Herein, we report epitaxial growth of p-type Ni doped gallium chromium oxide thin film on Al2O3 substrates and studied its band alignment properties with that of the substrate. Thin films are grown using the magnetron-sputtering technique. Synchrotron-based XRD measurements, performed in the coplanar and non-coplanar geometries, confirm high-quality single domain epitaxial growth of p-type α-GaCrO3:Ni. Pendellosung oscillations around the Bragg peak and transmission electron microscopy reveal the high interfacial quality of p-type α-GaCrO3:Ni films with the substrate. Thin film, thickness ∼200 nm, shows around 70% average transmission. The values of valence band and conduction band offsets are determined to be 2.79 ± 0.2 and 0.51 ± 0.2 eV, respectively, which confirm straddling gap band alignment at the heterojunction. This type of alignment creates a threshold barrier for the selective charge carriers and is useful in enhancing the performance of a wide range of devices, including UV photodetectors, metal oxide semiconductor high electron mobility transistors, and light emitters.

Electrical conductivity and optical transparency were traditionally considered mutually exclusive properties of any material until the ground-breaking discovery of a new class of efficient materials called transparent conducting oxides (TCOs). TCOs possess a diverse range of bandgaps, enabling efficient light transmission in the visible spectrum while possessing an abundance of free charge carriers, giving them excellent electrical conductivity. In these TCOs, the host oxides contain an oxygen “2p” orbital-derived valence band, and the metal's “s” orbital primarily defines the conduction band.1 The spherically shaped vacant “s” orbital displays large spatial distributions, which leads to a highly dispersed conduction band characterized by wide bandwidth and small effective mass. When these materials are doped with suitable donors, whose energy levels lie just below the conduction band minima of host oxide, it facilitates the excitation of charge carriers to conduction band minima, which enhances the conductivity of n-type TCO materials.2 A wide range of n-type type TCOs has been extensively studied such as Sn-doped In2O3 (ITO), Al-doped ZnO (AZO), and F-doped SnO2 (FTO), which have led to advancement in transparent electrodes used in flat panel displays, photovoltaic cells, etc. and an active layer for transparent thin film transistors (TFTs), UV light-emitting diodes (LEDs), gas sensors, UV photodetectors, and other transparent electronic devices.2–4 Significant developments in n-type TCOs have also prompted the studies on complementary p-type TCOs, which could help in the development of an entirely transparent electronics and devices such as Metal Oxide Semiconductor High Electron Mobility Transistors (MOSHEMTs), HEMTs, and UV photodetectors. Transparent p-n heterojunction devices are regarded as more efficient structures, allowing for easy integration with everyday technology and leading to devices with higher energy efficiency, responsivity, and external quantum efficiency.5–8 

Development of p-type TCOs is challenging due to the localized nature of the oxygen 2p-orbital derived valence band, which leads to difficulty in introducing shallow acceptors (due to high electro-negativity of oxygen), and the presence of large hole effective mass decreases hole mobility significantly. To overcome these issues, Hosono et al. proposed a strategy of “chemical modulation of valence band” (CMVB) in which the “s” or “d” orbitals are designed to hybridize with O 2p orbitals, thus modifying the valence band such that valence band maximum (VBM) becomes more dispersive. This leads to a reduction in the effective mass and consequently, improved hole mobility.9 Using this concept, several p-type TCOs have been discovered, such as oxides of Ni 3d8, Cu 3d10, Cr 3d3, and other post-transition metals.10 NiO thin film, the very first known p-type TCO, was reported by Sato et al. in 1993 with a bandgap ranging from 3.5 to 3.9 eV and a work function of approximately 5.3 eV thereby making it a widely used p-type semiconductor.11,12 The p-type conductivity in NiO is attributed to Ni vacancies, which occur in the presence of excess oxygen. The hybridization between oxygen 2p orbital and Ni 3d orbital is believed to be a major reason for hole mobility in NiO. However, the coloration effect in the thin film suppresses its optical transparency.11 The discovery of Cu-based delafossite p-type TCOs leads to a significant development in this area.13 Among all Cu-based TCOs, CuCrO2 thin film shows superior electrical conductivity;14 however, a small bandgap at room temperature again constrains the optical transparency in Cu-based p-type TCOs. Similar to NiO, Cr-based oxides have also been identified as promising p-type TCO. α-Cr2O3 is typically a highly insulating material, because of the localized behavior of its “d” orbital, with a large bandgap of 3.4 eV.15 The electrical conductivity in α-Cr2O3 has been reported to be enhanced through doping of Mg, N, and Ni while preserving its transparency.16–18 

