then mounted on an extended hemispherical silicon lens for high antenna efficiency. The performance of the integrated detector has been characterized at 200 GHz. The detector module can simultaneously measure the two orthogonal linear polarized components of the incident THz waves, and hence obtain the polarization direction. On the basis of this polarimetric detection, polarization-resolved imaging of a Gaussian beam, as well as birefringent samples, was performed using the integrated detectors. The angular resolution of the polarization detection has been demonstrated to be as small as 3°. The single-pixel detector demonstrated here is amenable to array applications for THz focal-plane arrays that will have a significant impact on a wide range of remote sensing, though-barrier imaging, and detection/identification applications.

A 200 GHz Fully Integrated, Polarization-Resolved Quasi-Optical Detector Using Zero-Bias Heterostructure Backward Diodes

We report the design, fabrication, and prototype demonstration of a 200-GHz polarization-resolved quasi-optical detector employing monolithically integrated zero-bias heterostructure backward diodes (HBDs). In this design, polarization resolution is achieved by orthogonally integrating the detectors with a planar dual-polarization annular-slot antenna. The detector chip was fabricated and mounted on an extended hemispherical Si lens to enhance antenna efficiency in the millimeter-wave to terahertz region. The responsivity and radiation patterns of the detector were characterized experimentally; good agreement with theoretical calculations was obtained. By measuring the outputs of two orthogonal HBDs as a function of the polarization angle of the incident wave, the planar detector detects intensity and resolves polarization. On the basis of the polarimetric measurement, we further demonstrate the polarization imaging capability of the proposed detector by using it as an imaging system. The detector is promising for developing terahertz polarimetric sensors and imaging arrays in chemical sensing, biomedical imaging, and radio astronomy applications.

A G-Band Monolithically Integrated Quasi-Optical Zero-Bias Detector Based on Heterostructure Backward Diodes Using Submicrometer Airbridges

This paper presents the design, fabrication, and characterization of a zero-bias quasi-optical terahertz detector based on monolithically integrated heterostructure backward diodes (HBDs) for operation at G-band. The reported detectors consist of HBDs with $0.7 \times 0.7 ~\mu \text{m}^{2}$ active device area and submicrometer-scale airbridges, integrated with lens-coupled high-impedance planar folded dipole antennas. Measurements of the HBD detector show that a peak-measured detector sensitivity of approximately 2400 V/W and a minimum noise equivalent power (NEPmin) of 2.14 pW/ $\surd $ Hz have been obtained at 170 GHz. If an antireflection coating was used on the lens, a sensitivity of approximately 3500 V/W and NEPmin of 1.48 pW/ $\surd $ Hz is projected. The radiation patterns of the quasi-optical detector in both $E$ - and $H$ -planes have been measured, and good agreement has been achieved between simulation and measurement. The performance of this detector can be further improved by scaling the HBD device active area. The reported approach using monolithically integrated heterostructure backward tunneling diodes and submicrometer airbridges is promising for developing high performance and compact detectors and focal-plane arrays for millimeter-wave and terahertz sensing and imaging applications.

Advanced Terahertz Sensing and Imaging Systems Based on Integrated III-V Interband Tunneling Devices

Owing to the unprecedented development in terahertz (THz) sources and detectors in the last decade, technologists and researchers have intensified their efforts to develop advanced THz sensing and imaging systems with superior performance and unique functionalities. One of the key elements to realize such systems is the ability to monolithically integrate high-performance semiconductor devices with THz antennas and other passive structures. In recent years, devices based on interband tunneling in III-V heterostructures have emerged as promising candidates for THz detection that offer extremely high nonlinearity, high sensitivity, low noise, fast response, and room temperature operation. In this paper, we first review the development of heterostructure backward tunnel diodes (HBDs) in the InAs/AlSb/AlGaSb material system that have been demonstrated with detection sensitivity that outperforms the current state of the art (e.g., the fundamental limit of Schottky diodes) and with noise-equivalent power (NEP) below 0.2 pW/Hz½. We then present the monolithic integration of HBDs with planar folded dipole antennas (FDAs) using submicrometer-scale airbridges to achieve optimized impedance matching for high-performance and compact THz detectors and focal-plane array (FPA) imaging systems. In addition, the potential of using HBDs for realizing THz systems with advanced functionalities such as spectroscopic FPAs (using frequency-tunable THz antennas) and polariametric detection/imaging systems will be discussed. Finally, the integration of HBDs into waveguides for more advanced THz sensing and imaging (e.g., a six-port reflectometer for near-field imaging) will be discussed.

