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Abstract
Vertically aligned tellurium-nitrogen co-doped ZnO micro-/nano-rods with a hexagonal symmetry were fabricated via the chemical vapor transport method. The as-grown samples exhibited excellent crystalline. The incorporation of tellurium and nitrogen was confirmed by X-ray photoelectron spectra, and we found tellurium is beneficial to reinforce the nitrogen doping efficiency. Combining with Raman and variable temperature photoluminescence characterizations, we have demonstrated the suppression of zinc interstitial related shallow donor defects due to tellurium incorporation. Meanwhile, the intensity of the emission at 3.311 eV, which was ascribed to the radiative recombination of the free electron to the zinc vacancy related shallow acceptor states, was enhanced in the photoluminescence spectra of the sample with higher tellurium and nitrogen concentration. Our results show that tellurium-nitrogen co-doping might be a possible path for realizing reliable p-type one-dimensional ZnO nanostructures.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Zinc oxide (ZnO) with a direct wide band gap and large exciton binding energy has attracted substantial research interests, and exhibits a wide application in the various filed, such as photodetector [1], lasing [2], solar cell [3],and biological fluorescent imaging [4]. Recently, Zheng et al. [5] fabricated a heterojunction device based gallium doped ZnO single crystal, which realizes selective photovoltaic response to vacuum-ultraviolet light and increases the response speed and responsivity by one order of magnitude. However, the relatively high activation energy, low solubility of the acceptor, and the severe compensation from undesired donors, such as native point defect zinc interstitial (Zni) and oxygen vacancy (VO), make it difficult to realize stable p-type conductivity in ZnO. Among all the p-type doping elements, nitrogen (N) is widely considered the most promising candidate since it has a similar atomic radius and electronegativity with O. And many tangible evidences of N-doped p-type ZnO materials have been reported [6,7]. Meanwhile, the origin of shallow acceptor states in N-doped p-type ZnO is controversial because calculations concluded that N substituting on O site (NO) is actually a deep acceptor [8,9]. Then, many papers have a tendency to assign the unidentified shallow acceptor in ZnO to zinc vacancy (VZn) related complex, including VZn-NO [10] and VZn clusters [11].
On the other hand, the isovalent-acceptor co-doping strategy was exploited to improve p-type doping efficiency in ZnO. This co-doping concept actually arose from the case of N-doped zinc selenide (ZnSe), and it was reported that using tellurium (Te) as the isoelectronic element could increase the N solubility in ZnSe [12], which has been ascribed to the reduction in the NSe formation energy by the electrons transferring from Te 5p to NSe acceptor level on the growing surface [13]. In the case of ZnO, Te doping may cause the improvement of crystalline quality and suppression of formation of point defects by making a stoichiometric balance, since ZnTe is anion-rich while ZnO is cation-rich [14,15]. Moreover, it is expected that the more covalent nature of the Zn-Te bond may improve acceptor mobility as well as raise the valence band energy, resulting in the facile ionization of the acceptors [16].
Porter et al. had reported the enhancement of N incorporation into ZnO with Te addition during pulsed laser deposition process, and they obtained semi-insulating ZnO films with relatively low electron concentration compared to nominally un-doped and N-doped samples [14]. Via Te-N co-doping techniques, Park et al. [16] and Tang et al. [17] successively reported the realization of p-type conduction in ZnO films by molecular beam epitaxy and metal-organic chemical vapor deposition, respectively. And the co-doped Te atom could take O site which is the second-nearest neighbor to N, forming stable N-Zn-Te bond and reducing the formation energy of NO [17]. In addition, through a first-principle calculation, Yao et al. [18] found out that the N-related impurity band is broadened by Te co-doping, resulting in smaller effective-mass of free holes and shallower acceptor energy level, which has evidently explained the experimental findings. Recently, our group conducted an extensive investigation on the roles of Zni related donor defects by annealing of a batch of Te-N co-doping ZnO films, and we found that Te co-doping has a notable effect on suppressing the shallow donor defects [19].
ZnO nanostructures, specifically one-dimensional ZnO nanostructures, are the subject of increasing interest due to their potential applications in the field of short wavelength optoelectronics [20]. Lately, via the chemical vapor transport method, we have successfully synthesized N-doped and Te-doped ZnO micro-/nano-rods with excellent crystalline [21,22]. Then in this work, Te-N co-doped ZnO micro-/nano-rods were fabricated and characterized, and our results show that the Te-N co-doping might be a possible path for realizing reliable p-type one-dimensional ZnO nanostructures.
