Layered carbon nitride (g-C3N4) is a novel semiconducting and functional material for optoelectronic applications. The physical and chemical properties of g-C3N4 films differ depending on the preparation atmosphere. Herein, we deposited g-C3N4 films under a mixed oxygen (O2)-nitrogen (N2) gas atmosphere and studied their effects on the carrier transport properties. Although no significant change in the film orientation was observed, the deposition rate decreased as the O2 gas ratio in the mixed N2/O2 atmosphere increased. Despite their thinness, the luminescence intensity of g-C3N4 films deposited under an O2-containing atmosphere increased by 3.5–5.0 times compared to that deposited under an N2 atmosphere. With respect to voltage application, carrier transport owing to the thermionic emission and/or direct tunneling initially followed ohmic conduction, followed by insufficient trap-filled conduction. As the applied voltage increased further, fully trap-filled conduction was confirmed owing to Fowler–Nordheim tunneling. Moreover, the conductivity type could be changed to p-type and n-type using N2 gas and mixed N2/O2 gas atmospheres, respectively, during film deposition. In addition to the intrinsic transport properties, the intentionally formed Schottky barrier also affected the carrier transport; therefore, the diode-like rectifying behavior of the current density was achieved.

Layered carbon nitrides with various chemical compositions, such as g-C3N4,1–3 g-C2N,4,5 and g-C3N5,6,7 have received considerable attention as environmentally friendly functional materials because they are metal-free and/or non-toxic. These in-plane structures comprise C–N networks consisting of hexagonal sp2 and sp3 bonds stacked together by van der Waals forces. Powder-formed g-C3N4 facilitates hydrogen acquisition from water under visible-light irradiation, which is a metal-free photocatalyst.8–10 In addition to this application concept, g-C3N4 has also been investigated as a semiconducting material with a bandgap energy of 2.8 eV. The bandgap energy can be controlled by incorporating substitutional atoms into the g-C3N4 structure,11,12 which facilitates its use in the same manner as typical compound semiconductors. In addition, electronic carrier transport by electric fields has been demonstrated for crystalline films.13,14 Therefore, its potential for optoelectronic applications, such as solar cells and light emitting devices, have been investigated.15–18 To fabricate most semiconductor devices, conductivity control, including carrier type (p- or n-type), is an important aspect. A pristine g-C3N4 is usually an n-type semiconductor owing to the presence of numerous –NH/NH2 groups as donors on the sheet edge.19 However, the replacement of donor groups with acceptor groups may convert it to a p-type semiconductor.20 In addition, previous reports have demonstrated doping control for g-C3N4 by several methods, such as the incorporation of foreign (acceptor or donor) atoms21,22 and charge transfer from the layered surface.23 However, doping control within the structure would be effective from the viewpoint of stability and fabrication of heterojunctions.

In contrast, crystalline films can be deposited via chemical vapor deposition (CVD).24 The current through the film originates from the hole contribution, and the conductivity is p-type.25,26 Relatively high growth temperatures have been adopted to develop crystallized films, which are deposited under an N2 gas atmosphere. Although the chemical composition (N/C ratio) of ideal g-C3N4 is 1.33, that of the film is slightly lower, that is, C-rich or N-poor. Thus, it is possible that the effect of acceptor doping due to anti-site defect that is a substitution of N lattice with C atom.27 If the carrier type can be efficiently controlled via film deposition, electronic devices based on g-C3N4 film can be realized for developing novel semiconducting materials.

Notably, inert Ar or N2 gas flow is typically used for the CVD of g-C3N4 films.24,28 However, the physical properties can be changed by using a different ambient gas.29 Alternatively, powders for photocatalytic application are mostly synthesized in an air atmosphere and exhibit n-type conductivity.30–32 Therefore, we fabricated a thin film in an air atmosphere. The linear current–voltage (IV) characteristic from an ohmic contact between gold (Au) and g-C3N4 film deposited under an N2 gas atmosphere has previously been reported.25 In contrast, a Schottky contact between Au and g-C3N4 film prepared under an air atmosphere was indicated because an exponentially nonlinear IV characteristic was obtained, as shown in Fig. S1a. According to the Schottky–Mott rule, a significant change in the Fermi energy of the g-C3N4 film is suggested, and the conductivity type may be converted from p-type to n-type. In addition, the clear rectification characteristic of the diode performance shown in Fig. S1b was obtained from the stacked structure of the pn-junction, which is the van der Waals junction of the g-C3N4 films prepared under N2 gas and air atmospheres. Therefore, film deposition under air conditions should effectively control the n-type conductivity. Because air primarily consists of N2 (78.08%) and O2 (20.95%), a detailed investigation into the effect of changing these ratios can allow control over the conductivity of g-C3N4 films.

Herein, we report the CVD of g-C3N4 films in atmosphere containing O2 at various ratios. The lattice structure remained almost unchanged, regardless of the O2 gas ratio during film deposition. However, the N/C ratio of the deposited films approached that of ideal g-C3N4 (1.33), and the luminescence intensity improved. Although the carrier transport properties along the out-of-plane direction indicated n-type conductivity, anomalous carrier transport based on the space-charge-limited conduction (SCLC) model was observed. Moreover, in addition to the intrinsic carrier transport properties, the impact of the intentionally formed Schottky barrier at the interface was confirmed. These results demonstrate the effect of O2 gas on the physical properties and provide novel insights into the synthesis and electronic properties of g-C3N4 films.

