Solar Energy Materials & Solar Cells AgGaO
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electrical conduction in -AgGaO
2 delafossite thin films
Optoelectronics Devices Laboratory, Department of Physics, Cochin University of Science and Technology,
Kochi 682022, India
Received 28 December 2005; accepted 26 March 2006; published online 23 May 2006
Thin films of delafossites of -AgGaO
were prepared on -Al
0001 and on Si 100 single crystal substrates by pulsed laser deposition. The films have a band gap of 4.12 eV and a transparency of more than 50% in the visible region. The electrical conductivity at 300 K was 3.2 10
-4 S cm -1 . The positive sign of Seebeck coefficient +70 V K -1 demonstrated the p-type conduction in the films. Transparent p-n heterojunctions on a glass substrate having a structure glass/ITO/n-ZnO/p-AgGaO
were fabricated. The ratio of forward to reverse current was more than 100 in the range of -2 to +2 V. © 2006 American Institute of Physics. DOI: 10.1063/1.2204757 Transparent conducting oxides TCO™s find wide range of applications.
1 Most known TCOâ„¢s such as Sn doped In 2 O 3 or SnO 2 are n-type conductors. There has been considerable interest in finding p-type electrical conductivity in wide band gap semiconductors. These wide band gap p-type semicon- ducting oxides with n-type transparent conducting oxides can lead to the development of UV and blue emitting light emit- ting diodes. The report of p-type conductivity in CuAlO 2
Kawazoe et al.
has aroused much interest in A I B III O 2 delafossite A
I =Cu, Ag, Pt and Pd; B III =Al, Ga, In, Fe, Co, Sc, and rare earths . Since the report of p-type conductivity in CuAlO
films, reports followed by observation of p-type conductivity in transparent of CuScO 2 , 3 CuGaO 2 , 4 CuYO 2
and CuCrO 2 . 6 Bipolarity has been reported in the CuInO 2 delafossite
with the realization of transparent p-n homo- junction. All oxide transparent p-n junctions and ultraviolet emitting diodes were fabricated using n-ZnO and p-SrCuO
The recent improvement in the growth of high quality p-type ZnO Ref. 9 has resulted in p-n ZnO based junctions and p-n ZnO light emitting diodes.
Rectifying behavior has been reported in delafossite oxide based struc- tures, such as p-CuYO
/i-ZnO/n-ZnO Ref. 12 p-i-n junc- tion, n-ZnO/p-CuAlO 2
Ref. 13 and 14 p-n junction, and n-CuInO 2
:Sn/p-CuInO 2 :Ca homojunction. 15 All the p-type delafossites reported so far are all based on copper delafos- sites except for AgCoO 2 .
Acceptor doping of AgInO
17 has not been successful in inducing p-type conductivity. Among wide band gap semiconductors, p-type TCOâ„¢s are particularly difficult to dope.
