Giant and Controllable Photoplasticity and Photoelasticity in Compound Semiconductors

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Giant and Controllable Photoplasticity and Photoelasticity in Compound Semiconductors

Jiahao Dong, Yifei Li, Yuying Zhou, Alan Schwartzman, Haowei Xu, Bilal Azhar, Joseph Bennett, Ju Li, and R. Jaramillo

Phys. Rev. Lett. 129, 065501 – Published 3 August 2022

See synopsis: Semiconductors in the Spotlight

Abstract

We show that the wide-band gap compound semiconductors ZnO, ZnS, and CdS feature large photoplastic and photoelastic effects that are mediated by point defects. We measure the mechanical properties of ceramics and single crystals using nanoindentation, and we find that elasticity and plasticity vary strongly with moderate illumination. For instance, the elastic stiffness of ZnO can increase by greater than 40% due to blue illumination of intensity $1.4 mW / {cm}^{2}$ . Above-band-gap illumination (e.g., uv light) has the strongest effect, and the relative effect of subband gap illumination varies between samples—a clear sign of defect-mediated processes. We show giant optomechanical effects can be tuned by materials processing, and that processing dependence can be understood within a framework of point defect equilibrium. The photoplastic effect can be understood by a long-established theory of charged dislocation motion. The photoelastic effect requires a new theoretical framework which we present using density functional theory to study the effect of point defect ionization on local lattice structure and elastic tensors. Our results update the longstanding but lesser-studied field of semiconductor optomechanics, and suggest interesting applications.

Received 5 November 2021
Accepted 26 June 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.065501

Physics Subject Headings (PhySH)

Defects Elasticity Mechanical & acoustical properties Photoinduced effect Plasticity

Condensed Matter, Materials & Applied Physics

synopsis

Semiconductors in the Spotlight

Published 3 August 2022

A new model suggests that lattice defects are responsible for the way some semiconductors become harder under illumination.

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Authors & Affiliations

Jiahao Dong^1,*, Yifei Li ^1,*, Yuying Zhou ^1,2, Alan Schwartzman ¹, Haowei Xu³, Bilal Azhar¹, Joseph Bennett ⁴, Ju Li ^1,3, and R. Jaramillo ^1,†

¹Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
²Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800 China
³Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁴Department of Chemistry and Biochemistry, University of Maryland Baltimore County, Baltimore, Maryland 21250, USA

^*These authors contributed equally to this work.
^†rjaramil@mit.edu

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Vol. 129, Iss. 6 — 5 August 2022

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Figure 1
Giant optomechanical effects in wide-band-gap II-VI compounds measured by nanoindentation. (a) Hardness and (b) Young’s modulus ( $E$ ) measured in the dark and under blue illumination by an LED with center wavelength 470 nm; irradiance at the sample surface is $1.4 mW / {cm}^{2}$ . The data shown include three successive cycles of darkness or illumination, and at each condition the measurement is repeated at 36 distinct locations, for a total of 108 separate measurements each for dark and illuminated conditions. The individual data are plotted directly; errorbarrs report average and standard deviation. Orange bars indicate the average change between dark and illuminated conditions. (c)–(e) Additional details provided for measurements on CdS. (c) Hardness during successive conditions of dark ( $D$ ) and blue illumination ( $B$ ). The data and error bars report the average and standard deviation of the 36 measurements performed at each condition; orange line traces the averages. (d) Dependence of $E$ on the color of illumination, from red to uv; result of measurements in the dark are also shown. The irradiance is different for each condition: red $(660 nm, 1.9 eV) = 3.3 mW / {cm}^{2}$ ; green $(530 nm, 2.3 eV) = 0.9 9 mW / {cm}^{2}$ ; blue $(470 nm, 2.6 eV) = 1.4 mW / {cm}^{2}$ ; violet $(420 nm, 3.0 eV) = 1.9 mW / {cm}^{2}$ ; uv $(365 nm, 3.4 eV) = 22 mW / {cm}^{2}$ . (e) Depth dependence of the photoelastic effect, showing dark conditions (gray) and blue illumination (blue); illumination conditions are as in (a)–(c). Error bars on individual points represent the standard deviation of repeated measurements, and the solid lines are guides to the eye. The orange vertical bars indicate the photoelastic effect.
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Figure 2
Controlling giant optomechanical effects through sample processing. (a) Hardness and (b) Young’s modulus ( $E$ ) measured in the dark and under illumination by an LED for samples processed with different annealing conditions. Illumination conditions are the same as in Fig. 1. Gray and blue symbols indicate individual measurements taken during three successive dark or illumination cycles, totaling 108 separate measurements for each condition. Gray and blue lines trace the average for each condition, and the orange bars highlight the change upon illumination. The samples are labeled “orig.” for the original, as-received samples, “ann. 1” for vacuum-annealed samples, “ann. 2” for sulfur-annealed samples, and “homo” for homoepitaxial ZnO.
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Figure 3
Theoretical results for the change in the crystal lattice and elastic properties upon sulfur vacancy double ionization in CdS and ZnS. (a)–(b) Visualizing the lattice changes in CdS through the sequence: $p r i s t i n e \to V_{S}^{\times} \to V_{S}^{• •}$ . Cd atoms shown in magenta, S shown in yellow; the missing sulfur atom and its coordination polyhedron are shown in red. (a) Change upon introducing a neutral sulfur vacancy. Black dashed line traces the ${Cd}_{4}$ tetrahedron in the pristine lattice, red polyhedron highlights the ${Cd}_{4}$ arrangement after introducing the vacancy and relaxing the lattice. (b) Change upon ionizing the sulfur vacancy. Black dashed line traces the ${Cd}_{4}$ tetrahedron in the lattice with neutral $V_{S}^{\times}$ , red polyhedron highlights the ${Cd}_{4}$ arrangement after ionizing and relaxing the lattice. (c)–(d) As in (a)–(b), but for the case of ZnS; Zn atoms are shown in gray. (e)–(f) $Γ$ -point phonon DOS presented as histograms; negative-frequency modes that drive lattice relaxation are indicated with red arrows. (e) Data for pristine CdS, and after creating a sulfur vacancy but before relaxing the lattice [indicated as excited state $(V_{S}^{\times})^{*}$ ]. (f) Data for a relaxed lattice with a neutral vacancy, and after ionizing the vacancy but before relaxing the lattice [indicated as excited state ${(V_{S}^{• •})}^{*}$ ]. (g) Change in Cd-Cd and Zn-Zn distances around a sulfur vacancy, relative to the pristine lattice. Distances contract around $V_{S}^{\times}$ and expand around $V_{S}^{• •}$ , as illustrated in (a)–(d). Error bars present the span of results calculated for varying [ $V_{S}$ ], up to 6.25% and 8.33% missing sulfur atoms for the case of CdS and ZnS, respectively. (h) Change in $c_{11}$ for neutral and doubly ionized vacancies in CdS, for a range of missing sulfur atoms fractions [ $V_{S}$ ]. The photoelastic response is the difference between the neutral (gray, representing dark conditions) and ionized (blue, representing illuminated conditions) data, colored in orange. The orange dashed line marks [ $V_{S}$ ] for the original CdS sample, determined by ICP-OES. (i) As in (h), but for the case of ZnS.
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