2. Experimental details
Te-N co-doped ZnO micro-/nano-rods were synthesized on a high-quality ZnO template via chemical vapor transport method in a horizontal tube furnace without any catalyst. The detailed growth process has been reported in our previous work [21,22]. In brief, the source material was a mixture of high-purity ZnO and graphite powders. N2 was used as the carrier gas, while nitrous oxide was employed as both N and O precursors. High purity commercial tellurium dioxide (TeO2) powders were added into the source material for the purpose of Te incorporation. Finally, we got three Te-N co-doped samples with the TeO2:ZnO:C weight ratio of 1:15:15, 1:10:10, and 1:5:5, denoted as sample A, B, and C, respectively.
The morphology of Te-N co-doped ZnO micro-/nano-rods were characterized by scanning electron microscopy (SEM, COXEM EM-30) operating at 15 kV. High-resolution X-ray diffraction (XRD), Philips X'pert Pro diffractometer equipped with a Cu-ray source, was used to investigate the crystallinity of the samples. Then, the compositional properties of the Te-N co-doped samples were analyzed by X-ray photoelectron spectrometry (XPS, Thermo Scientific K-Alpha) with an Al Kα x-ray monochromatic source at 1486.6 eV. And the binding energy was calibrated by the carbon (C) 1s peak at 285 eV. The vibrational properties of the samples were recorded by a JOBIN YVON HR800 Raman system in the backscattering geometry with 514 nm radiation at room temperature (RT). Temperature-dependent (TD) photoluminescence (PL) spectra excited by a He–Cd laser with a wavelength of 325 nm were recorded to analyze the samples’ optical characterizations.
3. Results and discussion
Via SEM characterization, we found the three samples have similar morphology. Figure 1 shows the SEM image from top view of sample A, and high-density Te-N co-doped ZnO micro-/nano-rods with hexagonal symmetry are vertically aligned on the template, similar to the results of our previous work [22]. Considering we didn’t use any catalyst during the growing process, we think the growth mechanism of ZnO micro-/nano-rods is not the traditional vapor-liquid-solid mechanism, but the so-called self-catalyzed vapor-liquid-solid mechanism as reported in literature [23]. The vaporized zinc via carbothermal reduction in the center of the furnace is carried by N2 to lower temperature regions downstream, where it deposits in the form of a liquid droplet on the surface of template. Then, the liquidized zinc reacts with oxygen and forms ZnO, which further serves as seeds for ZnO micro-/nano-rods growth, and the diameters of ZnO micro-/nano-rods are predecided by the different sizes of these liquid zinc droplets.
The high-quality ZnO homo-template grown on sapphire is beneficial for the orientation of Te-N co-doped ZnO micro-/nano-rods as shown in the SEM image, which is confirmed by XRD investigation. Figure 2 shows the XRD patterns of three samples, and we only see the intense ZnO (0002) diffraction peaks except the weak sapphire (0006) reflections coming from the substrate, indicating that the co-doped ZnO micro-/nano-rods are oriented the c-axis with a well-ordered wurtzite structure.
As we mentioned in the experimental details, high purity TeO2 powders were added into the mixture of ZnO and graphite powders for the purpose of Te incorporation. And according to a literature [24], the following reactions can be written for the doping process:
With increasing TeO2:ZnO weight ratio in the reaction source material, we got sample A, B, and C. In order to verify Te incorporation and investigate the chemical configuration of the elements, XPS was performed on the three as-grown samples, and the measurement results are shown in Fig. 3. The binding energy scale was calibrated by C 1s peak at 285 eV as reference.
Fig. 3 XPS lines of Te-N co-doped ZnO micro-/nano-rods. (a) Te 3d spectra. (b) O 1s spectra. (c) Zn LMM auger lines. (d) N 1s spectra. (e) Intensity ratio of Te 3d lines and Zn LMM auger lines in (a) and intensity ratio of Zni and lattice Zn in (c).
Figure 3(a) illustrates Te 3d spectra of the three samples. The peaks around 583 and 572 eV are Te 3d3/2 and 3d5/2 lines, respectively, which can be ascribed to the Zn-Te bond [25] and confirms Te incorporation in all three as-grown samples. Meanwhile we assign the peaks around 586 and 576 eV with similar peak intensity and width in all three samples to Zn LMM auger lines due to the use of Al Kα x-ray source in the XPS analysis system. In addition, with the XPS measurement data, the Te/O ratio of sample A, B, and C are 0.02, 0.06, and 0.15, respectively, indicating Te concentration in Te-N co-doped ZnO micro-/nano-rods increases with the increasing TeO2 content in the reaction source material.
