WO2019155444A1

WO2019155444A1 - Semiconductor devices with two iii‑oxide layers having different phases and method of production

Info

Publication number: WO2019155444A1
Application number: PCT/IB2019/051125
Authority: WO
Inventors: Xiaohang Li; Kuang-Hui LI; Haiding SUN; Gary S. Tompa; Serdal Okur; Thomas Salagaj
Original assignee: King Abdullah University Of Science And Technology
Priority date: 2018-02-12
Filing date: 2019-02-12
Publication date: 2019-08-15

Abstract

A semiconductor device is formed having a second III-oxide layer on a first III-oxide layer. The first III-oxide layer having a first phase is formed by feeding a first group III element precursor, a first oxygen precursor, and molecules including hydrogen and/or chlorine ions into a growth chamber. The second III-oxide layer is formed on the first III-oxide layer by feeding a second group III element precursor, a second oxygen precursor, and molecules including hydrogen and/or chlorine ions into the growth chamber. The second III-oxide layer has a second phase, which is different from the first phase. The molecules including hydrogen and/or chlorine ions are fed at a first flow rate during the formation of the first III-oxide layer and are fed at a second flow rate during the formation of the second III-oxide layer. The first and second flow rates are different flow rates.

Description

SEMICONDUCTOR DEVICES WITH TWO lll-OXIDE LAYERS HAVING DIFFERENT PHASES AND METHOD OF PRODUCTION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/629,343, filed on February 12, 2018, entitled“A MULTIPLE-LAYER

STACKING STRUCTURE CONSISTING OF (Al, Ga, In) lll-OXIDE WITH

DIFFERENT PHASES,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the disclosed subject matter generally relate to methods for forming semiconductor devices with two Ill-oxide layers having different phases by controlling the feed rate of molecules comprising hydrogen and/or chlorine ions during the formation of the two Ill-oxide layers.

DISCUSSION OF THE BACKGROUND

[0003] Ill-nitride semiconductor materials (AIN, InN, GaN, and their alloys) are of great interest due to their direct and wide bandgap, which makes them suitable for power electronic and optoelectronic applications. Remarkable breakthroughs have been achieved in the past few decades. For example, (Al) GaN alloys have achieved a high critical electric field of ~3.5 MV/cm, which makes it one of the best candidates for power electronics. However, some semiconductor materials exhibit ionic bonding instead of covalent bonding. Semiconductor materials exhibiting ionic bonding include wide bandgap oxides of post-transitional metals that exhibit higher electrical breakdown strength, since higher electrical fields are needed to ionize charges. Among these oxides, Ga2C>3 has attracted attention due to its extremely high theoretical breakdown field of ~8 MV/cm, which is even higher than SiC (3 MV/cm) and GaN (3.8 MV/cm). Over 1 kV of reverse breakdown voltage in a vertical diode has been achieved using b-ΰ82q3 and a record-high critical field strength of 3.8 MV/cm has been reported in a homoepitaxially grown Ga203 MOSFET, already surpassing the GaN and SiC bulk theoretical field strengths.

[0004] In addition, due to its large intrinsic bandgap (4.5-5.1 eV), Ga203 is suitable for solar-blind deep ultraviolet (DUV) detection, and it can also be used as a UV-transparent template for other semiconductor materials. Furthermore, up to four-inch high-quality but costly native substrate produced by melt growth techniques are commercially available for large-scale device fabrication. Additionally, Ga203 thin film can be doped with n-type dopant by different growth techniques such as halide vapor phase epitaxy (FIVPE), molecular beam epitaxy (MBE), low-pressure chemical vapor deposition (LPCVD) and metal-organic chemical vapor deposition (MOCVD). However, Ga203 thin film has yet to be doped p-type.

