rus
Issue #1/2025
L. A. Mochalov, S. V. Telegin, E. A. Slapovskaya
Preparation and Properties Study of IGZO Thin Films Obtained by PECVD Method
Preparation and Properties Study of IGZO Thin Films Obtained by PECVD Method
DOI: 10.22184/1993-7296.FRos.2025.19.1.62.70
In this paper, the plasma-enhanced chemical vapor deposition (PECVD) method was used for the first time to obtain InGaZnO (IGZO) thin films with various stoichiometry, morphology, and phase composition. The films were synthesized using the setup described in detail in [1–5]. The precursors were elementary high-purity In, Ga, and Zn, the carrier gases were Ar and H2, and a mixture of (Ar-H2-O2) was used as a plasma-forming gas. Deposition was performed on the high-pure quartz glass substrates. The composition of the samples was determined by the energy-dispersive X-ray analysis. The obtained samples were also examined by the scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical profilometry methods. The electrical properties of the obtained films, such as the type, mobility, and carrier concentration, were established by the Hall effect measurements.
In this paper, the plasma-enhanced chemical vapor deposition (PECVD) method was used for the first time to obtain InGaZnO (IGZO) thin films with various stoichiometry, morphology, and phase composition. The films were synthesized using the setup described in detail in [1–5]. The precursors were elementary high-purity In, Ga, and Zn, the carrier gases were Ar and H2, and a mixture of (Ar-H2-O2) was used as a plasma-forming gas. Deposition was performed on the high-pure quartz glass substrates. The composition of the samples was determined by the energy-dispersive X-ray analysis. The obtained samples were also examined by the scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical profilometry methods. The electrical properties of the obtained films, such as the type, mobility, and carrier concentration, were established by the Hall effect measurements.
Preparation and Properties Study of IGZO Thin Films Obtained by PECVD Method
L. A. Mochalov, S. V. Telegin, E. A. Slapovskaya
Lobachevsky University, Nizhny Novgorod, Russia
In this paper, the plasma-enhanced chemical vapor deposition (PECVD) method was used for the first time to obtain InGaZnO (IGZO) thin films with various stoichiometry, morphology, and phase composition. The films were synthesized using the setup described in detail in [1–5]. The precursors were elementary high-purity In, Ga, and Zn, the carrier gases were Ar and H2, and a mixture of (Ar-H2-O2) was used as a plasma-forming gas. Deposition was performed on the high-pure quartz glass substrates. The composition of the samples was determined by the energy-dispersive X-ray analysis. The obtained samples were also examined by the scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical profilometry methods. The electrical properties of the obtained films, such as the type, mobility, and carrier concentration, were established by the Hall effect measurements.
Keywords: IGZO, PECVD, thin film transistors, gas sensors
Article received: 03.12.2024
Article accepted: 25.01.2025
1. INTRODUCTION
Indium gallium zinc oxide (IGZO) is one of the most promising multi-element transparent semiconductor materials that can replace the conventional amorphous silicon. It is transparent in the visible light range, has high thermal and chemical stability, as well as interesting electrophysical properties such as a large band-gap energy (about 3.5 eV at the room temperature [6], [7]), high carrier mobility reaching 30 cm2 V‑1 s‑1 [8], and ability to regulate the carrier concentration over a wide range.
Due to its unique properties, the thin amorphous InGaZnO films can be used to produce the field-effect transistors [9, 10], Schottky diodes [11], main electrodes for solar cells [12], gas sensors [13–15], photo- and X-ray detectors [16–18], data storage devices [19, 20], biosensors [21], flexible electronics elements [22, 23], and displays [24, 25].
The most popular production methods for the thin IGZO films include radiofrequency magnetron sputtering followed by annealing [26–29], centrifugation (spin-coating) [30, 31], and pulsed laser deposition [32, 33]. The disadvantages of these methods are insufficient chemical and phase homogeneity of the resulting materials, as well as impossibility of obtaining the samples with a wide range of gross composition.
The goal of this paper was to develop a new method for synthesizing thin films of the IGZO system using the high-purity elements directly as a source of macro-components, and the low-temperature nonequilibrium RF plasma discharge as an initiator of chemical interactions.
2. EXPERIMENT
The thin IGZO films were obtained by the low-pressure (0.1 torr) plasma-enhanced chemical vapor deposition (PECVD) on the high-pure quartz glass substrates. High-purity indium, gallium, and zinc were used as the original substances and put into the quartz reservoirs equipped with the external heaters. The source temperatures were 780 °C, 780 °C, and 265 °C for indium, gallium, and zinc, respectively. The source temperatures for gallium and zinc were previously determined in [34]. A photograph of the process and optical emission diagnostics are shown in Fig. 1.