Figure 3(b) shows the typical asymmetric O 1s peaks of Te-N co-doped ZnO micro-/nano-rods, which can be consistently fitted by three nearly Gaussian components, centered at 532, 531.3, and 529.8 eV, respectively, in all three samples. The main component can be attributed to the O-Zn bond, while the component on the high binding energy side is usually attributed to chemisorbed O species on the surface of ZnO material [26]. Moreover, according to literature [27], the component with the medium binding energy is associated with O2- ions in the O deficient regions within the ZnO lattice, and the intensity of this component may be connected with the variations of the concentration of intrinsic defect VO. Obviously, the intensity of this component increases with increasing Te concentration from sample A to C, suggesting that higher Te concentration could lead to more deep level defect VO in Te-N co-doped ZnO micro-/nano-rods. This phenomenon is probably due to the desorption of O-site Te atoms at high growth temperature, because it has been reported that Te atoms could be desorbed from Te-N co-doped ZnO films after annealing [17].
The Zn LMM auger lines of the three as-grown samples in the binding energy range of 490 and 505 eV are illustrated in Fig. 3(c). They have been fitted by two Gaussian components centered around 498 and 494.6 eV, which are attributed to zinc ions at lattice and interstitial sites, respectively [28]. Zni has been generally considered to act as a shallow donor, which could efficiently contribute electrons to the conduction band and be the source of n-type conductivity of ZnO material [29]. However, the isolated Zni is regarded as a fast diffuser with a low migration barrier, which is not stable at RT [30]. Furthermore, it was reported that an isolated Zni bonding with an extrinsic or native defect [31,32], or forming small Zni clusters [33] could be more stable in ZnO. Then, the intensity ratio of Zni related component and lattice Zn related component were calculated and shown in Fig. 3(e), which decreases with the increasing of Te concentration from sample A to C, suggesting Te incorporation could suppress Zni related shallow donors in Te-N co-doped ZnO micro-/nano-rods.
In addition, Fig. 3(d) shows the N 1s XPS spectra of the three as-grown samples. Probably due to low concentration, the N signal is not been observed in sample A and B. However, in sample C with the highest Te concentration, we can observer a clear N signal located around 399 eV, indicating that a certain amount of Te incorporation could increase N solubility in Te-N co-doped ZnO micro-/nano-rods. As we know, The bonding length of Zn-Te is larger than that of Zn-O, which will lead to a lattice expansion. As a result, the ZnO (0002) diffraction peak in Fig. 2 shifts to lower angle from sample A to B with increasing Te concentration. On the other hand, the bonding length of Zn-N is smaller than that of Zn-O, which will cause a lattice shrinking. Then, due to higher N concentration, the diffraction peak of sample C shifts to higher angle compared with sample B.
The vibrational properties of as-grown ZnO micro-/nano-rods were characterized by Raman analysis in back scattering geometry, which is very sensitive to the micro structural changes induced by intrinsic defects and impurities. Wurtzite ZnO has C46v symmetry with two formula units in the primitive cell, and the optical phonons at the Γ point of the Brillouin zone correspond to the irreducible representation: Γopt = A1 + E1 + 2E2 + 2B1, where E2 modes are nonpolar and Raman active only. The Raman spectra of the three Te-N co-doped samples recorded at RT are illustrated in Fig. 4, comparing with the result of N-doped sample in our previous work [21]. We observed the classical ZnO vibrational modes of low frequency E2 (E2(low)), 2E2(M), and high frequency E2 (E2(high)), located at 99, 332, and 438 cm−1, respectively. Furthermore, the additional modes located around 276, 510, 582, and 643 cm−1 are observed in both N-doped and three Te-N co-doped ZnO micro-/nano-rods.
Fig. 4 Raman spectra of N-doped and Te-N co-doped ZnO micro-/nano-rods recorded at RT. The inset is the intensity ratio of additional mode at 276 cm−1 and E2(high) mode at 438 cm−1 as a function of Te concentration in the samples.
