Abstract
Electron accumulation in transparent conductive oxides (TCOs), when driven by a gate voltage, is capable of inducing extremely strong electro-optic absorption at the telecommunication wavelength window due to the epsilon-near-zero (ENZ) effect and various waveguide modulators have been proposed in recent years. This paper conducts a comparative analysis of TCO modulators by reviewing several representative designs based on the uniform concentration accumulated carrier model and the classical continuous carrier distribution model. We also apply the quantum moment model to analyze the free carrier distribution of the TCO based metal-oxide-semiconductor (MOS) capacitor for the first time, and reveal significantly different device physics compared with previous simulation models. The quantum moment model predicts a much higher driving voltage in order to turn the TCO materials into ENZ and a stronger modulation strength compared with the classical model. Especially, the requirement of the higher gate voltage brings a great challenge to the insulator layer as the electric field in the insulator is exceeding the breakdown strength, which raises the concern of reliability. In order to evaluate the accuracy of different models, we compare the simulation results with two of the most recent experimental papers and show that the quantum model has a better match in terms of the electro-absorption rate and the differential driving voltage. However, the quantum moment model still cannot explain some other experimental results, which may be induced by different modulation mechanisms.
Highlights
In recent years, transparent conductive oxides (TCOs) have attracted escalating research interests as a new type of plasmonic materials [1,2]
TCOs, such as indium-tin oxide (ITO) and aluminum-zinc oxide (AZO), are a family of wide bandgap semiconductor oxide materials that can be degenerately doped to a high level, which are widely used in display industry [3] and photovoltaics [5]
We conducted a comparative analysis of TCO-EA modulators using uniform concentration free carrier model, classical semiconductor device model, and quantum moment model
Summary
Transparent conductive oxides (TCOs) have attracted escalating research interests as a new type of plasmonic materials [1,2]. It is possible to turn Si or III-V semiconductors into ENZ with extremely high free carrier concentration (>2 × 1021) based on Drude model, the required electric field will far exceed the breakdown strength of any known insulator material. Realizing the great potential to build wavelength-scale EA modulators, various groups have designed TCO based gate-driven ENZ modulators [14,15,16,17,18,19,20,21,22,23,24,25,26], which are exclusively built upon the concept of integrating metal-oxide-semiconductor (MOS) capacitor with silicon or plasmonic waveguides These modulators follow the same working principal: the gate voltage induces electron accumulation in the TCO materials, which dramatically increases the imaginary part κ or even turns TCOs into ENZ to achieve strong light absorption.
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