In the present letter, we report the growth of silver delafossite thin film with p-type conductivity and moderate transparency in the visible region. The p-type silver delafos- sites are the -AgGaO and the thin films were deposited by pulsed laser deposition PLD on silicon 100 and
-Al 2 O 3 0001 single crystal substrates. The electrical and optical properties were investigated and the results are pre- sented. The PLD grown AgGaO
2 films were used for the fabrication of transparent p-n heterojunction. The direct synthesis of -AgGaO
by conventional solid state reaction of the constituent oxides at high temperature was not successful. The sintered disks of -AgGaO
were prepared by two step process. NaGaO
, which has an ortho- rhombic structure, was prepared by solid state reaction
of stoichiometric amount of NaCO 3 and -Ga 2 O 3 . The reaction was carried out by successive heating at 650, 750, 850, 1000, and 1050 °C for 24 h at each temperature. The -NaGaO 2
thus obtained is transformed into -AgGaO 2 by reacting with excess molten AgNO 3 at 280 °C for 24 h under nitro- gen atmosphere. The AgGaO 2 obtained through the ion ex- change reaction has an orthorhombic structure. The excess AgNO 3 was removed by repeated washing with distilled wa- ter. The -AgGaO 2 is then converted into -AgGaO 2 by hydrothermal reaction in a Parr bomb at 250 °C. The length of the reaction was four days. The reagents used were -AgGaO
and KOH 1M solution. The -AgGaO
pre- pared by direct hydrothermal synthesis contains more silver impurities, where as phase pure -AgGaO 2 is obtained by the hydrothermal conversion of -AgGaO 2 into -AgGaO 2 . The result on the synthesis of phase pure -AgGaO 2 is being published elsewhere. 20 The -AgGaO 2 were pelletized by cold isostatic press and then sintered at 350 °C for 5 h in air. -AgGaO 2 thin films were prepared on silicon and Al
2 O 3 substrates by pulsed laser deposition. The third har- monics 355 nm of a Q-switched Nd:YAG yttrium alumi- num garnet laser Spectra Physics Quanta Ray GCR series was focused onto a rotating target. The repetition rate of the laser pulse was 10 Hz with a pulse width of 9 ns and the energy density of the laser was 1 J/cm
per pulse. The cham- ber was initially pumped down to a base pressure of 10 -6 mbar. Oxygen gas was then introduced into the cham- ber and the working pressure of oxygen was controlled at 0.01 mbar. The substrate to target distance was kept at 3.7 cm. The substrate temperature was kept at 250 °C for silicon substrates and 400 °C when Al
O 3 was used as sub- strates. The films were allowed to cool down to room tem- perature at the same oxygen pressure. The thickness of the deposited -AgGaO
2 films was measured using a stylus profiler Dektak 6M Stylus profiler as 180 nm. The crystalline nature of the films was identified by x-ray diffraction using Cu K line. Figure 1 shows the diffraction pattern of -AgGaO
2 a , -AgGaO 2 b , and the thin film c . The crystalline phase identified in the thin film sample was found to belong to -AgGaO 2 of R3 ¯ m space group. The impurity phase in the x-ray diffraction can be a
0003-6951/2006/88 21 /212103/3/$23.00
© 2006 American Institute of Physics
88, 212103-1
Downloaded 05 May 2007 to Redistribution subject to AIP license or copyright, see 2
identified as the 002 peak of the -AgGaO
. The high reso-
lution transmission electron micrograph HRTEM of the
films grown under the same deposition condi- tions of film growth on silicon substrates but on carbon coated copper grids is shown in Fig. 2. The nucleation and growth of the film in the form of nanorods with an average diameter of 20 nm and a length up to 270 nm were observed. The atomic scale images of the films inset of Fig. 2 show parallel line of ions at intervals of 2.225 Ã… for most of the grains. This lattice spacing coincides with d spacing of -AgGaO
104 . Similar growth has been observed in nano- crystalline CuAlO 2 . 21 The d spacing observed do not match with that of Ag
