(N-Doped) ZnO Nanostructures from a Dimethylformamide Aqueous

Jul 1, 2009 - by Zhang et al.30 Different positions of the blue-violet band have .... Sieber et al. ..... on Al-doped thin ZnO films, the optical gap ...
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J. Phys. Chem. C 2009, 113, 13643–13650

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Synthesis and Luminescence Properties of (N-Doped) ZnO Nanostructures from a Dimethylformamide Aqueous Solution Brigitte Sieber,*,† Hongqin Liu,‡,§ Gae¨lle Piret,‡ Jacky Laureyns,| Pascal Roussel,⊥ Bernard Gelloz,# Sabine Szunerits,‡ and Rabah Boukherroub*,‡ Laboratoire de Structure et Proprie´te´s de l’Etat Solide, UMR CNRS 8008, UniVersite´ Lille 1, Baˆtiment C6, 59655 VilleneuVe d’Ascq, France, Institut de Recherche Interdisciplinaire (IRI), USR CNRS 3078, Parc de la Haute Borne, 50 aVenue de Halley, BP 70478, 59658 VilleneuVe d’Ascq, France, Institut d’Electronique, de Microe´lectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Cite´ Scientifique, AVenue Poincare´, BP 60069, 59652 VilleneuVe d’Ascq, France, The Institute for Chemical Physics, School of Science, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China, Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), Baˆtiment C5 - UMR CNRS 8516, UniVersite´ Lille 1, 59655 VilleneuVe d’Ascq, France, UCCS, Equipe de Chimie du Solide, UMR CNRS 8181, ENSCL et UniVersite´ Lille 1, BP 90108, 59652 VilleneuVe d’Ascq, France, and DiVision of Electrical and Electronic Engineering, Faculty of Technology, Tokyo UniVersity of Agriculture and Technology, Koganei-shi, Tokyo 184 8588, Japan ReceiVed: April 16, 2009; ReVised Manuscript ReceiVed: June 10, 2009

The paper reports on the optical properties of ZnO nanostructures elaborated on a zinc foil substrate by a simple chemical approach. The doping type and density of the ZnO nanostructures were evaluated using electrochemical impedance spectroscopy. XRD diffraction patterns and Raman spectroscopy were used to study the structural properties evolution upon thermal annealing at 300 °C for 1 h in air. Their optical properties, probed by low temperature photoluminescence and room temperature cathodoluminescence (CL), are correlated to their electronic and structural properties. The luminescence of the nanorods is dominated by a broad near band edge emission located in the blue-violet region of the optical spectrum. Analysis of the CL spectra and monochromatic CL images show that the main luminescence has an extrinsic origin, which is tentatively assigned to nitrogen impurities. I. Introduction Zinc oxide (ZnO) is one of the most studied semiconductors due to its outstanding properties. It is a direct, wide bandgap semiconductor (Egap ) 3.36 eV) with a free exciton binding energy large enough (60 meV) to allow excitons to be stable at room temperature. ZnO is used in blue/UV optoelectronics,1 transparent electronics,2 piezoelectric transducers,3 photovoltaic applications,4 varistors,5 spintronic devices,6 and gas sensors.7 The preparation of ZnO nanostructures such as nanorods and nanowires has already been described in several reports. They have been elaborated by many different techniques such as reactive sputtering,8 thermal evaporation,9 spray pyrolysis,10 oxidation of Zn,11 pulsed laser deposition,12 chemical vapor transport and condensation,13 and metal organic chemical vapor deposition.14 Among these various techniques, wet chemical approaches have been increasingly used in the last years15-20 because they require neither sophisticated equipment nor vigorous experimental conditions. Thus, they became more widely used next to vapor-phase deposition techniques. Their optical properties have been explored by means of photoluminescence (PL),21-23 spatially and spectrally resolved cathodoluminescence (CL), * To whom correspondence should be addressed. E-mail: brigitte.sieber@ univ-lille1.fr (B.S.); [email protected] (R.B.). † UMR CNRS 8008. ‡ USR CNRS 3078 and UMR CNRS 8520. § Beijing Institute of Technology. | Baˆtiment C5 - UMR CNRS 8516. ⊥ UMR CNRS 8181. # Tokyo University of Agriculture and Technology.