Most of the studied p-type TCOs such as NiO and Cu2O show multi-domain growth instead of single-domain epitaxial growth on sapphire substrates, whereas most of the n-type TCOs show epitaxial growth on sapphire.19–23 The epitaxial growth of p-type TCOs on the sapphire substrate could open up several applications in fully transparent integrated p-n junction based devices.24 A high-quality transparent p-n junction has been reported using n-type Ni-doped p-type α-Cr2O3 and Al-doped ZnO.25 Substitution of Ga and Al in α-Cr2O3 increases its optical bandgap up to 3.95 and 5.3 eV, respectively.26,27 Ni doped α-GaCrO3 [α-(Ga0.79Ni0.01)Cr1.2O3] shows better conduction properties when compared to Ni doped α-Cr2O3 due to increased hybridization between Cr 3d and O 2p orbitals.28 The work function of Ni doped α-GaCrO3 has been reported to be approximately 5.2 eV, which shows its suitability for most of the perovskite and organic solar cells as a hole transport layer.28,29 Figure of merit (FOM) in TCOs is primarily determined by its optical transparency and electrical conductivity. Consequently, Ni doped α-GaCrO3 can have better FOM as compared to Ni doped α-Cr2O3.28 In this study, we report the epitaxial growth of Ni-doped α-GaCrO3 on sapphire substrate for the first time using RF magnetron sputtering technique. In view of the available reports of the epitaxial growth of n-type β-Ga2O3 on sapphire substrates,29 our results show a very good promise of integrating Ni-doped α-GaCrO3 with β-Ga2O3/Al2O3 for the development of an all oxide transparent p-n heterojunction. In several heteroepitaxial junctions with large lattice mismatch30,31 or growth of films on the substrates with different crystal structures,32,33 interfacial density of states significantly deteriorates the electrical characteristics with large leakage currents. A thin large bandgap insulating layer, e.g., Al2O3, MgO between the p and n layers can improve electrical characteristics of these junctions, as reported in other systems.34–37 Both β-Ga2O3 and Ni-doped α-GaCrO3 have showed good interfacial qualities with Al2O3,29 which indicates that Al2O3 will be a better choice as an insulating layer between these p and n type layers. These aspects have motivated us to study the crystalline quality and the interface of optimized epitaxial thin films of Ni-doped α-GaCrO3 deposited on Al2O3 substrates. XRD, XRR, AFM, and TEM measurements have been performed to assess the crystalline and interface quality. The composition of gallium and chromium in the thin film has been verified using EDS. The valence band offset for Ni-doped α-GaCrO3/Al2O3 heterostructures have also been determined using synchrotron-based photoelectron spectroscopy (PES) technique at plane undulator-based PES beamline (BL-10) of Indus-2 synchrotron source.

