Band-gap deformation potential and elasticity limit of semiconductor free-standing nanorods characterized in situ by scanning electron microscope-cathodoluminescence nanospectroscopy

Modern field-effect transistors or laser diodes take advantages of band-edge structures engineered by large uniaxial strain ε_zz, available up to an elasticity limit at a rate of band-gap deformation potential a_zz (= dE_g/dε_zz). However, contrary to a_P values under hydrostatic pressure, there is no quantitative consensus on a_zz 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 (ε_cc = r/R) and its gradient across a NR (dε_cc/dr = R^-1) are controlled by a NR local curvature (R^-1). The NR elasticity limit is evaluated sequentially (ε_cc = 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 E_g is evaluated from near-band-edge CL emission energy. Uniaxial a_cc (= dE_g/dε_cc) is evaluated at regulated surface depth, and the impact of R^-1 on observed a_cc is investigated. The a_cc 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 a_cc = -4 eV may originate from strain relaxation at the side surface under uniaxial stress. The nanoprobe-CL technique reveals an E_g(ε_ij) response to specific strain tensor ε_ij (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.