We discuss the structure of threading dislocations in α-Ga2O3 thin films grown on c- and m-plane sapphire substrates. The thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Two distinct types of threading dislocations are identified for each of the c- and m-plane α-Ga2O3 thin films. The c-plane α-Ga2O3 thin film shows Burgers vectors of and , while the m-plane α-Ga2O3 thin film displays Burgers vectors of and . This paper presents a detailed structure of the threading dislocations in α-Ga2O3, which has been little disclosed thus far mainly due to the difficulty in synthesizing the metastable α-Ga2O3.
I. INTRODUCTION
Within the last decade, gallium oxide (Ga2O3) has been intensively investigated as an attractive material for high-power, radio frequency electronic, and deep-ultraviolet optical device applications due to its large bandgap.1–3 β-Ga2O3, which is thermodynamically the most stable phase among the well-known five polymorphs, has experienced rapid device-oriented progress.2 Such an accelerated development in research of β-Ga2O3 is mainly due to the availability of high-quality β-Ga2O3 single-crystalline wafers, which allow homoepitaxy of the β-Ga2O3 thin film. In contrast, in spite of the absence of bulk single crystal, there has also been much attention to corundum-structured metastable α-Ga2O3 with the largest bandgap among the five polymorphs, i.e., 5.3–5.6 eV,4,5 because it can be heteroepitaxially grown on an inexpensive and widely available sapphire substrate.4,6–9 Also, α-Ga2O3 has advantages from a viewpoint of bandgap engineering: the bandgap of α-Ga2O3 can be tuned from 3.7 to 8.6 eV by alloying with isostructural α-Al2O3 and α-In2O3.10,11 Furthermore, not only α-Ga2O3-based Schottky barrier diodes (SBDs),3,12–18 metal–semiconductor field effect transistors (MESFETs),19 and metal–oxide–semiconductor field effect transistors (MOSFETs),15,20–22 but also pn heterojunction diodes using α-(IrxGa1−x)2O33,23–25 have been fabricated, confirming a potential of α-Ga2O3 as a power-semiconductor.
Although there have been a lot of reports on α-Ga2O3-based devices, numerous threading dislocations in α-Ga2O3 thin films on sapphire substrates induced by lattice mismatches at the interface (4.81% and 3.54% along the a- and c-axis, respectively) remain a serious issue.7,26,27 We previously demonstrated that threading dislocations with a high density of 1010–1011 cm−2 severely limit carrier mobility in α-Ga2O3 thin films on sapphire.28 Moreover, such dislocations could form leakage-current paths, causing a premature breakdown of high-voltage devices employing α-Ga2O3. Dang et al. demonstrated α-Ga2O3 SBDs employing α-(AlxGa1−x)2O3 single-layer and heterostructure buffers, whereas the presence of threading dislocations and a large ideality factor were observed.18 In contrast, it is worth mentioning that Maeda et al. have indicated that the additional current components caused by defects are small at moderated voltage range in an α-Ga2O3 quasi-vertical SBD.13 While there have been reports so far on the dynamics of dislocation and/or strain relaxation process in α-Ga2O3 thin films on sapphire substrates, there is still a lack of detailed knowledge about dislocations in α-Ga2O3 thin films on sapphire substrates, partly because the reports on the synthesis of metastable α-Ga2O3 have been limited due to the difficulty to optimize synthesis conditions.21,29 Recently, Myasoedov et al. investigated the formation of planar defects in relation to the dislocation structure in α-Ga2O3 thin films on c-plane sapphire substrates.30 The study revealed complex features of the defects in α-Ga2O3 thin films and highlighted the need for further research. For instance, further research could be conducted on the microscopic configuration and mechanical properties of dislocations, including the magnitude and direction of Burgers vectors and lattice displacement, the degree of the strain field, and the propagation behavior. To optimize material properties and device performance and to advance the practical applications of the α-Ga2O3/sapphire system, it is important to acquire insights into the dislocation behavior, i.e., the detailed description of the structure of dislocations as well as the relationship between dislocations and electrical properties.