Target pellet of Ni-doped α-(Ga0.79Ni0.01)Cr1.2O3, used in deposition, was synthesized by taking powders of Cr2O3 (purity 99.97%), Ga2O3 (purity 99.999%), and NiO (purity 99.99%) in the stoichiometric ratio. The powders were ball milled for 12 h at 200 rpm for homogeneous mixing. Subsequently, 28 mm diameter pellet was made by applying the force of 15 tons. The as prepared target was sintered for 12 h at 1600 °C to achieve the required density for sputtering. RF magnetron sputtering was used to deposit all the thin films on c-oriented sapphire substrates and are termed α-GaCrO3:Ni throughout the paper. Various parameters like gas ratio, growth temperature, and pressure were changed to optimize the growth of thin films. In this work, all the thin films were deposited on a sapphire substrate of 10  × 10 mm2. The substrate temperature was maintained at 750 °C during the growth. Various ratios of O2:Ar were taken to optimize the thin film growth. The best quality thin film growth was observed for the ratio 1:4. The pressure in the chamber kept constant at ∼3.8 × 10−2 mbar during the growth. The growth rate was determined to be 1 nm/min using XRR technique. Three samples having the thickness of around 200, 60, and 2 nm were grown and are denoted as samples A, B, and C, respectively. 50 nm amorphous Al2O3 thin film was also grown on the Si substrate and is denoted as sample D.38 These samples were used for various characterizations. Samples A and B were used for structural characterization to investigate the crystalline quality. Structural characterization was performed at 10 keV photon energy (λ = 1.24 Å) using BL-13 beamline of Indus-2 synchrotron source. BL-13 is equipped with an 8-circle x-ray goniometer (Make: Huber) and a scintillation detector (C400, Oxford Instruments) mounted at the detector arm of the goniometer with a crossed slit to measure the intensity of the scattered x ray, from the sample. X-ray reflectivity (XRR) measurement, using BL-13 beamline of Indus-2 synchrotron, was also carried out on sample B for the precise measurement of thickness and interface quality. Atomic force microscopy (AFM) was also done on sample A to check the surface roughness. Transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS) measurements were performed on sample A using the Philips CM200, 200 keV machine. Optical measurement was carried out on sample A for the bandgap determination using a UV-VIS-NIR transmission spectrometer (Carry 50) in the wavelength range of 190–1000 nm. To determine the nature of the charge carriers, whether they were holes (positive) or electrons (negative), Seebeck measurements were performed on sample A. To produce a temperature gradient (ΔTg), a controlled heat pulse was employed and simultaneously ΔV was measured. Details of the experimental setup can be found in Ref. 39. Core level and valence band of thin films and the substrate were investigated using the angle resolved photoelectron spectroscopy (ARPES) beamline, BL-10, Indus 2. All samples were cleaned by Ar-ion sputtering before the measurements. Data were collected with a pass energy of 30 eV. The flood gun was used to eliminate any charging effects and the binding energy of core level of physiosorbed carbon (C 1s peak ∼284.8 eV) on the surface of the layer was used as the energy reference.

Figure 1(a) shows the XRD results for sample A, taken in the co-planar geometry. The 2θ/ω scan reveals sharp and intense Bragg peaks at 2θ values of 31.68°, 33.20°, 66.15°, and 69.56°. Bragg peaks at 2θ values of 33.20° and 69.56° correspond to the (0006) and (00012) reflections of the Al2O3 substrate, respectively. The space group for both the substrate and α-GaCrO3:Ni is rhombohedral R-3c with different lattice parameters.26 Due to the different values of lattice parameters for α-GaCrO3:Ni, two additional peaks are observed at little lower 2θ values of 31.68° and 66.15° corresponding to (0006) and (00012) Bragg reflections of α-GaCrO3:Ni films. This reveals that the film exhibits growth in the same direction as that of the substrate. No other peaks are detected, signifying a good phase purity and oriented growth of the thin film along the (0001) direction of the substrate. Figure 1(b) illustrates the 2θ/ω scan of sample B in the co-planar geometry. The presence of Pendellosung oscillations around the α-GaCrO3:Ni Bragg peak (0006) is indicative of a well-defined sharp interface between the α-GaCrO3:Ni film and the Al2O3 substrate. The thickness of α-GaCrO3:Ni thin film can be evaluated from the separation of the Pendellosung oscillations by using the following relation:40 
t = λ 2 Δ θ cos θ B ,
(1)
where t is the thickness of the deposited film, λ = 1.24 Å, Δθ belongs to periodic oscillations, and θB is the Bragg angle associated with the peak. The α-GaCrO3:Ni thin film's thickness is determined using Eq. (1) to be around 60 nm.
FIG. 1.

(a) The 2θ/ω scan of α-GaCrO3:Ni films (sample A) deposited on Al2O3 (0001) oriented substrates in coplanar geometry, (b) high resolution 2θ scan near the (0006) reflection of α-GaCrO3:Ni films (sample B) on sapphire substrates, (c) the 2θ scan of α-GaCrO3:Ni films (sample A) in non-coplanar geometry, (d) Phi (Φ) scan for (10–14) asymmetric reflection of α-GaCrO3:Ni shows three peaks separated by 120° similar to Al2O3 substrates, which confirm the single domain epitaxial growth.

FIG. 1.

