- Email: [email protected]
ZnO junctions
Solid State Ionics 75 ( 1995) 179-186
ELSEVIER
Atmosphere sensitive CuO/ZnO junctions Kwang-Ki Baek *, Harry L. Tuller Centerfor Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract Rectifying CuO/ZnO heterojunctions were successfully fabricated by sputtering thin film p-type CuO onto n-type ZnO polycrystalline substrates. The thermally activated, forward-bias current increased with decreasing oxygen partial pressure pointing to a close correlation between barrier height and adsorbed oxygen at the interface. Both dc I-Vand ac impedance measurements were utilized to characterize the deviation from ideality of the characteristics and were found to increase with decreased oxygen partial pressure, (Po, ). An energy band diagram was synthesized and examined in light of the measurements performed in this study. Keywords: CuO/ZnO heterojunction; Copper oxide; Zinc oxide
1. Introduction Semiconducting or ptype
oxides
conductivity.
normally
Thus,
while
exhibit
either
it is possible
n to
prepare, e.g. n-type ZnO [ 1 ] or p-type NiO [ 2 1, it has not been possible to achieve reasonably high conductivities of the opposite sign in these oxides near ambient conditions. Consequently, junction studies have been limited to oxide semiconductor-metal Schottky barriers [3] or electrically active grain boundaries [ 4 1. Recently, however, studies of oxide p-n heterojunctions have been initiated. These have included CuO/ZnO [5], NiO/ZnO [6], CuOfSnO, [7], LnMO,/SnOz (Ln = rare earth, M = Cr, Co, Mn, Fe ) junctions [ 81. Many of these heterojunctions have been assembled in a rather primitive manner, relying on spring loading to achieve mechanical contact bePresent address: Hyundai Heavy Industries Co. Ltd., Hyundai Research Institute, Materials Research Dept., 1, Cheonha-Dong, Dong-Ku, Ulsan, Korea.
l
Elsevier Science B.V. SSDIOl67-2738(94)00172-3
tween adjacent polycrystalline, semiconducting pellets. Nevertheless, they exhibit well defined rectifying characteristics as evidenced by their reported current-voltage (Z-V) and capacitance-voltage ( CV) characteristics. Furthermore, they exhibit strong sensitivity to reducing gases and/or humidity [ 5-8 1, with the precise mechanisms for this sensitivity, however, remaining unclear. In this paper, we report preliminary results obtained on CuO/ZnO junctions prepared by thin film deposition of p-type CuO onto n-type polycrystalline ZnO substrates thereby bypassing the inherent poor reproducibility of mechanical heterocontacts. The temperature and oxygen partial pressure of the operating environment were controlled to evaluate their effect on the junction characteristics. A tentative energy band diagram of the CuO/ZnO heterojunction is proposed based both on experimental results obtained in this and other studies.
180
K.-K. Baek, H.L. Tuller /Solid State Ionics 75 (1995) 179-186
2. Experimental 2.1. Preparation of CuO/ZnO heterojunctlon Nominally undoped, polycrystalline ZnO pellets were prepared by sintering following the procedure described elsewhere [ 9 1. ZnO plates with dimensions of approximately 5 X 5 X 2.5 mm were cut from the pellets and used as substrates for CuO thin film deposition. The surfaces of these ZnO plates were polished with 1 urn diamond paste to a mirror finish and cleaned with acetone. CuO thin films were reactively sputtered onto these plates by RF magnetron sputtering (K.J. Lesker, Super MS System) from a commercially available Cu target (99.99% purity, K.J. Lesker). The pressure in the chamber prior to plasma sputtering was maintained at around 5 x 1O-’ Torr, while the substrate temperature did not exceed 100°C. X-ray diffraction measurements and cross-sectional scanning electron microscope (SEM) observation of the prepared CuO/ZnO heterojunction confirmed that the deposited thin films were single phase CuO with a columnar structure [IO]. CuO films deposited on glass slides under like conditions were evaluated for film thickness and thereby deposition rate with a prolilometer (Sloan DEKTAK II ). Resistivity values of 130-470 !&cm were obtained from four-point resistivity measurements [ 111 performed on several batches of CuO films. To ensure ptype semiconductive behavior, it was important to keep the stoichiometry of the CuO phase on the oxygen excess side, i.e. CuO, +x(x> 0). For this purpose, Ar/O* mixtures with a ratio of ( l/4) were selected as the reactive gas for plasma sputtering [ 12- 141. The sign of the thermoelectric voltage confirmed that the CuO thin films and ZnO pellets prepared for this study were p-type and n-type, respectively. Sputtering conditions including gas composition, and the physical properties of the sputtered CuO thin films obtained under these conditions are summarized in Table 1. 2.2. Electrical characterization The dc I- Vcurves were measured using a personal computer controlled pica-ammeter/de voltage source (Hewlett-Packard, #4 140B), For ohmic contacts,
silver paste was applied to the face of the p-type CuO thin films and baked at 150°C for one hour, while Ga-In eutectic alloy paste was applied to the n-type ZnO. The schematic diagram of the heterojunction sample used for electrical characterization is shown in Fig. 1. The electronic transport characteristics of the junction were evaluated by measuring the temperature-dependent I- Vcharacteristics under different gas atmospheres at temperatures ranging from 25-230°C. As for the gas environments, the following gases were used for each measurement; air, argon, oxygen, (nitrogen+ 1% H2), (nitrogen+ 5% CO). All gases were purchased premixed and analyzed by the supplier (AirCo). The steady state capacitance measurements were made in the frequency range from 100 to 13 x 1O6Hz at an oscillator voltage of 10 mV RMS using an impedance analyzer (Hewlett-Packard, #4192A) controlled by a PC. AC impedance measurements, as a function of dc offset voltage, were conducted at high frequency to evaluate the diffusion or built-in voltage, VD, formed at the heterojunction.
