Homojunction Interfacial Workfunction Internal Photoemission Research Articles

E-shaped patch antenna for GaAs-based broadband THz detectors

High-performance broadband terahertz (THz) photodetectors are of great interest driven by lots of applications. To improve the performance of the THz photodetector in a wide spectral coverage, the E-shaped patch antenna with three slots is proposed and studied in theory. GaAs-based homojunction interfacial work function internal photoemission (HIWIP) broadband THz detector is used as an example to demonstrate the effect in broadband detection. Due to the combined effect of the antenna and the microcavity, E-shaped patch antenna microcavity HIWIP increase the optical coupling efficiency in a broadband range from 2.1–20 THz by an average of four times. Considering the requirement to reduce the dark current, the etched E-shaped patch antenna microcavity HIWIP only retaining the active region beneath the antenna is further investigated. In contrast to the reference HIWIP, the coupling efficiency is improved by an average of 15 times in 2.3–20 THz. The high coupling efficiency of both structures leads to the significant improvement of the responsivity, noise equivalent power, and detectivity in a wide spectral coverage theoretically, which makes it possible to realize higher operating temperatures. Such a design scheme of E-shaped patch antenna structure provides a guideline for broadband THz detectors and can be used for other THz detectors.

Optimization of the Cryogenic Light-Emitting Diodes for High-Performance Broadband Terahertz Upconversion Imaging

High-performance terahertz (THz) imaging devices have drawn wide attention due to their significant application in a variety of application fields. Recently, the upconversion device based on the integrated homo-junction interfacial workfunction internal photoemission detector and light-emitting diode (HIWIP-LED) has emerged as a promising candidate for broadband THz upconversion pixelless imaging device. In this paper, systematical investigations on the cryogenic-temperature performances of the LED part in HIWIP-LED devices, including electroluminescence (EL) spectra and the EL efficiency, have been carried out by elaborating the radiative recombination mechanism in the quantum well, internal quantum efficiency, and the light extraction efficiency (LEE) both experimentally and theoretically. On this basis, we have further studied the operation mode of the HIWIP-LED and concluded that the LEE could directly determine the upconversion efficiency. A numerical simulation has been performed to optimize the LEE. Numerical results show that the device with a micro-lens geometry structure could significantly improve the LEE of the LED thereby increasing the upconversion efficiency. An optimal upconversion efficiency value of 0.12 W/W and a minimum noise equivalent power (NEP) of 14 pW/Hz1/2 are achieved using the micro-lens structure together with anti-reflection coating. This work gives a precise description of cryogenic LED performance in the HIWIP-LED device and provides an optimization method for the broadband HIWIP-LED THz upconversion pixelless imaging device.

Broadband THz to NIR up-converter for photon-type THz imaging

High performance terahertz imaging devices have drawn wide attention due to their significant application in healthcare, security of food and medicine, and nondestructive inspection, as well as national security applications. Here we demonstrate a broadband terahertz photon-type up-conversion imaging device, operating around the liquid helium temperature, based on the gallium arsenide homojunction interfacial workfunction internal photoemission (HIWIP)-detector-LED up-converter and silicon CCD. Such an imaging device achieves broadband response in 4.2–20 THz and can absorb the normal incident light. The peak responsivity is 0.5 AW−1. The light emitting diode leads to a 72.5% external quantum efficiency improvement compared with the one widely used in conventional up-conversion devices. A peak up-conversion efficiency of 1.14 × 10−2 is realized and the optimal noise equivalent power is 29.1 pWHz−1/2. The up-conversion imaging for a 1000 K blackbody pin-hole is demonstrated. This work provides a different imaging scheme in the terahertz band.

Realization of the high-performance THz GaAs homojunction detector below the frequency of Reststrahlen band

High-performance terahertz (THz) detectors are in great need in the applications of security, medicine, as well as in astronomy. A high responsivity p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) detector was demonstrated for a specific frequency (5 THz) below the frequency of the Reststrahlen band. The experimental results indicate that the optimized detector shows significant enhancement of the response below the Reststrahlen band in contrast to the conventional detectors. With the bottom gold layer serving as a perfect reflector, nearly 50% increment of responsivity and quantum efficiency was obtained further due to the cavity effect. Though very simple, such reflector design shows a satisfactory effect and is easy to be realized in practical applications. The resultant peak responsivity of the detector with a bottom reflector could be as high as 6.8 A/W at 1 V bias. The noise equivalent power is 2.3×10−12W/Hz1/2. Due to the absorption ability to normal incident light and high responsivity, the p-GaAs HIWIP detector is promising for the focal plane array and large-scale pixelless imaging applications.

