Abundant Acceptor Emission from Nitrogen-Doped ZnO Films

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Abundant Acceptor Emission from Nitrogen-Doped ZnO Films Prepared by Atomic Layer Deposition under Oxygen-Rich Conditions Elizbieta Guziewicz, E. Przezdziecka, Dmytro Snigurenko, Dawid Jarosz, Bartlomiej Slawomir Witkowski, Piotr D#u#ewski, and Wojciech Paszkowicz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04127 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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ACS Applied Materials & Interfaces

Abundant Acceptor Emission from Nitrogen-Doped ZnO Films Prepared by Atomic Layer Deposition under Oxygen-Rich Conditions

E. Guziewicz*, E. Przezdziecka, D. Snigurenko, D. Jarosz, B.S. Witkowski, P. Dluzewski, W. Paszkowicz Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, PL- 02668 Warsaw, Poland

Abstract. Nitrogen doped and undoped ZnO films were grown by thermal Atomic Layer Deposition (ALD) under oxygen rich conditions. Low temperature photoluminescence spectra reveal a dominant donor-related emission at 3.36 eV and characteristic acceptor-related emissions at 3.302 and 3.318 eV. Annealing at 800oC in oxygen atmosphere leads to conversion of conductivity from n to p-type, which is reflected in photoluminescence spectra. Annealing does not increase any acceptor-related emission in the undoped sample, while in the ZnO:N it leads to a considerable enhancement of the photoluminescence at 3.302 eV. The high resolution cathodoluminescence cross-section images show different spatial distribution of the donor-related and the acceptor-related emissions, which complementarily contribute to the overall luminescence of the annealed ZnO:N material. Similar area of both emissions indicates that the acceptor luminescence comes neither from the grain boundaries nor from stacking faults. Moreover, in ZnO:N the acceptor-emission regions are located along the columns of growth, which shows a perspective to achieve a ZnO:N material with homogeneous acceptor conductivity at least at the micrometer scale.

Keywords: zinc oxide, acceptor doping, nitrogen, Atomic Layer Deposition, luminescence

PACs numbers: 78.20.N-, 78.55.Et, 78.60.Hk, 78.66.Hf

*

corresponding author, e-mail: [email protected] 1 ACS Paragon Plus Environment


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1. Introduction Achieving a stable p-type conductivity of zinc oxide still encounters difficulties and is the main obstacle in a wide application of this material in optoelectronics. Acceptor doping of ZnO can be realized through group I dopants like Li, K, Na located at the zinc site or group V dopants like N, P, Sb or As substituting the oxygen atom.1-2 Density functional theory (DFT) calculations show relatively shallow acceptor levels of group I dopants in ZnO,1 but because of small size these atoms tend to occupy rather interstitial than substitutional positions where they act as donors. In turn, the atomic radii of such group V dopants as P, As and Sb considerably exceed the oxygen atom radius thus these elements likely form anti-sites to avoid stress where they are donors1.

However, numerous groups report on p-type

conductivity of ZnO:P, ZnO:As and ZnO:Sb.e.g.2-4 This has been explained by creation of acceptor-like defect-impurity complexes involving zinc vacancies5. After more than a decade of extensive search for the best acceptor dopant the investigations returned to the primal idea of nitrogen as the most promising element for p-type doping of ZnO. In fact, nitrogen and oxygen atomic radii are very similar, so it reveals a limited tendency to form antisites and/or interstitials in a ZnO lattice. Although first-principle calculations provide rather high values of nitrogen-related acceptor levels1,6 multiple research groups have reported on p-type conductivity of ZnO:N, which might indicate that nitrogen, like other V group dopants, P, Sb and As, interacts with native defects, creating defect-impurity centers that provide shallow acceptor levels. Therefore, in order to achieve an acceptor conductivity in ZnO it is important not only to effectively introduce nitrogen, but also to use such growth conditions which ensure a low level of donor-related states and an appropriate interaction with the remaining defects. The latter conditions strongly depend on growth parameters; growth temperature plays a crucial role, because it decides on the number of point and extended defects and can be the origin of transition between zinc-rich and oxygen-rich conditions,7 which, in turn, determine defects energetics in ZnO.8 In this context the temperature of growth seems to be a critical issue in obtaining the p-type ZnO, the problem which has not been properly addressed to date. Oxygen-rich conditions, which are beneficial for achieving the p-type conductivity, can be reached at very low (about 100oC) or, alternatively, at very high growth temperatures (above 700oC),7,9 while the overwhelming majority of efforts towards acceptor conductivity of ZnO is focused on samples grown at 400-600oC. The latter growth temperature range does not ensure oxygenrich conditions and thus seems to be not the best choice for a successful p-type doping. It has 2 ACS Paragon Plus Environment

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been shown, that growth temperature is important for nitrogen incorporation. In ZnO samples grown by CVD with the NH3 precursor a nitrogen concentration changes from 1019 cm-3 to 1021 cm-3, when growth temperature decreases from 600oC to 200oC.10 In the present work we realized acceptor doping of ZnO at very low growth temperature. We studied polycrystalline zinc oxide films, undoped and doped with nitrogen, grown by Atomic Layer Deposition (ALD) at 100oC. We investigated the optical properties of fabricated ZnO and ZnO:N films by low temperature photoluminescence (LT PL) and spatially resolved lowtemperature cathodoluminescence (SR-LTCL), looking for acceptor-related states and their activation after performed annealing processes. We show that the low temperature deposition might be an interesting new path in ZnO investigations that was only scarcely explored to date.

