We systematically analyzed the Al composition dependences of the structural properties of (Al Ga )2O3 thin films grown on -Ga2O3 (010) substrates. The crystal structure was characterized by x-ray diffraction, and the surface morphology was observed by reflection high-energy electron diffraction and atomic force microscopy. In the 100-nm-thick thin films, the crystallinity began to degrade and defects appeared on the surface when the Al composition exceeded about 0.16. The defects developed mainly along the direction and slightly along the direction as increased. The boundary where the thin film quality changed was close to a critical thickness curve calculated using the Matthews–Blakeslee model assuming the slip system of .
Gallium oxide (Ga2O3) has great potential for power electronics due to its ultrawide bandgap of 4.5–5.3 eV.1 Among the six polymorphs of Ga2O3 ( , , , , , and ), monoclinic -phase is thermodynamically the most stable, and bulk single crystals can be grown by conventional melt growth techniques.2,3 The availability of large-size substrates has driven diverse studies on -Ga2O3 for purposes related to thin film growth,1 device processing,4 and power device applications represented by Schottky barrier diodes5,6 and field-effect transistors (FETs).7,8
In addition to power device applications, -Ga2O3 can also be applied to wireless communication devices in harsh environments by taking advantage of the chemical stability and radiation hardness of its metal-oxide-semiconductor FETs (MOSFETs).9 Thus far, promising radio-frequency (RF) characteristics, such as a current-gain cutoff frequency of 10 GHz and a maximum oscillation frequency of 27 GHz, have been obtained for lateral -Ga2O3 MOSFETs with gate lengths of 80 and 200 nm, respectively.10 For an even higher RF performance, the challenge is how to suppress the short-channel effect (SCE). It occurs when the gate length is scaled down to less than 200–300 nm, and the representative phenomenon is a reduction of the threshold voltage. Solutions include thinning the channel layer and inserting a back-barrier layer. Here, (Al Ga )2O3 is suitable for the back-barrier layer of Ga2O3 MOSFETs because ( ) it can be easily grown on a -Ga2O3 substrate, after which a -Ga2O3 layer can be grown on it,11 and ( ) the bandgap of (Al Ga )2O3 is wider than that of Ga2O3.12 In fact, inserting the (Al Ga )2O3 back-barrier has successfully shifted the threshold voltage by +8 V, thus indicating its effectiveness in suppressing the SCE.13
When forming an epitaxial structure of Ga2O3 MOSFETs, an (Al Ga )2O3 layer is primarily grown on a -Ga2O3 substrate, and an unintentionally doped (UID) -Ga2O3 layer is grown thereon. In this upper layer, a channel will be formed in a later device process. It is, therefore, important to check the quality of the underlying (Al Ga )2O3 layer because it will influence the final device characteristics. Specifically, the following points must be carefully considered to grow the (Al Ga )2O3 layer. First, it is, of course, preferable to have good crystallinity and a flat surface because a Ga2O3 channel is formed on it. Second, a larger Al composition is better to widen the bandgap;12 however, attention must be paid to the Al solubility limit and the resultant decrease in crystallinity. Third, the (Al Ga )2O3 layer should be thick enough, but if the thickness exceeds the critical value and lattice relaxation occurs, the emergent dislocations could affect the device characteristics as a leakage current source. Here, we provide an additional explanation of why the thicker layer is required. In the Ga2O3 device processing, Si ion implantation doping can be utilized to form a channel and Ohmic contact regions in the UID Ga2O3 layer.14 While this technique improves the degree of freedom in device design, it also has downsides. For one, the implantation generates defects through which Fe diffusion from the Fe-doped Ga2O3 substrate to the channel layer is enhanced during the activation annealing after implantation.15 Moreover, the Gaussian-like depth profile of the implanted Si ions inevitably has a tail region below the channel layer. Thus, the (Al Ga )2O3 layer should be thick enough to counteract the Fe outdiffusion and to accommodate the implantation tail region. Note here that a high-quality (Al Ga )2O3 layer must be grown while maintaining the balance between the Al composition and the thickness, as there is also the matter of critical thickness.
In this work, to obtain high-quality (Al Ga )2O3 thin films with an adequate thickness ensuring sufficient back-barrier effects, we evaluated the crystal structure and surface morphology of the (Al Ga )2O3 thin films when the Al composition was systematically changed. Results showed that the crystallinity began to degrade and defects appeared on the surface when the Al composition exceeded about 0.16. As increased, the defects developed mainly along the direction and slightly along the direction. These experimental findings are briefly discussed alongside a comparison with the critical thickness.
