Influence of Oxygen Plasma on the Growth, Structure, Morphology

p-type transparent conducting CuBr thin films were grown by thermal evaporation of CuBr followed by oxygen plasma treatment. Efficient incorporation o...
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Influence of Oxygen Plasma on the Growth, Structure, Morphology, and Electro-Optical Properties of p‑Type Transparent Conducting CuBr Thin Films Rajani K Vijayaraghavan,*,† Anthony P. McCoy,§ Lalit Chauhan,§,∥ Aidan Cowley,†,‡ Richard J. H. Morris,⊥ Stephen Daniels,† and Patrick J. McNally‡ †

National Centre for Plasma Science and Technology, ‡The Rince Institute, School of Electronic Engineering, §School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9 Ireland ⊥ Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom S Supporting Information *

ABSTRACT: p-type transparent conducting CuBr thin films were grown by thermal evaporation of CuBr followed by oxygen plasma treatment. Efficient incorporation of oxygen into the CuBr films was revealed, and the influence of plasma exposure on the electrical, structural, optical, and electronic properties of CuBr films was investigated. Xray diffraction (XRD) analysis indicated the Zincblende structure of the oxygen plasma treated CuBr (OCB) films with the formation of nanocrystalline grains preferentially oriented along the (111) direction. p-type conducting OCB films show >85% average transmittance along with hole concentration, conductivity and Hall mobility values of ∼1019 cm−3, ∼1.5 S cm−1 and ∼0.45 cm2 V−1 s−1, respectively. The X-ray photoelectron spectra of the OCB films demonstrated that the plasma exposure resulted in a significant increase in the O 1s signal at the surface of OCB films. On the basis of the experimental results from the Hall measurements and X-ray photoemission, a possible explanation comprising the formation of a surface CuO layer is also proposed to elucidate the increase in the p-type conductivity of the OCB films. Strong room temperature emission of OCB films at around 416 nm was also observed, the intensity of which decreased with the increase of oxygen plasma exposure time. The results present oxygen plasma exposure as a simple and promising technique for the production of CuBr-based p-type materials for future transparent electronics.

1. INTRODUCTION

CuBr is a I−VII compound semiconductor material with great potential for short-wavelength applications due to its extremely large excitonic binding energy (∼108 meV) and direct band gap (∼3.1 eV at 300 K).10−12 It exhibits good transparency (>80%) throughout the major portion of the visible region (above 420 nm) and shows strong absorption in the UV-violet region of the spectrum.13 Although, there are numerous reports on the excitonic and nonlinear optical properties of CuBr,13−19 there are relatively few studies on the semiconducting properties of this material.20−22 The poor hole conductivity of as-deposited (ASD) CuBr films represents a major limitation for the adoption of these materials, and thus the deposition of good quality CuBr films with increased p-type conductivity would be an extremely important development for future applications of this material system. CuBr is naturally a p-type material, with the p-type conductivity originating either from copper deficiency (Cu1−δBr) or via doping with oxygen.20 The semiconducting properties of polycrystalline CuBr have been investigated by Knauth et al. using the Hall effect, capacitive measurements and

There is a rapidly growing demand for transparent conducting materials (TCMs) with good electrical conductivity and high transparency in the visible region of the electromagnetic spectrum. These materials have been extensively used for flexible electronics, displays, sensor arrays, and photovoltaics as transparent conducting electrodes.1−4 TCMs currently used in such applications include metal oxides, the most common of which is indium tin oxide (ITO). However, ITO suffers from higher cost, scarcity of In raw material, and its ceramic nature which leads to brittle and easily damaged films. It has been reported that transparent oxide based thin film transistors (TFTs) could be a key solution to the challenging requirements of improved display technologies.5,6 For example, transparent InGaZnO films are reported to be useful in TFTs for the production of next generation flat panel displays,5 similarly copper oxide film properties are investigated and their TFT applications are reported.7,8 As most of the current transparent conducting materials (TCMs) are n-type,9 their cutting-edge applications based on p−n junctions are limited due to the unavailability of suitable p-type TCMs. Hence it is very desirable to develop high performing p-type TCMs for future transparent electronics. © 2014 American Chemical Society

Received: June 1, 2014 Revised: September 8, 2014 Published: September 10, 2014 23226