3. Results and discussion 3.1. de current-voltage measurements Fig. 2 shows a typical Z-V curve of a CuO/ZnO heterojunction structure at 20°C in dry air. A rectifying character similar to a conventional p/n junction diode is observed. The current increased exponentially when CuO was positively biased (forward bias), while the reverse current is much reduced at low voltage followed by a linear dependence on reverse bias for larger values of bias. Substantial reverse currents only initiate at about - 4 V. These ZVcharacteristics confirm that ap/n heterojunction is formed at the interface of the CuO thin film and ZnO bulk plate and is consistent with the p-type nature of the CuO thin film and the n-type character of the ZnO bulk plate. The electronic transport characteristics of the junctions were evaluated by measuring the temperature-dependent I- I’ characteristics under different atmospheres. When current transport through the heterojunction is dominated by a thermal process, the
K.-K. Baek, H.L. Tuller /Solid State Ionics 75 (1995) 179- 186
181
Table 1 Sputtering conditions and properties of CuO thin films. Target:
cu (99.99%)
Initial vacuum pressure
5x
Deposition time: Plasma color: Average film thickness: Film color:
60 min. bright violet 6500 A black
Plasma gas: Power: Substrate temperature: Film resistivity:
20% Ar + 80% O2 300 w < 100°C 130-470 R cm
Ag Electrode
IO-‘Torr
mann constant, and T the absolute temperature. The reverse saturation current density (Jo) is given by
CuO (Thin Film)
Jr, =JoO exp( - @,/AkT)
,
(2)
where Qs is the barrier height determined by the dominant current transport mechanism. If tunneling dominates, the current-voltage relationship becomes instead [ 15 ] J=&(T)
In&a
Electrode
Fig. 1. Schematic diagram of the CuO/ZnO heterojunction. 0.010
0.005
I---
,
exp(ol’)
(3)
where (Yis a constant. Fig. 3 shows a plot of In J versus the forward bias voltage V in oxygen for various temperatures between 20°C and 198°C. Similar results are shown in Figs. 4 and 5 for measurements performed in air and argon, respectively. Several trends are apparent. First, J at constant bias is strongly temperature dependent consistent with the thermally activated model described in Eqs. ( 1) and (2 ). This becomes even more evident from a plot of In Jo versus reciprocal temper-4.00
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DC Bias (V)
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-14.00, (I.0
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DC Bias
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Fig. 3. Current density versus forward voltage characteristics of a CuO/ZnO heterojunction in oxygen at various temperatures.
182
K.-K. Baek. H.L. Tuller /SolidState
Ionics 75 (1995) 179-186
26°C .*
DC Bias
1000/T (K)
(V)
Fig. 4. Current density versus forward voltage characteristics of a CuO/ZnO heterojunction in air at various temperatures. -4.00 2 I 5°C -6.00 .
.
-8.00
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Fig. 6. Plot of In(&) versus 1000/T of a CuO/ZnO heterojunction under various gas atmospheres.
large forward bias conditions initiating at lower bias values for junctions with lower barrier heights. This appears to be consistent with the decreasing activation energies observed in Fig. 6 with decreasing PO, i.e. 0.69 eV in oxygen, 0.64 eV in air and 0.50 eV in argon. We can use Eq. (2) and the derived values of A to estimate values for the barrier heights (Pn which fall in the range of = l-l .5 eV. However, since the values of A differ so substantially from unity, we are hesitant to take these derived values too seriously. 3.2. ac impedance measurements
1
I
I
I
I
0.1
0.2
0.3
0.4
0.5
DC Bias
(V)
Fig. 5. Current density versus forward voltage characteristics of a CuO/ZnO heterojunction in argon at various temperatures.
ature shown in Fig. 6. Second, the extrapolated reverse saturation or leakage current is strongly atmosphere dependent, increasing with decreasing oxygen partial pressure as observed in Fig. 6. Lastly, the diode factor A, which is large and falls in the range of 2.23.1 at low V, also depends on temperature and atmosphere tending to increase with decreasing PO,. Furthermore, the lower the Paz, the greater the curvature observed in the In J versus V plots. This may reflect the onset of a series resistance contribution for
Fig. 7 shows a typical complex impedance spectrum of a CuO/ZnO heterojunction sample measured at 20°C in air with zero bias. The data points obtained over the frequency range from 100 Hz to I 3 MHz are fitted to a single semicircle which is somewhat depressed relative to the Z’ axis, showing capacitive dispersion. The semicircle is attributed to the electrically active interface in the heterojunction. The deviation of the high frequency intercept Z’ value from zero (approximately 60 a) is a series resistance which must include the bulk resistance. The dispersion may be due to the trapping states at the interface [ 161 although the exact cause of the dispersion is not yet clear. An additional small semicircle appears to exist at low frequency which could reflect a small electrode polarization.
