Zero-bias Responsivity Research Articles

Asymmetric Schottky contacts induced via localized ultrafast laser irradiation for ultrasensitive, self-powered, 2D photodetectors

Self-powered photodetectors hold great promise for addressing power-related challenges in the “Internet of Things (IoT)” era. However, the self-powered performances of a metal-semiconductor-metal (MSM) structure photodetector are inherently limited due to the presence of the symmetric Schottky contacts, leading to canceling photocurrents in opposite directions, while constructing an asymmetric MSM structure demands complicated design or sustained external fields. Herein, we propose a novel contact engineering strategy based on localized femtosecond (fs) laser irradiation, achieving an asymmetric band structure in an MSM-structure photodetector with pristine symmetric Schottky contacts. As a result, a MoS2 photodetector that exhibits exceptional photoresponse and self-powered performances is successfully fabricated. The device type transitions from a photoconductor to a photodiode, exhibiting a significant improvement in the rectification ratio from 1 to > 104. A comprehensive study on fs laser-based contact engineering is conducted by investigating atom diffusion, element variation, and Schottky barrier height variation. Notably, the photodetector demonstrates an exceptional zero-bias responsivity of 1.72 A/W, with a short circuit current of 37.2 nA and maximum output electric power of 1.67 nW. Our work provides a new strategy for modifying the energy band structure and facilitates the fabrication of high-performance self-powered photodetectors.

Electron affinity of metal oxide thin films of TiO2, ZnO, and NiO and their applicability in 28.3 THz rectenna devices

The holy grail of achieving efficient operation of infrared (IR) rectennas continues to be the realization of a high performance rectifier. In this paper, we have fabricated metal–insulator–metal (MIM) diodes based on TiO2, ZnO, and NiO thin films using shadow mask evaporation, photolithography, and sputtering. The electron affinities of oxides have been measured by a combination of variable angle spectroscopic ellipsometry and x-ray photoelectron spectroscopy, as well as deduction from the extraction of metal/oxide barrier heights of Fowler–Nordheim tunneling plots. Our results confirm a low value for the electron affinity of NiOx of ∼2.1–2.5 eV, which correlates with the high zero-bias dynamic resistance (RD0) of ∼500 kΩ of an associated MIM diode. These values render NiOx to be unsuitable for use in a rectenna device. Better performance has been observed from diodes based on TiO2 and ZnOx films. The best rectification performance was achieved for a Au/2.6 nm ZnOx/Cr diode, scaled down to 1 μm2 device area, showing a zero-bias dynamic resistance of RD0 = 71 kΩ, zero-bias responsivity β0 = 0.28 A/W, and a coupling efficiency of ηc = 2.4 × 10−5% for rectification at 28.3 THz. The main significance of this study is that it employs a methodology whereby key parameters of the MIM stack are derived from physical measurements, which are then used to assist in the fitting of electrical current–voltage data to produce a reliable appraisal of diode performance in an IR rectenna.

Long-wave infrared photothermoelectric detectors with ultrahigh polarization sensitivity

Filter-free miniaturized polarization-sensitive photodetectors have important applications in the next-generation on-chip polarimeters. However, their polarization sensitivity is thus far limited by the intrinsic low diattenuation and inefficient photon-to-electron conversion. Here, we implement experimentally a miniaturized detector based on one-dimensional tellurium nanoribbon, which can significantly improve the photothermoelectric responses by translating the polarization-sensitive absorption into a large temperature gradient together with the finite-size effect of a perfect plasmonic absorber. Our devices exhibit a zero-bias responsivity of 410 V/W and an ultrahigh polarization ratio (2.5 × 104), as well as a peak polarization angle sensitivity of 7.10 V/W•degree, which is one order of magnitude higher than those reported in the literature. Full linear polarimetry detection is also achieved with the proposed device in a simple geometrical configuration. Polarization-coded communication and optical strain measurement are demonstrated showing the great potential of the proposed devices. Our work presents a feasible solution for miniaturized room-temperature infrared photodetectors with ultrahigh polarization sensitivity.