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Mott−Schottky analysis.22 Mott−Schottky analysis of polycrystalline CuBr confirmed this material to be p-type with an acceptor concentration of (2 ± 1) 1016 cm−3.22 According to these studies, oxygen impurities can increase the hole concentration and hence can act as effective acceptors for CuBr. They reported a hole concentration and mobility of (9 ± 2) × 1017 cm−3 and (0.4 ± 0.1) cm2 V−1 s−1, respectively. The inclusion of oxygen in their Hall samples occurred due to atmospheric exposure. Again, it was noted that the incorporation of oxygen was mainly near the surface because of the very slow oxygen bulk diffusion at room temperature. In this work, we investigated the effect of oxygen plasma treatment as a method for the efficient incorporation of oxygen into the CuBr crystal, to improve the p-type conductivity of CuBr films. Although we have recently reported the use of p-type CuBr films for solar cell and diode applications,12 there was a lack of knowledge on the detailed mechanisms of oxygen incorporation into the film. In order to understand the effect of plasma exposure time on the structural, morphological, electrical, optical and photoemission properties of the films, they have been systematically analyzed using techniques such as XRD, atomic force microscopy (AFM), scanning electron microscopy (SEM), Hall effect, X-ray photoelectron spectroscopy (XPS), UV−vis absorption spectroscopy, photoluminescence spectroscopy (PL), and secondary ion mass spectrometry (SIMS). The investigations provide a good understanding of the growth, structural and optoelectronic properties and p-type conduction mechanism of novel OCB films, which will be essential for their use in future optoelectronic and photovoltaic applications. The important novel outcome from our work is the demonstration that we can use oxygen plasma exposure as a tool to effectively incorporate oxygen into a CuBr film to increase its p-type conductivity for use as a p-type TCM with good violet emission. In addition, a possible explanation for p-type conductivity enhancement is also proposed and verified.

Table 1. Values of the Plasma Parameters Used for the Oxygen Doping of the CuBr Films process parameters

used values

flow rate of oxygen (sccm) flow rate of Argon (sccm) chamber pressure (mbar) power (W)

80 20 6.6 × 10−2 300

∼0.9 nm. The optical properties such as absorption and PL of the films were investigated using a PerkinElmer Lambda 40 UV−vis spectrometer and a Jobin Yvon-Horiba LabRam800 spectrometer, respectively. The X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG Microtech electron spectrometer at a base pressure of 1 × 10−9 mbar. The photoelectrons were excited with a conventional Mg Kα (hν = 1253.6 eV) X-ray source and an electron energy analyzer operating at a 20 eV pass energy, yielding an overall resolution of ∼1.2 eV. The electrical properties are measured using a Hall measurement (HL 5500 PC) setup in the van der Pauw configuration in which CuBr/glass samples with gold ohmic contacts are used.

3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Properties. The structural properties of the ASD CuBr and the oxygen plasma exposed films for various time intervals (OCB1 (1 min), OCB3 (3 min), and OCB5 (5 min)) were investigated using XRD analysis. All films show preferential orientation along the (111) direction corresponding to a 2θ value of ∼27.1° (Figure 1a). Peaks appear at ∼45° and ∼53.3° corresponding to the (220) and (311) planes of CuBr, respectively. The observed peaks are in good agreement with a previous report on CuBr thin films.14 There was no indication of the generation of noticeable structural deformities in the zincblende CuBr upon exposure to oxygen plasma up to periods of 5 min. However, a slight variation in the full width at half-maximum (fwhm) of the (111) peak is observed with the plasma exposure of the films. Figure 1b demonstrates the changes in the fwhm of the CuBr (111) peak and the average crystal size as a function of the length of oxygen plasma exposure. The fwhm values were extracted by applying a Gaussian fit to the high resolution (111) XRD peaks. The average crystal sizes were calculated using the Scherrer equation.23 A decrease in the fwhm which correlates with an increase in the average crystal size is noted with the increase of the oxygen plasma exposure time. The fwhm decreases from ∼1090 arc sec for the ASD CuBr sample to ∼872 arc sec for the OCB5 sample. Conversely, the average crystal size increases from ∼31 nm for the ASD CuBr sample to ∼41 nm for the OCB5 film. This is most likely due to the bombardment by the energetic argon and oxygen species from the plasma, which induces a combinational effect similar to a low temperature annealing and plasma induced microstructural evolution of the film. During the plasma bombardment, the atoms on the surface and subsurface of the film gain extra energy and the increased mobility helps them to relax to lower free energy sites. This enhances the growth of the grains with lower free energy. A similar enhancement in the average grain size with oxygen plasma exposure has been reported in polycrystalline ITO films.24 Furthermore, the effect of oxygen plasma irradiation on the LiMn2O4 films has also been reported.25

