Enhancing Acceptor-Based Optical Behavior in Phosphorus-Doped

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Enhancing acceptor based optical behavior in PhosphorusDoped ZnO Thin Films using Boron as compensating species Sushama Sushama, Punam Murkute, Hemant Ghadi, and Subhananda Chakrabarti ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00082 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Electronic Materials

Enhancing acceptor based optical behavior in Phosphorus-Doped ZnO Thin Films using Boron as compensating species Sushama Sushama1, Punam Murkute2, Hemant Ghadi3, Subhananda Chakrabarti1* 1Department

of Electrical Engineering, Indian Institute of Technology, Mumbai 400076, India

2Centre

for Research in Nanotechnology and Science, Indian Institute of Technology, Mumbai 400076, India

3Electrical

Engineering, Ohio State University, Columbus, Ohio 43210, USA Email: [email protected]

Abstract Modulating nature of doping in oxide based semiconductors has always been an area of interest as it resulted in numerous technological developments. On working with ZnO, the principal challenge faced in its realistic utilization as an optoelectronic material lies in its default n-type of nature due to presence of native defects. Thus achieving p-type behaviour has been a tedious job and considerable efforts have been made over the last couple of decades. The incorporation of mono-dopants have yielded p-type ZnO of unstable reliability, thus spurring research on co-doping technology. In present study, we examined the effects of boron implantation time on the structural and optical properties of phosphorus-doped ZnO 1 ACS Paragon Plus Environment

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thin films, with the objective of realizing a material ideal for optoelectronic applications. Furthermore, we investigated the effects of annealing temperature on the behavior of codoped samples. Field emission gun scanning electron microscopy, high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence, and conductive atomic force microscopy results evidenced that boron implantation improved the solubility of the acceptor atom (i.e., phosphorus), which in turn improved the films’ acceptor-based optical emission. The various spectral data also indicated the presence and location of boron atoms in the films. Moreover, we realized a shallow acceptor energy level, with the minimum value being 54.63 meV from the valence band level. The acceptor-bound exciton peak was observed up to 300 K, indicating the feasibility of room-temperature applications of these films. In addition, compared with phosphorus doping, co-doping increased the photoluminescence intensity of the acceptor peaks. The co-doped samples also exhibited stability in the acceptor behavior with the signature of the acceptor bound peaks observed over the span of 13 months. Keywords: ZnO thin films; Plasma immersion ion implantation; Phosphorus; Boron; Codoping; Acceptor solubility; Donor passivation

1 Introduction With a wide direct bandgap of 3.37 eV, large exciton binding energy of 60 meV, favorable thermal stability, and controllable conductivity1, 2, ZnO is an important material in the semiconductor industry as well as in academia, where it has attracted substantial attention over the last few decades as a potential candidate for applications in short-wavelength (i.e., ultraviolet (UV)/violet/blue) optoelectronic devices, such as light-emitting diodes, lasers3, and UV detectors, based on thin films and nano-structures4,

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5, 6.

However, realizing p-type

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behavior in ZnO—which is ideal for such applications—is difficult given its asymmetric doping limitations7 and is thus an active area for research. Location of the elements in the periodic table defines their nature as the donor or acceptor for ZnO. In that line, the elements of the third column in the periodic table i.e. boron (B), Aluminium (Al), Gallium (Ga) and Indium (In) act as the donors to ZnO. B occupies the tetrahedral interstitial site8, or the zinc site9 giving BI or BZn in the crystal. Al, Ga and In are substituted at the zinc site10, 11. The elements of the fifth column in the periodic table i.e. Nitrogen (N), Phosphorus (P), Arsenic (As), and Antimony (Sb) act as the acceptors to ZnO. N, Ar, Sb substitute the oxygen site10 whereas P is reported to substitute both oxygen and zinc sites10, 11. PO provides deep acceptor level10 and PZn together with two zinc vacancies (VZn) provides PZn-2VZn complex, which has acceptor behaviour12. To get ideal p-type ZnO thin films, certain barriers inherent in ZnO—for example, native donor point defects, low dopant solubility, self-compensation, high dopant ionization energy, and unstable lattice—must be overcome13. Although numerous studies have realized p-type ZnO by using single dopants such as N14, P15, As16, Sb17, Li18, and Na19, the achieved p-type behaviors are unstable and short-lived20. Recent studies have examined the effects of acceptor-type defect complexes and dual-doping methodologies12, 21–23 on achieving p-type ZnO. Defect pairs such as NO–2VZn, PZn–2VZn, AsZn–NO, and PZn–4NO have been proposed as the main acceptor-type defect complexes in p-type ZnO24–27. However, in this regard, the formation mechanism of such complexes remains elusive. Co-doping is emerging as a promising technique for achieving p-type behavior in ZnO owing to its advantages of increasing dopant solubility and reducing ionization energy28. To this end, the pioneering works were undertaken by Yamamoto et al.29 in 1999; numerous studies have been undertaken since, with various dopant pairs being identified as capable of