Geometric filterless photodetectors for mid-infrared spin light

Free-space circularly polarized light (CPL) detection, requiring polarizers and wave plates, is well established, but such a spatial degree of freedom is unfortunately absent in integrated on-chip optoelectronics. The filterless CPL photodetectors reported so far suffer from an intrinsic small discrimination ratio, vulnerability to the non-CPL field components and low responsivity. Here we report a distinct paradigm of geometric photodetectors in the mid-infrared, exhibiting a substantial discrimination ratio of 84, a close-to-perfect CPL-specific response, a zero-bias responsivity of 392 V W−1 at room temperature and a detectivity of ellipticity down to 0.03° Hz−1/2. Our approach makes use of a plasmonic nanostructures array with judiciously designed symmetry, assisted by graphene ribbons, to electrically read their near-field optical information. This geometry-empowered recipe for infrared photodetectors provides a robust, direct, strict and high-quality solution to on-chip filterless CPL detection and unlocks new opportunities for integrated functional optoelectronic devices.

(Digital Presentation) Optimization of MIM Rectifiers for Terahertz Rectennas

Metal-Insulator-Metal (MIM) rectifiers comprising thin films of Al2O3, ZnO, NiO and Nb2O5 and metal configurations of Au/Au, Au/Zn and AuCr/AuCr, have been fabricated using atomic layer deposition and radio-frequency sputtering. The effect of device area scaling from 104 µm2 to 1 µm2 on rectification properties, in particular zero-bias dynamic resistance (R0) and zero-bias responsivity (β0) has been studied and found to be of critical importance in improving diode coupling efficiency. A significant increase of current has been found for Au/3.3 nm ZnO/Au diode when compared to the reference Au/3 nm Al2O3/Au diode, that resulted in obtaining the lowest R0 of 540 W for a device area of 104 µm2. The best performing device is found to be 1 µm2 AuCr/6.77 nm NiO/AuCr featuring (R0, b0) = (461 kW, 0.76 A/W) and a coupling efficiency of 1.5 ´ 10-5 %.

High zero-bias responsivity induced by photogating effect in asymmetric device structure

Graphene photodetectors based on photogating effect are promising due to the high photogain, but with the drawback of large dark current due to external bias. In this paper, we realized a graphene photodetector working at zero bias with high responsivity. By establishing asymmetric structure using CdS film on different part of graphene channel, the influence of asymmetric ratio on device performance were found to be opposite for external and zero bias. Without external bias, the responsivity can reach 0.26 A/W under visible light, resulting in a quantum efficiency of 51.5%, which is much larger than other zero-bias graphene photodetectors based on photothermoelectric effect. Furthermore, based on the asymmetric structure, infrared photoresponse beyond the CdS cut-off wavelength was observed at 980 nm wavelength at low temperature. This work provides a feasible scheme for the low energy consumption work of photodetectors with low dimensional materials.

Oxides for Rectenna Technology.

The quest to harvest untapped renewable infrared energy sources has led to significant research effort in design, fabrication and optimization of a self-biased rectenna that can operate without external bias voltage. At the heart of its design is the engineering of a high-frequency rectifier that can convert terahertz and infrared alternating current (AC) signals to usable direct current (DC). The Metal Insulator Metal (MIM) diode has been considered as one of the ideal candidates for the rectenna system. Its unparalleled ability to have a high response time is due to the fast, femtosecond tunneling process that governs current transport. This paper presents an overview of single, double and triple insulator MIM diodes that have been fabricated so far, in particular focusing on reviewing key figures of merit, such as zero-bias responsivity (β0), zero-bias dynamic resistance (R0) and asymmetry. The two major oxide contenders for MInM diodes have been NiO and Al2O3, in combination with HfO2, Ta2O5, Nb2O5, ZnO and TiO2. The latter oxide has also been used in combination with Co3O4 and TiOx. The most advanced rectennas based on MI2M diodes have shown that optimal (β0 and R0) can be achieved by carefully tailoring fabrication processes to control oxide stoichiometry and thicknesses to sub-nanometer accuracy.

CVD-Grown Monolayer Graphene-Based Geometric Diode for THz Rectennas.