2. Experimental conditions The undoped and nitrogen doped ZnO films were grown on a highly resistive silicon substrate by ALD at temperature of 100oC. The native SiO2 has not been removed from the substrate and is present as a ∼3 nm thick film at the ZnO/Si interface. Before the ALD growth, the substrates were sequentially rinsed in acetone, 2-propanol and deionized water. The ZnO films were obtained with diethylzinc (DEZn, Zn(C2H5)2) and deionized water (DI) as a zinc and oxygen precursors, whereas nitrogen served as a purging gas. Nitrogen was introduced into the ZnO films through ammonia water (NH4OH, 25% ammonium hydroxide MOS grade from Baker Analyzed) which was used as a nitrogen precursor at every fourth ALD cycle. All the precursors were kept at room temperature. The growth processes were preformed in a Savannah-100 reactor using 0.015 s pulsing times and 2 s purging times for all three precursors. The ZnO and ZnO:N films were deposited in 10 000 ALD cycles, which resulted in thickness of about 2 µm. Annealing processes were performed in a rapid thermal processing (RTP) system (Accu Thermo AW610 from Allwin21 Inc.) at 800oC in an oxygen atmosphere. Structural X-ray diffraction measurements were done with the Cu Kα1 radiation using a Bragg-Brentano powder diffractometer (X'Pert Pro Alpha1 MPD from Philips/PANalytical) equipped with an incident beam Ge(111) Johansson monochromator and a strip detector. Additionally, a Titan Cubed 80-300 Transmission Electron Microscope was used for imaging of the crystal lattice with a high resolution of 100 pm. 3 ACS Paragon Plus Environment


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Temperature dependent photoluminescence (PL) study was performed with the 302.8 nm line from a Coherent Innova 200 Argon laser. Scanning electron microscopy (SEM) images were taken with a Hitachi SU-70 system. High resolution CL spectra and submicroscopic CL maps were obtained at 5 K with a Gatan MonoCL-3 system synchronized with SEM. ZnO-ALD growth at low temperature regime The growth temperature used for deposition of a peculiar material by the ALD technique, depends on the applied chemical precursors. Organic precursors, like diethylzinc (DEZn), easily evaporate, so partial pressure sufficient for the ALD deposition is reached at lower temperature than in case of inorganic precursors. The so-called ‘ALD growth window’, which is the optimal growth temperature range assuring a conformal coverage, lies between 100 and 180oC when DEZn and water are used.11 Although the layer thickness is constant within the growth window provided the same number of ALD cycles is applied, physical properties of the obtained ZnO films strongly depend on temperature. It concerns, in particular, free carrier concentration which considerably increases when deposition temperature changes from 100 to 200oC, even inside the ALD growth window.11 The detailed X-ray photoelectron spectroscopy (XPS) and Rutherford Backscattering (RBS) measurements performed on ZnO-ALD films deposited at different temperatures show the clear correlation between a free electron concentration and the O to Zn ratio. This ratio increases by a few percent when deposition temperature is reduced from 200oC to 100oC. This means that low electron concentration, observed for ZnO films grown at the low temperature regime, is related to a higher oxygen content, i.e. the ALD growth at 100oC assures oxygen-rich conditions.7 Because of this, the growth temperature of 100oC is beneficial for acceptor doping and therefore it was used for deposition of ZnO samples investigated here.

3. Experimental section 3.1. Nitrogen doping In the ALD process precursors are alternatively introduced into the growth chamber and purging by inert gas (nitrogen in our case) occurs after each precursor dose. Doping in ALD can be realized in situ by changing one of the used precursors. The typical surface reaction between diethylzinc and water can be described as follows: C2H5 – Zn – C2H5 + H2O → ZnO + 2C2H6 ↑

{1} 4

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This reaction occurs at the surface of the growing film in two stages, every stage during introduction of each precursor. The first phase takes place, when DEZn is supplied: Surface – OH + C2H5 – Zn – C2H5 → Surface – O – Zn – C2H5 + C2H6 ↑

{2}

In the second phase, when deionized water is supplied: Surface – O – Zn – C2H5 + H2O → Surface – O – Zn – OH + C2H6 ↑

{3}

For nitrogen doping ammonia water was used as a non-metallic precursor alternatively with deionized water. At temperature of 100oC ammonium hydroxide is not stable, it decomposes into ammonia and water (NH4OH → NH3 + H2O), so ammonia water acts simultaneously as a nitrogen and oxygen precursor. Ammonia reacts at the surface according to the equation: Surface – Zn – C2H5 + NH3 → Surface – Zn – NH2 + C2H6 ↑

{4}

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1E20 1E19 1E18 0

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Fig.1. a) The schematic view of the ALD surface reaction leading to incorporation of the –NH group substituting oxygen. b) SIMS profile of nitrogen in a 2 µm thick ZnO:N film obtained when 50% of ALD cycles were conducted with NH4OH and 50% with deionized water. c) Nitrogen atomic concentration versus a number of NH4OH to H2O cycles.

