Localized versus delocalized states : Photoluminescence from electrochemically synthesized ZnO nanowires

from electrochemically synthesized ZnO nanowires T. Voss, C. Bekeny, J. Gutowski, R. Tena-Zaera, J. Elias, C. Lévy-Clément, I. Mora-Seró, and J. Bisquert Institute of Solid State Physics, University of Bremen, P.O. Box 330 440, D-28334 Bremen, Germany Institut de Chimie et Matériaux de Paris-Est (ICMPE), CNRS, UMR 7182, Bât. F, 2-8 rue Henri Dunant, 94320 Thiais, France Departament de Física, Universitat Jaume I, Av. de Vicent Sos Baynat s/n, 12071 Castelló de la Plana, Spain


I. INTRODUCTION
2][3] In general, the nanowires can be characterized as ZnO single crystals with a large aspect ratio, i.e., their length in c-axis direction is significantly exceeding all other lateral dimensions, with the latter ones being in the 10 to some hundred nanometer range.This implies that nanowires have a significantly increased surfaceto-volume ratio when compared to standard bulk crystals, resulting in considerably strong surface effects 4 in nanowirebased optoelectronic devices.
For many of such devices, a large scale, low-cost fabrication of high-quality and single crystalline ZnO nanowires with a high degree of c-axis orientation is desirable.Such demands are met by low-temperature chemical synthesis methods.7][8][9] Because of the aqueous environment, the limited purity of the involved chemicals and the low growth temperatures ͑especially when compared to typical growth temperatures used for vapor-phase epitaxial methods͒ is not a priori clear how the nanowire material compares to that of nanowires synthesized by, e.g., high-temperature vapor-liquid-solid processes. 10However, a detailed knowledge of the microscopic processes governing the optical and electronic properties of chemically synthesized nanowires is mandatory to understand, tailor, and optimize any optoelectronic device based on such nanowires.
In this paper, we report on the identification of the processes that contribute to the near-band-edge emission ͑NBE͒ in electrochemically synthesized nanowires.We show that the relatively high donor concentration in the as-deposited nanowires leads to emission from delocalized donor-band states as opposed to localized bound-exciton states where the latter ones usually dominate the optical properties of ZnO with moderate donor densities at liquid-helium temperature.We show that annealing of the nanowires at moderate conditions reduces the donor density by about an order of magnitude and results in the observation of sharp emission bands associated with bound-exciton transitions after the annealing process.

II. EXPERIMENTAL
The ZnO nanowire arrays were electrochemically deposited on conducting ͑10 ⍀ / sq͒ glass/ SnO 2 : F from Solaronix, covered by a ZnO nanocrystalline buffer layer.This layer was fabricated by two different techniques: Spray pyrolysis and electrodeposition. 6The sizes of nanocrystals determine the diameters of the nanowires because they act as nucleation centers for the nanowire growth.Details about the electrodeposition of the ZnO nanowire arrays can be found in Refs.6 and 7. Briefly, the electrodeposition was performed at 80 °C under constant potential ͑Ϫ1 V versus saturated calomel electrode͒.The electrolyte was an aqueous solution of ZnCl 2 ͑Flucka purity Ͼ98%͒ and KCl ͑Flucka purity Ͼ99.5%͒ saturated with bubbling oxygen 10 min before and during the experiment.The zinc chloride and potassium chloride concentrations ͓͑ZnCl 2 ͔ and ͓KCl͔͒ were 5 ϫ 10 −4 M and 0.1M, respectively, for all samples.
For optical characterization, a microphotoluminescence ͑PL͒ setup was used with a frequency-doubled Ti-sapphire laser with a repetition rate of 82 MHz at 3.54 eV as excitation source ͑excitation fluence ϳ10 J / cm −2 ͒.The sample was placed in a temperature-variable cryostat cooled with liquid helium.The laser light was focused on the sample through a microscope objective ͑spot diameter on sample: 2 m͒.The PL from the nanowires was collected via the same objective, spectrally resolved by a spectrometer ͑resolution 0.06 nm͒ and detected by a liquid-nitrogen-cooled charge coupled device camera.
To determine the donor density in the nanowires, electrochemical impedance spectroscopy ͑EIS͒ measurements were performed in a three-electrode cell using a Pt wire as counter electrode and a standard Ag/AgCl wire ͑3M KCl͒ as the reference electrode.A carbonate propylene electrolyte ͑0.1M LiClO 4 ͒ was used to avoid ZnO nanowire decomposition.The EIS data were obtained using a potentiostat ͑Autolab PGSTAT-30͒ equipped with a frequency-response analyzer.Each measurement was done by applying a 20 mV ac sinusoidal signal with the frequency ranging from 500 kHz down to 5 mHz superimposed to the applied constant bias.The donor density determined by the EIS is assumed to be equal to the density of the activated donors at room temperature where the EIS measurements are performed.For a detailed description of the EIS technique in which we carefully account for the influence of the buffer layer, see Ref. 11.
For the annealing process, ZnO samples were placed in a specially designed sample holder in an oven ͑Nabertherm, model L3/11/B170͒ and heated up to the required temperature.The sample holder was equipped with a gas inlet enabling the annealing process to be carried out in a specific atmosphere.The heating rate of the sample was 6 °C / min.The sample was annealed at 500 °C for 1 h, taken out of the oven and being allowed to cool down naturally to room temperature.