The films were deposited using a home-built hot-wall CVD system consisting of a commercially available tube furnace and a quartz tube.24,29 The epi-polished c-plane sapphire (α-Al2O3) substrate was cleaned with acetone, methanol, and deionized water by ultrasonic washing. Melamine (C3H6N6) powder, which was heated and supplied in gas phase, was used as the source molecule. A small glass tube sealed at one end was used to hold the precursor and substrate simultaneously and achieve a highly efficient supply of the vapor phase. The substrate and precursor were placed in a small glass tube in the CVD furnace, as illustrated in Fig. 1. Before deposition, the CVD furnace was evacuated using a vacuum pump and purged, and the film was deposited under a mixed N2/O2 gas flow at atmospheric pressure. The O2 ratio in the mixed N2/O2 atmosphere was between 0 and 30%, and the total flow rate was 2 L/min. The substrate temperature was varied from 550 to 600 °C, and the typical source temperature was ∼400 °C. The deposition time was maintained at 2.5 h. After the deposition, the source was cooled to room temperature (∼20 °C), and the substrate was cooled. The exhaust gas was removed using a water trap because the thermal decomposition of melamine produces ammonia. The films were characterized by the ω/2θ curve under symmetrical out-of-plane diffraction conditions using a multifunctional x-ray diffraction (XRD) instrument with CuKα radiation (SmartLab, Rigaku Corp.). X-ray photoelectron spectroscopy (XPS) was performed to characterize the chemical bond state using AlKα radiation as the x-ray source (PHI Quantera II™, ULVAC-PHI, Inc.). The deposition rate was estimated from the thickness, which was confirmed using scanning electron microscopy (SEM) with cold-field emission (SU8000, Hitachi High-Tech Corp.). To prevent charge-up during SEM observations, a few-nanometer-thick platinum film was deposited on the film surface. The thickness per a layer of g-C3N4 is ∼0.34 nm, including van der Waals gap. Microscopic photoluminescence (PL) studies were performed (LabRAM HR Evolution, HORIBA, Ltd.) using the 325 nm line of a helium–cadmium laser for optical excitation, with an excitation power of 0.9 mW and a spot size of ∼15.7 µm. The PL signals were dispersed using an 800 mm monochromator and a cooled charge-coupled device.

FIG. 1.

Schematic of the thermal CVD equipment consisting of the tube furnaces and a quartz tube for the deposition of the g-C3N4 film.

FIG. 1.

Schematic of the thermal CVD equipment consisting of the tube furnaces and a quartz tube for the deposition of the g-C3N4 film.

Close modal

Mechanically exfoliated films were used to investigate the carrier transport properties in the out-of-plane direction. Electrode patterns were formed directly via spin coating with an ultraviolet-sensitive positive-type resist using microscope-based exposure equipment (UAT-IA-53M-SET, Arms System Co., Ltd.). The Al and Au electrodes were deposited by vacuum evaporation (VPC-1100, ULVAC KIKO., Inc.) and electron beam evaporation (VI–43N, CANON ANELVA Corp.), respectively, while maintaining the substrate temperature. The properties of the fabricated devices were measured at room temperature (∼297 K) in ambient air using a probe station equipped with tungsten needles connected to a source-measure unit (2450, Keithley Instruments).

The typical XRD profiles of the g-C3N4 films deposited at 600 °C under N2 and mixed N2/O2 atmospheres are shown in Fig. 2(a). The 006 diffraction peak of the c-plane sapphire substrate occurs at 41.6°. The 002 and 004 diffraction peaks of g-C3N4 appear at ∼27.6° and 57.2°, respectively.33,34 The full width at half maxima (FWHMs) of XRD peaks were between 0.68° and 0.74° with different O2 ratios; therefore, no drastic degradation of crystallinity is observed. Only the characteristic peaks corresponding to the substrate and 002 family diffractions for the g-C3N4 films are found, indicating the deposition of highly ordered crystalline films. However, the deposition rate, which was estimated using the film thickness and deposition time, depended on the O2 gas ratio in the N2/O2 gas mixture [Fig. 2(b)]. The film thickness was measured using a cross-sectional SEM image (Fig. S2). The deposition rate increased as the substrate temperature was increased from 550 to 600 °C. Thermal polymerization was sufficiently achieved with increasing temperatures, which was approximately consistent with the results of previous studies on the powdered form.35 In addition, the deposition rate decreased as the O2 gas ratio in the ambient gas was increased, regardless of the substrate temperature. This indicates the suppression of film formation during thermal CVD using the melamine precursor.

FIG. 2.

(a) Wide-range XRD profiles for the g-C3N4 films on the c-plane sapphire substrate deposited at 600 °C under N2 and N2/O2 gas flows. The weak peaks at 37.5° correspond to the CuKβ line. (b) Deposition rate of g-C3N4 films depending on the O2 gas ratio in an N2/O2 gas atmosphere. The film thickness was estimated from the cross-sectional SEM image. (c) Chemical compositions of C, N, and O for the g-C3N4 films deposited at 600 °C under varying O2 ratio in the mixed N2/O2 gas atmosphere. (d) Room-temperature PL spectra for the g-C3N4 films deposited at 600 °C under N2 and N2/O2 gas atmospheres.

FIG. 2.

(a) Wide-range XRD profiles for the g-C3N4 films on the c-plane sapphire substrate deposited at 600 °C under N2 and N2/O2 gas flows. The weak peaks at 37.5° correspond to the CuKβ line. (b) Deposition rate of g-C3N4 films depending on the O2 gas ratio in an N2/O2 gas atmosphere. The film thickness was estimated from the cross-sectional SEM image. (c) Chemical compositions of C, N, and O for the g-C3N4 films deposited at 600 °C under varying O2 ratio in the mixed N2/O2 gas atmosphere. (d) Room-temperature PL spectra for the g-C3N4 films deposited at 600 °C under N2 and N2/O2 gas atmospheres.

