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Nanomedicine & Nanotechnology Open Access Research Article 11 min read

Effect of Substrate Temperature on Structure, Morphology and Optical Properties of β-Ga2O3 Thin Film Grown on GaN by MBE

Boyuan Feng, Gaohang He*, Zhengcheng Li, Ying Wu, Zhongming Zeng and Sunan Ding
* Corresponding author
ISSN: 2574-187X  10.23880/nnoa-16000208  Received: January 30, 2021  Published: February 25, 2021
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Keywords
β-Ga2O3 MBE GaN Template Substrate Temperature Influence
Abstract

The structure, morphology and optical properties of β-Ga2O3 thin films grown on GaN at various substrate temperature by ozone molecular beam epitaxy (MBE) are investigated in this work. (-201)-oriented β-Ga2O3 thin films are formed on c-plane GaN template substrate. When the substrate temperature increases, the crystal quality of β-Ga2O3 thin film improves, and a high-crystalline-quality β-Ga2O3 thin film is obtained at the substrate temperature of 700 °C. The Φ scans of X-ray diffraction is utilized to characterize the β-Ga2O3 thin film, from the result we find that the β-Ga2O3 thin film has a six-fold domain structure, attributed to the epitaxial relationships (β-Ga2O3 [010] // GaN [11-20] and β-Ga2O3 [102] // GaN [1-100]). Base on the morphology, it can be seen that the β-Ga2O3 thin film follows the island-growth model, and the size of the island increases as the substrate temperature increases. Furthermore, it is found that the defect related luminescence decreases with the increase of substrate temperature by analyzing the CL spectra, implying the improvement of crystal quality. The presented optimized β-Ga2O3 thin film grown on GaN template substrate should effectively promote the development of high reliable performance self-powered ultraviolet (UV) photodetector based on the Ga2O3/GaN heterojunction.

Boyuan Feng1,3, Gaohang He1*, Zhengcheng Li1,3, Ying Wu1, Zhongming Zeng2 and Sunan Ding1,3

RMS: Root-Mean-Square; SEM: Scanning Electron Microscope; GL: Green Luminescence; BL: Blue Luminescence; DAP: Donor-Acceptor Pair.

Introduction

Driven by its potential application on high power electronics and solar-blind ultraviolet (UV) photodetectors (PDs), gallium oxide in its most thermodynamically stable monoclinic structure, β-Ga2O3, gradually enters people’s field of vision. Compared to GaN and SiC, β-Ga2O3 has a wide band gap (4.3-4.9 eV), high breakdown electric field (8 MV/ cm) and excellent Baliga’s figure (3214) [1, 2, 3, 4, 5]. Therefore, β-Ga2O3 is attracting interest for solar-blind self-powered UV PDs [6, 7, 8, 9, 10, 11], Schottky barrier diodes (SBDs) [12], metal- semiconductor field-effect-transistors (MESFETs) [2] and metal-oxide-semiconductor field-effect-transistors (MOSFETs) [13, 14, 15].

The possibility of depositing β-Ga2O3 films on native substrates grown from the melt can allow synthesis of high-quality films and large-scale production, nonetheless, the high price hinders its possible application. Hence, heteroepitaxial β-Ga2O3 films have been obtained on several substrates, such as α-Al2O3 (0001) [16, 17], MgO (100) [18], GaN (0001) [19, 20], STO (100) [21] and KTaO3 (100) [22]. At the same time, the corresponding photodetectors have been prepared and achieved excellent performance [23]. Among them, Guo, et al. [9, 10] prepared a heterojunction by depositing n-type β-Ga2O3 thin film on p-type GaN by pulse laser deposition (PLD) to realize a super-high-performance self-powered UV photodetector, which helped solve the energy issues. However, the β-Ga2O3 thin film grown on GaN by molecular beam epitaxy (MBE) have been investigated to date seldom.

Recently, Nakagomi, et al. [20] reported that the orientation of β-Ga2O3 thin film formed on GaN template substrate was found to be (-201) β-Ga2O3 || (0001) GaN || (0001) sapphire and (010) β-Ga2O3 || {11-20} GaN, resulting in six-fold domain structure of the β-Ga2O3 layer. The β-Ga2O3 film grown on GaN substrate by metal organic chemical vapor deposition (MOCVD) is amorphous and transformed into (100) crystalline phase by annealing in oxygen atmosphere [24]. Li, et al. [19] reported the growth of vertical β-Ga2O3 nanowire arrays on GaN layers by MOCVD. The effect of the growth temperatures on the β-Ga2O3 thin film formed on GaN substrate remained unexplored, which is one of the crucial parameters for achieving high crystalline quality. In this work, the influence of substrate temperatures on the film quality is studied by using the ozone MBE. In addition, the epitaxial relationships, morphology and the optical properties are analyzed.