achieving p-type conductivity in ZnO, including Al–N30, Mg–N31, Ag–N32, Li–N33, B–N34, Be–N35, Ag–S36, Ga–N37, As–N38, Al–S39, and Li–F40. However, the stability of co-doping with nitrogen (N) remains problematic as N induces the donor defect state Ni over the span of time. We will briefly discuss about the theory behind the enhanced dopant solubility by using partially compensated co-doping. Figure 1(a) gives the band diagram of undoped ZnO. Since ZnO has n-type behavior, the fermi level EF1 is closer to the conduction band (CB). With the introduction of the acceptor dopants in the material, acceptor energy level Ea is formed in the bandgap which is closer to the valence band (VB) as shown in figure 1(b). Together with that fermi level is also shifted downwards, towards the VB, indicated by EF2. When the donor co-dopant is introduced together with the formation of the donor energy level Ed, the fermi level (EF3) is shifted upwards as shown by figure 1(c). For the reference, fermi level EF2 is also shown in the figure. Since the amount of acceptor atoms that are ionized is dependent on the difference between the fermi level and acceptor energy level as given by the equation41, Na― = Na

[

1

(

1 + g exp

Ea ― EF kT

)

]

……………………………………………………………………(1)

where, Na― gives the concentration of ionized acceptors, Na gives the concentration of the acceptors, g is the degeneracy factor, Ea is the acceptor energy level and EF is the fermi level. From the equation it can be seen that the as the value of (EF ― Ea) increases, the denominator value will decrease, increasing the value of Na― . As was observed from figure 1(c), (EF3 ― Ea ) is greater than (EF2 ― Ea), thus co-doping enhances the ionized acceptors thereby increasing the solubility of the acceptors.


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Reduction in the ionization energy can be explained through figure 1(d). When acceptors and donors are introduced in the semiconductor, they will experience strong attractive interaction due to the opposite charges as compared to the repulsive interaction between the identically charged acceptors. This interaction will lower down the acceptor energy level and raise the donor energy level, as a result of which the ionization energy of the acceptors will reduce42. Combining this theory with the above one, we can also say that this will further increase the value of (EF ― Ea) and hence further enhancing the acceptors’ solubility. Also, the selfcompensation mechanism observed for the ZnO can be explained through figure 1(d). Since, ZnO possess intrinsic donor defects, the introduction of acceptor dopants will lead to rise in the donor defect level closer to the CB, lowering their ionization energies and hence the behavior of self-compensation is observed. Co-doped ZnO films have been achieved through various techniques, such as pulsed laser deposition43, magnetron sputtering44, ultrasonic spray pyrolysis45, sol–gel method46, molecular beam epitaxy47, and ion implantation (II)48. Of these, II has the advantages of high productivity and precise in-situ control over the dopant profile, which is realized by controlling the ion beam current49. Unlike most methods, II can surpass the solid solubility limit and yield highly doped semiconductors50,

51

and hence, is employed in this study.