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In the next step of the ALD process, DEZn is introduced into the chamber and reacts with the NH2 groups at the surface as follows: Surface – Zn – NH2 + C2H5 – Zn – C2H5 → Surface – Zn – NH – C2H5 + C2H6 ↑

{5}

Because we used a solution of ammonium hydroxide, only a part of chemical radicals at the surface react according to equations {4} and {5}, whereas the rest according to {2} and {3}. The graphical representation of surface chemical reactions described above is shown in Fig. 1a. One can see that, as a result of a surface double exchange reaction with ammonia, the −NH group is introduced exactly at the oxygen site of the ZnO crystal lattice.12 This fact has two vital consequences. The first one is that the –NH impurity which has a bond with a Zn atom generally does not generate a hole, because nitrogen dopant is passivated by hydrogen. Therefore, an annealing process is necessary at this stage in order to remove hydrogen and activate the p-type conductivity. The second important finding is that using ammonia in the ALD process, we actually realize acceptor-donor co-doping, so we can expect a very effective nitrogen incorporation. This type of −NH co-doping has been proposed by Zhang et al. in 2001.13 They suggested that a very high N concentration can be reached without the formation of hole-killer defects, when nitrogen is introduced together with hydrogen, because (N+H) as a whole, has six valence electrons, identical to oxygen. It fact, we observe that high nitrogen concentration can be achieved when ammonia water is used in the low temperature ALD process. The SIMS measurements show that the maximum nitrogen concentration of 2⋅1021 at./cm3 can be reached when 25% ammonia water solution is used in double exchange reaction with diethylzinc in each ALD cycle. Nitrogen is uniformly distributed across the ZnO film as shown in Fig. 1b. Moreover, we can precisely regulate the amount of introduced nitrogen if ammonia water is used alternatively with deionized water precursor. In this case, the concentration of nitrogen can be scaled from 1018 at./cm3 to 1021 at./cm3 depending on the number of ammonia water cycles versus deionized water cycles (Fig. 1c). 3.2. Structural properties of ZnO and ZnO:N films Structural properties of ZnO films grown by ALD depend on temperature and such parameters of the ALD process as pulsing and purging time.14 For a low growth temperature (Tg) range (100-200oC) a polycrystalline growth is observed and the main factor determining a preferential orientation is temperature. For Tg = 200oC and above, the diffractograms show only one 002 peak which indicates that all the crystallites are oriented with c axis 6 ACS Paragon Plus Environment

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Fig.2. X-ray diffractograms of undoped (a) and nitrogen doped (b) ZnO films.

perpendicular to the surface. For lower Tg the diffractograms reveal several diffraction peaks corresponding to the wurzite-type crystal structure of ZnO. Transmission Electron Microscope (TEM) images depict a columnar structure with columns width depending on the film thickness and varying between 10 and 45 nm for films 70 and 400 nm thick, respectively.14 When Tg = 100oC is applied, a few crystallographic orientations are typically observed and the relative intensity of the XRD peaks depends mainly on the purging time.15 In the present study we applied short purging times (2s) for fabrication of the ZnO and ZnO:N films, which resulted in several diffraction peaks corresponding to [10.0], [00.2] and [11.0] crystallographic directions (see Fig. 2). The diffractograms show only peaks due to a wurtzite-type crystal structure of ZnO. The N-doped and undoped ZnO films are very similar (compare Fig. 2a and Fig. 2b). Both films are oriented mostly along the [11.0] direction. The only difference concerns the Full Width at Half Maximum (FWHM) of the diffraction peaks which indicates differences in a crystallite size. The Scherrer formula applied to the 110 peak a)

b)

c)

Fig.3. TEM image of the ZnO:N film grown at 100oC, (a) columnar growth of ZnO:N films, (b) a region near to the interface, (c) a region inside the ZnO:N crystallite. 7 ACS Paragon Plus Environment


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for undoped and N-doped ZnO films gives the crystallite size of 44 and 60 nm, respectively. Low temperature CL images presented in the next paragraph confirm that an average crystallite size after nitrogen doping is higher as will be presented in the paragraph 3.4. TEM images show that ZnO:N have a columnar structure like undoped ZnO films (see Fig. 3a). TEM also show a very uniform coverage of a used silicon substrate which is typical feature of all films deposited with the ALD technique. The ZnO and ZnO:N layers were deposited on a highly resistive silicon substrate without removing of a native silicon oxide surface layer. This SiO2 is visible at the TEM image of the ZnO/Si interface (see Fig. 3b). It has been found that it is about 3.45 nm thick. A good crystallographic quality of the films is observed in a high-resolution TEM images showing ordered rows of atoms inside the grains (Fig. 3c). A high samples’ quality is also confirmed by relatively narrow excitonic lines that appear in a low temperature luminescence as is presented in the next section. 3.3.