These additional modes have been observed in Raman spectra of various N-doped ZnO materials. However, the origin of these additional modes is still in debate. They were ascribed to local vibration modes due to NO [34] or N-related complexes [35], disorder induced Raman scattering [36], and impurity activated silent modes [37]. Recently, Raman spectra of ZnO samples with different composition of isotropic Zn were investigated by Gluba et al. [33]. With logical inference and theoretical calculation, they proposed that the origin of the mode at 274 cm−1 is small Zni clusters, which has no contradiction up to now. Therefore, it is reasonable to say, that the intensity of the mode at 276 cm−1 in our case is connected to the concentration of Zni related shallow donors in ZnO micro-/nano-rods. As shown in the inset of Fig. 4, we calculated the intensity ratio of the additional mode at 276 cm−1 and E2(high) mode at 438 cm−1, and we found the values of these four samples decrease with increasing Te concentration from N-doped sample to Te-N co-doped sample C. This variation trend implies that Zni related shallow donors in Te-N co-doped ZnO micro-/nano-rods are suppressed due to Te incorporation, confirming the XPS investigation results.
For a long time, N is thought to be the most suitable doping element for p-type ZnO. However, via ab initio density functional calculations, Zhang et al. [38] reported that the formation energy of shallow donor defect Zni in ZnO decreases with N incorporation, which could be the reason that Zni related additional modes frequently emerge in the Raman spectra of ZnO after N doping. It is generally known that, except relatively high activation energy and low solubility of the acceptor, the severe compensation from undesired donors is one of the biggest obstacles for stable p-type conduction in ZnO. Therefore, the notable effect on suppressing Zni related shallow donors of Te incorporation in co-doped ZnO micro-/nano-rods is beneficial for realizing reliable p-type one-dimensional ZnO nanostructures. We ascribed the mechanism of this suppressing effect to the increased VI/II ratio due to additional TeO2 powders in the source material for the growth of Te-N co-doped ZnO micro-/nano-rods, because the O-rich atmosphere could significantly elevate the formation energy of Zni [39]. In the other hand, the lattice deformation of Te-N co-doped ZnO micro-/nano-rods should be smaller than that of N-doped sample since the bonding length of Zn-Te is larger than that of Zn-O, while the bonding length of Zn-N is smaller than that of Zn-O. As a result, small lattice deformation can reduce the formation of interstitial, including Zni related shallow donors.
Finally, the optical properties of Te-N co-doped ZnO micro-/nano-rods were characterized by PL. Figure 5(a) shows the PL spectra of three Te-N co-doped samples recorded at 13 K, comparing with the result of N-doped sample. And the exciton related emissions in the range of 3.35 and 3.38 eV are zoomed in Fig. 5(b) for clarity. The green band (GB) emissions of N-doped sample exhibit fine structures, consisting of doublet with a fixed splitting energy, which are the radiative transitions from the ground and exited states of the shallow donor Zni recombining with the deep acceptor isolated VZn [21]. We can observe 9 longitudinal optical (LO) phonon replicas, indicating a strong electron-phonon coupling. Meantime, the GB emissions of Te-N co-doped samples don’t have this kind of fine structures. And especially in sample C with the highest Te concentration, the GB emissions become much stronger comparing with the near band edge (NBE) emissions. Considering the XPS analysis result that the concentration of VO increases with increasing Te concentration, and according to literature [40], we think deep level defect VO is responsible for the GB emissions of Te-N co-doped ZnO micro-/nano-rods.
Fig. 5 (a) PL spectra of N-doped and Te-N co-doped ZnO micro-/nano-rods recorded at 13 K. (b) The exciton related emissions in the range of 3.35 and 3.38 eV.
At the lower energy side of the NBE emissions, the PL spectrum of N-doped sample is dominated by the donor-acceptor pair (DAP) recombination around 3.240 eV and its 3 LO phonon replicas. While at the higher energy side, as denoted in Fig. 5(a) and 5(b), we identified the emissions at 3.311, 3.359, 3.363, 3.368, and 3.377 eV to free electrons to acceptors (eA0), excitons bound to neutral acceptors (A0X), excitons bound to neutral donors (D0X), surface bound excitons (SX), and free excitons (FX), respectively [21]. We believe the shallow donor in N-doped ZnO micro-/nano-rods is Zni related complex, probably small Zni clusters [33]. On the other hand, since calculations concluded that NO is actually a deep acceptor [8], many papers have a tendency to assign the shallow acceptor in N-doped ZnO to VZn related complex, such as VZn-NO [10] or VZn clusters [11]. As we observed in Fig. 3(d), relatively lower Te concentration in Te-N co-doped sample A and B didn’t increase N concentration prominently. Hence, the NBE emissions of sample A and B are similar to that of N-doped sample. However, except the D0X emission at 3.363 eV almost vanishes due to the suppression of Zni related shallow donors, which has been confirmed by XPS and Raman characterizations, there are some other changes in the NBE emissions of Te-N co-doped sample C with the highest Te and N concentration. First, the emission at 3.372 eV becomes the strongest emission in the PL spectrum. Second, the emissions around 3.311 and 3.240 eV become relatively stronger compared with the others samples. Therefore, we recorded TD-PL spectra of sample C to better understand these variations and the optical properties of Te-N co-doped ZnO micro-/nano-rods.
