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n-Zn1 − xAlxO transparent diode for potential application in “invisible electronics”
Thin Solid Films 515 (2007) 7324 – 7330 www.elsevier.com/locate/tsf
Fabrication and characterization of all-oxide heterojunction p-CuAlO2 + x/n-Zn1 − xAlxO transparent diode for potential application in “invisible electronics” A.N. Banerjee 1 , S. Nandy, C.K. Ghosh, K.K. Chattopadhyay ⁎ Thin Film and Nano-Science Laboratory, Department of Physics, Jadavpur University, Kolkata-700 032, India Received 28 January 2006; received in revised form 2 February 2007; accepted 19 February 2007 Available online 24 February 2007
Abstract Transparent p–n heterojunction diodes have been fabricated by p-type copper aluminum oxide (p-CuAlO2 + x) and n-type aluminum doped zinc oxide (n-Zn1 − xAlxO) thin films on glass substrates. The n-layers are deposited by sol-gel-dip-coating process from zinc acetate dihydrate (Zn (CH3COO)2·2H2O) and aluminum nitrate (Al(NO3)3·9H2O). Al concentration in the nominal solution is taken as 1.62 at %. P-layers are deposited onto the ZnO:Al-coated glass substrates by direct current sputtering process from a prefabricated CuAlO2 sintered target. The sputtering is performed in oxygen-diluted argon atmosphere with an elevated substrate temperature. Post-deposition oxygen annealing induces excess oxygen within the p-CuAlO2 + x films, which in turn enhances p-type conductivity of the layers. The device characterization shows rectifying current– voltage characteristics, confirming the proper formation of the p–n junction. The turn-on voltage is obtained around 0.8 V, with a forward-toreverse current ratio around 30 at ± 4 V. The diode structure has a total thickness of 1.1 μm and the optical transmission spectra of the diode show almost 60% transmittance in the visible region, indicating its potential application in ‘invisible electronics’. Also the cost-effective procedures enable the large-scale production of these transparent diodes for diverse device applications. © 2007 Elsevier B.V. All rights reserved. Keywords: Sputtering; Sol-gel-dip-coating; CuAlO2; Al-doped ZnO; P-type semiconducting oxide; Heterojunction transparent diode; Cost-effective procedures
1. Introduction In the field of opto-electronics device technology, ‘transparent electronics’ or ‘invisible electronics’ [1] becomes an important and emerging area of research, where a ‘functional’ window could be fabricated by both n- and p-types of transparent conducting oxides (TCO), which would transmit the visible portion of solar radiation yet generates electricity by the absorption of UV part of it. Thus, simultaneously these devices can act as ‘UV-shields’ as well as ‘electricity generators’ by the UV absorption. Although the n-type TCOs such as ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (A.N. Banerjee), [email protected] (K.K. Chattopadhyay). 1 Present address: The Center for Nanoscale Device Research, Department of Electrical and Computer Engineering, University of Nevada, Las Vegas, Nevada-89154, USA. 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.02.087
ZnO1 − x, In1 − xSnxO3, SnO2 − x, etc., and their doped versions are well known and widely used in many opto-electronic applications [2–6], the corresponding p-type counterpart was surprisingly missing for a long time until Kawazoe and coauthors [7] reported the synthesis of transparent p-type semiconducting delafossite CuAlO2 + x thin films by pulsed laser deposition (PLD) method. Thereafter p-TCO technology becomes a very important and emerging area of research and various research groups around the globe during last few years reported the syntheses and characterization of several delafossite and non-delafossite p-TCO thin films [8–15]. As far as the fabrication of transparent junctional devices are concerned for the development of ‘transparent’ or ‘invisible electronics’, Sato et al. [16] first reported the formation of semi-transparent p-NiO/ i-NiO/i-ZnO/n-ZnO structure, with only 20% visible transparency. Although this low transparency is not favorable for hightechnology applications, but still this report has scientific importance in the field of ‘invisible’ or ‘transparent electronics’.
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Later, various groups reported the fabrication of p–i–n as well as p–n homojunction and heterojunction diodes with considerably good electro-optical properties for potential applications in opto-electronic devices [17–25]. Hosono and co-authors reported the fabrication of p-SrCu 2 O 2 /n-ZnO heterojunction diodes and observed UV emission when an electrical current was injected through the junction [17–21]. They also observed visible light emission from a p–i–n structure of the form p-SrCu2O2/i-ZnO/n-ZnO, under a certain forward bias [20]. Jayaraj et al. [22] fabricated p-CuY1 − xCaxO2/n-Zn1 − xAlxO heterojunction diode and observed ∼40% transparency of the diode layers. Hoffman and co-authors [23] reported the formation of transparent p-CuY1 − xCaxO2/i-ZnO/n-ITO diode and obtained a forward-to-reverse current ratio as high as 60 in the −4 to +4 voltage range. Tonooka et al. [24] fabricated transparent n-ZnO/pCu-Al-O heterojunction diode and observed large photovoltaic effect under blue illumination. As far as the fabrications of p–n homojunctions are concerned, Yanagi and co-authors [25] prepared p-CuIn1 − xCaxO2/n-CuIn1 − xSnxO2 diode and observed rectifying characteristics with turn-on voltage ∼1.8 V. Also it is noteworthy that after the report of the possibility of the formation of p-type ZnO films [26,27], fabrication of all-ZnO p–n homojunction diodes attracted greater interest in the scientific community for its potential use in electroluminescent and laser devices. Several groups reported the fabrication of all-ZnO homojunction diodes [28–30] and observed rectifying I–V characteristics of the junctions. In this paper, we have reported the fabrication of transparent heterojunction diode made up of p-type copper aluminum oxide and n-type aluminum doped zinc oxide films (of the form p-CuAlO2 + x/n-Zn1 − xAlxO) and studied the electro-optical characteristics of the diode structure. Previously, we have synthesized CuAlO2 + x thin films by direct current (DC) sputtering and reactive DC sputtering techniques [11,15,31,32] and reported its good thermoelectric [33] and field-emission properties [11,34]. It is noteworthy that there is only one report (as far as literature survey depicts) on the CuAlO2-ZnO-based transparent diodes by Tonooka et al. [24], where they used costly PLD technique to deposit the layers. We have used sol-gel-dip-coating (SGDC) technique for the deposition of n-layer (ZnO:Al) and DC sputtering technique for p-layer. The importance of these two syntheses routes is that these methods are cost-effective and large-scale production is possible for diverse device applications. As the fabrication of all-transparent junctional devices is an important and emerging field of research in opto-electronic technology, this report may become an important addition in the development of ‘Invisible Electronics’. 2. Experimental procedure The all-TCO p–n heterojunction diode having the structure n-Zn1 − xAlxO/p-CuAlO2 + x was fabricated on glass substrates. The n-type layer was taken as aluminum doped zinc oxide films (ZnO:Al), which was deposited on commercial glass substrates (of size 18 mm × 8 mm) by sol-gel-dip-coating technique. Thereafter these n-layer-coated glasses were used