2. EXPERIMENTAL DETAILS CuBr thin films of thickness 380 ± 10 nm were deposited by the thermal evaporation of CuBr powder (Sigma-Aldrich, 99.999%) onto Si (2 × 2 cm, n-type (100), 1−10 Ω cm resistivity) and glass substrates (3 × 3 cm, VWR microscope slides) using an Edwards 306A vacuum evaporator with a base pressure of ∼5 × 10−7 mbar (distance from the evaporation source to the substrate was ∼10 cm and the evaporation was performed at a current flow of 2.2 A). Prior to deposition, the substrates were cleaned using acetone, methanol, and deionized water in an ultrasonic bath. The thin films of CuBr on Si and glass substrates were then transferred to a plasma chamber (Oxford Instruments RIE, distance between the plates = 6 cm, volume of the chamber used = 0.00369 m3), in which a mixture of O2 and Ar (4:1) was used as the working gas at an RF power and chamber pressure of 300 W and 6.6 × 10−2 mbar, respectively. The films were then exposed to oxygen plasma for various time intervals (1, 3, and 5 min). The plasma parameters used in the experiment are given in Table 1. X-ray diffraction (XRD) analysis was carried out using Cu Kα radiation of wavelength 1.54 Å from a Bruker D8 instrument to determine the crystal structure of the CuBr films using a Bragg−Brentano θ−2θ configuration. The morphological properties of the films were examined by AFM (Nano Rule) and SEM (Zeiss EVO LS-15). The diffusion of oxygen into the film was studied using SIMS with an ultralow energy, Atomika 4500 Ultra-Shallow SIMS, capable of depth profiling at a very high resolution of 23227

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Figure 1. (a) XRD pattern of the as-deposited and the 1, 3, and 5 min oxygen plasma treated CuBr films deposited on Si (100) substrates (b) Variation of fwhm and average crystal size as a function of plasma exposure time. All the scans were performed under the same experimental conditions.

The surface morphology of the ASD CuBr and OCB films was investigated using AFM (Figure 2) and SEM (Figure 3)

plasma exposure time is in agreement with the trend observed in the XRD analysis. The RMS roughness of the ASD CuBr film was 6.4 nm and it decreased to 3.4 nm for the OCB1 film and then increased to 8.3 and 9.4 nm, respectively, for OCB3 and OCB5 films. The smaller RMS roughness value of the OCB1 and the larger roughness values of the OCB3 and OCB5 samples compared to the ASD CuBr samples could be explained due to the effect of lower and higher surface sputtering as a result of the shorter and longer plasma exposure, respectively. Figure 3 shows the SEM images of the ASD CuBr and OCB samples. Similar SEM images were observed for the ASD CuBr (Figure 3a) and OCB1 (Figure 3b) films; however, OCB3 (Figure 3c) and OCB5 (Figure 3d) exhibit the occurrence of “black spots” (more in the OCB5) on the surface, representative of the formation of pits due to plasma induced surface damage. The formation of the surface pits are further confirmed by energy dispersive X-ray spectroscopic analysis (data not shown). The OCB5 sample further reveals the formation of bigger columnar grains which is in agreement with the AFM results. 3.2. SIMS Concentration Profiles and XPS Analysis. The presence and diffusion of oxygen in the OCB film is investigated using the concentration profile of oxygen in the film. The count intensities of various elements (O, Cu, and Br) present in the ASD CuBr and OCB1 samples are plotted against the sputtering time (Figure 4). Cu and Br profiles are almost uniform throughout both films. However, an order of magnitude increase in the oxygen count intensity in OCB1 with respect to the ASD CuBr film is demonstrated, indicating effective incorporation of oxygen in to the CuBr film as a result of oxygen plasma exposure. OCB3 and OCB5 samples have similar Cu and Br concentration profiles (Supporting Information, Figure S1). However, there was a slight increase in the count intensity of O, as a function of plasma exposure time. As the exposure time increases, the effect of surface sputtering also increases resulting in plasma induced surface damage in the film, which is detrimental for optoelectronic applications. The surface chemistry of the CuBr films were analyzed using XPS analysis and the results are shown in Figure 5. The ASD film contains a Cu:Br ratio of ∼1:1 as calculated using the integrated areas and elemental sensitivity factors of the Cu 2p3/2 and the Br 3d peaks, respectively. There is a very limitied oxygen signal detected in the ASD film as shown in Figure 5a. However, the oxygen plasma treatments result in a significant

Figure 2. AFM images of the (a) as deposited and (b) 5 min oxygen plasma exposed CuBr films deposited on Si (100) substrate. The imaging was performed under identical conditions and the scan size in both cases is equal to 500 nm × 500 nm.