III. RESULTS
In Fig. 1, the PL spectra of the NBE of an as-grown ZnO nanowire array at temperatures between T = 4 and 290 K are depicted.For this nanowire array, the density of activated donors at room temperature determined from EIS is N D =8 ϫ 10 19 cm −3 .The spectrum at T = 4 K shows two main broad emission bands: One centered at E = 3.328 eV with a linewidth of ⌬E = 23 meV ͑labeled D͒, and one at E = 3.366 eV with a linewidth of ⌬E = 18 meV ͑labeled DB͒.Both emission bands have asymmetric shapes with a smooth decrease toward lower energies.With increasing temperature, both the DB and D emission bands broaden and merge above T = 60 K while the whole NBE shifts toward lower energies, as expected for the NBE due to the temperature dependence of the band gap.Additional, significantly weaker emission bands at E ϳ 3.2 and E ϳ 3.1 eV can be observed at room temperature, which are most probably related to longitudinal optical phonon replica of the broad D and DB bands.
Figure 2 shows SEM micrographs of arrays of the ZnO nanowire samples with the ͑a͒ smallest and ͑b͒ largest diameters analyzed in this work.The variation in the diameter in the electrodeposition process was achieved by changing the grain size of the nanocrystals which act as seed crystals. 6his allows for the tailoring of the nanowire diameter while keeping all other parameters constant for the growth process of the nanowires.The mean value of the nanowire diameter was estimated from a statistical evaluation of the SEM micrographs. 9igure 3͑a͒ compares the low-temperature NBE spectra between 3.30 and 3.34 eV for three arrays of as-deposited ZnO nanowires with different mean diameters.The spectra are normalized to the maximum of the DB peak.The error bar on each spectrum indicates the variation of the intensity at various excitation positions on the ensemble, obtained due to dispersion in nanowire diameter in each sample. 9The average diameter d of the nanowires on each array is given in the figure.For smaller diameters, we observe an increasing contribution of the D emission band to the PL spectrum.Figure 3͑b͒ reveals an almost linear dependence of the emission intensity ratio ͑I D / I DB ͒ on the inverse diameter of the nanowire ͑1 / d͒.
Postgrowth annealing treatment of low-temperature grown nanowires under appropriate conditions can enormously improve their optical quality. 12In Fig. 4, the PL spectra of ZnO nanowires annealed at T = 500 °C for 60 min in air are depicted.The spectra are measured at temperatures between T = 7 and 140 K.After the annealing process, distinct and relatively sharp emission lines are detected at T = 7 K, contrasting the spectrum measured for the asdeposited ZnO nanowire array ͑compared to Fig. 1͒.From its temperature dependence and energy position, the emission shoulder at 3.376 eV is assigned to free-exciton transitions ͑FX͒. 2,13The main emission band D 0 X at 3.361 eV ͑linewidth ⌬E = 2 meV͒ is attributed to ͑localized͒ donor-bound exciton transitions. 2,13An additional emission band ͑B͒ is observed at 3.318 eV, its origin will be discussed in Sec.IV.The SD emission band at 3.331 eV results from recombination processes involving excitons bound to structural defects. 13The 1-LO-phonon replica of the FX ͑FX-1LO͒ and the B transitions ͑B-1LO͒ are visible at an energy separation of 72 meV to the lower energy side of their respective zerophonon lines. 2,10All bands shift to lower energies as the temperature increases due to the reduction of the band gap.The intensities of the D 0 X and B bands decrease while those of the FX and FX-1LO bands increase.The SD band is hardly visible above T = 40 K. Above T = 140 K, the FX-1LO emission band dominates the spectrum.

