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Al doping for bipolarity induction in transparent conducting CuInO2 and its application in diode fabrication
Materials Science & Engineering B 255 (2020) 114520
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Al doping for bipolarity induction in transparent conducting CuInO2 and its application in diode fabrication
T
Bindu G. Naira, Hilal Rahmana, Vikash Sharmab, G.S. Okramb, Uday Deshpandeb, V. Ganesanb, ⁎ Rachel Reena Philipa, a b
Thin Film Research Lab, Union Christian College, Aluva, Kerala, India UGC-DAE Consortium For Scientific Research, Khandwa Road, Indore 452 001, Madhya Pradesh, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Thin films Crystallization Electrical conductivity P-n junction diode
Al doping is used in transparent conducting CuInO2 (CIO:Al) thin films for producing bipolar electrical conductivity. The doped thin films of electrical conductivity ~2 to 4 S/cm and mobility 100 to 101 V/cm2 are deposited by oxygen plasma assisted reactive evaporation technique. The change in conductivity from n-to ptype with the variation in doping atomic percentage is confirmed by multiple techniques like hot probe, hall and Seebeck measurements. The as deposited amorphous films are found to assume 3R poly type delafossite structure after post air annealing at 673 K. The suitability of the doped films in transparent device fabrication is verified by construction and characterization of a diode with configuration FTO/n-CIO:Sn/p-CIO:Al/Ag.
1. Introduction The discovery of transparent conducting delafossite compounds especially Cu based compounds has initiated rapid technological advancements in invisible electronics. Now a days TCOs have wide applications in variety of fields such as window defrosters of automobiles and airplanes, electrically controlled electrochromic smart windows, automatically dimming rear view mirrors of automobiles, in-built transparent radio antennas of automobile windows, touch screen panels, transparent electrodes of solar cells, infrared reflectors, electroluminescent as well as liquid crystal displays and also in active bipolar devices like p-n hetero junction diodes, p-n homo junction diodes, UV emitting diodes and bipolar p-n junction transistors[1–4]. Development of functional p-n junctions solely using TCOs is a major goal for material scientists since this would open up the possibility of progress in transparent (invisible) electronics. This necessitates the development of good quality p and n-type TCO materials. Up to 1997 all the known TCOs were n-type in their conductivity. Invention of CuAlO2 by Kawazoe in 1997 triggered rapid technological advancements in invisible electronics. Reports on achievement of bipolarity by intrinsic or extrinsic defect formations in various TCOs in literature includes, p-type electrical conductivity due to some native defects in CuAlO2 [5], p-type electrical conductivity by Mg doped CuCrO2 due to substitution of Mg in Cr site (MgCr) [6], p and n-type polarity of CIO by the substitution of In sites by Ca and Sn respectively [1], p-type conductivity by CuScO2
⁎
Corresponding author. E-mail address: reenatara@rediffmail.com (R. Reena Philip).
https://doi.org/10.1016/j.mseb.2020.114520 Received 7 March 2019; Accepted 13 March 2020 Available online 27 March 2020 0921-5107/ © 2020 Elsevier B.V. All rights reserved.
films on intercalation of film with O2 [7]. These contributions found them a position in the field of ‘transparent electronics’ [8] where the combination of these p-n junctions led to the fabrication of a number of technologically useful devices. The methods such as cation exchange reaction [9–11], Pulsed Laser Deposition (PLD) [1,12,13] and magnetron sputtering [14,15] are conventionally used for the deposition of the films. It has been reported that the main limitation that restricts wide use of these films in device fabrication is their poor electrical conductivity compared to the other compounds. This limitation is circumvented through the deposition of both doped and un doped films with 3–5 orders of magnitude higher electrical conductivity than those reported by other researchers, through oxygen plasma enhanced reactive evaporation in our lab, which yielded highly crystalline films on post air annealing [16–18]. This paper reports a very interesting result of bipolarity obtained in CIO when Al is used as a dopant in an attempt to fabricate CIO films having improved electrical conductivity. The CIO films with lightly Al incorporated are found to exhibit n-type electrical conductivity while those with Al at% > 6 are found to possess p-type electrical conductivity. The conductivity variation is confirmed through multiple techniques such as hall coefficient, hot probe and Seebeck coefficient measurements. The diode fabricated using the p-type film so deposited as the counterpart of n-type CIO:Sn is characterized and are found to have the parameters like turn on voltage and rectification ratio in par with the existing transparent diodes. Moreover, the transparency of the
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XRD patterns of films are taken using a Bruker AXS D8 Advance X- ray diffractometer where Cu k-α line having λ = 1.54 Å is used as the source. An NS IVA (NS4 model) atomic force microscope (AFM) having a Silicon Nitride (Si3N4) tip is used in contact mode to detect the surface morphology of the film. An Ecopia Hall Effect Measurement system (HMS-3000) having van der Pauw electrode configuration is used for Hall measurements. Shimadzu 1800 UV–Vis spectrophotometer is used for optical characterizations of films. Room temperature as well as high temperature (303–703 K) electrical conductivity of films are measured in rotary vacuum (of the order of 10−2 Torr) using Keithley 2450 source meter with Kickstart interface. Ag paste is applied to ensure ohmic contact. Low temperature conductivity measurements are performed using a cryostat filled with liquid He. Four probe method is adopted for measuring electrical conductivity. Keithley source meter coupled with a Keithley nanovoltmeter is used for measurements with the help of Lakeshore temperature controller to regulate and monitor the temperature and the cooling has been done in a cryostat [19]. Details of characterization techniques are explained in the earlier reports [16–18]. To fabricate junction, the n-type layer of Sn (~6 at%) doped Copper Indium Oxide is deposited by the successive deposition of Sn at room temperature followed by the deposition of CIO over it by oxygen plasma assisted reactive evaporation technique at a substrate temperature 623 K. The commercially available highly conducting Fluorine Tin Oxide (FTO) glass substrates are suitably masked before the film deposition. FTO acts as the back ohmic contact to the junction. After post annealing the films at 673 K, amorphous Al doped CIO film (having ~7 at% Al) is deposited over the CIO coated FTO glass substrates at substrate temperature 623 K and post annealed at 673 K. Silver is deposited as top electrode on the junction by thermal evaporation. The junction characteristics are studied using Keithley 2450 source meter. The transmittance of the individual as well as of the fabricated diode is assessed using a Shimadzu 1800 UV–Vis spectrophotometer with wavelength in the range 250–1100 nm.