(a) The 2θ/ω scan of α-GaCrO3:Ni films (sample A) deposited on Al2O3 (0001) oriented substrates in coplanar geometry, (b) high resolution 2θ scan near the (0006) reflection of α-GaCrO3:Ni films (sample B) on sapphire substrates, (c) the 2θ scan of α-GaCrO3:Ni films (sample A) in non-coplanar geometry, (d) Phi (Φ) scan for (10–14) asymmetric reflection of α-GaCrO3:Ni shows three peaks separated by 120° similar to Al2O3 substrates, which confirm the single domain epitaxial growth.

Close modal

Figure 1(c) shows the 2θ/ω scan in a non-co-planar geometry for (10–14) asymmetric Bragg reflection. Sample A is tilted by an angle χ of approximately 38.3° to align the scattering plane normal to the investigated (10–14) crystal plane. The scan revealed prominent and well-defined sharp peaks at values of 28.04° and 57.98°, corresponding to (10–14) and (20–28) reflections of the Al2O3 substrate. Furthermore, three peaks with slightly lower intensity are observed at values of 26.78°, 55.42°, and 88.58°. These peaks are attributed to (10–14), (20–28), and (30–312) reflections of α-GaCrO3:Ni thin films, respectively. The absence of any other peaks reflects an excellent in-plane growth quality of the thin film on the substrate. Phi(φ) scans for (10–14) Bragg reflection of α-GaCrO3:Ni thin film and substrate are shown in Fig. 1(d), which reveal the in-plane orientation relationship between the substrate and the thin film. From the R-3c space group symmetry of α-GaCrO3:Ni and substrate, it is known that (10–14) Bragg reflection has threefold symmetry and hence three distinct peaks separated by 120° are observed. These results confirm the single domain epitaxial growth of GaCrO3:Ni thin films on the substrate. From Figs. 1(a) and 1(c), the lattice parameters a and c are deduced to be around 4.97 ± 0.01 and 13.55 ± 0.01 Å for the thin film sample, which are close to the value of bulk Ga0.9Cr1.1O3 (a = 4.98 ± 0.005 Å and c = 13.52 ± 0.005 Å).26 This indicates that films are relaxed and there is minimal strain in the thin films. X-ray reflectivity (XRR) measurement was also carried out on sample B, and the results are shown in Fig. 2. XRR measurement also indicates similar results, i.e., thickness around 60 nm with around 7 Å surface roughness. Surface topography of sample A was also measured by AFM, which confirms the 2D growth of the thin films with good surface quality as shown in Fig. 3. Root mean square roughness of the film is found to be around 1.2 nm.

FIG. 2.

XRR spectrum, taken at 10 keV photon energy, of sample B together with the fitting. Large numbers of well-defined Kiessig fringes indicate excellent chemical gradient at the interface, which suggests very less interface diffusion. Fitting of experimental data reveals the thickness of film around 59.5 ± 1 nm with an interface roughness of about 0.7 nm.

FIG. 2.

XRR spectrum, taken at 10 keV photon energy, of sample B together with the fitting. Large numbers of well-defined Kiessig fringes indicate excellent chemical gradient at the interface, which suggests very less interface diffusion. Fitting of experimental data reveals the thickness of film around 59.5 ± 1 nm with an interface roughness of about 0.7 nm.

Close modal
FIG. 3.

AFM images in 2D (upper) and 3D (lower) mode of sample A. Thin film shows 2D growth with good surface quality. Root mean square roughness of the film is around 1.2 nm.

FIG. 3.

AFM images in 2D (upper) and 3D (lower) mode of sample A. Thin film shows 2D growth with good surface quality. Root mean square roughness of the film is around 1.2 nm.

Close modal

Cross-sectional high resolution TEM (HRTEM) measurements on sample A were carried out to understand the growth mode on the microscopic level. Figure 4 shows the TEM images for α-GaCrO3:Ni/Al2O3 heterojunction. Figure 4(a) shows the low magnification image indicating α-GaCrO3:Ni film, Al2O3 substrate, and the interface between them. The thickness of α-GaCrO3:Ni film is determined to be ∼200 nm. A defect-free selected area electron diffraction (SAED) pattern is shown in Fig. 4(b), taken at the α-GaCrO3:Ni film region. The SAED pattern of film is indexed to [01–10] zone axis of α-GaCrO3:Ni. Figures 4(c) and 4(d) show cross-sectional HRTEM images corresponding to SAED. HRTEM images cover the interface region where different regions are marked. Plane spacing corresponds to (10–14) and (10–1–2) planes of α-GaCrO3:Ni film and substrate are shown in Fig. 4(d). The interface region between the film and the substrate is of very good quality and free from any significant defects. This validates the results observed in the XRD and XRR measurements. The stoichiometric ratio of gallium and chromium is confirmed by EDS and is found to be around 49:50 in atomic fraction (%) with 1% Ni. The corresponding EDS data are shown in Figure S1 in the supplementary material.

