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Abstract
N-doped p-type zinc oxide (ZnO) thin films were prepared by rapid thermal annealing (RTA) of nitrogen ion implanted high quality ZnO epitaxial layers. Annealing at 900 °C in a nitrogen atmosphere leads to the conversion of conductivity from n to p-type with a hole concentration of 9.60×1017 cm-3, which is reflected in photoluminescence spectra. To reveal the thermal activation and doping mechanism of this film, the samples were also analyzed by Raman scattering and X-ray photoelectron spectroscopy. The results indicate that elimination of the Zni related shallow donors and the formation of shallow acceptor complex No-VZn account for the stable p-type conductivity of N-doped ZnO. The shallow acceptor state is calculated at 0.161 eV above the valence band edge.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As II-VI compound semiconductor material, ZnO has attracted extensive attention in recent decades because of a wide direct band gap of about 3.3 eV at room temperature (RT) and a large exciton binding energy of about 60 meV. Due to its unique properties, ZnO has huge application prospects in short-wavelength optoelectronic devices such as blue/violet light-emitting diodes (LEDs), laser diodes (LDs), and ultraviolet detectors [1]. Meanwhile, as an alternative for ITO and FTO, ZnO also has important commercial applications such as in photovoltaics (PV) including solar cells, flat panel displays and touch-screen panels [2]. For instance, Al and Ga-doped ZnO with a textured surface structure as well as a high transmittance in the near infrared region are in practical use for transparent electrode applications in solar cells [3–5]. Additionally, the optical properties of Al-doped and H-doped ZnO in a very wide spectral ranges starting from ultraviolet all the way to terahertz (nm to mm) have also be studied by the researchers [6,7].
To realize the application of ZnO, the stable and reproductive n-type and p-type ZnO must be fabricated. Due to the doping asymmetry problem, high quality n-type ZnO can be achieved by doping Al, Ga, and other IIIA elements, but reliable p-type ZnO are difficult to prepare. Theoretically, the p-type ZnO can be achieved by replacing the Zn lattice with IA elements or replacing the oxygen (O) lattice with VA elements. Among these doping elements, nitrogen (N) is considered to be the most suitable p-type dopant since it has the similar atomic radius and electronegativity as O [8]. In recent years, multiple research groups have reported on the incorporation of N in ZnO, and several of them realized p-n and/or p-i-n homojunction violet LEDs based on the p-type N-doped ZnO thin films [9–14]; however, the reports on p-type conductivity in N-doped ZnO still remain controversial [15–20]. Experiments showed that N-doping has the feature of a shallow acceptor, with ionization energy in the range of 100-209 meV [15–17], which was speculated to be the contribution from substituting nitrogen at oxygen site (NO). However, first-principle calculations suggested that the isolated NO is actually an acceptor as deep as 1.3 eV above the valence band maximum (VBM) [18,19], significantly larger than the experimental values. Such disagreement implies that the p-type conductivity in the N-doped ZnO is probably not from the isolated No. N interacts with native defects, creating defect-impurity centers provide shallow acceptor levels also maybe account for the observed p-type conductivity in experiments. In 2012, Liu et al. theoretically proposed that the p-type conductivity of N-doped ZnO may originate from the shallow acceptor defect complex composed of substitutional NO and zinc vacancy (VZn-NO), which is thought to evolve from the metastable NZn-VO double donor [20]. However, the evolvement faces a barrier of >1.6 eV to overcome. Experimentally, Reynolds et al. deem that the hole carriers of N-doped ZnO originate from the contribution of VZn-No-H+, and its ionization energy is approximately 130 meV [21]. Subsequently, Amini et al. further showed that NO-VZn is a valid acceptor because Hi in NO-VZn-H+ will occupy a shallower hole level, leaving only two empty deep acceptors states [22]. Therefore, the mechanism for the p-type N-doped ZnO thin films is still complicated and confusing. If VZn-NO complex does provide shallow acceptor levels in ZnO, in order to achieve an acceptor conductivity in ZnO it is important not only to effectively introduce N, but also to use such growth conditions and other techniques to ensure a high level of VZn states and an appropriate interaction between NO and VZn to form as VZn-NO complex centers. However, there are a variety of donor defects in N-doped ZnO that will compensate for effective acceptors. In N-doped ZnO materials, Zn interstitial (Zni), H interstitial (Hi) [23], N2 molecules on O sites [(N2)O] [24] and Zni-NO donor complexes [25] are common donor defects, but it is gratifying that these donor defects might be eliminated and suppressed through annealing. In summary, the realization of p-type conductivity of N-doped ZnO requires not only the formation of an effective shallow acceptor, but also the suppression of donor compensation as much as possible.