Close modal

For further investigation, the chemical compositions of g-C3N4 films deposited at 600 °C under a mixed N2/O2 gas atmosphere with various O2 ratios were analyzed using XPS as shown in Fig. 2(c). The raw spectral data of the C1s, N1s, and O1s core signals at various deposition temperatures are shown in Fig. S3.36–39 No systematic trend is observed for the O composition of the films regardless of the O2 ratio. In addition, no significant difference is observed even when deposited under an N2 gas atmosphere. Therefore, changing the atmosphere during deposition to mixed N2/O2 gas did not affect the incorporation of O atoms into the film. However, the C composition decreased and approached the ideal value as the O2 ratio in the mixed N2/O2 gas increased. A highly ordered crystal lattice was obtained by using a relatively high substrate temperature for depositing g-C3N4. Hence, the decomposition of the C–N bonds was enhanced, resulting in the residual of C atoms. These results suggest their removal by O2 gas, which is more effective at higher ratios. Figure 2(d) shows the room-temperature PL spectra of g-C3N4 films deposited at 600 °C in N2 gas and mixed N2/O2 gas atmospheres. As the spectral shape is similar, the luminescence origin remains unchanged.24,40 Incidentally, the potential of the g-C3N4 film as a luminescent material is comparable to that of gallium nitride for group-III nitrides.29 The PL intensity was improved by 3.5–5.0 times owing to deposition under a mixed N2/O2 gas atmosphere, despite the lower film thickness. In particular, non-radiative recombination caused by point defects originated from sp3 C–N bonds decreased as the N/C ratio approached the ideal value.24 Thus, crystallinity may be effectively improved by depositing films using N2 gas mixed with O2 gas.

The carrier transport properties along the out-of-plane direction were investigated. After the evaporation of the bottom Al electrode on an insulating substrate (α-Al2O3), the g-C3N4 film deposited under an N2/O2 gas atmosphere was exfoliated and transferred to the bottom Al electrode. Subsequently, the top Al electrode was formed. Au was used as the electrode metal for the films deposited under an N2 gas atmosphere. Figure 3(a) shows the typical current density–voltage (JV) characteristics of the g-C3N4 film deposited under an N2/O2 gas atmosphere. The film was deposited at 550 °C under the O2 gas ratio of 20% in the mixed N2/O2 gas atmosphere. The film thickness was 142 nm, and the device size was 9.88 × 10−6 cm2, which was assumed to be the top contact area [inset of Fig. 3(a)]. Other JV curves for various deposition conditions, including the N2 gas atmosphere, are shown in Fig. S4. A double logarithmic plot was used to fit the data points, which revealed three dominant components with different slopes ni and two inflection points (transition voltages, Vtr and VTFL). The first region at a low applied voltage (V < Vtr) implies Ohm’s law, that is, the slope n1 is 1. Specifically, this characteristic is represented by
J=1ρLV=qnμLV,
(1)
where ρ is the resistivity, q is the elementary charge, n is the carrier density of the free charge carriers in thermal equilibrium, μ is the carrier mobility, and L is the spacing between electrodes (film thickness). The first transition voltage (Vtr) was 2.5 V and is given by41–43,
Vtr=89qnLε0εrθ,
(2)
where ε0 and εr denote the permittivity of free space and the dielectric constant of g-C3N4 (5–6) estimated experimentally from Fig. S5, while θ is the ratio of the free carrier density to the total carrier density. The slope n2 was ∼2 at the applied voltage between Vtr and VTFL. This trend is attributed to SCLC, indicating that the charge traps are filled by the injected carriers. However, the number of carriers is insufficient for completely filling the traps in this region. As the Fermi level of g-C3N4 moves above the trap levels with increasing injected carriers, the traps are gradually filled.44,45 The current density in the region can be quantitatively analyzed as follows:41–43,
JTFL=98ε0εrμθV2L3.
(3)
The second transition voltage (VTFL) was experimentally obtained as 5.0 V, indicating that the traps were completely filled by high-density carrier injection. This can also be estimated using valuable parameters based on the SCLC model as follows:41–43,
VTFL=qL2Nt2ε0εr.
(4)
Therefore, we estimated the total trap density as Nt = 1.64–1.97 × 1017 cm−3 for this deposition condition and comparable values for other deposition conditions. Traps are known to originate from surface defects and structural disorders.46 For layered materials, heterointerfaces comprising a film and an electrode metal are formed across the van der Waals gap (a weak contact interface), which acts as an ultrathin potential barrier (<0.5 nm) and may generate interface defects. Therefore, the Nt estimated in this study includes this aspect in addition to the conventional origins. All the traps were filled as the applied bias voltage exceeded VTFL. Thus, the injected carriers moved directly to the band edge, causing a sudden increase in the current density. At high applied voltages (VTFL < V), the slope n3 was 4.6, indicating a trap-filled and strongly injected carrier transport.47 
FIG. 3.

(a) Typical JV characteristics along the out-of-plane direction for the g-C3N4 films deposited at 550 °C under an O2 gas ratio of 20% in the N2/O2 gas atmosphere. The inset shows the optical microscopy image of the measured device. (b) ln(J/E2)–1/E plot for estimating the carrier transport properties. (c) Schematic images of the carrier transport mechanism. Left: ohmic conduction with TE and/or direct tunneling below Vtr; center: insufficient trap-filled conduction between Vtr and VTFL; right: fully trap-filled conduction with FN tunneling above VTFL.

