growth of thick gallium oxide on the various …

GROWTH OF THICK GALLIUM OXIDE ON THE VARIOUS

SUBSTRATES BY HALIDE VAPOR PHASE EPITAXY A.V. Kremleva1*, Sh.Sh. Sharofidinov1,2, A.M. Smirnov1, E. Podlesnov1, M.V. Dorogov1,

M.A. Odnoblyudov1,3, V.E. Bougrov1, A.E. Romanov1,2 1ITMO University, Kronverksky pr. 49., St. Petersburg, 197101, Russia 2Ioffe Physical-Technical Institute of the Russian Academy of Sciences,

Polytechnicheskaya 26, St. Petersburg, 194021, Russia 3Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya 29, St. Petersburg, 195251, Russia

*e-mail: [email protected] Abstract. In this work, we report on the results of structural characterization of α- and β-Ga2O3 layers grown on various substrates: Al2O3, AlN/Al2O3 and AlN by halide vapor phase epitaxy. Scanning electron microscopy analysis is used to control the thickness of the Ga2O3 layers. The maximum achieved thickness is 46 µm, this is for α-Ga2O3 grown on the bulk AlN substrate. X-ray analysis is used to find the crystallographic orientation of the layers and to characterize the structure quality of the samples. Measured full-width at high-maximum of rocking curves of 15' and 56' confirms crystal quality of the grown α-Ga2O3 and β-Ga2O3 layers on AlN and AlN/Al2O3 substrates, correspondingly. Using AlN as substrate material or buffer AlN layer leads to an increase of Ga2O3 layer thickness and crystal quality in comparison to the layers grown on Al2O3 substrate. Keywords: Wide-bandgap semiconductor, monoclinic gallium oxide, trigonal gallium oxide, halide vapor phase epitaxy, scanning electron microscopy, X-ray analysis 1. Introduction In recent years, wide-bandgap semiconductors became key materials in electrical engineering. The rapid development of electrical networks, industrial control, automotive, and consumer electronics causes a huge demand for power electronics devices, which are based on wide-bandgap semiconductor materials [1,2]. One of the most promising materials of this class is gallium oxide (Ga2O3), which exists in five polymorphic phases. Among them, the most stable are monoclinic β-Ga2O3 phase with a band gap ~ 4.9 eV [3] and trigonal α-Ga2O3 phase with a band gap ~ 5.2 eV [4]. Ga2O3 has several advantages compared to other wide-bandgap semiconductor materials such as silicon carbide (SiC) or gallium nitride (GaN). For example, theoretical calculations and computer simulations show that the Schottky diode Au/β-Ga2O3 has a smaller reverse current and high breakdown voltage in comparison with Schottky diodes Au/GaN and Ni/SiC [5-10], that is also confirmed by experimental data [11]. In general, Ga2O3 is a promising candidate for creating improved semiconductor devices, such as Schottky diodes [12-18] and MOS transistors [19-24].

The processing of high-quality semiconductor materials for power electronics devices is impossible without conducting studies of their electronic, structural, and mechanical properties. The structural properties of Ga2O3 are nontrivial due to the complexity of the

Materials Physics and Mechanics 44 (2020) 164-171 Received: April 18, 2020

http://dx.doi.org/10.18720/MPM.4422020_2 © 2020, Peter the Great St. Petersburg Polytechnic University © 2020, Institute of Problems of Mechanical Engineering RAS

structure of the crystal lattice and may even be somewhat unexpected, especially in the case of nanostructures based on it. In this work, we propose to use an Al2O3, AlN/Al2O3 and AlN substrates for halide vapor phase epitaxy (HVPE) growth of high quality of thick α-Ga2O3 and β-Ga2O3 layers and characterize structure properties of grown layers. 2. Experimental details α-Ga2O3 and β-Ga2O3 layers were grown on Al2O3, AlN/Al2O3 and AlN substrates in a home-made hot wall atmospheric pressure HVPE reactor. The differences between α- and β- phase layer growth was dictated by the process temperature: 550°C – 650°C for α-Ga2O3 and 1050°C for β-Ga2O3.The reactor consisted of a 75 mm quartz tube. During the growth HCl and air were used as precursors, Ar was used as a transport gas. The HCl flow through the source was varied from 200 sccm to 600 sccm. The maximal HCl flow was limited by the range of the mass flow controller used, therefore, a higher flow was not investigated. The air precursor was produced by liquefaction and evaporation of atmospheric air and therefore consisted mainly of oxygen 20%, argon 79%, nitrogen and other noble gases 1%. Impurities such as moisture, carbon dioxide, hydrocarbons, etc. were present only in trace amounts (ppm range). The air flow was kept constant at 2 slm. Ar flow through the reactor was 6 slm. The GaCl vapor (Ga source) was synthesized in-situ upstream in the reactor by the reaction of metallic gallium (99.9999%) and gaseous HCl (99.999 %) at 600 – 850°C. The GaCl yield formation was estimated at more than 80%. After all preparatory processes have been completed, the vapors were transported to the reactor deposition zone where they were mixed with air to produce Ga2O3. The experiment time ranged from 30 to 60 min for different groups of samples. Under these conditions, the deposition rate was in the range from 1 to 70 µm/h. After the growth, the substrate was cooled down to room temperature under Ar flow.