In the present work, we realized acceptor doping of ZnO thin films by rapid thermal annealing (RTA) of nitrogen ion implanted high quality ZnO epitaxial layers. We investigated the properties of N-doped ZnO films by Hall, Raman, X-ray photoelectron, and low temperature photoluminescence (PL), looking for the acceptor-related states and their thermal activation mechanism upon thermal annealing. Our experimental results suggest that the elimination the Zni related shallow donors and meanwhile the formation of shallow acceptor complex VZn-NO upon appropriate thermal annealing condition will causes the p-type conductivity in ZnO.
2. Experimental details
High quality ZnO epitaxial layers grown on c-sapphire substrates using pulsed laser deposition (PLD) from Nanovation were used in this study. Details about the growth technique are available elsewhere [26]. The as-deposited ZnO thin films are (002) preferred orientation with the full width at half maximum (FWHM) of about 0.1 degrees. The thickness of the layers is about 100 nm by SEM measurement, as show in Fig. 1. Before the ion implantation, the ZnO thin films were implanted with 30 keV N+ ions at a dose of 5×1016 cm−2 under normal incidence. The ion beam was raster scanned over the surface to produce a laterally homogeneous implantation. Subsequently, the ZnO layer was cut into pieces with dimensions of 5 mm×5 mm×1 mm. After ion implantation, the specimen underwent rapid thermal annealing (RTA) for 3 minutes in a nitrogen atmosphere at a temperature of 800-950 °C to repair lattice damage and activate the dopant. The surface roughness of all sample was tested by AFM, as shown in Fig. 2, and all samples have an average roughness of less than 5 nm.
Subsequently, the Hall effect measurement system (RH2035, Phys Tech) was used to investigated the electrical properties and indium electrode was used to form a good ohmic contact. Chemical bonding state of samples were characterized by X-ray photoelectron spectroscopy (XPS) and all binding energies were calibrated by the C 1s peak at 284.6 eV. Raman scattering (Raman) test was performed using LabRAM HR Evolution and a 532 nm Ar+ laser as the excitation source, all tests were completed in the form of backscatter at room temperature. Secondary ion mass spectrometry (SIMS) was used to study the depth distribution of N impurities. The ZnO film was etched using Cs ions (1 keV energy) sputtering. PL measurements were performed on a FLS920 fluorescence spectrometer (Edinburgh Instruments), using the 325 nm line of a He-Cd laser as the excitation light. For the low temperature (5 K) PL test, place the sample in a low temperature chamber and use a closed loop liquid helium refrigeration system to lower the sample temperature.
3. Results and discussion
The distribution of the implanted nitrogen ions simulated by using the stopping and range of ions in matter (SRIM) code is shown in Fig. 3(a). Moreover, the distribution of N impurity in ZnO layer was measured by SIMS, as shown in Fig. 3(b). It can be seen that N ions were successfully implanted into the ZnO thin films. In the process of ZnO implanted with nitrogen ions simulated by the SRIM code, the corresponding Zn and O vacancy depth profiles are also shown in Fig. 3(a). That means the implantation of high energy of N ions can induce quite a bit of vacancy defects in ZnO matrix, especially the VZn is desired for realization of acceptor doping in ZnO:N films according to theoretical calculations [20,22]. At the same time, the generation of oxygen vacancy (VO) also contributes to N ions form as NO upon a certain condition. Therefore, an appropriate post-annealing treatment is expected to activate the N-related acceptor and simultaneously repair the lattice defects induced by ion implantation.
Fig. 3. (a) The distribution of the implanted nitrogen atoms calculated with the SRIM code, and the corresponding Zn and O vacancy depth profiles are also given. (b) SIMS depth profiles of N, Zn, O and Al atoms in as-implanted N-doped ZnO film.
The electrical properties of the nitrogen ion implanted ZnO thin films annealed at different temperatures are summarized in Table 1. Sample S0, the as-deposited ZnO sample, shows n-type conductivity with a carrier concentration of 4.38×1018 cm−3 and a mobility of 19.6 cm2V−1s−1. Nitrogen ion implantation induced the ZnO thin films (S1) exhibiting high resistance, which indicates that the as-implanted ZnO samples have been highly compensated. Post-annealing is expected to activate the nitrogen dopants and meanwhile ensure a low level of donor-related states to reduce the self-compensation effect. While for the samples annealed at 850 °C (S3) and 900 °C (S4), the carrier type converted from negative to positive, which evidences a hole-dominant transport in the ZnO thin films. However, the films are high resistance again after annealing at 950 °C (S5). These results reveal that has an annealing temperature window for the p-type film formation. The best p-type conductivity was obtained for the samples annealed at 900 °C with a high hole concentration of 9.60×1017 cm−3, a low resistivity of 0.37 Ω cm, and a mobility of 17.6 cm2V−1s−1 at room temperature. Most importantly, unlike the sample S3 converted into n-type after several days, the p-type conductivity of the sample S4 is very stable. Even after three months, no reduction of hole conduction was found. The stability of p-type conductivity for the N-doped ZnO films observed in our experiments is significantly better than the previously reported studies in the literature (usually only lasts for two weeks) [27,28]. Herein, it should be noted that the changes in the films morphologies such as grains size and grain boundaries may also impact the carrier transport properties of the films, which is not considered in this work, but would be interesting for further study.