K.-K. Back, H.L. Tuller /Solid State Ionics 75 (1995) 179-186
n
looIl
2000 2'
3000
4000
SO00
(Ohms)
Fig. 7. Complex impedance spectrum of a CuO/ZnO heterojunction with zero bias in air at 20°C. The high frequency segment is enlarged in the insert.
2500
.
.
.
1500 -
lOOI)'.
l
l
400
200
0 2000 -
. .
.
l
.
l
.
2OOO Z'
3nOO
(Ohms)
Fig. 8. Complex impedance spectra of a CuO/ZnO heterojunction at 80°C obtained in oxygen and argon atmospheres. The high frequency segment is enlarged in the insert.
Complex impedance spectra taken at 80°C under oxygen and argon atmospheres respectively are shown in Fig. 8. As the gas environment changes from oxygen to argon, the interface resistance, equal to the diameter of the semicircle in the (Z’-Z”) plot, decreases significantly. Furthermore, the spectra in argon is no longer represented by a single semicircle
183
but rather by a superposition of a smaller semicircle at high and a larger semicircle at low frequencies. The shift from oxygen to argon also results in an increase in the high frequency intercept on the real axis. The more complex nature of the impedance spectra under reducing conditions may explain the high values of A obtained from the dc Z-V measurements. The low frequency intercept on the real axis of the impedance plot corresponds in principle to the dc resistance value. If the spectra corresponded to an equivalent circuit dominated by the heterojunction one would expect good correspondence between the measured I- Vcharacteristics and those predicted by the model. However, if bulk and electrode resistances become comparable to those of the interface, substantial deviations from the predicted Z-P’ characteristics are expected as observed in argon (see Figs. 5 and 8 ). The degradation of the rectifying character of the junction in argon cannot be connected with the reduction of the CuO film to Cu10 or Cu metal since CuO is thermodynamically stable to much lower Po,‘s at these low temperatures [ 17 1. A more likely explanation is related to a reduction in the oxygen adsorbed at the CuO/ZnO interface. We will return to this issue below. The ac impedance spectrum obtained in oxygen atmosphere, on the contrary, maintains a single semicircle as shown in Fig. 8. This indicates that the heterojunction interface dominates the overall electrical properties of the device. Fig. 9 shows the effect of reverse dc bias on the complex impedance spectra of the heterojunction measured at 140°C in oxygen. As the dc offset bias increases, the semicircle’s radius or equivalently, the interface resistance, decreases at higher dc voltages consistent with the I- L’characteristics of the heterojunction obtained under the same conditions. The voltage independent high frequency intercept Z’ value (approximately 30 Q ) is believed to correspond to the ohmic bulk resistance. From the dc voltage dependence of the complex impedance spectra, the junction capacitance was obtained as a function of reverse bias voltage in air at 20°C. The capacitance decreased with increasing dc reverse bias voltage. As shown in Fig. 10 the plot of C-* versus Vmeasured at 0.1 MHz was found to yield a linear dependence on reverse voltage up to - 0.5 V as expected for diode-like heterojunctions in the low voltage region. This indicates an abrupt junction with
K.-K. Baek, H.L. Tuller/SolidState Ionics 75 (1995) 175186
184
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3.3. Band diagram of CuO/ZnO heterojunction
Z’ (Ohms)
Fig. 9. Complex impedance spectra of a CuO/ZnO heterojunction in oxygen at 140°C with bias offset voltages of 0 V, -0.5 V, and - 1 V. The high frequency segment is enlarged in the insert.
61...‘...‘...‘...1 -0.4 -0.6 DC
-0.2
where NAand ND are the acceptor and donor concentrations in pCu0 and n-ZnO, respectively, and e, and ez are their respective dielectric constants, VDis the diffusion voltage, and Vis the bias voltage. From Eq. (4), the diffusion voltage ( VD) is obtained from the x-axis intercept, assuming that the interface states cannot follow the applied signal. At 0.1 MHz, V,= 1.55 V was obtained. This value can be compared with a value of 1.06 V reported earlier for a CuO/ZnO mechanical heterojunction [ 51. However, there are other studies on the CuO/ZnO heterojunction with thin film structures suggesting that VDmay be higher than the estimated value of 1.05 v [ 191.