Fig. 1. Post air annealed A2 film at 673 K (A21), Inset: X-ray diffraction patterns of as deposited CIO:Al films - A1, A2 and A3.
junction is ~60% which espouses its application in transparent electronics.
2. Materials and methods Successful incorporation of Al in CIO lattice has been achieved by a two stage process, that is, oxygen plasma enhanced reactive evaporation technique followed by post air annealing. In the first step, pure Al is evaporated using a helical boat of tungsten (W) onto ultrasonically cleaned quartz and sodalime glass substrates keeping them at room temperature (303 K). During deposition, pressure inside the vacuum coating unit is ensured to be of the order of 10−5 Torr. After this 99.999% pure Cu and 99.99% pure In are evaporated from two independently heated molybdenum boats in oxygen (0.03 Torr) plasma ambience keeping substrates at a temperature ~623 ± 5 K. After deposition, films are kept at the same elevated substrate temperature for an hour to ensure complete reaction of elements and compound formation. Slightly brownish uniform films obtained are well characterized compositionally, morphologically and structurally using EDAX, AFM and XRD respectively. To assess composition, JEOL Model JED - 2300 scanning electron microscope equipped with EDAX instrument is used.
3. Results and discussion 3.1. Compositional and structural analysis Stoichiometric Al doped CIO sample films (CIO:Al) with Al at% 2.07 ± 0.02, 6.87 ± 0.02 and 12.02 ± 0.02, as detected by EDAX, are selected for this study and the samples are coded as A1, A2 and A3 respectively. The as deposited CIO:Al films are found to be amorphous even after in situ annealing at elevated substrate temperature (623 K) in
Fig. 2. (I) XPS wide spectra of CIO:Al film with Al at% 6.87 (A2), (II) Detailed XPS of A2. 2
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Fig. 3. FESEM of CIO:Al films - As deposited amorphous films A1, A2 and A3 and air annealed A2 films A21.
Fig. 4. AFM images of A1, A2, A3 and A21.
crystallite size are a = 3.28 Å, c = 17.54 Å and 13 ± 1 nm respectively for the post air annealed CIO:Al film (A21). The structure and oxidation states of elements in the compound are further assessed using high resolution X- ray Photoelectron Spectroscopic measurements depicted in Fig. 2. The XPS spectrum of Cu2p3/2 is fitted with two separate peaks as shown in Fig. 2(II-a). The prominent one at 932.8 eV represents Cu–O binding and the other less intense peak at 934.1 eV indicate traces of Cu in 2+ oxidation state
vacuum for 1 h as evidenced by the inset of Fig. 1, whereas post air annealing turn them crystalline. Fig. 1 gives the XRD of the crystalline film with Al at% ̴ 6.8 (A21) obtained on post air annealing at a temperature 673 K. XRD pattern of air annealed CIO:Al film is given in Fig. 1. All the peaks except the one at 22.700 of A21 are identified as that of the delafossite CIO with R3m space group and preferential orientation along (1 0 1) plane [JCPDS 53-0954]. The peak at 22.70° corresponds to that of Cu2In2O5. Estimated lattice parameters and 3
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Fig. 5. (a) Tauc plots of A1, A2 and A3, (b) Transmittance curves of A1, A2 and A3.
size ~35 nm in all the amorphous films. FESEM image of A21 (annealed at 673 K), shows the distribution of non-uniformly agglomerated particles having sizes ranging from 30 nm to 69 nm over the surface. To get a clear picture on the surface morphology, AFM microprofiles of the CIO:Al films are taken (Fig. 4). The as deposited films show an average particle size ± 55 nm and rms surface roughness 4.7–12.6 nm. AFM image of A21 shows closely packed arrangement of agglomerated nanoparticles. The average roughness estimated from AFM micrograph is 5.75 nm for A21 film. 3.2. Optical analysis From the absorbance spectra taken in the range 250–1100 nm, the energy band gaps for the Al doped films are found from Tauc relation to be direct but lesser than that of the band gap ~3.83–3.90 eV for stoichiometric CIO films [18]. The band gap is found to vary as 3.29 eV → 3.05 eV → 3.01 eV for A1 → A2 → A3 (Fig. 5(a)), which shows that tuning of band gap is possible through Al doping. On annealing the transmittance improves of the order of 75% in the entire range (Inset of Fig. 6) Tauc plot reveals an enhancement in band gap (Fig. 7) to ~3.65 eV for the crystalline film which shows a shift in band gap occurs with on ordered arrangement of atoms on air annealing [22].
Fig. 6. Tauc plot of air annealed CIO:Al film – A21. Inset: Transmittance.