In this study, we report the relationship between thickness-dependent threading dislocations and electrical properties along the lateral directions as well as structural properties around the dislocation core at the atomic scale in α-Ga2O3 thin films on c- and m-plane sapphire substrates. We chose these two plane orientations because they have been commonly used for α-Ga2O3-based devices.13,14,16,18–20,22–25 We show that the carrier mobility is apt to increase with the increase in the film thickness accompanied by a decrease in threading dislocation density. Furthermore, by using plan-view transmission electron microscopy (TEM) observation and by comparing the results with those derived from previous cross-sectional TEM studies,27,28,30–32 it is found that one pure edge-type dislocation and one mixed-type dislocation are present in the α-Ga2O3 thin film on the c-plane sapphire substrate, while two mixed-type dislocations are present in the α-Ga2O3 thin film on the m-plane sapphire substrate. We adopted Si-doped α-Ga2O3 films to evaluate the relationship between threading dislocation density and electrical properties of α-Ga2O3 films. On the other hand, we conducted TEM observations for undoped α-Ga2O3 thin films without Si doping to clarify the structure of dislocations. This is because we consider that analyses on undoped α-Ga2O3 films, which should be purer than doped ones, are needed to obtain the essential information about the structural and mechanical properties of dislocations in an α-Ga2O3 film on a sapphire substrate. As suggested by studies on Sn- and Ge-doped α-Ga2O3 films on c- and m-plane sapphire substrates, respectively,17,33 the threading dislocation density in Si-doped α-Ga2O3 films on sapphire substrates may be different from that in undoped α-Ga2O3 films. Nevertheless, from a viewpoint of qualitative discussion on the relationships between threading dislocations and electrical properties, there would be no significant inaccuracy in the discussion even if the data on the threading dislocations in undoped α-Ga2O3 films were assumed to be approximately equal to those in Si-doped α-Ga2O3 films.
II. EXPERIMENTAL
Eight Si-doped α-Ga2O3 films with thicknesses ranging from 617 to 1422 nm and a 100 nm-thick undoped α-Ga2O3 thin film were grown on c-plane sapphire substrates by mist chemical vapor deposition (CVD). Additionally, six Si-doped α-Ga2O3 films with thicknesses ranging from 385 to 1322 nm and a 100 nm-thick undoped α-Ga2O3 thin film were grown on m-plane sapphire substrates by mist CVD. Tris(acetylacetonato)gallium(III) and chloro(3-cyanopropyl)dimethylsilane were used as precursors of Ga and Si, respectively. As a reaction source, we used an aqueous solution of these precursors with the addition of a small amount of hydrochloric acid, which helped to dissolve the precursors completely. For the Si-doped α-Ga2O3 thin films, the molar ratio of Si to Ga, [Si]/[Ga], in the solution was fixed at 4 × 10−4. The growth temperature was set to 500 °C and the carrier gas was oxygen.
Threading dislocation densities of the 14 Si-doped α-Ga2O3 thin films were evaluated by x-ray diffraction (XRD) rocking-curve measurements in both symmetric and skew-symmetric geometries using the Cu Kα1 radiation (Rigaku, SLX-2500K). Surface morphology of the 14 Si-doped α-Ga2O3 thin films was evaluated utilizing atomic force microscopy (AFM) (SHIMADZU, SPM-9700HT). Electron concentrations and mobilities of the 14 Si-doped α-Ga2O3 thin films were evaluated by Hall effect measurements (Toyo Corp., ResiTest8300) with a DC magnetic field of 0.67 T at room temperature. For the Hall measurements, Ti (30 nm)/Au (50 nm) electrodes with van der Pauw configuration were deposited on the α-Ga2O3 thin films using electron beam evaporation. The structure of threading dislocations in the two undoped α-Ga2O3 thin films was characterized by plan-view TEM with an acceleration voltage of 200 kV and high-angle annular dark-field scanning TEM (HAADF-STEM) with an acceleration voltage of 300 kV (Thermo Scientific Titan 60-300). Both plan-view samples for the TEM and HAADF-STEM observations were prepared by focused-ion-beam milling followed by back thinning from the substrate side. Strain analysis around the core of dislocations was performed using geometric phase analysis (GPA) with CrysTBox software.34
III. RESULTS AND DISCUSSION
A. Impacts of thickness-dependence of threading dislocation density on electrical properties
Threading dislocation density in Si-doped α-Ga2O3 thin films on sapphire substrates as a function of the film thickness. The threading dislocation density of the Si-doped α-Ga2O3 thin films was evaluated by the XRD rocking-curve measurements in symmetric and skew-symmetric geometries. Blue blank circles and triangles correspond to the density of edge- and screw-type threading dislocations, respectively, in α-Ga2O3 thin films on c-plane sapphire substrates. Red blank circles and triangles correspond to the density of edge- and screw-type threading dislocations, respectively, in α-Ga2O3 thin films on m-plane sapphire substrates.