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as the substrates for the deposition of p-layer (p-CuAlO2 + x film) by DC sputtering technique. Six independent junctions (1 mm × 1 mm) were fabricated on a single substrate using proper masking. Details of the deposition procedures are furnished below. 2.1. Deposition of n-layer ZnO films were deposited on glass substrates by SGDC route from a starting solution of zinc acetate dihydrate (Zn(CH3 COO)2·2H2O) and isopropyl alcohol (Pri-OH). Since zinc acetate has low solubility in isopropyl alcohol, diethanolamine (DEA) was added (with [DEA] / [Zn2+] = 1.5) to get transparent solution and to keep the solution stable in dip-coating process. Doping of Al was done by the addition of controlled amount of aluminum nitrate (Al(NO3)3·9H2O) to the solution. Then the resultant solution was stirred and refluxed, keeping the temperature at 343 K for 1 h. The atomic ratio of Al/Zn in the initial solution was varied from 0.32% to 1.62% and the concentration of zinc acetate was fixed at 0.85 mol/L. Distilled water (with [H2O] / [Zn2+] = 14) and acetic acid (with [H+] / [Zn2+] = 2) were added for better stability of the solution and to avoid gelation or precipitation. The pH of the solution was kept around 7.0. Lastly, stirred and refluxed solution was aged for half an hour to get the resultant solution. Then the ultrasonically cleaned glass substrates were dipped vertically into the solution and withdrawn at a speed of 8 cm/min to coat them with the required material. The coated substrates were dried at room temperature for 10 min and heated at ∼ 423 K for 10 min in open atmosphere for gelation. This process was repeated for 2–3 times for getting a desired thickness. Finally the films were heated at 573 K for 1 h to obtain crystalline Zn1 − xAlxO films. The details of the deposition conditions were reported elsewhere [35]. It is to be noted that although the Al concentration in the starting solution was varied from 0.32% to 1.62% to get Zn1 − xAlxO films with varied electrical and optical properties, but for the fabrication of the diode, those films were chosen which were having Al concentration of 1.62% in the starting solution. This is because of the better comparability of the electrical and optical properties of these films with the corresponding p-layer (CuAlO2 + x films). 2.2. Deposition of p-layer The n-layer-coated glass was used as the substrate in the DC sputtering process to deposit p-CuAlO2 + x thin film. Mica masks were used on the n-Zn1 − xAlxO-coated glass substrates for preferential deposition of p-CuAlO2 + x layers on desired position. Initially, solid-state reaction between stoichiometric ratios of Cu2O and Al2O3 powder at 1400 K produced CuAlO2 powder. This powder was then pressed into a pellet and was used as a target for DC sputtering. The sputtering unit was evacuated by standard rotary-diffusion arrangement up to a base pressure of 10− 4 Pa. The pellet was arranged properly by aluminum holder to act as upper electrode and the negative terminal of the DC power supply unit was connected to it. N-layer-coated glass substrates were placed on the lower
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electrode and connected to the ground of the power supply. The electrode distance was taken as 1.8 cm. Ar and O2 (3:2 vol. ratio) were taken as sputtering gas and the sputtering was done at an elevated substrate temperature (∼ 453 K) to achieve high crystallinity in the film. The sputtering voltage, current density, deposition pressure and deposition time were set to 1.1 kV, 10 mA/cm2 , 20 Pa and 4 h, respectively. Postdeposition annealing of the film (at 473 K) for 30 min in an O2 atmosphere (at a pressure of 20 Pa) was performed to induce nonstoichiometry in the film for enhancing p-type conductivity. Details of the deposition conditions were reported elsewhere [15,31]. A schematic diagram of the diode structure is given in Fig. 1. 2.3. Characterization Structural properties of the films were studied by X-ray diffractometer (XRD, BRUKER, D8, ADVANCE) using the Cu Kα (1.5406 Å) radiation operating at 40 kV, 40 mA range with a normal θ–2θ scanning mode. Optical properties were measured by UV-Vis-NIR spectrophotometer (SHIMADZU-UV-3101PC). All electrical measurements were done by standard fourprobe method using Keithley-6514 electrometer under vacuum condition (∼ 0.1 Pa). For ohmic contacts, evaporated silver electrodes were used with proper masking in both types of layers, which showed linear I–V characteristics over a wide range of voltages and temperatures. Thereafter the electrical connections were made by Cu leads with silver paints, as shown in Fig. 1.
Fig. 2. XRD patterns of (a) p-CuAlO2 and (b) n-ZnO films. Lines and circles represent the reference patterns of corresponding materials (Ref. [37]).
also observed by Kawazoe and Yanagi et al. [7,36], for their pulsed laser deposited CuAlO2 + x films deposited on sapphire substrates. Two small peaks of (003) and (018) orientations are also observed in the pattern. This pattern closely reflects the rhombohedral crystal structure with R3¯m space group [37]. The XRD patterns for the films deposited on glass substrates are broader and the peak intensities are lesser than what we have previously reported for the films deposited on Si substrate
3. Results and discussion For the proper fabrication of a transparent rectifying junction, comparability of the electrical and optical properties of both n- and p-layers is the most important criteria, which must be dealt with considerable attention. So we have described the properties of both the layers individually and then furnished the opto-electrical properties of the junction. 3.1. Properties of p-CuAlO2 + x layer Fig. 2, pattern a, shows XRD spectrum of the as-deposited p-CuAlO2 + x thin film on glass substrate. The XRD pattern shows a preferential (006) orientation. Similar orientation was
Fig. 1. Schematic diagram of n-Zn1 − xAlxO/p-CuAlO2 + x diode structure.
Fig. 3. (a) Optical transmission spectra of the p-CuAlO2 + x film. Inset: Determination of bandgaps. (b) Optical transmission spectra of n-Zn1 − xAlxO film. Inset: Determination of direct bandgap.
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▪
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Fig. 4. Temperature variation of conductivity of (●) p-CuAlO2 and ( ) n-Zn1 − x AlxO films.
Fig. 5. Optical transmission spectra of the n-Zn1 − xAlxO/p-CuAlO2 + x diode deposited on glass substrate.
