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The further investigation of N-doped β-Ga2O3 thin films with native defects for Schottky-barrier diode
Journal of Alloys and Compounds 812 (2020) 152026
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The further investigation of N-doped b-Ga2O3 thin films with native defects for Schottky-barrier diode Suzhen Luan a, Linpeng Dong b, Xiaofan Ma b, Renxu Jia b, * a b
Xi'an University of Science and Technology, Xi'an, 710054, China Xidian University, Xi'an, 710071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2019 Received in revised form 24 August 2019 Accepted 26 August 2019 Available online 26 August 2019
The structural, electronic and optical properties of b-Ga2O3 thin films with different N ion concentrations are investigated. With N concentrations increasing, the crystallization degrades and even polycrystalline appears. After Ar annealing treatment, the crystallization of N-doped b-Ga2O3 thin films improved owing to recovery of the implantation-damaged crystals. The N-doped Ga2O3 thin films were used to construct vertical Schottky diodes comprising Ga2O3:N/n-Ga2O3. The net carrier concentrations of these devices degrade one-two orders due to the compensation effect of more N implantation, which suggests that hole could be mobile although N is a deep acceptor dopant. The phenomenon could be explained by the decrease of the ionization energy of N dopant due to the complex defects NGa2O3VO (N-doped b-Ga2O3 with O vacancy) and NGa2O3VGa (N-doped b-Ga2O3 with Ga vacancy). Thus, the development of N ion implantation for b-Ga2O3 has its potential opportunity for b-Ga2O3 power devices. © 2019 Elsevier B.V. All rights reserved.
Keywords: Gallium oxide N-doped Native defects Electronic properties
1. Introduction There have been strong interests in developing high power device based on wide-bandgap compound semiconductors since Si technology is approaching fundamental performance limitations. Recently, b-Ga2O3 has shown its great promise for next-generation power device applications due to its ultra-wide bandgap (4.9eV), higher breakdown field (8 MV/cm), higher Baliga's figures of merit (over 3000) and lower cost for mass production [1e7]. Metal-oxide-semiconductor field effect transistor (MOSFET) is one of the most widely used power devices because of its low driving power, switching loss and on-resistance. The research of Ga2O3-based MOSFET devices is mainly focused on the depletion mode (D-mode) device and the enhancement mode (E-mode) devices are rarely reported [8e19]. However, for high-voltage and low-dissipation power devices, the E-mode MOSFET is the optical choice. Due to the shallow donor level (Ec-0.04eV) formed by oxygen vacancies, the unintentionally doped b-Ga2O3 exhibits n-type conductivity [20]. At the same time, the achievement of p-type bGa2O3 thin films remains challenge largely due to the compensation with oxygen vacancies, the larger effective mass and self-
* Corresponding author. E-mail address: [email protected] (R. Jia). https://doi.org/10.1016/j.jallcom.2019.152026 0925-8388/© 2019 Elsevier B.V. All rights reserved.
trapped effect of holes which hampers E-mode MOSFET power devices application. Up to now, theoretical and experimental studies were carried out [21e25], but failed to obtain high quality p-type Ga2O3 films. Nitrogen (N) element is considered to be an effective p-type dopant [26e28]. Besides, the ionic radius and electronic structure of N3 are close to O2, which make it a promising p-type dopant in bGa2O3. However, only N-doped b-Ga2O3 nanowires with a p-type electric conductivity have been reported [29]. We had reported the structural, electronic and optical properties of N-doped b-Ga2O3 with first-principles calculations based on density functional theory (DFT) [30]. It was found that the compensation mechanism and interaction of N dopants with native defects in N-doped b-Ga2O3. The introduction of N atoms formed a variety of complexes with native defects, in particular oxygen vacancies and Ga interstitials. Thus, the band structure calculations found that N dopants act as deep acceptors (Evþ1.33eV) but cannot be effective p-type dopants. Therefore, it is worth further exploring experimentally whether Ndoped Ga2O3 films can be obtained with p-type Ga2O3 films, in order to be better applied in the field of MOSFET power devices. In this paper, we make experimental studies of b-Ga2O3 thin films with different N-implanted concentrations. The structure, surface morphology, electrical and optical properties of these films were characterized by Atom force microscope (AFM), Raman, X-ray diffraction (XRD) and photo-luminescence spectroscopy (PL).