FIG. 3.

(a) Typical JV characteristics along the out-of-plane direction for the g-C3N4 films deposited at 550 °C under an O2 gas ratio of 20% in the N2/O2 gas atmosphere. The inset shows the optical microscopy image of the measured device. (b) ln(J/E2)–1/E plot for estimating the carrier transport properties. (c) Schematic images of the carrier transport mechanism. Left: ohmic conduction with TE and/or direct tunneling below Vtr; center: insufficient trap-filled conduction between Vtr and VTFL; right: fully trap-filled conduction with FN tunneling above VTFL.

Close modal
The ln(J/E2)–1/E plot for studying the detailed carrier transport property is illustrated in Fig. 3(b). The film was prepared at 550 °C under the O2 gas ratio of 20% in a mixed N2/O2 gas atmosphere. Other curves for various deposition conditions, including the N2 gas atmosphere, are shown in Fig. S6. The schematic images of the carrier transport properties are presented in Fig. 3(c). A change from thermionic emission (TE) and/or direct tunneling to Fowler–Nordheim (FN) tunneling at 0.35 MV/cm (4.9 V) is evident, which is consistent with VTFL. The FN tunneling can be described using48–50,
lnJFNE2=lnq28πhΦFNm0m*8π2m*(qΦFN)33qh1E,
(5)
where h, m0, m*, and ΦFN denote the Planck constant, free electron mass, effective mass of g-C3N4 (0.53 and 0.72 for electrons and holes, respectively),51 and the barrier height between the electrode and film for FN tunneling. An ohmic-like behavior between the films deposited under a mixed N2/O2 gas atmosphere and an Al electrode was observed, and ΦFN was estimated to be ∼34.1 meV. Electrons were lightly injected by the TE over ΦFN at the low applied voltages between 0 and VTFL. Then, by increasing the applied voltage to VTFL, the traps were completely filled, and electrons were strongly injected by FN tunneling. Therefore, according to the Schottky–Mott rule, the deposition of g-C3N4 films under a mixed N2/O2 gas atmosphere shows that n-type conductivity, in which electrons contribute to electrical conduction, can be controlled. In contrast, films deposited under an N2 gas atmosphere with an Au electrode exhibited an ohmic-like behavior (Fig. S4). The estimated ΦFN was ∼83 × 10−3 meV. Thus, the deposition of g-C3N4 films under an N2 gas atmosphere shows that the film conductivity becomes a p-type, in which holes contribute to electrical conduction.

The carrier type and resistivity depending on the deposition conditions are summarized in Table I. When this table is blank, no current is obtained. The detection limit of the current is below 1 × 10−12 A for our measurement setup. For the deposition in the N2 gas atmosphere, p-type conductivity was confirmed, as discussed above. In addition, the resistivity decreased with increasing deposition temperatures, whereas the conductivity type was maintained. The compositions of C and N increased and decreased, respectively, with changing deposition temperatures.15 Therefore, the acceptor doping attributed to C self-doping into anti-sites may be enhanced, resulting in low resistivity.27 In contrast, the change in conductivity type from p-type to n-type was confirmed as the O2 gas ratio in the mixed N2/O2 gas atmosphere increased during film deposition. The adsorption of residual NH and/or NH2 molecules that function as donors was more likely to occur during deposition under an O2-containing gas atmosphere than under an N2 gas atmosphere.19 In addition, the effect of anti-site doping was reduced by C removal in the films. The resistivity decreased with increasing O2 gas ratios at deposition temperatures above 575 °C. The resistivity of the film deposited at 600 °C under an O2 gas ratio of 20% in the mixed N2/O2 gas atmosphere is significantly low, which may be attributed to improved contact resistance between the film and the electrode.

TABLE I.

Electronic properties (carrier type and resistivity) for the g-C3N4 films deposited at various temperatures and O2 gas ratios in a mixed N2/O2 gas atmosphere.