Low temperature photoluminescence Optical measurements are a valuable tool for investigation

of

doping

in

ZnO,

because

acceptor-, donor- and defect-related emission lines might shed some light on the origin of observed conductivity. Photoluminescence (PL) spectra of intrinsic ZnO crystals and thin films exhibit intensive emission in two main regions: (i) near band-edge (NBE) luminescence in the region 3.30 –3.37 eV, (ii) defect related emission bands in the “green” region 2.1 – 2.8 eV.16-17 It is believed that the latter one is caused by intrinsic point defects like oxygen vacancy (VO), zinc vacancy (VZn) and/or zinc interstitial (Zni).18 NBE luminescence has excitonic origin and consists on emission lines from neutral donors Fig.4. Low temperature PL spectra of as grown and annealed (a) ZnO:N and (b) ZnO films. The intensities are normalized to the most intensive line.

(DoX) and/or ionized donors (D+X) bound excitons observed between 3.35 and 3.37 eV, possibly with their phonon repetitions, and acceptors-related AoX line at 3.31 – 3.35 eV. 8


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Donor-acceptor pair (DAP) emission band is typically situated around 3.24 eV.16-17 Doping with group V elements is usually accompanied by enhancement of luminescence intensity around 3.30-3.32 eV. This emission band is usually assigned to free electron-acceptor transition (FA) or recombination of excitons bound to deep neutral acceptors (A0X). However, this emission line is omnipresent in ZnO irrespective on the chemical nature of dopant and sometimes even without any intentional doping. In the latter case, the 3.31 eV luminescence was related to stacking faults.19 In the present study, low temperature photoluminescence (LT PL) was measured on ZnOALD films grown at 100oC and doped with nitrogen at the level of 1019 cm-3, i.e. using ammonia water in every fourth ALD cycle. After 3 min. RTP annealing in oxygen atmosphere at 800oC these films showed the p-type conductivity with carrier concentration 4.5⋅1016 cm-3 and mobility 17.3 cm2/V⋅s as obtained in room temperature Hall measurements. The PL spectra taken at 10 K for (a) nitrogen doped and (b) undoped ZnO films show two emission bands, one at about 3.36 eV and the second one between 3.30 and 3.32 eV (Fig. 4a and 4b, green lines). The FWHM of excitonic peaks at ∼ 3.36 eV of all samples is 4-7 meV, which is comparable with FWHM typically reported for ZnO samples obtained at much higher growth temperature, e.g. at 900 °C by RF magnetron sputtering (7 meV)20 and with the ZnO material obtained by MOCVD (6 meV) at 250–600 °C21, although for epitaxial ZnO films grown by MBE, the reported FWHM of excitonic peak can be as narrow as ~0.08 meV22-23, though much higher FWHM values for MBE samples (∼3 meV) are also reported.24 In the “as grown” ZnO sample two excitonic peaks located at 3.361 and 3.367 eV can be seen (Fig. 4b). The former can be related to donor bound exciton transitions DoX associated to H, while the latter one to DoX or D+X related to another donor.16 Both excitonic peaks are also present after RTP annealing. However, the intensity of the 3.367 eV line significantly decreases, which indicates that annealing at 800oC in oxygen atmosphere leads to disappearance of some donor states. Interestingly, the excitonic line at 3.367 eV is not observed in the “as grown” nitrogen-doped sample pointing at passivation of related donor states by nitrogen doping. The 3.361 eV PL line is visible in “as grown” ZnO:N. This PL line can be related to the I4 transition and interpreted as an exciton bound to neutral shallow H donor.25-26 After 3 min. post-growth RTP annealing at 800oC in an oxygen atmosphere, the donor DoX peak at energy of 3.361 eV disappears and a new excitonic peak at 3.359 eV arises. The origin of this PL line can be 9 ACS Paragon Plus Environment