The mixture was used as a carrier gas through a zinc source, argon was used as a carrier gas through the indium and gallium sources, and high-purity oxygen was used as a reactive gas. The low-temperature nonequilibrium RF plasma discharge with a frequency of 40.68 MHz and a power of 150 W was applied to initiate chemical reactions. The morphology and elemental composition of the obtained InGaZnO thin film samples were analyzed by the scanning electron microscopy method using a JEOL JSM-IT300LV microscope with X-ray microanalysis (X-MaxN 20 detector). The phase composition of thin IGZO films was studied using the X-ray phase analysis by an X-ray powder diffractometer LabX XRD‑6100 (Shimadzu, Japan) within the range of 20–75° with a pitch of 0.01°. The Hall effect studies of thin IGZO films were performed at a constant current (0.1–1 mA) in the Van der Pauw geometry using a Nanometrics HL5500PC setup that included a power source with a wide adjustment range of generated current, a voltmeter-electrometer with a high input resistance, and a permanent magnet (with the magnetic induction between the poles of 5170 G). The setup allowed measurements of resistance and the Hall effect of the samples with a sheet resistance of 0.1–1011 Ohm/sq, the charge carrier concentrations of 107–1020 cm‑3, and charge carrier mobility of 0.1–106 cm2/V∙s. The optical properties of the samples were studied by a Cary 5000 UV–Vis-NIR spectrophotometer (Agilent Technologies), operating in the spectral range of 175–3300 nm. A tungsten halogen lamp was used as a visible light source, and a deuterium arc lamp was used as a UV radiation source.
3. RESULTS AND DISCUSSION
3.1. Study of Elemental Composition and Surface Morphology of the Thin IGZO Film Samples
Fig. 2 and 3 show appearance of the IGZO system samples with the approximate compositions of 2:1:2, 1:1:1 and 2:1:1, the results of elemental analysis of the films, and a typical map of element distribution over the surface. The data obtained indicate that the resulting films have a fairly homogenous smooth structure and uniform distribution of elements over the surface.
The results of the influence study of the ratio of elements in the final film on the surface morphology are shown in Fig. 4.
According to the data obtained by the SEM method, the final film composition has a significant effect on the surface morphology. With an increase in the indium content in the film, the size of the structure-forming elements is decreased, and the surface becomes smoother. The film with the composition of In52Ga25Zn23O (~2:1:1) has the most homogenous surface that is also confirmed by the data obtained by the AFM method (Fig. 5).
The average arithmetic roughness of the films was 3.2–5.4 nm. Thus, based on the data obtained, it can be concluded that in this study, the IGZO film surface morphology depends on the indium content in the final film, since with an increase in its content, the size of the structure-forming elements is decreased, and the surface became smoother.
3.2. Structural Determination of the Thin IGZO Film Samples by the X-ray Diffraction Methods
Fig. 6 shows the results of X-ray phase analysis of IGZO samples with the approximate element ratios of 2:1:2, 1:1:1 and 2:1:1.
Regardless of the ratio of macro-components, all samples had an amorphous structure. The significant peak values indicating a crystalline structure of the samples were not found.
3.3. Determination of Carrier Mobility, Type and Concentration of Carriers by the Hall Effect Measurements in the Thin IGZO Films
In order to determine the carrier mobility, its type and concentration, in the IGZO samples with an approximate element ratio of 2:1:2; 1:1:1 and 2:1:1, the ohmic contacts were developed by soldering In at a temperature of 250 °C. Fig.7 shows the results of Hall measurements, namely the carrier concentration and mobility, their dependence on the ratio of macro-components.
All samples under study were the n-type semiconductors. The best carrier concentration (≈1·1016 cm‑3) and mobility (≈44 cm2/V·s) were achieved in the IGZO sample with a macro-component ratio of 2:1:1 (In52Ga25Zn23O). This sample also had the smoothest surface (Fig. 4c). The electrical properties of the samples were probably influenced by the indium content in the final film, since with an increase in its concentration, the carrier concentration and mobility were also increased. This dependence is consistent with the results obtained by other researchers [34].
Based on the obtained data, it can be concluded that the ratio of macro-components 2:1:1 in the thin IGZO films is optimal due to the increased indium content. The samples with such a composition have the highest concentration and mobility of carriers.
