(Color online) AFM images (a) and TOF-SIMS Mg profiles normalized to 1 (b) of simultaneously grown homo- and heteroepitaxial SQWs. The equally long arrows in (b) illustrate the larger width of the top barrier compared to the first barrier for heteroepitaxial samples.
(Color online) Comparison of (a) symmetric 002 and (b) asymmetric 101 HRXRD ω-rocking-curves of homo- and heteroepitaxial samples, (c) measured (upper curves) and simulated (lower curves) 2θ – ω-scans of two heteroepitaxial SQW structures.
PL spectra measured at T = 4.2 K for SQWs of various well widths heteroepitaxially grown at TSubst = 460 °C (series I) and at TSubst = 540 °C (series II), as well as homoepitaxially grown at TSubst = 540 °C (series III). The SQW emission is marked by arrows. The maximum of the Zn1 − xMgxO emission was used to determine the Mg concentration x according to Eq. (1). In contrast to homoepitaxial SQWs, emission lines related to extended defects (DBX, D) are observed for heteroepitaxial samples.
(Color online) Temperature-dependent PL decay times for the homo- (full circles) and heteroepitaxial SQW (full squares) with a well width of 2.0 nm. The corresponding normalized PL intensities are shown in the inset by the open symbols.
(Color online) Temperature-dependent PL spectra of a heteroepitaxial (a) and a homoepitaxial SQW (b) with well width dW = 2.0 nm. The SQW emission consists of a contribution from the recombination of LX and from FX, whose peak variation for increasing temperature is represented by the dashed lines (guide for the eye). In the homoepitaxial case, the LX peak prevails up to room temperature. For the heteroepitaxial sample, the D-band disappears for T > 60 K.
(Color online) (a) FX transition energy of homoepitaxial SQWs at 4.2 K (full circles). The experimental values are compared to simulations performed using nextnano3 without internal electric field and with a constant exciton binding energy (continuous line), for an internal field of 40 kV/cm (dotted line) and 150 kV/cm (short-dashed line) including a well width dependent correction of the exciton binding energy as well as for a field of 150 kV/cm with a residual charge carrier concentration of n = 5 · 1018 cm−3 (long-dashed line). (b) Determination of the activation energies E 1 and E 2 for two homoepitaxial SQWs with well widths dW of 2.0 nm and 6.5 nm by fitting the integrated PL intensity with Eq. (2). (c) Exciton binding energy calculated according to Ref. 52 (line style as in (a)) and plotted alongside the activation energies E 2. The continuous line marks the exciton binding energy in bulk ZnO.
Activation energies E 1 and E 2 obtained by fitting the temperature dependence of the integrated PL intensity of simultaneously grown homo- and heteroepitaxial SQWs with Eq. (2).
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