However, as ion bombardment damages the film surface, annealing is necessary to ameliorate this damage as well as to activate the dopants. In this study, we have co-doped ZnO thin films with phosphorus (P) and boron (B) through II and investigated the effects of boron incorporation on the optical emission peaks, structural, elemental and electrical properties of the films compared to only phosphorus doped

ZnO

thin

photoluminescence

films (PL)

through

high-resolution

measurements,

and

X-ray

high-resolution

diffraction X-ray

(HRXRD), photoelectron

spectroscopy (HRXPS), field emission gun scanning electron microscopy (FEG-SEM) and 5 ACS Paragon Plus Environment


conductive atomic force microscopy (C-AFM). Further, we have also studied the effects of varying B dopant concentration on the above mentioned properties of phosphorus doped ZnO thin films. To the best of the authors’ knowledge, this is the first such experimental work to be reported. Phosphorus is a preferred p-type dopant because of its large solubility relative to other dopants and its shallow acceptor level in ZnO; however, p-ZnO achieved in this manner has stability concerns. An ideal donor co-dopant is one that is not a p-type killer but is a reactive dopant that facilitates p-dopant incorporation and activation52. Boron, one such dopant, is used in this study as the donor co- dopant.

2 Experimental details RCA-cleaned n-type silicon with a orientation and a high resistivity of approximately 500 Ω-cm was used as the substrate to deposit ZnO thin films through radio-frequency (RF) sputtering. Before deposition, the substrate was treated with 2% HF solution to remove native oxide. During deposition, argon and oxygen (in the ratio of 4:1) were used as the sputtering and reactive gases, respectively. The process chamber was maintained at a pressure of 2.2 × 10−2 mbar, and deposition was performed at 400°C for 60 min to obtain films of approximately 500-nm thick. After deposition, boron and phosphorus dopants were implanted into the thin films through plasma immersion ion implantation (PIII) technique. Phosphorus was implanted for 70 s23, 53, followed by boron implantation at three implantation durations— 2, 4, and 10 s—to obtain samples C, D, and E, respectively (Table 1); the as-grown and 70-s P-implanted ZnO films are referred to as samples A and B, respectively. Phosphorus and boron plasma were generated using phosphine and diborane gases, respectively. A high negative voltage of 2 kV was applied to the samples during implantation, and the power was maintained at 900 W. The process chamber pressure, RF frequency, and 6 ACS Paragon Plus Environment

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ON time to generate plasma were maintained at 0.1 mbar, 5 kHz, and 10 µs, respectively. After implantation, the samples were subjected to rapid thermal annealing in oxygen ambient at 700°C and 900°C for 10 s to reduce implantation damage and to activate the dopants. The morphological characterizations on the samples were carried out using JEOL JSM FEGSEM system (model: JSM-7600F). The samples were structurally characterized using HRXRD, with the source of X-ray being CuKα1 radiation. Elemental analysis was conducted using electron spectroscopy for chemical analysis (ESCA; AXIS Supra) with a monochromatic (AlKÎ±) 600-W X-ray source. To study the effects of co-doping on ZnO thin films, the film samples were optically characterized through temperature-dependent (18–300 K) PL measurements using 325-nm He–Cd laser. The temperature of the Si array detector was regulated using a Lakeshore temperature controller. Electrical properties of the samples were captured by c-AFM measurement using model: MFP3D Origin, Asylum Instruments.

3 Results and discussion 3.1 Morphological characteristics 3.1.1 FEG-SEM measurements Figure 2 shows the SEM images for the samples A, B, C, D, and E to capture the effect of boron phosphorus co-doping on the surface morphology of ZnO thin films. Figure 2(a) gives the SEM images of the samples for the un-annealed case. From the figure, it can be observed that the as compared to the as-deposited sample A, the implanted samples B, C, D and E showed deterioration in the crystallinity of the film with the incorporation of the dopants in the film. With annealing, as observed from 2 (b), the crystallinity of the co-doped samples was observed to improve with the enhancement in the grain size of the implanted films. 7 ACS Paragon Plus Environment


