Origin of the nonradiative decay of bound excitons in GaN nanowires

Christian Hauswald, Pierre Corfdir, Johannes K. Zettler, Vladimir M. Kaganer, Karl K. Sabelfeld, Sergio Fernández-Garrido, Timur Flissikowski, Vincent Consonni, Tobias Gotschke, Holger T. Grahn, Lutz Geelhaar, and Oliver Brandt

Phys. Rev. B 90, 165304 – Published 8 October 2014

Abstract

We investigate the origin of the fast recombination dynamics of bound and free excitons in GaN nanowire ensembles by temperature-dependent photoluminescence spectroscopy using both continuous-wave and pulsed excitation. The exciton recombination in the present GaN nanowires is dominated by a nonradiative channel between 10 and 300 K. Furthermore, bound and free excitons in GaN NWs are strongly coupled even at low temperatures resulting in a common lifetime of these states. By solving the rate equations for a coupled two-level system, we show that one cannot, in practice, distinguish whether the nonradiative decay occurs directly via the bound or indirectly via the free state. The nanowire surface and coalescence-induced dislocations appear to be the most obvious candidates for nonradiative defects, and we thus compare the exciton decay times measured for a variety of GaN nanowire ensembles with different surface-to-volume ratio and coalescence degrees. The data are found to exhibit no correlation with either of these parameters, i.e., the dominating nonradiative channel in the GaN nanowires under investigation is neither related to the nanowire surface, nor to coalescence-induced defects. Hence we conclude that nonradiative point defects are the origin of the fast recombination dynamics of excitons in GaN nanowires.

Received 8 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.165304

Authors & Affiliations

Christian Hauswald^*, Pierre Corfdir, Johannes K. Zettler, Vladimir M. Kaganer, Karl K. Sabelfeld^†, Sergio Fernández-Garrido, Timur Flissikowski, Vincent Consonni^‡, Tobias Gotschke^§, Holger T. Grahn, Lutz Geelhaar, and Oliver Brandt

Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5–7, 10117 Berlin, Germany

^*Author to whom correspondence should be addressed: hauswald@pdi-berlin.de
^†Permanent address: Institute of Computational Mathematics and Mathematical Geophysics, Russian Academy of Sciences, and Novosibirsk State University, Lavrentiev Prosp. 6, 630090 Novosibirsk, Russia.
^‡Present address: LMGP, Univ. Grenoble Alpes - CNRS, 38000 Grenoble, France.
^§Present address: OSRAM Opto Semiconductors GmbH, Leibnizstr. 4, 93055 Regensburg, Germany.

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Vol. 90, Iss. 16 — 15 October 2014

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Images

Figure 1
(a) Top-view scanning electron micrograph of the representative GaN NW ensemble (sample R). (b) Histogram representing the distribution of equivalent disk diameters $d_{disk}$ calculated from the NW top facet area $A$ . A fit with a shifted Gamma distribution to the data yields $〈 d_{disk} 〉 = 99$ nm. (c) Histogram showing the circularity $C$ of 265 randomly selected NW top facets. Employing a threshold for the circularity of $ζ_{A} = 0.762$ (vertical dashed line) in order to distinguish between uncoalesced NWs and coalesced aggregates, a coalescence degree of $σ_{c} = 0.89$ is obtained [28].
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Figure 2
Low-temperature (10 K) cw PL spectrum of sample R. The spectrum is dominated by the recombination of A excitons bound to neutral donors $[(D^{0}, X_{A})]$ at 3.471 eV. Transitions attributed to the recombination of B-excitons bound to neutral donors $[(D^{0}, X_{B})]$ at 3.475 eV and free A excitons $(X_{A})$ at 3.478 eV are also detected. The origin of the TES/ $(U, X)$ band at 3.45 eV is discussed in the text. The dashed boxes indicate a 3-meV wide spectral window centered around the $(D^{0}, X_{A})$ and $X_{A}$ transitions used to spectrally integrate the low-temperature TRPL transients depicted in the inset. The solid line in the inset represents a fit to the $(D^{0}, X_{A})$ transient using a single exponential decay convoluted with the system response function yielding an effective decay time of 0.16 ns within the first nanosecond. The dashed line is a guide to the eye and shows that the $X_{A}$ transient has the same effective lifetime as the $(D^{0}, X_{A})$ state within the error margin of the fit.
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Figure 3
(a) Temperature-dependent cw PL spectra of sample R (the spectra have been shifted vertically for clarity). With increasing temperature, the bound exciton states are progressively delocalized, until the spectrum is dominated by the recombination of free A excitons $(X_{A})$ at room temperature (300 K). The grey line is a guide to the eye and highlights the spectral shift of the $X_{A}$ transition with increasing temperature $T$ . (b) Evolution of the spectrally integrated PL intensity of the donor-bound and free exciton states between 10 and 300 K. The straight line shows that the integrated intensity decreases proportional to $T^{- 3 / 2}$ with increasing $T$ . (c) Effective $(τ_{eff})$ and radiative $(τ_{rad})$ lifetime obtained from temperature-dependent PL transients. The calculated values for $τ_{rad}$ increase proportional to $T^{3 / 2}$ , while $τ_{eff}$ remains almost constant between 10 and 300 K. (d) Dependence of $τ_{eff}$ on the excitation fluence at $T = 10 K$ .
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Figure 4
(a) and (b) Schematic energy diagrams visualizing Eqs. (2) and (3) for the two different cases “A” and “B,” respectively. The two states involved are denoted by $| n_{i} 〉$ , and the crystal ground state is represented by $| 0 〉$ . The red solid arrows symbolize the effective decay channel, which dictates the dynamics of the coupled system, while the green dotted arrows represent purely radiative transitions. (c) Simulated TRPL transients of the $(D^{0}, X_{A})$ and $X_{A}$ given by $γ_{i, r} n_{i}$ , respectively, employing the effective decay rates $γ_{i}$ as summarized in Table 1 and radiative decay rates $γ_{i, r}$ for the free and bound exciton of 0.1 and 1 ${ns}^{- 1}$ , respectively.
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Figure 5
Overview of the effective $(D^{0}, X_{A})$ lifetime obtained by fitting the short component of the respective TRPL transient recorded at low temperatures with a single-exponential decay for 19 different GaN NW ensembles. Solid symbols represent samples grown using the self-induced growth mode, while the open symbols are samples grown by SAG [26]. (a) Dependence of the effective lifetime $τ_{eff}$ on the parameter $〈 d^{*} 〉$ . The solid line depicts the expected lifetimes for an effective surface recombination velocity of $\tilde{S} = 6.3 \times 10^{3}$ cm/s [13]. The horizontal bars indicate the typical standard deviation of the $d^{*}$ distribution in the NW ensemble for the self-induced growth (upper bar) and SAG (lower bar). (b) Dependence of $τ_{eff}$ on the coalescence degree $σ_{C}$ of the sample.
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