[31]. Also no peaks of starting materials (e.g. Cu2O and Al2O3) and any other reactant species (such as CuAl2O4) have been found which conclusively indicate that the reactants were completely mixed to form the proper phase of copper aluminum oxide. Optical properties of the p-CuAlO2 + x thin films, deposited on glass substrates, are shown in Fig. 3. The trace shows almost 80% transmittance of the layer in the visible portion of solar radiation. The film thickness was measured as 500 nm from cross-sectional scanning electron microscopy (not shown here). From the transmittance data, using Manifacier model [38] we have calculated the absorption coefficients (α) at the region of strong absorption. The fundamental absorption, which corresponds to electron excitation from valance band to the conduction band, can be used to determine the nature and value of the optical bandgap. The relation between the absorption coefficients (α) and the incident photon energy (hν) can be written as [39],
Fig. 4, curve a, represents the lnσ vs. 1000/T plot of the CuAlO2 + x film on glass substrate from room temperature (300 K) to 575 K. The straight-line nature of the Arhenius plot indicates the thermally activated conduction, as often found in semiconductors. Room temperature conductivity of the film was obtained as 0.09 S cm− 1, which is comparable to that obtained by Kawazoe et al. [7]. From the slope of the graph we get the value of activation energy (Ea) which corresponds to the minimum energy required to transfer carriers from acceptor level to the valence band and the value of Ea comes out as 0.27 eV. Defect chemistry plays an important role for the increase in p-type conductivity of CuAlO2 thin film. Metal deficit (or excess oxygen) within the crystallite sites of the material enhances the p-type conductivity [15,33,41]. This deviation from the stoichiometric composition of the components can be induced by regulating the post-deposition annealing time (ta) in oxygen atmosphere. In our previous papers [15,33], we have reported an increase in the excess oxygen content within the CuAlO2 + x thin films as well as an increase in the p-type conductivity of the films with the increase in the post-deposition oxygen annealing time. This observation supports the above argument of suspected p-type conduction in p-CuAlO2 + x films caused by excess oxygen. In those articles [15,33], we have reported the amount of excess oxygen within the film, postannealed for 30 min (which is identical to the present work) to be around 0.5 at % over stoichiometric value (i.e. x is as low as 1/100 in CuAlO2 + x). Hall measurements of the film show positive Hall-coefficient, confirming p-type nature of the film and the value of hole concentration (np) is found to be ∼ 3.0 × 1017 cm− 3 along with the Hall mobility value of 2.03 cm− 2 V− 1 S− 1.
ðahmÞ1=n ¼ Aðhm−Eg Þ
ð1Þ
where A is a constant and Eg is the bandgap of the material and exponent n depends on the type of transition. For indirect allowed transition, n = 2; for direct allowed, n = 1/2. To determine the possible transitions, (αhν)1/n vs. hν were plotted for different values of n. The (αhν)2 vs. hν and (αhν)1/2 vs. hν plots are shown in the inset of Fig. 3a. Extrapolating the linear portions of the graphs to the hν axis, we have obtained the direct and indirect allowed bandgap values as 3.81 eV and 2.1 eV, respectively. These values are comparable to that reported previously by Kawazoe et al. [7] and Yanagi et al. [36] and also fall within the range theoretically calculated by Robertson et al. (3.91 eV and 2.1 eV) [40].
Table 1 Comparison of different electrical and optical properties of individual p-CuAlO2+x layer and n-Zn1 − xAlxO layer Layers
Thickness (nm)
Average visible transmittance (%)
Direct bandgap (eV)
Room temperature conductivity (S cm− 1)
Activation energy (eV)
Hall mobility (cm− 2 V− 1 S− 1)
Carrier concentration (cm− 3)
n-Zn1 − xAlxO p-CuAlO2+x
600 500
∼ 80 ∼ 80
3.31 3.81
0.08 0.09
0.55 0.27
2.14 2.03
2.6 × 1017 3.0 × 1017
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For the fabrication of the heterojunction transparent all-oxide diode, Zn1 − xAlxO was chosen as the n-type layer by us as well as by others groups [17–22] because of its several advantages: the first is its suitability for low temperature deposition [3, 42]. It is important to deposit crystalline Zn1 − xAlxO films on glass substrates to fabricate all-transparent junction for the potential applications in ‘invisible circuits’. Secondly, the electron concentration of Zn1 − xAlxO films can easily be controlled by varying Al doping concentration during deposition. This is necessary in order to match the carrier concentrations with those positive holes in p-CuAlO2 + x, which is more difficult to control. Fig. 6. Current–voltage characteristics of p-CuAlO2 + x/n-Zn1 − xAlxO diode.
3.2. Properties of Zn1 − xAlxO layer XRD pattern of Zn1 − xAlxO film deposited on glass substrate for 1.62% of Al is shown in Fig. 2, curve b. The peaks are identified according to JCPDS File Card # 36-1451 [37] and originate from (100), (002), (101), (102), (110) and (103) reflections of hexagonal ZnO. Fig. 3b shows the optical transmission spectra of 1.62% Al doped ZnO film. Similar to p-CuAlO2 + x film, this film also shows almost 80% visible transmittance. The film thickness was measured to be around 600 nm. Calculation of direct bandgap according to Eq. (1) gives a value of 3.31 eV (shown in the inset of Fig. 4b), which is slightly less than that of the corresponding p-CuAlO2 + x layer. The temperature dependence of conductivity (σ) of 1.62% Al doped ZnO film was measured in the temperature range of 300– 575 K. The experimental logσ vs. 1000/T plot is shown in Fig. 4, curve b. Activation energy (Ea) calculated from the slope of the middle portion of the curve gives a value of 0.55 eV with a room temperature conductivity (σRT) of 0.08 S cm− 1. Hall effect measurements of the film give the value of carrier concentration (ne) as 2.6 × 1017 cm− 3, which is comparable to the corresponding p-CuAlO2 + x film (3 × 1017 cm− 3). The Hall mobility value is calculated to be ∼2.14 cm− 2 V− 1 S− 1. A comparative study of different electro-optical properties of the individual n-Zn1 − xAlxO film and p-CuAlO2 + x film is furnished in Table 1.