-0.0
On the basis of the experimental data and reported values regarding energy band diagrams of CuO and ZnO, a tentative band diagram of a CuO/ZnO heterojunction can be constructed. From the resistivity measurements of the ZnO pellet (tie% 150-200 cm’/ V~~[l]),adonordensity(No)of-3.5XlO’~cm-~ is estimated. A band gap (I?,) of 3.3 eV with an energy difference between the conduction band edge and the Fermi level (S,= EC-E,) of N 0.05 eV have been reported for ZnO, while the electron affinity (X,,) was reported to be 4.2 eV [ 20,2 1 ] From the resistivity measurements of CuO thin films and a reported hole mobility of flP= 0.1 cm*/ V.s [22], an acceptor density (NA) of -2.1 x lOI cme3 is estimated. To obtain the energy difference between the conduction band edge and the Fermi level (6,=&-E,), the following relation is used [I41
0.;
NA=2(2nm*kTlh2)3’2
Bias (V)
Fig. 10. Plot of C2 versus V for a CuO/ZnO heterojunction at 100 kHz in air at 20°C.
the depletion region in the vicinity of the heterojunction interface broadening with increasing reverse bias. Under these conditions the capacitance per unit area is also described by conventional heterojunction theory [I81
exp(&/kT)
.
(5)
By taking a hole effective mass, ( m*/mO) = 7.9 [ 201 S, is estimated to be N 0.12 eV above the valence band. A band gap (I&,) of 1.35 eV with an electron affinity (x,) of 4.07 eV are reported by Koffyberg and Benko for CuO [ 221. From these values, the diffusion voltage ( V,) is determined to be 1.05 eV, and the conduction band edge discontinuity (A&) is determined through the relation
cZ={(qN,N,~,~2)/2(~,N*+~*hTD)}I(VD-~/), (4)
AE,=E,,-I/,-&-6,.
(6)
K.-K. Baek, H.L. Tuller /Solid State Ionics 75 (I 995) 179-186
VACUUM
LEVEL
185
tra confirmed an increasing deviation from ideal junction behavior with decreased PO,. This appears to be connected with increasing contributions from the device series resistance as the junction barrier height decreases. An energy band diagram of the CuO/ZnO junction was synthesized from data obtained in this and other studies. The estimated diffusion potential of 1.05 eV was lower than the 1.55 eV value extracted in this study from C-V measurements. The discrepancy is suspected to be related to the existence of interface states neglected in the derivation of the energy diagram.
% 8
Acknowledgement
NA = 2.lxlOl7cm-3
ND = 3.5x1016cm-3
Fig. 11. Synthesized energy band diagram of a p&O/n-ZnO heterojunction.
Putting these values into Eq. (6 ) yields AE, = 0.12 eV. Based on these values and neglecting interface states, the synthesized band diagram for the CuO/ ZnO heterojunction is shown in Fig. 11. While the above energy band diagram provides a first order estimate of the CuO/ZnO heterojunction, it fails to include contributions from interface states which are evidenced in the dispersion in capacitance with frequency and the larger derived value for the diffusion voltage. Further studies are required to clarify the role that the interface states play in influencing the junction characteristics.
4. Summary CuO/ZnO heterojunctions were successfully fabricated by sputtering thin film p-type CuO onto ntype ZnO polycrystalline substrates. Well defined ZYrectifying characteristics were obtained which were characterized by thermionic excitation over a potential barrier rather than tunneling. The forward bias current increased with decreasing oxygen partial pressure indicating a close correlation between barrier height and adsorbed oxygen at the interface. Both the dc I- Vcharacteristics and the ac impedance spec-
This work was supported by the National Science Foundation through the MIT Center of Materials Science and Engineering under Grant #90-22933DMR. HLT thanks the Organizing Committee of the International Symposium on Interfaces in Ionic Materials, Schloss Ringberg, Germany, March 7-l 1, 1994, for their kind invitation to attend the conference.
References [ I] A.R. Hutson, Phys. Rev. 108 (1957) 222. [2] E.J.W. Verwey, P.W. Haaijman,F.C. RomeijnandG.W.van Oosterhaut, Philips Res. Rep. 5 (1950) 173. [ 31 M.H. Sukkar and H.L. Tuller, in: Surface and Near-Surface Chemistry of Oxide Materials, eds. J. Nowotny and L.-C. Dufour (Elsevier, Amsterdam, 1988) pp. 61 l-667. [4] H.J. Leamy, GE. Pike and C.H. Seager, eds., Grain Boundaries in Semiconductors (North Holland, Amsterdam 1982). [ 51 Y. Nakamura, N. Yoshioka, M. Miyayama, H. Yanagida, T. Tsurutani and Y. Nakamura, J. Electrochem. Sot. 137 (1990) 940. [6] R.A. Marra, Y. Nakamura, S. Fujitsu, and H. Yanagida, J. Am. Ceram. Sot. 69 ( 1986) C143. [ 71 J. Tamaki, T. Maekawa, N. Miura, and N. Yamazoe, Sensors Actuators B 9 (1992) 197. [ 81M. Mitsuoka, A. Otofuji and T. Arakawa, Sen. Act. B 9 ( 1992) 205. [9] K.-K. Baek, A. Broniatowski and H.L. Tuller, in: Intergranular and Interphase Boundaries in Materials, eds. P. Komninou and A. Rocher, Mater. Sci. Forum, Vols. 126128 (1993) pp. 737-774. [lo] K.-K. Baek, PhD Thesis, (Massachusetts Institute of Technology, Cambridge, MA, January 1994).