0%10%20%30%
550 °C p-type 1.03 × 1010 Ωm n-type 7.65 × 109 Ωm n-type 8.33 × 109 Ωm n-type 1.27 × 1010 Ωm 
575 °C p-type 5.04 × 109 Ωm  n-type 7.23 × 109 Ωm n-type 4.05 × 109 Ωm 
600 °C p-type 3.82 × 108 Ωm ⋯ n-type 3.85 × 105 Ωm  
0%10%20%30%
550 °C p-type 1.03 × 1010 Ωm n-type 7.65 × 109 Ωm n-type 8.33 × 109 Ωm n-type 1.27 × 1010 Ωm 
575 °C p-type 5.04 × 109 Ωm  n-type 7.23 × 109 Ωm n-type 4.05 × 109 Ωm 
600 °C p-type 3.82 × 108 Ωm ⋯ n-type 3.85 × 105 Ωm  
The carrier transport properties of g-C3N4 in an asymmetric electrode structure were also investigated. After the evaporation of the Au electrode on an insulating substrate (α-Al2O3), a 138-nm-thick g-C3N4 film was exfoliated and then transferred onto the Au electrode. Subsequently, an Al electrode was formed. The film was prepared at 550 °C under the O2 gas ratio of 20% in a mixed N2/O2 gas atmosphere. The device size was of 1.68 × 10−5 cm2, which was assumed to be the top contact area. Figure 4(a) shows the JV curve on a semi-logarithmic scale for the current density. A rectifying behavior is achieved, implying the impact of the Schottky barrier at the Au/g-C3N4 interface on carrier transport. A turn-on voltage Vth of ∼0.5 V and a current density of 2.91 × 10−7 A/cm2 with a −10 V reverse bias voltage are observed. These characteristics can be analyzed using a simple thermionic emission model defined as
J=J0expqVDnkBT1=A*T2expqΦSBkBT×expqVDnkBT1.
(6)
Here, J0, VD, n, kB, and T are the saturation current density, applied voltage, empirical ideality factor, Boltzmann’s constant, and absolute temperature (293 K), respectively. Furthermore, A* = 4πm0m*kB2q/h3 is the effective Richardson constant, while ΦSB is the Schottky barrier height. From this equation, the parameters ΦSB and n were estimated to be 0.563 eV and 7.53, respectively. The Schottky barrier diode behavior was achieved; however, a relatively low operating performance was confirmed. Therefore, we investigated the SCLC model of the device. Figure 4(b) shows the JV characteristics in a double logarithmic plot. The current density first exhibited an exponential behavior up to Vth (0.5 V), after which it showed a linear behavior based on Ohm’s law up to 3.7 V. Subsequently, it slightly followed a quadratic function up to 5.7 V, indicating in a trap-filling trend based on the SCLC model. Then, the slope was 3.2 as the applied voltage increased further. In addition, FN tunneling was confirmed by the ln(J/E2)–1/E plot [inset of Fig. 4(b)]. The electron transport mechanism changed from TE and/or direct tunneling to FN tunneling at 0.38 MV/cm (5.2 V), which is reasonably consistent with the transition voltage from the quadratic function to the larger slope region. The estimated Nt of 1.64 × 1017 cm−3 is consistent with that of the symmetrical electrode structure. Thus, electron transport is strongly affected by the Schottky interface up to Vth, and at applied voltages higher than Vth, the trap-filling conduction dominates. This study suggests that the effect of a typical semiconductor interface can be obtained with g-C3N4, and the carrier transport properties can be controlled.
FIG. 4.

(a) JV characteristics of the Al/g-C3N4/Au structure with a semi-logarithmic plot of the current density and (b) a double logarithmic plot. The insets of (a) and (b) show the photograph of the measured device and the ln(J/E2)–1/E plot, respectively.

FIG. 4.

(a) JV characteristics of the Al/g-C3N4/Au structure with a semi-logarithmic plot of the current density and (b) a double logarithmic plot. The insets of (a) and (b) show the photograph of the measured device and the ln(J/E2)–1/E plot, respectively.

Close modal

g-C3N4 films were deposited via thermal CVD using a melamine precursor in a mixed N2/O2 gas atmosphere. Highly ordered layered stacking along the c-axis was confirmed. The deposition rate decreased with an increasing O2 ratio in the mixed N2/O2 gas atmosphere. However, the incorporation of O into g-C3N4 films was independent of the O2 ratio in the mixed N2/O2 gas atmosphere. The C composition changed from excess to the ideal as the O2 ratio in the mixed N2/O2 gas increased. Therefore, film deposition in an oxygen-containing atmosphere affected the removal of residual C atoms. In addition, the room-temperature PL intensity increased for the films deposited in a mixed N2/O2 gas atmosphere because of reduced non-radiative recombination owing to point defects. The carrier transport properties of the symmetrical electrode structure of Al/g-C3N4/Al first followed ohmic conduction and was then confirmed to be insufficient trap-filled conduction and fully trap-filled conduction with two transition voltages. The trap density was estimated to be of the order of 1017 cm−3, owing to surface defects, structural disorders, and a slightly weak contact interface. In addition, TE and/or direct tunneling and FN tunneling with the barrier height of 34.1 meV were confirmed, leading to the significant ohmic conduction. Therefore, the g-C3N4 films deposited in the mixed N2/O2 gas atmosphere exhibited n-type conductivity, and the resistivity was controllable by changing the O2 gas ratio. Moreover, the carrier transport properties of the asymmetric electrode structure of Al/g-C3N4/Au were also investigated. The Schottky barrier height (0.563 eV) at the Au/g-C3N4 interface affected the electron transport up to the threshold voltage (∼0.5 V), resulting in the rectifying behavior of the current density. As the applied voltage increased, the electron transport properties resemble those of the symmetric electrode structure. Thus, our work offers novel experimental insights into g-C3N4 films for developing chemically and physically functional materials.

The supplementary material contains the I–V characteristics of the g-C3N4 film deposited under an air atmosphere; cross-sectional SEM images; signals from the x-ray photoelectron spectroscopy; J–V curves under various deposition conditions; C–f characteristic of the g-C3N4 film; and ln(J/E2)–1/E plots under various deposition conditions.

This work was partially supported by JSPS KAKENHI (Grant No. JP21K14194), a research grant from the Mazda Foundation (Grant No. 21KK-206), and a research grant for the Iketani Science and Technology Foundation (Grant No. 035108-A). The authors thank M. Obata of the “Research Initiative for Supra-Materials at Shinshu University” for assistance with the XPS analysis. Part of this work was carried out using the analysis facilities at the “Global Aqua Innovation Center at Shinshu University (AICS).”

The authors have no conflicts to disclose.

Kota Higuchi: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Masaki Tachibana: Data curation (supporting); Formal analysis (supporting); Investigation (supporting); Writing – original draft (supporting). Noriyuki Urakami: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Writing – original draft (lead); Writing – review & editing (lead). Yoshio Hashimoto: Investigation (supporting); Writing – original draft (supporting); Writing – review & editing (supporting).