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interpreted as a line related to a another donor transition.16,27 Because of uncertainty related to this line width, the unambiguous interpretation of this emission requires an additional study. As can be seen, the annealing process of nitrogen doped sample not only activate acceptors but affects donor states as well. It might involve a complex interaction of acceptors, donors and native point defects causing a raise of the PL line at 3.359 eV. Moreover, a new peak at 3.355 eV arises (Fig. 4a). The PL line at this energy has been previously observed in ZnO:N films, related to nitrogen acceptor and interpreted as the exciton bound to neutral acceptor AoX transition.28-29 Apart from the excitonic emission described above, the ZnO and ZnO:N films show photoluminescence in the region between 3.30 and 3.32 eV. This PL band has been variously assigned in the literature to many different acceptor-related transitions like acceptor-bound excitons, donor-acceptor pairs (DAPs) and free electron to neutral acceptor (FA). In fact, more than one transition contributes to the luminescence at this energy, the two mentioned above derive from acceptor states, but the first LO phonon replica of the dominant excitonic state also appears at about 3.287 eV (marked as a dashed line). It is worth noting, that the intensity of this acceptor-related PL differently responds to annealing, depending on whether the ZnO sample was doped with nitrogen or not. It should be also mentioned that because of a strong emission appearing at ∼3.31 eV, the phonon replica of this peak are also relatively intensive. Because of that, the DAP emission at 3.24 eV, commonly observed in ZnO:N, overlaps with phonon replica of the 3.31 eV emission and, as a consequence, it cannot be clearly distinguished in our spectra. Two emission lines at 3.302 and 3.318 eV are observed in both as grown and annealed ZnO:N samples. Intensity of the peak at 3.302 eV considerably increases after annealing of ZnO:N (Fig. 4a), whereas this effect is not observed for the undoped ZnO sample (Fig. 4b). The presence of the acceptor-related line and its significant increase after annealing points at thermal activation of a nitrogen dopant. As a result, the relative intensity of acceptor- and donor-related PL after annealing changes considerably and after annealing the acceptorrelated emission dominates the overall spectrum. Thermal activation of acceptor-related PL can be related with the breakdown of a nitrogen-hydrogen chemical bond and hydrogen removal from the ZnO:N-ALD film, as was described in the previous paragraph. In case of undoped ZnO, the situation is quite different, as the relative intensity of donor- and acceptor-related peaks of undoped ZnO sample before and after annealing are very similar 10 ACS Paragon Plus Environment

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(Fig. 4b) and we do not observe any thermal activation process as it was for the nitrogendoped sample. The temperature dependence of PL emission of annealed ZnO:N and ZnO films is graphically depicted in Fig. 5(a) and 5(b), respectively. Both figures show PL spectra of annealed ZnO and ZnO:N films taken at 10 K (top figures 5a and 5b) together with a temperature dependence of PL intensity. For both annealed samples (N-doped and undoped) the peak at 3.318 eV is observed up to room temperature, whereas this at 3.302 eV rapidly decreases with temperature showing a blue-shift characteristic of DAP or FA transitions (Fig. 5a and 5b). Additionally, the freeexciton (FX) peak at about 3.37 eV (at 10 K) is visible up to ∼300 K in all measured samples. 3.4. Low temperature cathodoluminescence A deeper insight into the nature of the acceptor-related states provides analysis of cathodoluminescence (CL) spectra and images measured at 10 K.

Fig.5. Low temperature and temperature dependence PL of annealed (a) ZnO:N, (b) undoped ZnO samples. Red color indicates high intensity peaks, whereas blue color reflects low PL intensity, (c) average PL (top) and CL (bottom) spectra from ZnO and ZnO:N samples annealed at 800oC, spectra normalized to the ~3.36 eV line.

The LT CL spectra averaged over ∼ 10 µm2 look very similar to the LT PL, where the signal was collected from the area of a laser spot (∼0.5 mm2) plus the excitonic diffusion area. It is 11 ACS Paragon Plus Environment


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shown in Fig. 5c, where emissions from annealed ZnO and ZnO:N are compared. The existing minor differences like slightly different peaks intensities or small shifts arise from different type of excitation used in CL and PL. In general, PL utilizes photons providing considerably lower excitation energies than electrons used in CL, and in PL each absorbed photon produces only one excited electron. Additionally, the luminescence peaks intensities

depend on

excitation power thus some peaks can be most intensive than others in comparison with CL, where excitation power is much higher than in PL. The LT CL show two emission regions, one at ∼3.36 eV and the second at ∼3.30 eV (Fig. 5c, bottom), as it is observed in the corresponding LT PL (Fig 5c, top). Thermal activation of acceptors after annealing of ZnO:N is reflected in LT CL spectra as well. The observed one to one correspondence between PL and CL spectra confirms that in the annealed ZnO:N layer the intensity of the luminescence near to the 3.3 eV is relatively higher than in the case of the undoped ZnO sample. However, the LT CL signal can be also measured with a high spatial resolution thanks to a Gatan MonoCL-3 system synchronized with SEM used in the experiment. It allows obtaining unique

Fig.6. (color online) The cross section view of annealed ZnO:N (top panel) and ZnO (botton panel) samples: (a) and (d) SEM images; (b) and (e) low temperature CL maps-red color represent CL at 375 nm (3.305 eV) and green one at 370 nm (3.36 eV); (c) and (f) two dimensional maps of CL (5 kV) come from the points along the blue lines marked in Fig. (b) and (e), the color scale shows intensity of the PL signal. 12 ACS Paragon Plus Environment