Experiments

The β-Ga2O3 thin films on (0001) Ga-plane GaN template (4 μm on (0001) sapphire substrate) were deposited by a commercial MBE (Octoplus-O 400, Komponenten) with liquid Ga (99.9999%) and ozone as the Ga source and O source. The GaN template substrate was cleaned by ultrasonic agitation in acetone, isopropyl alcohol and deionized water for 15min each, followed by drying with nitrogen gas blowing. During the growth, the ozone pressure of growth chamber was maintained at 5×10-6mbar, and the Ga flux was 0.14Å/s which was detected by a quartz monitor crystal. The substrates were heated to growth temperature (500℃ ~ 700℃), and then growth for 3h.

X-ray diffraction (XRD; D8 Advanced, Bruker) was carried out to check out the crystalline orientations and crystal quality. The film thickness was determined by cross- section observation with a focused ion beam (FIB; Scios, FEI), meanwhile, the surface morphology of the film was obtained from the plane view. The surface roughness of the film was analyzed by atomic force microscopy (AFM; Asylum Research MFP-3D, Oxford). The optical properties of β-Ga2O3 films was characterized by scanning cathodoluminescence (CL; Delmic Sparc, FEI).

Results and Discussion

The XRD θ-2θ scans of β-Ga2O3 thin films prepared with different substrate temperatures are shown in Figure 1(a). After the deposition of β-Ga2O3 thin films with the temperature changed from 500 to 700 °C, three diffraction peaks appear and locate at 18.96°, 38.36° and 58.09°, respectively, which correspond to the (-201), (-402) and (-603) lattice planes of monoclinic β-Ga2O3 (PDF# 43-1012). This indicates that the (-201) planes in the β-Ga2O3 thin film are parallel to the (0001) GaN surface. In addition, when the substrate temperature is 500 °C, the unwanted plane of (-801) is shown in the XRD θ-2θ scan, demonstrating the polycrystalline nature of this film. As the increase of substrate temperature, the (-801) plane disappears, indicating that when the substrate temperature above 550 °C, the β-Ga2O3 thin film is grown with the single orientation along (-201) lattice plane on the GaN template substrate. Figure 1(b) shows the XRD rocking curves of the (-201) plane of the β-Ga2O3 thin films prepared at different growth temperatures. And the full width at half-maximum (FWHM) values of rocking curves are plotted as a function of the substrate temperature in Figure 1(c). Apparently, the FWHM value monotonically decreases with increasing growth temperature and down to 1.67° at 700 °C, implying excellent crystal quality and unique (-201) out-plane orientation of the thin film.

Figure 1: (a) θ-2θ XRD patterns of β-Ga2O3 thin films deposited on GaN template at various substrate temperatures. (b) XRD rocking curves of the (-201) plane of the β-Ga2O3 films and (c) the dependence of rocking curves FWHM values on the substrate temperature.
Click to enlarge
Figure 1: (a) θ-2θ XRD patterns of β-Ga2O3 thin films deposited on GaN template at various substrate temperatures. (b) XRD rocking curves of the (-201) plane of the β-Ga2O3 films and (c) the dependence of rocking curves FWHM values on the substrate temperature.

Figure 2(a) shows the XRD Φ-scan results for the β-Ga2O3 {-401} diffraction of the film grown at 700 ℃. It can be seen that there are 6 diffraction peaks, derived from the β-Ga2O3 {-401} diffraction with the rotation angle offset 30°. Furthermore the 6 peaks appear every 60° are observed from the six peaks which associated with the {11-22} diffractions of GaN. This result indicates that the β-Ga2O3 thin film has a six-fold domain structure and an epitaxial relationship with the GaN. The schematic diagram of epitaxial relationship is presented in Figure 2(b). Base on it one can get that epitaxial relationships between (-201) plane of β-Ga2O3 (orange) and (0001) plane of GaN (blue) are β-Ga2O3 [10] // GaN [11, 12, 13, 14, 15, 16, 17, 18, 19, 20] and β-Ga2O3 [102] // GaN [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]. The appearance of six-fold domain structure is due to the three-fold rotation symmetry corresponds to the epitaxial relationship and the originally two-fold β-Ga2O3 epitaxial growth in the three different directions at same rates [20].