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Finally, current-voltage (IV), capacitance-voltage (CV) and X-ray photo-electron spectroscopy (XPS) characteristics of vertical SBD devices comprising Ga2O3: N/n-Ga2O3 films were investigated. 2. Experiments The n-type b-Ga2O3 substrates with crystal orientation (201), thickness of 680 mm and Si-doping concentration of 3.9 1017cm3 are plotted into eight pieces of 1 cm 1 cm. Then Nitrogen ions were implanted into six n-type b-Ga2O3 substrates of these. In order to get uniform distribution, each target concentration of N ion is adopted three energies and doses. The energies of N ion implantation are 30/150/400 keV and the implantation doses are 3.1 1013cm2, 3.1 1014cm2, 3.1 1015 cm2, resulting in the N concentrations of 1 1018 cm3, 1 1019 cm3 and 1 1020 cm3, respectively. These three samples are denoted as N1Ga2O3, N2Ga2O3 and N3Ga2O3, respectively. Then these samples were annealed at 900 C for 50 min in Ar atmosphere to recover the damaged crystal structure and activate the implanted dopants. The annealed samples were denoted as N1Ga2O3eAr, N2Ga2O3eAr and N3Ga2O3eAr. As a comparison, the undoped b-Ga2O3 with and without annealing are denoted as UndopedGa2O3 and UndopedGa2O3eAr, respectively. The structure, surface morphology and optical properties of these films were characterized by AFM, Raman, XRD and PL spectra. Then, SBDs comprising Ga2O3: N/n-Ga2O3 films are fabricated that Ti/Au electrodes are patterned-top and blanked-bottom to form Schottky and ohmic contacts, respectively. Finally, currentvoltage (IV), capacitance-voltage (CV) and XPS characteristics of vertical SBD devices comprising Ga2O3: N/n-Ga2O3 films were investigated. 3. Results and discussion 3.1. Electrical structures Cross-session AFM analysis of the as-implanted with Si and N3Ga2O3 b-Ga2O3 samples were performed to exam the microstructures of the N-doped b-Ga2O3. The image of N-undoped Ga2O3 before annealing are shown in Fig. 1(a), from which no clear grainlike structures are found expect for the defect dots due to Si implantation. With N ions implanting, the bright defect dots are heavier in Fig. 1(c). After annealing at 900 C, the crystallization of these samples are improved well, the details in that the RMS decreases and the grain size increase. In comparison, the significantly higher crystal damage with N-rich cluster in the Nþþ-doped films
was likely due to the higher implantation energy and dose used for N. Raman measurements of N-doped b-Ga2O3 samples before and after annealing are shown in Fig. 2. Raman activation modes of bGa2O3 are as follows: stretching and bending modes of GaO4 tetrahedron at high frequency (770-500 cm1); deformation modes of GaO6 octahedron at medium frequency (480-310 cm1); vibration and translation modes of tetrahedron and octahedron chains at low frequency (200 cm1) [31]. Fig. 2 (a) shows that the Raman peaks change mainly in the high frequency region before and after ion implantation and annealing. Therefore, N ion implantation mainly affects GaO4 tetrahedron. Fig. 2 (b) shows that after N ion implantation, the Raman peaks near 631.9 cm1 in UndopedGa2O3 film disappear, while the intensity of the Raman peaks at 658.9 cm1 increases and blue shifts to the high frequency region. It is explained that the Raman bands of 653 cm1 and 767 cm1 are attributed to the symmetrical stretching bands of GaO4 tetrahedron, while the Raman bands near 631.9 cm1 are determined to be O-Ga-O bending dies [32]. The peak at 631.9 cm1 decrease due to more simple defects and less complex defects on Ga2O3 surface with N ions concentration increasing. In addition, the enhancement of Raman band near 658.9 cm1 and the appearance of new Raman band near 767 cm1 may be due to the disorderly activation of Raman scattering. After annealing, the crystallization quality of Ndoped Ga2O3 thin films improves. Fig. 3 shows the XRD results of b-Ga2O3 with different N concentration before and after annealing. The spectrum of the asimplanted sample after annealing manifested several peaks, indicating the presence of tensile strain relative to the normal substrate due to Si redistribution while the sample growing preferentially with monocrystal structure along the (201) plane [33]. With the increase of N implantation dose, it can be found that when the implantation dose is maximum, new diffraction peaks of 2q appear at 35.2 and 45.86 , corresponding to GaN (101) plane and Ga2O3 (600) plane [34] respectively. After Ar annealing, it can be found that the crystallization quality of all samples improve due to atoms migrating to normal lattice positions and strain relief. The XPS spectrum of N-doped b-Ga2O3 samples in the bonding energy range of 0e1200eV is shown in Fig. 4 (a). The large image in Fig. 4(b) shows that the O1s signal peaks of UndopedGa2O3 sample do not meet the Gauss distribution. we decomposed them into two fitting peaks for 531.3eV and 532.45eV, that the signal peak at 531.3eV mainly from oxygen in GaeO bond and the signal peak at 532.45eV are from adsorbed oxygen on the surface of b-Ga2O3 [35e37], which disappears under annealing atmosphere. The
Fig. 1. Cross-session AFM images of b-Ga2O3 (a) UndopedGa2O3, (b) UndopedGa2O3eAr, (c) N3Ga2O3, (d) N3
Ga2O3-
Ar and (e) the RMS of all samples before and after annealing.