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images showing a distribution of acceptor- and donor-related emissions over the sample cross-section and thus to get insight into the origin of acceptor- and donor-related emissions. For this purpose, a CL detector was set to register the 3.36 eV emission or, alternatively, the 3.30 eV emission and two obtained images have been superimposed on each other, wherein each emission was highlighted in a different color. In Fig. 6a and 6d we present the SEM cross-section images of the annealed ZnO:N (top) and ZnO (bottom) samples, respectively. Exactly the same cross-section areas have been probed by LT CL (Fig. 6b and 6e). According to two LT CL contributions shown in Fig. 5c, the maps showing donor-related (3.36 eV (370 nm), green color) and acceptor-related (3.305 eV (375 nm), red color) emissions have been superimposed and presented in Fig. 6b (for ZnO:N) and Fig. 6e (for ZnO). Additionally, the CL intensities at different wavelengths measured along the blue lines shown in Fig. 6b and 6e are presented in Fig. 6c and 6f. The latter maps reflect the previous conclusion derived from photoluminescence investigations that for the annealed ZnO:N sample the acceptor-related emission is stronger than for the undoped ZnO. Indeed, the CL emissions in the region 374 – 376 nm (energy between 3.297 and 3.315 eV) is much more intensive for the nitrogen doped film (Fig. 6c) than for the undoped ZnO (Fig. 6f). It is additional fingerprint of a thermal activation of the nitrogen-related acceptor state observed in the PL investigations.

4. Discussion A few important conclusions can be derived from the presented CL images. The first one concerns a spatial distribution of acceptor and donor-related regions in undoped and N-doped samples. It is clearly seen that acceptor and donor emissions derive from different spatial regions of the samples. It is also evident, as seen in Fig. 6b and 6e, that acceptor-related luminescence does not derive from grain boundaries. The latter conclusion is also supported by the XRD study presented in the paragraph 5, which reveals a larger grain size in the nitrogen-doped sample. This means that an area of grain boundaries in the ZnO:N film is lower than in ZnO, so grain boundaries cannot decide on the observed enhancement of acceptor luminescence.



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Moreover, in the annealed ZnO film both acceptor and donor-related regions occupy similar spatial areas which are randomly distributed over the sample cross-section (Fig. 4c), whereas in ZnO:N the regions with acceptor emission prevail, are arranged along the columns of growth, and average acceptor-related PL and CL are considerably more intensive, pointing at much higher concentration of acceptor states. Such a spatial distribution of acceptor regions should allow, at least partially, to explain the problem of ambiguous results of Hall effect measurements which often provide results depending on the sample geometry. In fact, in Hall measurements the related electrical characteristics are taken across the samples, where acceptor and donor regions are randomly distributed, therefore one might obtain ambiguous results. Similar situation was already reported in scanning capacitance spectroscopy measurements of ZnO samples co-doped with nitrogen and arsenic.30-31 On the other hand, contrary to these results, good electrical characteristics have been reported for ZnO-based homojunctions,32 where a carrier transport is realized across the ZnO films, i.e. along the columns. A spatial distribution of acceptor regions along the columns of growth, as seen in Fig. 6b, allows for obtaining nanostructure homojunctions, though conductivity in perpendicular direction is not fully satisfactory. In fact, the ZnO homojunction fabricated with annealed ZnO:N and “as grown” ZnO films, both obtained in the ALD processes described above, shows a rectification ratio at the level of 105 at ±2 V, as was reported in our recent work.32 The last but not least conclusion concerns the stacking faults problem that focused a lot of scientific attention in the last few years.19,33-34 The spatial-resolved CL studies on epitaxial ZnO films show that the 3.31 eV emission line is rather related to stacking faults than to a peculiar acceptor dopant19 and can be observed for undoped epitaxial films as well. This called into a question the possibility of obtaining an acceptor conductivity with carriers mobility high enough for electronic applications. The results presented here lead to a more optimistic conclusion. The CL results presented above suggest that we might expect a considerable localization of carriers in the planar direction while carriers’ transport across the ZnO films could be much easier. Moreover, the p-type emission comes from entire columns and is not related to line-directed structural defects like stacking faults.


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Spatial separation of different conductivity types suggests that, at least locally, it is possible to achieve the p-type conductivity without concomitant donors and offers some hope to obtain a homogeneous material with acceptor conductivity at least at a nanometer scale.

5. Conclusions Thermal atomic layer deposition was employed to deposit ZnO and ZnO:N films at 100oC. The films are polycrystalline with crystallite size 44-60 nm and the dominant [11.0] crystallographic orientation. The RTP annealing at 800oC in oxygen atmosphere leads to activation of acceptor states which is observed as a significant enhancement of photo- and cathodoluminescence lines at 3.302 eV and 3.318 eV. The LT CL cross-section images show a complicated picture of acceptor- and donor-related emissions, which are related to a microstructure of the films. In the annealed ZnO:N film, the intensity of acceptor-related emission is considerably higher and micro-areas showing this emission are arranged along the columns of growth. Macroscopically the sample exhibits p-type conductivity with hole concentration of 4.5⋅1016 cm-3 and mobility 17.3 cm2/V⋅s as obtained from room temperature Hall measurements. The ZnO homojunction fabricated with annealed ZnO:N and “as grown” ZnO films shows a high rectification ratio (105 at ±2V). In conclusion, it was shown that low deposition temperature, assuring oxygen-rich conditions, and nitrogen-hydrogen co-doping assure abundant acceptor states in ZnO after post-grown annealing and thus are beneficial for achieving p-type.