[0005] Ga203 can form five different polymorphs designated as a, b, g, d and e, with b-ΰ82q3 being the most thermally stable one. This characteristic makes it possible to produce both single crystals and epitaxial b-ΰ32q3 films. One approach for producing b-ΰ32q3 epitaxial films is by vapor phase epitaxy, for example by using trimethylgallium (TMGa) or triethylgallium (TEGa) and O2 as precursors in a MOCVD or by using gaseous GaC and O2 in a HVPE reactor. The main difference between these two techniques is that HVPE uses high chlorine concentrations to produce very high growth rates. The b-q82q3 has a monoclinic structure, resulting in difficulty growing high-quality b-q82q3 films on hetero-substrates, i.e., sapphire and silicon (Si). Thus, the majority of existing b-Ga203-based power devices have been developed on expensive (-201 ) b-ΰ82q3 native substrate.

[0006] The £-Ga203 was first synthesized in 1952 using CVD, but it was one of the meta-stable phases of Ga203 and transformed to b-phase at temperatures as low as 500 °C. In contrast to b-ΰ32q3, there are only a limited number of reports on the MOCVD-grown phase-pure £-Ga203. Most of the reported MOCVD-grown Ga203 has been grown above 550 °C, which results in the formation of only b-phase. Recently, single-phase £-Ga203 thin films grown on c-plane sapphire by MOCVD have been reported, as well as on a less lattice-mismatch substrate of 6H-SiC.

[0007] Studies of successful heteroepitaxial growth of metastable £-Ga203 on sapphire typically use HVPE technique, which has a fast growth rate of

approximately a few micrometers per hour. Researchers were able to achieve phase-pure £-Ga203 on c-plane sapphire, GaN (0001), AIN (0001 ) and b-ΰ82q3 (-201 ) by this technique at 550 °C. Because the £-Ga203 is a highly symmetric hexagonal structure close to the Ill-nitrides, there is an assumption £-Ga203 can be integrated with nitrides to form heterojunctions for optoelectronic devices. Thus, a precise control of the growth rate of e- Ga203 by using MOCVD is necessary.

[0008] a-Ga203 is another less-studied metastable phase of Ga203. a-Ga203 was found to have the largest bandgap among all forms of Ga203, and hence the highest electrical breakdown field compared with other phases. a-Ga2C>3 has corundum-like structure the same as a-Al203, and thus a-Ga2C>3 could be obtained on sapphire (a-AteOs) substrate due to the similarity in lattice structure between the epilayer and the substrate. However, most of the studies on a-Ga203 grown on c-plane sapphire involved mist chemical vapor deposition (CVD) and HVPE. Pure MOCVD-grown a-Ga203 thin film on sapphire has not been reported to date.

Nevertheless, Reference [1] discloses that pseudomorphic a-Ga203 layer having a thickness of three monolayers can be stabilized by strain present at the interface between c-plane sapphire and b-ΰ32q3, indicating a-Ga203 has a great potential to be used as novel buffer layers for epitaxial growth of other phases of Ga203 on sapphire. Reference [1] notes that the three monolayer thick pseudomorphic a-Ga203 layer is formed regardless of growth method, and thus does describe particular conditions that can be employed to intentionally grow b-ΰ82q3 on a-Ga203. Further, the technique disclosed in Reference [1] is limited to a structure in which b-ΰ82q3 is grown on a-Ga203. Additionally, the technique disclosed in Reference [1] is limited to growing the a-Ga203 layer having a thickness of three monolayers and does not describe how to grow thicker layers of a-Ga203.

[0009] Thus, it would be desirable to provide for the ability to control the formation of at least two layers with different phases of a Ill-oxide for a

semiconductor device. Further, it would be desirable to form two layers with different phases of a Ill-oxide in which the different phases can be formed in any order.

Additionally, it would be desirable to grow Ill-oxide layers having thickness greater than three monolayers, if desired. SUMMARY

[0010] According to one embodiment there is a method for forming a semiconductor device. A first Ill-oxide layer having a first phase is formed by feeding a first group III element precursor, a first oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into a growth chamber. A second Ill-oxide layer is formed on the first Ill-oxide layer by feeding a second group III element precursor, a second oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber. The second Ill-oxide layer has a second phase, which is different from the first phase. The molecules comprising hydrogen and/or chlorine ions are fed at a first flow rate during the formation of the first Ill- oxide layer and the molecules comprising hydrogen and/or chlorine ions are fed at a second flow rate during the formation of the second Ill-oxide layer. The first and second flow rates are different flow rates.