Integration and fabrication of high-performance Sb-based heterostructure backward diodes with submicron-scale airbridges for terahertz detection

In this work, the authors report integration and fabrication of high-performance Sb-based heterostructure backward diodes (HBDs) with planar folded dipole antennas (FDAs) using submicron-scale airbridges for terahertz (THz) detection. By integrating HBDs into FDAs, high detector responsivity of 20 000 V/W at 200 GHz and 9500 V/W at 585 GHz could be potentially achieved due to the optimized impedance matching between the antenna and HBD detector. In order to minimize interconnect parasitics, the HBD integration is accomplished using submicron-scale airbridges. Electromagnetic simulations coupled to device models show that by introducing submicron-scale airbridges and optimizing the device layout, parasitic capacitance and spreading resistance can be significantly reduced. This allows performance nearly equal to the intrinsic device performance to be obtained. To achieve this level of performance, a novel fabrication and integration process has been developed. The process includes mix-and-match electron beam and optical lithography to span the size scales required for both the FDA and small-area HBDs, and offers high accuracy and reproducibility while requiring fewer critical fabrication steps compared to conventional hybrid integration techniques. Devices fabricated using this process have obtained a record-high device curvature coefficient of −58 V−1, indicating the quality of the devices that can be achieved. The process is scalable—in terms of device size, frequency range, and array size—enabling the development of THz focal plane arrays in a wide frequency range (100 GHz to beyond 1 THz).

A Broadband Focal Plane Array Camera for Real-time THz Imaging Applications

We present a large-format, sub-millimeter-resolution, focal plane array sensor for THz imaging. Each pixel in the sensor array consists of broadband THz antennas monolithically integrated with ultra-fast heterostructure backward diodes for THz sensing. With the aid of in-house hybrid electromagnetic modeling tools, the focal plane array is optimized for diffraction limited image resolution and conjugate impedance matching for highest THz sensitivity. The camera is designed to operate in the 0.6–1.2 THz band with 5 frames-per-second image acquisition speeds, making it ideal for THz imaging applications, such as security screening, non-destructive evaluation and chemical, pharmaceutical, and medical imaging. The simulation results are validated by measurements to demonstrate sub-millimeter resolution with a pixel optical responsivity of 600 V/W at 0.7 THz.

A PHYSICS-BASED TUNNELING MODEL FOR SB-HETEROSTRUCTURE BACKWARD TUNNEL DIODE MILLIMETER-WAVE DETECTORS

InAs / AlSb / GaSb heterostructure backward tunnel diodes are sensitive millimeter-wave detectors that achieve their high sensitivity by leveraging interband tunneling current in a broken-gap heterostructure. A new physics-based tunneling model that accurately reproduces experimental results is proposed in this work. This model, which uses Kane's eight-band k · p band structures and tunneling coefficients calculated using the transfer matrix method, predicts IV curves that closely match measured data. It also predicts critical device parameters for detector performance (including curvature coefficient and junction resistance) as a function of device structure, and good agreement with experiment results has been obtained. It accurately predicts the junction resistance's exponential dependence on tunnel barrier thickness, as well as more subtle effects such as the impact of cathode doping profile adjustment and anode ternary composition. The predictive capabilities of this model are promising to provide guidance for future detector heterostructure optimization and may be useful in modeling the characteristics of other heterostructure designs and devices using 6.1 Å lattice constant materials.