Threading dislocation density in Si-doped α-Ga2O3 thin films on sapphire substrates as a function of the film thickness. The threading dislocation density of the Si-doped α-Ga2O3 thin films was evaluated by the XRD rocking-curve measurements in symmetric and skew-symmetric geometries. Blue blank circles and triangles correspond to the density of edge- and screw-type threading dislocations, respectively, in α-Ga2O3 thin films on c-plane sapphire substrates. Red blank circles and triangles correspond to the density of edge- and screw-type threading dislocations, respectively, in α-Ga2O3 thin films on m-plane sapphire substrates.
Second, the relationship between threading dislocation density and electrical properties in Si-doped n-type α-Ga2O3 thin films on sapphire substrates was explored. Figure 2 presents carrier concentrations and mobilities as a function of threading dislocation density in the Si-doped α-Ga2O3 thin films on c- and m-plane sapphire substrates, respectively. It should be noted that the samples presented in Fig. 2 are the same as those presented in Fig. 1. As depicted in Fig. 2, both carrier mobilities and carrier concentrations are apt to decrease as the threading dislocation density increases for both the α-Ga2O3 thin films on c- and m-plane substrates. The result indicates that the threading dislocation directly restricts the carrier mobility in terms of the dislocation scattering mechanism. Also, it is suggested that dangling bonds along the threading dislocations likely act as acceptor centers, which trap electrons to reduce the carrier concentrations in n-type α-Ga2O3 like n-type GaN.37–40
Carrier concentration and mobility as a function of threading dislocation (TD) density in Si-doped α-Ga2O3 thin films on sapphire substrates. The carrier concentration and mobility in Si-doped α-Ga2O3 thin films were obtained by the Hall effect measurements. Blue blank circles and triangles correspond to carrier concentration in α-Ga2O3 thin film on c- and m-plane sapphire substrates, respectively. Red blank circles and red blank triangles correspond to mobility in α-Ga2O3 thin film on c- and m-plane sapphire substrates, respectively.
Carrier concentration and mobility as a function of threading dislocation (TD) density in Si-doped α-Ga2O3 thin films on sapphire substrates. The carrier concentration and mobility in Si-doped α-Ga2O3 thin films were obtained by the Hall effect measurements. Blue blank circles and triangles correspond to carrier concentration in α-Ga2O3 thin film on c- and m-plane sapphire substrates, respectively. Red blank circles and red blank triangles correspond to mobility in α-Ga2O3 thin film on c- and m-plane sapphire substrates, respectively.
