Band-Gap Deformation Potential and Elasticity Limit of Semiconductor Free-Standing Nanorods Characterized in Situ by Scanning Electron Microscope–Cathodoluminescence Nanospectroscopy

Abstract

Modern field-effect transistors or laser diodes take advantages of band-edge structures engineered by large uniaxial strain εzz, available up to an elasticity limit at a rate of band-gap deformation potential azz (= dEg/dεzz). However, contrary to aP values under hydrostatic pressure, there is no quantitative consensus on azz values under uniaxial tensile, compressive, and bending stress. This makes band-edge engineering inefficient. Here we propose SEM-cathodoluminescence nanospectroscopy under in situ nanomanipulation (Nanoprobe-CL). An apex of a c-axis-oriented free-standing ZnO nanorod (NR) is deflected by point-loading of bending stress, where local uniaxial strain (εcc = r/R) and its gradient across a NR (dεcc/dr = R(-1)) are controlled by a NR local curvature (R(-1)). The NR elasticity limit is evaluated sequentially (εcc = 0.04) from SEM observation of a NR bending deformation cycle. An electron beam is focused on several spots crossing a bent NR, and at each spot the local Eg is evaluated from near-band-edge CL emission energy. Uniaxial acc (= dEg/dεcc) is evaluated at regulated surface depth, and the impact of R(-1) on observed acc is investigated. The acc converges with -1.7 eV to the R(-1) = 0 limit, whereas it quenches with increasing R(-1), which is attributed to free-exciton drift under transversal band-gap gradient. Surface-sensitive CL measurements suggest that a discrepancy from bulk acc = -4 eV may originate from strain relaxation at the side surface under uniaxial stress. The nanoprobe-CL technique reveals an Eg(εij) response to specific strain tensor εij (i, j = x, y, z) and strain-gradient effects on a minority carrier population, enabling simulations and strain-dependent measurements of nanodevices with various structures.

Non-linear optical deformation potentials in uniaxially strained ZnO microwires

“…Following the model presented by Watanabe et al,12 the change of the deformation potential for the used bending radii is about 1% at most and within the determined uncertainty. Additionally, the observed non-linearity is similar for different bending radii and microwires with diameter 1.5 lm <d < 7.3 lm.…”

“…3(a) and 3(b)), which was also observed in previous experiments 7 and by other groups. 9,12 For larger values of c , this linear relationship does not hold anymore. In this regime, we observe that the magnitude of the energy shift decreases with increasing strain and depends on the sign of c , i.e.…”

“…Thus, the determination of the deformation potentials and the emission characteristics of strained microand nanowires have been investigated. [6][7][8][9][10][11][12][13] For small applied strain values (jj < 1:5%), a linear shift of the exciton energy as a function of the applied strain was observed. However, the reported slope of this energy shift differs strongly in the literature.…”

Strain Gradient Modulated Exciton Evolution and Emission in ZnO Fibers

“…The evolution of the free excitons and bound excitons are quite different. First, the free excitons excited by the incident e-beam drifted towards and concentrated at the narrow bandgap within their lifetime in the presence of a large bandgap gradient3452. As a result, the recombination of drifted free excitons and free exciton interaction emission formed an asymmetric energy-intensity distribution along the cross-section and a nonlinear redshift of the total NBE band of the ZnO fiber.…”

Scanning cathodoluminescence microscopy: applications in semiconductor and metallic nanostructures

“…In addition to traditional optical measurements, the scanning near-field optical microscopy (SNOM) presents a higher spatial resolution 2 , but suffers from an unavoidable interaction between the scanning probe and the material surface. CL microscopy, on the other side, providing a label-free optical imaging technique with deep subwavelength resolution, has achieved significant interests for the mineralogy [3][4][5] , semiconductor physics [6][7][8] , and many other research fields like the tracking of cellular processes in biology 9,10 . CL was also found extensive applications as the emission source in cathode-ray-tube computer monitors and televisions for the industry.…”