[0011] According to another embodiment there is a semiconductor device having a first Ill-oxide layer having a first phase and a second Ill-oxide layer arranged on top of the first-ill oxide layer and having a second phase, which is different from the first phase. The first phase of the first Ill-oxide layer is a beta phase, an epsilon phase, or a combination of beta and epsilon phases. The second phase of the second Ill-oxide layer is an alpha phase, a beta phase, an epsilon phase, or a combination of beta and epsilon phases.

[0012] According to a further embodiment there is a method for forming a semiconductor device. A first phase for a first Ill-oxide layer and a second phase for the second Ill-oxide layer are determined. The second Ill-oxide layer is formed on the first Ill-oxide layer by first feeding a first group III element precursor, a first oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into a growth chamber and subsequently feeding a second group III element precursor, a second oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber. A feed rate of the molecules comprising hydrogen and/or chlorine ions into the growth chamber to form the first and second Ill-oxide layers is based on the determined phase of the first and second Ill-oxide layers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

[0014] Figure 1 is a flowchart of a method for forming a semiconductor device according to embodiments;

[0015] Figure 2 is a block diagram of a semiconductor device according to embodiments;

[0016] Figure 3 is a graph of hydrogen chloride flow rate versus deposition rate according to embodiments;

[0017] Figure 4 is a graph of x-ray diffraction (XRD) patterns of gallium oxide films grown under different flow rates of hydrogen chloride according to

embodiments; and

[0018] Figure 5 is a flowchart of a method for forming a semiconductor device according to embodiments.

DETAILED DESCRIPTION

[0019] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of Ill-oxide materials. [0020] Reference throughout the specification to“one embodiment” or“an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases“in one embodiment” or“in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0021] Figure 1 is a flowchart of a method for forming a semiconductor device according to embodiments. Initially, a first Ill-oxide layer having a first phase is formed by feeding a first group III element precursor, a first oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into a growth chamber (1 10). Next, a second Ill-oxide layer is formed on the first Ill-oxide layer by feeding a second group III element precursor, a second oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber (step 120). The second Ill-oxide layer has a second phase, which is different from the first phase. The phases of the first and second Ill-oxide layers can be one or a combination of a-, b-, e-, d-, or g-phases. The particular phases of the first and second Ill-oxide layers can be defined by controlling the feed rate of the molecules comprising hydrogen and/or chlorine ions during the formation of each of the Ill-oxide layers. For example, the molecules comprising hydrogen and/or chlorine ions are fed at a first flow rate during the formation of the first Ill-oxide layer and the molecules comprising hydrogen and/or chlorine ions are fed at a second flow rate during the formation of the second Ill-oxide layer and the first and second flow rates are different flow rates. In addition to, or as an alternative to one of more of, the precursors mentioned above, water can also be fed into the chamber during the formation of one or both layers in any of the disclosed techniques.

[0022] The first group III element precursor can include indium, boron, aluminum, or gallium, and the second group III element precursor can include a different one of indium, boron, aluminum, and gallium. Alternatively, the first and second group III element precursors can include a same one of indium, boron, aluminum, or gallium. Further, the first and/or second Ill-oxide layers can be alloys, and thus a third group III element precursor can be fed into the growth chamber during the formation of the first Ill-oxide layer and/or a fourth group III element precursor can be fed into the growth chamber during the formation of the second Ill-oxide layer. Further, the formation of the Ill-oxide layer can include controlling the feed rate of molecules comprising both hydrogen and chlorine ions in the form of hydrogen chloride during the formation of one or more of the Ill-oxide layers. Because the phase of the Ill-oxide layer can be selected by the flow rate of molecules comprising hydrogen and/or chlorine ions, the Ill-oxide layer can be grown to any thickness, and thus the disclosed method can form an alpha phase Ill-oxide layer having a thickness greater than three monolayers, whereas the technique disclosed in Reference [1] is limited to a thickness of three monolayers.