Here, it is noticeable that while threading dislocation density decreases by only half, the Hall carrier concentration and mobility increase by an order of magnitude. Farahani et al. reported a similar phenomenon for SnO2 films on sapphire substrates with a threading dislocation density of ≥1010 cm−2.41 Possible explanations for this phenomenon are as follows. The XRD measurements provide information on the averaged density of threading dislocations in an entire film. This means that the upper (surface) and lower (interface) regions of a film with a lower and higher dislocation density, respectively, as observed in the cross-sectional TEM studies,27,28 are not distinguished but are averaged. On the other hand, it is possible that charge carriers may transport more readily in the upper region than in the lower region due to the fact that the lower region is more electrically resistive because of the higher dislocation density. Consequently, the Hall carrier concentration and mobility of the thicker films may be less susceptible to the influence of threading dislocations than expected from the dislocation density estimated by the XRD, whereas those of the thinner films may be more significantly affected. Furthermore, it is known that the occupation factor of acceptor centers along the dislocation lines increases as the dislocation density increases.42 As a result, the dislocation scattering and the trapping of electrons are enhanced more significantly than expected from the dislocation densities. On the other hand, for systems with a dislocation density of ≥1010 cm−2, including GaN and α-Ga2O3 on sapphire substrates, the ionized impurity scattering becomes influential above carrier concentrations of late 1018 cm−3. Here, it should be noted that the carrier concentration depends on threading dislocation density.43,44 In this study, the carrier concentration of the α-Ga2O3 films is an order of 1018 cm−3, suggesting that the carrier mobility may be affected by the ionized impurity scattering. However, we consider that the dislocation scattering still plays an important role, as has been reported for SnO2 on the sapphire substrate with a carrier concentration of around 1018 cm−3.41 Also, unintentional acceptor impurities, such as Mg and Fe, can influence the electrical properties. However, the concentration of such unintentional acceptor impurities is much less than that of acceptor centers along the dislocation lines. Thus, this effect is considered to be not so significant in the context of this study.
This study confirms that as the film thickness increases, the threading dislocation density decreases, which affects the electrical properties of α-Ga2O3 thin films on sapphire substrates. In other words, increasing the film thickness and decreasing the threading dislocation density can result in a minimal effect of the threading dislocations on the electrical properties of α-Ga2O3 thin films. Here, we discuss a film thickness that is optimal for α-Ga2O3 devices. Previously, we proposed that dislocation densities below ∼1 × 107–1 × 108 cm−2 would be required for lightly doped drift layers in α-Ga2O3 devices to be insensitive to the effect of dislocation scattering.28 Recently, Oshima et al. have demonstrated that the dislocation density in α-Ga2O3 films is reduced via rapid growth at low temperatures.45 The film-thickness dependence of dislocation density indicated that the dislocation density would be within the range of 1 × 107–1 × 108 cm−2 at a film thickness of ∼1 × 103 μm.45 Furthermore, they demonstrated that the combination of the technique with epitaxial lateral overgrowth resulted in a dislocation density of 1.1 × 107 cm−2 at a film thickness of ∼1 × 102 μm.45 Therefore, the optimal film thickness for α-Ga2O3 devices is currently considered to be above ∼1 × 102–1 × 103 μm, which may become lower with the application of modified techniques in the future.
Although the threading dislocation density and its trend as a function of the film thickness are roughly consistent between the α-Ga2O3 thin films on c- and m-plane sapphire substrates, the mobility in the α-Ga2O3 thin films on m-plane sapphire substrates appears to be slightly higher than that on c-plane sapphire substrates. This trend has also been reported elsewhere.44 The variation in mobility based on plane orientations may not be attributed to the anisotropy in the effective mass of electrons, but rather to differences in the structure of dislocations and other defects at the atomic scale. This is because some first-principles calculations have concluded the nearly isotropic effective mass in α-Ga2O3.46–48 Also, Yamaguchi pointed out that the observed anisotropy in the n-type semiconducting state in β-Ga2O3 should not be attributed to the properties of a perfect lattice.49 Such estimation requires the characterization of threading dislocations structure in α-Ga2O3 at the atomic scale as follows.
B. TEM characterization of threading dislocations structure at the atomic scale
We discuss the structures of dislocations in undoped α-Ga2O3 films on c- and m-plane sapphire substrates at the atomic scale based on the results derived from the plane-view TEM and HAADF-STEM observations along with previous cross-sectional two-beam TEM studies.27,28,30–32 Figures 3(a) and 3(b) illustrate plane-view TEM images of the undoped α-Ga2O3 films on the c-plane sapphire substrate viewed along the [0001] direction and on the m-plane sapphire substrate viewed along the direction, respectively. Threading dislocations can be seen in both films. From three plane-view TEM images including Figs. 3(a) and 3(b), we calculated the threading dislocation density by counting the number of dislocations and then dividing them by the area. The threading dislocation densities for the α-Ga2O3 thin films on c- and m-plane sapphire substrates were estimated to be about (8.6 ± 3.0) × 1010 and (7.1 ± 3.0) × 1010 cm−2, respectively. Those values are roughly consistent with those estimated by the XRD rocking-curve measurements.