Figure 2: (a) XRD Φ-scan patterns for (top) {11-26} planes of Al2O3 substrate, (middle) {11-22} planes of GaN template and (bottom) {-401} planes of β-Ga2O3 film grown at 700℃. (b) Schematic diagram of epitaxial relationship between β-Ga2O3 (-201) plane (orange) and GaN (0001) plane (blue).
Click to enlarge
Figure 2: (a) XRD Φ-scan patterns for (top) {11-26} planes of Al2O3 substrate, (middle) {11-22} planes of GaN template and (bottom) {-401} planes of β-Ga2O3 film grown at 700℃. (b) Schematic diagram of epitaxial relationship between β-Ga2O3 (-201) plane (orange) and GaN (0001) plane (blue).

Figure 3 shows the cross-sectional scanning electron microscope (SEM) images of the β-Ga2O3 films prepared with different substrate temperatures and fabricated by FIB. The carbon layer and platinum layer are covered on the surface to are used to protect the surface before FIB. It can be seen that the thickness of film is stable around 144 nm. However, the film thickness decreases to 112 nm with the substrate temperature set at 700 ℃. This may be due to the enhanced of the desorption or evaporation rate of volatile Ga2O suboxide from the substrate when the temperature exceeds 700 ℃ [16, 25].

Figure 3: The cross-sectional SEM images of the β-Ga2O3 films deposited at different substrate temperatures. And the thickness of β-Ga2O3 films as a function of the substrate temperature.
Click to enlarge
Figure 3: The cross-sectional SEM images of the β-Ga2O3 films deposited at different substrate temperatures. And the thickness of β-Ga2O3 films as a function of the substrate temperature.

The SEM plane-view images of β-Ga2O3 thin films grown at various temperatures are shown in Figure 4. It can be clearly observed that the β-Ga2O3 thin film grown on GaN template substrate follows the island-growth model, and the island size of β-Ga2O3 thin film increases as the substrate temperature increases. We believe that the island size difference is ascribed to the growth mechanism with different substrate temperature. When the substrate temperature is fixed at 500 ℃, the adatoms do not have enough mobility, which limits the atomic migration distance and determine the size of the island. Thus, the polycrystalline thin film forms, which is consisted with the XRD results as shown in Figure 1(a). It is notable that the surface prefers a low free energy, the appeared morphology should be spherical shaped island, if the substrate structure is not similar to that of the epitaxial film. Hence, considering the influence of the substrate lattice structure, island structure appears as has been observed in the SEM images. Upon increasing the substrate temperature, the increased atomic mobility causes the radius of island to become larger, and nearby island begins to join together as shown in the SEM images.

Figure 5 displays the AFM morphologies and roughness of β-Ga2O3 thin films. The change of morphology is consistent with the SEM images with the substrate temperature increase. Corresponding to the changes of the surface morphology, the root-mean-square (RMS) roughness of the films increases when the substrate temperature does not exceed 650 ℃. The maximum roughness of the films is 6.15 nm, indicating smooth surface.