(Fig. 2(II-a)). The high intensity of peak at 932.8 eV indicates that Cu mainly exists in CuInO2 phase. A less intense shake-up line between Cu2p3/2 and Cu2p1/2 at around 943 eV is also confirm the presence of Cu2+ due to the presence of the compound Cu2In2O5. The XPS spectrum in the range 70–82 eV consists of three peaks for the best fit, in which peak around 73.6 eV represents Al2p and that around 75.6 eV and 77.9 eV corresponds to Cu3p split orbits respectively (Figure.(II-d)) [20]. Out of the three peaks for the best fit of O1s, peak at 529.5 eV represents O-In bond, 531.1 eV corresponds to O-Cu bond and the less intense peak at 530.6 eV is that of O-Al binding (Fig. 2(II-c)) [20]. Due to the proximity of Al2p peak to Cu 3p peaks, a careful fitting is necessary to extract the Al peak. This has been explained by different researchers who have worked on CuAlO2 compound [20,21]. Microstructural analyses from FESEM images (Fig. 3) show agglomerated particles, embedded in a pool of very fine particles whose size is hard to estimate. The agglomerated particles possess an average
3.3. Electrical conductivity studies Electrical conductivity type that has been assessed by hot probe method reveals that Al incorporated samples possess bipolarity depending on the Al content in the CIO compound. This is further confirmed by the Hall effect measurements performed at room temperature (303 K) and thermopower measurements. The heavily Al doped CIO is obtained as p-type while lightly doped remains n-type as in undoped stoichiometric CIO [18]. According to reports, CuAlO2 is a p-type material while stoichiometric CIO is n-type [14,23,24]. Theoretical calculations attributes this to the atomic size effect of Al (0.53 Å) and In (0.8 Å) in the compound [24]. In the present work, an assessment of composition reveals that as Al doping percentage in CIO increases the In at% is decreasing correspondingly whereas Cu% is remaining almost a
Table 1 Room temperature electrical conductivity and other hall parameters of as deposited Al doped CIO films. Films
A1 A2 A3
Electrical conductivity
Carrier density
Mobility
Hall coefficient
Seebeck Parameters at room temperature
σ (S/cm)
n (cm−3)
μ (cm2/Vs)
RH (cm3/C)
Seebeck Coeff. S(μV/K)
Fermi level EF (eV)
Density of states NC/NV (cm−3)
2.25 3.68 3.45
1.28 × 1019 1.12 × 1018 5.84 × 1017
1.09 20.40 37.28
−4.86 × 10−1 +5.55 × 100 +1.07 × 101
−193.3 +5.7 +53.4
0.458 0.032 0.027
5.18 × 1026 4.19 × 1023 1.64 × 1018
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Fig. 7. Resistivity (ρ) – Temperature (T) plots of A1, A2 and A3. Inset of each graph represents ρ–T plots of A1, A2 and A3 above room temperature (303 K to ~435 K).
Fig. 8. (I) Arrhenius plot of air annealed CIO:Al film-A21 and (II) Thermopower vz Temperature curves of A1, A2 and A3.
careful analysis of the resistivity data in the temperature range 25 K to ~435 K (Fig. 7), reveals that Mott’s Variable Range Hopping mechanism, Seto's Grain Boundary Model and Arrhenius thermally activated type of conduction mechanism control the electrical conductivity of these films in distinct temperature regions [18]. The data in Table 1 shows that electrical conductivity of the films remains in the same order of magnitude with Al doping though the conductivity type changes from n to p on increasing the doping percentage. But on air annealing, electrical conductivity decreases by one order of magnitude to around 1.06 × 10−1 S/cm. The hot probe and Hall measurements confirms there is no change in the conductivity type on air annealing. But the hole density is found to be decreased to 5.1 × 1017 cm−3 and mobility is reduced to 1.3 cm2/Vs for the crystalline A21 compared to the amorphous A2. An activation energy of 361 ± 3 meV is obtained for the crystalline film from the Arrhenius fit in the range of temperature 370–421 K Fig. 8 (1). The conductivity type of the films are further confirmed by taking the Seebeck coefficient (S) of films. Fig. 8(II) represents S-T curves of A1, A2 and A3 over the temperature range 220–320 K. In the case of lightly Al doped CIO film A1, Seebeck coefficient falls abruptly from −30 µV/K to −270 µV/K when temperature increases from 220 K to 320 K whereas it is almost steady at +2.5 µV/K in the case of A2. In the case of film A3, S increases quickly to +52 µV/K and remains constant in the range 235–308 K. −ve sign of Seebeck coefficient of A1 affirms its n-type conductivity whereas +ve sign of Seebeck coefficients indicates p-type conductivity of A2 and A3. This has been confirmed by hot probe and Hall measurements. Analyses of S-T graphs of CIO:Al films show
Fig. 9. Cross sectional FESEM of CIO:Sn/CIO:Al junction.
constant. This suggests an effective substitution of Al with smaller atomic size in the sites of In which might be leading to the realignment of valence and conduction band edges. According to Nie et al such realignment could cause conductivity type conversion from n to p or vice versa [24]. Room temperature electrical conductivity and transport parameters obtained from Hall measurements are depicted in Table 1. A 5
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Fig. 10. (a) Schematic diagram of CIO:Sn/CIO:Al junction and (b) Transmittance curves of (i) n-CIO:Sn layer, (ii) p-CIO:Al layer and (iii) n-CIO:Sn/p-CIO:Al junction.
Fig. 11. (i) I-V curve and (ii) semilog plot of n-CIO:Sn/p-CIO:Al junction.