S. Luan et al. / Journal of Alloys and Compounds 812 (2020) 152026
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Fig. 2. Raman spectra of N-doped Ga2O3 samples w/wo annealing (a) all region of Raman shift, (b)high frequency region of Raman shift.
recombination of electrons by donor (Si) level and holes by acceptor (N) level. It was conceivable that the donor (Si) and the acceptor (N) paired up by Coulombic interactions to form immobile ionic complexes such as Siþ-N-, which deduced the role of the N ion as compensating active charge-carrying acceptor impurities in the Ga2O3 film. Extra emission peaks arise at 476 nm and 524 nm corresponds to blue-green light maybe due to NGa2O3Gai and NGa2O3VO related with N-2p states and the defective states caused by intrinsic defects [30,38,39]. After annealing, the PL peak of N-doped Ga2O3 films tends to be the same as that of UndopedGa2O3 samples, which indicates that defects caused by ion implantation decrease and crystallization quality improves. 3.3. Electric properties
Fig. 3. XRD patterns of N-doped b-Ga2O3.
energy peak for Ga3d of UndopedGa2O3 is located at 20.75eV [in Fig. 4(c)]. The energy peak for Ga2p is located at 1119eV and 1146eV, with a difference of 27eV [in Fig. 4 (d)], in accordance with the literature reported. With the increase of N concentrations, the peak for O1s at 531.3eV shifts to the high binding energy side while the peaks for Ga3d and Ga2p shift right and become weaker, which may be due to the formation of GaN.
3.2. Optical properties The optical properties can be described by the PL spectra which can be explained by band-to-band transition. The roomtemperature photoluminescence (PL) spectra of as-implanted and N-doped b-Ga2O3 samples with different N ions concentrations excited with 260 nm laser are shown in Fig. 5 (a).It is noted that a luminescence peak of the UndopedGa2O3 sample exhibits at 369 nm, which is mainly caused by the electron transition from the donor level formed by the Si ion, intrinsic defect oxygen vacancy and the Ga interstitial atom to the acceptor level formed by the Ga vacancy [29,30]. After N ion implantation, four distinct photo-luminescence peaks appear in the visible range and slight red shifts manifest.The broad peaks can be decomposed into four single peaks with Gauss distribution [in Fig. 5(b)]. The peak of 386 nm corresponds to ultraviolet light and that of 430 nm corresponds to purple light [38,39], which are in main due to photons emitted by the
Electrical properties of the vertical Schottky diodes comprising Ga2O3:N/n-Ga2O3 were characterized by current-voltage (IeV) and capacitance-voltage (CeV) analysis. Firstly, IeV curves of all samples are confirmed to satisfy the characteristics of Schottky diodes as follows:
qU 1 1 I ¼ IS exp nkT 2
(1)
where Is is the reverse saturated current, q is the electron charge, U is the applied voltage, n is the correction factor, k is the constant parameter of motion and T is the temperature. Then, the CeV curves of each sample were tested and converted to 1/C2eV curves. According to the formula:
2 1 2ðVbi þ VR Þ ¼ C eεs Nd
(2)
where Vbi is built-in voltage, VR is the reverse voltage, εs is the dielectric constant of b-Ga2O3 (εs ¼ 10 is used here [40]) and Nd is the net carrier concentration. The 1/C2eV and IeV curves of N3Ga2O3 device are shown in Fig. 6 (a). All the samples satisfy the IeV and 1/C2eV characteristics of Schottky diodes. The carrier concentration of each sample is extracted from the 1/C2eV curve as shown in Fig. 6 (b). The UndopedGa2O3 device has a net donor concentration Nd ~2.24 1017cm3 as determined by CeV measurements. The Ti barrier height derived from the voltage intercept in the 1/C2 versus voltage plot was ~1.1eV. With the increase of N injection dose, the net carrier concentration decreases 1e2 orders of magnitude gradually, which suggest that hole could be mobile with N implantation. The phenomenon could be explained by the decrease of
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Fig. 4. XPS spectrum of annealed N-doped Ga2O3 samples of (a) full spectrum, (b)O1s peak, (c) Ga3d peak, and (d) Ga2p peak.(these binding energies corrected by the binding energy of 284.8eV in C1s state).
