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Electrically and optically active charge carrier traps in silicon-doped few-layer GaSe

M. Bissolo, R. Li, M. Ogura, Z. Sofer, S. Polesya, D. Han, A. W. Holleitner, C. Kastl, G. Koblmüller, H. Ebert, E. Zallo, J. J. Finley

TL;DR

This work addresses how electrically active deep-level defects limit and tailor the performance of 2D GaSe when Si-doped, which is crucial for next-generation optoelectronic and photovoltaic devices. It leverages a multi-technique approach—DLTS, DLOS, TAS, SSPC, and CV in a GaSe MIS capacitor, complemented by DFT-HSE06 defect calculations—to map trap energies across the full bandgap and assign them to specific Si-related defects and Ga vacancies. The study identifies multiple trap levels, including $0.31$ eV, $0.88$ eV, $1.40$ eV below the CBM, a near-VBM trap at $0.16$ eV above, and a $0.26$ eV below CBM donor, with a bandgap of $2.16$ eV; optical techniques corroborate several of these levels, while DFT links them to Si_Ga, Si_Se, and V_Ga configurations. The results establish a comprehensive defect map and a robust framework for defect characterization in doped 2D vdW semiconductors, informing defect-engineering strategies for GaSe-based devices and broader PTMC materials.

Abstract

Understanding defects in atomically thin van der Waals (vdW) semiconductors is essential for advancing their use in next-generation optoelectronic and photovoltaic devices. Here, we apply a combination of various impedance spectroscopy techniques to two-dimensional (2D) vdW GaSe doped with silicon (Si) to reconstruct deep trap states across the full bandgap. Deep-level transient spectroscopy reveals three distinct deep states 0.31, 0.88, and 1.40 eV below the conduction band edge. Complementary deep-level optical spectroscopy and photocapacitance measurements identify three deep states at 1.4 and 1.8 eV below the conduction band edge, and 2.0 eV above the valence band edge, with thermal admittance spectroscopy providing additional verification and further resolving two trap states, at 0.16 eV above the valence band edge and at 0.26 eV below the conduction band edge. By comparing the experimentally extracted ionization energies with the predictions of density functional theory, our results attribute these trap states primarily to Si-related defects and metal vacancies. This work presents a comprehensive defect map of Si-doped GaSe, providing critical insights into carrier trapping mechanisms that are essential for optimizing the design of 2D material-based devices for industrial applications.

Electrically and optically active charge carrier traps in silicon-doped few-layer GaSe

TL;DR

This work addresses how electrically active deep-level defects limit and tailor the performance of 2D GaSe when Si-doped, which is crucial for next-generation optoelectronic and photovoltaic devices. It leverages a multi-technique approach—DLTS, DLOS, TAS, SSPC, and CV in a GaSe MIS capacitor, complemented by DFT-HSE06 defect calculations—to map trap energies across the full bandgap and assign them to specific Si-related defects and Ga vacancies. The study identifies multiple trap levels, including eV, eV, eV below the CBM, a near-VBM trap at eV above, and a eV below CBM donor, with a bandgap of eV; optical techniques corroborate several of these levels, while DFT links them to Si_Ga, Si_Se, and V_Ga configurations. The results establish a comprehensive defect map and a robust framework for defect characterization in doped 2D vdW semiconductors, informing defect-engineering strategies for GaSe-based devices and broader PTMC materials.

Abstract

Understanding defects in atomically thin van der Waals (vdW) semiconductors is essential for advancing their use in next-generation optoelectronic and photovoltaic devices. Here, we apply a combination of various impedance spectroscopy techniques to two-dimensional (2D) vdW GaSe doped with silicon (Si) to reconstruct deep trap states across the full bandgap. Deep-level transient spectroscopy reveals three distinct deep states 0.31, 0.88, and 1.40 eV below the conduction band edge. Complementary deep-level optical spectroscopy and photocapacitance measurements identify three deep states at 1.4 and 1.8 eV below the conduction band edge, and 2.0 eV above the valence band edge, with thermal admittance spectroscopy providing additional verification and further resolving two trap states, at 0.16 eV above the valence band edge and at 0.26 eV below the conduction band edge. By comparing the experimentally extracted ionization energies with the predictions of density functional theory, our results attribute these trap states primarily to Si-related defects and metal vacancies. This work presents a comprehensive defect map of Si-doped GaSe, providing critical insights into carrier trapping mechanisms that are essential for optimizing the design of 2D material-based devices for industrial applications.
Paper Structure (9 sections, 4 equations, 4 figures, 1 table)

This paper contains 9 sections, 4 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Results
  3. Discussion
  4. Conclusions
  5. Experimental Section
  6. Crystal growth
  7. Sample Fabrication
  8. Impedance and Optical Measurements
  9. Theoretical Methods

Figures (4)

  • Figure 1: (a) Schematic (left) and optical micrograph (right) of the GaSe metal-insulator-semiconductor (MIS) device (left). (b) Forward and backward CV sweeps at 500 Hz and 700 kHz (room temperature). The grey dotted line indicates the position of half the geometric capacitance. (c) Heat map of the DLTS measurement for temperatures ranging between 100 K and 300 K and (d) Arrhenius plot (black points) of the temperature-dependent lifetime of the identified defects with respective fits (red lines). The dashed green lines in (c) are computed from the Arrhenius fits in (d). To limit measurement duration, temperatures below 257 K were limited to measured lifetimes of a maximum of 100 ms.
  • Figure 2: (a) Deep Level Optical Spectroscopy (DLOS) and (b) steady-state photocapacitance (SSPC) of the GaSe MIS device. The black dashed lines in (a) show the fitting results using Lucosky's model. The gray line in (b) represents the derivative function used to determine the transitions.
  • Figure 3: (a) CV measurements taken at frequencies ranging from 20 Hz to 700 kHz. (b) Temperature-dependent CV profiling at 500 Hz. (c) Acceptor energy as a function of voltage, as extracted from the Thermal Admittance Spectroscopy (TAS) measurement in the inversion regime. The inset shows the Arrhenius plot of the detrapping lifetime for -4, -5, and -6 V, plotted as $1000/T$ vs. $\ln\!\left(1/\omega T^{3/2}\right)$. (d) Arrhenius plots of the detrapping lifetime at various voltages in the accumulation regime. (e) Acceptor energy as a function of voltage, as extracted from (d). The dashed red line is a fit of the voltage-dependent chemical potential $\mu(V)$. The dotted black line indicates $\mu(V)$ for a semiconductor with no gap states. (f) Defect density close to the conduction band. The energy position ($\mu(V)$) is computed from the fit in (e). The red line is a fit using the sum of an exponential function and a Gaussian.
  • Figure 4: Comparison between experimentally measured (left) and DFT-calculated (right) mid-gap states and band-edge positions relative to the CBM. Left: Deep levels detected experimentally by DLTS (continuous blue), DLOS (dotted blue), SSPC (continuous gray), and TAS (dotted gray). Right: Defect levels for various point defects calculated by DFT. The colour of each marker indicates the defect site: green for interstitial positions, yellow for selenium sites, and red for gallium sites. Solid black lines indicate the CBM and VBM.