IV. DISCUSSION
The results presented in the previous section demonstrate that, without postgrowth annealing treatment, the donor concentration in electrochemically synthesized ZnO nanowires is usually relatively high, especially when compared to ZnO nanowires synthesized by high-temperature vapor-transport techniques for which carrier densities on the order of 10 17 cm −3 have been reported. 14For our samples, the value N D =8ϫ 10 19 cm −3 determined by EIS readily explains the broad linewidth of the DB emission in the PL spectrum of Fig. 1.In the presence of such high donor densities, a band of donor states is formed due to the overlap of the wave functions of neighboring donors. 12rocesses take place between this band and the valence band ͑band-to-band transitions͒ as opposed to transitions involving localized ͑bound-͒exciton states ͑in nominally undoped ZnO known as D 0 X transitions͒ which usually dominate the optical spectra of ZnO crystals at low temperatures. 2,12,13he Mott criterion 15 states that there exists a critical donor density N C beyond which the wave functions of the donors are so close to each other that they overlap and form a band of donor states.N C is calculated from the equation a B is the exciton Bohr radius ͑a B ZnO = 1.8 nm for the A and B excitons͒. 16Therefore, we find a critical donor density of N C = 2.4ϫ 10 18 cm −3 .For the electrochemically deposited nanowires, N D Ͼ N C , which supports the above given interpretation that the PL emission originates from delocalized donor-band states.
As can be seen in Fig. 3, the relative intensity of the D band ͑normalized to the DB intensity I DB ͒ scales with the inverse of the nanowire diameter according to I D / I DB ϰ 1 / d; the relative intensity increases by a factor of 3.5 when the diameter of the nanowire is reduced from 125 to 70 nm.We therefore assign the D peak to recombination processes involving surface states which increasingly contribute to the overall PL emission for smaller wire diameters as the surface-to-volume ratio increases.
The quality-improving influence of thermal annealing is demonstrated in Fig. 4 by the recovery of sharp excitonrelated transition lines.The donor density determined for the annealed samples is 4 ϫ 10 18 cm −3 at T = 300 K, a factor of 10 less than N D in the as-deposited nanowires.N D determined for the annealed samples is therefore of the same order as the above calculated critical donor density N C thus explaining the observation of exciton-related recombination process.The reduction in N D during the annealing process is due to the out-diffusion of the residual shallow donors, most probably Zn interstitials. 17,18Furthermore, the absence of the D emission band might be attributed to surface reconstruction in the annealed nanowires.
The B emission band observed at 3.318 eV shows a similar temperature dependence as the main D 0 X band.We carried out intensity dependent measurements ͑not shown here͒ in which both the B and the D 0 X bands showed a linear dependence on the excitation fluence.This suggests that both emission bands have a similar origin 19 ͑i.e., donor-bound ex-citons͒ and both bands are visible as long as the excitonic transitions determine the emission spectrum.Therefore, we conclude that the B emission band results from a donorbound exciton transitions with the donor involved being a deep one, most probably a Cl-related complex state.More investigations are in progress to identify its nature.
All in all, the near-band-edge PL from electrochemically deposited ZnO nanowires can be completely understood by considering the donor densities determined by EIS.The presence of a band of donor states and, thus, band-to-band transitions as opposed to transitions involving localized boundexciton states is a crucial point to consider when discussing the performance of nanowire-based solar cells, light-emitting diodes, and other optoelectronic devices.In particular, our results demonstrate that for as-grown ECD nanowires, excitonic effects should play no role for the PL emission processes due to the high concentration of activated donors.Furthermore, distinguishable and localized excitonic transitions, similar to those observed in ZnO nanowires and crystals grown by high-temperature vapor-transport processes, are clearly visible in the PL spectra after annealing the samples in air.This can be attributed to a significant reduction of the donor density by the still relatively mild annealing process.

V. CONCLUSIONS
In summary, we have shown that the broad NBE observed at 3.366 eV in electrochemically deposited ZnO nanowires results from band-to-band transitions.We have explained the absence of excitonic states and the formation of a band of donor states by considering the Mott criterion.Donor densities higher than 8 ϫ 10 19 cm −3 have been determined for the as-deposited ZnO nanowires at room temperature.Furthermore, a second emission band D scales with the diameter of the nanowires and is attributed to recombination processes involving surface states.Annealing the nanowires at 500 °C for 60 min in air results in a reduction of the donor density by about one order of magnitude and the observation of sharp emission bands associated with bound-exciton transitions.The results show a very good correlation of the electrical and optical properties of the electrochemically deposited ZnO nanowires.

FIG. 3 .
FIG. 3. ͑a͒: Low temperature PL spectra of as-deposited ZnO nanowire arrays with different average diameters.The error bar on each spectrum indicates the variation in the intensity at various excitation positions on the ensemble.͑b͒: Dependence of the intensity ratio of the D and DB bands ͑I D / I DB ͒ on the inverse of the nanowire diameter 1 / d.