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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/locate/jnoncrysol
Low-temperature sol-gel ZrHfO2-PMMA hybrid dielectric thin-films for
metal oxide TFTs
⁎
M.G. Syamala Raoa, , M.A. Pacheco-Zuñigab, L.A. Garcia-Cerdab, G. Gutiérrez-Herediac,
J.A. Torres Ochoaa, M.A. Quevedo Lópezd, R. Ramírez-Bona,d
a
Centro de Investigación y de Estudios Avanzados del IPN, Unidad Querétaro, Apdo. Postal 1-798, 76001, Querétaro, Querétaro, Mexico
Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna Hermosillo No. 140, 25253 Saltillo, Coahuila, Mexico
c
Centro de Investigaciones en Óptica, A.C.Lomas del Bosque 115, Lomas del Campestre, 37150 León, Guanajuato, Mexico
d
Department of Materials Science and Engineering, The University of Texas at Dallas, 800 W. Campbell Road, Richardson 75080, TX, United States
b
A R T I C LE I N FO
A B S T R A C T
Keywords:
Sol-gel
Hybrid gate dielectric
Low-temperature
a-IGZO
ZnO
TFTs
In this work, we developed a novel inorganic-organic hybrid gate dielectric by combining zirconium and hafnium components to form zirconium hafnium oxide strongly linked with poly methyl methacrylate (ZrHfO2PMMA) and deposited at low temperature (200 °C) by sol-gel method. The obtained 108-nm thick high-quality
hybrid gate dielectric showed an exceptionally low surface roughness (0.9-nm), a low leakage current density
(7.7 × 10−6 A/cm2) and reasonable dielectric properties such as gate capacitance along with dielectric constant
(77 nF/cm2 & 9.4 @1 kHz) respectively. To examine the ZrHfO2-PMMA hybrid dielectric electrical properties we
constructed TFTs with room temperature r.f sputtered n-type metal oxide semiconductors, a-IGZO and ZnO. The
bottom gate fabricated a-IGZO TFTs driving at as low as below 6 V, the extracted field effect mobilities are
2.45 cm2/V. s, a low threshold voltage 1.2 V with large current ON/OFF ratio 107 respectively. On the other
hand, for comparison we employed ZnO TFTs by applying same hybrid dielectric system, the obtained parameters of bottom gate ZnO TFTs were good field effect saturation mobility of 12.8 cm2/V. s, threshold voltage of
1.8 V and current ON/OFF ratio of 103.
1. Introduction
In recent years, thin-film transistors (TFTs) have gained a lot attention due to their multiple areas of applications such as displays [1]
(television, mobile and computer screens), sensors [2], RFID smart
cards [2] and many others. To make true these potential applications,
metal oxide semiconductors-based thin-film transistors (MOTFTs) have
been successfully developed as alternative to a-Si based TFTs [3], which
are commonly employed in these applications. Recently, a variety of
metal oxide semiconductors including ZnO [4–6], ZTO [7, 8], a-IGZO
[9, 10], IZO [11] and In2O3 [11] have been extensively studied for
high-efficiency TFT applications, because of their reliability, high field
effect mobility and high optical transparency. Among these, ZnO is a
conventional semiconductor material which has been studied for decades for different applications, including as active channel in TFTs [12,
13]. On the other hand, Indium-Gallium-Zinc-Oxide (IGZO) [14, 15]
has emerged as one of the most promising semiconductor channel
materials in high-performance TFTs, because of its low temperature
processing and reliable electrical device properties. Therefore, this
⁎
semiconductor material is the focus of current interest because it has
been recognized as one of the most significant for future flexible electronic devices.