Table 1. Results of RT Hall measurements of ZnO and N-doped ZnO films annealed at different temperatures.
Figure 4 presents the RT Raman scattering spectra of the N implanted ZnO thin films annealed at different temperatures. Regarding the distribution of the Raman vibration modes, the vibration modes at 416, 429, 448, 576 and 750 cm−1 (marked as Psub) originated from the sapphire substrate. The Raman scattering peak at 100 cm−1 is assigned to the low-frequency E2 mode [E2(Low)], and the peak appears at 378 cm−1 can be assigned to the A1(TO) mode. Our results are consistent with previous Raman studies [29,30]. In addition, additional vibration modes such as 275 cm−1, 510 cm−1, 584 cm−1, and 644 cm−1 can be observed in N-doped sample. These vibration modes are generally not present in doped ZnO [31]. In the past decade, the source of these vibration modes has always been controversial. The researchers attributed these modes to nitrogen related local vibrational modes (LVM) in ZnO [32], host lattice defects [33] or zinc interstitial (Zni) related defect complexes (Zni-No) (275 cm−1) and (Zni-Oi) (510 cm−1) [34]. Most recently, Gluba et al. pointed out that the vibrational mode at 275 cm−1 could be caused by the vibration of Zn-Zn in the small cluster of zinc interstitials (about 3-9 zinc atoms) [35]. At the same time, the other three additional vibration modes coexist with the 275 cm−1 mode were also detected by the above researchers [32–35]. Therefore, the additional vibration mode at 275 cm−1 should be attributed to the local vibration mode related to the Zni defects. The interstitial Zn, as an intrinsic donor defect, is a typical hole killer in the ZnO materials. Considering it has a small diffusion barrier (0.55 eV) [36], at low temperature of ∼170 K, so the isolated Zni defects could be easily annealed out [37]. In our experiment, it can been found that the intensity of the 275 cm−1 vibration modes in the sample annealed at 800 °C reduced significantly, and which was not observed for the samples annealed above 850°C. These results imply that the Zni intrinsic donor defects have been annealed out thoroughly at high temperature. While the shallow acceptor defects are dominant in the ZnO samples, the N doped ZnO thin films would covert from n-type to p-type conductivity.
To study the effect of annealing on the N chemical states, the N 1s core-level XPS spectra of different samples were measured, as shown in Fig. 5. Three peaks around 395.8, 396.8 and 403.5 eV were detected. According to the literatures, the peak at ∼395.8 and ∼ 396.8 eV are assigned to the N-Zn bond in N- and O- rich local environments (α- and β-NO), respectively [38]. In fact, the ∼395.8 eV peak is usually observed in heavily N-doped ZnO, and its position is very consistent with that of the N-Zn band in Zn3N2 [39,40]. The peak around 399.2 eV can been assigned to the overlap of the N-H component and the C-N component [41], while the peak at ∼ 403.5 eV is attributed to the (N2)O [42]. The variation of N 1s core-level XPS spectrum for the samples annealed at different temperatures reflects the evolution of N local chemical states in ZnO:N films. Interestingly, when the samples were annealed at 850 °C and 900 °C, it is clearly seen that the N 1s peaks at 396.8 eV is dominate, which means that the effective substitutional doping (NO) have been formed in the ZnO thin films [43,44]. While for the sample annealed at 950 °C, the out-diffusion of N ions upon high temperature leads to the disappearance of any N-related XPS signals from the sample S5.