The data that support the findings of this study are available from the corresponding author and will be provided upon reasonable request.

1.
X. J.
Bai
,
C. B.
Cao
,
X. Y.
Xu
, and
Q.
Yu
, “
Synthesis and characterization of crystalline carbon nitride nanowires
,”
Solid State Commun.
150
,
2148
2153
(
2010
).
2.
Y.
Zheng
,
J.
Liu
,
J.
Liang
,
M.
Jaroniec
, and
S. Z.
Qiao
, “
Graphitic carbon nitride materials: Controllable synthesis and applications in fuel cells and photocatalysis
,”
Energy Environ. Sci.
5
,
6717
6731
(
2012
).
3.
Y.
Zhang
,
Q.
Pan
,
G.
Chai
,
M.
Liang
,
G.
Dong
,
Q.
Zhang
, and
J.
Qiu
, “
Synthesis and luminescence mechanism of multicolor-emitting g-C3N4 nanopowders by low temperature thermal condensation of melamine
,”
Sci. Rep.
3
,
1943
(
2013
).
4.
J.
Mahmood
,
E. K.
Lee
,
M.
Jung
,
D.
Shin
,
I.-Y.
Jeon
,
S.-M.
Jung
,
H.-J.
Choi
,
J.-M.
Seo
,
S.-Y.
Bae
,
S.-D.
Sohn
,
N.
Park
,
J. H.
Oh
,
H.-J.
Shin
, and
J.-B.
Baek
, “
Nitrogenated holey two-dimensional structures
,”
Nat. Commun.
6
,
6486
(
2015
).
5.
N.
Fechler
,
N. P.
Zussblatt
,
R.
Rothe
,
R.
Schlögl
,
M.-G.
Willinger
,
B. F.
Chmelka
, and
M.
Antonietti
, “
Eutectic syntheses of graphitic carbon with high pyrazinic nitrogen content
,”
Adv. Mater.
28
,
1287
1294
(
2016
).
6.
P.
Kumar
,
E.
Vahidzadeh
,
U. K.
Thakur
,
P.
Kar
,
K. M.
Alam
,
A.
Goswami
,
N.
Mahdi
,
K.
Cui
,
G. M.
Bernard
,
V. K.
Michaelis
, and
K.
Shankar
, “
C3N5: A low bandgap semiconductor containing an azo-linked carbon nitride framework for photocatalytic, photovoltaic and adsorbent applications
,”
J. Am. Chem. Soc.
141
,
5415
5436
(
2019
).
7.
C.
Hu
,
Y.-H.
Lin
,
M.
Yoshida
, and
S.
Ashimura
, “
Influence of phosphorus doping on triazole-based g-C3N5 nanosheets for enhanced photoelectrochemical and photocatalytic performance
,”
ACS Appl. Mater. Interfaces
13
,
24907
24915
(
2021
).
8.
J.
Hong
,
X.
Xia
,
Y.
Wang
, and
R.
Xu
, “
Mesoporous carbon nitride with in situ sulfur doping for enhanced photocatalytic hydrogen evolution from water under visible light
,”
J. Mater. Chem.
22
,
15006
15012
(
2012
).
9.
W.-J.
Ong
,
L.-L.
Tan
,
Y. H.
Ng
,
S.-T.
Yong
, and
S.-P.
Chai
, “
Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: Are we a step closer to achieving sustainability?
,”
Chem. Rev.
116
,
7159
7329
(
2016
).
10.
D.
Masih
,
Y.
Ma
, and
S.
Rohani
, “
Graphitic C3N4 based noble-metal-free photocatalyst systems: A review
,”
Appl. Catal. B: Environ.
206
,
556
588
(
2017
).
11.
M.
Kosaka
,
N.
Urakami
, and
Y.
Hashimoto
, “
Formation of graphitic carbon nitride and boron carbon nitride film on sapphire substrate
,”
Jpn. J. Appl. Phys.
57
,
02CB09
(
2018
).
12.
N.
Urakami
,
M.
Kosaka
, and
Y.
Hashimoto
, “
Chemical vapor deposition of boron-incorporated graphitic carbon nitride film for carbon-based wide bandgap semiconductor materials
,”
Phys. Status Solidi B
257
,
1900375
(
2020
).
13.
Y.
Noda
,
C.
Merschjann
,
J.
Tarábek
,
P.
Amsalem
,
N.
Koch
, and
M. J.
Bojdys
, “
Directional charge transport in layered two-dimensional triazine-based graphitic carbon nitride
,”
Angew. Chem.
131
,
9494
9498
(
2019
).
14.
P. C.
Patra
and
Y. N.
Mohapatra
, “
Dielectric constant of thin film graphitic carbon nitride (g-C3N4) and double dielectric Al2O3/g-C3N4
,”
Appl. Phys. Lett.
118
,
103501
(
2021
).
15.
Q.
Guo
,
M.
Wei
,
Z.
Zheng
,
X.
Huang
,
X.
Song
,
S.-B.
Qiu
,
X.-b.
Yang
,
X.
Liu
,
J.
Qiu
, and
G.
Dong
, “
Full-color chemically modulated g-C3N4 for white-light-emitting device
,”
Adv. Opt. Mater.
7
,
1900775
(
2019
).
16.
S.
Pareek
,
S.
Waheed
,
A.
Rana
,
P.
Sharma
, and
S.
Karak
, “
Graphitic carbon nitride quantum dots (g-C3N4) to improve photovoltaic performance of polymer solar cell by combining Förster resonance energy transfer (FRET) and morphological effects