FIG. 4.

(a) Low resolution transmission electron microscopy (TEM) image of α-GaCrO3:Ni and Al2O3 heterojunction (sample A), (b) selected area electron diffraction (SAED) pattern taken from the α-GaCrO3:Ni region, which is indexed to [01–10] zone axis. (c) High resolution TEM (HRTEM) image showing α-GaCrO3:Ni layer, Al2O3 substrate, and the interface between them. (d) HRTEM image indicating (10–14) and (10–1–2) planes along with plane spacing. Interfacial regions are indicated by blue arrows and yellow lines are used to denote interplanar distances.

FIG. 4.

(a) Low resolution transmission electron microscopy (TEM) image of α-GaCrO3:Ni and Al2O3 heterojunction (sample A), (b) selected area electron diffraction (SAED) pattern taken from the α-GaCrO3:Ni region, which is indexed to [01–10] zone axis. (c) High resolution TEM (HRTEM) image showing α-GaCrO3:Ni layer, Al2O3 substrate, and the interface between them. (d) HRTEM image indicating (10–14) and (10–1–2) planes along with plane spacing. Interfacial regions are indicated by blue arrows and yellow lines are used to denote interplanar distances.

Close modal

The UV-VIS-NIR data in transmission mode for the epitaxial α-GaCrO3:Ni film (sample A) are measured in the wavelength range of 190–1000 nm whose results are presented in Fig. 3. In the transmission spectrum, some extra features are observed along with the thickness oscillation in the visible range. These features lie below the bandgap region and are attributed to the weak d-d* transitions of Cr3+ multiplet, which makes the film to appear slightly green in color. The average transmittance (Tavg) in the visible region is determined by averaging the transmission values at incident photon wavelengths 420, 490, 560, 630, and 700 nm. The average transmittance value for the epitaxial α-GaCrO3:Ni film (sample A) is found to be ∼70% in the visible range, as shown in Fig. 5. If the contribution of the substrate is removed, then the Tavg value will be ∼20% higher. The inset in Fig. 5 displays a Tauc's plot between (αhν)2 and energy E. The bandgap is obtained by fitting the linear portion of the spectrum near the leading absorption edge. The estimated bandgap of α-GaCrO3:Ni film is approximately 4.1 eV, which is the highest reported value among p-type transparent oxide thin films. This leads to an enhanced optical transparency of the films and therefore an improved figure of merit (FOM). The nature of the charge carriers (hole or electron) was also verified by Seebeck measurements (∼+22 μV/K), which confirms the p-type nature of the thin film similar to that reported in bulk case.28 

FIG. 5.

Optical transmittance spectra of α-GaCrO3:Ni thin film (∼200 nm) on Al2O3 substrate. The inset shows (αhν)2 vs energy (E) curve where linear fit to the leading edge is also drawn to determine the bandgap of α-GaCrO3:Ni thin film.

FIG. 5.

Optical transmittance spectra of α-GaCrO3:Ni thin film (∼200 nm) on Al2O3 substrate. The inset shows (αhν)2 vs energy (E) curve where linear fit to the leading edge is also drawn to determine the bandgap of α-GaCrO3:Ni thin film.

