P -type electrical conduction in α-AgGaO2 delafossite thin films

Article abstract of DOI:10.1063/1.2204757

Thin films of delafossites of α-AgGa O2 were prepared on α- Al2 O3 (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 μV K-1) demonstrated the p -type conduction in the films. Transparent p-n heterojunctions on a glass substrate having a structure glass/ ITOn-ZnOp-AgGa O2 were fabricated. The ratio of forward to reverse current was more than 100 in the range of -2 to +2 V.

Full text of DOI:10.1063/1.2204757

p -type electrical conduction in α - Ag Ga O 2 delafossite thin films
K. A. Vanaja, R. S. Ajimsha, A. S. Asha, and M. K. Jayaraj
Citation: Applied Physics Letters 88, 212103 (2006); doi: 10.1063/1.2204757
View online: http://dx.doi.org/10.1063/1.2204757
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APPLIED PHYSICS LETTERS 88, 212103 ͑2006͒
p-type electrical conduction in ␣-AgGaO delafossite thin films
2
a͒
K. A. Vanaja, R. S. Ajimsha, A. S. Asha, and M. K. Jayaraj
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 2006͒
Thin films of delafossites of ␣-AgGaO were prepared on ␣-Al O ͑0001͒ and on Si ͑100͒ single
2
2
3
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
−
4
−1
−1
3
.2ϫ10 S cm . The positive sign of Seebeck coefficient ͑+70 ␮V K ͒ 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
2
than 100 in the range of −2 to +2 V. © 2006 American Institute of Physics.
͓
DOI: 10.1063/1.2204757͔
Transparent conducting oxides ͑TCO’s͒ find wide range
prepared by two step process. NaGaO , which has an ortho-
rhombic structure, was prepared by solid state reaction of
2
1
19
of applications. Most known TCO’s such as Sn doped In O
2
3
or SnO are n-type conductors. There has been considerable
stoichiometric amount of NaCO and ␤-Ga O . The reaction
2
3
2
3
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-
was carried out by successive heating at 650, 750, 850, 1000,
and 1050 °C for 24 h at each temperature. The ␤-NaGaO2
thus obtained is transformed into ␤-AgGaO by reacting
2
with excess molten AgNO at 280 °C for 24 h under nitro-
3
ting diodes. The report of p-type conductivity in CuAlO by
gen atmosphere. The AgGaO obtained through the ion ex-
change reaction has an orthorhombic structure. The excess
2
2
2
I
III
Kawazoe et al. has aroused much interest in A B O
2
I
III
delafossite ͑A =Cu, Ag, Pt and Pd; B =Al, Ga, In, Fe, Co,
Sc, and rare earths͒. Since the report of p-type conductivity
AgNO was removed by repeated washing with distilled wa-
3
ter. The ␤-AgGaO is then converted into ␣-AgGaO by
2
2
in CuAlO films, reports followed by observation of p-type
hydrothermal reaction in a Parr bomb at 250 °C. The length
of the reaction was four days. The reagents used were
␤-AgGaO and KOH ͑1M͒ solution. The ␣-AgGaO pre-
2
3
4
5
conductivity in transparent of CuScO , CuGaO , CuYO ,
2
2
2
6
and CuCrO . Bipolarity has been reported in the CuInO
2
2
2
2
7
delafossite with the realization of transparent p-n homo-
junction. All oxide transparent p-n junctions and ultraviolet
pared by direct hydrothermal synthesis contains more silver
impurities, where as phase pure ␣-AgGaO is obtained by
2
emitting diodes were fabricated using n-ZnO and
the hydrothermal conversion of ␤-AgGaO into ␣-AgGaO .
2
2
8
p-SrCuO . The recent improvement in the growth of high
The result on the synthesis of phase pure ␣-AgGaO is being
2
2
2
0
quality p-type ZnO ͑Ref. 9͒ has resulted in p-n ZnO based
published elsewhere. The ␣-AgGaO were pelletized by
2
1
0,11
junctions and p-n ZnO light emitting diodes.
Rectifying
cold isostatic press and then sintered at 350 °C for 5 h in air.
␣-AgGaO2 thin films were prepared on silicon and
Al O substrates by pulsed laser deposition. The third har-
behavior has been reported in delafossite oxide based struc-
tures, such as p-CuYO /i-ZnO/n-ZnO ͑Ref. 12͒ p-i-n junc-
2
2
3
tion, n-ZnO/p-CuAlO ͑Ref. 13 and 14͒ p-n junction, and
monics ͑355 nm͒ of a Q-switched Nd:YAG ͑yttrium alumi-
num garnet͒ laser ͑Spectra Physics Quanta Ray GCR series͒
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
2
1
5
n-CuInO :Sn/p-CuInO :Ca homojunction. All the p-type
2
2
delafossites reported so far are all based on copper delafos-
1
6
sites except for AgCoO . Acceptor doping of AgInO ͑Ref.
2
2
2
1
7͒ has not been successful in inducing p-type conductivity.
Among wide band gap semiconductors, p-type TCO’s are
particularly difficult to dope.
energy density of the laser was 1 J/cm per pulse. The cham-
ber was initially pumped down to a base pressure of
1
8
−
6
10 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 was used as sub-
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
2
2
3
by pulsed laser deposition ͑PLD͒ on silicon ͑100͒ and
strates. The films were allowed to cool down to room tem-
perature at the same oxygen pressure.
␣
-Al O ͑0001͒ single crystal substrates. The electrical and
2 3
optical properties were investigated and the results are pre-
The thickness of the deposited ␣-AgGaO films was
2
sented. The PLD grown AgGaO films were used for the
fabrication of transparent p-n heterojunction.
measured using a stylus profiler ͑Dektak 6M Stylus profiler͒
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 ͑a͒, ␣-AgGaO ͑b͒, and the
2
The direct synthesis of ␣-AgGaO by conventional solid
2
state reaction of the constituent oxides at high temperature
2
2
was not successful. The sintered disks of ␣-AgGaO were
thin film ͑c͒. The crystalline phase identified in the thin film
2
¯
sample was found to belong to ␣-AgGaO of R3m space
2
a͒Electronic mail: mkj@cusat.ac.in
group. The impurity phase in the x-ray diffraction can be
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On: Thu, 11 Dec 2014 00:22:55
0