Figure 3. SEM images of the (a) ASD CuBr, (b) OCB1, (c) OCB3, and (d) OCB5 films. Presence of black spots in the OCB3 and OCB5 images indicate the formation of pits as a result of plasma induced surface damage for longer exposed films.

analyses. The larger grain sizes (>100 nm) measured from AFM analysis compared to that from XRD are most likely due to the presence of agglomerated grains as observed in the AFM image.26 Figure 2a demonstrates the presence of well-packed and more uniform grains in the ASD CuBr in contrast with the appearance of nonuniform and damaged grains in the OCB5 (Figure 2b). The increase of the average grain size with the 23228

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932.2 eV can also be detected in all samples suggesting CuBr is still present within the sampling depth of the XPS after plasma treatments. The Br 3d spectra was peak fitted using a Voigt line profile with a Shirley-Sherwood background and is shown in Figure 5c. The peak has overlapping spin−orbit components which are separated by 1.04 eV. The profile of the Br 3d peak is very similar in all the films suggesting that the Br present in all the samples is in an identical chemical state (CuBr). However, the presence of CuBr2 cannot be conclusively ruled out as the peak profiles and binding energy positions of the Br 3d in both CuBr and CuBr2 are almost identical27,28 and cannot be sufficiently resolved. It is important to note that there are multiple component peaks within the O 1s as shown in Figure 5d, which have not been peak fitted. Given the Br 3d peak profile remains unchanged after oxygen treatment, any additional oxygen component peaks in the O 1s are likely due to C−O or H−O contaminant species at the surface. 3.3. Optical and Electrical Properties. The optical properties of the ASD CuBr and the OCB samples were investigated using room temperature absorption, transmission, and PL studies. Both the ASD CuBr and OCB1 films show an average transmittance of ∼90% at wavelengths above ∼420 nm (Figure 6). However, the average transmittance decreases with oxygen plasma exposure time, and it is above 70% for OCB5. The interference fringes present are due to the interference between the air/CuBr and CuBr/glass substrate interfaces. Since the transmittance spectra of the OCB films do not show

Figure 4. SIMS depth profile of the ASD CuBr and OCB1 films deposited on Si substrate.

increase in the O 1s signal intensity due to the incoportaion of oxygen into the CuBr films as seen in the OCB1, OCB3 and OCB5 survey scans. The Cu 2p spectra shown in Figure 5b indicate the presence of CuO (Cu2+) species in the plasma treated samples which was not present in the ASD film. It should be noted that the ASD Cu 2p peak profile is very similar to that of metallic Cu which is in agreement with Vasquez.27 The original CuBr Cu 2p peak at a binding energy position of

Figure 5. XPS spectra of the as deposited (ASD) CuBr films and three oxygen plasma exposures of 1 (OCB1), 3 (OCB3), and 5 min (OCB5). (a) Survey scans, (b) Cu 2p spectra, (c) Br 3d and Cu 3p spectra, and (d) O 1s spectra. 23229

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to the effect of a surface oxide layer formation as well as plasma induced surface damage. The luminescence properties of the ASD CuBr and the OCB films are investigated and shown in Figure 8. The free exciton

Figure 6. Transmittance spectra of the ASD CuBr, OCB1, OCB3, and OCB5 films deposited on glass substrates. The spectra were recorded under identical experimental conditions.

significant influence on the transmission edges with respect to the ASD CuBr films, it is expected that the thickness of the formed CuO layer on the OCB film surface would be minimum. Additionally, CuBr signals are detected in all four samples within the sampling depth (∼7 nm) of the XPS in section 3.2, suggesting plasma treatment is surface localized. A typical absorption spectrum of the OCB1 sample is shown in the inset of Figure 7. Both the ASD CuBr and the OCB

Figure 8. Room temperature PL spectra of ASD CuBr, OCB1, and OCB5 samples.