Acknowledgements. The work was supported by the Polish NCN project DEC-2012/07/B/ST3/03567. The author EP was supported by the NCN project DEC-2013/09/D/ST3/03750. References (1)Park C.H.; Zhang S.B.; Wei S.H. Origin of p-type Doping Difficulty in ZnO: The Impurity Perspective. Phys. Rev. B 2002, 66, 073202 (1-3). (2) Przeździecka E.; Kamińska E.; Korona K.P.; Dynowska E.; Dobrowolski W.; Jakieła R.; Kłopotowski Ł.; Kossut J. Photoluminescence Study and Structural Characterization of ptype ZnO Doped by N and/or As Acceptors. Semicond. Sc. Technol. 2007, 22, 10-14. (3)Xiu X.; Yang Z.; Mandalapu L.J.; D. T. Zhao; D.T.; Liu J.L. High-mobility Sb-doped ptype ZnO by Molecular-Beam epitaxy. Appl. Phys. Lett. 2005, 87, 152101 (1-3).



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(16) Meyer B.K., Alves H., Hofmann D.M., Kriegseis W., Forster D., Bartram F., Christen J., Hoffmann A., Straßburg M., Dworzak M., Haboeck U., and Rodina A.V. Bound Exciton and Donor-acceptor Pair Recombinations in ZnO. Phys. stat. sol. (b) 2004, 241, 231-260. (17) Wang L. and Giles N.C. Temperature Dependence of the Free-exciton Transition Energy in Zinc Oxide by Photoluminescence Excitation Spectroscopy. J. Appl. Phys. 2003, 94, 973-978. (18) Lin B.; Fu Z. and Jia Y. Green Luminescent Center in Undoped Zinc Oxide Films Deposited on Silicon Substrates. Appl. Phys. Lett. 2001, 79, 943-945. (19) Schirra M.; Schneider R.; Rieser A.; Prinz G.M.; Feneberg M.; Biskupek J.; Kaiser U.; Krill C.E.; Thonke K.; Sauer R. Stacking Fault Related 3.31 eV Luminescence at 130 meV Acceptors in Zinc Oxide. Phys. Rev. B 2008, 77, 125215 (1-10). (20) Hwang D.-K.; Kim H.-S.; Lim J.-H.; Oh J.-Y.; Yang J.-H.; Park S.-J. Study of Photoluminescence of Phosphorus-doped p-type ZnO Thin Films Grown by Radiofrequency Magnetron Sputtering. Appl. Phys. Lett. 2005, 86, 151917 (1-3). (21) Gorla C.R.; Emanetoglu N.W.; Liang S.; Mayo W.E.; Lu Y.; Wraback M.; Shen H. Structural, Optical and Surface Acoustic Wave Properties of Epitaxial ZnO Films Grown on (0012) Sapphire by Metalorganic Chemical Vapor Deposition. J. Appl. Phys. 1999, 85, 2595-2602. (22) Zeuner A.; Alves H.; Hofmann D.M.; Meyer B.K. ; Heuken M. ; Bläsing J. and Krost A. Structural and optical properties of epitaxial and bulk ZnO. Appl. Phys. Lett. 2002, 80, 2078-2080. (23) Wagner M.R.; Schulze J.-H.; Kirste R.; Cobet M.; Hoffmann A.; Rauch C.; Rodina A.V.; Meyer B.K.; Rőder U.; Thonke K. Γ7 valence band symmetry related hole fine splitting of bound excitons in ZnO observed in magneto-optical studies. Phys. Rev. B 2009, 80, 205203 (1-6). (24) Chen Y.; Bagnall D.M.; Koh H.-j.; Park K.-t.; Hiraga K.; Zhu Z.; Yao T. Plasma Assisted Molecular Beam Epitaxy of ZnO on c-plane Sapphire: Growth and Characterization. Journal of Applied Physics 1998, 84, 3912-3918. (25) Rodina A.V.; Strassburg M.; Dworzak M.; Haboeck U.; Hoffmann A.; Zeuner A.; Alves H.R.; Hofmann D.M.; Meyer B.K. Magneto-optical Properties of Bound Excitons in ZnO. Phys. Rev.B 2004, 69, 125206 (1-9). (26) Lavrov E.V.; Herkoltz F.; Weber J. Identification of Two Hydrogen Donors in ZnO. Phys. Rev.B 2009, 79, 165201 (1-13). (27) Meyer B.K.; Sann J.; Lautenschläger S.; Wagner M.R.; Hoffman A. Ionized and Neutral Donor-bound Excitons in ZnO. Phys. Rev. B 2007,76, 184120 (1-4). (28) Wang D., Zhao D., Wang F., Yao B., Shen D. Nitrogen-doped ZnO Obtained by Nitrogen Plasma Treatment. Phys. status solidi (a) 2015, 212, 846-50.