3.4. Study of Optical Properties of the IGZO Thin Film Samples. Determination of the Band-gap Energy by the Tauc Method
To study the optical properties in the range of 260–1 000 nm, the IGZO thin films with an approximate element ratio of 2:1:2; 1:1:1 and 2:1:1 were deposited on the glass substrates. The transmission spectra are shown in Fig. 8 (left). All samples have a similar optical transmission coefficient of about 80%. The optical transmission edge of the films was located at a wavelength of about 300 nm.
The optical transmission spectrum in the UV and visible range for the IGZO sample with a macro-component ratio of 2:1:1 was reconstructed in the Tauc coordinates to determine the band-gap energy by the Tauc method. The linear section of the Tauc plot was selected and extrapolated to the intersection with the abscissa axis (Fig. 8 (right)). The determined band-gap energy value was ≈3.62 eV that was slightly larger than the tabular value, probably due to the increased indium content in the film.
Thus, the optical transmission coefficient of the IGZO thin film samples in the wavelength range of 260–1000 nm was at the level of 80%, the optical transmission edge was at a wavelength of 300 nm. The obtained value of the band-gap energy was about 3.62 eV.
4. CONCLUSIONS
For the first time, the thin InGaZnO films of various stoichiometry, morphology and phase composition were obtained by the plasma-enhanced chemical vapor deposition (PECVD). The average arithmetic roughness of the films was 3.2–5.4 nm. The surface morphology of the IGZO films depended on the indium content in the final film, since with an increase in its content, the size of the structure-forming elements was decreased, and the surface became smoother. Regardless of the ratio of macro-components, all samples had an amorphous structure, no intense peak values were found in the diffraction patterns. The electrical properties of the samples were also affected by the indium content in the final film, since with an increase in its concentration, the concentration and mobility of carriers were also increased. The optical transmission coefficient of the samples was at the level of 80%, the optical transmission edge was located at a wavelength of 300 nm. The determined band-gap energy value was about 3.62 eV. Based on the data obtained, it can be concluded that the optimal ratio of macro-components in the IGZO thin films is 2:1:1.
FUNDING
These studies were supported by the Russian Science Foundation, grant No. 22-13-00053 (https://rscf.ru/project/22-13-00053/) and the educational Design Center of Electronics of the Lobachevsky University.
AUTHORS
Mochalov L. A., Dr of Sciences (Tech.), Associate Professor, Head of Laboratory of High-Purity Materials Technology, Research Institute of Chemistry, N. I. Lobachevsky Nizhny Novgorod State University; e-mail: mochalovleo@gmail.com; Nizhny Novgorod city, Russia
ORCID: 0000-0002-7842-8563
Telegin S. V., Cand. of Sciences (Chem.), Associate Professor, Senior Researcher, Laboratory of High-Purity Materials Technology, Research Institute of Chemistry, N. I. Lobachevsky Nizhny Novgorod State University; e-mail: telegin@chem.unn.ru; Nizhny Novgorod city, Russia
ORCID: 0000-0002-4960-3502
Slapovskaya E. A., Engineer, Laboratory of High-Purity Materials Technology, Research Institute of Chemistry, Lobachevsky Nizhny Novgorod State University; e-mail: slapovskaya@unn.ru; Nizhny Novgorod city, Russia
ORCID: 0009-0008-0670-2253
L. A. Mochalov, S. V. Telegin, E. A. Slapovskaya
Lobachevsky University, Nizhny Novgorod, Russia
In this paper, the plasma-enhanced chemical vapor deposition (PECVD) method was used for the first time to obtain InGaZnO (IGZO) thin films with various stoichiometry, morphology, and phase composition. The films were synthesized using the setup described in detail in [1–5]. The precursors were elementary high-purity In, Ga, and Zn, the carrier gases were Ar and H2, and a mixture of (Ar-H2-O2) was used as a plasma-forming gas. Deposition was performed on the high-pure quartz glass substrates. The composition of the samples was determined by the energy-dispersive X-ray analysis. The obtained samples were also examined by the scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical profilometry methods. The electrical properties of the obtained films, such as the type, mobility, and carrier concentration, were established by the Hall effect measurements.
Keywords: IGZO, PECVD, thin film transistors, gas sensors
Article received: 03.12.2024
Article accepted: 25.01.2025
1. INTRODUCTION
Indium gallium zinc oxide (IGZO) is one of the most promising multi-element transparent semiconductor materials that can replace the conventional amorphous silicon. It is transparent in the visible light range, has high thermal and chemical stability, as well as interesting electrophysical properties such as a large band-gap energy (about 3.5 eV at the room temperature [6], [7]), high carrier mobility reaching 30 cm2 V‑1 s‑1 [8], and ability to regulate the carrier concentration over a wide range.