Close modal
Band alignment and band offset at active interfaces are critical factors that impact the performance of optoelectronic devices. The valence band offset of the α-GaCrO3:Ni/Al2O3 heterojunction is determined using Kraut's method.41 In this approach, the valence band offset is evaluated using the relation
Δ E V = E VBM Ga 3 d E VBM Al 2 p ,
Δ E V = Δ E C L ( E Al 2 p E VBM ) A l 2 O 3 + ( E Ga 3 d E VBM ) GaCr O 3 : Ni ,
(2)
Δ E C = E CVM Ga 3 d E CVM Al 2 p = Δ E V + E G GaCr O 3 : Ni E G A l 2 O 3 ,
(3)
where ΔECL is the difference between the core levels energies of Ga 3d and Al 2p measured on the α-GaCrO3:Ni/Al2O3 heterojunction (sample C) by the PES experiments. (EAl2p – EVBM)Al2O3 and (EGa3d – EVBM)GaCrO3 are the energy differences of core levels of Al 2p and Ga 3d from their respective valence band maximum (VBM). All the calculations are performed by taking the valence band of Al2O3 as a reference. These energy levels have been measured using sample A and sample D. The survey spectra are shown in Figure S2 in the supplementary material. VBM value is obtained by linear fitting and extrapolating the leading valence band edge. The peak of the core energy level positions is obtained using a linear combination of Gaussian and Lorentzian functions with Shirley's background subtraction using XPSPEAK41 software. The uncertainties in the energy positions of core level energy and VBM determination are ±0.08 and ±0.1 eV, respectively. The obtained energy values of VBM and core levels for different samples are listed in Table I. Figure 6(a) shows the PES spectrum and fitting of VBM and core level, obtained using a synchrotron source beam of 156 eV photon energy, for the Al2O3 layer deposited on silicon (001) substrate (sample D). VBM is found to be 3.80 eV and the core level energy of the Al 2p peak is found to be 75.23 eV. The energy difference of the core level Al 2p from the VBM, i.e., (EAl2p – EVBM)Al2O3 is found to be 71.43 eV, which is in good agreement with the reported values.42,43 The bandgap of the Al2O3 layer is obtained from the difference between the core-level energy and the onset of inelastic loss of the oxygen peak using 615 eV photon energy,44 as shown in Fig. 6(b). The bandgap of the Al2O3 layer is found to be around 7.4 eV, which is close to the reported values in the literature.40, Figure 6(c) shows the valence band and core level spectrum of Ga 3d and Cr 3p for the 200 nm thick film of α-GaCrO3:Ni (sample C), obtained using a synchrotron beam of 615 eV photon energy. The VBM is estimated to be 0.65 eV, and the core level energy of Cr 3p is determined to be 43.98 eV. Ga 3d core level peak is fitted with the three features at 20.25, 19.64, and 22.19 eV, which are related to Ga3+ (stoichiometric Ga2O3) and Ga+ (sub-stoichiometric Ga environment), and O 2s characteristic, respectively.45,46 The energy separation of core level energy of Ga 3d from the VBM, i.e., (EGa3d – EVBM)GaCrO3 is equal to 19.60 eV, and the energy separation of core level energy of Cr 3p from the VBM, i.e., (ECr3p – EVBM)GaCrO3 is found to be 43.33 eV. Figure 6(d) shows the PES spectrum of a 2 nm thin layer of α-GaCrO3: Ni deposited on an Al2O3 substrate (sample C), obtained using a synchrotron beam of 615 eV photon energy. The core level energy difference of Al 2p with the core level energy of Ga 3d and Cr 3p is evaluated. The core level energy position of Al 2p is found to be at 74.57 eV, which is close to the reported values in the literature.43 On the other hand, the Ga 3d core level energy peaks fit to three features at 19.95, 18.88, and 22.72 eV. These peaks correspond to Ga3+ (stoichiometric Ga2O3) and Ga+ (sub-stoichiometric Ga environment) and O 2s characteristic, respectively.45,46 Chromium 3p core level peak is observed at 43.77 eV.43 The core level energy difference, i.e., ΔECL for Al 2p and Ga 3d core level is determined to be ∼54.62 eV, and ΔECL for Al 2p and Cr 3p core levels is found to be ∼30.8 eV.
FIG. 6.

(a) Photoelectron spectroscopy (PES) spectrum from ≈50 nm Al2O3 layer on Si (001) substrate measured by 156 eV photon energy of synchrotron source. The valence band maximum (VBM) is determined by the linear fit to the leading valence band edge. The energy difference of the Al 2p core level from the VBM is also marked. (b) The bandgap of Al2O3 layer is also determined from the difference of O 1 s core level and onset to inelastic loss peak using 615 eV photon energy of synchrotron source. (c) PES spectrum from ≈ 200 nm thick GaCrO3 layer on Al2O3 measured by 615 eV photon energy of synchrotron source. VBM is determined by the linear fit to the leading valence bad edge. Ga 3d core level has been fitted by three features. The energy differences of Ga 3d and Cr 3p core levels from the VBM are also marked. (d) PES spectrum from ≈2 nm thick GaCrO3 layer on Al2O3 (sample C) measured by 615 eV photon energy of synchrotron source. The core levels of Ga 3d and Cr 3p corresponding to the GaCrO3 layer and Al 2p corresponding to Al2O3 are observed. The Ga 3d core level has been fitted to three features related to bulk, surface, and satellite contributions. The energy difference of Ga 3d and Cr 3p with Al 2p core levels is also marked.