that the density of states in these films are greater than the carrier concentration deduced from Hall measurements. This gives a clear evidence to the non-degenerate nature of these films. Position of Fermi level below the conduction band minimum in the case of A1 and above the valence band maximum in the case of A2 and A3 and density of states near fermi level are estimated from the Seebeck coefficient data and are depicted in Table 1. The application of the p-CIO: Al in transparent pn-junction fabrication is tested by the construction of a junction with configuration FTO/n-CIO:Sn/p-CIO:Al/Ag. The cross sectional FESEM micrograph of the junction is depicted in Fig. 9. Photograph of the fabricated junction is given as inset of Fig. 10(b). The details of preparation of n-CIO:Sn and its characterization are given elsewhere[18]. This layer has an electrical conductivity of ~6 S/cm, mobility ~2.5 cm2/Vs, carrier concentration ~2 × 1019 cm−3 and optical band gap of ~3.6 eV. The lattice constant, c ~17.4 Å and surface roughness ~5 nm. The p-type CIO:Al is the annealed A21 characterized above. The thicknesses ~570 nm and ~394 nm of the two distinct layers of n-CIO:Sn and p-CIO:Al is measured from the FESEM micrograph (Fig. 9). Fig. 10(a) represents the schematic diagram of this junction. Response of junction to electromagnetic waves in the range 250 nm to 1100 nm is measured (Fig. 10(b)) and it is found that the diode shows a transparency 70% at 700 nm and then falls to 55% at 550 nm and then to 28% at 400 nm. I-V curve of the CIO:Sn/CIO:Al junction is depicted in Fig. 11(i). The junction gives a rectification ratio ̴ 9.0 at ± 0.8 V. The other parameters such as turn on voltage, ideality factor (n) and reverse saturation
current (I0) are estimated to be 0.62 V, 4.0 and ~23 mA respectively from the semilog plot of the junction depicted in Fig. 11(ii). The turn on voltage is comparable to that of the commonly used Si diodes and the transparency is good enough to consider this as a transparent diode. The transparency reported for CIO:Sn/CIO:Ca junction by Yanagi et al is 30–60% in the visible region and the turn on voltage and rectification ratios they reported are 1.8 V and 10 at ± 4 V [28]. The ideality factor here is higher than the expected for an ideal diode but this is the general case in diodes with compounds as junction layers. Mayes et al reports an ideality factor of 5.6 and turn on voltage 4.2 V for their AlGaN/AlN heterojunction which is but not transparent [25]. They attribute this non ideal behaviour of junction to some sort of tunneling conduction of holes to empty valence band tails and vacant acceptor impurity bands [25]. Shah et al reports high values of ideality factor of 6.9 and 4 for their AlGaN/GaN superlattice [26]. They assume that one of the reasons for the high value of ideality factor in their diode is the metal semiconductor contact. While considering TCO based diodes, Wang et al reports ideality factors 1.09 and 6.4 in two distinct regions of their graph for the ZnO/diamond heterojunction [27]. Taking into consideration, the diode parameters, this is work opens a new avenue for the use of the delafossite doped CIO in transparent electronics.
4. Conclusion Isovalent Al is successfully incorporated into the lattice of CIO thin films by activated reactive evaporation technique. Amorphous as6
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deposited CIO:Al films are found to exhibit delafossite structure after post air annealing of the films at 673 K. Composition is assessed by EDAX analysis and is corroborated by XPS measurement. Optical measurements yield a bandgap of 3.29 ± 0.02 eV, 3.05 ± 0.01 and 3.01 ± 0.01 eV in case of unannealed A1, A2 and A3 which is observed to be dialated on post air annealing. The as deposited films show an electrical conductivity of the order of 100 S/cm whereas air annealing decreases conductivity to ~10−1 S/cm. All- transparent delafossite transparent p-n homojunction with structure FTO/n-CIO:Sn/p-CIO:Al/Ag is fabricated and characterized. Diode ideality factor, reverse saturation current and turn on voltage of the diodes are determined as 4, ~23 mA and 0.62 V respectively for the junction. The diode is found to exhibit a transmittance of 50% at 500 nm which is found to increase upto 75% on reaching the higher wavelength region of visible spectra 680 nm.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements B G Nair acknowledges UGC for sanctioning fellowship under FDP. RRP acknowledges KSCSTE (Ref. No. KSCSTE/5131/2017-SRSPS) and DST (File No. ECR/2016/00708) for funding respectively through SRS and SERB major projects. The authors would like to acknowledge Dr. S. N. Potty and P. Prabeesh C-MET, Thrissur for Hall measurements, T M J M Govt College for optical measurements and Er Mohan Gangrade, UGC-DAE Consortium for Scientific Research Indore for AFM measurements. References [1] Hiroshi Yanagi, Tomomi Hase, Shuntaro Ibuki, Kazushige Ueda, Hideo Hosono, Bipolarity in electrical conduction of transparent oxide semiconductor CuInO2 with delafossite structure, Appl. Phys. Lett. 78 (11) (2001) 1583–1585, https://doi.org/ 10.1063/1.1355673. [2] Atsushi Kudo, Hiroshi Yanagi, Kazushige Ueda, Hideo Hosono, Hiroshi Kawazoe, Yoshihiko Yano, Fabrication of transparent p-n heterojunction thin film diodes based entirely on oxide semiconductors, Appl. Phys. Lett. 75 (18) (1999) 2851–2853, https://doi.org/10.1063/1.125171. [3] Hiromichi Ohta, Ken-ichi Kawamura, Masahiro Orita, Masahiro Hirano, Nobuhiko Sarukura, Hideo Hosono, Current injection emission from a transparent p-n junction composed of p-SrCu2O2/n -ZnO, Appl. Phys. Lett. 77 (4) (2000) 475–477, https://doi.org/10.1063/1.127015. [4] John F Wager, Transparent electronics, Science 300 (5623) (2003) 1245–1246, https://doi.org/10.1126/science.1085276. [5] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, P-type electrical conduction in transparent thin films of CuAlO2, Nature 389 (1997) 939–942, https://doi.org/10.1038/40087. [6] R. Nagarajan, A.D.A. Draeseke, W. Sleight, J. Tate, P-type conductivity in CuCr1− x MgxO2 films and powders, J. Appl. Phys. 89 (2001) 8022–8025, https://doi.org/10. 1063/1.1372636. [7] N. Duan, A.W. Sleight, M.K. Jayaraj, Janet Tate, Transparent p-type conducting CuScO2+ x films, Appl. Phys. Lett. 77 (9) (2000) 1325–1326, https://doi.org/10. 1063/1.1289906. [8] Gordon Thomas, Materials science: invisible circuits, Nature 389 (6654) (1997) 907, https://doi.org/10.1038/39999.