(Al Ga )2O3 thin films were grown on Fe-doped semi-insulating -Ga2O3 (010) substrates by plasma-assisted molecular beam epitaxy (MBE). Ga and Al were evaporated from effusion cells. The beam equivalent pressure of Ga (BEP ) was fixed at Torr, while that of Al (BEP ) was varied from to Torr to change the Al composition . Oxygen plasma was generated by an RF plasma cell. The oxygen flow rate and the plasma power were set to 2.0 sccm and 250 W, respectively. The growth temperature of 630 C was monitored by a pyrometer. The temperature was used as optimized for the growth of unintentionally doped (UID) -Ga2O3 thin films on -Ga2O3 (010) substrates.16 The UID -Ga2O3 thin films grown below 500 C and above 700 C had rough surfaces due to three-dimensional growth and step bunching, respectively. In contrast, the films grown at 550–650 C had atomically smooth surfaces. In addition, the growth rate decreased above 700 C because of the decomposition of Ga2O3 to volatile suboxides.17 For the above reasons, the growth temperature of 630 C was determined. The film thickness was estimated to be about 100 nm by a surface profiler, and the growth rate was determined to be 0.33 m/h. The crystal structures of the samples were characterized by x-ray diffraction (XRD) using Cu K radiation. The surface morphology was observed by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The RHEED patterns were recorded at 300 C during the cooling process after the growth, while the AFM observation was carried out at room temperature in air.
III. RESULTS AND DISCUSSION
Figure 1(a) shows -2 XRD patterns of the (Al Ga )2O3/ -Ga2O3 (010) samples. The (020) diffraction peaks of the -(Al Ga )2O3 thin films monotonically shift to the higher angle side as the Al composition increases. Here, was estimated from the (020) diffraction peak separation between the substrate and the coherently grown (shown later) thin film in the -2 pattern.18, Figure 1(b) shows the relationship between and the Al BEP ratio. Since is linearly in proportion to the Al BEP ratio, Al atoms were efficiently incorporated into the thin films. In monoclinic -Ga2O3, Ga ions occupy the octahedral and tetrahedral sites. The results of a theoretical calculation to assess the phase stability of (Al Ga )2O3 alloy suggested that the octahedral site was preferentially occupied by Al up to .12 Ionic radii of sixfold coordinated Ga and Al are 0.62 and 0.535 Å, respectively.19 This means that the lattice constant becomes smaller with increasing . Further, the thin film is subjected to in-plane tensile strain on the substrate, leading to a contraction along the out-of-plane direction. Therefore, it is reasonable that the diffraction peaks of the -(Al Ga )2O3 thin films monotonically shift to the higher angle side with increasing . Clear thickness fringes observed in the -2 XRD patterns indicate the formation of sharp interfaces, from which the film thicknesses were estimated to be 100–110 nm. These values are in good agreement with the ones expected from the growth rate. In Fig. 1(c), for the -(Al Ga )2O3 thin films, the (020) diffraction peak intensities and the full widths at half maximum (FWHMs) of the (020) rocking curve peaks are plotted as a function of . Above , the peak intensity decreases and the rocking curve is broadened, indicating the lowering of the crystallinity due to approaching the solubility limit.
According to a previous work on the growth of (Al Ga )2O3 thin films, -phase may be stabilized with relatively high Al compositions.20 However, all the films in this study were grown without phase segregation.21 The solubility limit of Al in -Ga2O3 increases for higher growth temperature, however, it appears around in the case of the MBE growth.22 Since MBE operates in ultrahigh vacuum, the growth temperature is limited to C to reduce the decomposition of Ga2O3 into volatile suboxides.17 As a result, the value of is limited. On the other hand, since metal organic chemical vapor deposition (MOCVD) can operate at low to medium vacuum, the growth temperature can be increased to higher temperatures, leading to the higher solubility of Al. For instance, MOCVD growth at 880 C has demonstrated that -(Al Ga )2O3 thin films were grown on -Ga2O3 (010) substrates up to .20
Figures 1(d)–1(h) show reciprocal space mappings (RSMs) of the (Al Ga )2O3/ -Ga2O3 (010) samples measured around the asymmetric (420) diffraction. The peak of the (Al Ga )2O3 thin film is located at the same as that of the substrate, which indicates that these films were coherently grown on the substrates. In the sample, as the peak of the (Al Ga )2O3 thin film is broadened in the direction, the crystallinity is certainly lowered, as shown in Fig. 1(c). In addition, the peak of the -Ga2O3 substrate is also broadened in the direction for the sample, while the peaks are sharp for the samples. This implies a degradation of the substrate crystallinity (discussed later).
