GaN nanorods grown on Si (111) substrates and exciton localization
© Park et al; licensee Springer. 2011
Received: 23 August 2010
Accepted: 12 January 2011
Published: 12 January 2011
We have investigated exciton localization in binary GaN nanorods using micro- and time-resolved photoluminescence measurements. The temperature dependence of the photoluminescence has been measured, and several phonon replicas have been observed at the lower energy side of the exciton bound to basal stacking faults (I 1). By analyzing the Huang-Rhys parameters as a function of temperature, deduced from the phonon replica intensities, we have found that the excitons are strongly localized in the lower energy tails. The lifetimes of the I 1 and I 2 transitions were measured to be < 100 ps due to enhanced surface recombination.
PACS: 78.47.+p, 78.55.-m, 78.55.Cr, 78.66.-w, 78.66.Fd
The wide band gap semiconductor, GaN, and its heterojunction systems with AlGaN, has been intensively investigated during the past decade and has shown to be a very useful material for developing light emitting diodes, laser diodes, and high-power and high-temperature electronic devices [1, 2]. It features a parabolic lowest conduction band with a band gap energy of approximately 3.4 eV. The separation between the conduction band and the nearest satellite valley is approximately 1.4 eV. Due to the properties of its constituents, it is also characterized by high-energy optical phonons (ħ ωLO ≈ 92 meV).
Carrier localization in III-nitride materials caused by compositional fluctuations in ternary alloys leads to tailing of the energy bands that is observed in both absorption and photoluminescence (PL) spectra. Furthermore, it has been claimed that this localization gives rise to the high quantum efficiency commonly found in III-nitrides by preventing the carriers from reaching the dislocation level (which acts as many non-radiative recombination centers) [3, 4]. These localizations are caused by alloy fluctuations in ternary semiconductors such as AlGaN [5, 6] and InGaN [7, 8] and in multi-quantum well structures such as AlGaN/GaN  and InGaN/GaN . In binary systems such as GaN nanorods, clear identification of exciton localization with an appropriate analysis has rarely been reported. Exciton localization features have, however, been identified on basal stacking faults (BSFs) in a-plane epitaxial laterally overgrown GaN both experimentally  and in numerical calculations .
In this paper, we report on exciton localization within binary semiconductor GaN nanorods that are grown directly on Si(111) substrates. Time-integrated micro-photoluminescence and time-resolved photoluminescence (TRPL) experiments were carried out in order to study the optical properties of the GaN nanorods. The Huang-Rhys (H-R) parameters were calculated from the phonon replica intensities in order to understand exciton localization due to potential fluctuations.
Sample preparation and characterization
A commercial micro-PL spectroscopy system (Renishaw Wotton-under-Edge, UK) was used for photoluminescence measurements. The excitation source was a He-Cd laser operating at 325 nm. This was focused to a spot size of approximately 0.8 μm2 (marked with the dotted circle in Figure 1b) on the sample by a 36× reflecting objective positioned above a continuous-flow helium cryostat which housed the sample. The same objective was used to both focus the incident beam and to collect the resulting luminescence, which was subsequently directed to a spectrometer with a spectral resolution of approximately 700 μeV and a spatial resolution of 0.8 μm. The signal was detected using a charge-coupled device detector. For TRPL measurements, a frequency-tripled pulsed Ti:Al2O3 laser (100 fs at 76 MHz) was used to excite the samples at a wavelength of 266 nm. A commercial time-correlated single-photon counting system was used for detection.
Results and discussion
In addition to these two peaks, there is a broad emission (full width at half maximum (FWHM), approximately 30 meV) at 3.417 eV, which has been labeled I 1 0. This has been attributed to both extended structural defects located at the bottom of the nanorods  and the recombination of excitons bound to the BSFs. The BSFs are produced during the initial phase of the growth and propagate perpendicularly to the c-axis from the substrate toward the surface of the sample [18, 19]. Indeed, densely merged GaN nanorods on the surface (as shown in Figure 1) may result in the formation of the localized defects.
Here, E LO is the LO phonon energy and V(q) is the matrix element for the interaction between the exciton and the phonon, with wave vector q. The H-R parameter is therefore a quantitative measure of the exciton-phonon coupling strength [20, 21] and, by extension, a measure of the degree of localization (localized excitons have a stronger interaction with phonons as their wavefunctions contain large q components ).
In summary, we have investigated the exciton localization in bulk-like GaN nanorods by micro- and time-resolved photoluminescence measurements. In the temperature-dependent photoluminescence measurements, several phonon replicas at the lower energy side of the exciton bound to the BSFs (I 1 0) are observed. By analyzing the H-R parameter as a function of temperature deduced from the phonon replica intensities, we found that the excitons are strongly localized in the lower energy tails. For the I 1 transition, the value of S 1/S 0 slightly decreases when the temperature increases from 4.2 to 30 K and then rapidly increases with further temperature increase, up to a value of 75 K. The PL decay times for the emissions at 3.468 and 3.417 eV were measured to be 92.2 and 68.1 ps, respectively. These fast decays are due to surface recombination, which is enhanced due to the large surface to volume ratio of the columns. It is finally concluded that exciton localizations in III-nitride materials can be observed not only in ternary alloys but also in binary semiconductors.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) (grant numbers K20901000002-09E0100-00210, 2010-0023856, and 2009-0076332). M. Holmes acknowledges funding from the Engineering and Physical Sciences Research Council (EPSRC) U.K. (GR/S82176/01).
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