O or -AgGaO 2 . Energy dispersive x-ray analysis shows that the ratio of Ag/Ga is 1.02, which is close to the atomic ratio of -AgGaO 2 . The grains in the films grown on silicon substrates may be very small. Such nanos- cale particle and small sample thickness may be the reason for the very weak signal in the x-ray diffraction pattern. Figure 3 shows the optical transmittance of the -AgGaO 2 films in the visible region. The films have more than 50% transparency in the visible region. The inset shows the plot of h 2 vs h , where is the absorption coeffi- cient and h is the photon energy. The optical band gap is estimated as 4.12 eV. The dc electrical conductivity of the samples was studied in the range of 50 to 300 K. The room temperature conductivity of the undoped AgGaO 2 films was 3.2 10 -4 S cm -1 . The activation energy at high tem- perature is 68 meV. The vs 1/T plot is not well fitted by a straight line shown in Fig. 4 . However, the log T 1/2 vs 1/T 1/4 plot inset of Fig. 4 is close to a straight line, sug- gesting that a variable range hopping 22 is dominant in posi- tive hole conduction at the top of valence band which is observed in similar delafossite materials. 4,6 The type of car- riers responsible for conduction are holes which were iden- tified from the Seebeck coefficient measurement. The posi- tive Seebeck coefficient of 70 V K -1 at room temperature indicates that the conduction is p type. The x-ray diffraction shows the presence of -AgGaO 2 impurity phase and the film is composed of nanosize particles. The conductivity of the -AgGaO 2 bulk 2.5 10 -7 S cm -1 Ref. 23 is smaller than that of -AgGaO 2 bulk samples 1 10 -6 S cm -1 Ref. 20 pelletized under similar conditions. The contribution from the impurity phase for the conductivity of the film may be very small. The Ag + ions contributing to the conductivity have been estimated by measuring the transference number using the dc polarization method. 24 The electron beam evaporated gold forms the blocking electrodes. The variation of current has been noted under a steady dc potential of FIG. 1. The x-ray diffraction pattern of a -AgGaO 2 powder, b -AgGaO 2
, powder, and c
thin film. * indicates 002 peak of the impurity -AgGaO 2 phase. FIG. 2. TEM picture of -AgGaO 2 thin film grown on carbon coated copper grid and the inset shows the atomic scale image of the film. FIG. 3. Color online Transmission spectra of the -AgGaO 2 thin film. Inset shows the plot of h
vs h . FIG. 4. Conductivity vs 1/T and inset shows log T 1/2 vs 1/T 1/4 of the AgGaO 2 thin film. 212103-2 Vanaja et al. Appl. Phys. Lett. 88, 212103 2006 Downloaded 05 May 2007 to Redistribution subject to AIP license or copyright, see 3 500 mV over a time of 2 h. The variation in current is very small and the estimated transference number is 0.03, indicat- ing that the ionic contribution to conductivity is negligible. The possible application of p-type TCO has been demon- strated by fabricating a p-n junction. The transparent hetero- junction diodes have a structure glass/ITO/n-ZnO/p
. The indium tin oxide thin film was deposited by rf magnetron sputtering as described else where. 25 The un- doped ZnO was deposited onto the ITO coated glass sub- strates by PLD at an oxygen partial pressure of 10 -4 mbar and at a laser power of 2 J/cm 2 for 20 min, resulting in a film of 200 nm thickness. The target to substrate distance was 5.5 cm and the substrate temperature was kept at 400 °C. The ZnO films deposited by PLD have a transpar- ency greater than 85% in the visible region, and the conduc- tivity is 44 S cm -1 . Depositing the p-type AgGaO 2 over the ZnO completed the device. The ITO/ZnO contact is Ohmic inset of Fig. 5 . The typical current voltage I-V character- istics of the p-n heterojunction diode is shown in Fig. 5. The n-ZnO/p-AgGaO
junction shows a rectifying characteris- tics with the forward current to reverse current ratio larger than 100 at applied voltage of -1.5 to +1.5 V. The turn on voltage of the device varied from 0.9 to 1.1 V from junction to junction. In conclusion, we have grown wide band gap -AgGaO 2 p-type conducting thin film by pulsed laser depo- sition. The room temperature conductivity was measured as
3.2 10 -4 S cm -1 and the optical band gap was estimated as 4.12 eV. A transparent p-n junction thin film diode on glass sub- strate was fabricated using p-type -AgGaO 2 and n-ZnO. This work is Supported by Board of Research in Nuclear Science, Government of India. One of the authors K.A.V. thanks Department of Science and Technology for the finan- cial assistance under women scientist scheme. The authors thank Professor P. V. Sathyam and Ummanda of Institute of Physics, Bhubaneswar for the TEM images and Dr. L. M. Kukreja of Raja Ramanna Centre for Advanced Technology, Indore for discussion.
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FIG. 5. The current voltage characteristics for the AgGaO
/n-ZnO p-n
heterojunction. Inset shows Ohmic nature of ITO/ZnO contact.
Vanaja et al.
Appl. Phys. Lett. 88, 212103 2006
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