Figure 4: It can be clearly observed that the β-Ga2O3 thin film grown on GaN template substrate follows the island-growth model, and the island size of β-Ga2O3 thin film increases as the substrate temperature increases. We believe that the island size difference is ascribed to the growth mechanism with different substrate temperature. When the substrate temperature is fixed at 500 ℃, the adatoms do not have enough mobility, which limits the atomic migration distance and determine the size of the island. Thus, the polycrystalline thin film forms, which is consisted with the XRD results as shown in Figure 1(a). It is notable that the surface prefers a low free energy, the appeared morphology should be spherical shaped island, if the substrate structure is not similar to that of the epitaxial film. Hence, considering the influence of the substrate lattice structure, island structure appears as has been observed in the SEM images. Upon increasing the substrate temperature, the increased atomic mobility causes the radius of island to become larger, and nearby island begins to join together as shown in the SEM images.
Click to enlarge
Figure 4: It can be clearly observed that the β-Ga2O3 thin film grown on GaN template substrate follows the island-growth model, and the island size of β-Ga2O3 thin film increases as the substrate temperature increases. We believe that the island size difference is ascribed to the growth mechanism with different substrate temperature. When the substrate temperature is fixed at 500 ℃, the adatoms do not have enough mobility, which limits the atomic migration distance and determine the size of the island. Thus, the polycrystalline thin film forms, which is consisted with the XRD results as shown in Figure 1(a). It is notable that the surface prefers a low free energy, the appeared morphology should be spherical shaped island, if the substrate structure is not similar to that of the epitaxial film. Hence, considering the influence of the substrate lattice structure, island structure appears as has been observed in the SEM images. Upon increasing the substrate temperature, the increased atomic mobility causes the radius of island to become larger, and nearby island begins to join together as shown in the SEM images.
Figure 5: AFM images of the surface morphology for β-Ga2O3 films deposited at 500, 550, 600, 650 and 700 ℃. And the RMS values of surface roughness of β-Ga2O3 films as a function of substrate temperature. The room-temperature CL results from the β-Ga2O3 thin films are found to be strongly dependent on the substrate temperatures as shown in Figure 6. Three major light emission features are obvious from each spectrum and the energy correspond to 2.4 eV, 2.8-3.0 eV and 3.2- 3.6 eV, named as green luminescence (GL) band, blue luminescence (BL) band and UV band, respectively. The UV band is generally assigned to recombination of free electrons and self-trapped holes [26-28]. And the BL band may result from defect related luminescence, attributed to donor-acceptor pair (DAP) recombination. Possible donors are intrinsic point defects such as oxygen vacancies (Vo) and interstitial Ga (Gai), and possible acceptors are Ga vacancies (VGa), Vo-VGa complexes [26-28]. Thus, the change of CL spectra is considered to be caused by the variation of crystal quality of the β-Ga2O3 thin films. As the substrate temperature increases, the luminescence of BL band and GL band gradually decreases, while the UV band luminescence increases, illustrating the improvement of crystal quality. In order to further quantitatively compare the luminescence ratio, each spectrum is fitted by four peaks, as shown in Figure 7(a)-(e). Note that the UV is fitted by the two peaks because the photogenerated holes can self-trap onto two different O sites [29,30]. Figure 7f shows the summation area of BL an GL peaks with respect to the area of the UV peak as a function of the substrate temperature. The relative intensity ratio of BL and GL to UV decreases with the increasing of substrate temperature, and the ratio reaches the lowest value when the substrate temperature is 700℃. This result implies that among these substrate temperatures, the β-Ga2O3 thin film has the best crystal quality at the substrate temperature of 700℃, which is consistent with the XRD rocking curves as shown in Figure 1b.
Click to enlarge
Figure 5: AFM images of the surface morphology for β-Ga2O3 films deposited at 500, 550, 600, 650 and 700 ℃. And the RMS values of surface roughness of β-Ga2O3 films as a function of substrate temperature. The room-temperature CL results from the β-Ga2O3 thin films are found to be strongly dependent on the substrate temperatures as shown in Figure 6. Three major light emission features are obvious from each spectrum and the energy correspond to 2.4 eV, 2.8-3.0 eV and 3.2- 3.6 eV, named as green luminescence (GL) band, blue luminescence (BL) band and UV band, respectively. The UV band is generally assigned to recombination of free electrons and self-trapped holes [26-28]. And the BL band may result from defect related luminescence, attributed to donor-acceptor pair (DAP) recombination. Possible donors are intrinsic point defects such as oxygen vacancies (Vo) and interstitial Ga (Gai), and possible acceptors are Ga vacancies (VGa), Vo-VGa complexes [26-28]. Thus, the change of CL spectra is considered to be caused by the variation of crystal quality of the β-Ga2O3 thin films. As the substrate temperature increases, the luminescence of BL band and GL band gradually decreases, while the UV band luminescence increases, illustrating the improvement of crystal quality. In order to further quantitatively compare the luminescence ratio, each spectrum is fitted by four peaks, as shown in Figure 7(a)-(e). Note that the UV is fitted by the two peaks because the photogenerated holes can self-trap onto two different O sites [29,30]. Figure 7f shows the summation area of BL an GL peaks with respect to the area of the UV peak as a function of the substrate temperature. The relative intensity ratio of BL and GL to UV decreases with the increasing of substrate temperature, and the ratio reaches the lowest value when the substrate temperature is 700℃. This result implies that among these substrate temperatures, the β-Ga2O3 thin film has the best crystal quality at the substrate temperature of 700℃, which is consistent with the XRD rocking curves as shown in Figure 1b.