TEM images of the undoped α-Ga2O3 thin film on (a) the c-plane sapphire substrate viewed along the [0001] direction and (b) the m-plane sapphire substrate viewed along the direction.
TEM images of the undoped α-Ga2O3 thin film on (a) the c-plane sapphire substrate viewed along the [0001] direction and (b) the m-plane sapphire substrate viewed along the direction.
Figures 4(a) and 4(d) present HAADF-STEM images of the threading dislocation cores in the undoped α-Ga2O3 thin film on the c-plane sapphire substrate viewed along the [0001] direction. Since a brighter spot in the HAADF-STEM image corresponds to a heavier atom,50 the bright spots denote the position of Ga columns. As for the α-Ga2O3 thin films on the c-plane sapphire substrate, two distinct types of threading dislocations are found as shown in Figs. 4(a) and 4(d). By drawing a Burgers circuit, an edge component (be) of the dislocation in Fig. 4(a) is determined to be . The threading dislocations with be along the direction in α-Ga2O3 films on c-plane sapphire substrates were also observed by cross-sectional two-beam TEM under the diffraction vector (g) of .30,31 As is not a translation vector of the corundum structure, the dislocation should have a screw component (bs) along the [0001] direction to make it a perfect dislocation. Cross-sectional two-beam TEM images under g = 0006 suggested the presence of threading dislocations with bs along the c-axis.27,30–32 Thus, since a vector of corresponds to the smallest translation vector with , the dislocation in Fig. 4(a) is expected to have bs = 1/3[0001] and to be a mixed dislocation with Burgers vector (b) of . The perfect mixed dislocations with were also observed in isostructural α-Al2O3.51,52 On the other hand, be of the dislocation in Fig. 4(d) is determined to be , which dissociates into two dislocations with . Based on the fact that corresponds to a translation vector of the corundum structure, the threading dislocation in Fig. 4(d) is considered to be a pure edge dislocation, which can be estimated by cross-sectional two-beam TEM under .27 It has also been reported that there exists a perfect dislocation with which dissociates into two partial dislocations with in α-Al2O3.52–55 Myasoedov et al. indicated the formation of dislocation loops bounded by partial dislocations with along with the formation of extended stacking faults in the α-Ga2O3 thin films on the c-plane sapphire substrate, which may also correspond to the case presented in Fig. 4(d).
HAADF-STEM images of threading dislocation cores in the undoped α-Ga2O3 thin film on the c-plane sapphire substrate with projected Burgers vectors of (a) and (d) viewed along the [0001] direction, and those on the m-plane sapphire substrate with projected Burgers vectors of (g) and (j) be = 1/3[0001] viewed along the direction. Bragg-filtered HAADF-STEM images of (b) the threading dislocation cores shown in (a), (e) shown in (d), (h) shown in (g), and (k) shown in (j). ɛxx strain field calculated based on (c) the lattice fringes shown in (b), (f) shown in (e), and (l) shown in (k). (i) ɛyy strain field calculated based on the lattice fringes shown in (h).
HAADF-STEM images of threading dislocation cores in the undoped α-Ga2O3 thin film on the c-plane sapphire substrate with projected Burgers vectors of (a) and (d) viewed along the [0001] direction, and those on the m-plane sapphire substrate with projected Burgers vectors of (g) and (j) be = 1/3[0001] viewed along the direction. Bragg-filtered HAADF-STEM images of (b) the threading dislocation cores shown in (a), (e) shown in (d), (h) shown in (g), and (k) shown in (j). ɛxx strain field calculated based on (c) the lattice fringes shown in (b), (f) shown in (e), and (l) shown in (k). (i) ɛyy strain field calculated based on the lattice fringes shown in (h).