Along with semiconductors, gate dielectric materials also play a
crucial role and have strong influence on the TFTs electrical performance. That is why recently there has been intensive research to develop new materials for dielectric gate applications. The arising of
flexible electronic devices has imposed the low temperature processing
constraint to these materials, limiting the use of high k inorganic dielectrics, which otherwise have shown to be the best performance dielectric gates. Thus, at this regard, high k inorganic materials are still
very relevant for flexible electronics applications, where a lot of research has been focused on the decreasing the processing temperature
down to achieve compatibility with plastic substrates. Several high-k
inorganic oxides such as HfO2 [16–18], ZrO2 [19–21], Al2O3 [22], TiO2
[23] and Ta2O5 [24] are among the emerging alternative gate dielectric
materials because they fulfil the required dielectric characteristics for
TFT applications, such as high capacitance, dielectric constant and dielectric breakdown strength. Because of the similarity between their
Corresponding author.
E-mail address: grao@cinvestav.mx (M.G. Syamala Rao).
https://doi.org/10.1016/j.jnoncrysol.2018.08.014
Received 12 July 2018; Received in revised form 7 August 2018; Accepted 9 August 2018
0022-3093/ © 2018 Published by Elsevier B.V.
Please cite this article as: Syamala Rao, M.G., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.08.014
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
M.G. Syamala Rao et al.
2. Experimental procedure
physical and chemical properties, HfO2 and ZrO2 compounds are
known as twin oxides [25]. This similarity obeys to the same crystalline
structure of both oxides, which in turn can be explained by the chemical
similarity of Hf and Zr atoms. HfO2 has energy band gap of 5.86 eV and
15–30 dielectric constant, meanwhile the corresponding values for ZrO2
are quite similar 5.8 eV and 16–25, respectively. The great interest on
these materials for dielectric gate layer applications is strongly supported on their remarkable high dielectric constant, allowing high
performance of TFTs at low operating voltages [26, 27]. Nonetheless,
for these metal oxide gate dielectrics exhibit top performance, high
thermal annealing temperature (500 °C) is required, making unviable
their deposition on plastic substrates. To overcome such drawback,
high-k binary metal oxides have mixed with a third metal to obtain
ternary alloys such as HfAlOx [29], ZrAlOx [30], LaZrOx [31], HfLaOx
[32], HfSiOx [33] with improved dielectric properties, at lower processing temperatures. However, for good TFTs performance, these
ternary dielectric gate layers still need annealing treatments at temperatures above 300 °C. Therefore, the high processing temperatures is
also the bottleneck for the applications of these dielectric materials in
flexible electronics.
Dielectric materials based on organic-inorganic hybrid films and
polymer nanocomposites have recently emerged as promising alternative for dielectric gate layers. These materials result from the combination of organic and inorganic phases with an appropriate bonding
at molecular scale. The main purpose in the design of a hybrid material
is to take advantage on the complementary properties of both constituents to obtain a new material with predetermined properties. The
challenge for this is to accomplish the proper link between the organic
and inorganic phases, which otherwise are by nature incompatible the
most of times. As materials for dielectric gate applications, hybrid
materials offer processing at low temperatures by simple solution
methods, like those for organic materials. Furthermore, by choosing
conveniently the organic and inorganic components, the resulting hybrid dielectrics can achieve excellent dielectric and mechanical properties even when deposited on flexible substrates. High k materials are
natural choice as inorganic component for hybrid dielectric materials
with high capacitance. In addition to their adequate dielectric properties for TFT applications, hybrid dielectric layers have shown very flat,
smooth and crack-free surface, allowing a better interface with the
active semiconductor layers. This is a very important characteristic
because the surface properties of the dielectric gate layer are among the
most key factors which strongly influence the TFTs electrical performance. There are several reports in literature about the low temperature processing of hybrid dielectrics for applications as dielectric layers
in TFTs [34–38].
In this work, we developed a low temperature sol-gel process to
obtain a novel organic-inorganic hybrid dielectric by combining hafnium and zirconium compounds to form hafnium zirconium oxide
mixed with ploy methyl methacrylate (ZrHfO2-PMMA), in a crosslinked hybrid network. As coupling agent, to achieve compatibility
between organic and inorganic phases, it was used 3-glycidoxypropyltrimethoxysilane (GPTMS). Hybrid dielectric layers of ZrHfO2-PMMA
were deposited from precursors solutions by spin coating on several
types of substrates and then annealed at 200 °C. After dielectric characterization in MIM structures, the hybrid layers were tested as dielectric gate layers in two types of TFTs assembled on ITO-coated glass
substrates using either ZnO or a-IGZO semiconductor films, both deposited at room temperature by r.f. sputtering, as the active channel
layer. The analysis of the electrical response of these ZrHfO2-PMMA
hybrid gate-based TFTs shows, in both cases, very good performance
with parameters comparable to other similar devices with different
dielectric gate materials, including inorganic ones.