The TD-PL spectra (13 to 120 K) of sample C in the range of 3.2 and 3.4 eV, and the energy positions of the NBE emissions as a function of temperature are depicted in Fig. 6(a) and 6(b), respectively. The strongest emission at 3.372 eV should be excitons related recombination, because the continuous redshift of this emission is owing to the band gap shrinkage with increasing temperature, which could be well fitted by the Varshni’s equation as plotted in Fig. 6(b). In addition, Fig. 6(c) shows the temperature dependence of the integrated intensity of the emission at 3.372 eV (normalized to the value at 13 K), which can be described by a phenomenological expression:
where I(13 K) is the integrated intensity at the lowest measuring temperature 13 K, C is a constant, and Eloc is the exciton localization energy. With the fitting curve in Fig. 6(c), the value of Eloc in our case is determined to be 9.8 meV. Therefore, we believe the emission at 3.372 eV in sample C is bound excitons (BX) recombination, as denoted in Fig. 6.Fig. 6 (a) TD-PL spectra of Te-N co-doped sample C in the range of 13 and 120 K. (b) The energy positions of the NBE emissions as a function of temperature. (c) The integrated intensity of the emission at 3.372 eV (normalized to the value at 13 K) as a function of temperature and its fitting curve.
The emission around 3.31 eV is a characteristic optical feature observed in a great variety of ZnO materials, with all types of assignments [41]. Via the TD-PL spectra of Te-N co-doped sample C, we found there is a 0.5kBT offset between the energy position of the emission at 3.311 eV (red solid line) and the excitons related emissions (green dash line) in Fig. 6(b), which can be used as an evidence to judge whether an emission is free electrons to shallow acceptors or not. Hence, we assigned the emission at 3.311 eV in the PL spectra of sample C to eA0, same as the situation in the other three samples. The enhancement of eA0 emission suggests that a certain amount of Te and N incorporation is beneficial for the formation of shallow acceptors in ZnO micro-/nano-rods. Meanwhile, the emission around 3.24 eV of sample C can be fitted by two components, centered at 3.253 and 3.238 eV, respectively, as shown in Fig. 6(a). Considering the energy spacing between 3.238 and 3.311 eV, and the same variation tendency of the energy position as a function of temperature, we ascribed the emission at 3.238 eV to the first LO phonon replica of eA0. In addition, the energy of the DAP recombination is represented as follow: EDAP(T) = EG(T) - (ED + EA) + e2/4πεr, where EG, ED, EA, e, ε, and r are band gap, donor binding energy, acceptor binding energy, elementary electric charge, dielectric constant, and the distance of DAP, respectively. It has been illustrated that the DAP with larger distance exhibit a higher probability for thermal dissociation, and the fractions of large distant DAP decrease with increasing temperature, while the energy of the DAP band gradually shifts to higher energies [42]. As plotted in Fig. 6(b), the energy of the emission at 3.253 eV gradually shifts blue until 60 K, which is the typical behavior of DAP recombination. Consequently, more shallow acceptors in sample C decrease the distance of DAP compared with the other three samples, which increases the energy of the DAP recombination. At last, due to the enhancement of shallow acceptors, we assigned the emission at 3.359 eV of sample C to A0X.
4. Conclusions
In summary, the growth of vertically aligned Te-N co-doped ZnO micro-/nano-rods was carried out via chemical vapor transport method. Te incorporation has a notable effect on suppressing Zni related shallow donors, which has been confirmed by XPS and Raman characterizations. And a certain amount Te concentration could increase N solubility in the co-doped ZnO micro-/nano-rods. In addition, the enhancement of eA0 emission and the blueshift of the energy of DAP recombination suggest that Te-N co-doping is beneficial for the formation of shallow acceptors.
Funding
National Natural Science Foundation of China (Nos. 61504057, 61574075, 61674077, and 61322403); Natural Science Foundation of Jiangsu Province (Nos. BK20150585 and BK20130013); the Fundamental Research Funds for the Central Universities (No. 2632018FY03).
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