[0023] Figure 2 is a block diagram of a semiconductor device according to embodiments. As illustrated, the semiconductor device 200 includes a first Ill-oxide layer 210 having a first phase. A second Ill-oxide layer 220, which is arranged on top of the first Ill-oxide layer 210, has a second phase, which is different from the first phase. In one embodiment, the first phase of the first Ill-oxide layer 210 is a beta phase, an epsilon phase, or a combination of beta and epsilon phases, and the second phase of the second Ill-oxide layer 220 is an alpha phase, a beta phase, an epsilon phase, or a combination of beta and epsilon phases. In other embodiments, the phases of the first and second Ill-oxide layers can be one or a combination of a-, b-, e-, d-, or g-phases.

[0024] The Ill-oxides for the first and/or second layer can be indium oxide (InO), boron oxide (BO), aluminum oxide (AIO), or gallium oxide (GaO). Depending upon which particular Ill-oxides are used for the first and second Ill-oxide layers, the semiconductor device can be an optoelectronic device (e.g., a distributed Bragg reflector (DBR), distributed feedback (DFB), super lattice structure, or multiple quantum wells (MQWs) for waveguiding, modulator, detector, laser, and light emitting diode (LED) applications) or an electronic power device (e.g., MOSFET or MESFET).

[0025] Although Figure 2 illustrates a semiconductor device having two Ill-oxide layers with different phases, the semiconductor device can include additional layers, as well as a substrate. These additional layers can be additional Ill-oxide layers having the same or different phases as the two Ill-oxide layers, which can be grown to achieve a particular phase using the same techniques as are disclosed in connection with the first and second Ill-oxide layers. The additional layers can also be Ill-nitride layers. Further, the additional layer(s) can be interposed between the first and second Ill-oxide layers. For example, a third Ill-oxide layer on the second Ill-oxide layer can be formed on the second Ill-oxide layer 220, which can be achieved by feeding a third group III element precursor, a third oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber following the formation of the second III- oxide layer 220. The third Ill-oxide layer can have a third phase, which can be the same or different from the first or second phases.

[0026] Figure 3 is a graph of hydrogen chloride flow rate versus deposition rate according to embodiments and Figure 4 is a graph of x-ray diffraction (XRD) patterns of gallium oxide films grown under different flow rates of hydrogen chloride according to embodiments. These graphs were produced by growing a Ill-oxide layer in a reactor using metal-organic chemical phase epitaxy (MOCVD).

Specifically, the layer grown to produce these graphs was a gallium oxide (Ga2C>3) layer. In one example, the reactor can comprise a 16-inch diameter stainless steel chamber and a single rotating disc reactor with a 13-inch platter. The gasses were fed into the chamber from a showerhead at the top of the chamber. Four different Ill-oxide layers were separately grown on different sapphire substrates during the experiments, with hydrogen chloride (HCI) flow rates (also referred to as feed rates) of 0, 5, 10, 30, and 60 seem. Each growth run lasted one hour during which the chamber pressure and temperature were maintained at 45 Torr and 600° C, respectively. Triethylgallium (TEGa) and oxygen (O2) were used as precursors for the gallium oxide layer and argon (Ar) was used as the carrier gas. It should be recognized, however, that other precursors and carrier gasses can be used within the scope of this disclosure.

[0027] As illustrated in Figure 3, the deposition rate (also referred to as growth rate) exhibited a linear increase with the increase of the hydrogen chloride flow rate, reaching the highest point of ~1 pm/hour at 30 seem. After this high point, the growth rate started to decrease with the increase of the hydrogen chloride flow rate and dropped to approximately 400 nm/hour at 60 seem. However, the deposition rate of approximately 400 nm/hr with a hydrogen chloride flow rate of 60 seem is still significantly higher than the deposition rate of approximately 100 nm/hour when no hydrogen chloride is feed into the chamber (i.e., 0 seem). Thus, the deposition rate can be increased by feeding at least some hydrogen chloride into chamber during the formation of a Ill-oxide layer.