2.1. Materials synthesis and Thin-film formation
For the preperation of the sol-gel hybrid solutions, hafnium chloride
(HfCl4 98%), zirconium chloride (ZrCl4 99.99%), methyl methaacrylate
(MMA 98%), 3-glycidoxy propyl trimethoxy silane (GPTMS 98%), were
used as inorganic and organic precursors and coupling agent, respectively. Benzoyl peroxide (BPO 98%) was used to start the polymerization of MMA, ethyl alcohol (99.99%) and 2-propanol (99%) were used
as solvents and HNO3 (65.1%) as catalyzer. All the reactives were
purchased to Sigma-Aldrich and were used without further purification.
The HfCl4 and ZrCl4 precursor solutions were prepared seperately by
dissolving in ethanol and subsequently adding HNO3 and deionized (DI)
water to promote hydrolysis and condensation. The final concentration
of inorganic solution was made at 1:1 (Zr:Hf) molar ratio. For the
preparation of hybrid sloution the hydrolyzed GPTMS and PMMA solutions were simultaneously added into this inorganic solution then
continuously stirring for 4 h to complete hybrid solution formation.
Before deposition, the hybird solution was filtered through 0.2 μm PTFE
filter. The hybrid films deposition was done on various types of substrates by spin coating process at 5000 rpm for 30 s. After deposition,
the films were dried on hot plate at 1000C for the evaporation of solvents and subsequently thermally annealed at 200 °C for 12 h in a
conventional oven to complete the hybrid thin film formation.
2.2. Thin-film characterization
The resulting ZrHfO2-PMMA hybrid film surface morphology and
thickness measurements were performed by field emission scanning
electron microscopy (FE-SEM) (JSM e7610F). The surface roughness
on various morphologies was investigated by atomic force microscopy
(AFM) (Nanoscope IV). The thermal behaviour of hybrid powders obtained from the hybrid precursor solutions was performed by thermogravimetric analysis (TGA) (TA SDT-421 Q600). The identification of
chemical functional groups in the hybrid films was obtained by using
Fourier transform infrared spectroscopy (FTIR) (Gx Perkin Elmer) and
their chemical bonding were confirmed by X-ray photo electron spectroscopy (XPS) (Intercovamex-XPS110).
2.3. Capacitors (MIM) and TFTs fabrication
To examine the electrical properties of the ZrHfO2-PMMA hybrid
films, metal-insulator-metal (MIM) capacitor structures were fabricated
on ITO-coated glass substrates and completed with evaporated top gold
contacts, using a shadow mask with 0.002 cm2 circular area. The capacitance-voltage (CeV) was measured on these MIM structures employing a HP 4284A analyser at frequencies in the 1 kHz to 1 MHz
range. The leakage current-voltage (IeV) was measured by using a
4200 Keithely semiconductor analyser. For the fabrication of TFTs, ITO
coating on glass substrates was used as bottom gate electrode, on which
the hybrid dielectric gate layer was deposited. Then, the ZnO or IGZO
thin films as active layers were deposited by r.f. sputtering at room
room temperature, and finally patterned Al source and drain electrodes
were deposited on top of the semiconductor layer by means of e-beam
evaporation (Temescal BJD-1800) through shadow mask in US-class
clean room environment. The electrical response of the assemlbed TFTs
was investigated by measuring IeV curves, in dark at room atmosphere,
with a 4200 Keithely semiconductor analyser.
3. Result and discussion
3.1. Structural and chemical characterization
To understand the thermal behaviour of ZrHfO2-PMMA hybrid gate
dielectric, thermo gravimetric analysis (TGA) was conducted on sol-gel
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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
M.G. Syamala Rao et al.