We also performed the EDS elemental mapping for the samples B, C, D, and E to capture the percentage of phosphorus incorporated. It can be observed from the table 1 that as compared to as-implanted phosphorus mono-doped sample B, there was increase in the amount of phosphorus incorporated in the as-implanted co-doped samples. Further with increase in implantation time, i.e. from sample C to E, there was increase in the solubility of phosphorus in the samples as observed by the percentage incorporated. 3.2 Structural characteristics 3.2.1 HRXRD measurements HRXRD measurements (𝜃-2𝜃 scan) were performed to study the effects of co-doping on the crystalline properties of ZnO thin films which is presented in figure 3. Figure 3 (a) and (b) shows the polycrystalline nature of samples A, B, C, D, and E with the dominant orientation for the cases of as-implanted and annealed at 900oC, respectively. The result for the samples annealed at 700oC is added in the supplementary data, which also showed the similar behavior. These results support the observation made from the SEM analysis. Figure 4 gives the rocking curve analysis for the samples A, B, C, D, and E for the following conditions: (a) un-annealed, and (b) annealed at 900oC. The data corresponding to samples annealed at 700oC is included in the supplementary section. Figure 4 (a) shows the comparative plots of rocking curve for the c-axis orientation and the observed 𝜔 value was around 17.16°, 17.18°, 17.17°, 17.22°, and 17.16° for samples A–E, respectively, as listed in table 2. As evident, with phosphorus doping, the peak shifted toward higher angles, indicating a decrease in the lattice constant due to the substitution of Zn atoms by P atoms; these results are in agreement with the reported theory54. After co-doping with boron, the peak shifted toward lower angles for sample C, indicating an increase in the lattice constant. Per Vegard’s law, the lattice constant as well as the overall behavior of the lattice of a crystal 8 ACS Paragon Plus Environment

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are dependent on the (i) concentration of the dopants and native defects and (ii) difference between the ionic radii of the host and dopant atoms55. Following boron incorporation in the film, the concentrations of dopants, antisites, and interstitials will increase; consequently, the energy gap of the semiconductor will decrease and the interatomic distance will increase, which in turn will increase the lattice constant, shifting the peak towards lower angle. This explanation also accounts for the location of the boron in the lattice: In the lattice of a ZnO film, boron can (i) occupy the octahedral interstice, (ii) replace zinc, (iii) replace oxygen, or (iv) replace phosphorus atoms56. Because the ionic radius of B3+ (0.23 Å) is smaller than those of Zn2+ (0.74 Å), O2− (1.40 Å), and P3+ (0.44 Å), the substitution of these atoms by boron results in a reduction in the lattice constant; the spectra obtained in this study indicate the absence of this behavior. Thus, we infer that boron atoms occupy the octahedral interstice in the ZnO lattice. For sample D, the peak shifted toward higher angle, possibly due to the dominance of phosphorus substitution at the zinc site, which is caused by the improvement of phosphorus solubility of the film. For sample E, again the peak shifted toward lower angles, indicating the dominance of B incorporation at the interstitial site, which is due to the increase in boron concentration. Among the co-doped samples, the as-implanted sample E exhibited the largest shift toward lower angles, affirming the aforementioned hypothesis as sample E has the highest boron concentration among the studied samples. As shown in Figure 4 (b), 4 (c) and Table 2, the peaks of all annealed co-doped samples shifted toward higher angles, indicating that the improved solubility of the phosphorus dopants increased the substitution of zinc atoms by phosphorus atoms. Consequently, the second factor affecting the lattice constant became dominant, shifting the peak toward higher angles. This substitution of the phosphorus at the zinc site is essential factor for the PZn-2VZn complex, which, as mentioned above is acceptor in nature for ZnO. Since the substitution was