186
K.-K. Baek, H.L. Tuller /Solid State Ionic5 75 (I 995) 179- 186
[ II] L.J. Van Der Pauw, Philips Res. Rept. 13 ( 1958) 1. [ 121 V.F. Drobny and D.L. Pulfrey, Thin Solid Films 6 I ( 1979) 89.
[ 131 P. Boher, P. Houdy, C. Schiller and L.J. Van Ijzendoorn, Thin Solid Films 174 ( 1989) 91.
[ 141 T. Nakamura, H. Yamamoto and M. Tanaka, Physica B 165/166(1990)1489.
[ 151 SM. Sze, Physics of Semiconductor Devices, 2nd Ed. (Wiley, New York 199X ) p. 18. [ 161 G.E. Pike, in: Grain Boundaries in Semiconductors, eds. H.J. Leamy, G.E. Pike and C.H. Seager (North-Holland, Amsterdam, 1980) pp. 369-380.
[ 171 T.B. Reed, Free Energy of Formation of Binary Compounds (The MIT Press, Cambridge, MA, 197 I ) [ 181 SM. Sze, Physics of Semiconductor Devices, 2nd. Ed. (Wiley, New York, 1992 ) p. 124. [ 191 T. Suzuki, T. Yamazaki, T. Kageyama and T. Yata, J. Mat. Sci.24(1989)2110. [20] P.S. Nayar, J. Appl. Phys. 53 (1982) 1069. [21] P.S. Nayar and A. Catalano, Appl. Phys. Lett. 39 ( 1981) 105. [22] F.P. Koffyberg and F.A. Benko, J. Appl. Phys. 53 (1982) 1173.
ELSEVIER
Atmosphere sensitive CuO/ZnO junctions Kwang-Ki Baek *, Harry L. Tuller Centerfor Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract Rectifying CuO/ZnO heterojunctions were successfully fabricated by sputtering thin film p-type CuO onto n-type ZnO polycrystalline substrates. The thermally activated, forward-bias current increased with decreasing oxygen partial pressure pointing to a close correlation between barrier height and adsorbed oxygen at the interface. Both dc I-Vand ac impedance measurements were utilized to characterize the deviation from ideality of the characteristics and were found to increase with decreased oxygen partial pressure, (Po, ). An energy band diagram was synthesized and examined in light of the measurements performed in this study. Keywords: CuO/ZnO heterojunction; Copper oxide; Zinc oxide
1. Introduction Semiconducting or ptype
oxides
conductivity.
normally
Thus,
while
exhibit
either
it is possible
n to
prepare, e.g. n-type ZnO [ 1 ] or p-type NiO [ 2 1, it has not been possible to achieve reasonably high conductivities of the opposite sign in these oxides near ambient conditions. Consequently, junction studies have been limited to oxide semiconductor-metal Schottky barriers [3] or electrically active grain boundaries [ 4 1. Recently, however, studies of oxide p-n heterojunctions have been initiated. These have included CuO/ZnO [5], NiO/ZnO [6], CuOfSnO, [7], LnMO,/SnOz (Ln = rare earth, M = Cr, Co, Mn, Fe ) junctions [ 81. Many of these heterojunctions have been assembled in a rather primitive manner, relying on spring loading to achieve mechanical contact bePresent address: Hyundai Heavy Industries Co. Ltd., Hyundai Research Institute, Materials Research Dept., 1, Cheonha-Dong, Dong-Ku, Ulsan, Korea.
l
Elsevier Science B.V. SSDIOl67-2738(94)00172-3
tween adjacent polycrystalline, semiconducting pellets. Nevertheless, they exhibit well defined rectifying characteristics as evidenced by their reported current-voltage (Z-V) and capacitance-voltage ( CV) characteristics. Furthermore, they exhibit strong sensitivity to reducing gases and/or humidity [ 5-8 1, with the precise mechanisms for this sensitivity, however, remaining unclear. In this paper, we report preliminary results obtained on CuO/ZnO junctions prepared by thin film deposition of p-type CuO onto n-type polycrystalline ZnO substrates thereby bypassing the inherent poor reproducibility of mechanical heterocontacts. The temperature and oxygen partial pressure of the operating environment were controlled to evaluate their effect on the junction characteristics. A tentative energy band diagram of the CuO/ZnO heterojunction is proposed based both on experimental results obtained in this and other studies.
