Journal of Luminescence 260 (2023) 119873
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Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Fast and broadband photoresponse in CdO thin film
L.M.B. Vargas a, M.J. da Silva a, S.de Castro a, A.L.C. Silva b, A.B. Paiva b, M.D. Teodoro b,
M.P.F. de Godoy b, D.A.W. Soares a, M.L. Peres a, *
a
b
Instituto de Física e Química, Universidade Federal de Itajubá, Itajubá, MG, CEP 37500-903, Brazil
Departamento de Física, Universidade Federal de São Carlos, São Carlos, SP, CEP 13565-905, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
CdO
Photoresponse
Photoluminescence
Annealing
Thermal annealing is recognized as an effective tool to manage the structural and electronic properties of oxide
thin films. Combined with the atmospheric environment control, additional issues concerning surface effects act
to change the performance of oxides for photodetectors purposes. The present investigation systematically
studies morphological and optical properties and electrical transport of CdO thin films using an as-grown
reference and annealed samples at 500 ◦ C under O2 and N2 atmospheres. As-grown CdO presents fast and
high photoresponses in a wide range of the visible spectrum, with an amplitude that reaches nearly 3000%. Also,
a transition from negative to positive photoresistance was observed for temperatures below 230 K. From the
analysis of the photoluminescence spectra, we noted two activation energies for as-grown CdO, whereas
annealed samples presented only one. Therefore, the additional defect level, reduced after annealing, is
responsible for the huge amplitude as well as the negative to positive transition in the photoresistance of asgrown CdO film.
1. Introduction
Modern gadgets employ widely transparent conductive oxides (TCO)
in flat plane displays, which are just one of the several applications of
this class of materials [1–3]. Despite the limiting toxicity of cadmium
(Cd), the high visible transparency and electrical conductivity of CdO
are very attractive for the development of transparent electrodes,
transparent thermal mirrors, solar cells, flat-panel displays, and other
upcoming applications [4–6]. Its cubic structure with a 4.7 Å lattice
parameter associated with a direct bandgap around 2.4 eV and degen­
erate n-type behavior are also interesting to match the silicon technol­
ogy. However, synthesis conditions or post-annealing procedures play a
significant role in the properties of CdO, such as nonlinear optical
properties [7–9]. In chemical-based synthesis, the nature of thin-film
morphologies and nanostructures presents changes in the optical
absorbance as well as significant signatures in photoluminescence
emissions [9–11].
In addition to optical transparency, the complementary phenomenon
of photoresponse (photoconductivity) exhibits exciting features in
semiconductors. Light can increase the number of free carriers in a
semiconductor when electrons are excited either from the valence to the
conduction band or from defect levels. Furthermore, the defects which
collaborate with electrical conduction, such as interstitial or vacancies,
are also responsible for anomalous effects such as persistent photocon­
ductivity and negative photoconductivity [12–15].
The present work investigates the electro-optical properties of CdO
thin films grown on silicon (Si) substrates by spray pyrolysis. The pho­
toresponse (PR) measurements performed on the as-grown CdO film
revealed a fast PR for several wavelengths at room temperature. An
analysis of PR for different temperatures indicates a huge PR increase of
nearly 3000% at 77 K. Besides, a transition from negative to positive PR
was observed as temperature lowered. We performed the annealing
process on the samples for different atmospheres to investigate the
origin of the effects observed.
2. Sample preparation
CdO thin films were grown on top of semi-isolated (001) silicon
substrates by spray pyrolysis technique employing an aqueous solution
containing cadmium acetate dehydrate (Cd(CH3COO)2⋅H2O - Synth
A1005.01.AE). The highly diluted solution with a molarity of 4 × 10− 3 is
pumped at a flux of 0.25 mL/min in a professional spray nozzle by
Spraying Systems Co. Dry compressed air gas at 1 bar atomizes the so­
lution on top of heated substrates at 300 ◦ C. After dropping the substrate
* Corresponding author.
E-mail address: marcelos@unifei.edu.br (M.L. Peres).
https://doi.org/10.1016/j.jlumin.2023.119873
Received 6 February 2023; Received in revised form 24 March 2023; Accepted 9 April 2023
Available online 18 April 2023
0022-2313/© 2023 Elsevier B.V. All rights reserved.
