Theory of tunneling and transport in emerging narrow-gap semiconductor alloys
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Date
2024
Authors
Das, Sarita
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University College Cork
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Abstract
There is a high demand for narrow-band gap semiconductor alloys to support the development of advanced devices such as high-sensitivity photodiodes (PDs) and light sources (lasers and LEDs) for the application-rich wavelength range from 3 to 5 $\mu$m. These applications include environmental and industrial process monitoring, free-space optical communications, and imaging for aerospace and defence applications.
Therefore, there is a need to access the electronic and optical properties of these alloys. For instance, the suppression of dark current is crucial to achieving high-sensitivity PDs to improve the detector signal-to-noise ratio. Leakage currents linked to band to band tunneling (BTBT) plays a substantial role in the dark current in PDs based on narrow-gap semiconductors. Determining the effectiveness of a device also involves looking into the electron transport properties of such alloys. Two types of emerging narrow-band gap alloys are investigated in this thesis, namely (i) highly mismatched dilute nitride and bismide alloys, and (ii) Ge$_{1-x}$Sn$_{x}$ alloys.
We first theoretically analyse BTBT in highly-mismatched, narrow-gap dilute nitride and bismide alloys. Replacing As by N (or Bi) introduces localised impurity states into the conduction (valence) band, whose impact on the band structure can be described by a band-anticrossing (BAC) interaction between the localised impurity states and the host conduction (valence) band. For this class of semiconductors, the assumptions underpinning the widely-employed Kane model of BTBT in an applied electric field break down, due to strong band edge nonparabolicity resulting from the BAC interactions. Via numerical calculations based on the Wentzel-Kramers-Brillouin approximation, we demonstrate that BAC leads, at a fixed band gap, to a reduced (increased) BTBT current at low (high) applied electric field compared to that in a conventional InAs$_{1-y}$Sb$_{y}$ alloy. Our analysis reveals that BTBT in InN$_{x}$As$_{1-x}$ and InAs$_{1-z}$Bi$_{z}$ is governed by a field-dependent competition between the impact of N (Bi) incorporation on (i) the dispersion of the evanescent Bloch band linking the valence and conduction band edges, which dominates at low field strengths, and (ii) the conduction (valence) band dispersion, which dominates at high field strengths. The implications of our results for applications in long-wavelength avalanche photodiodes (APDs) and tunneling field-effect transistors (TFETs) are discussed.
In addition to this study, we investigate the evolution of the electron transport properties of Ge$_{1-x}$Sn$_{x}$ alloys as a function of increasing Sn content, as the alloy goes from indirect-gap to direct-gap. Ge$_{1-x}$Sn$_{x}$ has attracted significant interest for applications in electronic and photonic devices as a (narrow-gap) direct-gap semiconductor compatible with existing CMOS processing. We use the Boltzmann transport equation to calculate as a function of Sn composition the evolution of the electron transport properties in an applied electric field. The intra and inter-valley alloy scattering rates are determined from tight-binding calculations. At extremely low electric field, we predict an increase of approximately two orders of magnitude in electron mobility in direct-gap alloys with higher Sn composition. However, our calculations predict that the electron mobility at these high Sn compositions decreases rapidly with increasing field, as the electrons are quickly accelerated to higher energy, where they can scatter from the $\Gamma$ valley with low density of states (DOS) to the L valleys, with their much higher DOS. We calculate that the contribution of alloy scattering causes the mobility to drop below that of Ge for all values of $x$ considered once the field exceeds 50 V cm$^{-1}$.
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Keywords
Band to band tunneling , Highly-mismatched nitride and bismide alloys , Band anti-crossing , Field-depedent electron transport , GeSn alloys , Electron mobility , Drift velocity , APDs , TFETs
Citation
Das, S. 2024. Theory of tunneling and transport in emerging narrow-gap semiconductor alloys. PhD Thesis, University College Cork.