,”
Nano Express
1
,
010057
(
2020
).
17.
H.
Zhang
,
D.
Zheng
,
Z.
Cai
,
Z.
Song
,
Y.
Xu
,
R.
Chen
,
C.
Lin
, and
L.
Guo
, “
Graphitic carbon nitride nanomaterials for multicolor light-emitting diodes and bioimaging
,”
ACS Appl. Nano Mater.
3
,
6798
6805
(
2020
).
18.
S.
Pareek
,
S.
Waheed
, and
S.
Karak
, “
Graphitic carbon nitride nanosheets: Dual functional charge selective cathode/anode interface layer for polymer solar cells
,”
ACS Appl. Energy Mater.
6
,
554
563
(
2023
).
19.
T.-F.
Yeh
,
C.-Y.
Teng
,
S.-J.
Chen
, and
H.
Teng
, “
Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination
,”
Adv. Mater.
26
,
3297
3303
(
2014
).
20.
G.
Liu
,
G.
Zhao
,
W.
Zhou
,
Y.
Liu
,
H.
Pang
,
H.
Zhang
,
D.
Hao
,
X.
Meng
,
P.
Li
,
T.
Kako
, and
J.
Ye
, “
In situ bond modulation of graphitic carbon nitride to construct p–n homojunctions for enhanced photocatalytic hydrogen production
,”
Adv. Funct. Mater.
26
,
6822
6829
(
2016
).
21.
G.
Liu
,
P.
Niu
,
C.
Sun
,
S. C.
Smith
,
Z.
Chen
,
G. Q.
Lu
, and
H.-M.
Cheng
, “
Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3N4
,”
J. Am. Chem. Soc.
132
,
11642
11648
(
2010
).
22.
Z.
Cao
,
Y.
Jia
,
Q.
Wang
, and
H.
Cheng
, “
High-efficiency photo-Fenton Fe/g-C3N4/kaolinite catalyst for tetracycline hydrochloride degradation
,”
Appl. Clay Sci.
212
,
106213
(
2021
).
23.
A.
Mehtab
and
T.
Ahmad
, “
Investigating the spatial charge density flow and molecular structure of g-C3N4 photocatalyst from a computational perspective
,”
Appl. Catal., A
659
,
119190
(
2023
).
24.
N.
Urakami
,
M.
Kosaka
, and
Y.
Hashimoto
, “
Thermal chemical vapor deposition and luminescence property of graphitic carbon nitride film for carbon-based semiconductor systems
,”
Jpn. J. Appl. Phys.
58
,
010907
(
2019
).
25.
K.
Takashima
,
N.
Urakami
, and
Y.
Hashimoto
, “
Electronic transport and device application of crystalline graphitic carbon nitride film
,”
Mater. Lett.
281
,
128600
(
2020
).
26.
N.
Urakami
,
K.
Ogihara
,
H.
Futamura
,
K.
Takashima
, and
Y.
Hashimoto
, “
Demonstration of electronic devices in graphitic carbon nitride crystalline film
,”
AIP Adv.
11
,
075204
(
2021
).
27.
Y.
Li
,
Z.
He
,
L.
Liu
,
Y.
Jiang
,
W.-J.
Ong
,
Y.
Duan
,
W.
Ho
, and
F.
Dong
, “
Inside-and-out modification of graphitic carbon nitride (g-C3N4) photocatalysts via defect engineering for energy and environmental science
,”
Nano Energy
105
,
108032
(
2023
).
28.
Y.-P.
Yuan
,
W.-T.
Xu
,
L.-S.
Yin
,
S.-W.
Cao
,
Y.-S.
Liao
,
Y.-Q.
Tng
, and
C.
Xue
, “
Large impact of heating time on physical properties and photocatalytic H2 production of g-C3N4 nanosheets synthesized through urea polymerization in Ar atmosphere
,”
Int. J. Hydrogen Energy
38
,
13159
13163
(
2013
).
29.
N.
Urakami
,
K.
Takashima
,
M.
Shimizu
, and
Y.
Hashimoto
, “
Thermal chemical vapor deposition of layered carbon nitride films under a hydrogen gas atmosphere
,”
CrystEngComm
25
,
877
883
(
2023
).
30.
J.
Wang
,
J.
Huang
,
H.
Xie
, and
A.
Qu
, “
Synthesis of g-C3N4/TiO2 with enhanced photocatalytic activity for H2 evolution by a simple method
,”
Int. J. Hydrogen Energy
39
,
6354
6363
(
2014
).
31.
S.
Chen
,
C.
Wang
,
B. R.
Bunes
,
Y.
Li
,
C.
Wang
, and
L.
Zang
, “
Enhancement of visible-light-driven photocatalytic H2 evolution from water over g-C3N4 through combination with perylene diimide aggregates
,”
Appl. Catal., A
498
,
63
68
(
2015
).
32.
N.-N.
Vu
,
C.-C.
Nguyen
,
S.
Kaliaguine
, and
T.-O.
Do
, “
Synthesis of g-C3N4 nanosheets by using a highly condensed lamellar crystalline melamine-cyanuric acid supramolecular complex for enhanced solar hydrogen generation
,”
ChemSusChem
12
,
291
302
(
2019
).
33.
S.
Matsumoto
,
E.-Q.
Xie
, and
F.
Izumi
, “
On the validity of the formation of crystalline carbon nitrides, C3N4
,”
Diamond Relat. Mater.
8