For THz rectennas, ultra-fast diodes are required. While the metal–insulator–metal (MIM) diode has been investigated in recent years, it suffers from large resistance and capacitance, as well as a low cut-off frequency. Alternatively, a geometric diode can be used, which is more suitable due to its planar structure. However, there is only one report of a THz geometric diode based on a monolayer graphene. It is based on exfoliated graphene, and thus, it is not suitable for mass production. In this work, we demonstrate chemical vapor deposition (CVD)-grown monolayer graphene based geometric diodes, which are mass-producible. The diode’s performance has been studied experimentally by varying the neck widths from 250–50 nm, the latter being the smallest reported neck width for a graphene geometric diode. It was observed that by decreasing the neck widths, the diode parameters such as asymmetry, nonlinearity, zero-bias resistance, and responsivity increased within the range studied. For the 50 nm neck width diode, the asymmetry ratio was 1.40 for an applied voltage ranging from −2 V to 2 V, and the zero-bias responsivity was 0.0628 A/W. The performance of the diode was also verified through particle-in-cell Monte Carlo simulations, which showed that the simulated current-voltage characteristics were consistent with our experimental results.

Towards an optimal MIIM diode for rectennas at 10.6 μm

We investigate, for the first time, optimization of the responsivity of metal-insulator-insulator-metal (MIIM) diodes by carefully considering materials' properties. The diode's resistance is fixed at 100 Ω in order to match the nano antenna's impedance and to increase the total efficiency of the rectifying antenna (rectenna). The optimization is performed at zero-bias responsivity and resistance to ensure zero-bias operation. The diode's resistance and responsivity are calculated from the simulated current-voltage characteristics using Airy functions-based transfer matrix method (TMM) that was verified using experimental results. The design parameters of barrier heights for each insulator and metals, their dielectric constants, and the difference in the metals' work functions, are optimized to enhance the overall diode's performance. Optimal results are presented showing the beneficial effect of simultaneously varying these design parameters. Suggestions for implementing these optimal values with real materials are proposed and presented. Finally, the MIIM capacitance effect on the rectenna cut-off frequency is investigated to ensure that proposed MIIM diodes allow the rectenna to operate efficiently in the IR regime.

Applicability of Sc2O3 versus Al2O3 in MIM rectifiers for IR rectenna

Impedance matching between the terahertz antenna and ultra-fast Metal-Insulator-Metal (MIM) diode is a crucial issue in realizing rectenna technology for harvesting infrared (IR) energy spectrum. In this paper, scandium oxide (Sc2O3)-based MIM diodes were fabricated using magnetron sputtering and their rectification performance compared to state-of-the-art Au/Al2O3/Au diodes. The fabricated Al/Sc2O3/Al diode has exhibited around two orders of magnitude lower zero-bias dynamic resistance (RD0 = 956 kΩ) and high zero-bias responsivity (β0 = 1 A/W) in advance to Au/Al2O3/Au diode. The results point to applicability of scandium oxide in MIM rectifiers for IR rectenna.

Trap-related frequency dispersion of zero-bias microwave responsivity at low temperature in GaN-based self-switching diodes

The zero-bias microwave detection capability of self-switching diodes (SSDs) based on AlGaN/GaN is analyzed in a wide temperature range, from 10 K to 300 K. The measured responsivity shows an anomalous enhancement at low temperature, while the detected voltage exhibits a roll-off in frequency, which can be attributed to the presence of surface and bulk traps. To gain a deep insight into this behavior, a systematic DC and AC characterization of the diodes has been carried out in the mentioned temperature range. DC results confirm the existence of traps and AC measurements allow us to identify their properties. In particular, impedance studies enable to distinguish two types of traps: at the lateral surfaces of the channel, with a wide spread of relaxation times, and in the bulk.

InSb Avalanche Photodiodes on GaAs Substrates for Mid-Infrared Detection

We present indium antimonide-based devices for mid-infrared (mid-IR) detection with enhanced sensitivity. InSb devices will be useful for many applications, such as gas sensing and imaging. InSb avalanche photodiodes (APDs) monolithically integrated with GaAs substrates were fabricated with diameters ranging from 90 to $200~\mu \text{m}$ and extensively characterized at temperatures ranging from 77 K to 300 K. At 120 K a zero-bias responsivity of 2 A/W was measured, corresponding to a quantum efficiency of 55%. An experimental gain value of 10 at a reverse bias of −3 V was obtained at 120 K, which to the best of our knowledge, is the highest ever reported for InSb APDs. These results pave the way for the development of a monolithically integrated mid-IR array with added gain and wavelength tunability.