observed to enhance for samples C, D, and E, thus it can be inferred that boron co-doping improved concentration of this complex in phosphorus doped ZnO samples. As-implanted samples C, D, and E exhibited broadening of the full width at half maximum (FWHM) relative to sample B; this is because implantation increases crystal defects. The FWHM of samples C and D increased progressively, in line with the aforementioned explanation, but this increasing trend was not seen in sample E. This result is attributable to grain size, as sample E has the largest grains (Table 2): With increase in implantation duration, fewer grain boundary–related defects are induced because boron occupies the interstitial position, narrowing the FWHM. As grain size increases, the number of grain boundary–related defects decreases, narrowing the XRD spectra. With annealing, the FWHM of co-doped samples decreased over the annealing temperature range, indicating that annealing improved sample crystallinity. Table 2 lists the grain size and c-axis strain derived from the XRD data57 for samples B, C, D, and E, both as-implanted as well as annealed at 700°C and 900°C. All calculations were performed considering the wurtzite structure of ZnO thin films. After co-doping, the grain size of samples C, D, and E decreased compared to that of sample B; this is due to the deterioration of the crystalline quality of the film, which in turn is caused by the lattice distortion induced by additional boron implantation. The grain sizes of the co-doped samples increased with annealing, indicating improvement in crystallinity of the film. The largest grains were seen in sample E annealed at 900°C. The negative strain value (i.e., compressive strain) affirms that the lattice constant decreases with doping. Strain was observed to decrease on co-doping indicating increase in lattice constant which was also evidenced from the peak shift. The annealed samples exhibited compressive strain due to the relatively higher incorporation of P atoms at Zn sites as mentioned earlier. In summary, HRXRD data revealed the location of B atoms in the crystal as well as improvements in the solubility of P atoms 10 ACS Paragon Plus Environment

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post boron implantation. For sample E, co-doping resulted in improved grain boundaries. Codoping degraded the crystalline quality of the ZnO films relative to only phosphorus-doped films; nevertheless, annealing drastically improved the crystallinity, keeping their candidature safe for optoelectronic application. 3.3 Elemental characteristics 3.3.1 HRXPS measurement To study the elemental composition as well as the chemical and binding energy states in the films, the samples were characterized through XPS measurements. Figure 5 shows the binding energy corresponding to the O 1s peaks for samples A, B, C, D, and E for the following conditions: (a) un-annealed, (b) annealed at 700°C, and (c) annealed at 900°C. Figure 5(a) shows O 1s peaks at 531.79 (peak 1), 532.06 (peak 2), and 533.39 (peak 3) eV for sample B; at 531.50, 532.93, and 533.45 eV for sample C; at 531.78, 533.08, and 533.23 eV for sample D; and at 531.70, 532.66, and 532.66 eV for sample E. The peaks at around 531 eV are attributable to the Zn–O bond58, that around 532 eV to the P–O bond58, and those around 533 eV to the OH− ions53. Peak 1 increasingly shifts leftward on comparing sample B with samples C, D, and E, indicating a reduction in the bonding energy between Zn and O atoms with boron implantation; these results can be explained using the XRD data: Because the inter-atomic distance increases due to co-doping, Zn and O atoms are loosely bonded to each other and thus have low bonding energy. Moreover, the FWHM reduced with co-doping, indicating a change in the coordination number contributing to the shape of the peak. The binding energy of peak 2 shifted rightward after co-doping, indicating enhanced bonding between the P and O atoms. This can be explained using the shielding effect and the solubility of the phosphorus atoms. The nuclear charge of the oxygen on 1s orbital electrons is shielded by the outer second orbital electrons; as co-doping introduces more phosphorus 11 ACS Paragon Plus Environment