Figure 5: AFM images of the surface morphology for β-Ga2O3 films deposited at 500, 550, 600, 650 and 700 ℃. And the RMS values of surface roughness of β-Ga2O3 films as a function of substrate temperature. The room-temperature CL results from the β-Ga2O3 thin films are found to be strongly dependent on the substrate temperatures as shown in Figure 6. Three major light emission features are obvious from each spectrum and the energy correspond to 2.4 eV, 2.8-3.0 eV and 3.2- 3.6 eV, named as green luminescence (GL) band, blue luminescence (BL) band and UV band, respectively. The UV band is generally assigned to recombination of free electrons and self-trapped holes [26, 27, 28]. And the BL band may result from defect related luminescence, attributed to donor-acceptor pair (DAP) recombination. Possible donors are intrinsic point defects such as oxygen vacancies (Vo) and interstitial Ga (Gai), and possible acceptors are Ga vacancies (VGa), Vo-VGa complexes [26, 27, 28]. Thus, the change of CL spectra is considered to be caused by the variation of crystal quality of the β-Ga2O3 thin films. As the substrate temperature increases, the luminescence of BL band and GL band gradually decreases, while the UV band luminescence increases, illustrating the improvement of crystal quality. In order to further quantitatively compare the luminescence ratio, each spectrum is fitted by four peaks, as shown in Figure 7(a)-(e). Note that the UV is fitted by the two peaks because the photogenerated holes can self-trap onto two different O sites [29, 30]. Figure 7f shows the summation area of BL an GL peaks with respect to the area of the UV peak as a function of the substrate temperature. The relative intensity ratio of BL and GL to UV decreases with the increasing of substrate temperature, and the ratio reaches the lowest value when the substrate temperature is 700℃. This result implies that among these substrate temperatures, the β-Ga2O3 thin film has the best crystal quality at the substrate temperature of 700℃, which is consistent with the XRD rocking curves as shown in Figure 1b.

Figure 6: CL spectra of β-Ga2O3 films grown at different substrate temperatures.
Click to enlarge
Figure 6: CL spectra of β-Ga2O3 films grown at different substrate temperatures.
Figure 7: (a)-(e) CL spectra with curve-fitting results of β-Ga2O3 thin films grown at different substrate temperatures. (f) The dependence of the ratio (the summation area of BL and GL peaks to the area of UV peaks) on the substrate temperature.
Click to enlarge
Figure 7: (a)-(e) CL spectra with curve-fitting results of β-Ga2O3 thin films grown at different substrate temperatures. (f) The dependence of the ratio (the summation area of BL and GL peaks to the area of UV peaks) on the substrate temperature.

Conclusion

In conclusion, β-Ga2O3 thin films are grown on c-plane GaN template substrate by ozone MBE. Phase-pure (-201)-oriented β-Ga2O3 thin film can be formed with the substrate temperature exceeded 550 ℃, and the crystal quality improves with increasing substrate temperature. The epitaxial relationships are confirmed as β-Ga2O3 [10] // GaN [11, 12, 13, 14, 15, 16, 17, 18, 19, 20] and β-Ga2O3 [102] // GaN [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100], and result six-fold domain structure in β-Ga2O3 thin film. Because the difference in crystal structure between the film and the substrate, the β-Ga2O3 thin film follows the island-growth model, and the size of the island increases as the substrate temperature increases. In addition, CL spectra variations are observed that the defect related luminescence decreases with the increase of substrate temperature, claiming the improvement of crystal quality which are consistent with the results of XRD rocking curves. The presented optimized β-Ga2O3 thin film grown on GaN template substrate should effectively promote development of reliable high performance self-powered UV photodetector based on the Ga2O3/GaN heterojunction.

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@article{boyuan2021,
  title   = {Effect of Substrate Temperature on Structure, Morphology and
Optical Properties of β-Ga2O3 Thin Film Grown on GaN by MBE},
  author  = {Boyuan Feng, Gaohang He, Zhengcheng Li, Ying Wu, Zhongming
Zeng and Sunan Ding},
  journal = {Nanomedicine & Nanotechnology Open Access},
  year    = {2021},
  volume  = {6},
  number  = {1},
  doi     = {10.23880/nnoa-16000208}
}
Boyuan Feng, Gaohang He, Zhengcheng Li, Ying Wu, Zhongming
Zeng and Sunan Ding (2021). Effect of Substrate Temperature on Structure, Morphology and
Optical Properties of β-Ga2O3 Thin Film Grown on GaN by MBE. Nanomedicine & Nanotechnology Open Access, 6(1). https://doi.org/10.23880/nnoa-16000208
TY  - JOUR
TI  - Effect of Substrate Temperature on Structure, Morphology and
Optical Properties of β-Ga2O3 Thin Film Grown on GaN by MBE
AU  - Boyuan Feng, Gaohang He, Zhengcheng Li, Ying Wu, Zhongming
Zeng and Sunan Ding
JO  - Nanomedicine & Nanotechnology Open Access
PY  - 2021
VL  - 6
IS  - 1
DO  - 10.23880/nnoa-16000208
ER  -