[0028] Further, as illustrated in Figure 4, the phase of the Ill-oxide layer is based on the feed rate of the hydrogen chloride. Specifically, when no hydrogen chloride or 5 seem of hydrogen chloride were fed into the chamber while feeding the precursors, the gallium oxide layer had a beta- (b-) phase. More specifically, when no hydrogen chloride is fed into the chamber while feeding the precursors, the formed layer exhibited diffraction peaks at 18.96°, 38.38°, and 59.15°, which are assigned to the low and higher order diffractions of beta-phase gallium oxide

^-Ga2C>3). When 5 seem of hydrogen chloride is fed into the chamber while feeding the precursors, the same three peaks were exhibited, as well as additional beta-phase gallium oxide peaks at 31 .02° (-1 10) and 64.89° (-204), which further confirms the beta-phase. Thus, if a beta-phase Ill-oxide layer is desired, 5 seem of hydrogen chloride can be fed into the chamber to achieve an increased deposition rate (i.e., as illustrated in Figure 3, an increase from ~ 100 nm/hour to ~ 400 nm/hour).

[0029] When the feed rate of the hydrogen chloride was increased to 10 seem, the Ill-oxide layer exhibited a combined epsilon- (e-) and beta-phase structure.

Specifically, the Ill-oxide layer exhibited new diffraction peaks at 19.23°, 38.93 °, and 59.93° corresponding to 002, 004 and 006 diffractions of £-Ga2C>3, respectively.

These results indicate that the Ga2C>3 film transformed into a mixture of b- and £-Ga203 and the growth rate continued to rise.

[0030] When the feed rate of the hydrogen chloride was increased to 30 seem, the Ill-oxide layer exhibited an epsilon-phase structure. At the flow rate of 30 seem, a phase-pure £-Ga203, with only diffraction peaks at 19.23°, 38.93°, and 59.93°, was obtained. The full-width at half-maximum (FWHM) of the rocking curve of the 006 diffraction peak of b-ΰ82q3 was measured without HCI flow (0.812°). This value was more than two times larger than that of the £-Ga203 (0.420°), implying that the crystal quality of £-Ga203 was improved compared to b-ΰ82q3 under such growth conditions. Consistent with the XRD results, an obvious improvement in the surface morphology was observed. Scanning electron microscope (SEM) images of the phase-pure £-Ga203 formed as disclosed herein showed a flat surface with surface roughness about 2.4 nm measured by AFM in a 1 c 1 pm scan area.

[0031] When the feed rate of the hydrogen chloride was increased to 60 seem, the Ill-oxide layer exhibited a combined epsilon- and alpha- (a-) phase, which is evidenced by a new diffraction peak appearing at 40.15°, which corresponds to the (006) reflection of a-Ga203, coexisting with other three main peaks from e- Ga203 but with much lower peak intensity. Thus, the film formed at 60 seem has a dominant phase of a-Ga203. The FWFIM of the rocking curve of the (006) peak of the a-Ga203 has the lowest value of 0.212°, among all samples, indicating highest crystallinity.

[0032] Thus, as will be appreciated from the graphs in Figures 3 and 4, feeding hydrogen chloride into the reaction chamber while feeding the precursors not only increases the deposition rate but also can be used to control the phase of the layer formed during the growth process.

[0033] Impurity depth profiles were measured by secondary ion mass spectrometry (SIMS) for the samples with 0, 30 and 60 seem flow rate of HCI.

Similar behaviors of each element are observed. The carbon (C) impurity concentrations in the specimen are almost the same. The chlorine concentration in all the Ga2C>3 film shows similar values, as well as its background level in the sapphire substrates. Normally, the chlorine concentration can be detected in the HVPE-grown Ga2C>₃ film likely originating from the precursor, even though the concentration was quite low. However, it was found that the HCI flow rate does not affect the chlorine concentration in the films. Thus, it is believed that the HCI acts as a catalyst for the growth since it does not incorporate in Ga203 but actively evolves during the growth.