which offers a higher spatial resolution,24-28 or by the combination of both PL and CL.29 In this paper, we report on the synthesis, electronic, morphological, and structural characterizations, and optical properties of ZnO nanostructures elaborated by a simple chemical approach. The synthesis was performed following a slightly modified approach of the simple and mild strategy for largearea fabrication of high-quality ZnO nanorod arrays proposed by Zhang et al.30 Different positions of the blue-violet band have been observed for different nanostructures.31 Because not all of them correspond to the expected excitonic transition, they are likely related to defects.31 In this study, we put a special focus on the analysis of the shape and the origin of the dominant blue-violet emission band. We show that the intense blue-violet band is not always related to an excitonic transition. The influence of postannealing at 300 °C in air has also been studied. II. Experimental Section A. Materials. Zinc foils (99.9%, 0.25 mm thick), dimethylformamide (DMF), carbonate propylene, and lithium perchlorate (LiClO4) were obtained from Aldrich and used without further purification. B. Preparation of ZnO Nanostructures. Zinc substrates (zinc foil cut into 1 × 1 cm2) were ultrasonically degreased in ethanol, propanol, and water before use. The clean zinc foil was immersed in a 5% dimethylformamide (DMF) aqueous solution and heated up to 95 °C (oil bath) for 24 h, washed with water, and finally dried in an oven at 130 °C for 1 h. Some of the samples were annealed afterward at 300 °C in air during 1 h.32

10.1021/jp903504w CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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C. Characterizations of the Nanostructured ZnO Substrates. Scanning Electron Microscopy (SEM). SEM images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter and a high efficiency In-lens SE detector. Raman Spectroscopy. Raman measurements were carried out at room temperature using a microspectrometer LABRAM Jobin-Yvon. The 1 µm spot diameter on the sample surface was produced by the 514.5 nm line of a 10 mW Ar+ ion laser. Raman spectra are recorded in backscattering geometry with the incident and scattered light (not polarized) propagating parallel to the c-axis. Electrochemical Impedance Spectroscopy (EIS). EIS experiments were performed using an Autolab potentiostat 30 (Eco Chemie, Utrecht, The Netherlands). The Zn/ZnO nanowires interface was sealed against the bottom of a single compartment electrochemical cell (V ) 5 mL) by means of a rubber O-ring (the electrical contact was made to a copper plate through the Zn). A platinum sheet and an AgCl-modified Ag wire were used as counter and reference electrodes, respectively. EIS was performed using the following parameters: amplitude of 20 mV; frequency range of 10 kHz-1 Hz, potential range: -0.8-0.8 V. The electrolyte was carbonate propylene/LiClO4 (0.1 M) to avoid ZnO decomposition.32,33 X-ray Diffraction. The XRD patterns were obtained from θ-θ scans in a Bruker D8 XRD operating at 50 kV and 30 mA with a Cu anticathode (λ ) 1.5418 Å). Photoluminescence. PL spectra were measured while the samples were in a cryostat under vacuum. The temperature was varied from 10 to 300 K. An optical multichannel analyzer (resolution: 1 nm) and the fourth harmonic line (266 nm) of a YAG laser (pulse duration: 12 ps; repetition rate: 10 Hz; power: 4 mW; spot diameter: 6 mm) were used for detection and excitation, respectively. Each spectrum was acquired during 10 s in order to average over many laser pulses. The area of the sample probed by the detector was about 200 µm in diameter. Cathodoluminescence. The CL experiments were performed at 300 K in a Hitachi 4700 FESEM equipped with a Gatan parabolic mirror. The accelerating voltage of the electron beam was 8 kV which corresponds to an electron penetration depth of 0.3 µm.34 The beam current is in the range 100-200 pA and the working distance is equal to 12.4 mm. This corresponds to a focused beam spot size close to 30-50 nm. The spectral resolution of the CL system is equal to 10 meV.

∆

Zn2+ + 2OH- f Zn(OH)2 98 ZnO + H2O

(2)