3.3. Properties of n-Zn1 − xAlxO/p-CuAlO2 + x diode The optical transmission spectrum of the n-Zn1 − xAlxO/ p-CuAlO2 + x diode is shown in Fig. 5. As mentioned earlier, the thicknesses of both p-layer and n-layer are 500 nm and 600 nm, respectively, making the total device thickness as 1100 nm. The visible transparency of the diode is around 60%. Also, as obvious, a comparison of this spectra with that shown in Fig. 3a and b, we have observed that the starting point of the fundamental absorption region of the diode structure is comparable to that of n-Zn1 − xAlxO layer, which is having lower bandgap energy (3.31 eV). Previously, Tonooka et al. [24] obtained the average visible transmittance of their n+-ZnO/n-ZnO/p-Cu-Al-O diode around 60%. As far as other all-oxide transparent diodes are concerned, Sato et al. [16] reported 20% visible transmittance for their p-NiO/i-NiO/i-ZnO/n-ZnO structure, Kudo et al. [18] obtained 70–80% visible transmittance for p-SrCu2O2/n-ZnO diode, Hoffman and co-authors [23] reported 35% to 65% visible transmittance in a p-CuY1 − x CaxO2/n-Zn1 − xAlxO/n+-ITO heterojunction diode, Yanagi et al. [25] obtained 60% to 80% transmittance for their p-CuIn1 − x CaxO2/n-CuIn1 − xSnxO2 homojunction diode in the visible region. The current–voltage characteristics of the all-transparent heterojunction diode are shown in Fig. 6. The curve shows the rectifying properties, indicating proper formation of the junction. Maximum current obtained at 5 V is around 1 μA and the turn-on voltage obtained ∼ 0.8 V. However, it varied from 0.6 V to 1.0 V from junction to junction. This indicates
Fig. 7. Schematic representation of approximate equilibrium energy-band diagram of p-CuAlO2 + x/n-Zn1 − xAlxO diode. Energy levels are not exactly to the scale.
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considerable reproducibility of the junctions. The forward bias current is greater than the reverse bias current by approximately a factor of 30 at ± 4 V. A small leakage current as low as 30 nA was observed at a reverse bias of − 4 V. Previously, Tonooka and co-authors [24] reported the average turn on voltage of their n-ZnO/p-Cu-Al-O diode ∼ 0.5 V, which is comparable to ours. Generally, for heterojunction diodes, the structural imperfections at grain boundaries as well as at the interface deteriorate the efficiency of the diode [18]. Also another important point to be mentioned about the inherent difficulty in manufacturing these all-oxide diodes is that the p- and n-layers must be produced under oxidizing and reducing conditions, respectively, so that optimal processing for one type is detrimental to the other [22]. All these facts must be addressed with considerable attention for diverse applications of these heterojunction alloxide transparent diodes in the field of “invisible electronics’, which is the further course of our research work. Fig. 7 represents an approximate equilibrium energy-band diagram for the transparent p-CuAlO2 + x/n-Zn1 − xAlxO diode. The n-Zn1 − xAlxO has a bandgap of 3.31 eV (cf. inset of Fig. 3b). The position of Fermi level of both p- and n-type materials are obtained from thermo-electric power (TEP) measurements of the materials. For p-CuAlO2 films this value is around 200 meVand for n-ZnO:Al films it is around 280 meV, which were given in our previous literatures [33,35]. The activation energy values are obtained from Fig. 4. But the sketching of the energy-band diagram of pCuAlO2 + x needs some discussions. From the optical measurements, both direct and indirect bandgaps are found to exist for pCuAlO2 + x film (cf. inset of Fig. 3a). Now if we draw the band diagram of the p-side in terms of the direct bandgap value (3.81 eV), then the depletion barrier height would be approximately around 3.50 eV. But this value is quite larger than the turn-on field observed in the I–V characteristics of the diode, which is around 0.8 V. So there must be some midgap energy band present in the player, which decreases the effective bandgap. Therefore we have focused our attention on the indirect bandgap of the p-CuAlO2 + x film. Now redrawing the equilibrium energy-band diagram in terms of the indirect bandgap value of p-layer, which is around 2.1 eV (as obtained from optical data and shown in the inset of Fig. 3a and drawn in terms of thick-dotted line in Fig. 7), the depletion barrier height comes out as ∼1.7 eV. Although this value is smaller than that of previous one, but still it is twice that of the observed turn-on voltage. This inconsistency between the turn-on voltage and the barrier height may be explained in the following way: investigation of previous literatures about the band structure calculations of Mattheis [43] and experimental findings of Cava et al. [44] of similar delafossite p-CuYO2 + x material, we see the existence of some midgap impurity bands within the material due to the interstitial oxygen doping, which decreases the effective bandgap of the material. In a similar way it can be argued that in our pCuAlO2 + x thin films, excess oxygen intercalation and probably some unintentional impurity incorporation may give rise to some new and deep states within the bandgap via self-compensation [45], which further reduces the effective bandgap of the material, so also the barrier height. This might provide an explanation of the low turn-on voltage of the p-CuAlO2 + x/n-Zn1 − xAlxO heterojunction diode.
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4. Conclusions All-oxide transparent diode of the form p-CuAlO2 + x/n-Zn1 − x AlxO has been fabricated successfully. The diode shows around 60% transmittance in the visible region. The current–voltage characteristics show rectifying nature, indicating proper formation of the junction. The turn-on voltage is obtained around 0.8 V. Maximum current obtained at 5 V is around 1 μA and the forwardto-reverse current ratio is obtained approximately ∼30 at ±4 V. The moderate transparency and low turn-on voltage of the heterojunction all-oxide transparent diode indicates its potential application in ‘transparent’ or ‘invisible electronics’. Acknowledgments The authors wish to thank the Department of Science and Technology (D.S.T.), Government of India, for financial support. ANB and CKG gratefully acknowledge Council of Scientific and Industrial Research (C. S. I. R.), Government of India, for awarding them a senior research fellowship and a junior research fellowship, respectively, during the execution of the work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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[25] H. Yanagi, K. Ueda, H. Ohta, M. Orita, M. Hirano, H. Hosono, Solid State Commun. 121 (2002) 15. [26] T. Yamamoto, H.K. Yoshida, Jpn. J. Appl. Phys. 38 (1999) L166. [27] M. Joseph, H. Tabata, T. Kawai, Jpn. J. Appl. Phys. 38 (1999) L2505. [28] T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [29] S. Tüzemen, G. Xiong, J. Wilkinson, B. Mischuck, K.B. Ucer, R.T. Williams, Physica B 308–310 (2001) 1197. [30] D.-K. Hwang, K.-H. Bang, M.-C. Jeong, J.-M. Myoung, J. Cryst. Growth 254 (2003) 449. [31] A.N. Banerjee, S. Kundoo, K.K. Chattopadhyay, Thin Solid Films 440 (2003) 5. [32] A.N. Banerjee, R. Maity, K.K. Chattopadhyay, Mater. Lett. 58 (2003) 10. [33] A.N. Banerjee, R. Maity, P.K. Ghosh, K.K. Chattopadhyay, Thin Solid Films 474 (2005) 261. [34] A.N. Banerjee, K.K. Chattopadhyay, Appl. Surf. Sci. 225 (2004) 243. [35] R. Maity, S. Kundoo, K.K. Chattopadhyay, Sol. Energy Mater. Sol. Cells 86 (2005) 217.