180
K.-K. Baek, H.L. Tuller /Solid State Ionics 75 (1995) 179-186
2. Experimental 2.1. Preparation of CuO/ZnO heterojunctlon Nominally undoped, polycrystalline ZnO pellets were prepared by sintering following the procedure described elsewhere [ 9 1. ZnO plates with dimensions of approximately 5 X 5 X 2.5 mm were cut from the pellets and used as substrates for CuO thin film deposition. The surfaces of these ZnO plates were polished with 1 urn diamond paste to a mirror finish and cleaned with acetone. CuO thin films were reactively sputtered onto these plates by RF magnetron sputtering (K.J. Lesker, Super MS System) from a commercially available Cu target (99.99% purity, K.J. Lesker). The pressure in the chamber prior to plasma sputtering was maintained at around 5 x 1O-’ Torr, while the substrate temperature did not exceed 100°C. X-ray diffraction measurements and cross-sectional scanning electron microscope (SEM) observation of the prepared CuO/ZnO heterojunction confirmed that the deposited thin films were single phase CuO with a columnar structure [IO]. CuO films deposited on glass slides under like conditions were evaluated for film thickness and thereby deposition rate with a prolilometer (Sloan DEKTAK II ). Resistivity values of 130-470 !&cm were obtained from four-point resistivity measurements [ 111 performed on several batches of CuO films. To ensure ptype semiconductive behavior, it was important to keep the stoichiometry of the CuO phase on the oxygen excess side, i.e. CuO, +x(x> 0). For this purpose, Ar/O* mixtures with a ratio of ( l/4) were selected as the reactive gas for plasma sputtering [ 12- 141. The sign of the thermoelectric voltage confirmed that the CuO thin films and ZnO pellets prepared for this study were p-type and n-type, respectively. Sputtering conditions including gas composition, and the physical properties of the sputtered CuO thin films obtained under these conditions are summarized in Table 1. 2.2. Electrical characterization The dc I- Vcurves were measured using a personal computer controlled pica-ammeter/de voltage source (Hewlett-Packard, #4 140B), For ohmic contacts,
silver paste was applied to the face of the p-type CuO thin films and baked at 150°C for one hour, while Ga-In eutectic alloy paste was applied to the n-type ZnO. The schematic diagram of the heterojunction sample used for electrical characterization is shown in Fig. 1. The electronic transport characteristics of the junction were evaluated by measuring the temperature-dependent I- Vcharacteristics under different gas atmospheres at temperatures ranging from 25-230°C. As for the gas environments, the following gases were used for each measurement; air, argon, oxygen, (nitrogen+ 1% H2), (nitrogen+ 5% CO). All gases were purchased premixed and analyzed by the supplier (AirCo). The steady state capacitance measurements were made in the frequency range from 100 to 13 x 1O6Hz at an oscillator voltage of 10 mV RMS using an impedance analyzer (Hewlett-Packard, #4192A) controlled by a PC. AC impedance measurements, as a function of dc offset voltage, were conducted at high frequency to evaluate the diffusion or built-in voltage, VD, formed at the heterojunction.
3. Results and discussion 3.1. de current-voltage measurements Fig. 2 shows a typical Z-V curve of a CuO/ZnO heterojunction structure at 20°C in dry air. A rectifying character similar to a conventional p/n junction diode is observed. The current increased exponentially when CuO was positively biased (forward bias), while the reverse current is much reduced at low voltage followed by a linear dependence on reverse bias for larger values of bias. Substantial reverse currents only initiate at about - 4 V. These ZVcharacteristics confirm that ap/n heterojunction is formed at the interface of the CuO thin film and ZnO bulk plate and is consistent with the p-type nature of the CuO thin film and the n-type character of the ZnO bulk plate. The electronic transport characteristics of the junctions were evaluated by measuring the temperature-dependent I- I’ characteristics under different atmospheres. When current transport through the heterojunction is dominated by a thermal process, the
K.-K. Baek, H.L. Tuller /Solid State Ionics 75 (1995) 179- 186
181
Table 1 Sputtering conditions and properties of CuO thin films. Target:
cu (99.99%)
Initial vacuum pressure
5x
Deposition time: Plasma color: Average film thickness: Film color:
60 min. bright violet 6500 A black
Plasma gas: Power: Substrate temperature: Film resistivity:
20% Ar + 80% O2 300 w < 100°C 130-470 R cm
Ag Electrode
IO-‘Torr
mann constant, and T the absolute temperature. The reverse saturation current density (Jo) is given by
CuO (Thin Film)
Jr, =JoO exp( - @,/AkT)
,
(2)
where Qs is the barrier height determined by the dominant current transport mechanism. If tunneling dominates, the current-voltage relationship becomes instead [ 15 ] J=&(T)
In&a
Electrode
Fig. 1. Schematic diagram of the CuO/ZnO heterojunction. 0.010
0.005
I---
,
exp(ol’)
(3)
where (Yis a constant. Fig. 3 shows a plot of In J versus the forward bias voltage V in oxygen for various temperatures between 20°C and 198°C. Similar results are shown in Figs. 4 and 5 for measurements performed in air and argon, respectively. Several trends are apparent. First, J at constant bias is strongly temperature dependent consistent with the thermally activated model described in Eqs. ( 1) and (2 ). This becomes even more evident from a plot of In Jo versus reciprocal temper-4.00
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-14.00, (I.0
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DC Bias
(V)
Fig. 3. Current density versus forward voltage characteristics of a CuO/ZnO heterojunction in oxygen at various temperatures.
182
K.-K. Baek. H.L. Tuller /SolidState
Ionics 75 (1995) 179-186
26°C .*
DC Bias
1000/T (K)
(V)
Fig. 4. Current density versus forward voltage characteristics of a CuO/ZnO heterojunction in air at various temperatures. -4.00 2 I 5°C -6.00 .