Global Optimization of Rectennas for IR Energy Harvesting at 10.6μm

The impedance mismatch between a rectenna's two components, the antenna and the rectifying diode, represents a serious impediment to improving the rectenna's efficiency. Also, the diode's capacitance and the device equivalent resistance determine the rectenna's cutoff frequency. For the detection of the terahertz-frequency signals, ultrafast rectifying metal–insulator–metal diodes are feasible rectifiers. Here, we investigate the global optimization of the rectenna's efficiency working at 10.6 μ m by taking the diode materials’ properties into consideration. Our proposed objective function considers the impedance matching between antenna and diode, the device cutoff frequency, and the diode responsivity which governs the rectification efficiency. The optimal parameter set reveals a 5.5% coupling efficiency and a zero-bias responsivity of 6.4 A/W. The desirable material properties and geometry are prescribed. The fabrication of Ti-TiO2/ZnO-Al metal–insulator–insulator–metal diode based on the optimal results is carried out and characterized.

Polarization-Graded AlGaN Solar-Blind p-i-n Detector With 92% Zero-Bias External Quantum Efficiency

We report on record high zero-bias external quantum efficiency (EQE) of 92% for back-illuminated Al0.40Ga0.60N p-i-n ultra-violet (UV) photodetectors on sapphire. The zero-bias responsivity measured 211 mA/W at 289 nm, which is the highest value reported for solar-blind, p-i-n detectors realized over any epitaxial wide band-gap semiconductor. This is also the first report for a p-i-n detector, where a polarization-graded Mg-doped AlGaN layer is utilized as the p-contact layer. The devices exhibited a ten-orders of magnitude rectification, a low reverse leakage current density of 1 nA/cm2 at 10 V, a high $\text{R}_{{0}}\text{A}$ product of $1.3\times 10^{{11}} \Omega $ .cm2 and supported fields exceeding 5 MV/cm. The light-to-dark current ratio and the UV-to-visible rejection ratio for the detectors exceeded six-orders of magnitude and the thermal noise limited detectivity (D*) measured $6.1\times 10^{{14}}$ cmHz1/2W−1. The state-of-the-art performance parameters can be attributed to a high crystalline quality absorbing AlGaN epi-layer resulting from the use of an AlN/AlGaN superlattice buffer and an improved p-contact via polarization grading.

MBE-Grown <inline-formula> <tex-math notation="LaTeX">$\beta$ </tex-math> </inline-formula>-Ga2O3-Based Schottky UV-C Photodetectors With Rectification Ratio ~107

In this letter, we demonstrate high-performance vertical solar-blind Schottky photodetectors on MBE-grown homoepitaxial (010)-oriented β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films. The structure, consisting of (100 nm) β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /(60 nm) n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , was grown on a Fe-doped insulating (010) β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. Ni/Au and indium were used as the Schottky and Ohmic contacts, respectively. The devices exhibited a rectification ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> with turn-on voltage ~1 V and an ideality factor of 1.31. The extracted Schottky barrier height was 1.4 eV. The photodetectors showed low dark current of 0.3 nA at 5 V with a photo-to-dark current ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 0 V. The devices exhibited a zero-bias responsivity of 4 mA/W at 254 nm corresponding to an external quantum efficiency ~3 %, with a UV-to-visible rejection ratio >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , showing true solar-blind operation. The transient response of the devices indicated rise/fall times of ~100 ms. Temperature-dependent current-voltage characteristics agree with the thermionic emission model.

Avoiding erroneous analysis of MIM diode current-voltage characteristics through exponential fitting

Accurate fitting of measured current-voltage [I(V)] data is crucial to the correct analysis and understanding of metal-insulator–metal (MIM) diodes, especially for optical rectennas. With the commonly used polynomial fitting of the I(V) data, the order of the fit can drastically affect the diode performance metrics such as resistance, responsivity, and asymmetry. Additionally, the resulting fitting coefficients provide no useful parameters. An exponential-based equation can fit the I(V) data well, can avoid artifacts from the choice of order of the polynomial, and allows for the accurate calculation of diode performance metrics directly from the fitting coefficients. Connecting the performance metrics to fitting coefficients shows a correspondence between zero-bias responsivity and asymmetry at any given voltage.