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Al doping for bipolarity induction in transparent conducting CuInO2 and its application in diode fabrication
T
Bindu G. Naira, Hilal Rahmana, Vikash Sharmab, G.S. Okramb, Uday Deshpandeb, V. Ganesanb, ⁎ Rachel Reena Philipa, a b
Thin Film Research Lab, Union Christian College, Aluva, Kerala, India UGC-DAE Consortium For Scientific Research, Khandwa Road, Indore 452 001, Madhya Pradesh, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Thin films Crystallization Electrical conductivity P-n junction diode
Al doping is used in transparent conducting CuInO2 (CIO:Al) thin films for producing bipolar electrical conductivity. The doped thin films of electrical conductivity ~2 to 4 S/cm and mobility 100 to 101 V/cm2 are deposited by oxygen plasma assisted reactive evaporation technique. The change in conductivity from n-to ptype with the variation in doping atomic percentage is confirmed by multiple techniques like hot probe, hall and Seebeck measurements. The as deposited amorphous films are found to assume 3R poly type delafossite structure after post air annealing at 673 K. The suitability of the doped films in transparent device fabrication is verified by construction and characterization of a diode with configuration FTO/n-CIO:Sn/p-CIO:Al/Ag.
1. Introduction The discovery of transparent conducting delafossite compounds especially Cu based compounds has initiated rapid technological advancements in invisible electronics. Now a days TCOs have wide applications in variety of fields such as window defrosters of automobiles and airplanes, electrically controlled electrochromic smart windows, automatically dimming rear view mirrors of automobiles, in-built transparent radio antennas of automobile windows, touch screen panels, transparent electrodes of solar cells, infrared reflectors, electroluminescent as well as liquid crystal displays and also in active bipolar devices like p-n hetero junction diodes, p-n homo junction diodes, UV emitting diodes and bipolar p-n junction transistors[1–4]. Development of functional p-n junctions solely using TCOs is a major goal for material scientists since this would open up the possibility of progress in transparent (invisible) electronics. This necessitates the development of good quality p and n-type TCO materials. Up to 1997 all the known TCOs were n-type in their conductivity. Invention of CuAlO2 by Kawazoe in 1997 triggered rapid technological advancements in invisible electronics. Reports on achievement of bipolarity by intrinsic or extrinsic defect formations in various TCOs in literature includes, p-type electrical conductivity due to some native defects in CuAlO2 [5], p-type electrical conductivity by Mg doped CuCrO2 due to substitution of Mg in Cr site (MgCr) [6], p and n-type polarity of CIO by the substitution of In sites by Ca and Sn respectively [1], p-type conductivity by CuScO2
⁎
Corresponding author. E-mail address: reenatara@rediffmail.com (R. Reena Philip).
https://doi.org/10.1016/j.mseb.2020.114520 Received 7 March 2019; Accepted 13 March 2020 Available online 27 March 2020 0921-5107/ © 2020 Elsevier B.V. All rights reserved.
films on intercalation of film with O2 [7]. These contributions found them a position in the field of ‘transparent electronics’ [8] where the combination of these p-n junctions led to the fabrication of a number of technologically useful devices. The methods such as cation exchange reaction [9–11], Pulsed Laser Deposition (PLD) [1,12,13] and magnetron sputtering [14,15] are conventionally used for the deposition of the films. It has been reported that the main limitation that restricts wide use of these films in device fabrication is their poor electrical conductivity compared to the other compounds. This limitation is circumvented through the deposition of both doped and un doped films with 3–5 orders of magnitude higher electrical conductivity than those reported by other researchers, through oxygen plasma enhanced reactive evaporation in our lab, which yielded highly crystalline films on post air annealing [16–18]. This paper reports a very interesting result of bipolarity obtained in CIO when Al is used as a dopant in an attempt to fabricate CIO films having improved electrical conductivity. The CIO films with lightly Al incorporated are found to exhibit n-type electrical conductivity while those with Al at% > 6 are found to possess p-type electrical conductivity. The conductivity variation is confirmed through multiple techniques such as hall coefficient, hot probe and Seebeck coefficient measurements. The diode fabricated using the p-type film so deposited as the counterpart of n-type CIO:Sn is characterized and are found to have the parameters like turn on voltage and rectification ratio in par with the existing transparent diodes. Moreover, the transparency of the
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XRD patterns of films are taken using a Bruker AXS D8 Advance X- ray diffractometer where Cu k-α line having λ = 1.54 Å is used as the source. An NS IVA (NS4 model) atomic force microscope (AFM) having a Silicon Nitride (Si3N4) tip is used in contact mode to detect the surface morphology of the film. An Ecopia Hall Effect Measurement system (HMS-3000) having van der Pauw electrode configuration is used for Hall measurements. Shimadzu 1800 UV–Vis spectrophotometer is used for optical characterizations of films. Room temperature as well as high temperature (303–703 K) electrical conductivity of films are measured in rotary vacuum (of the order of 10−2 Torr) using Keithley 2450 source meter with Kickstart interface. Ag paste is applied to ensure ohmic contact. Low temperature conductivity measurements are performed using a cryostat filled with liquid He. Four probe method is adopted for measuring electrical conductivity. Keithley source meter coupled with a Keithley nanovoltmeter is used for measurements with the help of Lakeshore temperature controller to regulate and monitor the temperature and the cooling has been done in a cryostat [19]. Details of characterization techniques are explained in the earlier reports [16–18]. To fabricate junction, the n-type layer of Sn (~6 at%) doped Copper Indium Oxide is deposited by the successive deposition of Sn at room temperature followed by the deposition of CIO over it by oxygen plasma assisted reactive evaporation technique at a substrate temperature 623 K. The commercially available highly conducting Fluorine Tin Oxide (FTO) glass substrates are suitably masked before the film deposition. FTO acts as the back ohmic contact to the junction. After post annealing the films at 673 K, amorphous Al doped CIO film (having ~7 at% Al) is deposited over the CIO coated FTO glass substrates at substrate temperature 623 K and post annealed at 673 K. Silver is deposited as top electrode on the junction by thermal evaporation. The junction characteristics are studied using Keithley 2450 source meter. The transmittance of the individual as well as of the fabricated diode is assessed using a Shimadzu 1800 UV–Vis spectrophotometer with wavelength in the range 250–1100 nm.