Zn2+ ions produced under these conditions are continuously released in the DMF aqueous solution, leading to zinc hydroxide Zn(OH)2 precipitation on the Zn surface. Working at an elevated temperature (95 °C in the present work) ensures that a homogeneous nucleation process in solution and Zn(OH)2 does not block the further dissolution of zinc and allows the conversion of the hydroxide into ZnO. Figure 1A shows a SEM image of a freshly grown ZnO sample. ZnO nanostructures appear as hexagonal prisms with a faceted hexagonal end face or faceted pyramids with a diameter ranging from 60-600 nm. Annealing the ZnO nanostructures up to 300 °C in air for 1 h does not induce any significant change in their morphology (Figure 1B). B. Determination of the Charge Carrier Concentration. The charge carrier concentration was determined using electrochemical impedance spectroscopy (EIS) in a carbonate propylene electrolyte (0.1 M LiClO4) to avoid ZnO decomposition as described recently.32 An equivalent circuit, Rs(RZnCZn)(RZnOCZnO), comprising the electrolyte resistance Rs in series with two parallel association of a resistance R with a capacitance C describes the electrochemical system under investigation. The RZnCZn couple presents the non-ZnO coated Zn interface, while RZnOCZnO rationalizes the electrode structure comprising ZnO nanostructures and non-ZnO coated Zn. Figure 2 shows the change of C-2 values versus to the interface applied potential. From the orientation of the slope of the Mott-Schottky plots the doping type of the ZnO nanostructures can be determined.

III. Results and Discussion A. Morphology of the ZnO Nanostructures. ZnO nanostructures investigated in this work were prepared by chemical oxidation of Zn foils in a 5% dimethylformamide (DMF) aqueous solution at 95 °C as reported recently.30,32 The oxidation of metallic Zn by naturally dissolved oxygen in water is slow due to the formation of a passive oxide layer. The presence of DMF in the aqueous solution accelerates significantly the oxidation process of metallic Zn. The underlying mechanism for the ZnO nanostructures formation can be rationalized in eqs 1-2.

DMF 1 Zn + O2 + H2O 98 Zn2+ + 2OH2

(1)

Figure 1. SEM images of as-grown ZnO nanostructured film before (A) and after postannealing at 300 °C for 1 h (B).

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Figure 2. Mott-Schottky plots of as-grown ZnO nanowires before (b) and after thermal postannealing for 1 h at 300 °C (9). Solution: LiClO4 (0.1 M)/carbonate propylene, ∆E ) 20 mV.

Figure 3. X-ray diffraction patterns of as prepared ZnO nanostructures (black line) and after postannealing in air at 300 °C for 1 h (gray line). *Peak due to Zn substrate.

The positive slope in Figure 2 is characteristic of an n-type semiconductor. Extrapolating the linear part of C2--E curves allows determining the flat band potential EFB, while from the slope the apparent donor density ND can be determined using eq 3.

1 2 ) (EZnO + EFB) 2 C qεZnOεoA2ND

(3)

where q is the electron charge (1.6 × 10-19 C), εZnO is the dielectric constant of ZnO (εZnO ) 10), εo is the permittivity of free space (8.85 × 10-14 F cm-1), A is the active surface, ESC is the potential difference across the ZnO space-charge region, and EFB is the flat band potential.32 A linear behavior observed at potentials more positive than ∼0.0 V and a flat band potential of EFB ∼ -0.13 ( 0.01 V/Ag/AgCl was determined for asgrown samples. Annealing at 300 °C for 1 h induces an anodically shift of the flat band potential and a EFB ∼ 0.17 ( 0.01 V/Ag/AgCl was determined. Assuming a total active surface area of A ) 1.00 cm2,32 a donor concentration of ND ) 3.90 ( 0.88 × 1019 cm-3 was calculated for as grown ZnO nanostructured interfaces, which decreased to ND ) 1.36 ( 0.1 × 1019 cm-3 after annealing the substrate at 300 °C for 1 h in air. C. Structural Properties: XRD Patterns and MicroRaman Spectra. XRD diffraction patterns (Figure 3) show that the nanostructures have a wurtzite structure and that they are

Figure 4. Raman spectra of freshly grown ZnO nanorods (black lines) and after postannealing in air at 300 °C for 1 h (gray lines): (a) full spectra between 250 and 750 cm-1 and (b) enlargement of the spectra between 420 and 690 cm-1.