Metal-insulator-metal diodes with sub-nanometre surface roughness for energy-harvesting applications

For ambient radio-frequency (RF) energy harvesting, the available power levels are quite low, and it is highly desirable that the rectifying diodes do not consume any power at all. Contrary to semiconducting diodes, a tunnelling diode – also known as a metal-insulator-metal (MIM) diode – can provide zero-bias rectification, provided the two metals have different work functions. This could result in a complete passive rectenna system. Despite great potential, MIM diodes have not been investigated much in the GHz-frequency regime due to challenging nano-fabrication requirements. In this work, we investigate zero-bias MIM diodes for RF energy-harvesting applications. We studied the surface roughness issue for the bottom metal of the MIM diode for various deposition techniques such as sputtering, atomic layer deposition (ALD) and electron-beam (e-beam) evaporation for crystalline metals as well as for an amorphous alloy, namely ZrCuAlNi. A surface roughness of sub-1nm has been achieved for both the crystalline metals as well as the amorphous alloy, which is vital for the reliable operation of the MIM diode. An MIM diode comprising of a Ti-ZnO-Pt combination yields a zero-bias responsivity of 0.25V−1 and a dynamic resistance of 1200Ω. Complete RF characterisation has been performed by integrating the MIM diode with a coplanar waveguide transmission line. The input impedance varies from 100Ω to 50Ω in the frequency range of between 2GHz and 10GHz, which can be easily matched to typical antenna impedances in this frequency range. Finally, a rectified DC voltage of 4.7mV is obtained for an incoming RF power of 0.4W at zero bias. These preliminary results of zero-bias rectification indicate that complete, passive rectennas (a rectifier and antenna combination) are feasible with further optimisation of MIM devices.

High performance MIIM diode based on cobalt oxide/titanium oxide

Optical rectennas for infrared energy harvesting commonly incorporate metal/double-insulator/metal diodes. Required diode characteristics include high responsivity and low resistance near zero bias with a sub-micron area, which have not been obtainable simultaneously. Diodes based on a new material set, Co/Co3O4/TiO2/Ti and an area of 0.071 μm2, provide a median maximum responsivity of 4.1 A/W, a median zero-bias responsivity of 1.2 A/W, and a median resistance of 14 kΩ. The highest performing diode has a maximum responsivity of 4.4 A/W, a zero-bias responsivity of 2.2 A/W, and a resistance of 18 kΩ.

InGaN Visible Photodiodes With Improved Performance Using Oxidized Ir Schottky Contact

Oxidized iridium (Ir) was employed as the Schottky contact of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> N photodiodes. The Schottky barrier height (SBH) increases from 0.70 eV for the as-deposited Ir contacts to 0.91 eV for the contacts annealed at 500 ° C in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambient, while the dark current at -3 V bias decreases from 4.65×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to 2.29×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> A. A zero-bias responsivity of 32 mA/W was obtained at 410 nm in the devices with the oxidized contacts, corresponding to an external quantum efficiency of 10%. Analysis results indicate that the increased SBH and decreased leakage current can be mainly ascribed to two factors: 1) the higher work function of the IrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> contacts formed by oxidation annealing and 2) the reduction in the background electron concentration near the IrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> N interface caused by the generation of compensating group-III vacancies during the oxidation process. Besides, the oxidation of group-III elements may also play a minor role in the performance improvement.

Photovoltaic Schottky ultraviolet detectors fabricated on boron-doped homoepitaxial diamond layer

A photovoltaic diode using tungsten carbide (WC) Schottky contact for deep ultraviolet (DUV) light detection is developed using a lightly boron-doped homoepitaxial diamond layer. The photodiode shows a quick dc response time lower than an instrumental time constant of 0.3 s. The responsivity varies linearly with the DUV light intensity. A discrimination ratio of 105 between 210 nm and visible light is achieved at zero bias. The short-circuit photocurrent at 220 nm decreases with increasing the temperature from 300 to 373 K, whereas the Schottky barrier height under illumination remains unchanged. Numerical and experimental analyses reveal that the interface states between diamond and WC play a major role in reducing the zero-bias responsivity at elevated temperatures.