Design of a top mirror for the n-GaAs homojunction far-infrared/terahertz detectors

The top mirror of the resonant cavity enhanced homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR)/terahertz (THz) detector is investigated. Aiming at an existing half-optimized n-GaAs HIWIP FIR/THz detector, two designs of the top mirror are investigated to increase the quantum efficiency. The genuine or equivalent single layer is proved to be unqualified as the top mirror. A design based on the two-dimensional (2D) periodical reversed pyramidal structure of intrinsic GaAs is proposed as the top mirror. The resulted quantum efficiency can be as high as 29.0%. Some elementary thoughts and a tentative fabricating solution of this 2D periodical reversed pyramidal intrinsic GaAs top mirror are suggested. The designing ideas of this kind of top mirror may also be applied to other FIR/THz devices for reference.

Performance optimization of resonant cavity enhanced n-GaAs homojunction far-infrared detectors: A theoretical study

The n-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detector is investigated to improve the quantum efficiency by simulation. The main structure, the bottom mirror, and the top mirror for the resonant cavity enhanced detector are optimized step by step. Two designs of the bottom mirror are suggested and compared. One consists of a bottom contact layer and one period of undoped/doped GaAs layers; the other is composed of a bottom contact layer, an undoped GaAs layer, and a gold layer. The results show that both mirrors enhance the quantum efficiency significantly and the gold mirror seems to be a better choice if not considering the technical difficulty. Preliminary study of the top mirror is also conducted. The top mirror shows a satisfactory effect under the condition that the reflectivity of the bottom mirror is high enough. The resulting quantum efficiency can be as high as 29.0% theoretically, which is much higher than p-GaAs and Si HIWIP FIR that was ever reported.

Imaging Characteristics and Quantum Efficiency: Key Properties for Pixelless Far-Infrared Semiconductor Upconversion Imaging Devices

Employing the concept of photon frequency upconversion, we have proposed two types of semiconductor upconverters to realize pixelless far-infrared (FIR) imaging. The upconverter consists of a GaAs FIR homojunction interfacial work-function internal photoemission detector integrated with a GaAs/AlGaAs near-infrared light-emitting diode (LED), or GaN/AlGaN mid-IR/FIR dual-band heterojunction detector with a GaN/AlGaN violet LED. We have studied in detail the dependences of the modulation transfer function and quantum efficiency of the upconverters on various device parameters. Through the application of resonant cavity to the GaAs-based upconverter, we can expect sharp and high-resolution image and improved quantum efficiency. In addition, due to the large light absorption in the GaN/AlGaN heterojunction detector, the quantum efficiency of the GaN-based upconverter is much higher than that of GaAs-based one.

Resonant-Cavity-Enhanced Far-Infrared Upconversion Imaging Devices

We have carried out a detailed investigation on the application of resonant cavities to the photon-frequency-upconversion-based far-infrared (FIR) semiconductor imaging devices. The employment of a bottom mirror (BM) enhances the FIR photon absorption efficiency and, therefore, increases the quantum efficiency of GaAs homojunction interfacial work- function internal photoemission (HIWIP) FIR detectors. Significant improvement of the extraction efficiency could be achieved in resonant cavity enhanced (RCE) GaAs-AlGaAs near-infrared (NIR) light-emitting diodes (LEDs) through redirecting as many NIR photons as possible into the escape cone. Under the optimal structural parameters, we have predicted that the upconversion quantum efficiency of the integrated HIWIP-BM-RCE-LED imaging device could be boosted to 5-6 times of the normal HIWIP-LED upconverter without any resonant cavities. As a consequence of few reincarnation cycles needed by NIR photons to escape in the photon recycling process, we can further expect sharp and high-resolution imaging in HIWIP-BM-RCE-LED

Far-infrared upconversion imaging devices: Imaging characteristics and quantum efficiency

We have carried out an investigation of imaging characteristics and quantum efficiency of far-infrared (FIR) semiconductor imaging devices. The realization of the FIR imaging employs the concept of photon frequency upconversion in GaAs homojunction interfacial work-function internal photoemission (HIWIP) FIR detectors integrated with GaAs∕AlGaAs near-infrared light-emitting diodes (LEDs). Satisfying images have been expected through the analysis of modulation transfer function of the system, where the FIR detector parameters play key roles in the low spatial frequency image contrast, while those of LEDs dominate in the high spatial frequency range. We have examined in detail the dependence of the quantum efficiency on the emitter layer number, thickness, doping concentration, and applied bias of the FIR detectors, as well as the active layer thickness and internal quantum efficiency of the LEDs. The present study has also yielded an optimal structure for the integrated HIWIP-LED FIR imaging devices.

Quantum structures for multiband photon detection

AbstractThe work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.

Near- and far-infrared p-GaAs dual-band detector

A dual-band homojunction interfacial workfunction internal photoemission infrared detector that responds in both near- and far-infrared (NIR and FIR) regions is reported. In the p+-i-p+ detector structure, the emitter is carbon doped to 1.5×1019cm−3, and a 1μm thick GaAs layer acts as the barrier, followed by another highly p-doped GaAs contact layer. The NIR response is due to the interband transition in GaAs barrier layer and the threshold wavelength observed at 0.82μm is in good agreement with the 1.51eV band gap of GaAs at 4.2K. The intraband transition giving rise to FIR response is observed up to 70μm. Interband responsivity was (under 100mV reverse bias at 20K) ∼8A∕W at 0.8μm, while the intraband responsivity was ∼7A∕W. The detector has peak detectivities D*∼6×109 and 5×109cmHz1∕2∕W at 0.8 and 57μm wavelengths, respectively, under 100mV reverse bias at 20K.