FIG. 6.

(a) Photoelectron spectroscopy (PES) spectrum from ≈50 nm Al2O3 layer on Si (001) substrate measured by 156 eV photon energy of synchrotron source. The valence band maximum (VBM) is determined by the linear fit to the leading valence band edge. The energy difference of the Al 2p core level from the VBM is also marked. (b) The bandgap of Al2O3 layer is also determined from the difference of O 1 s core level and onset to inelastic loss peak using 615 eV photon energy of synchrotron source. (c) PES spectrum from ≈ 200 nm thick GaCrO3 layer on Al2O3 measured by 615 eV photon energy of synchrotron source. VBM is determined by the linear fit to the leading valence bad edge. Ga 3d core level has been fitted by three features. The energy differences of Ga 3d and Cr 3p core levels from the VBM are also marked. (d) PES spectrum from ≈2 nm thick GaCrO3 layer on Al2O3 (sample C) measured by 615 eV photon energy of synchrotron source. The core levels of Ga 3d and Cr 3p corresponding to the GaCrO3 layer and Al 2p corresponding to Al2O3 are observed. The Ga 3d core level has been fitted to three features related to bulk, surface, and satellite contributions. The energy difference of Ga 3d and Cr 3p with Al 2p core levels is also marked.

Close modal
TABLE I.

Binding energy (BE) positions of core levels, VBM, and their energy differences (ΔE) for ≈200 nm thick layer of α-GaCrO3:Ni, ≈2 nm thick layer of α-GaCrO3: Ni /Al2O3, and ≈50 nm thick layer of Al2O3/Si. The uncertainties in the energy positions of VBM and core level energy determination are ±0.1 and ±0.08 eV, respectively. The error in the determination of core level binding energy differences and position of the core levels with respect to VBM is around ±0.11 and ±0.13 eV, respectively.

SampleStateBE (eV)ΔE (eV)
∼200 nm α-GaCrO3:Ni (sample A) VBM 0.65 
Ga 3d 20.25 (EGa3d – EVBM) GaCrO3 = 19.60 
Cr 3p 43.98 (ECr3p – EVBM) GaCrO3= 43.33 
2 nm α-GaCrO3:Ni (sample C) Al 2p 74.57 
Ga 3d 19.95 (EAl2p – EGa3d) GaCrO3= 54.62 
Cr 3p 43.77 (EAl2p – ECr3p) GaCrO3= 30.8 
∼50 nm Al2O3 (sample D) VBM 3.80 
Al 2p 75.23 (EAl2p – EVBM) Al2O3= 71.43 
SampleStateBE (eV)ΔE (eV)
∼200 nm α-GaCrO3:Ni (sample A) VBM 0.65 
Ga 3d 20.25 (EGa3d – EVBM) GaCrO3 = 19.60 
Cr 3p 43.98 (ECr3p – EVBM) GaCrO3= 43.33 
2 nm α-GaCrO3:Ni (sample C) Al 2p 74.57 
Ga 3d 19.95 (EAl2p – EGa3d) GaCrO3= 54.62 
Cr 3p 43.77 (EAl2p – ECr3p) GaCrO3= 30.8 
∼50 nm Al2O3 (sample D) VBM 3.80 
Al 2p 75.23 (EAl2p – EVBM) Al2O3= 71.43 