luminescence in the ASD CuBr bulk material is attributed to the lowest energy triplet exciton, named Zf, whose energy is slightly lower than that of the Z1,2 free exciton, due to the exchange interaction.30 The high intensity luminescence peak of CuBr originates from the Zf free exciton.31 The spectra demonstrate a strong emission at around 2.97 eV (∼417 nm), which is in close agreement with the previous reports on the room temperature PL studies in CuBr.13,32 There was no significant reduction of the intensity of the PL peaks in the OCB1 and OCB3 films with respect to the ASD CuBr film. The fwhm of the strong PL spectra of CuBr and OCB1 obtained using Gaussian fitting is ∼8.5 nm, indicating the good optical and surface quality of the films. Both the OCB1 and OCB5 samples exhibit asymmetric tailing to the long wavelength side, which is attributable to the presence of plasma induced damage and surface states. The massive reduction in the luminescence intensity and the broadening of the PL peak in the OCB5 sample is also expected due to the surface damage as a result of significant oxygen and argon ion bombardment on the film surface owing to the longer plasma exposure. The effect of PL quenching due to surface defects was previously reported in Si wafers.33 The electrical properties of ASD CuBr and OCB films deposited on glass substrates are determined by Hall effect measurements using the van der Pauw method. Figure 9 shows the variation of electrical conductivity, hole concentration, and Hall mobility (inset) as a function of oxygen plasma exposure time. The ASD CuBr films are very poor conductors (3 min) due to plasma induced surface damage generation. The results show that oxygen plasma exposure can be used as a simple technique for the deposition of CuBr-based p-type TCM, which could find potential applications in future transparent electronics.

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ASSOCIATED CONTENT

* Supporting Information S

SIMS concentration profile of the OCB5 films. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Variation of carrier concentration, conductivity, and Hall mobility (inset) as a function of oxygen plasma exposure time.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

OCB5 sample, which is expected due to the reduced grain boundary scattering owing to the highest average grain size of this sample. The major mechanism that is responsible for the enhancement in the conductivity of the OCB samples is the growth of the surface oxide layer as a result of oxygen plasma exposure, which is obvious from the XPS data (Figure 5). CuO films have already been reported as a p-type semiconductor with a high hole concentration of ∼1019 cm−3.35 The variation of the conductivity and carrier concentration values as a function of the plasma exposure time (Figure 9) is in agreement with the trend in the Cu2+ (CuO) peak intensities, which supports the aforementioned proposed explanation. The slight reduction in the Cu2+ XPS peak intensity as well as the decrease in the respective conductivity and hole concentration of the OCB5 sample could be attributed to the increased sputtering of the sample as a result of longer plasma exposure, which could possibly remove the formed CuO layer from the surface. It should be noted that the increase in the conductivity of the OCB films due to the contribution from the introduction of substitutional oxygen onto the Br sites to create acceptor levels in the CuBr lattice cannot be excluded. However, evidence for this mechanism is extremely difficult to acquire due to the large oxygen signal present in the CuO layer.

∥

Department of Physics, University of Petroleum and Energy Studies, Bidholi, Dehradun, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of Science Foundation Ireland′s Research Frontiers Programme (06/RFP/ENE/027) and Enterprise Ireland’s Commercialisation Programme (CFTD/ 07/IT/331). This work was also part-funded by the Irish Higher Education Authority PRTLI “INSPIRE” project, and Science Foundation Ireland′s Strategic Research Cluster Programme (“Precision” 08/SRC/I1411).

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REFERENCES

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4. CONCLUSIONS Oxygen plasma exposure as an effective method to incorporate oxygen into the CuBr lattice to make it highly p-type conducting has been demonstrated. OCB films have been presented as an alternative for p-type TCMs with an added advantage of good violet emission. The influence of oxygen plasma exposure on the growth and properties of transparent ptype conducting CuBr film has been investigated. XRD results confirmed that all OCB films are crystallized in the zincblende structure with texturing along the (111) direction. p-type conducting OCB films exhibit >85% transmittance along with hole concentration, conductivity, and mobility values of ∼1019 cm−3, ∼1.5 S cm−1, and ∼0.45 cm2 V−1 s−1, respectively. The enhancement in the p-type conductivity of the CuBr films as a result of oxygen plasma exposure was explained mainly due to the formation of CuO on the surface of the CuBr films which is known to be a good p-type semiconductor. The proposed explanation was further verified using the XPS data, in which there is an increase in the Cu2+ peak intensity up to the OCB3 sample followed by a decrease in the OCB5 sample. SIMS data 23231

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