Fig. 2. FTIR spectra of ZrHfO2-PMMA hybrid dielectric thin film annealed at
200 °C.
Fig. 1. Thermo gravimetric analysis (TGA) of ZrHfO2-PMMA hybrid powder
and PMMA.
vibration modes of HfeO and ZreO [38] bonds, confirming the formation of the mixed inorganic phase in the hybrid material. Thus, the
FTIR analysis is consistent with the formation of an organic-inorganic
hybrid material which components are joined through the coupling
agent molecules.
As complement to FTIR analysis, X-ray photo electron spectra (XPS)
measurements were employed to further investigate the chemical bonds
in the ZrHfO2-PMMA hybrid films. Fig. 3 (a) shows the XPS survey scan
of the hybrid film and it reveals all the chemical elements which it
contains, namely Hf, Zr, C, O and Si. Following, there are shown the
high resolution XPS spectra around the energy region of each one of
these elements. Fig. 3. (b) displays the XPS spectrum of Hf 4f region,
where a broad band with shoulders is observed. This signal, as well as
those in the subsequent XPS spectra, were deconvoluted by means of
Gaussian functions (shown in the spectra), and the results are plotted as
continuous lines. This way, four peaks at binding energies 15.4, 17.1,
17.8 and 19.4 eV can fit the spectrum in the Hf 4f energy region. All
these peaks correspond to the HfeO bond [40]. Fig. 3. (c) shows the
core level spectrum of Zr 3d which can be deconvoluted into two peaks
at binding energies of 183.5 eV related to Zr 3d5/2 and 186 eV related to
Zr 3d3/2, evidencing the ZreO ionic bond in the hybrid network [41].
Fig. 3. (d) shows the XPS spectrum in the O 1 s region. In this case, two
peaks, at binding energy of 532.7 and 531 eV fit the spectrum. The
binding energy peak at 531 eV is related to the ionic bond of oxygen
with either of metals and Si to form strong M-O-Si bond [ 42] (M = Zr,
Hf) in the hybrid film. Meanwhile, the higher binding energy peak at
532.7 eV can be assigned to oxygen bonded to H in the hydroxide (-OH)
[41] species. For the carbon, one single band was observed at 284.9 eV
energy associated to C]Oe[42] bond in the polymer. Finally, in Fig. 3.
(f) it is shown the XPS spectrum in the Si 2p region. The two peaks in
the deconvolution at 100.1 and 102 eV energy binding are assigned to
SieO and M-O-Si (M = Zr, Hf) [43] bonds, respectively. The XPS analysis of the hybrid films agrees with the FTIR measurements regarding
the identification of the functional chemical groups, and complements
this information providing the energy binding in these groups.
synthesized hybrid powders cured at 200 °C for 12 h and the results are
shown in Fig. 1. For comparison, the TGA curve for PMMA is also
plotted in this graph. The TGA curve of the hybrid material shows that
the weight loss takes place in three stages labelled as 1,2, 3 and indicated by the dotted lines. In the first stage, from room temperature to
about 250 °C, there is a small weight loss of 6.7%, which is produced by
the extraction of residual solvents trapped in the hybrid network due to
the low annealing temperature. Next, in the temperature range from
250 to 400 °C, there is a strong weight drop down to 75%. By comparison with the TGA curve of pure PMMA, which completely decomposes in this same temperature range, this major weight loss can be
attributed to the decomposition of the PMMA in the hybrid material.
Unlike PMMA, the weight of the hybrid sample keeps decreasing until it
stabilizes at about 600 °C. In this stage, the weight loss has been attributed to the decomposition of organic component, which is strongly
linked to inorganic one and consequently the polymer chains need more
energy to be broken. This is a clear evidence of the strong bonding
achieved between organic and inorganic phases in the hybrid material.
At the highest temperature in this curve, the final weight of the sample
is 62.4% and can be assigned to the amount of inorganic component
contained in the hybrid material. Therefore, the thermal decomposition
depicted by TGA analysis provides a rough estimation of the remaining
solvents, organic and inorganic weight percentages contained in the
hybrid material, and the results are shown in Fig.1.