.
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Fig. 6. Plot of In(&) versus 1000/T of a CuO/ZnO heterojunction under various gas atmospheres.
large forward bias conditions initiating at lower bias values for junctions with lower barrier heights. This appears to be consistent with the decreasing activation energies observed in Fig. 6 with decreasing PO, i.e. 0.69 eV in oxygen, 0.64 eV in air and 0.50 eV in argon. We can use Eq. (2) and the derived values of A to estimate values for the barrier heights (Pn which fall in the range of = l-l .5 eV. However, since the values of A differ so substantially from unity, we are hesitant to take these derived values too seriously. 3.2. ac impedance measurements
1
I
I
I
I
0.1
0.2
0.3
0.4
0.5
DC Bias
(V)
Fig. 5. Current density versus forward voltage characteristics of a CuO/ZnO heterojunction in argon at various temperatures.
ature shown in Fig. 6. Second, the extrapolated reverse saturation or leakage current is strongly atmosphere dependent, increasing with decreasing oxygen partial pressure as observed in Fig. 6. Lastly, the diode factor A, which is large and falls in the range of 2.23.1 at low V, also depends on temperature and atmosphere tending to increase with decreasing PO,. Furthermore, the lower the Paz, the greater the curvature observed in the In J versus V plots. This may reflect the onset of a series resistance contribution for
Fig. 7 shows a typical complex impedance spectrum of a CuO/ZnO heterojunction sample measured at 20°C in air with zero bias. The data points obtained over the frequency range from 100 Hz to I 3 MHz are fitted to a single semicircle which is somewhat depressed relative to the Z’ axis, showing capacitive dispersion. The semicircle is attributed to the electrically active interface in the heterojunction. The deviation of the high frequency intercept Z’ value from zero (approximately 60 a) is a series resistance which must include the bulk resistance. The dispersion may be due to the trapping states at the interface [ 161 although the exact cause of the dispersion is not yet clear. An additional small semicircle appears to exist at low frequency which could reflect a small electrode polarization.
K.-K. Back, H.L. Tuller /Solid State Ionics 75 (1995) 179-186
n
looIl
2000 2'
3000
4000
SO00
(Ohms)
Fig. 7. Complex impedance spectrum of a CuO/ZnO heterojunction with zero bias in air at 20°C. The high frequency segment is enlarged in the insert.
2500
.
.
.
1500 -
lOOI)'.
l
l
400
200
0 2000 -
. .
.
l
.
l
.
2OOO Z'
3nOO
(Ohms)
Fig. 8. Complex impedance spectra of a CuO/ZnO heterojunction at 80°C obtained in oxygen and argon atmospheres. The high frequency segment is enlarged in the insert.
Complex impedance spectra taken at 80°C under oxygen and argon atmospheres respectively are shown in Fig. 8. As the gas environment changes from oxygen to argon, the interface resistance, equal to the diameter of the semicircle in the (Z’-Z”) plot, decreases significantly. Furthermore, the spectra in argon is no longer represented by a single semicircle
183
but rather by a superposition of a smaller semicircle at high and a larger semicircle at low frequencies. The shift from oxygen to argon also results in an increase in the high frequency intercept on the real axis. The more complex nature of the impedance spectra under reducing conditions may explain the high values of A obtained from the dc Z-V measurements. The low frequency intercept on the real axis of the impedance plot corresponds in principle to the dc resistance value. If the spectra corresponded to an equivalent circuit dominated by the heterojunction one would expect good correspondence between the measured I- Vcharacteristics and those predicted by the model. However, if bulk and electrode resistances become comparable to those of the interface, substantial deviations from the predicted Z-P’ characteristics are expected as observed in argon (see Figs. 5 and 8 ). The degradation of the rectifying character of the junction in argon cannot be connected with the reduction of the CuO film to Cu10 or Cu metal since CuO is thermodynamically stable to much lower Po,‘s at these low temperatures [ 17 1. A more likely explanation is related to a reduction in the oxygen adsorbed at the CuO/ZnO interface. We will return to this issue below. The ac impedance spectrum obtained in oxygen atmosphere, on the contrary, maintains a single semicircle as shown in Fig. 8. This indicates that the heterojunction interface dominates the overall electrical properties of the device. Fig. 9 shows the effect of reverse dc bias on the complex impedance spectra of the heterojunction measured at 140°C in oxygen. As the dc offset bias increases, the semicircle’s radius or equivalently, the interface resistance, decreases at higher dc voltages consistent with the I- L’characteristics of the heterojunction obtained under the same conditions. The voltage independent high frequency intercept Z’ value (approximately 30 Q ) is believed to correspond to the ohmic bulk resistance. From the dc voltage dependence of the complex impedance spectra, the junction capacitance was obtained as a function of reverse bias voltage in air at 20°C. The capacitance decreased with increasing dc reverse bias voltage. As shown in Fig. 10 the plot of C-* versus Vmeasured at 0.1 MHz was found to yield a linear dependence on reverse voltage up to - 0.5 V as expected for diode-like heterojunctions in the low voltage region. This indicates an abrupt junction with
K.-K. Baek, H.L. Tuller/SolidState Ionics 75 (1995) 175186
184
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Z’ (Ohms)
Fig. 9. Complex impedance spectra of a CuO/ZnO heterojunction in oxygen at 140°C with bias offset voltages of 0 V, -0.5 V, and - 1 V. The high frequency segment is enlarged in the insert.