Figures 4(g) and 4(j) illustrate HAADF-STEM images of threading dislocation cores in the undoped α-Ga2O3 thin film on the m-plane sapphire substrate viewed along the direction. It is revealed that there are two types of threading dislocations for the α-Ga2O3 thin film on the m-plane sapphire substrate. By drawing a Burgers circuit, be of the dislocation in Fig. 4(g) is determined to be , while that in Fig. 4(j) is to be be = 1/3[0001], which dissociates into two dislocations with be = 1/6[0001]. In our previous study, high-density threading dislocations with be along the direction in α-Ga2O3 films on m-plane sapphire substrates were also observed by cross-sectional two-beam TEM under .28 Conversely, only obscure contrast was seen in the TEM image under .28 These results suggest that the density of the threading dislocations with is much higher than that with be = 1/3[0001]. Here, neither nor be = 1/3[0001] is a translation vector, thus, the dislocations should have a screw component (bs) along the direction to make them a perfect dislocation. Cross-sectional two-beam TEM images under illustrated high-density threading dislocations with bs along the m-axis.28 Since vectors of and correspond to translation vectors of the corundum structure, the dislocations in Figs. 4(g) and 4(j) are expected to have and , respectively, and to be mixed dislocations with and , respectively. The perfect mixed dislocations with and were observed in α-Al2O3.51,52,56 Here, we further discuss the structure of the threading dislocations with shown in Fig. 4(j). Since the perfect dislocations with have been reported to dissociate into two partial dislocations with in α-Al2O3,52,56 the two dissociated dislocations shown in Fig. 4(j) should have to be . Additionally, it has been reported that the perfect dislocations with dissociate into two partial dislocations with in α-Al2O3.51,52 Since the Burgers vectors of the two dissociated dislocations in Fig. 4(j) have no component, they are expressed as and . Therefore, the dissociation reaction of the dislocation in Fig. 4(j) may also correspond to the case.52 It should be noted that the perfect dislocation with which dissociates into three partial dislocations with was reported in α-Al2O3,52,57 However, this type of dislocation was not observed in the present study.
To clearly distinguish the dislocation cores, we performed the inverse fast Fourier transform for each HAADF-STEM image. Figures 4(b), 4(e), 4(h), and 4(k) exhibit Bragg-filtered images of the threading dislocation cores presented in Figs. 4(a), 4(d), 4(g), and 4(j), respectively. The , , , and (0006) Bragg spots were selected to obtain Figs. 4(b), 4(e), 4(h), and 4(k), respectively. Figures 4(b), 4(e), 4(h), and 4(k) illustrate the introduction of extra half-planes , , , and directions, respectively, from each location of the dislocation core. Finally, we carried out the strain analysis using the GPA as shown in Figs. 4(c), 4(f), 4(i), and 4(l). The , , and , (0006) Bragg spots were chosen for the calculations of strain field in the c- and m-plane α-Ga2O3 thin films, respectively. The x- and y-axes are set according to the indications in Fig. 4. We show the ɛxx strain field in Figs. 4(c), 4(f), and 4(l) because the plane orientations of extra half-planes in Figs. 4(b), 4(e), and 4(k) are parallel to the x axis, while the ɛyy strain field is shown in Fig. 4(i) because the plane orientation in Fig. 4(h) is parallel to the y axis. The color scale indicates the magnitude of each strain. It can be seen that the strains are negative and compressive in the regions to which extra half-planes are inserted, whereas the strains are positive and tensile in the opposite regions for all the types of threading dislocations. From a viewpoint of the relationship between these dislocations and electrical properties, it is considered that these strains around the dislocation core have a significant impact on the carrier mobility, that is, these strains restrict the carrier mobility. In contrast, possible charged dangling bonds along threading dislocation lines also should limit carrier mobility, although further research is needed to detect charged states around the dislocation cores.