The hybrid thin-films were further characterized by FTIR spectroscopy to identify the chemical functional groups that were formed in
the hybrid network after processing. The FTIR spectrum of the ZrHfO2PMMA film is shown in the Fig. 2 a) 4000–800 cm−1 and b)
500–380 cm−1 wavenumber range. The broad absorption peak centred
at about 3400 cm−1 is assigned to OeH stretching vibrations associated
to hydroxyl groups from either the incomplete condensation of the inorganic component or remaining solvents. Very weak signals at
2935 cm−1 and 2895 cm−1 are produced by symmetric and asymmetric
vibration modes of methyl groups from remaining solvents used in the
synthesis process. The signals related to the PMMA organic phase are
observed as low intensity, narrow band at 1640 cm−1 attributed to
vibration mode of the C]O group in the PMMA [36]. On the other side,
there is a strong narrow band at 1127 cm−1 which can be related to
vibration mode of either Si-O-Si bond in the coupling agent molecules
or the metal oxides coupled with the coupling agent forming M-O-Si
bond [39]. On the other hand, in the low wavenumber region, Fig. 2 b),
the bands observed at 387 cm−1 and 410 cm−1 are assigned to
3.2. Surface morphology of hybrid gate dielectric
The SEM image in Fig. 4 a) shows the surface morphology of the
hybrid film, where it is observed its homogeneity, flatness and
smoothness. In the inset, the cross-sectional SEM image of the hybrid
film shows its thickness uniformity with a value around 108 nm. On the
other hand, the surface roughness of the hybrid film was investigated
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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
M.G. Syamala Rao et al.
Fig. 3. (a) XPS survey scan (b) Hf 4f spectra (c) Zr 3d spectra (d) O 1 s spectra (e) C 1 s spectra (f) Si 2p spectra of ZrHfO2-PMMA hybrid dielectric film annealed at
200 °C.
measured in the applied voltage range of −5 to 5 V. Since the insulator
hybrid film thickness is 108 nm, the corresponding range of applied
electric field is −463 to 463 kV/cm. Fig. 5 b) shows the capacitance (C)
per unit area versus voltage (V) measured in the MIM structures at
different applied frequencies from 1 kHz to 1 MHz. It can be observed a
slight variation in capacitance values due to organic residuals which are
trapped in the hybrid film as determined from TGA and FTIR analysis.
This is because the low processing temperature of our hybrid films. By
using the equation for the capacitance of a parallel plates capacitor, the
dielectric constant of the hybrid dielectric layer was determined form
the CeV measurements. The values for the dielectric constant of the
ZrHfO2-PMMA hybrid films were 9.4, 8.2, 7.8 and 7.2 at 1 kHz, 10 kHz,
100 kHz and 1 MHz, respectively. The high capacitance and dielectric
values of our low temperature solution processed hybrid dielectric
layers can be related with the high dielectric constant of both inorganic
components, representing potential, important advantages for the operation of TFT electronic devices at low voltages.
by atomic force microscopy (AFM) in the tapping mode. Fig. 4 b) shows
a 5 × 5 μm AFM image of the hybrid film surface. The root mean square
(RMS) roughness of the hybrid film measured on this area was very low,
0.9 nm. Furthermore, the AFM image also shows a homogeneous surface without defects such as cracks or pores. The characteristics of the
hybrid film surface observed by SEM and AFM techniques are very
relevant for gate dielectric applications. The amorphous nature of the
hybrid film without grain boundaries can reduce leakage current and its
flat and smooth surface, very similar to polymer dielectrics, can improve the interface with the semiconductor active layer in a TFT, where
the electric transport takes place. As a result, gate dielectric layers with
such morphology characteristics have been reported in several works
about high-performance TFTs.