L.M.B. Vargas et al.
Journal of Luminescence 260 (2023) 119873
temperature to 220 ◦ C, the flux is interrupted, and the layers are
annealed up to 300 ◦ C when the cycle restarts. The formation of CdO
layers occurs due to the dehydration of bound water followed by the
decomposition and oxidation of Cd liberating CO2 and H2O as:
the samples, we performed Hall effect measurements in an automated
Keithley 237 Hall effect system with magnetic fields up to 1.0 T. For
photoresistance measurements, we used light-emitting diodes (LEDs)
with wavelengths λ = 398, 449, 568, 591, 634, and 940 nm and a
constant excitation current of 9 mA.
Cd(CH3 OO)2 • 2H2 O → Cd(CH3 OO)2 + 2H2 O
3. Results and discussion
Cd(CH3 OO)2 + 4O2 → CdO + 4CO2 + 3H2 O
The as-grown sample is regarded as the reference, and the samples
were annealed in controlled atmospheres of oxygen and nitrogen in a
tubular oven. The thicknesses of the samples are around 450 nm. The
annealing procedure starts at room temperature at a 40 ◦ C/min ramp up
to 300 ◦ C. After this step, the temperature rate is reduced to 25 ◦ C/min
up to 500 ◦ C. At this temperature, the samples are annealed for 1 h, and
then the temperature drops slowly in the gas atmosphere until the
ambient temperature.
The X-ray diffraction measurements were carried out using a Shi­
madzu XRD-6100 diffractometer equipped with a Cu–Kα radiation
source (λ = 1.5406 Å) in a Bragg–Brentano geometry (see Fig. 1(a) in
supplementary material). Scanning electron microscopy (SEM) and en­
ergy dispersive x-ray (EDX) measurements (see Fig. 1(b)–(e) in supple­
mentary material) were obtained using a Shimadzu electronic
microscope model SS-550 equipped with a secondary electron (SE) de­
tector with an acceleration voltage of 15 kV and exposed in a high
vacuum (10− 5 mbar). Electrical resistance measurements were per­
formed in the range of 10–300 K using a Physical Property Measurement
System (PPMS) from Quantum Design (see Fig. 2(a) in supplementary
material). The electric contact preparation follows the van der Pauw
geometry, using gold (Au) wires soldered with indium (In) contacts. We
used the standard four-probe AC lock-in technique with a constant
excitation current of 1.0 μA. To determine the carrier concentration of
Fig. 1(a) presents the PR as a function of time, in the temperature
range of 77–300 K, for the as-grown CdO sample using the blue LED
where the instants of LED switched on/off are indicated by the arrows.
Here, PR = (R − R0 )/fR0 , where R is the electrical resistance under
illumination, R0 is the electrical resistance in dark conditions, and f is
the normalized intensity of LED emissions concerning the red one, cor­
recting the PR for each wavelength intensity. In this figure, it is possible
to observe a clear transition from negative to positive PR as temperature
decreases below 240 K, and a considerable amplitude at 77 K is
observed, increasing nearly 3000% in comparison to the value in dark
conditions. In Fig. 1(b), we show the maximum amplitude of the PR
taken at 8 min as a function of temperature. In this picture, one observes
that the transition occurs around 230 K. As found in the literature, the
positive PR (negative photoconductivity) effect could result from the
interplay between the generation and recombination rates significantly
altered by defect levels [13]. We performed an annealing process in the
CdO films under different atmospheres to investigate the presence of
defect levels. This procedure could influence the defect levels role in the
PR measurements and electrical transport. Fig. 1(c) presents the carrier
concentration obtained from Hall measurement for the as-grown and
annealed samples. Samples annealed under N2 and O2 atmospheres
presented an increase in the carrier concentration as temperature
decreased. In contrast, the as-grown sample presented a slight increase
Fig. 1. (a) PR for the as-grown sample using the blue LED in the temperature range of 77–300 K. (b) Maximum amplitude of the PR at 8 min as a function of
temperature. In this picture, one observes that the transition occurs around 230 K. (c) Measured carrier concentration for the as-grown and annealed samples.
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L.M.B. Vargas et al.