Next, we examine the surface morphologies of the as-grown (Al Ga )2O3 thin films. Figures 2(a)–2(e) show RHEED patterns for – . The sharp streaky patterns indicate atomically smooth surfaces. However, a closer look at the surface using AFM revealed an interesting difference. Figures 2(f)–2(j) show surface AFM images of the series of (Al Ga )2O3 thin films. For , , and , the surfaces are flat, and the root mean square roughness (R ) is comparable to that of the as-supplied -Ga2O3 (010) substrate (data not shown). For , although the R value is similar to those for – , small defects are sparsely observable, as indicated by the white arrows in Fig. 2(i), which are aligned along the direction. When is further increased to , more and larger defects appear across the surface [Fig. 2(j)]. According to the AFM images, the defects developed mainly along the direction and slightly along the direction as increases and approaches the solubility limit.
Here, we discuss the experimental results for the case of . It is seemingly inconsistent that the RHEED pattern was streaky [Fig. 2(e)] while the AFM image showed a defective surface [Fig. 2(j)]. However, since the RHEED patterns and the AFM images were observed at 300 C and room temperature, respectively, it is likely that the defects were generated during the cooling process after the growth due to the difference in thermal expansion coefficients between the film and substrate, which is a well-known phenomenon. For instance, the defect densities were increased during the cooling process after the growth from to cm in the GaAs/Si and GaP/Si systems.23 Moreover, the AFM images indicate that the defects developed in specific crystal orientations [Figs. 2(i) and 2(j)]. Regarding thin film defects, it is known that defects on substrates are one of the origins. When defects, which are sources of dislocations, are present on substrates, the dislocations are inherited by epitaxial thin films grown thereon. For -Ga2O3 substrates, defects along various directions are known to occur,24 and defects along the and directions have also been reported.25 Therefore, the defects observed on the (Al Ga )2O3 surfaces in the current study might have been partly triggered by defects that were originally present on the substrates. Note here that not all the defects on the film surface originated from the pre-existing defects in the substrate because the defect density observed in the AFM image was cm , while that in the substrate is cm .26 Returning to the RSM [Fig. 1(h)], not only the thin film peak but also the substrate peak was broadened in the direction. This implies a degradation in the substrate crystallinity. As the RSM indicates that the thin film is not relaxed, we presume that the strain energy accommodated in the (Al Ga )2O3 thin film is locally released by the generation of the defects, which helps the defect-free regions to remain coherently grown. In this case, the sample with exceeds the critical thickness even though the lattice relaxation was not detected by the RSM. Regarding the broadening of the substrate peak in the RSM, one possible scenario is that the defects generated in the (Al Ga )2O3 thin film propagated back to the Ga2O3 substrate, resulting in the degradation of the crystallinity. In terms of this assumption, a similar situation has been reported in tensile-strained SiGe thin films grown on Ge-on-Si (111).27,28
The parameters necessary to calculate are now all present. Figure 3 shows the curves as a function of and compares them with the experimental results for the (Al Ga )2O3 thin films. The boundary, where the crystallinity lowering was confirmed by XRD and the appearance of defects was observed by AFM ( ), is close to the curve obtained using the M–B model for the slip system of . Regarding the deviation from the calculated , it is generally known that the value is overestimated by the P–B model and is larger than the estimation by the M–B model when the lattice mismatch is below 1 , as in this study.35 In designing the -Ga2O3/(Al Ga )2O3 epitaxial structure for device prototyping, it is, thus, useful to adopt the Al composition and the (Al Ga )2O3 layer thickness estimated by the M–B model in anticipation of a small safety margin.
In summary, we aimed to improve the RF characteristics of lateral Ga2O3 MOSFETs by inserting the (Al Ga )2O3 back barrier. As a preliminary step, the crystal structure and surface morphology of 100-nm-thick (Al Ga )2O3 thin films were evaluated when the Al composition was changed. For , the crystallinity was lowered and the defects appeared across the surface. The defects developed mainly along the direction and slightly along the direction as increased. The boundary where the crystallinity and the surface morphology changed in the experiments was close to the critical thickness curve calculated using the M–B model assuming the slip system of . This finding will be useful to design -Ga2O3/(Al Ga )2O3 epitaxial structures for future device prototyping.
This work was supported in part by the Development Program, “R&D of ICT Priority Technology” of the Ministry of Internal Affairs and Communications, Japan (JPMI00316).
Conflict of Interest
The authors have no conflicts to disclose.
Takumi Ohtsuki: Conceptualization (equal); Data curation (lead); Formal analysis (lead); Investigation (lead); Methodology (lead); Validation (equal); Writing – original draft (lead); Writing – review & editing (equal). Masataka Higashiwaki: Conceptualization (equal); Funding acquisition (lead); Project administration (lead); Resources (lead); Supervision (lead); Validation (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.