212103-2
Vanaja et al.
Appl. Phys. Lett. 88, 212103 ͑2006͒
FIG. 1. The x-ray diffraction pattern of ͑a͒ ␤-AgGaO powder, ͑b͒
2
*
␣
-AgGaO , powder, and ͑c͒ ␣-AgGaO thin film. indicates ͑002͒ peak of
2
2
the impurity ␤-AgGaO phase.
2
FIG. 3. ͑Color online͒ Transmission spectra of the ␣-AgGaO thin film.
2
2
identified as the ͑002͒ peak of the ␤-AgGaO . The high reso-
Inset shows the plot of ͑␣h␯͒ vs h␯.
2
lution transmission electron micrograph ͑HRTEM͒ of the
␣
-AgGaO films grown under the same deposition condi-
2
perature is 68 meV. The ␴ vs 1/T plot is not well fitted by a
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. 2͒ show
parallel line of ions at intervals of 2.225 Å for most of the
grains. This lattice spacing coincides with d spacing of
1/2
straight line ͑shown in Fig. 4͒. However, the log ␴T vs
1/4
1
/T plot ͑inset of Fig. 4͒ is close to a straight line, sug-
22
gesting that a variable range hopping is dominant in posi-
tive hole conduction at the top of valence band which is
observed in similar delafossite materials. The type of car-
4,6
riers responsible for conduction are holes which were iden-
tified from the Seebeck coefficient measurement. The posi-
␣
-AgGaO ͑104͒. Similar growth has been observed in nano-
−1
2
tive Seebeck coefficient of 70 ␮V K at room temperature
2
1
crystalline CuAlO . The d spacing observed do not match
2
indicates that the conduction is p type. The x-ray diffraction
with that of Ag O or ␤-AgGaO . Energy dispersive x-ray
2
2
shows the presence of ␤-AgGaO impurity phase and the
2
analysis shows that the ratio of Ag/Ga is 1.02, which is close
film is composed of nanosize particles. The conductivity of
to the atomic ratio of ␣-AgGaO . The grains in the films
−7
−1
2
the ␤-AgGaO bulk ͑2.5ϫ10 S cm ͒ ͑Ref. 23͒ is smaller
2
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
−6
−1
than that of ␣-AgGaO bulk samples 1ϫ10 S cm ͑Ref.
2
2
0͒ 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
␣
-AgGaO films in the visible region. The films have more
2
have been estimated by measuring the transference number
than 50% transparency in the visible region. The inset shows
24
using the dc polarization method. The electron beam
2
the plot of ͑␣h␯͒ vs h␯, where ␣ is the absorption coeffi-
evaporated gold forms the blocking electrodes. The variation
of current has been noted under a steady dc potential of
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 films
2
−
4
−1
was 3.2ϫ10 S cm . The activation energy at high tem-
1
/2
1/4
FIG. 2. TEM picture of ␣-AgGaO thin film grown on carbon coated copper
FIG. 4. Conductivity ␴ vs 1/T and inset shows log ␴T vs 1/T of the
2
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grid and the inset shows the atomic scale image of the film.
AgGaO thin film.
2
On: Thu, 11 Dec 2014 00:22:55

212103-3
Vanaja et al.
Appl. Phys. Lett. 88, 212103 ͑2006͒
3
4
.2ϫ10−4 S cm−1 and the optical band gap was estimated as
.12 eV.
A transparent p-n junction thin film diode on glass sub-
strate was fabricated using p-type ␣-AgGaO and n-ZnO.
2
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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2
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1
7
͑
inset of Fig. 5͒. The typical current voltage ͑I-V͒ character-
1
1
8
9
istics of the p-n heterojunction diode is shown in Fig. 5. The
n-ZnO/p-AgGaO junction shows a rectifying characteris-
2
0͑
1981͒.
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2
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2
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In conclusion, we have grown wide band gap
24
25
␣
-AgGaO p-type conducting thin film by pulsed laser depo-
2
sition. The room temperature conductivity was measured as
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