Fig. 1. Post air annealed A2 film at 673 K (A21), Inset: X-ray diffraction patterns of as deposited CIO:Al films - A1, A2 and A3.
junction is ~60% which espouses its application in transparent electronics.
2. Materials and methods Successful incorporation of Al in CIO lattice has been achieved by a two stage process, that is, oxygen plasma enhanced reactive evaporation technique followed by post air annealing. In the first step, pure Al is evaporated using a helical boat of tungsten (W) onto ultrasonically cleaned quartz and sodalime glass substrates keeping them at room temperature (303 K). During deposition, pressure inside the vacuum coating unit is ensured to be of the order of 10−5 Torr. After this 99.999% pure Cu and 99.99% pure In are evaporated from two independently heated molybdenum boats in oxygen (0.03 Torr) plasma ambience keeping substrates at a temperature ~623 ± 5 K. After deposition, films are kept at the same elevated substrate temperature for an hour to ensure complete reaction of elements and compound formation. Slightly brownish uniform films obtained are well characterized compositionally, morphologically and structurally using EDAX, AFM and XRD respectively. To assess composition, JEOL Model JED - 2300 scanning electron microscope equipped with EDAX instrument is used.
3. Results and discussion 3.1. Compositional and structural analysis Stoichiometric Al doped CIO sample films (CIO:Al) with Al at% 2.07 ± 0.02, 6.87 ± 0.02 and 12.02 ± 0.02, as detected by EDAX, are selected for this study and the samples are coded as A1, A2 and A3 respectively. The as deposited CIO:Al films are found to be amorphous even after in situ annealing at elevated substrate temperature (623 K) in
Fig. 2. (I) XPS wide spectra of CIO:Al film with Al at% 6.87 (A2), (II) Detailed XPS of A2. 2
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Fig. 3. FESEM of CIO:Al films - As deposited amorphous films A1, A2 and A3 and air annealed A2 films A21.
Fig. 4. AFM images of A1, A2, A3 and A21.
crystallite size are a = 3.28 Å, c = 17.54 Å and 13 ± 1 nm respectively for the post air annealed CIO:Al film (A21). The structure and oxidation states of elements in the compound are further assessed using high resolution X- ray Photoelectron Spectroscopic measurements depicted in Fig. 2. The XPS spectrum of Cu2p3/2 is fitted with two separate peaks as shown in Fig. 2(II-a). The prominent one at 932.8 eV represents Cu–O binding and the other less intense peak at 934.1 eV indicate traces of Cu in 2+ oxidation state
vacuum for 1 h as evidenced by the inset of Fig. 1, whereas post air annealing turn them crystalline. Fig. 1 gives the XRD of the crystalline film with Al at% ̴ 6.8 (A21) obtained on post air annealing at a temperature 673 K. XRD pattern of air annealed CIO:Al film is given in Fig. 1. All the peaks except the one at 22.700 of A21 are identified as that of the delafossite CIO with R3m space group and preferential orientation along (1 0 1) plane [JCPDS 53-0954]. The peak at 22.70° corresponds to that of Cu2In2O5. Estimated lattice parameters and 3
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Fig. 5. (a) Tauc plots of A1, A2 and A3, (b) Transmittance curves of A1, A2 and A3.
size ~35 nm in all the amorphous films. FESEM image of A21 (annealed at 673 K), shows the distribution of non-uniformly agglomerated particles having sizes ranging from 30 nm to 69 nm over the surface. To get a clear picture on the surface morphology, AFM microprofiles of the CIO:Al films are taken (Fig. 4). The as deposited films show an average particle size ± 55 nm and rms surface roughness 4.7–12.6 nm. AFM image of A21 shows closely packed arrangement of agglomerated nanoparticles. The average roughness estimated from AFM micrograph is 5.75 nm for A21 film. 3.2. Optical analysis From the absorbance spectra taken in the range 250–1100 nm, the energy band gaps for the Al doped films are found from Tauc relation to be direct but lesser than that of the band gap ~3.83–3.90 eV for stoichiometric CIO films [18]. The band gap is found to vary as 3.29 eV → 3.05 eV → 3.01 eV for A1 → A2 → A3 (Fig. 5(a)), which shows that tuning of band gap is possible through Al doping. On annealing the transmittance improves of the order of 75% in the entire range (Inset of Fig. 6) Tauc plot reveals an enhancement in band gap (Fig. 7) to ~3.65 eV for the crystalline film which shows a shift in band gap occurs with on ordered arrangement of atoms on air annealing [22].
Fig. 6. Tauc plot of air annealed CIO:Al film – A21. Inset: Transmittance.