[36] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J. Appl. Phys. 88 (2000) 4159. [37] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1967, Card # 35-1401, # 36-1451. [38] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002. [39] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall. Inc., New Jersy, 1971, p. 34. [40] J. Robertson, P.W. Peacock, M.D. Towler, R. Needs, Thin Solid Films 411 (2002) 96. [41] K. Koumoto, H. Koduka, W.S. Seo, J. Mater. Chem. 11 (2001) 251. [42] A. Barker, S. Crowther, D. Rees, Sens. Actuators, A, Phys. 58 (1997) 229. [43] L.F. Mattheis, Phys. Rev., B 48 (1993) 18300. [44] R.J. Cava, H.W. Zandbergen, A.P. Ramirez, H. Tagaki, C.T. Chien, J.J. Krajewski, W.F. Peck Jr., J.V. Waszczak, G. Meigs, R.S. Roth, L.F. Schneemeyer, J. Solid State Chem. 104 (1993) 437. [45] F.A. Kröger, The Chemistry of Imperfect Crystal, North Holland, Amsterdam, 1974.
Fabrication and characterization of all-oxide heterojunction p-CuAlO2 + x/n-Zn1 − xAlxO transparent diode for potential application in “invisible electronics” A.N. Banerjee 1 , S. Nandy, C.K. Ghosh, K.K. Chattopadhyay ⁎ Thin Film and Nano-Science Laboratory, Department of Physics, Jadavpur University, Kolkata-700 032, India Received 28 January 2006; received in revised form 2 February 2007; accepted 19 February 2007 Available online 24 February 2007
Abstract Transparent p–n heterojunction diodes have been fabricated by p-type copper aluminum oxide (p-CuAlO2 + x) and n-type aluminum doped zinc oxide (n-Zn1 − xAlxO) thin films on glass substrates. The n-layers are deposited by sol-gel-dip-coating process from zinc acetate dihydrate (Zn (CH3COO)2·2H2O) and aluminum nitrate (Al(NO3)3·9H2O). Al concentration in the nominal solution is taken as 1.62 at %. P-layers are deposited onto the ZnO:Al-coated glass substrates by direct current sputtering process from a prefabricated CuAlO2 sintered target. The sputtering is performed in oxygen-diluted argon atmosphere with an elevated substrate temperature. Post-deposition oxygen annealing induces excess oxygen within the p-CuAlO2 + x films, which in turn enhances p-type conductivity of the layers. The device characterization shows rectifying current– voltage characteristics, confirming the proper formation of the p–n junction. The turn-on voltage is obtained around 0.8 V, with a forward-toreverse current ratio around 30 at ± 4 V. The diode structure has a total thickness of 1.1 μm and the optical transmission spectra of the diode show almost 60% transmittance in the visible region, indicating its potential application in ‘invisible electronics’. Also the cost-effective procedures enable the large-scale production of these transparent diodes for diverse device applications. © 2007 Elsevier B.V. All rights reserved. Keywords: Sputtering; Sol-gel-dip-coating; CuAlO2; Al-doped ZnO; P-type semiconducting oxide; Heterojunction transparent diode; Cost-effective procedures
1. Introduction In the field of opto-electronics device technology, ‘transparent electronics’ or ‘invisible electronics’ [1] becomes an important and emerging area of research, where a ‘functional’ window could be fabricated by both n- and p-types of transparent conducting oxides (TCO), which would transmit the visible portion of solar radiation yet generates electricity by the absorption of UV part of it. Thus, simultaneously these devices can act as ‘UV-shields’ as well as ‘electricity generators’ by the UV absorption. Although the n-type TCOs such as ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (A.N. Banerjee), [email protected] (K.K. Chattopadhyay). 1 Present address: The Center for Nanoscale Device Research, Department of Electrical and Computer Engineering, University of Nevada, Las Vegas, Nevada-89154, USA. 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.02.087
ZnO1 − x, In1 − xSnxO3, SnO2 − x, etc., and their doped versions are well known and widely used in many opto-electronic applications [2–6], the corresponding p-type counterpart was surprisingly missing for a long time until Kawazoe and coauthors [7] reported the synthesis of transparent p-type semiconducting delafossite CuAlO2 + x thin films by pulsed laser deposition (PLD) method. Thereafter p-TCO technology becomes a very important and emerging area of research and various research groups around the globe during last few years reported the syntheses and characterization of several delafossite and non-delafossite p-TCO thin films [8–15]. As far as the fabrication of transparent junctional devices are concerned for the development of ‘transparent’ or ‘invisible electronics’, Sato et al. [16] first reported the formation of semi-transparent p-NiO/ i-NiO/i-ZnO/n-ZnO structure, with only 20% visible transparency. Although this low transparency is not favorable for hightechnology applications, but still this report has scientific importance in the field of ‘invisible’ or ‘transparent electronics’.