By putting all the values in Eq. (2), valence band offset (ΔEV) is obtained ∼ 2.79 ± 0.2 eV, which indicates that the VBM of film is 2.7 eV above the VBM of Al2O3. Similarly, the value of conduction band offset (ΔEC) using Eq. (3) is determined to be approximately −0.51 ± 0.2 eV, which indicates that the CBM of thin film is 0.51 eV below the CBM of the Al2O3. The values of ΔEV and ΔEC are also determined by considering the Cr 3p core level and are found to be ∼2.70 ± 0.2 and ∼−0.60 ± 0.2 eV respectively. With this knowledge, the diagram of band alignment considering the Ga 3d core level is presented in Fig. 7 and it reveals that the band alignment of the α-GaCrO3:Ni/Al2O3 heterojunction is of type I band alignment. A similar band alignment considering Cr 3p core level is shown in Fig. S3 in the supplementary material. Such types of band alignments have been reported for various heterojunctions such as Zn1−xMgxO/ZnO, n-ZnO/p-Si, p-NiO/n-GaN, and p-NiO/Al2O3 heterojunction.40,47–49 In these structures, one type of charge carrier faces a threshold barrier due to high band offset value, resulting in a favorable condition for selective charge carrier transport. This makes them a promising semiconductor for a wide range of applications, including UV photodetectors, MOSHEMTs, and light-emitting diodes.48–50 A revolutionary breakthrough in such optoelectronic devices can be achieved by the integration of complementary p-type TCO material, which will be exemplified by α-GaCrO3:Ni/Al2O3 heterojunction.

FIG. 7.

Band diagram of α-GaCrO3:Ni and Al2O3 heterojunction, where the bandgaps of α-GaCrO3:Ni and Al2O3 are marked. The core level energy difference between Al 2p and Ga 3d and the energy difference between VBM and respective core levels of Al 2p and Ga 3d are also marked. The valence and conduction band offsets are indicated. The band diagram shows a type-I band alignment between α-GaCrO3:Ni and Al2O3 heterojunction.

FIG. 7.

Band diagram of α-GaCrO3:Ni and Al2O3 heterojunction, where the bandgaps of α-GaCrO3:Ni and Al2O3 are marked. The core level energy difference between Al 2p and Ga 3d and the energy difference between VBM and respective core levels of Al 2p and Ga 3d are also marked. The valence and conduction band offsets are indicated. The band diagram shows a type-I band alignment between α-GaCrO3:Ni and Al2O3 heterojunction.

Close modal

In conclusion, we have deposited the epitaxial thin layer of α-GaCrO3:Ni on α-Al2O3 and investigated its band alignment properties. XRD measurements show good quality epitaxial nature of the film, with single domain growth. Pendellosung oscillations and HRTEM images confirm the high interfacial and crystalline qualities of α-GaCrO3:Ni thin film. The bandgap of α-GaCrO3:Ni is determined to be 4.1 eV using UV Visible Spectroscopy, which contributes to an average transparency of approximately 70%, and Seebeck measurements confirm the p-type character of the thin films. This is the highest bandgap reported for any Cr-based p-type materials. The values of valence band and conduction band offsets at the α-GaCrO3:Ni/Al2O3 heterojunction are found to be 2.79 ± 0.2 and −0.51 ± 0.2 eV, respectively, with type I band alignment, characterized as straddling gap band alignment. This band alignment makes it a suitable system to be explored in designing of remarkable optoelectronic devices such as UV photodetectors, MOSHEMTs, and light emitting diodes.

See the supplementary material for EDS data, PES survey scans, and Band alignment diagram.

The authors acknowledge Dr. S. D. Singh, RRCAT for providing growth facilities and Mr. Arijeet Das for helping in the XRR fittings. The authors are thankful to Dr. Chandrachur Mukherjee, RRCAT, Indore for optical transmission measurements and Dr. Satyaban Bhunia and Dr. Mrinmay K. Mukhopadhyay of Surface Physics and Material Science Division, SINP, Kolkata for their kind support for HRXRD measurements at GIXS beamline (BL-13) Indus-2, RRCAT, Indore, India. The authors also acknowledge Dr. Dinesh Kumar Shukla from UGC-DAE Consortium for Scientific Research, Indore for providing facilities for Seebeck measurements.

The authors have no conflicts to disclose.

Rishav Sharma: Data curation (lead); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (lead). Kiran Baraik: Data curation (equal). Himanshu Srivastava: Data curation (equal); Formal analysis (equal). Satish Kumar Mandal: Data curation (equal). Tapas Ganguli: Writing – review & editing (equal). Ravindra Jangir: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Investigation (equal); Resources (lead); Supervision (lead); Validation (lead); Writing – review & editing (equal).

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

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