mostly oriented along the c-axis. This indicates that this axis corresponds to the highest growth rate. Figure 4 shows the Raman spectra of the ZnO nanostructures before and after annealing at 300 °C. They consist of several bands and most of them correspond to Raman-active phonon modes of wurtzite ZnO with a C6V symmetry. The Raman active zone-center optical phonons predicted by the group theory are 2A1 + 2E2 + 2E1 + 2B1, where E1 and E2 are double degenerate modes.35 Phonon modes of A1 and E1 symmetry are both Raman and IR active, the E2 mode is only Raman active, and the B1 mode is silent (forbidden for both Raman and infrared excitations). Nonpolar phonon modes with symmetry E2 have two frequencies: E2 (high) associated with oxygen atoms and E2 (low) associated with Zn sublattice. The phonons of A1 and E1 symmetry are polar phonons and, hence, exhibit different frequencies for the transverse-optical (TO) and longitudinaloptical (LO) phonons. All these vibration modes have been reported in Raman spectra of bulk ZnO.35 In the backscattering configuration, only the E2 (high), E2 (low), and A1 (LO) modes are allowed, following Raman selection rules. The E2 (low) mode, located at about 100 cm-1, is out of range of our spectra, which start at 200 cm-1. In both specimens, the dominant Raman line is located at 437-438 cm-1 (Figure 4) and corresponds to the E2 (high) vibration mode. This is a second indication of the wurtzite structure of the nanorods with a main orientation along the c-axis, as previously observed by XRD. Because no

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significant variation of the position of E2 (high) mode has been observed, we can exclude the presence of strain in the nanorods. Figure 4 shows that many forbidden vibration modes are present in the spectra: the 333 cm-1 mode that corresponds to a second order (E2 (high) - E2 (low)),35 the 380 and 417 cm-1 peaks, which can be assigned to the A1 (TO) and the E1 (TO) modes, respectively. The observation of such unexpected peaks is the first evidence of the presence of a structural and/or doping induced disorder within the nanostructured ZnO substrate. The broadband visible in Figure 4 which ranges from 450 to 630 cm-1 examined more carefully can be divided into two parts, one from 450 to 520 cm-1 and the other one from 520 to 630 cm-1. First, the intensity of the shoulder between 520 and 630 cm-1 remains unchanged after annealing since it can be either increased or decreased. In fact, it seems to be dependent on the size of the nanorod from which the micro-Raman spectrum is issued, but we do not have enough results at the present time to give a definitive conclusion. Its shape changes from one place to another in such a way that it looks like being composed of different modes. A combination of the forbidden E1 (LO) mode at 580 cm-1 and the allowed A1 (LO) mode expected at 574 cm-1, respectively, could compose the high Raman shift side. These values of LO modes are indicative since they can be shifted when the crystal is disordered.36 The B1 (high) silent mode could also be involved in the shoulder since it is located close to the A1 (LO) and the E1 (LO) modes of ZnO.37 But, because it usually comes along with the B1 (low) silent mode at 275 cm-1, which is not observed in our spectra, it may not be present.37 The lower part of the broadband (Figure 4), which ranges from 450 to 520 cm-1 and whose intensity is very important in freshly grown specimens, is observed to decrease after postthermal annealing at 300 °C. Also its intensity fluctuates from place to place on each specimen. Its precise origin is not so easily determined since it is not composed of any apparent Raman peak. In N-doped ZnO samples grown by CVD on GaN substrate38 as well as in Ga-N codoped ZnO layers,39 an extra Raman peak has been observed at 510 cm-1 and related to the presence of nitrogen.38,39 But following the analysis by Manjon et al.37 and the results of ab initio calculations,35 this peak could be assigned to the 2B1 (low) second-order. The 2B2 (low) mode close to 520 cm-1 could also be present in few spectra in Figure 4.37 The activation of all the forbidden vibration modes in our samples as well as the existence of the broadband from 450 to 630 cm-1 should be due to the breakdown of the translational crystal symmetry by defects and impurities. Considering the high doping level of the specimens, a doping-induced disorder can be thus proposed. Its main effect is obviously the occurrence of a broad Raman band in our samples in the spectral range of 450-520 cm-1. The decrease of its intensity after annealing as well as that of the intensity of the forbidden modes A1 (TO) and E1 (TO) modes (Figure 4) could be related to the measured decrease of the doping level, and probably to the reduction of the doping induced disorder. This suggestion is confirmed by the observation of a broadened E2 (high) peak, whose line width decreases after annealing from 9.2 to 8.2 cm-1,40 but remains larger than the 6-7 cm-1 usually observed in undoped ZnO substrates.38 As proposed previously,32 the reduction of the disorder together with the reduction of the doping level after postannealing at 300 °C is most likely due to the out diffusion of shallow donors such as interstitial zinc atoms Zni and hydrogen

Sieber et al.