Journal of Luminescence 260 (2023) 119873
temperature-dependent PL spectra of CdO as-grown and annealed in O2
and N2 atmospheres, respectively. The spectra of the films exhibit a
broad emission band in the visible range. At low temperatures, the
emission peaks are centered at 2.16 eV, 2.23 eV, and 2.69 eV for asgrown, O2 and N2 annealed samples, respectively. The presence of
optical-active defect states contributes to spectra broadening, as noted
in the high-energy tail for as-grown and O2-annealed samples. The redyellow is typical for CdO, although blue shift absorptions are observed
for CdO grown by MOCVD in high oxygen conditions [7]. It is worth
noting that the lower energy tail in the N2-annealed sample includes the
stronger emission of ~2.2 eV observed in the former samples.
The band emission maxima also present exciting features. Fig. 3
exhibits the temperature dependence of the band maxima as a function
of temperature for all samples. We can observe that the emission peaks
show a systematic redshift with increasing temperature for all samples,
which is stronger in sample annealed with N2. However, the band
maximum of the as-grown sample exhibits an anomaly around 220 K,
where a redshift is observed when the temperature decreases from 220 K
to 150 K (see arrow in Fig. 3(a)). It is worth noting that this temperature
is close to the transition from negative to positive PR, indicating that this
effect can occur due to the competition between optical active defect
states and band-band recombination.
The quenching of photoluminescence intensity I is well described by
the Arrhenius-like expression:
I = I0 / (1 + γ exp( − Ea / kB T))
(1)
where I0 , kB , γ, and Ea are the intensity at T = 0 K, the Boltzmann
constant, a constant related to carrier lifetimes, and the activation en­
ergy, respectively. For annealed samples, this expression fits the in­
tensity profile resulting in Ea = (13 ±1) meV, for the sample annealed in
O2 (inset of Fig. 3(b)), and Ea = (11 ±1) meV for the sample annealed in
N2 (inset of Fig. 3(c)) atmosphere. As the expression does not fit well for
the as-grown sample, the behavior needs an adjustment concerning the
presence of an additional defect level. In such case, for the as-grown
sample (inset of Fig. 3(a)), the best fit for the band intensity is ach­
ieved using a double-channel:
Fig. 2. Temperature-dependent PL spectra of CdO (a) as-grown and annealed
samples in (b) O2 and (c) N2 atmospheres.
from 300 to 250 K, and a drop between 250 and 200 K. This region is
close to the temperature where the transition from negative to positive
PR was observed, as indicated in Fig. 1(b). The drop in the carrier
concentration can indicate an additional energy level in the as-grown
sample, which is not present in the annealed samples, acting as a trap­
ping level. The presence of defect levels that could act as trapping levels
can be further investigated using photoluminescence measurements.
PL spectra were measured in a temperature range of 10–295 K when
excited by a 355 nm solid-state laser. Fig. 2(a)–(c) show the
I = I0 / (1 + γ 1 exp( − Ea1 / kB T) + γ2 exp( − Ea2 / kB T))
(2)
From the fitting, we extracted Ea1 = (43 ±3) meV e Ea2 =
(16 ±1) meV. These results indicate that only the as-grown sample has a
defect level with an activation energy of 43 meV, which is the possible
cause of the anomaly observed in Fig. 3(a). We attribute the transition
from negative to positive PR, and the drop observed in the carrier
Fig. 3. Temperature dependence of the maximum peak of the PL spectrum as a function of temperature for the as-grown and annealed samples treated with O2 and
N2 atmospheres in (a), (b), and (c), respectively. It is possible to observe that the emission peaks show a linear blue shift with decreasing temperature for all samples.
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L.M.B. Vargas et al.
Journal of Luminescence 260 (2023) 119873
concentration for the as-grown sample, to the existence of a deep level
with activation energy Ea1 = (43 ±3) meV. The lower activation en­
ergies found for all samples (16 meV, 13 meV, and 11 meV) are probably
of the same nature, which was slightly altered due to the annealing
process.
At room temperature, we also investigated the PR sensitivity for
different wavelengths, varying from ultraviolet to infrared range. The
annealed samples do not present PR for any of the used illuminations.