Furthermore, to understand the structural properties of the α-Ga2O3 thin films on sapphire substrates, it is inevitable to consider the relationship between misfit and threading dislocations because high-density misfit dislocations have been also observed at the interfaces of α-Ga2O3 films on c- and m-plane sapphire substrates.7,26,27,58 Misfit dislocations are strongly connected to threading dislocations. In general, threading dislocations in the films that originate from preexisting threading dislocations in the substrates glide and form misfit dislocations when the film thickness reaches the critical value.59 Moreover, in systems with large lattice mismatches, other threading dislocations have been reported to be generated by the coalescence of initially grown islands with misfit dislocations,60 the intersection of two misfit dislocations,61 and the nucleation of dislocation half-loops.62 Regarding α-Ga2O3 thin films on c-plane sapphire substrates, Ma et al. indicated that the line density of misfit dislocations matched that of edge-type threading dislocations. Moreover, the high-density misfit dislocation is a consequence of significant lattice mismatches between an α-Ga2O3 film and a sapphire substrate (4.81% and 3.54% along the a- and c-axis, respectively). For α-Ga2O3 films on c-plane sapphire substrates, the in-plane symmetry is isotropic with respect to the hexagonal structure, and the in-plane lattice mismatch is governed by the mismatch along the a axis, that is, 4.81%. In contrast, for α-Ga2O3 films on m-plane sapphire substrates, the in-plane symmetry is anisotropic. Consequently, the lattice mismatches along both the a- and c-axis contribute to the overall in-plane lattice mismatch. The different strain states induced by the different in-plane lattice mismatches are considered to affect the nucleation processes of threading dislocations as well as misfit dislocations. Thus, from this perspective, the distinct characteristics of the threading dislocations between α-Ga2O3 films on c- and m-plane sapphire substrates need to be evaluated. Therefore, clarifying the specific relationship between the threading and misfit dislocations in α-Ga2O3 films on sapphire substrates will be the focus of our future work.
IV. CONCLUSIONS
We discussed the relationship between the thickness-dependent threading dislocation density and electrical properties along the lateral directions as well as structural characteristics around the dislocation core at the atomic scale in α-Ga2O3 thin films on c- and m-plane sapphire substrates. We found that the thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Furthermore, we found two types of threading dislocations for the α-Ga2O3 thin film on the c-plane sapphire substrate. One has , the other which dissociates into . The former should have bs = 1/3[0001] to be a mixed dislocation, while the latter is a pure edge dislocation. As for the α-Ga2O3 thin film on m-plane sapphire substrate, two types of them were found. One has , the other be = 1/3[0001], which dissociates into be = 1/6[0001]. Both of them should be mixed dislocations, that is, the former should have , while the latter . For all the types of dislocations found in this study, there exists compressive strain in the regions where extra half-planes are present, whereas tensile strain in the opposite regions.
SUPPLEMENTARY MATERIAL
See the supplementary material for the details of the XRD and AFM analyses.
ACKNOWLEDGMENTS
This work was, in part, supported by the Ministry of Internal Affairs and Communications (MIC) under a grant entitled “R&D of ICT Priority Technology (JPMI00316)” and “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grant No. JPMXP1223HK0049 (Hokkaido University).
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Hitoshi Takane: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (equal); Methodology (lead); Visualization (lead); Writing – original draft (lead). Shinya Konishi: Investigation (equal); Project administration (supporting); Writing – review & editing (supporting). Yuichiro Hayasaka: Investigation (equal); Resources (equal); Writing – review & editing (supporting). Ryo Ota: Investigation (equal); Resources (equal); Writing – review & editing (supporting). Takeru Wakamatsu: Investigation (supporting); Writing – review & editing (supporting). Yuki Isobe: Investigation (supporting); Writing – review & editing (supporting). Kentaro Kaneko: Investigation (supporting); Writing – review & editing (supporting). Katsuhisa Tanaka: Funding acquisition (lead); Investigation (supporting); Project administration (lead); Resources (equal); Supervision (lead); Writing – review & editing (lead).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding authors upon reasonable request.