Due to its unique properties, the thin amorphous InGaZnO films can be used to produce the field-effect transistors [9, 10], Schottky diodes [11], main electrodes for solar cells [12], gas sensors [13–15], photo- and X-ray detectors [16–18], data storage devices [19, 20], biosensors [21], flexible electronics elements [22, 23], and displays [24, 25].
The most popular production methods for the thin IGZO films include radiofrequency magnetron sputtering followed by annealing [26–29], centrifugation (spin-coating) [30, 31], and pulsed laser deposition [32, 33]. The disadvantages of these methods are insufficient chemical and phase homogeneity of the resulting materials, as well as impossibility of obtaining the samples with a wide range of gross composition.
The goal of this paper was to develop a new method for synthesizing thin films of the IGZO system using the high-purity elements directly as a source of macro-components, and the low-temperature nonequilibrium RF plasma discharge as an initiator of chemical interactions.
2. EXPERIMENT
The thin IGZO films were obtained by the low-pressure (0.1 torr) plasma-enhanced chemical vapor deposition (PECVD) on the high-pure quartz glass substrates. High-purity indium, gallium, and zinc were used as the original substances and put into the quartz reservoirs equipped with the external heaters. The source temperatures were 780 °C, 780 °C, and 265 °C for indium, gallium, and zinc, respectively. The source temperatures for gallium and zinc were previously determined in [34]. A photograph of the process and optical emission diagnostics are shown in Fig. 1.
The mixture was used as a carrier gas through a zinc source, argon was used as a carrier gas through the indium and gallium sources, and high-purity oxygen was used as a reactive gas. The low-temperature nonequilibrium RF plasma discharge with a frequency of 40.68 MHz and a power of 150 W was applied to initiate chemical reactions. The morphology and elemental composition of the obtained InGaZnO thin film samples were analyzed by the scanning electron microscopy method using a JEOL JSM-IT300LV microscope with X-ray microanalysis (X-MaxN 20 detector). The phase composition of thin IGZO films was studied using the X-ray phase analysis by an X-ray powder diffractometer LabX XRD‑6100 (Shimadzu, Japan) within the range of 20–75° with a pitch of 0.01°. The Hall effect studies of thin IGZO films were performed at a constant current (0.1–1 mA) in the Van der Pauw geometry using a Nanometrics HL5500PC setup that included a power source with a wide adjustment range of generated current, a voltmeter-electrometer with a high input resistance, and a permanent magnet (with the magnetic induction between the poles of 5170 G). The setup allowed measurements of resistance and the Hall effect of the samples with a sheet resistance of 0.1–1011 Ohm/sq, the charge carrier concentrations of 107–1020 cm‑3, and charge carrier mobility of 0.1–106 cm2/V∙s. The optical properties of the samples were studied by a Cary 5000 UV–Vis-NIR spectrophotometer (Agilent Technologies), operating in the spectral range of 175–3300 nm. A tungsten halogen lamp was used as a visible light source, and a deuterium arc lamp was used as a UV radiation source.
3. RESULTS AND DISCUSSION
3.1. Study of Elemental Composition and Surface Morphology of the Thin IGZO Film Samples
Fig. 2 and 3 show appearance of the IGZO system samples with the approximate compositions of 2:1:2, 1:1:1 and 2:1:1, the results of elemental analysis of the films, and a typical map of element distribution over the surface. The data obtained indicate that the resulting films have a fairly homogenous smooth structure and uniform distribution of elements over the surface.
The results of the influence study of the ratio of elements in the final film on the surface morphology are shown in Fig. 4.
According to the data obtained by the SEM method, the final film composition has a significant effect on the surface morphology. With an increase in the indium content in the film, the size of the structure-forming elements is decreased, and the surface becomes smoother. The film with the composition of In52Ga25Zn23O (~2:1:1) has the most homogenous surface that is also confirmed by the data obtained by the AFM method (Fig. 5).
The average arithmetic roughness of the films was 3.2–5.4 nm. Thus, based on the data obtained, it can be concluded that in this study, the IGZO film surface morphology depends on the indium content in the final film, since with an increase in its content, the size of the structure-forming elements is decreased, and the surface became smoother.
3.2. Structural Determination of the Thin IGZO Film Samples by the X-ray Diffraction Methods
Fig. 6 shows the results of X-ray phase analysis of IGZO samples with the approximate element ratios of 2:1:2, 1:1:1 and 2:1:1.