Figure 5. Typical CL spectrum recorded on freshly grown ZnO nanostructures. The spectrum has been obtained on a (10 × 10) µm2 area.

atoms. But Zni are fast diffusers and, thus, unstable in n-type ZnO.41 Hydrogen atoms have already been proposed to induce disorder;42 no change in our Raman spectra and impedance measurements has been observed when the annealing temperature is lower than 300 °C. Thus, hydrogen atoms are the most probable impurities responsible for the doping induced disorder and its reduction. Indeed, it was reported that for temperatures above 125 °C, all interstitial hydrogen atoms (Hi) diffuse out without the dissociation of hydrogen atoms on oxygen site (HO).43 The HO atoms start to evolve into Hi at around 225 °C and completely diffuse out at temperatures above 475 °C.43 Specifically, hydrogen atoms should be responsible for the broad Raman band observed between 450 and 520 cm-1. We also propose to associate the spatial variation of the intensity of the Raman band between 450 and 520 cm-1 to doping level fluctuations related to the strength of the doping induced disorder. D. Optical Properties of ZnO Nanostructures. The luminescence emitted by the specimens is mainly located in the UV part of the spectrum, as shown in Figure 5. In the following, we will focus on the origin of the UV band. Since its energy peak is located close to the stoichiometric band gap value expected at 3.36 eV at RT, it will be named NBE (near band edge). The NBE band shown in Figure 6 is rather broad: its average full width at half-maximum (fwhm) is 150 and 158 meV in freshly grown and postannealed samples, respectively. These values are much larger than the thermal broadening equal to 1.8kBT, that is, to 45 meV at 300 K,44 and that the homogeneous broadening of 40 meV expected at RT in high quality ZnO bulk samples.45 Several mechanisms can contribute to the broadening of the NBE band: (i) homogeneous and/or inhomogeneous strain that arises from stoichiometric fluctuations and thermal expansion differences between the substrate and the nanowires, (ii) disorder in the nanowires, (iii) band tailing, indirect band-to-band and band-to-impurity transitions. We have already seen (§ IIIB) that the presence of strain can be excluded (mechanism i). Disorder in the nanowires (mechanism ii) has been evidenced in freshly grown samples by Raman spectroscopy, which also showed that it was strongly reduced after annealing at 300 °C. On one hand the average broadening of the NBE CL band is nearly constant before and after annealing. On the other hand it is as large at 4 K than at 300 K (105 meV at 10 K and 150 meV at 300 K, as detected by PL; Figure 7). This implies that disorder is certainly important in the ZnO

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Figure 6. Two types of CL spectra recorded on ZnO nanostructures at 300 K: (a) fit of the spectrum with two Gaussian curves (2G-spectra); (b) fit of the spectrum with two Gaussian curves (3G-spectra); (b) experimental data.

nanorods before and after annealing, and that the broadening of the whole NBE peak could have another origin that the H-related disorder as previously proposed from Raman experiments. The third mechanism which could also explain the widening of the NBE band holds usually in highly doped semiconductors and therefore should be examined here more carefully. When the ionized donor density (in n-type semiconductor) is larger than the effective density of states neff of the conduction band (CB; degeneracy limit), excessive electrons fill the low states of the CB. The Fermi level being above the bottom of the CB, indirect transitions become possible because the k selection rules are violated as a result of an enhanced carrier scattering by the ionized impurities. At low temperature, the band broadens asymmetrically with a low energy tail following a functional dependence (E - Egap) and a steeper high energy tail, but at room temperature (RT) the band tends to a more Gaussian shape.46,47 This band-filling effect is known as the Burstein-Moss (BM) effect and leads to a blue shift of the energy position of the optical bands.48,49 A close examination of all the NBE CL bands such as that shown in Figure 6 shows that the spectral peak positions are always shifted toward low energies by about 150 meV from the stoichiometric band gap value expected at 3.36 eV at RT.50 Additionally, the shape of the NBE bands is not Gaussian at RT and their low energy tail does not follow the (E - Egap) dependence at low temperature. Therefore we can conclude that the broadening of the whole NBE band is

Figure 7. Temperature dependence of (a) PL spectra and (b) energy of the main PL peak.