3.3. Gate dielectric properties
The dielectric properties of ZrHfO2-PMMA hybrid films were characterized on metal-insulator-metal (MIM) capacitor structures fabricated on ITO-coated glass substrates. Fig. 5 a) shows the leakage current
density (J) vs voltage (V) measured on the MIM structure. The measured leakage current density is in the range of 10−6–10−8 A/cm2 when
3.4. Semiconductors growth and characterization of TFTs
To examine the proper function of ZrHfO2-PMMA hybrid films as
Fig. 4. (a) FESEM image of hybrid dielectric thin-film (b) AFM topographic image of ZrHfO2-PMMA hybrid dielectric.
4
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
M.G. Syamala Rao et al.
Fig. 5. (a) leakage current vs voltage and (b) capacitance vs voltage of inorganic-organic ZrHfO2-PMMA hybrid dielectric thin-films performed on MIM
capacitor devices.
Fig. 6. AFM 3-D images of (a) a-IGZO (b) ZnO semiconductor growth on hybrid
gate dielectrics (c) schematic cross section view of bottom gate thin-film transistor (TFT).
dielectric gate layer, TFTs devices were assembled with a-IGZO and
ZnO semiconductors as channel layers. The deposition of both semiconductors layers (around 65 nm thickness measured by ellipsometry)
was made by r.f. magnetron sputtering at room temperature on the
hybrid dielectric gate layer. The semiconductor layer growth on the
hybrid dielectric one was analyzed by AFM. Fig. 6 shows the 3D AFM
image of a) IGZO and b) ZnO semiconductor layers on the hybrid dielectric. In both cases, the observed surface is homogeneous and
smooth with low rms roughness of 2.2 and 1.2 nm for IGZO and ZnO
films, respectively. With these characteristics of the semiconductor
layers, it is expected a smooth dielectric-semiconductor interface which
is beneficial for the charge transport in the semiconductor channel,
improving the TFTs performance. The TFTs were completed by evaporating Al top contacts on the semiconductor layers as source and
drain electrodes. The patterning of source and drain electrodes was
made by means of shadow mask process. The schematic structure of the
TFT is shown in Fig. 6 c).
The electrical response of the ZrHfO2-PMMA dielectric gate-based
TFTs was analyzed from output IDS-VDS and transfer IDS-VGS curves,
measured at room temperature. For the devices with IGZO channel
layer, these results are shown in Fig. 7 a) and b), respectively. The family of IDS-VDS curves in Fig. 7 a), measured in the drain voltage range
from 0 to 10 V in steps of gate voltage of 1 V, shows the linear behaviour of the drain current at low voltage and then its saturation at relatively low voltage. The corresponding transfer IDS-VGS curve in Fig. 7
b) is plotted in semi-log scales (right axis), where the lowest value of the
drain current at lower gate voltage is of the order of 10−10 A. This is the
drain current of the device when it is turned off (IOFF, off-current).
When the gate voltage increases a few volts (6 V), the drain current
increases to the order of 10−3 A, indicating that the devices turns on.
This is the drain current of the device when it is turned on (ION, oncurrent). Therefore, the current ratio between the on and off state of the
device is 107. The subthreshold slope (SS) can be determined from this
data by using the following Eq. 1:
SS = (∂ (log IDS )/ ∂VGS
and the result was 0.68 V/dec. On the other side, in Fig. 7 b) it is also
plotted the square root IDS versus VGS (right y-axis) curve. The threshold
voltage (Vth) and field effect mobility (μFE) of the device can be extracted from this curve by fitting the experimental data to the Eq. 2:
IDS = (WCG /2 L) μ FE (VGS − Vth )2
where W (500 μm) and L (60 μm) are the width and length of the
channel, and CG is the gate capacitance of the hybrid dielectric gate. By
substituting all the values, the calculated saturated mobility is
2.45 cm2/ V. s and the threshold voltage 1.2 V. The obtained field effect
saturation mobilities are extracted by using metal-insulator-semiconductor (MIS) structure and as seen in fig. S1(ESI). Interestingly, the
properties of the a-IGZO TFT by using low temperature ZrHfO2-PMMA
hybrid dielectric are comparable to other reported based on high
-temperature inorganic gate dielectrics. Xifeng et al.[15] reported solution processed ZrO2 as gate dielectric for a-IGZO TFTs with the mobilities of 0.8 cm2/V.s, threshold voltage 0.1 V and current ON/OFF
ratio was 104 respectively. On the other hand, Shao et al. [44] reported
with HfO2 as gate dielectric using anodic oxidation technique for IGZO
TFTs with the mobilities of 8.1 cm2/V.s, threshold voltage −0.15 V and
current ON/OFF ratio 107 respectively. Thus, it is expected that all
electrical parameters (e.g. μFE, Vth, Ion/Ioff, S·S) of our a-IGZO TFTs
are attributed to the favourable dielectric/semiconductor interface as
confirmed by the AFM topographic image of a-IGZO on hybrid gate
dielectric as shown in fig.6. (a).