atoms into the film, the number of P atoms surrounding the O atoms increases, resulting in increased sharing of the valence electrons and reduced screening of the nuclear charge. These actions increase the binding energy of the core electron. Among the co-doped samples, the binding energy for P–O bond increased from samples C to D and then decreased for sample E. This decrease could be due to boron doping leading to the formation of bonds between boron and phosphorus atoms, thus deteriorating the attraction between phosphorus and oxygen atoms, thereby reducing their binding energy. This phenomenon is strongest in sample E as it has the highest boron concentration. With increase in boron doping, the intensity of the peak was observed to get reduced, suggesting a decrease in the number of O atoms on the surface because implantation forces the surface atoms to move deeper. Furthermore, the number of OH− ions increased on co-doping of ZnO film by boron, as evidenced by the rightward shift of peak 3. Among the co-doped samples, the peaks increasingly shifted leftward from samples C to E, indicating a progressive reduction in the number of OH− ions with increase in boron implantation duration. On annealing, all three peaks exhibited ambiguous behaviors; this result warrants further analysis. Figure 6 presents the binding energy corresponding to P 2p peaks for samples A, B, C, D, and E for the following conditions: (a) un-annealed, (b) annealed at 700°C and, (c) annealed at 900°C. Figure 6(a) depicts the P 2p peaks for the as-implanted samples. For sample B, the peak is at around 133.13 eV, corresponding to the P–O bond in PO3−4 ions59. For sample E, the peaks are at around 134.08 and 134.76 eV; the first peak is attributable to the P–O bond in PO3− ions59, whereas the second can be attributed to the P–B bond; this is because boron is less electronegative than is oxygen and hence the P–B bond has lower binding energy than does the P–O bond53. For samples C and D, the peaks were of very low intensity, almost at the level of noise; this is due to the deterioration of the film surface, as evidenced by the XRD data. The intensity of the peaks deceased with co-doping, indicating a reduction in the 12 ACS Paragon Plus Environment

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number of phosphorus atoms on the surface owing to boron bombardment. Among the codoped samples, compared with the as-implanted samples, both peaks of sample E shifted rightward on annealing at 700°C (Figure 6 (b)) and 900°C (Figure 6 (c)). This behavior can be explained using the XRD data, which revealed that the inter-atomic distance decreases with annealing, which in turn strengthens the bonding between the atoms, thereby increasing the binding energy. Figure 7 presents the B 1s peaks for the co-doped samples C, D, and E for the following conditions: (a) un-annealed, (b) annealed at 700°C, and (c) annealed at 900°C. As evident from Figure 7 (a) and 7 (b), the B 1s peak was absent in the as-implanted samples C and D, and these peaks appeared only after annealing, confirming the presence of boron in the samples. The figure shows the presence of three types of boron species. For as-implanted sample E, the peaks were at 189.49 eV (peak 1), 191.67 eV (peak 2), and 192.88 eV (peak 3); these are attributable to the BHx–O bond60, B–P bond, and B–O bond (pertaining to the B3+ state)61, respectively. The peak at around 191.67 eV is attributable to the B–P bond for reasons similar to those provided for the P 2p peaks, with phosphorus being less electronegative than is oxygen. Regarding annealed sample E, compared with as-implanted samples, all peaks shifted rightward on annealing at 700°C (Figure 7 (b)) and 900°C (Figure 7 (c)); this is because of the increase in the binding energy induced by the reduction in the inter-atomic distance, as explained earlier. On annealing at 700°C and 900°C, the binding energies of peaks corresponding to P–B and P–O bonds for samples C, D, and E exhibited anomalous behavior, and this result warrants further analysis. In summary, the XPS data confirmed the incorporation of boron atoms in the film in line with the XRD results. Effect of co-doping on the formation and behaviour of Zn-O, P-O and P-B bonds has been discussed here with further report on the effect of annealing on them. Effect of crystal matrices on the binding energy of the peaks has also been talked about in this section. 13 ACS Paragon Plus Environment


3.4 Optical properties 3.4.1 PL measurement Figure 8 presents the low-temperature PL plots for samples A, B, C, D and E for the following conditions: (a) un-annealed, (b) annealed at 700°C, and (c) annealed at 900°C. The as-implanted co-doped samples C, D, and E showed photoluminescence peaks in the 2.8–3.6 eV range (Figure 8 (a)); furthermore, compared with phosphorus-doped sample B, the nonradiative emissions from these films decreased after boron implantation, possibly due to complex formation after co-doping, as evidenced by the relatively narrow PL spectra in the UV region (the spectra typically broadens on implantation owing to damage induced by the implantation process). This result indicates the annihilation of defects by some physical phenomena. Co-doping reduces the formation energy of the complexes; this could be the dominant factor in the narrowing of the spectra of the co-doped samples. Among the codoped samples, increase in boron implantation duration decreased the spectral width and increased the photoluminescence intensity in the UV region; in other words, the optical properties of the films improved with increase in boron concentration, affirming the enhancement in PL intensity as a result of suppression of donor defects. Figure 8 (b) and 8 (c) shows the effects of annealing temperature on co-doped samples; the low-temperature PL spectra indicated that annealing improved the crystalline quality of samples C, D, and E compared with sample B. Defect peaks were suppressed in the 2.9–3.5 eV range for samples annealed at 700°C and in the 3.0–3.4 eV range for samples annealed at 900°C; in addition, the peaks of samples C–E narrowed relative to that of sample B. Among the co-doped samples, the photoluminescence peaks of sample E were sharper than those of samples C and