not connected with the BM effect, implying that the electron concentration in the areas from which are issued the NBE bands is lower than that measured by impedance spectroscopy, that is, 3.9 1019 cm-3. This seems quite reasonable because it has already been shown that nonradiative recombinations are dominant at very high doping level.51 The depth underneath the nanostructures surface probed with the two techniques is very different. Impedance spectroscopy probes the surface of the nanostructures. In CL, the information depth is evaluated to 300 nm for an accelerating voltage of 8 kV.34 Also the monochromatic (and also polychromatic) CL images recorded on all the samples reveal a spatial heterogeneity of the luminescence intensity (Figure 8). The broad shape of the NBE band renders its origin difficult to determine, but it is worth trying because it seems to be a general feature of ZnO nanorods prepared at low temperature.52,53 The fact that the spectral peak position is red-shifted from the band-to-band (BB) transition expected in stoichiometric ZnO together with the absence of obvious BM effect allow us to propose that it is more related to a extrinsic transition than to an intrinsic BB one. Such a possibility is corroborated by the observation of spectra such as that shown in Figure 6, which exhibit a more symmetrical shape. This kind of NBE CL spectrum was only recorded in the freshly grown nanostructured

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Figure 8. SEM images of as-prepared ZnO nanostructures (a) secondaries electrons mode; (b and c) monochromatic CL images recorded at (b) 3.228 eV (spectrum peak), (c) 3.024 eV.

ZnO; it represents about half of the spectra. From the shape of the spectrum displayed in Figure 6, it is now obvious that the band corresponds to the convolution of several transitions. The spectra of the type 1 (Figure 6a) and of type 2 (Figure 6b) are easily convoluted by 2 and 3 Gaussian bands, respectively. In the following they will be named 2G- and 3G-spectra. The first two Gaussian bands with the lowest energies are labeled Band V-bands and that with the highest energy UV-band. 3G spectra were observed in CL spectra but not in PL spectra; this is likely due to the smaller volume probed in the CL experiments. The V-band is always the predominant band of the NBE emission. The fit of the spectra such as those in Figures 6 is also justified by the observation of featureless and broad NBE bands even at low temperature (Figure 8) by monochromatic CL images which can appear different when recorded with the B- and the V-bands (Figure 8) and also by a spatial dependence of the V-band to B-band intensity ratio. The results of the fits of the 300 K CL-spectra are given in Table 1. In the following we analyze the NBE band by first studying the UV-band in the freshly grown sample, thanks to its occurrence in the CL spectra. The UV-band ranges from 3.289 to 3.363 eV with a mean value at 3.337 eV. Because it is just between the values expected for an excitonic and a BB transition in stoichiometric ZnO (3.30 and 3.36 eV, respectively), it should correspond to an intrinsic transition. The excitonic transition is favored in low doped semiconductors, whereas the BB transition

Sieber et al. is favored in highly doped semiconductors. This shows that the determination of the UV-band origin is not so straightforward because it requires the knowledge of the local electron concentration. Thus, it is further discussed below by also taking into account the influence of the electron density on the optical band gap. When the carrier density in the conduction band exceeds the Mott critical density for the semiconductor-metal transition, a narrowing of the band gap (BGN) is induced by the band tailing resulting from the merging of the donor band with the conduction band, leading to a red shift of the optical bands. This occurs in parallel to the BM effect. Additionally, the carriers can be localized by the local variations of the ionized impurities density, resulting in a band tailing of both the valence and conduction bands, and thus an increased BGN effect. When the carrier density becomes even larger, electron-electron exchange and Coulomb interactions as well as the screened electron-ion interactions become significant, enhancing the BGN effect and thus the red shift of the optical bands. Because the BGN effect is expected to compete with the BM effect, the relation between the carrier density and the position of the NBE peak is not so straightforward to establish. The optical experiments performed have often lead to contradictory conclusions: by increasing the donor density in the degenerated range, a blue shift of the absorption edge and of the PL band was always observed, as a result of the BM effect. But this blue shift could be preceded or followed by a red shift in the range 8 × 1018 to 2 × 1020 cm-3. And a fix experimental value of the carrier concentration at which the blue shift changes to a red shift (or inverse) has not been found. Calculations have shown that the optical band gap is constant for electron concentrations lower than 2 × 1019 cm-3.52 Beyond that concentration, the blue shift should be preceded by a very slight red shift of a few meV. From optical absorption measurements on Al-doped thin ZnO films, the optical gap was found to increase with the electron concentration approximately as n2/3 for ne < 4.2 × 1019 cm-3,53 followed by a sudden decrease at 5.4 × 1019 cm-3 and an increase at about 1020 cm-3. Also, the value of the Mott density found in the literature varies from 3 × 1017 cm-3 to 5.5 × 1019 cm-3 and even to 7 × 1019 cm-3.51,53-56 The discrepancy being very large, we have to assume a reasonable value of the Mott density in the following. We take the value of 3 × 1017 cm-3 calculated by Klingshirn et al.57 When the electron concentration is above the Mott density value of 3 × 1017 cm-3, the excitonic transition should progressively be replaced by the BB transition. The value of the energy peak of the excitonic transition should remain unchanged since the decrease of its binding energy is compensated by the red shift of the band gap,57 whereas that of the BB transition most probably undergoes a red shift. When the electron density reaches a value of 5 × 1018 cm-3 that corresponds to the onset of a degenerate population,57 the BB transition should blue shift a little as a result of a small BM effect. Thus, a precise identification of the nature of the NBE band is not so straightforward. In the following we make the assumption that the UV band corresponds to the BB transition since a variation of its energy has been experimentally observed. The quenching of the free exciton recombination could also result from the presence of hydrogen as a shallow donor.18 In the case of a nondegenerate electron density, the energy of the transition being located at Eg+kT/2,44 the optical gap probed in our experiments should vary from 3.25 to 3.324 eV. This corresponds to a shrinkage of the gap by the BGN effect. Thus,