,
1175
1182
(
1999
).
34.
F.
Fina
,
S. K.
Callear
,
G. M.
Carins
, and
J. T. S.
Irvine
, “
Structural investigation of graphitic carbon nitride via XRD and neutron diffraction
,”
Chem. Mater.
27
,
2612
2618
(
2015
).
35.
I.
Papailias
,
T.
Giannakopoulou
,
N.
Todorova
,
D.
Demotikali
,
T.
Vaimakis
, and
C.
Trapalis
, “
Effect of processing temperature on structure and photocatalytic properties of g-C3N4
,”
Appl. Surf. Sci.
358
,
278
286
(
2015
).
36.
C.
Ronning
,
H.
Feldermann
,
R.
Merk
,
H.
Hofsäss
,
P.
Reinke
, and
J.-U.
Thiele
, “
Carbon nitride deposited using energetic species: A review on XPS studies
,”
Phys. Rev. B
58
,
2207
2215
(
1998
).
37.
Y.
Li
,
J.
Zhang
,
Q.
Wang
,
Y.
Jin
,
D.
Huang
,
Q.
Cui
, and
G.
Zou
, “
Nitrogen-rich carbon nitride hollow vessels: Synthesis, characterization, and their properties
,”
J. Phys. Chem. B
114
,
9429
9434
(
2010
).
38.
H.-J.
Shin
,
K. K.
Kim
,
A.
Benayad
,
S.-M.
Yoon
,
H. K.
Park
,
I.-S.
Jung
,
M.-H.
Jin
,
H.-K.
Jeong
,
J. M.
Kim
,
J.-Y.
Choi
, and
Y. H.
Lee
, “
Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance
,”
Adv. Funct. Mater.
19
,
1987
1992
(
2009
).
39.
R.
Sadri
,
M.
Hosseini
,
S. N.
Kazi
,
S.
Bagheri
,
N.
Zubir
,
K. H.
Solangi
,
T.
Zaharinie
, and
A.
Badarudin
, “
A bio-based, facile approach for the preparation of covalently functionalized carbon nanotubes aqueous suspensions and their potential as heat transfer fluids
,”
J. Colloid Interface Sci.
504
,
115
123
(
2017
).
40.
Z.
Gan
,
Y.
Shan
,
J.
Chen
,
Q.
Zhang
,
S.
Nie
, and
X.
Wu
, “
The origins of the broadband photoluminescence from carbon nitrides and applications to white light emitting
,”
Nano Res.
9
,
1801
1812
(
2016
).
41.
J. G.
Simmons
, “
Conduction in thin dielectric films
,”
J. Phys. D: Appl. Phys.
4
,
613
657
(
1971
).
42.
F.-C.
Chiu
, “
A review on conduction mechanisms in dielectric films
,”
Adv. Mater. Sci. Eng.
2014
,
578168
.
43.
E.
Saloma
,
S.
Alcántara
,
N.
Hernández-Como
,
J.
Villanueva-Cab
,
M.
Chavez
,
G.
Pérez-Luna
, and
J.
Alvarado
, “
Photoelectric effect on an Al/SiO2/p-Si Schottky diode structure
,”
Mater. Res. Express
7
,
105902
(
2020
).
44.
S.
Ghatak
and
A.
Ghosh
, “
Observation of trap-assisted space charge limited conductivity in short channel MoS2 transistor
,”
Appl. Phys. Lett.
103
,
122103
(
2013
).
45.
Y. S.
Shin
,
K.
Lee
,
Y. R.
Kim
,
H.
Lee
,
I. M.
Lee
,
W. T.
Kang
,
B. H.
Lee
,
K.
Kim
,
J.
Heo
,
S.
Park
,
Y. H.
Lee
, and
W. J.
Yu
, “
Mobility engineering in vertical field effect transistors based on van der Waals heterostructures
,”
Adv. Mater.
30
,
1704435
(
2018
).
46.
S.
Baidyaroy
and
P.
Mark
, “
Analytical and experimental investigation of the effects of oxygen chemisorption on the electrical conductivity of CdS
,”
Surf. Sci.
30
,
53
68
(
1972
).
47.
D.
Joung
,
A.
Chunder
,
L.
Zhai
, and
S. I.
Khondaker
, “
Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets
,”
Appl. Phys. Lett.
97
,
093105
(
2010
).
48.
M.
Lenzlinger
and
E. H.
Snow
, “
Fowler-Nordheim tunneling into thermally grown SiO2
,”
J. Appl. Phys.
40
,
278
283
(
1969
).
49.
C.
Lu
,
X.
Zhang
,
X.
Xie
,
S.
Feng
,
I.
Diagne
,
A.
Khan
, and
S.
Noor Mohammad
, “
Interface states mediated reverse leakage through metal/AlxGa1-xN/GaN Schottky diodes
,”
J. Vac. Sci. Technol., B
26
,
1987
1992
(
2008
).
50.
I.
Jabbari
,
M.
Baira
,
H.
Maaref
, and
R.
Mghaieth
, “
Evidence of Pool-Frenkel and Fowleer-Nordheim tunneling transport mechanisms in leakage current of (Pd/Au)/Al0.22Ga0.78N/GaN heterostructure
,”
Solid State Commun.
314–315
,
113920
(
2020
).
51.
W.
Yu
,
J.
Chen
,
T.
Shang
,
L.
Chen
,
L.
Gu
, and
T.
Peng
, “
Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically defined junction for H2 production
,”
Appl. Catal. B: Environ.
219
,
693
704
(
2017
).

Supplementary Material