[0034] Density function theory (DFT) calculations were performed to verify and explore how HCI flux can tune the phases of Ga203. The growth of the crystal Ga203 is understood as following the Ostwald’s description: the Ga_xO_y initially forms nuclei, which is a rate-limiting step, where the thermodynamic factors control the whole process. The reactants then grow on the nuclei, and the kinetic factors may govern this process. Therefore, the DFT calculations were based on the assumption that the nuclei may determine the morphology of the final product, and the calculations focus on the relative energies corresponding to the thermodynamic controlled process. [0035] First, the relative energy corresponding to the a-, b-, e-phases of Ga2C>3 were calculated and listed in Table 1 :

Table 1

[0036] Consistent with the experiments, the b-phase Ga2C>3 has the lowest energy, indicating without any modification, it can be the dominant product. This result matches the earlier studies in which they have also shown that the formation energy has the tendency of b<e<a during the Ga203 synthesizing process, which indicates that b-phases Ga203 should be the primary product after processing ( see References [2], [3], and [4]). The hydrogen atoms were then gradually doped into the material to achieve the relatively hydrogenated Ga₄06H. With the increment amount of doped hydrogen, the relative energy for epsilon phase decreased, indicating epsilon phase material may be more favorable. This is consistent with experimental results, that the HCI flux may offer the hydrogen resource and produce more epsilon-phase material. This is based on the fact that because hydrogen is quite light and can leave the material by forming H2O under reaction conditions, the hydrogen may work as the catalysis to tune the structure. Further incrementing the hydrogen resource results in an even smaller relative energy between different phases. Considering the precision of the DFT calculations and the model, the results may show the trend that doping hydrogen into the material can shift the relative energy of different phases of Ga2C>3. Thus, as the HCI flow continues to rise, it is expected that the difference in free energy between the b, e, and a becomes smaller due the involvement of HCI in the reaction. Eventually, the e-phase becomes more stable and then the a-phase after flowing 30 seem and 60 seem of the HCI gas, respectively.

[0037] A detailed microstructure analysis was performed to investigate these films, which involved cross-sectional HAADF STEM images showing three distinct nano-structures due to different HCI. Specifically, HAADF STEM imaging of FIB cross-section samples grown under HCI flow rates of 0, 30 and 60 seem was performed. The cross-sectional HAADF STEM images showed three distinct nano-structures due to different HCL flows, i.e., 0, 30, and 60 seem.

[0038] In the film without HCI flow, a columnar shape of b-q32q3 along the growth direction was observed, which indicates a columnar growth mode that might lead to a rough surface. The contact between the b-q32q3 film and sapphire substrate exhibited epitaxial relationships generally, with the b-q32q3 [-201 ] crystal direction parallel to the sapphire [0001] crystal direction. In particular, the b-q32q3 film displayed a nano-domain structure including twining structures as well. Due to the large lattice mismatch between (-201 ) b-q32q3 and c-plane sapphire, inevitably, disoriented domains can form during the growth process. The sample grown with HCI flow rate of 30 seem shows a flat top with pure-phase s-Ga2C>3. However, it was observed that two domains nucleated at the s-Ga203/sapphire interface with a sharp vertical boundary. In contrast to the intermixing texture between different crystal domains in the b-ΰ32q3, the domains in the s-Ga203 are highly aligned and well-maintained along the growth direction, as confirmed by the high-resolution STEM (HR-STEM) imaging performed far away from the interface. An alternated texture of the domains in the pure-phase £-Ga2C>3 was observed.

[0039] The STEM image of the film formed using an HCL flow of 60 seem showed a smooth film although some three-dimensional (3D) grains were observed on the surface. The growth rate of the film was estimated to be approximately 0.4 pm/h. These three-dimensional grains originated at the interface with the substrate and their surface density was independent of the growth time.

[0040] HR-STEM images at the interface with a flat surface were collected. The a-Ga2C>3 exhibits a clear epitaxial growth over the sapphire substrate. The corresponding symmetry relationships were determined to be [10-10] a-Ga203 II [10-10] sapphire, indicating that a-Ga203 has a uniform growth rate thus flat surface.

[0041] The crystal structure of the 3D-grains during the film growth was also investigated. As the 3D-grains grew laterally and vertically faster than the rest of epilayers, the grain-size increased continuously, becoming more protruding with the growth time, as observed in the bottom and the top part at the boundary between the a- and £-Ga203. To avoid the formation of such 3D-grains embedded in the a-Ga203 film matrix, it is necessary to improve the growth conditions at the early growth stage, during the nucleation, in order to suppress the formation of the 3D grains.