61...‘...‘...‘...1 -0.4 -0.6 DC
-0.2
where NAand ND are the acceptor and donor concentrations in pCu0 and n-ZnO, respectively, and e, and ez are their respective dielectric constants, VDis the diffusion voltage, and Vis the bias voltage. From Eq. (4), the diffusion voltage ( VD) is obtained from the x-axis intercept, assuming that the interface states cannot follow the applied signal. At 0.1 MHz, V,= 1.55 V was obtained. This value can be compared with a value of 1.06 V reported earlier for a CuO/ZnO mechanical heterojunction [ 51. However, there are other studies on the CuO/ZnO heterojunction with thin film structures suggesting that VDmay be higher than the estimated value of 1.05 v [ 191.
-0.0
On the basis of the experimental data and reported values regarding energy band diagrams of CuO and ZnO, a tentative band diagram of a CuO/ZnO heterojunction can be constructed. From the resistivity measurements of the ZnO pellet (tie% 150-200 cm’/ V~~[l]),adonordensity(No)of-3.5XlO’~cm-~ is estimated. A band gap (I?,) of 3.3 eV with an energy difference between the conduction band edge and the Fermi level (S,= EC-E,) of N 0.05 eV have been reported for ZnO, while the electron affinity (X,,) was reported to be 4.2 eV [ 20,2 1 ] From the resistivity measurements of CuO thin films and a reported hole mobility of flP= 0.1 cm*/ V.s [22], an acceptor density (NA) of -2.1 x lOI cme3 is estimated. To obtain the energy difference between the conduction band edge and the Fermi level (6,=&-E,), the following relation is used [I41
0.;
NA=2(2nm*kTlh2)3’2
Bias (V)
Fig. 10. Plot of C2 versus V for a CuO/ZnO heterojunction at 100 kHz in air at 20°C.
the depletion region in the vicinity of the heterojunction interface broadening with increasing reverse bias. Under these conditions the capacitance per unit area is also described by conventional heterojunction theory [I81
exp(&/kT)
.
(5)
By taking a hole effective mass, ( m*/mO) = 7.9 [ 201 S, is estimated to be N 0.12 eV above the valence band. A band gap (I&,) of 1.35 eV with an electron affinity (x,) of 4.07 eV are reported by Koffyberg and Benko for CuO [ 221. From these values, the diffusion voltage ( V,) is determined to be 1.05 eV, and the conduction band edge discontinuity (A&) is determined through the relation
cZ={(qN,N,~,~2)/2(~,N*+~*hTD)}I(VD-~/), (4)
AE,=E,,-I/,-&-6,.
(6)
K.-K. Baek, H.L. Tuller /Solid State Ionics 75 (I 995) 179-186
VACUUM
LEVEL
185
tra confirmed an increasing deviation from ideal junction behavior with decreased PO,. This appears to be connected with increasing contributions from the device series resistance as the junction barrier height decreases. An energy band diagram of the CuO/ZnO junction was synthesized from data obtained in this and other studies. The estimated diffusion potential of 1.05 eV was lower than the 1.55 eV value extracted in this study from C-V measurements. The discrepancy is suspected to be related to the existence of interface states neglected in the derivation of the energy diagram.
% 8
Acknowledgement
NA = 2.lxlOl7cm-3
ND = 3.5x1016cm-3
Fig. 11. Synthesized energy band diagram of a p&O/n-ZnO heterojunction.
Putting these values into Eq. (6 ) yields AE, = 0.12 eV. Based on these values and neglecting interface states, the synthesized band diagram for the CuO/ ZnO heterojunction is shown in Fig. 11. While the above energy band diagram provides a first order estimate of the CuO/ZnO heterojunction, it fails to include contributions from interface states which are evidenced in the dispersion in capacitance with frequency and the larger derived value for the diffusion voltage. Further studies are required to clarify the role that the interface states play in influencing the junction characteristics.
4. Summary CuO/ZnO heterojunctions were successfully fabricated by sputtering thin film p-type CuO onto ntype ZnO polycrystalline substrates. Well defined ZYrectifying characteristics were obtained which were characterized by thermionic excitation over a potential barrier rather than tunneling. The forward bias current increased with decreasing oxygen partial pressure indicating a close correlation between barrier height and adsorbed oxygen at the interface. Both the dc I- Vcharacteristics and the ac impedance spec-
This work was supported by the National Science Foundation through the MIT Center of Materials Science and Engineering under Grant #90-22933DMR. HLT thanks the Organizing Committee of the International Symposium on Interfaces in Ionic Materials, Schloss Ringberg, Germany, March 7-l 1, 1994, for their kind invitation to attend the conference.
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