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(29) Przezdziecka E.; Kaminska E.; Korona K.P.; Dynowska E.; Dobrowolski W.; Jakiela R.; Klopotowski L.; Kossut J. Photoluminescence Study and Structural Characterization of ptype ZnO Doped by N and/or As Acceptors. Semicond. Sc. Technol. 2007, 22, 10-14. (30) Krtschil A.; Dadgar A.; Oleynik N.; Bläsing J.; Diez A.; Krost A. Local p-type Conductivity in Zinc Oxide Dual-doped with Nitrogen and Arsenic. Appl. Phys. Lett. 2005, 87, 262105 (1-3). (31) Dadgar A.; Krtschil A.; Bertram F.; Giemsch S.; Hempel T.; Veit P.; Diez A.; Oleynik N.; Clos R.; Christen J.; Krost A. ZnO MOVPE Growth: From Local Impurity Incorporation Towards p-type Doping. Superlat. Microstr. 2005, 38, 245-255. (32) Snigurenko D.; Kopalko K.; Krajewski T.A.; Jakiela R.; Guziewicz E. Nitrogen Doped ptype ZnO Films and p-n Homojunction. Semicond. Sci. Technol. 2015, 30, 015001 (1-6). (33) Thonke K.; Schirra M.; Schneider R.; Reiser A.; Prinz G.M.; Feneberg M.; Biskupek J.; Kaiser U.; Sauer R. The Role of Stacking Faults and Their Associated 0.13 eV Acceptor State in Doped and Undoped ZnO Layers and Nanostructures. Microel. J. 2009, 40, 210214. (34) Sieber B.; Addad A.; Szunerits S.; Boukherroub R. Stacking Faults-Induced Quenching of the UV Luminescence in ZnO. J. Phys. Chem. Letters 2010, 1(20), 3033-3038.


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Figure captions

Fig.1. a) The schematic view of the ALD surface reaction leading to incorporation of the –NH group substituting oxygen. b) SIMS profile of nitrogen in a 2 µm thick ZnO:N film obtained when 50% of ALD cycles were conducted with NH4OH and 50% with deionized water. c) Nitrogen atomic concentration versus a number of NH4OH to H2O cycles. Fig.2. X-ray diffractograms of undoped (a) and nitrogen doped (b) ZnO films. Fig.3. TEM image of the ZnO:N film grown at 100oC, (a) columnar growth of ZnO:N films, (b) a region near to the interface, (c) a region inside the ZnO:N crystallite. Fig.4. Low temperature PL spectra of as grown and annealed (a) ZnO:N and (b) ZnO films. The intensities are normalized to the most intensive line. Fig.5. Low temperature and temperature dependence PL of annealed (a) ZnO:N, (b) undoped ZnO samples. Red color indicates high intensity peaks, whereas blue color reflects low PL intensity, (c) average PL (top) and CL (bottom) spectra from ZnO and ZnO:N samples annealed at 800oC, spectra normalized to the ~3.36 eV line. Fig.6. (color online) The cross section view of annealed ZnO:N (top panel) and ZnO (botton panel) samples: (a) and (d) SEM images; (b) and (e) low temperature CL maps-red color represent CL at 375 nm (3.305 eV) and green one at 370 nm (3.36 eV); (c) and (f) two dimensional maps of CL (5 kV) come from the points along the blue lines marked in Fig. (b) and (e), the color scale shows intensity of the PL signal.

Brief description of the supplementary material: Electron concentration versus growth temperature dependence for ZnO films grown by ALD, oxygen to zinc content in ZnO-ALD films measured by XPS, 2x2 µm SEM image of a surface of ZnO-ALD film, FWHM of the excitonic photoluminescence, SIMS profile of nitrogen incorporated into the ZnO films, I-V characteristic of the ZnO-ALD based homojunction



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Graphic for manuscript


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Low temperature and temperature dependence PL of annealed (a) ZnO:N, (b) undoped ZnO samples. Red color indicates high intensity peaks, whereas blue color reflects low PL intensity, (c) average PL (top) and CL (bottom) spectra from ZnO and ZnO:N samples annealed at 800oC, spectra normalized to the ~3.36 eV line. 201x288mm (300 x 300 DPI)



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Graphical abstract 338x190mm (96 x 96 DPI)


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(color online) The cross section view of annealed ZnO:N (top panel) and ZnO (botton panel) samples: (a) and (d) SEM images; (b) and (e) low temperature CL maps-red color represent CL at 375 nm (3.305 eV) and green one at 370 nm (3.36 eV); (c) and (f) two dimensional maps of CL (5 kV) come from the points along the blue lines marked in Fig. (b) and (e), the color scale shows intensity of the PL signal. 254x173mm (150 x 150 DPI)


Abundant Acceptor Emission from Nitrogen-Doped ZnO Films

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