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TABLE 1: Average Values of the Energy Band Positions and their FWHM of the Gaussian Transitions that Compose the Two Kinds of NBE CL Spectra

freshly grown 300 °C annealed

B-band ECL (eV)

B-band fwhm (meV)

V-band ECL (eV)

V-band fwhm meV)

UV-band ECL (eV)

UV-band fwhm (meV)

ICLV-band/ ICLB-band

ICLV-band/ ICLUV-band

3.131 3.147

220 208

3.214 3.225

129 130

3.337

87

3.1 2.6

23

in our samples, the electron density should be larger than the Mott density (3 × 1017 cm-3) but smaller than 5 × 1018 cm-3. A spatial variation of the doping level could account for the observed fluctuation of the BB energy transition, in agreement with the conclusions drawn from the analysis of the Raman spectra. The V-band, in the 3G-spectra, is located at about 120 meV below the intrinsic UV-band: its energy peak ranges from 3.178 to 3.247 eV, with a mean value of 3.214 eV. Its intrinsic nature seems unlikely since it should give rise to a strong localization of carriers, a process that can be excluded. So the extrinsic nature of the V-band should now be considered. At room temperature, the donors are ionized then rendering more probable a free to bound transition than a donor-acceptor pair (DAP) one.18 Also, in a n-type material, the donor-hole type transition (D°h) is very improbable.58 In the case of the (eA°) transition with a peak at energy EeA, the energy level EA of the acceptor is given by EA ) Egap - EeA + kT/2. By assuming that the UV-band corresponds to a BB transition, one finds that EA varies from 108 to 155 meV (with a mean value of 128 meV) above the valence band maximum. The lowest value of the energy level EA (108 meV) corresponds to the lowest value of the BB transition. The energy level EA is of the order of that usually attributed to nitrogen on a substitutional oxygen site (NO) in the ZnO lattice,59,60 even if it is a little smaller. A different location of the hydrogen atoms around the nitrogen atoms61 or a larger doping level in our samples could explain this result. The close position of the V-band energy peak in 2G and 3G spectra allows us to suggest that the origin of the V-band in all the spectra and samples is related to nitrogen. This is corroborated by the enhancement of the V-band intensity observed after exposure to a NH3 plasma treatment. The formation of NO and related complexes is very likely in ZnO as shown by first-principles calculations.62 Furthermore, SIMS analysis of the nanostructured ZnO substrate indicates the presence of hydrogen and nitrogen. Concerning the B-band, it seems reasonable to assume that it is of the donor-acceptor pair (DAP) type since it is located about 80 meV below the V-band. The energy of a DAP transition is given by EDA ) Eg - ED - EA + e2/4πεr, where ε is the dielectric constant and r is the pair separation. The average Coulomb energy ∆E ) e2/4πεr can be very roughly estimated by assuming ) (3/4πNA)1/3.63 Applied to two V-bands detected in the 3G-spectra, we find that nitrogen could be also involved in the B-band: for instance, an EDA energy of 3.09 eV, corresponding to that of the V-band, is found for Ed ) 0.05 eV, Ea ) 0.11 eV, Na ) 1014-1015 cm-3, or Ed ) 0.1 eV, Ea ) 0.11 eV, Na ) 5 × 108 cm-3. For EDA ) 3.14 eV, one finds Ed ) 0.035 eV, Ea ) 0.155 eV, Na ) 1015-1016 cm-3, or Ed ) 0.05 eV, Ea ) 0.155 eV, Na ) 5 × 1017 cm-3. So, the donor could be either hydrogen located at 35 meV below the conduction band and/or a deeper donor like interstitial zinc atoms (Zni) for instance. It is also important to note that the B-band seems to be linked with the V-band in the sense that they always occur together.

