Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources

Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications. Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m. However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5−10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time. This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Green’s function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design. The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed. To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 μm2 and 49 μm2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis. In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications

In_0.53Ga_0.47As/AlAs Double-Barrier Resonant Tunnelling Diodes with High-Power Performance in the Low-Terahertz Band

We report about an In0.53Ga0.47As/AlAs doublebarrier resonant tunnelling diode (RTD) epitaxial structure that features high-power capabilities at low-terahertz frequencies (∼ 100−300 GHz). The heterostructure was designed using a TCAD-based quantum transport simulator and experimentally investigated through the fabrication and characterisation of RTD devices. The high-frequency RF power performance of the epitaxial structure was analysed based on the extracted small-signal equivalent circuit parameters. Our analysis shows that a 9 µm2, 16 µm2, and 25 µm2 large RTD device can be expected to deliver around 2 mW, 4 mW, and 6 mW of RF power at 300 GHz

Ohmic contacts optimisation for high-power InGaAs/AlAs double-barrier resonant tunnelling diodes based on a dual-exposure E-beam lithography approach

No abstract available

Micro-PL analysis of high current density resonant tunneling diodes for THz applications

Low-temperature micro-photoluminescence (μPL) is used to evaluate wafer structural uniformity of current densities >5mA/μm2 InGaAs/AlAs/InP resonant tunneling diode (RTD) structures on different length scales. Thin, highly strained quantum wells (QWs) are subject to monolayer fluctuations, leading to a large statistical distribution in their electrical properties. This has an important impact on the RTD device performance and manufacturability. The PL spot size is reduced using a common photolithography mask to reach a typical high Jpeak for a given RTD mesa size (1 ∼ 100 μm2). We observe that for lower strain-budget samples, the PL line shape is essentially identical for all excitation/collection areas. For higher strain-budget samples, there is a variation in the PL line shape that is discussed in terms of a variation in long-range disorder brought about by strain relaxation processes. The RTD operating characteristics are discussed in light of these findings, and we conclude that strain model limits overestimate the strain budget that can be incorporated in these devices. We also highlight μPL as a powerful nondestructive characterization method for RTD structures

An Advanced TCAD Quantum Transport-based Simulation Approach for InP RTD Devices Simulation

No abstract available

Modelling, Design, Fabrication, and Characterisation of In_0.53Ga_0.47As/AlAs Double-Barrier Resonant Tunnelling Diodes for Low-Terahertz Sources

No abstract available

Terahertz communications with Resonant Tunnelling Diodes: status and perspectives

This chapter aims to provide a description of the state-of-the-art of the rapidly developing solid-state electronics for terahertz (THz) communications with a focus on the resonant tunnelling diode (RTD) technology. The other key technologies include those that are transistor-based, the so-called THz monolithic integrated circuits (TMICs), and photonics-based solutions, which utilise the uni-travelling carrier photodiode (UTC-PD) for generating the THz signal through photo-mixing. Only an overview of these other technologies will be provided. The chapter will present the status of THz RTD oscillator sources, which are the key component in RTD-based THz transmitters (Tx) and receivers (Rx). An overview of the technical challenges is provided and possible strategies for future progress of the technology are discussed

Epitaxial Structure Simulation Study of In_0.53Ga_0.47As/AlAs Double-Barrier Resonant Tunnelling Diodes

We present an epitaxial structure simulation study of In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes (RTD) employing Atlas TCAD quantum transport simulation software developed by SILVACO, which is based on the non-equilibrium Green’s function (NEGF) formalism. We analyse how epitaxial layers design impacts the heterostructure static current density-voltage (JV) characteristic, including barriers, quantum well (QW), and lightly-doped spacers, as well as the employment of a high-bandgap emitter region. Our analysis shows that, while barriers and QW thicknesses have a strong impact on the current operation of the RTD device, accurate asymmetric spacers design can trade-off between the voltage span and relative position of its negative differential resistance (NDR) region, while a high-bandgap alloy at the emitter side lowers the RTD bias point. This work will guide in optimising the RTD epitaxial structure in order to maximise its RF power performance at low-terahertz frequencies (~ 100−300 GHz)

A High-Power InP Resonant Tunnelling Diode Heterostructure for 300-GHz Oscillator Sources

A high-power double-barrier resonant tunnelling diode (RTD) epitaxial structure in InP technology is reported. The heterostructure exhibits moderate available current density ΔJ≃1.4 mA/μm2 and large voltage swing ΔV≃1.2 V, resulting in a maximum RF power PRF,max≃0.31 mW/um2 , and over 530 GHz bandwidth, being 25 μm2 , 36 μm2 , and 49 μm2 large RTD devices expected to deliver up to 5 mW, 7 mW, and 10 mW at 300 GHz, respectively. Distributed inductors in both coplanar and microstrip geometry are designed through full 3D electromagnetic simulations, proving the feasibility of the proposed approach for the practical realisation of high-power 300-GHz oscillator sources employing low-cost optical lithography

Resonant tunnelling diodes high-speed terahertz wireless communications - a review

Resonant tunnelling diode (RTD) technology is emerging as one of the promising semiconductor-based solid-state technologies for terahertz (THz) wireless communications. This paper provides a review of the state-of-the-art, with a focus on the THz RTD oscillator, which is the key component of RTD-based THz transmitters and coherent receivers. A brief summary on the device principle of operation, technology, modelling, as well as an overview of oscillator design and implementation approaches for THz emitters, is provided. A new insight to device evaluation and to the reported oscillator performance levels is also given, together with brief remarks on RTD-based THz detectors. Thereafter, an overview of the reported wireless links which utilise an RTD in either transmission or reception, or in both roles, is given. Highlight results include the record single-channel wireless data rate of 56 Gb/s employing an all RTD-based transceiver, which demonstrates the potential of the technology for future short-range communications. The paper concludes with a discussion of the current technical challenges and possible strategies for future progress