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Later, various groups reported the fabrication of p–i–n as well as p–n homojunction and heterojunction diodes with considerably good electro-optical properties for potential applications in opto-electronic devices [17–25]. Hosono and co-authors reported the fabrication of p-SrCu 2 O 2 /n-ZnO heterojunction diodes and observed UV emission when an electrical current was injected through the junction [17–21]. They also observed visible light emission from a p–i–n structure of the form p-SrCu2O2/i-ZnO/n-ZnO, under a certain forward bias [20]. Jayaraj et al. [22] fabricated p-CuY1 − xCaxO2/n-Zn1 − xAlxO heterojunction diode and observed ∼40% transparency of the diode layers. Hoffman and co-authors [23] reported the formation of transparent p-CuY1 − xCaxO2/i-ZnO/n-ITO diode and obtained a forward-to-reverse current ratio as high as 60 in the −4 to +4 voltage range. Tonooka et al. [24] fabricated transparent n-ZnO/pCu-Al-O heterojunction diode and observed large photovoltaic effect under blue illumination. As far as the fabrications of p–n homojunctions are concerned, Yanagi and co-authors [25] prepared p-CuIn1 − xCaxO2/n-CuIn1 − xSnxO2 diode and observed rectifying characteristics with turn-on voltage ∼1.8 V. Also it is noteworthy that after the report of the possibility of the formation of p-type ZnO films [26,27], fabrication of all-ZnO p–n homojunction diodes attracted greater interest in the scientific community for its potential use in electroluminescent and laser devices. Several groups reported the fabrication of all-ZnO homojunction diodes [28–30] and observed rectifying I–V characteristics of the junctions. In this paper, we have reported the fabrication of transparent heterojunction diode made up of p-type copper aluminum oxide and n-type aluminum doped zinc oxide films (of the form p-CuAlO2 + x/n-Zn1 − xAlxO) and studied the electro-optical characteristics of the diode structure. Previously, we have synthesized CuAlO2 + x thin films by direct current (DC) sputtering and reactive DC sputtering techniques [11,15,31,32] and reported its good thermoelectric [33] and field-emission properties [11,34]. It is noteworthy that there is only one report (as far as literature survey depicts) on the CuAlO2-ZnO-based transparent diodes by Tonooka et al. [24], where they used costly PLD technique to deposit the layers. We have used sol-gel-dip-coating (SGDC) technique for the deposition of n-layer (ZnO:Al) and DC sputtering technique for p-layer. The importance of these two syntheses routes is that these methods are cost-effective and large-scale production is possible for diverse device applications. As the fabrication of all-transparent junctional devices is an important and emerging field of research in opto-electronic technology, this report may become an important addition in the development of ‘Invisible Electronics’. 2. Experimental procedure The all-TCO p–n heterojunction diode having the structure n-Zn1 − xAlxO/p-CuAlO2 + x was fabricated on glass substrates. The n-type layer was taken as aluminum doped zinc oxide films (ZnO:Al), which was deposited on commercial glass substrates (of size 18 mm × 8 mm) by sol-gel-dip-coating technique. Thereafter these n-layer-coated glasses were used
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as the substrates for the deposition of p-layer (p-CuAlO2 + x film) by DC sputtering technique. Six independent junctions (1 mm × 1 mm) were fabricated on a single substrate using proper masking. Details of the deposition procedures are furnished below. 2.1. Deposition of n-layer ZnO films were deposited on glass substrates by SGDC route from a starting solution of zinc acetate dihydrate (Zn(CH3 COO)2·2H2O) and isopropyl alcohol (Pri-OH). Since zinc acetate has low solubility in isopropyl alcohol, diethanolamine (DEA) was added (with [DEA] / [Zn2+] = 1.5) to get transparent solution and to keep the solution stable in dip-coating process. Doping of Al was done by the addition of controlled amount of aluminum nitrate (Al(NO3)3·9H2O) to the solution. Then the resultant solution was stirred and refluxed, keeping the temperature at 343 K for 1 h. The atomic ratio of Al/Zn in the initial solution was varied from 0.32% to 1.62% and the concentration of zinc acetate was fixed at 0.85 mol/L. Distilled water (with [H2O] / [Zn2+] = 14) and acetic acid (with [H+] / [Zn2+] = 2) were added for better stability of the solution and to avoid gelation or precipitation. The pH of the solution was kept around 7.0. Lastly, stirred and refluxed solution was aged for half an hour to get the resultant solution. Then the ultrasonically cleaned glass substrates were dipped vertically into the solution and withdrawn at a speed of 8 cm/min to coat them with the required material. The coated substrates were dried at room temperature for 10 min and heated at ∼ 423 K for 10 min in open atmosphere for gelation. This process was repeated for 2–3 times for getting a desired thickness. Finally the films were heated at 573 K for 1 h to obtain crystalline Zn1 − xAlxO films. The details of the deposition conditions were reported elsewhere [35]. It is to be noted that although the Al concentration in the starting solution was varied from 0.32% to 1.62% to get Zn1 − xAlxO films with varied electrical and optical properties, but for the fabrication of the diode, those films were chosen which were having Al concentration of 1.62% in the starting solution. This is because of the better comparability of the electrical and optical properties of these films with the corresponding p-layer (CuAlO2 + x films). 2.2. Deposition of p-layer The n-layer-coated glass was used as the substrate in the DC sputtering process to deposit p-CuAlO2 + x thin film. Mica masks were used on the n-Zn1 − xAlxO-coated glass substrates for preferential deposition of p-CuAlO2 + x layers on desired position. Initially, solid-state reaction between stoichiometric ratios of Cu2O and Al2O3 powder at 1400 K produced CuAlO2 powder. This powder was then pressed into a pellet and was used as a target for DC sputtering. The sputtering unit was evacuated by standard rotary-diffusion arrangement up to a base pressure of 10− 4 Pa. The pellet was arranged properly by aluminum holder to act as upper electrode and the negative terminal of the DC power supply unit was connected to it. N-layer-coated glass substrates were placed on the lower
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electrode and connected to the ground of the power supply. The electrode distance was taken as 1.8 cm. Ar and O2 (3:2 vol. ratio) were taken as sputtering gas and the sputtering was done at an elevated substrate temperature (∼ 453 K) to achieve high crystallinity in the film. The sputtering voltage, current density, deposition pressure and deposition time were set to 1.1 kV, 10 mA/cm2 , 20 Pa and 4 h, respectively. Postdeposition annealing of the film (at 473 K) for 30 min in an O2 atmosphere (at a pressure of 20 Pa) was performed to induce nonstoichiometry in the film for enhancing p-type conductivity. Details of the deposition conditions were reported elsewhere [15,31]. A schematic diagram of the diode structure is given in Fig. 1. 2.3. Characterization Structural properties of the films were studied by X-ray diffractometer (XRD, BRUKER, D8, ADVANCE) using the Cu Kα (1.5406 Å) radiation operating at 40 kV, 40 mA range with a normal θ–2θ scanning mode. Optical properties were measured by UV-Vis-NIR spectrophotometer (SHIMADZU-UV-3101PC). All electrical measurements were done by standard fourprobe method using Keithley-6514 electrometer under vacuum condition (∼ 0.1 Pa). For ohmic contacts, evaporated silver electrodes were used with proper masking in both types of layers, which showed linear I–V characteristics over a wide range of voltages and temperatures. Thereafter the electrical connections were made by Cu leads with silver paints, as shown in Fig. 1.
Fig. 2. XRD patterns of (a) p-CuAlO2 and (b) n-ZnO films. Lines and circles represent the reference patterns of corresponding materials (Ref. [37]).
also observed by Kawazoe and Yanagi et al. [7,36], for their pulsed laser deposited CuAlO2 + x films deposited on sapphire substrates. Two small peaks of (003) and (018) orientations are also observed in the pattern. This pattern closely reflects the rhombohedral crystal structure with R3¯m space group [37]. The XRD patterns for the films deposited on glass substrates are broader and the peak intensities are lesser than what we have previously reported for the films deposited on Si substrate
3. Results and discussion For the proper fabrication of a transparent rectifying junction, comparability of the electrical and optical properties of both n- and p-layers is the most important criteria, which must be dealt with considerable attention. So we have described the properties of both the layers individually and then furnished the opto-electrical properties of the junction. 3.1. Properties of p-CuAlO2 + x layer Fig. 2, pattern a, shows XRD spectrum of the as-deposited p-CuAlO2 + x thin film on glass substrate. The XRD pattern shows a preferential (006) orientation. Similar orientation was
Fig. 1. Schematic diagram of n-Zn1 − xAlxO/p-CuAlO2 + x diode structure.