The crystal structure of Ga2O3 layers was analyzed by means X-ray diffraction (XRD). A DRON-8 X-ray diffractometer in a slit configuration with fine focus X-ray tube with a copper anode and a NaI (Tl) scintillation detector and Ni-beta filter was used in these studies. The geometrical features of the samples were investigated by scanning electron microscope (SEM) MIRA3 TESCAN. 3. Results and Discussion We studied the set of the HVPE grown heterostructures and chose four typical samples: α-Ga2O3/Al2O3, β-Ga2O3/Al2O3, β-Ga2O3/AlN/Al2O3, and α-Ga2O3/AlN. At the first stage, we focused on α- and β-Ga2O3 layers grown on the Al2O3. At the next stage, we analyzed β-Ga2O3 layer grown on Al2O3 substrate with AlN buffer layer. Finally, we have studied α-Ga2O3 layer on AlN substrate. The growth parameters of the samples and their thicknesses are given in Table 1 and are discussed below through the text. Table 1. Growth parameters and layer thicknesses Name of samples growth temperature, °C growth time, min thickness, µm α-Ga2O3/Al2O3 550 30 5 β-Ga2O3/Al2O3 1050 30 11 β-Ga2O3/AlN/Al2O3 1050 30 31 α-Ga2O3/AlN 650 60 46

α- and β-phase of Ga2O3 grown on Al2O3. Figure 1(a) shows a SEM image of the cross-section of the α-Ga2O3 layer obtained on c-plane oriented Al2O3 substrate. From Fig. 1(a), it was found that the α-Ga2O3 layer has the thickness of 5 µm. Figure 1(b) shows the

Growth of thick gallium oxide on the various substrates by halide vapor phase epitaxy 165

α-Ga2O3 layer surface. The surface is without any visible roughness. The black spots in the image are the contrast of carbon tape which holds the sample in the microscope.

According to the X-ray diffraction data presented in Fig. 2, the α-Ga2O3 layer is polycrystalline and contains peaks from α-Ga2O3 (110), (210) and (006) reflections. It should be noted, in addition to the peaks of α-phase, the X-ray diffraction pattern also shows peaks from the β-Ga2O3 (201) and (603) reflections, which, according to the literature data [19], accompany α-Ga2O3 growth. However, the most intense peak is (006) reflection of α-Ga2O3. Figure 2(b) shows the rocking curve for α-Ga2O3 (006) reflection, the full-width at high-maximum (FWHM) was approximately 34'.

Fig. 1. SEM images of α-Ga2O3/Al2O3 heterostructure. (a) cross-section, (b) plane-view

Fig. 2. XRD data for α-Ga2O3/Al2O3 heterostructure. (a) XRD pattern, (b) the rocking curve for the (006) reflection of α-Ga2O3

Figure 3(a) shows a SEM image of the cross-section of the β-Ga2O3 layer grown on

c-plane oriented Al2O3 substrate. Figure 3(a) illustrates that the β-Ga2O3 layer has the thickness of 11 µm. Figure 3(b) shows the β-Ga2O3 layer surface. The surface has a lot of

5 µm

α-Ga2O3

Al2O3

5 µm

5 µm

α-Ga2O3

(a) (b)

(b) (a)

166 A.V. Kremleva, Sh.Sh. Sharofidinov, A.M. Smirnov, E. Podlesnov, M.V. Dorogov et al.

particles. The particles are responsible for the surface root mean square (RMS) roughness 400 – 500 nm.