The results for the ZrHfO2-PMMA hybrid dielectric gate TFTs with
ZnO active channel are shown in Fig. 7 c) and 7 d). The family curves of
these devices, Fig. 7 c), also display good saturation current at low
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M.G. Syamala Rao et al.
Fig. 7. (a-b) Output (ID versus VD) and Transfer (ID versus VG) characteristics of a-IGZO TFTs. (c-d) output characteristics (ID versus VD) and Transfer (ID versus VG) of
ZnO TFTs.
subthreshold slope of 0.68 V/dec. These TFTs operated at low voltages
under 6 V. Furthermore, to check the versatility of our novel hybrid
gate dielectric we also fabricated ZnO TFTs as channel layer. For these
devices, the performance was not as good as the a-IGZO ones, with
mobility of 12.8 cm2/Vs, threshold voltage of 2.5 V, current on/off ratio
of 103 and subthreshold slope of 3 V/dec. Finally, our low-temperature
solution processed a-IGZO TFT provides excellent performance with
feasible electrical properties and reliable for future flexible low voltage
electronic devices.
Table 1
Electrical performance of a-IGZO and ZnO TFTs with ZrHfO2-PMMA gate dielectric.
Dielectric
Semiconductor
μFET (cm2/
V.s)
Vth (V)
ION/IOFF
SS (V/dec)
ZrHfO2PMMA
a-IGZO
ZnO
2.45
12.8
1.2
2.5
107
103
0.68
3
voltage. Meanwhile, from the transfer curve, Fig. 7 d), the following
electrical parameters were determined: mobility 12.8 cm2/Vs are extracted by using metal-insulator-semiconductor (MIS) and as seen in fig.
S2 (ESI), threshold voltage 2.5 V, on-off current ratio 103, subthreshold
slope 3 V/dec. The electrical characteristics of both a-IGZO and ZnO
TFTs are summarized in Table 1.
Acknowledgements
The helpful technical support of Carlos Alberto Avila Herrera and
Adair Jimenez Nieto, and the partial financial support of CONACYTMexico (Project numbers 242549 and 271031) are greatly acknowledged. The support of the Fulbright Scholar Program is also acknowledged (RRB).
3.5. Conclusion
Appendix A. Supplementary data
In summary, we reported here the main characteristics of a novel
inorganic-organic ZrHfO2-PMMA hybrid dielectric material processed
at low-temperature (200 °C) by sol-gel solution method, and its applications to TFTs as dielectric gate layer. The ZrHfO2-PMMA dielectric
films are amorphous, highly transparent, exhibit smooth surface with
very low surface RMS roughness. FTIR and XPS measurements corroborated the formation of organic and inorganic components linked by
coupling agent molecules in the hybrid network. From TGA measurements the amounts of organic and inorganic components as well as
some solvents remnants were determined. The dielectric properties of
the hybrid films determined on MIM structures showed leakage current
comparable to other types of dielectric materials and good capacitance
with dielectric constant of 9.4 at 1 kHz. The assembled a-IGZO bottom
gated TFTs with ZrHfO2-PMMA hybrid dielectric gate exhibited good
electrical performance with high mobility of 2.45 cm2/Vs, large current
on/off ratio of 107, very low threshold voltage of 1.2 V with good
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.jnoncrysol.2018.08.014.
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