(Fig. 2(II-a)). The high intensity of peak at 932.8 eV indicates that Cu mainly exists in CuInO2 phase. A less intense shake-up line between Cu2p3/2 and Cu2p1/2 at around 943 eV is also confirm the presence of Cu2+ due to the presence of the compound Cu2In2O5. The XPS spectrum in the range 70–82 eV consists of three peaks for the best fit, in which peak around 73.6 eV represents Al2p and that around 75.6 eV and 77.9 eV corresponds to Cu3p split orbits respectively (Figure.(II-d)) [20]. Out of the three peaks for the best fit of O1s, peak at 529.5 eV represents O-In bond, 531.1 eV corresponds to O-Cu bond and the less intense peak at 530.6 eV is that of O-Al binding (Fig. 2(II-c)) [20]. Due to the proximity of Al2p peak to Cu 3p peaks, a careful fitting is necessary to extract the Al peak. This has been explained by different researchers who have worked on CuAlO2 compound [20,21]. Microstructural analyses from FESEM images (Fig. 3) show agglomerated particles, embedded in a pool of very fine particles whose size is hard to estimate. The agglomerated particles possess an average
3.3. Electrical conductivity studies Electrical conductivity type that has been assessed by hot probe method reveals that Al incorporated samples possess bipolarity depending on the Al content in the CIO compound. This is further confirmed by the Hall effect measurements performed at room temperature (303 K) and thermopower measurements. The heavily Al doped CIO is obtained as p-type while lightly doped remains n-type as in undoped stoichiometric CIO [18]. According to reports, CuAlO2 is a p-type material while stoichiometric CIO is n-type [14,23,24]. Theoretical calculations attributes this to the atomic size effect of Al (0.53 Å) and In (0.8 Å) in the compound [24]. In the present work, an assessment of composition reveals that as Al doping percentage in CIO increases the In at% is decreasing correspondingly whereas Cu% is remaining almost a
Table 1 Room temperature electrical conductivity and other hall parameters of as deposited Al doped CIO films. Films
A1 A2 A3
Electrical conductivity
Carrier density
Mobility
Hall coefficient
Seebeck Parameters at room temperature
σ (S/cm)
n (cm−3)
μ (cm2/Vs)
RH (cm3/C)
Seebeck Coeff. S(μV/K)
Fermi level EF (eV)
Density of states NC/NV (cm−3)
2.25 3.68 3.45
1.28 × 1019 1.12 × 1018 5.84 × 1017
1.09 20.40 37.28
−4.86 × 10−1 +5.55 × 100 +1.07 × 101
−193.3 +5.7 +53.4
0.458 0.032 0.027
5.18 × 1026 4.19 × 1023 1.64 × 1018
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Fig. 7. Resistivity (ρ) – Temperature (T) plots of A1, A2 and A3. Inset of each graph represents ρ–T plots of A1, A2 and A3 above room temperature (303 K to ~435 K).
Fig. 8. (I) Arrhenius plot of air annealed CIO:Al film-A21 and (II) Thermopower vz Temperature curves of A1, A2 and A3.
careful analysis of the resistivity data in the temperature range 25 K to ~435 K (Fig. 7), reveals that Mott’s Variable Range Hopping mechanism, Seto's Grain Boundary Model and Arrhenius thermally activated type of conduction mechanism control the electrical conductivity of these films in distinct temperature regions [18]. The data in Table 1 shows that electrical conductivity of the films remains in the same order of magnitude with Al doping though the conductivity type changes from n to p on increasing the doping percentage. But on air annealing, electrical conductivity decreases by one order of magnitude to around 1.06 × 10−1 S/cm. The hot probe and Hall measurements confirms there is no change in the conductivity type on air annealing. But the hole density is found to be decreased to 5.1 × 1017 cm−3 and mobility is reduced to 1.3 cm2/Vs for the crystalline A21 compared to the amorphous A2. An activation energy of 361 ± 3 meV is obtained for the crystalline film from the Arrhenius fit in the range of temperature 370–421 K Fig. 8 (1). The conductivity type of the films are further confirmed by taking the Seebeck coefficient (S) of films. Fig. 8(II) represents S-T curves of A1, A2 and A3 over the temperature range 220–320 K. In the case of lightly Al doped CIO film A1, Seebeck coefficient falls abruptly from −30 µV/K to −270 µV/K when temperature increases from 220 K to 320 K whereas it is almost steady at +2.5 µV/K in the case of A2. In the case of film A3, S increases quickly to +52 µV/K and remains constant in the range 235–308 K. −ve sign of Seebeck coefficient of A1 affirms its n-type conductivity whereas +ve sign of Seebeck coefficients indicates p-type conductivity of A2 and A3. This has been confirmed by hot probe and Hall measurements. Analyses of S-T graphs of CIO:Al films show
Fig. 9. Cross sectional FESEM of CIO:Sn/CIO:Al junction.
constant. This suggests an effective substitution of Al with smaller atomic size in the sites of In which might be leading to the realignment of valence and conduction band edges. According to Nie et al such realignment could cause conductivity type conversion from n to p or vice versa [24]. Room temperature electrical conductivity and transport parameters obtained from Hall measurements are depicted in Table 1. A 5
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Fig. 10. (a) Schematic diagram of CIO:Sn/CIO:Al junction and (b) Transmittance curves of (i) n-CIO:Sn layer, (ii) p-CIO:Al layer and (iii) n-CIO:Sn/p-CIO:Al junction.
Fig. 11. (i) I-V curve and (ii) semilog plot of n-CIO:Sn/p-CIO:Al junction.