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D (Figure 8 (b) and 8 (c)), indicating stronger annihilation of defects in the former, further affirming the HRXRD finding that annealing improves film crystallinity. Donor–acceptor pair (DAP) peaks at around 3.24 eV62 were observed for as-implanted and 700°C-annealed samples C and D, respectively, but not for sample E (Figure 8 (a) and 8 (b), respectively), affirming that an increase in boron concentration passivates donor impurities in the film, thereby increasing the solubility of phosphorus atoms. This peak was absent in 900°C-annealed sample D as well (Figure 8 (c)), indicating passivation of donor impurities post annealing due to activation of the acceptor dopants. Herein, we discuss the mechanism pertaining to the formation of acceptor and donor defect levels following co-doping as well as its effect on the self-compensation phenomenon (see Figure 9). As-grown ZnO has native donor defect levels (indicated by level ED in figure 9 (a)), and achieving p-type behavior by adding acceptor atoms in the film triggers the selfcompensation effect; thus, to achieve p-type behavior, the films must have a substantially higher concentration of acceptor atoms than donor defects. This creates a surplus of holes in the films, realizing p-type behavior. Adding phosphorus to the films leads to the formation of an acceptor level (indicated by energy level EA in the figure 9 (b)) in the bandgap as well as deals with the phenomenon of self-compensation by passivating the donor defects, as observed from figure 9 (c) where cross (x) shows the passivation effect. By contrast, adding boron improves phosphorus incorporation and in addition to native donors, forms a donor level in the bandgap (as shown in figure 9 (e), with the addition of new energy level ED2, with previous donor level now referred as ED1). As seen by PL spectra corresponding to figure 9 (d), no DAP emission was observed for as-implanted Sample B, suggesting the lack of detectable emission from the donor and acceptors possibly due to self-compensation and donor passivation effect. With boron incorporation, the DAP peak was apparent for samples C and D, affirming the presence of both donor level (related to boron) and acceptor level 15 ACS Paragon Plus Environment


(related to phosphorus) as observed from the PL spectra corresponding to figure 9 (e). This suggests that in addition of creating a donor defect level, ED2, boron doping improves phosphorus solubility shown by increased hole concentration in EA level in figure 9 (e). This enables the sample to retain sufficient hole concentration even after passivation effect, as can be seen from figures 9 (f) and (g). For samples D and E, the intensity of the peak increased, indicating improved phosphorus solubility due to the increase in boron concentration. On annealing, the peak disappeared for sample D (at 900°C) and sample E (at 700°C and 900°C), evidencing that annealing passivated donor impurities by activating the acceptor dopants and that dopant activation energy reduced as boron concentration increased (as observed for sample E). Free-acceptor (FA) peaks at around 3.31 eV53, 63, 64 was observed for the samples B, C, D, and E, as exhibited by figure 6 and increase in the intensity of the peak was observed for the codoped samples. For as-implanted case, as observed from figure 8 (a), the peak was dominant for sample E but with annealing the dominance of the peak emerged for all the co-doped samples. This observation can be related to the increase in the solubility of the phosphorus atoms with boron co-doping thereby enhancing the emission which as was also concluded from the DAP peak analysis for sample E. On annealing, more acceptor dopants are activated, facilitating increased free emission. Among the co-doped samples, the intensity of this peak increased with annealing temperature as a result of dopant activation, and sample C exhibited the highest intensity, indicating the higher concentration of free acceptor defect level in this sample. Samples C, D, and E exhibited acceptor-bound exciton (A°X) peaks at around 3.35 eV65 (Figure 8 (b) and 8 (c)). The signature of this peak was observed for all implanted samples (samples B to E) but only after annealing, further evidencing the effect of annealing on activating acceptor-based excitonic emission, which improves the optical characteristics of 16 ACS Paragon Plus Environment