[0042] Finally, the transmission of the films with pure b- and £-Ga203 and a-dominant Ga203 samples were collected. The spectra were converted into a Tauc plot ((ahv)² vs. hv) where a denotes the absorption coefficient in order to determine the direct optical bandgap of the Ga203 samples. This was done by extrapolating the linear part of (ahv)² vs. hv to the horizontal axis. The band gap of the b-, e-, and a-dominant Ga2C>3 film was determined to be 4.98, 4.86, and 5.14 eV, respectively, which is consistent with the reported values for the Ga2C>3 with different phases.

[0043] Although the experiments described above involved gallium oxide (Ga203), other Ill-oxides can be used, as well as Ill-oxide alloys, for example, a ternary alloy such as (Al_xGai-_x)203 or a quaternary alloy such as (Al_xGa_ylni-_x-y)203. Further, although the experiments were performed using HCI, the different phased Ill-oxide layers can be grown using molecules comprising either hydrogen ion or chlorine ion.

[0044] The disclosed HCI-enhanced MOCVD process can offer a new approach to achieve low-cost, large-scale hetero-epitaxy Ga203 films for optical and power device applications, as well as other types of Ill-oxide films.

[0045] The disclosed embodiments provide semiconductor devices comprising two Il l-oxide layers having different phases and methods for forming such semiconductor devices. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. Flowever, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0046] Although the features and elements of the present exemplary

embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0047] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

References

[0048] [1 ] R. Schewski, G. Wagner, M. Baldini, D. Gogova, Z. Galazka, T.

Schulz, T. Remmele, T. Markurt, H. von Wenckstern, M. Grundmann, O. Bierwagen, P. Vogt, and M. Albrecht, (2015), Epitaxial stabilization of pseudomorphic a-Ga203 on sapphire (0001 ), Applied Physics Express, Vol. 8, 01 1 101 . 10.7567/APEX.8.01 1 101 .

[0049] [2] H. He, R. Orlando, M. A. Blanco, and R. Pandey, First-principles study of the structural, electronic, and optical properties of Ga203 in its monoclinic and hexagonal phases Phys. Rev. B 74, 195123 (2006).

[0050] [3] S. Yoshioka, H. Hayashi, A. Kuwabara, F. Oba, K. Matsunaga, and I.

Tanaka, Structures and energetics of Ga203 polymorphs, J. Phys.: Condens. Mater 19, 34621 1 (2007).

[0051] [4] F. P. Sabino, L. N. de Oliveira, and J. L. F. Da Silva, Role of atomic radius and d-states hybridization in the stability of the crystal structure of M2O3 (M=AI, Ga, In) oxides, Phys. Re. B 90, 155206 (2014).

Claims

WHAT IS CLAIMED IS:

1. A method for forming a semiconductor device, the method comprising:

forming (1 10) a first Ill-oxide layer (210) having a first phase by feeding a first group III element precursor, a first oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into a growth chamber; and

forming (120) a second Ill-oxide layer (220) on the first Ill-oxide layer (210) by feeding a second group III element precursor, a second oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber, wherein the second Ill-oxide layer (220) has a second phase, which is different from the first phase,

wherein the molecules comprising hydrogen and/or chlorine ions are fed at a first flow rate during the formation of the first Ill-oxide layer (210) and the molecules comprising hydrogen and/or chlorine ions are fed at a second flow rate during the formation of the second Ill-oxide layer (220), wherein the first and second flow rates are different flow rates.

2. The method of claim 1 , wherein

the formation of the first Ill-oxide layer further comprises feeding a third group III element precursor into the growth chamber while feeding the first group III element precursor, the first oxygen precursor, and the molecules comprising hydrogen and/or chlorine ions into the growth chamber; and

the formation of the second Ill-oxide layer further comprises feeding a fourth group III element precursor into the growth chamber while feeding the second group IN element precursor, the second oxygen precursor, and the molecules comprising hydrogen and/or chlorine ions into the growth chamber.