Fig. 3. (a) Optical transmission spectra of the p-CuAlO2 + x film. Inset: Determination of bandgaps. (b) Optical transmission spectra of n-Zn1 − xAlxO film. Inset: Determination of direct bandgap.
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▪
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Fig. 4. Temperature variation of conductivity of (●) p-CuAlO2 and ( ) n-Zn1 − x AlxO films.
Fig. 5. Optical transmission spectra of the n-Zn1 − xAlxO/p-CuAlO2 + x diode deposited on glass substrate.
[31]. Also no peaks of starting materials (e.g. Cu2O and Al2O3) and any other reactant species (such as CuAl2O4) have been found which conclusively indicate that the reactants were completely mixed to form the proper phase of copper aluminum oxide. Optical properties of the p-CuAlO2 + x thin films, deposited on glass substrates, are shown in Fig. 3. The trace shows almost 80% transmittance of the layer in the visible portion of solar radiation. The film thickness was measured as 500 nm from cross-sectional scanning electron microscopy (not shown here). From the transmittance data, using Manifacier model [38] we have calculated the absorption coefficients (α) at the region of strong absorption. The fundamental absorption, which corresponds to electron excitation from valance band to the conduction band, can be used to determine the nature and value of the optical bandgap. The relation between the absorption coefficients (α) and the incident photon energy (hν) can be written as [39],
Fig. 4, curve a, represents the lnσ vs. 1000/T plot of the CuAlO2 + x film on glass substrate from room temperature (300 K) to 575 K. The straight-line nature of the Arhenius plot indicates the thermally activated conduction, as often found in semiconductors. Room temperature conductivity of the film was obtained as 0.09 S cm− 1, which is comparable to that obtained by Kawazoe et al. [7]. From the slope of the graph we get the value of activation energy (Ea) which corresponds to the minimum energy required to transfer carriers from acceptor level to the valence band and the value of Ea comes out as 0.27 eV. Defect chemistry plays an important role for the increase in p-type conductivity of CuAlO2 thin film. Metal deficit (or excess oxygen) within the crystallite sites of the material enhances the p-type conductivity [15,33,41]. This deviation from the stoichiometric composition of the components can be induced by regulating the post-deposition annealing time (ta) in oxygen atmosphere. In our previous papers [15,33], we have reported an increase in the excess oxygen content within the CuAlO2 + x thin films as well as an increase in the p-type conductivity of the films with the increase in the post-deposition oxygen annealing time. This observation supports the above argument of suspected p-type conduction in p-CuAlO2 + x films caused by excess oxygen. In those articles [15,33], we have reported the amount of excess oxygen within the film, postannealed for 30 min (which is identical to the present work) to be around 0.5 at % over stoichiometric value (i.e. x is as low as 1/100 in CuAlO2 + x). Hall measurements of the film show positive Hall-coefficient, confirming p-type nature of the film and the value of hole concentration (np) is found to be ∼ 3.0 × 1017 cm− 3 along with the Hall mobility value of 2.03 cm− 2 V− 1 S− 1.
ðahmÞ1=n ¼ Aðhm−Eg Þ
ð1Þ
where A is a constant and Eg is the bandgap of the material and exponent n depends on the type of transition. For indirect allowed transition, n = 2; for direct allowed, n = 1/2. To determine the possible transitions, (αhν)1/n vs. hν were plotted for different values of n. The (αhν)2 vs. hν and (αhν)1/2 vs. hν plots are shown in the inset of Fig. 3a. Extrapolating the linear portions of the graphs to the hν axis, we have obtained the direct and indirect allowed bandgap values as 3.81 eV and 2.1 eV, respectively. These values are comparable to that reported previously by Kawazoe et al. [7] and Yanagi et al. [36] and also fall within the range theoretically calculated by Robertson et al. (3.91 eV and 2.1 eV) [40].
Table 1 Comparison of different electrical and optical properties of individual p-CuAlO2+x layer and n-Zn1 − xAlxO layer Layers
Thickness (nm)
Average visible transmittance (%)
Direct bandgap (eV)
Room temperature conductivity (S cm− 1)
Activation energy (eV)
Hall mobility (cm− 2 V− 1 S− 1)
Carrier concentration (cm− 3)
n-Zn1 − xAlxO p-CuAlO2+x
600 500
∼ 80 ∼ 80
3.31 3.81
0.08 0.09
0.55 0.27
2.14 2.03
2.6 × 1017 3.0 × 1017
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For the fabrication of the heterojunction transparent all-oxide diode, Zn1 − xAlxO was chosen as the n-type layer by us as well as by others groups [17–22] because of its several advantages: the first is its suitability for low temperature deposition [3, 42]. It is important to deposit crystalline Zn1 − xAlxO films on glass substrates to fabricate all-transparent junction for the potential applications in ‘invisible circuits’. Secondly, the electron concentration of Zn1 − xAlxO films can easily be controlled by varying Al doping concentration during deposition. This is necessary in order to match the carrier concentrations with those positive holes in p-CuAlO2 + x, which is more difficult to control. Fig. 6. Current–voltage characteristics of p-CuAlO2 + x/n-Zn1 − xAlxO diode.
3.2. Properties of Zn1 − xAlxO layer XRD pattern of Zn1 − xAlxO film deposited on glass substrate for 1.62% of Al is shown in Fig. 2, curve b. The peaks are identified according to JCPDS File Card # 36-1451 [37] and originate from (100), (002), (101), (102), (110) and (103) reflections of hexagonal ZnO. Fig. 3b shows the optical transmission spectra of 1.62% Al doped ZnO film. Similar to p-CuAlO2 + x film, this film also shows almost 80% visible transmittance. The film thickness was measured to be around 600 nm. Calculation of direct bandgap according to Eq. (1) gives a value of 3.31 eV (shown in the inset of Fig. 4b), which is slightly less than that of the corresponding p-CuAlO2 + x layer. The temperature dependence of conductivity (σ) of 1.62% Al doped ZnO film was measured in the temperature range of 300– 575 K. The experimental logσ vs. 1000/T plot is shown in Fig. 4, curve b. Activation energy (Ea) calculated from the slope of the middle portion of the curve gives a value of 0.55 eV with a room temperature conductivity (σRT) of 0.08 S cm− 1. Hall effect measurements of the film give the value of carrier concentration (ne) as 2.6 × 1017 cm− 3, which is comparable to the corresponding p-CuAlO2 + x film (3 × 1017 cm− 3). The Hall mobility value is calculated to be ∼2.14 cm− 2 V− 1 S− 1. A comparative study of different electro-optical properties of the individual n-Zn1 − xAlxO film and p-CuAlO2 + x film is furnished in Table 1.