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the ZnO thin films. For the co-doped samples, this peak was observed after annealing at 700°C, but for sample B, it was observed only after annealing at 900°C, indicating that codoping improves the acceptor-based UV emission characteristic of the samples. This observation is in line with the HRXRD findings. The presence of these peaks supports that co-doping with boron improves the near-band-edge emission of the samples, which is a component of UV emission, thus enhancing the feasibility of the use of these films in optoelectronic devices. The co-doped samples also showed an emission peak attributable to interstitial boron (BI) at around 3.0 eV66 (Figure 8 (a)), confirming the HRXRD findings regarding the location of the boron atom in co-doped ZnO. Peak intensity increased from samples C to E, which was expected as increase in implantation duration increases the dopant concentration. Figure 10 presents the temperature-dependent PL spectra for samples C, D, and E annealed at 900°C performed to validate the peaks and capture the behaviour of the samples towards exposure to temperature. The FA peak did not shift over the temperature range, but the peak intensity reduced, as reported in the literature53; this behavior is characteristic of FA peaks67. The DAP and A°X peaks redshifted, as reported in68: with change in temperature, the positions of the exciton peak maxima follow the changes in the bandgap of the crystal. Furthermore, the A°X peak persisted up to 300 K, making these films potential candidates for room-temperature optoelectronic applications. With increase in temperature, the intensity of the peaks reduced and the peaks broadened and this decrease in the luminescence intensity is described by the Arrhenius equation69: 𝐼(𝑇) =

𝐼0 [1 + 𝐶 𝑒

( ― 𝐸𝐴 𝑘𝑇)

……………………………………………………...………...............(2)

]

where I0 is the initial integrated PL peak intensity at T = 18 K, EA is the acceptor activation energy, and C is the peak fitting constant. Using this equation, EA for the FA peak was 17 ACS Paragon Plus Environment


calculated to be around 64±0.35, 137±0.09, and 55±0.37 meV for samples C, D, and E, respectively. For sample B, EA was 125 meV, affirming the presence of shallower acceptor levels in co-doped films. The acceptor energy level with co-doping has been reported to be 119 meV70; thus, per the authors’ best knowledge, 55±0.37 meV is the lowest acceptor energy level reported yet. We also performed the reliability study of the co-doped samples. Figure 11 gives the low temperature (18 K) PL measurement for the samples C, D, and E, annealed at 900oC performed after the duration of 13 months. The plots are shown in the semi-log format to give a clear comparative picture. The vertical shift in the spectra is due to the signal to noise (SNR) ratio variation, which is a tool factor. For sample C (figure 11 (a)), we could observe that the peaks DAP, FA and AoX which had their signatures in the first set of measurement, existed in the spectra obtained post second measurement as well. Similar results were observed for samples D and E as well (figures 11 (b) and (c)), where the FA and AoX peaks showed their presence in the second spectra captured after 13 months. These results thus show the stability in acceptor based behaviour ZnO achieved as the effect of co-doping.

3.5 Electrical properties 3.5.1 c-AFM measurement I-V characteristics measured through c-AFM for the un-doped sample A and implanted samples B, C, D, and E, annealed at 900oC, is shown in figure 12. As can be observed from the figure, p-n junction type of behaviour was shown by the implanted samples. The OFF current was high for the samples since the characterization was performed on directly on the films without any fabricated contacts. The ON current for samples B, and C did not show much variation with boron co-doping which can be related to the presence of donor levels in 18 ACS Paragon Plus Environment

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both the samples, as observed from PL study with the presence of DAP peak. This leads to lower concentration of the holes in the samples thereby giving low current. For boron codoped samples D, and E, the I-V characteristic showed that the ON current reached the compliance value at the voltage

Enhancing Acceptor-Based Optical Behavior in Phosphorus-Doped

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