High degree of polarization of the near-band-edge photoluminescence in ZnO nanowires
© Jacopin et al; licensee Springer. 2011
Received: 13 April 2011
Accepted: 19 August 2011
Published: 19 August 2011
We investigated the polarization dependence of the near-band-edge photoluminescence in ZnO strain-free nanowires grown by vapor phase technique. The emission is polarized perpendicular to the nanowire axis with a large polarization ratio (as high as 0.84 at 4.2 K and 0.63 at 300 K). The observed polarization ratio is explained in terms of selection rules for excitonic transitions derived from the k·p theory for ZnO. The temperature dependence of the polarization ratio evidences a gradual activation of the X C excitonic transition.
PACS: 78.55.Cr, 77.22.Ej, 81.07.Gf.
Keywordszinc oxide nanowire photoluminescence polarization
One-dimensional nanoscale semiconductors have recently attracted considerable attention as promising candidates for innovative device applications. Their high surface to volume ratio can be exploited for the development of a new generation of chemical and biological sensors [1–3]. The wide bandgap (3.37 eV) of ZnO associated with its large exciton binding energy (60 meV) also makes it one of the most promising materials for photonic devices, such as light-emitting diodes  and lasers . Thanks to the spatial separation of photogenerated carriers, UV photodetectors with a very high photoconductive gain based on ZnO nanowires (NWs) have been demonstrated . It has been shown that the photodetection properties of ZnO NWs depend on the light polarization .
The photoluminescence of ZnO is typically composed of a near-band-edge (NBE) peak due to excitonic recombination and of a broad emission band in the visible range related to deep defect states [8–10]. The polarization properties of the luminescence of ZnO have been studied in bulk crystals [11–15]. However, these studies provided no theoretical explanation of the polarization behavior, especially of its temperature dependence. In the specific case of NWs, several studies have been carried out, but they were focused on the interpretation of the different behavior of defect and NBE luminescence [16, 17]. As shown in other semiconductor NW systems , the polarization dependence in NWs results from two competitive phenomena: bulk crystal symmetry (imposing polarization perpendicular to c-axis)  and dielectric contrast in thin NWs (privileging polarization parallel to the NW axis) [7, 20–23].
In this work, we have studied the polarization-resolved microphotoluminescence (μ-PL) of ZnO nanowires. We measured the polarization dependence of the NBE luminescence for temperature from 4.2 to 300 K. The experimental results are interpreted in the framework of the k·p model, allowing for the evaluation of the polarization ratio for each exciton type in bulk ZnO. The temperature dependence of the polarization ratio evidences a gradual activation of the X C excitonic transition.
ZnO NWs are prepared by means of vapor transport process, in which the source material is vaporized and transported by a gas carrier towards the substrates where it condenses . The experimental setup consists of a furnace capable to reach temperatures needed for oxide evaporation, a vacuum-sealed alumina tube connected to a vacuum pump, an automated valve, and a mass flow meter to control pressure and carrier flux. Adjusting the deposition conditions such as temperature of evaporation and carrier gas composition and flux, one-dimensional nanostructures can be obtained.
Platinum catalyst particles are firstly dispersed onto silicon substrates by DC magnetron sputtering at a working pressure of 5 × 10-3 mbar and 50 W applied power. The source material is positioned at the middle of the alumina tube and evaporated at a temperature of 1,370°C at a pressure of 100 mbar. The platinum catalyzed substrates are placed onto an alumina holder and positioned inside the tube in an area corresponding to a temperature T = 660°C. Furnace heating from room temperature to 1,370°C lasts 1.5 h. During furnace heating and cooling, a reverse Ar gas flow (from the substrates to the powder) is applied to avoid uncontrolled mass deposition under transient conditions. Once the desired temperature is reached, the deposition conditions are kept for 15 min, and afterwards, the furnace is cooled down to room temperature.
Results and discussions
For μ-PL studies, single NWs were detached by ultrasound bath from their substrates and dispersed in ethanol on Si substrates patterned with alignment marks. The surface density of NWs is controlled by dispersion in the range of 1 to 5 × 106 NWs/cm2, which is low enough to avoid simultaneous optical excitation of several wires with different orientations. The dispersed NWs do not show any bending and are free of strain. Polarization-resolved μ-PL experiments have been performed in the temperature interval of 4.2 to 300 K. The samples were cooled down in a continuous-flow liquid He cryostat and excited by means of a frequency-doubled continuous-wave Ar++ ion laser at 244 nm. The laser was focused on the substrate surface in a spot with a diameter of 3 μm by means of a UV microscope objective with 0.4 numerical aperture. The excitation power was set in the range of 10 to 50 μW. The sample was imaged through a UV-sensitive camera in order to visualize the luminescence spot and to locate the NW with respect to the alignment marks. μ-PL spectra were measured using a Jobin Yvon HR460 spectrometer (Horiba Ltd., Tokyo, Japan) with a 600- or 1,800-grooves/mm grating and a charge-coupled device camera. The energy resolution of the setup during these experiments is around 1 meV. In order to analyze the polarization of the single NW emission, a linear polarizer was placed at the entrance of the spectrometer. For each individual NW, a series of spectra was collected at different angles of the polarizer axis, which was varied over the whole interval 0° to 360° with a 15° step. The orientation of the NW with respect to the polarizer axis, as well as its isolation from other dispersed nanowires, has been assessed by SEM measurements performed after the optical characterization. The experiment was carried out on ten NWs, yielding a good reproducibility.
where I π and I σ are the integrals of the PL intensity for the π and σ polarizations, respectively.
Normalized interband squared momentum-matrix element
E// c-axis [ZnO value]
E ┴ c-axis [ZnO value]
at different temperatures, where I(E) p is the PL intensity at energy E in the p polarization. Figure 4 reports the P(E) for 70 and 150 K. (The temperature range is restricted within 70 to 150 K due to the extremely low signal above 3.38 eV at low temperatures and to the decrease of the overall luminescence intensity at high temperature). For the energy region between 3.28 and 3.38 eV, the signal arises from the X A and thermally activated X B excitonic transitions. Therefore, the polarization ratio remains high (>0.85) in this interval and is nearly temperature independent. At higher energy, around 3.41 eV, the polarization ratio decreases in correspondence of the emission of the X C exciton. It should be noted that in spite of the weak signal in the spectral range corresponding to the X C emission, the signal-to-noise ratio is about 20 at 3.40 eV. Therefore, the maximum possible error on the polarization ratio induced by the noise is less than 0.1. With increasing temperature, the dip in the P(E) dependence is amplified and progressively shifts towards lower energy. This behavior reflects the progressive thermal activation of the X C excitonic emission and the ZnO bandgap reduction described by the Varshni law .
In conclusion, we have studied the optical properties of ZnO nanowires grown by evaporation technique. Nanowires have defect-free single crystalline structure as shown by high-resolutions TEM (HRTEM) analysis. The nanowires are characterized by an intense photoluminescence with a spectral broadening below 2 meV. We have investigated the polarization dependence of the near-band-edge photoluminescence in ZnO strain-free nanowires. They exhibit a polarization ratio as high as 0.84. We show that these observations are consistent with the k·p theory and with the exciton selection rules. In particular, the weak dependence of the integrated polarization ratio P is a consequence of the large energy difference between X A and X C excitons. However, the analysis of the energy-resolved polarization ratio P(E) at different temperatures allows for the observation of the progressive activation of the X C exciton.
near band edge
scanning electron microscopy
transmission electron microscopy.
This work was supported by the French ANR agency under the programs ANR-08-NANO-031 BoNaFo and ANR-08-BLAN-0179 NanoPhotoNit.
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