3.3. Properties of n-Zn1 − xAlxO/p-CuAlO2 + x diode The optical transmission spectrum of the n-Zn1 − xAlxO/ p-CuAlO2 + x diode is shown in Fig. 5. As mentioned earlier, the thicknesses of both p-layer and n-layer are 500 nm and 600 nm, respectively, making the total device thickness as 1100 nm. The visible transparency of the diode is around 60%. Also, as obvious, a comparison of this spectra with that shown in Fig. 3a and b, we have observed that the starting point of the fundamental absorption region of the diode structure is comparable to that of n-Zn1 − xAlxO layer, which is having lower bandgap energy (3.31 eV). Previously, Tonooka et al. [24] obtained the average visible transmittance of their n+-ZnO/n-ZnO/p-Cu-Al-O diode around 60%. As far as other all-oxide transparent diodes are concerned, Sato et al. [16] reported 20% visible transmittance for their p-NiO/i-NiO/i-ZnO/n-ZnO structure, Kudo et al. [18] obtained 70–80% visible transmittance for p-SrCu2O2/n-ZnO diode, Hoffman and co-authors [23] reported 35% to 65% visible transmittance in a p-CuY1 − x CaxO2/n-Zn1 − xAlxO/n+-ITO heterojunction diode, Yanagi et al. [25] obtained 60% to 80% transmittance for their p-CuIn1 − x CaxO2/n-CuIn1 − xSnxO2 homojunction diode in the visible region. The current–voltage characteristics of the all-transparent heterojunction diode are shown in Fig. 6. The curve shows the rectifying properties, indicating proper formation of the junction. Maximum current obtained at 5 V is around 1 μA and the turn-on voltage obtained ∼ 0.8 V. However, it varied from 0.6 V to 1.0 V from junction to junction. This indicates
Fig. 7. Schematic representation of approximate equilibrium energy-band diagram of p-CuAlO2 + x/n-Zn1 − xAlxO diode. Energy levels are not exactly to the scale.
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considerable reproducibility of the junctions. The forward bias current is greater than the reverse bias current by approximately a factor of 30 at ± 4 V. A small leakage current as low as 30 nA was observed at a reverse bias of − 4 V. Previously, Tonooka and co-authors [24] reported the average turn on voltage of their n-ZnO/p-Cu-Al-O diode ∼ 0.5 V, which is comparable to ours. Generally, for heterojunction diodes, the structural imperfections at grain boundaries as well as at the interface deteriorate the efficiency of the diode [18]. Also another important point to be mentioned about the inherent difficulty in manufacturing these all-oxide diodes is that the p- and n-layers must be produced under oxidizing and reducing conditions, respectively, so that optimal processing for one type is detrimental to the other [22]. All these facts must be addressed with considerable attention for diverse applications of these heterojunction alloxide transparent diodes in the field of “invisible electronics’, which is the further course of our research work. Fig. 7 represents an approximate equilibrium energy-band diagram for the transparent p-CuAlO2 + x/n-Zn1 − xAlxO diode. The n-Zn1 − xAlxO has a bandgap of 3.31 eV (cf. inset of Fig. 3b). The position of Fermi level of both p- and n-type materials are obtained from thermo-electric power (TEP) measurements of the materials. For p-CuAlO2 films this value is around 200 meVand for n-ZnO:Al films it is around 280 meV, which were given in our previous literatures [33,35]. The activation energy values are obtained from Fig. 4. But the sketching of the energy-band diagram of pCuAlO2 + x needs some discussions. From the optical measurements, both direct and indirect bandgaps are found to exist for pCuAlO2 + x film (cf. inset of Fig. 3a). Now if we draw the band diagram of the p-side in terms of the direct bandgap value (3.81 eV), then the depletion barrier height would be approximately around 3.50 eV. But this value is quite larger than the turn-on field observed in the I–V characteristics of the diode, which is around 0.8 V. So there must be some midgap energy band present in the player, which decreases the effective bandgap. Therefore we have focused our attention on the indirect bandgap of the p-CuAlO2 + x film. Now redrawing the equilibrium energy-band diagram in terms of the indirect bandgap value of p-layer, which is around 2.1 eV (as obtained from optical data and shown in the inset of Fig. 3a and drawn in terms of thick-dotted line in Fig. 7), the depletion barrier height comes out as ∼1.7 eV. Although this value is smaller than that of previous one, but still it is twice that of the observed turn-on voltage. This inconsistency between the turn-on voltage and the barrier height may be explained in the following way: investigation of previous literatures about the band structure calculations of Mattheis [43] and experimental findings of Cava et al. [44] of similar delafossite p-CuYO2 + x material, we see the existence of some midgap impurity bands within the material due to the interstitial oxygen doping, which decreases the effective bandgap of the material. In a similar way it can be argued that in our pCuAlO2 + x thin films, excess oxygen intercalation and probably some unintentional impurity incorporation may give rise to some new and deep states within the bandgap via self-compensation [45], which further reduces the effective bandgap of the material, so also the barrier height. This might provide an explanation of the low turn-on voltage of the p-CuAlO2 + x/n-Zn1 − xAlxO heterojunction diode.
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4. Conclusions All-oxide transparent diode of the form p-CuAlO2 + x/n-Zn1 − x AlxO has been fabricated successfully. The diode shows around 60% transmittance in the visible region. The current–voltage characteristics show rectifying nature, indicating proper formation of the junction. The turn-on voltage is obtained around 0.8 V. Maximum current obtained at 5 V is around 1 μA and the forwardto-reverse current ratio is obtained approximately ∼30 at ±4 V. The moderate transparency and low turn-on voltage of the heterojunction all-oxide transparent diode indicates its potential application in ‘transparent’ or ‘invisible electronics’. Acknowledgments The authors wish to thank the Department of Science and Technology (D.S.T.), Government of India, for financial support. ANB and CKG gratefully acknowledge Council of Scientific and Industrial Research (C. S. I. R.), Government of India, for awarding them a senior research fellowship and a junior research fellowship, respectively, during the execution of the work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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