Annealing of the freshly grown sample in air at 300 °C leads to a slight blue shift (11-13 meV) of the two bands (see Table 1). This can be easily explained if we remember that the electron density n is in a range (3 × 1017-5 × 1018 cm-3) such that the band gap decreases with n. Then, the blue shift of all the transitions could result from the decrease of n as it has been detected by impedance spectroscopy. IV. Conclusion ZnO nanostructures were synthesized by a chemical dissolution of a zinc foil in a 5% DMF aqueous solution at 95 °C. The as-grown nanorods crystallize in the wurtzite structure with the c-axis as the main growth axis. No modification of the morphology could be evidenced after postannealing of the nanostructures in air at 300 °C for 1 h. However, disorder in the as-grown nanorods could be observed from the Raman and luminescence spectra. Its reduction after annealing at 300 °C suggests that hydrogen atoms were involved in the disorder. Luminescence spectra exhibit a major near band edge (NBE) emission band tentatively assigned to nitrogen atoms in substitutional oxygen sites. Analysis of the CL spectra and monochromatic CL images show that the main luminescence has an extrinsic origin. Acknowledgment. Claude Vanmansart (LSPES) is greatly acknowledged for his participation to the CL experiments. H.L. thanks the Chinese government for the China Scholarship Council Award. References and Notes (1) Kim, H. S.; Lugo, F.; Pearton, S. J.; Norton, D. P.; YWang, Y.-L.; Ren, F. Appl. Phys. Lett. 2008, 92, 112108. (2) Song, D. Y.; Aberle, A. G.; Xia, J. Appl. Surf. Sci. 2002, 195, 291. (3) Wang, X.; Zhang, J.; Zhu, Z.; Zhu, J. Appl. Surf. Sci. 2007, 253, 3168–3173. (4) Zhao, Q.; Yu, M.; Xie, T.; Peng, L.; Wang, P.; Wang, D. Nanotechnology 2008, 19, 245706. (5) Subasri, R.; Asha, M.; Hembram, K.; Rao, G. V. N.; Rao, T. N. Mater. Chem. Phys. 2009, 115, 677–684. (6) Cheng, C.; Xu, G.; Zhang, H.; Luo, Y. Mater. Lett. 2008, 62, 1617– 1620. (7) Son, J. Y.; Lim, S. J.; Cho, J. H.; Seong, W. K.; Kim, H. Appl. Phys. Lett. 2008, 93, 053109. (8) Tu¨zemen, S.; Gu¨r, E.; Dogan, S. J. Phys. D: Appl. Phys. 2008, 41, 135309. (9) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (10) Bacaksiz, E.; Yılmaz, S.; Parlak, M.; Varilci, A.; Altunbas, M. J. Alloys Compd. 2009, 478, 367–370. (11) Chao, L.-C.; Liau, C.-C.; Lin, S.-J.; Lee, J.-W. J. Vac. Sci. Technol., B 2008, 26, 2601. (12) Premkumar, T.; Manoravi, P.; Panigrahi, B. K.; Baskar, K. Appl. Surf. Sci. 2009, 255, 6819–6822. (13) Reiser, A.; Raeesi, V.; Prinz, G. M.; Schirra, M.; Feneberg, M.; Ro¨der, U.; Sauer, R.; Thonke, K. Microelectron. J. 2009, 40, 306–308. (14) Rosina, M.; Ferret, P.; Jouneau, P.-H.; Robin, I.-C.; Levy, F.; Feuillet, G.; Lafossas, M. Microelectron. J. 2009, 40, 242–245. (15) Zhang, Z.; Yu, H.; Shao, X.; Han, M. Chem.sEur. J. 2005, 11, 3149. (16) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. Nano Lett. 2005, 5, 1231. (17) Ahsanulhaq, Q.; Umar, A.; Hahn, Y. B. Nanotechnology 2007, 18, 115603.

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