The effects of light–heavy hole transitions on the cutoff wavelengths of far infrared detectors

Results are presented on the effects of doping variation on the cutoff wavelength ( λ c) of homojunction interfacial workfunction internal photoemission far infrared detectors. The behavior at low doping (<10 19 cm −3) is well predicted by the free carrier absorption model used previously. However at high doping the observed λ c is much shorter than the values predicted by the workfunction obtained from Arrhenius plots. An explanation for the reduced λ c in the high doping region is presented using a model for depletion of the heavy hole band due to direct transitions from the heavy hole to light hole band.

QUANTUM STRUCTURES FOR FAR-INFRARED DETECTION

FIR photon detector development starting from the extrinsic detectors for LWIR to FIR wavelengths are presented. Several other types of IR detectors, including the cut-off wavelength extension into the FIR range for quantum well infrared photodetectors (QWIPs), are summarized. Efforts in developing p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared detectors and the most reason developments on GaAs/AlGaAs Heterojunction interfacial workfunction internal photoemission (HEIWIP) far-infrared detectors are presented.

Effect of interface states on negative capacitance characteristics in GaAs homojunction far-infrared detectors

Bias-, frequency- and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. The origin of this effect is believed to be due to the carrier capture and emission at interface states, and has been confirmed by a comparison study of capacitance characteristics on p-GaAs HIWIP detectors with different interface state densities. A fitting data based on charging–discharging current and the inertial conducting current model show good agreement with the experimental observations.

Low dark current far infrared detector with an optical cavity architecture

Here we report designs for performance improvements of homojunction interfacial workfunction internal photoemission (HIWIP) detectors for different far infrared regions. A design is given to reduce dark current to about 10 e /s for a 300 μm cut-off detector at 1.3 K, at a bias field of 500 V/cm by adjusting the thickness of the intrinsic layer to eliminate tunneling component. The intrinsic region thickness and the bottom contact are used to obtain an optical cavity effect thus increasing the absorption to almost 100%. Increased responsivity due to the optical cavity effect is experimentally verified by using three detector structures, with three different cavity structures.

GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared detectors

The recent development of p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (>40 μm) detectors is briefly reviewed. The emphasis is on the detector performance, which includes responsivity, quantum efficiency, bias effects, cut-off wavelength, uniformity, noise, and negative capacitance characteristics. Promising results indicate that p-GaAs HIWIP detectors have great potential to become a strong competitor in far-infrared applications.

Effect of interface states on the performance of GaAs p+-i far-infrared detectors

Interface states have been shown to have an appreciable effect on the performance of p-GaAs multilayer (p+-i-p+-i-…) homojunction interfacial work function internal photoemission (HIWIP) far-infrared detectors. In this article, a comparison of detector performance was made of p-GaAs HIWIP detectors with different interface state densities, with the emphasis on the detector’s dark current, noise, and capacitance characteristics.

Recent Progress in Mid- and Far-Infrared Semiconductor Detectors

The recent developments of semiconductor infrared detectors in extending the wavelength coverage and improving the focal plane array (FPA) performance are reviewed. The emphasis is on the GeSi/Si heterojunction infrared photoemission detectors (HIPs), GaAs/AlGaAs quantum well infrared photodetecots (QWIPs), Si, Ge and GaAs blocked impurity band detectors (BIBS), and Si and GaAs homojunction interfacial work-function internal photo-emission (HIWIP) far-infrared (FIR) detectors. The advantages, current status, and potential limitations of these infrared detectors have also been discussed.

Negative capacitance of GaAs homojunction far-infrared detectors

Bias, frequency and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. Unlike in other devices, even up to 1 MHz in HIWIP, the negative capacitance value keeps increasing with frequency, giving a stronger effect. The origin of this effect is believed to be due to the carrier capture and emission at interface states. Fitting data based on charging-discharging current and the inertial conducting current model show good agreement with the experimental observations.

Low-frequency noise and interface states in GaAs homojunction far-infrared detectors

Low-frequency noise characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared (FIR) detectors are reported. The noise was found to exhibit 1/f behavior related to interface states at frequencies below 1 kHz and frequency independent shot noise at higher frequencies. The noise expressions correctly predict the dark current noise behavior, and provide a means of estimating both the gain and energy distribution of the interface states. The interface state density is estimated to be in the order of 10/sup 11/ cm/sup -2/. It has been shown that the estimated gain and noise equivalent power are in good agreement with the previous results obtained via optical measurements.