Photoluminescence excitation spectroscopy of quantum wire-like dislocation states in ZnS

TL;DR

The paper tests ab initio predictions that ZnS dislocations host $1$-D dispersive electronic states with optically active transitions, effectively forming quantum-wire-like dislocation cores. It combines SEM-ECCI-based quantification of dislocation densities with room-temperature ellipsometry, low-temperature PL, and PLE to identify sub-bandgap features linked to specific dislocation cores through $E_{gap}^{dln}$. Four PL-related sub-bandgap peaks ($DE_2$–$DE_4$) correlate with the threading dislocation density and align within ~200 meV of the predicted $E_{gap}^{dln}$, while two additional peaks ($DE_5$, $DE_6$) match $S^-_{90^\circ}$ core predictions; PLE shows efficient below-bandgap excitation for $DE_2$–$DE_5$, indicating radiative 1D core transitions. Overall, the work provides experimental evidence for quantum-wire-like dislocation core states in ZnS, supporting the integration of extended defects into photonic functionality of wide-gap semiconductors and offering a pathway to engineer optical properties via dislocation content.

Abstract

Recent \textit{ab initio} calculations predict 1D dispersive electronic bands confined to the atomic scale cores of dislocations in the wide bandgap (3.84 eV) semiconductor ZnS. We test these predictions by correlating sub-bandgap optical transitions with the density of dislocations formed during strain relaxation in epitaxial ZnS grown on GaP. The densities for four predicted partial dislocations are quantified using scanning electron microscopy-based electron channeling contrast imaging. Room-temperature ellipsometry reveals absorption peaks that scale with dislocation density and align with theoretical predictions. Low-temperature photoluminescence spectra show deep emission peaks matching dislocation 1D band-to-band transitions. Photoluminescence excitation spectroscopy reveals six distinct emission lines with contrasting excitation dependence. Four peaks (2.78, 2.41, 2.20, 1.88 eV), assigned to dislocations, exhibit only modest suppression ($\leq$5$\times$) when excited below the ZnS bandgap, while two other peaks (3.11, 1.53~eV) are strongly quenched ($>$10$\times$). These findings support the existence of efficient, 1D band-to-band radiative transitions within quantum wire-like dislocation core states in ZnS, distinct from typical non-radiative deep-level defects in wide-gap semiconductors.

Figures (8)

• Figure 1: (a) Schematic of surface terminating threading dislocations (TDs) connected to strain-releiving misfit dislocations (MDs) at the heteroepitaxial ZnS/GaP (001) interface, which are imaged through electron channeling contrast imaging (ECCI),fonseca_montenegro_log-normal_2024 an example image of which is projected onto the interface. The core structure of MDs and TDs is locked to the line directions, enabling quantification of the densities of Zn-core ($TD_{Zn}^+$), S-core ($TD_{S}^-$), and neutral($TD_{ZnS}^0$). The full dislocations identified by ECCI, dissociate into four possible partial dislocations, with either a Zn-rich or S-rich core, and either pure edge ($90^\circ$) or mixed ($30^\circ$) character. (b) Linear density of TD's ($\rho_{TD}$) as a function of film thickness ($h$) for the four partials: $Zn_{30^\circ}^+$, $S_{30^\circ}^-$, $Zn_{90^\circ}^+$, and $S_{90^\circ}^-$, see text. Lines guide the eye.
• Figure 2: DFT predicted dislocation band gap ($E_{gap}^{dln}$) and optical absorption in strain-relaxed ZnS. (a) DFT predicted interband transitions between partial dislocations (PD's) and conduction (CB) and valence band (VB) states.genlik_origin_2024 The electron energy-momentum (E-k) diagram plotted from the $\Gamma$ point along the PD line direction for the four PDs formed during ZnS epitaxial strain relaxation. Color indicates 1D bands localized at the PD cores ($Zn_{90 \degree}^+$-red, $Zn_{30 \degree}^+$-pink, $S_{90 \degree}^-$-blue, and $S_{30 \degree}^-$-light blue), while black lines are 3D bulk bands. $E_{gap}^{dln}$ energies (eV) for direct (solid) and indirect (dashed) transitions are plotted. (b) Absorption coefficient ($\alpha$) as a function of photon energy ($E_{ph}$) for epitaxial ZnS of various thicknesses. (c) Sub-band gap absorption peak intensity $A_{1-4}$ plotted as a function of the linear threading dislocation density ($\rho_{TD}$), from Fig. \ref{['fig:densities']}(b), where the PD data are chosen by best matching $E_{gap}^{dln}$ to absorption peak energy ($E_\alpha$).
• Figure 3: Steady-state photoluminescence spectroscopy of 15 nm, 20 nm, 25 nm, and 50 nm ZnS epilayers. (a) Thickness dependent PL spectra where PL intensity has been normalized by the fitted ($A_0, X$) peak height (see Methods). (b) PL spectra from part (a) where a Gaussian fit routine has been employed to fit individual PL peaks, labeled $DE_{1-4}$ by decreasing peak energy. (c) Plot of normalized deep level emission peak intensity vs. ($\rho_{TD}$) for each of the fitted PL peak. Plots are labeled based off of best match between measured PL peak energy and DFT calculated $E_{gap}^{dln}$.
• Figure 4: Photoluminescence excitation spectroscopy (PLE) of dislocation-correlated sub-bandgap emission in ZnS. (a) 2D plot of photoluminescence (PL) intensity as a function of photon detection energy ($E_{ph}$) and excitation energy ($E_{exc}$). The fitted PL peak heights are normalized by their value at $E_{HH-FX}$. (b) Fitted peak heights of $DE_2$, $DE_3$, $DE_4$, and $DE_5$, and (c) $DE_1$ and $DE_6$ as a function of ($E_{exc}$). (d) Line cuts with fitted peaks of 2D PLE data in (a), for $E_{exc}$=$E_{HH-FX}$ and $E_{exc}$= 3.60 eV. Schematic real space diagram and energy band diagram displaying absorption and emission processes in dislocations for (e) $E_{exc}\geq E_g$, and (f) $E_{exc}< E_g$.
• Figure 5: Thickness dependent band edge photoluminescence spectra fit with Gaussian functions. The noise floor is marked for the 15 nm sample spectrum which makes it difficult to conclusively identify any of the phonon replicas of the $(e, A)$ transition peak.