3. The method of claim 1 , further comprising:

forming a third Ill-oxide layer on the second Ill-oxide layer by feeding a third group III element precursor, a third oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber, wherein the third Ill-oxide layer has a third phase, which is the same or different from the first or second phases.

4. The method of claim 1 , wherein one of the first and second phases is a beta phase and the other of the first and second phases is an epsilon phase.

5. The method of claim 1 , wherein one of the first and second phases is a combination of beta and epsilon phases and the other of the first and second phases is an epsilon or beta phase.

6. The method of claim 1 , wherein one of the first and second phases is a combination of alpha and epsilon phases and the other of the first and second phases is an epsilon or beta phase.

7. The method of claim 1 , wherein one of the first and second phases is a combination of beta and epsilon phases and the other of the first and second phases is a combination of alpha and epsilon phases.

8. The method of claim 1 , wherein the first group III element precursor includes indium, boron, aluminum, or gallium, and the second group III element precursor includes a different one of indium, boron, aluminum, and gallium.

9. The method of claim 1 , wherein the first and second group III element precursors include a same one of indium, boron, aluminum, or gallium.

10. The method of claim 1 , wherein the first and second oxide layers are formed by controlling the feed rate of molecules comprising hydrogen and chlorine ions in the form of hydrogen chloride during the formation of the first and second oxide layers.

1 1. A semiconductor device, comprising:

a first Ill-oxide layer having a first phase (210); and

a second Ill-oxide layer (220) arranged on top of the first Ill-oxide layer and having a second phase, which is different from the first phase,

wherein the first phase of the first Ill-oxide layer (210) is a beta phase, an epsilon phase, or a combination of beta and epsilon phases, and wherein the second phase of the second Ill-oxide layer (220) is an alpha phase, a beta phase, an epsilon phase, or a combination of beta and epsilon phases.

12. The semiconductor device of claim 1 1 , wherein the first phase is a beta phase and the second phase is an epsilon phase.

13. The semiconductor device of claim 1 1 , wherein the first phase is a

combination of beta and epsilon phases and the second phase is an epsilon or beta phase.

14. The semiconductor device of claim 1 1 , wherein the first phase is a

combination of alpha and epsilon phases and the second phase is an epsilon or beta phase.

15. The semiconductor device of claim 1 1 , wherein the first phase is a

combination of beta and epsilon phases and the second phase is a combination of alpha and epsilon phases.

16. A method for forming a semiconductor device, the method comprising:

determining (510) a first phase for a first Ill-oxide layer (210) and a second phase for the second Ill-oxide layer (220); forming (510) the second Ill-oxide layer (220) on the first Ill-oxide layer (210) by first feeding a first group III element precursor, a first oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into a growth chamber and subsequently feeding a second group III element precursor, a second oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber,

wherein a feed rate of the molecules comprising hydrogen and/or chlorine ions into the growth chamber to form the first (210) and second (220) Ill-oxide layers is based on the determined phase of the first (210) and second (220) Ill-oxide layers.

17. The method of claim 16, further comprising:

determining third phase for a third Ill-oxide layer; and

forming the third Ill-oxide layer on the second Ill-oxide layer by feeding a third group III element precursor, a third oxygen precursor, and molecules comprising hydrogen and/or chlorine ions into the growth chamber after forming the second Ill-oxide layer,

wherein a feed rate of the molecules comprising hydrogen and/or chlorine ions into the growth chamber to form the third Ill-oxide layer is based on the determined phase of the third Ill-oxide layer.

18. The method of claim 16, wherein the first and second phases are different phases and the feed rates of the molecules comprising hydrogen and/or chlorine ions into the growth chamber to form the first and second Ill-oxide layers are different feed rates.

19. The method of claim 16, wherein

the formation of the second Ill-oxide layer further comprises feeding a fourth group III element precursor into the growth chamber while feeding the second group III element precursor, the second oxygen precursor, and the molecules comprising hydrogen and/or chlorine ions into the growth chamber.

20. The method of claim 16, wherein one of the first and second phases is a beta phase and the other of the first and second phases is an epsilon phase.