The behavior of both intrinsic and external defects caused by doping can be reflected by variation of excitonic recombination by low-temperature PL. To determine the origin of the emission peaks in the spectra between N-doped ZnO thin films before (S1) and after annealing (S4), low temperature PL studies were performed and the results are shown in Fig. 6. For the as-implanted sample (S1), multiple emission peaks were detected. The emission peak at 3.377 eV is usually associated with free exciton (FX) [45], and the emission peak centered at 3.360 eV is assigned to the neutral donor bound exciton radiative recombination (D0X) [46]. Previously, the emission peak of 3.36 eV was assigned to excitons bound to Al donors [47]. However, the same exciton emission has also been observed by researchers in some non-Al-doped ZnO materials [47,48]. Most recently, Yao et al. observe the emission peak at ∼3.36 eV in Te-doped ZnO microrods and assign it to D0X associated with Zni related defect complex [49]. Actually, the ionization energy (ED) of donor defects related to the PL peak at 3.36 eV can be calculated by Haynes’ rule, which is about 44 meV, in consistent with the ionization energy of the theoretically calculations [50]. Together with the Raman scattering measurements, hence we assign that the emission peak at 3.360 eV should be associated with Zni related shallow donors. Moreover, the emission peak at 3.329 eV is attributed to the acceptor-bound exciton (A0X), while the emission peak located at 3.307 eV and 3.275 eV named as FA1 and FA2 are ascribed to the free electron to different acceptor transitions. According to pervious literatures, the donor-acceptor pair (DAP) emission band is typically situated around 3.24 eV [51]. Here we attribute the emission peak at 3.235 eV to the recombination of DAP (marked as DA2P). Besides, the lower energy peaks on the left side of the 3.199 eV and 3.158 eV refer to the longitudinal optical (LO) phonon replica of FA2 (FA2-LO) and the phonon line of DA2P (DA2P-LO), respectively. When the samples were RTA annealed at 900 °C, it is evident that both the D0X and FA1 observed in S1 completely disappear, while a considerable enhancement of the photoluminescence of A0X, FA2, and DA2P emission peak is detected. Traditionally, the 3.31 eV luminescence is ascribed to (e, A0) transitions of conduction band electrons to acceptors. However, Fonoberov et al. [52] attribute the emission peak at 3.31 eV in ZnO nanocrystals to acceptor bound excitons (A0X), and they also suggest that the zinc vacancies or surface defects can act as acceptor impurities. Most recently, Yao et al. [53] attribute the emission peak at 3.31 eV in N-doped ZnO microrods to the radiative recombination of free electrons to acceptors, which is originated from the shallow acceptor of zinc vacancy clusters. At the same time, Pal et al. [47] detected the 3.313 eV PL peak in ion irradiated polycrystalline ZnO powders and pointed out the emission is related to the complex of VZn. Furthermore, they also find that the emission peak at 3.313 eV can be reduced through annealing, which is due to the thermally induced VZn migration and recovery takes place at high temperature annealing. In our experiment, the 3.31 eV PL peak appeared in the as-implanted ZnO thin films, but vanished when the films were annealed at 900 °C. Thus, we consider that the PL peak at 3.31 eV is from the transition of the conduction band to the VZn cluster acceptor level. Tuomisto et al. [54] also found that the incorporation of N can generate stable vacancy defects in ZnO through PAS experiments. Therefore, we speculate that the FA1 is associated with the VZn cluster related shallow acceptor that was generated by nitrogen implantation. While for the FA2 located at 3.275 eV, its acceptor energy can be calculated from Eq. (1):
where the temperature-dependent transition EFA is approximately 3.285 eV at 14 K and the intrinsic band gap Egap is about 3.43 eV at 14 K [45]. Therefore, the value of EA is calculated to be 0.161 eV. Based on the first-principles calculations, Liu et al. [20] pointed out that the VZn-NO acceptor complex is the effective acceptor of p-ZnO, and the transition energy ɛ(0/−) of this complex defect is about 110-160 meV. Most recently, Amini et al. further mentioned that the annealing process could lead to the formation of the shallow acceptor VZn-NO complex [22]. Considering the fact that the NO and VZn are present in the sample, thus we believe that they would form as VZn-NO complex during the annealing process. Meanwhile, the thermal treatment also suppresses the Zni related shallow donors. Thus the S4 exhibits high quality p-type conductivity.>4. Conclusions
In summary, we report on the realization of high quality p-type N-doped ZnO thin films using nitrogen implantation and a rapid thermal annealing (RTA) process. The samples were comprehensively characterized by a combination of Hall effect, Raman scattering, X-ray photoelectron, and photoluminescence (PL) measurements. It is observed that the N-doped ZnO thin films annealed at 900 °C exhibit a high hole concentration of 9.60×1017 cm−3 at room temperature. Both Raman and PL reveal the thermal treatment suppresses the Zni related shallow donors in ZnO. Moreover, PL studies reveal a shallow acceptor energy level of 0.161 eV above the valance band. Our experimental results suggest that the p-type conductivity of N-doped ZnO is originated from the shallow acceptor complex VZn-NO. These results has positive significance for clarifying the p-type conductivity mechanism of N-doped ZnO thin films.
Funding
National Natural Science Foundation of China (NSFC) (51472038, 51502030); Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501112); the opening project of Chongqing Key Laboratory of Micro /Nano Materials Engineering and Technology (KFJJ1301); Natural Science Foundation of Chongqing (cstc2017jcyjAX0393); China Postdoctoral Science Foundation (2016M600726); Postgraduate Scientific Project of Chongqing University of Arts and Sciences (CUAS) (M2018ME07).
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