A Novel Approach for Improving Off-Axis Pixel Performance of Terahertz Focal Plane Arrays

We present an antenna engineering approach to significantly improve off-axis detection properties of terahertz focal plane arrays (FPAs) behind an extended-hemispherical lens. In particular, antennas in the off-axis pixels in a large FPA were redesigned to exhibit tilted receiving patterns. The new antenna structure corrects for beam distortions in off-axis sensors due to the small lens size. Specifically, this approach is well suited for monolithic antenna structures integrated with high-sensitivity heterostructure backward diodes. Although the approach is demonstrated for 100- and 500-GHz FPAs, the presented technique is equally applicable to other planar antenna topologies and sensor materials that operate well into the terahertz regime. In addition, standard RF impedance-matching techniques were used to improve responsivity performance of the diode-coupled FPAs. The beam-correction techniques allow for a factor-of-5 larger size FPA for a fixed lens size. We also note that a tradeoff between Gaussicity and directivity becomes necessary for larger lenses. Thus, increasing the size of the FPA, without resorting to increasing the lens size, enables improved coupling efficiencies (Gaussicity) as well as better pixel resolution (directivity).

TEMPERATURE-DEPENDENT MICROWAVE PERFORMANCE OF SB-HETEROSTRUCTURE BACKWARD DIODES FOR MILLIMETER-WAVE DETECTION

The temperature dependence of heterostructure backward diodes based on the InAs/AlGaSb/GaSb material system for millimeter-wave detection has been investigated experimentally. Measured dc curvatures of 36 V-1 at 298 K and 74 V-1 at 4.2 K have been obtained. Variable-temperature on-wafer s-parameters to 110 GHz reveal that the junction capacitance of a typical 2×2 μ m 2 area device decreases from 18 fF at 298 K to 11 fF at 77 K, while the junction resistance decreases from 13.9 kΩ to 10.2 kΩ. Directly measured voltage sensitivities at 50 GHz of 3650 V/W and 7190 V/W were obtained at 298 K and 4.2 K, respectively, consistent with the expected value from measured dc curvature. A 1 dB compression point of 18.5 μW and 7.2 μW at 298 K and 77 K, respectively, was measured. A physical model based on self-consistent Poisson-Schrödinger equation solutions was obtained to explain the experimental observations, and suggests the ways to further improve the device performance.

Bias and Temperature Dependence of Sb-Based Heterostructure Millimeter-Wave Detectors With Improved Sensitivity

Nearly lattice-matched InAs/AlSb/GaSb-based heterostructure backward diodes for zero-bias millimeter wave detection were fabricated and measured. A record-high curvature, /spl gamma/=39.1 V/sup -1/, at zero bias was measured. On-wafer sensitivity measurements from 1 to 110 GHz gave a record-high average sensitivity of 3687 V/W for zero-bias operation. Further enhancement of detector sensitivity was observed with applied dc bias, with a sensitivity of 7996 V/W obtained for a 0.9 /spl mu/A bias. Extrapolating the conjugately-matched measured sensitivity suggests that 1000 V/W should be achievable at a record-high 541 GHz. The temperature dependence of detector sensitivity was evaluated from measured dc current-voltage characteristics and gave expected sensitivities ranging from 3910 V/W at 293 K to 7740 V/W at 4.2 K.

High-performance antimonide-based heterostructure backward diodes for millimeter-wave detection

Small-area antimonide-based backward diodes for zero-bias millimeter-wave detection have been fabricated and tested. The devices were fabricated using high-resolution I-line stepper lithography, allowing accurate control of the small device active area required for operation at W-band. The devices exhibit excellent measured performance from 1-110 GHz, with responsivities when driven from a 50-/spl Omega/ source of 2540 V/W at 95 GHz. This translates to a projected responsivity of 11.5 /spl times/ 10/sup 3/ V/W at 95 GHz for a conjugately matched detector. The compression characteristics of the detectors have been measured, with 1.2 dB of responsivity compression for an input power of 8 /spl mu/W.