Figure 4 contains the XRD data for the β-Ga2O3/Al2O3 heterostructure which epitaxial layer demonstrates the peaks only for (201) , (402) and (603) reflections. The substrate exhibits peaks only for (006), (0012) reflections of Al2O3. The peak at 2ϴ ≈ 59.09° was the farthest diffraction peak from β-Ga2O3 layer. The FWHM for (603) reflection was 87'. These data indicate that β-Ga2O3 does not have a high crystal quality.

Fig. 3. SEM images of β-Ga2O3/Al2O3 heterostructure. (a) cross-section, (b) plane-view

Fig. 4. XRD data for β-Ga2O3/Al2O3 heterostructure. (a) XRD pattern, (b) the rocking curve for the (603) reflection of β-Ga2O3

β-Ga2O3 grown on AlN/Al2O3. Figure 5(a) shows a SEM image of the cross-section of

the β-Ga2O3 layer grown on Al2O3 substrate with AlN buffer. Figure 5(a) indicates that the thickness of the β-Ga2O3 layer is 30 µm. Figure 5(b) shows the surface of the β-Ga2O3 layer with RMS roughness which approximately about 1 µm. The surface roughness can be connected with the reverse reaction with the formation of GaCl3 and its deposition on the layer surface during the growth process, on the other hand, it can be explained by the

10 µm

11 µm β-Ga2O3

Al2O3

(a)

10 µm

β-Ga2O3

(b)

(b) (a)


formation of β-Ga2O3 crystals on the substrate surface and their coalescence to the uniform layer with increasing temperature.

Figure 6 shows the XRD data for the β-Ga2O3/AlN/Al2O3 heterostructure. All observed layer peaks correspond to the β-phase of Ga2O3 in reflection from (201) plane series [25]. The rocking curve is obtained for (603) reflection of β-Ga2O3, the FWHM was approximately 56', see Fig. 6(b).

Fig. 5. SEM images of β-Ga2O3/AlN/Al2O3 heterostructure (a) cross-section, (b) plane-view

Fig. 6. XRD data for β-Ga2O3/AlN/Al2O3 heterostructure. (a) XRD pattern, (b) the rocking for the (603) reflection of β-Ga2O3

α-Ga2O3 grown on AlN. We have obtained thick α-Ga2O3 layer on the AlN substrate.

AlN substrate was grown on the c-plane Al2O3 substrate and separated from it. Figure 7(a) shows SEM image of the cross-section of the α-Ga2O3 layer, which thickness is 46 µm. Figure 7b shows the surface of the α-Ga2O3 layer. The surface looks as made of accrete columns.

X-ray diffraction data for the α-Ga2O3 layer grown on the AlN substrate are presented in Fig. 8. The layer has peaks only for (110) plane series, it means that obtained α-Ga2O3 layer

20 µm

β-Ga2O3

AlN

31 µm β-Ga2O3

(a) (b)

30 µm

10 µm

Al2O

3

(b) (a)


was monocrystal. Figure 8(b) shows the rocking curve for (220) reflection of α-Ga2O3, the FWHM was approximately 15'.

Fig. 7. SEM images for α-Ga2O3/AlN heterostructure. (a) cross-section, (b) plane-view

Fig. 8. XRD data for α-Ga2O3/AlN heterostructure. (a) XRD pattern, (b) the rocking curve for the (220) reflection of α-Ga2O3

4. Conclusions We have compared the layers of α- and β-phases of Ga2O3 grown on Al2O3, AlN/Al2O3, and AlN substrates. We have shown that the use of AlN buffer layer or bore AlN substrate leads to increases of Ga2O3 layer thickness compared with the thickness of layers grown on the Al2O3 substrate without the buffer. The maximal achieved Ga2O3 layer thickness was 46 µm for heterostructure with bulk AlN substrate. Structural analysis has shown that Ga2O3 layer quality is better for heterostructures with AlN buffer layer or AlN substrate. Measured FWHM of rocking curves of 15' and 56' confirms crystal quality of the grown α-Ga2O3 and β-Ga2O3 layers, correspondingly. Acknowledgments. A.M. Smirnov and A.V. Kremleva are thankful to the Russian Science Foundation (project No. 19-79-00349) for support of this work.

20 µm

α-Ga2O3

AlN

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10 µm

α-Ga2O3 (a) (b)

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