that the density of states in these films are greater than the carrier concentration deduced from Hall measurements. This gives a clear evidence to the non-degenerate nature of these films. Position of Fermi level below the conduction band minimum in the case of A1 and above the valence band maximum in the case of A2 and A3 and density of states near fermi level are estimated from the Seebeck coefficient data and are depicted in Table 1. The application of the p-CIO: Al in transparent pn-junction fabrication is tested by the construction of a junction with configuration FTO/n-CIO:Sn/p-CIO:Al/Ag. The cross sectional FESEM micrograph of the junction is depicted in Fig. 9. Photograph of the fabricated junction is given as inset of Fig. 10(b). The details of preparation of n-CIO:Sn and its characterization are given elsewhere[18]. This layer has an electrical conductivity of ~6 S/cm, mobility ~2.5 cm2/Vs, carrier concentration ~2 × 1019 cm−3 and optical band gap of ~3.6 eV. The lattice constant, c ~17.4 Å and surface roughness ~5 nm. The p-type CIO:Al is the annealed A21 characterized above. The thicknesses ~570 nm and ~394 nm of the two distinct layers of n-CIO:Sn and p-CIO:Al is measured from the FESEM micrograph (Fig. 9). Fig. 10(a) represents the schematic diagram of this junction. Response of junction to electromagnetic waves in the range 250 nm to 1100 nm is measured (Fig. 10(b)) and it is found that the diode shows a transparency 70% at 700 nm and then falls to 55% at 550 nm and then to 28% at 400 nm. I-V curve of the CIO:Sn/CIO:Al junction is depicted in Fig. 11(i). The junction gives a rectification ratio ̴ 9.0 at ± 0.8 V. The other parameters such as turn on voltage, ideality factor (n) and reverse saturation
current (I0) are estimated to be 0.62 V, 4.0 and ~23 mA respectively from the semilog plot of the junction depicted in Fig. 11(ii). The turn on voltage is comparable to that of the commonly used Si diodes and the transparency is good enough to consider this as a transparent diode. The transparency reported for CIO:Sn/CIO:Ca junction by Yanagi et al is 30–60% in the visible region and the turn on voltage and rectification ratios they reported are 1.8 V and 10 at ± 4 V [28]. The ideality factor here is higher than the expected for an ideal diode but this is the general case in diodes with compounds as junction layers. Mayes et al reports an ideality factor of 5.6 and turn on voltage 4.2 V for their AlGaN/AlN heterojunction which is but not transparent [25]. They attribute this non ideal behaviour of junction to some sort of tunneling conduction of holes to empty valence band tails and vacant acceptor impurity bands [25]. Shah et al reports high values of ideality factor of 6.9 and 4 for their AlGaN/GaN superlattice [26]. They assume that one of the reasons for the high value of ideality factor in their diode is the metal semiconductor contact. While considering TCO based diodes, Wang et al reports ideality factors 1.09 and 6.4 in two distinct regions of their graph for the ZnO/diamond heterojunction [27]. Taking into consideration, the diode parameters, this is work opens a new avenue for the use of the delafossite doped CIO in transparent electronics.
4. Conclusion Isovalent Al is successfully incorporated into the lattice of CIO thin films by activated reactive evaporation technique. Amorphous as6
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deposited CIO:Al films are found to exhibit delafossite structure after post air annealing of the films at 673 K. Composition is assessed by EDAX analysis and is corroborated by XPS measurement. Optical measurements yield a bandgap of 3.29 ± 0.02 eV, 3.05 ± 0.01 and 3.01 ± 0.01 eV in case of unannealed A1, A2 and A3 which is observed to be dialated on post air annealing. The as deposited films show an electrical conductivity of the order of 100 S/cm whereas air annealing decreases conductivity to ~10−1 S/cm. All- transparent delafossite transparent p-n homojunction with structure FTO/n-CIO:Sn/p-CIO:Al/Ag is fabricated and characterized. Diode ideality factor, reverse saturation current and turn on voltage of the diodes are determined as 4, ~23 mA and 0.62 V respectively for the junction. The diode is found to exhibit a transmittance of 50% at 500 nm which is found to increase upto 75% on reaching the higher wavelength region of visible spectra 680 nm.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements B G Nair acknowledges UGC for sanctioning fellowship under FDP. RRP acknowledges KSCSTE (Ref. No. KSCSTE/5131/2017-SRSPS) and DST (File No. ECR/2016/00708) for funding respectively through SRS and SERB major projects. The authors would like to acknowledge Dr. S. N. Potty and P. Prabeesh C-MET, Thrissur for Hall measurements, T M J M Govt College for optical measurements and Er Mohan Gangrade, UGC-DAE Consortium for Scientific Research Indore for AFM measurements. References [1] Hiroshi Yanagi, Tomomi Hase, Shuntaro Ibuki, Kazushige Ueda, Hideo Hosono, Bipolarity in electrical conduction of transparent oxide semiconductor CuInO2 with delafossite structure, Appl. Phys. Lett. 78 (11) (2001) 1583–1585, https://doi.org/ 10.1063/1.1355673. [2] Atsushi Kudo, Hiroshi Yanagi, Kazushige Ueda, Hideo Hosono, Hiroshi Kawazoe, Yoshihiko Yano, Fabrication of transparent p-n heterojunction thin film diodes based entirely on oxide semiconductors, Appl. Phys. Lett. 75 (18) (1999) 2851–2853, https://doi.org/10.1063/1.125171. [3] Hiromichi Ohta, Ken-ichi Kawamura, Masahiro Orita, Masahiro Hirano, Nobuhiko Sarukura, Hideo Hosono, Current injection emission from a transparent p-n junction composed of p-SrCu2O2/n -ZnO, Appl. Phys. Lett. 77 (4) (2000) 475–477, https://doi.org/10.1063/1.127015. [4] John F Wager, Transparent electronics, Science 300 (5623) (2003) 1245–1246, https://doi.org/10.1126/science.1085276. [5] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, P-type electrical conduction in transparent thin films of CuAlO2, Nature 389 (1997) 939–942, https://doi.org/10.1038/40087. [6] R. Nagarajan, A.D.A. Draeseke, W. Sleight, J. Tate, P-type conductivity in CuCr1− x MgxO2 films and powders, J. Appl. Phys. 89 (2001) 8022–8025, https://doi.org/10. 1063/1.1372636. [7] N. Duan, A.W. Sleight, M.K. Jayaraj, Janet Tate, Transparent p-type conducting CuScO2+ x films, Appl. Phys. Lett. 77 (9) (2000) 1325–1326, https://doi.org/10. 1063/1.1289906. [8] Gordon Thomas, Materials science: invisible circuits, Nature 389 (6654) (1997) 907, https://doi.org/10.1038/39999.
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