Regardless of the ratio of macro-components, all samples had an amorphous structure. The significant peak values indicating a crystalline structure of the samples were not found.
3.3. Determination of Carrier Mobility, Type and Concentration of Carriers by the Hall Effect Measurements in the Thin IGZO Films
In order to determine the carrier mobility, its type and concentration, in the IGZO samples with an approximate element ratio of 2:1:2; 1:1:1 and 2:1:1, the ohmic contacts were developed by soldering In at a temperature of 250 °C. Fig.7 shows the results of Hall measurements, namely the carrier concentration and mobility, their dependence on the ratio of macro-components.
All samples under study were the n-type semiconductors. The best carrier concentration (≈1·1016 cm‑3) and mobility (≈44 cm2/V·s) were achieved in the IGZO sample with a macro-component ratio of 2:1:1 (In52Ga25Zn23O). This sample also had the smoothest surface (Fig. 4c). The electrical properties of the samples were probably influenced by the indium content in the final film, since with an increase in its concentration, the carrier concentration and mobility were also increased. This dependence is consistent with the results obtained by other researchers [34].
Based on the obtained data, it can be concluded that the ratio of macro-components 2:1:1 in the thin IGZO films is optimal due to the increased indium content. The samples with such a composition have the highest concentration and mobility of carriers.
3.4. Study of Optical Properties of the IGZO Thin Film Samples. Determination of the Band-gap Energy by the Tauc Method
To study the optical properties in the range of 260–1 000 nm, the IGZO thin films with an approximate element ratio of 2:1:2; 1:1:1 and 2:1:1 were deposited on the glass substrates. The transmission spectra are shown in Fig. 8 (left). All samples have a similar optical transmission coefficient of about 80%. The optical transmission edge of the films was located at a wavelength of about 300 nm.
The optical transmission spectrum in the UV and visible range for the IGZO sample with a macro-component ratio of 2:1:1 was reconstructed in the Tauc coordinates to determine the band-gap energy by the Tauc method. The linear section of the Tauc plot was selected and extrapolated to the intersection with the abscissa axis (Fig. 8 (right)). The determined band-gap energy value was ≈3.62 eV that was slightly larger than the tabular value, probably due to the increased indium content in the film.
Thus, the optical transmission coefficient of the IGZO thin film samples in the wavelength range of 260–1000 nm was at the level of 80%, the optical transmission edge was at a wavelength of 300 nm. The obtained value of the band-gap energy was about 3.62 eV.
4. CONCLUSIONS
For the first time, the thin InGaZnO films of various stoichiometry, morphology and phase composition were obtained by the plasma-enhanced chemical vapor deposition (PECVD). The average arithmetic roughness of the films was 3.2–5.4 nm. The surface morphology of the IGZO films depended on the indium content in the final film, since with an increase in its content, the size of the structure-forming elements was decreased, and the surface became smoother. Regardless of the ratio of macro-components, all samples had an amorphous structure, no intense peak values were found in the diffraction patterns. The electrical properties of the samples were also affected by the indium content in the final film, since with an increase in its concentration, the concentration and mobility of carriers were also increased. The optical transmission coefficient of the samples was at the level of 80%, the optical transmission edge was located at a wavelength of 300 nm. The determined band-gap energy value was about 3.62 eV. Based on the data obtained, it can be concluded that the optimal ratio of macro-components in the IGZO thin films is 2:1:1.
FUNDING
These studies were supported by the Russian Science Foundation, grant No. 22-13-00053 (https://rscf.ru/project/22-13-00053/) and the educational Design Center of Electronics of the Lobachevsky University.
AUTHORS
Mochalov L. A., Dr of Sciences (Tech.), Associate Professor, Head of Laboratory of High-Purity Materials Technology, Research Institute of Chemistry, N. I. Lobachevsky Nizhny Novgorod State University; e-mail: mochalovleo@gmail.com; Nizhny Novgorod city, Russia
ORCID: 0000-0002-7842-8563
Telegin S. V., Cand. of Sciences (Chem.), Associate Professor, Senior Researcher, Laboratory of High-Purity Materials Technology, Research Institute of Chemistry, N. I. Lobachevsky Nizhny Novgorod State University; e-mail: telegin@chem.unn.ru; Nizhny Novgorod city, Russia
ORCID: 0000-0002-4960-3502
Slapovskaya E. A., Engineer, Laboratory of High-Purity Materials Technology, Research Institute of Chemistry, Lobachevsky Nizhny Novgorod State University; e-mail: slapovskaya@unn.ru; Nizhny Novgorod city, Russia
ORCID: 0009-0008-0670-2253
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