This occurs, probably, due to the higher carrier concentration present in
these samples compared to the as-grown (see Table 1 in Supplementary
material), i.e., the photogenerated carriers in these samples have no
significant contribution to the total electrical conductivity. On the other
hand, the as-grown sample presented fast PR in the whole spectra of
illumination used. Fig. 4(a) presents the PR measured for the as-grown
sample for several light wavelengths, and a clear squared shape PR is
observed for all colors. The instant when the LEDs are switched on and
off is indicated in the figure. Fig. 4(b) shows the maximum amplitude of
the PR observed in Fig. 4(a), taken at 8 min. From this figure, it is clear
that the amplitude of the PR increases (the negative values indicate that
resistance decreases under illumination) as the wavelength increases up
to 565 nm (green light) and decreases again to higher wavelengths. We
also verified the reproducibility of the PR, which is essential for devel­
oping sensor devices. Fig. 4(c) shows 3 cycles for each wavelength,
indicating that the PR is reproducible in the whole illumination spectra.
It is interesting to note that green, ultraviolet, and blue colors presented
a lower noise level than the other colors as their energies are above
bandgap. The fast PR in broadband of light spectra for the as-grown CdO
sample, as presented in Fig. 4, indicates the potential for application of
this material to electro-optical device sensors that can operate from
room temperature down to cryogenic temperatures with fast response
and low noise level.
We can also evaluate the performance of the photoresistance of the
as-grown sample extracting the response (τresp ) and recovery (τrec ) times
considering 90% of maximum conductivity and 90% of minimum con­
ductivity (see Fig. 3 in the supplementary material). The obtained values
are τresp = (0.80 ± 0.01) s and τrec = (1.90 ± 0.01) s. We observe that τrec
is nearly twice higher than τresp . This indicates that there is a small
persistent effect, which is not strong enough to alter the reproducibility
of the effect (see Fig. 4).
4. Conclusions
In this work, we investigated the influence of annealing on the
electrical transport, morphological, photoluminescence, and photo­
resistance properties of CdO samples as-grown and annealed at 500 ◦ C
under O2 and N2 atmospheres. We found that the morphology of the
samples is very sensitive to the atmosphere, which also affects the
electrical properties. The annealed samples presented smaller electrical
resistance and higher carrier concentration than the as-grown sample.
The PL measurements showed a systematic redshift as temperature in­
creases, excepting an anomaly observed in the as-grown sample. The
analysis of the PL spectra revealed common activation energies around
11–16 meV for all samples, but the as-grown presents an additional level
with 43 meV, suppressed by annealing. In addition, only the as-grown
sample presented fast PR, which exhibits a transition from negative to
positive photoresistance as temperature decreases. The low electrical
resistivity and fast PR in a wide range of visible spectrum for the asgrown CdO sample reveal its potential application as new opticalelectronic devices using the cost-effective spray-pyrolysis technique.
Fig. 4. (a) PR measurement for the as-grown sample for several light wavelengths. (b) The maximum amplitude of the PR observed in (a), taken at 8 min. (c) 3 cycles
of PR for each wavelength, indicating that the PR is reproducible in the whole spectra of illumination.
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L.M.B. Vargas et al.
Journal of Luminescence 260 (2023) 119873
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L. M. B. Vargas: Data curation (equal); Formal analysis (equal);
Investigation (equal); Writing – original draft (equal). M. J. da Silva:
Data curation (equal); Formal analysis (equal). S. de Castro: Data
curation (equal); Formal analysis (equal); Investigation (equal); Meth­
odology (equal); Writing – review & editing (equal). A. L. C. Silva and
A. B. Paiva: Data curation (equal), Formal analysis (equal); Investiga­
tion (equal). M. D. Teodoro: Data curation (equal); Formal analysis
(equal); D. A. W. Soares: Data curation (equal); Formal analysis (equal);
M. L. Peres: Formal analysis (equal); Resources (equal); Writing – re­
view & editing (equal). M. P. F. de Godoy: Conceptualization (equal);
Data curation (equal); Supervision (equal); Writing – review & editing
(equal).
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
This research was partially funded by Conselho Nacional de Desen­
volvimento Científico e Tecnológico – CNPq (grant 309230/2020-9).
Graduate fellowships from CAPES and CNPq are recognized as essential
to the scientific contributions of MJS, LMBV, ALCS, and ABP.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jlumin.2023.119873.
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