Fig. 5. (a)PL spectra of N-doped Ga2O3 samples,and(b)decomposition spectra of N1Ga2O3 sample.
the ionization energy of N dopant due to the complex defects NGa2O3VO (N-doped b-Ga2O3 with O vacancy) and NGa2O3VGa (Ndoped b-Ga2O3 with Ga vacancy) in our reported work [30]. In other words, N atoms become easily ionization with complex native defects thus the compensation effect enhances. The net carrier concentrations of the samples decrease after annealing, probably due to more N ions activated by the high temperature annealing, which enhances the compensation effect. 4. Conclusion We investigated the electronics structures, electrical and
optical properties of N-doped b-Ga2O3 thin films by AFM, XRD, XRS, PL, CeV and IeV measurements. The crystallization of bGa2O3 thin films with N concentration increasing degrades and new defect levels appear. Under Ar annealing treatment, the crystal qualities of all samples improve due to recovery of damaged crystal structure. The electrical characteristics of Ndoped Schottky diodes comprising Ga2O3:N/n-Ga2O3 are investigated by means of Capacitance and currents plot voltage. The net carrier concentrations of these devices degrade one-two orders due to the compensation effect of more N implantation, which suggests that hole could be mobile although N is a deep acceptor. In other words, N ion is a weak p-type acceptor and has a
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Fig. 6. (a) 1/C2eV curves of N3Ga2O3 SBD (the IeV curve insected), and (b)Carrier concentration of N-doped Ga2O3 samples.
compensation effect, suggesting its potential opportunities for bGa2O3 power devices. Acknowledgement Authors would like to acknowledge that this work is supported by the National Natural Science Foundation of China (Grant No. 61974119, 51602241 and 61834005) and Science Foundation of Xi'an University of Science and Technology (Grant No. 2018QDJ036). References [1] L.P. Dong, J.G. Yu, Y.M. Zhang, R.X. Jia, Elements (Si, Sn, and Mg) doped aGa2O3: first-principles investigations and predictions, Comput. Mater. Sci. 156 (2019) 273e279. [2] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal bGa2O3 (010) substrates, Appl. Phys. Lett. 100 (2012), 013504. [3] Z. Galazka, R. Uecker, D. Klimm, K. Irmscher, M. Naumann, M. Pietsch, A. Kwasniewski, R. Bertram, S. Ganschow, M. Bickermann, Method for growing beta phase of gallium oxide (b-Ga2O3) single crystals from the melt contained within a metal crucible, ECS J.Solid State Sci. Technol. 6 (2017) Q3007. [4] Y.J. Lv, X.Y. Zhou, S.B. Long, X.B. Song, Y.G. Wang, S.X. Liang, Z.Z. Zhao, T.T. Han, X. T, Z.H. Feng, H. D, X.Z. Zhou, Y.T. Yu, S.J. Cai, M. Liu, Source-field-Plated bGa2O3 MOSFET with record power figure of merit of 50.4 MW/cm2, IEEE Electron. Device Lett. 40 (1) (2019) 83e86. [5] J.G. Yu, Z.Z. Nie, L.P. Dong, L. Yuan, D.J. Li, Y. Huang, L.C. Zhang, Y.M. Zhang, R.X. Jia, Influence of annealing temperature on structure and photoelectrical performance of b-Ga2O3/4H-SiC heterojunction photodetectors, J. Alloy. Comp. 798 (2019) 458e466. [6] Y. Lei, H.p. Zhang, R.X. Jia, L.X. Guo, Y.M. Zhang, Y.M. Zhang, Energy-band Alignment of (HfO2)X (Al2O3)1-X gate dielectrics deposited by atomic layer deposition on b-Ga2O3 (-201), Appl. Surf. Sci. 433 (2018) 530e534. [7] S.Z. Luan, L.P. Dong, R.X. Jia, L. Yuan, Y.M. Zhang, Investigation of the structural, anisotropic elastic and electronic properties of b-Ga2O3 and a-Ga2O3 under pressures, J. Cryst. Growth 505 (2019) 74e81. [8] H.P. Zhang, R.X. Jia, L. Yuan, X.Y. Tang, Y.M. Zhang, Y.M. Zhang, Leakage current conduction mechanisms and electrical properties of atomic-layer-deposited HfO2/Ga2O3 MOS capacitors, J. Phys. D Appl. Phys. 51 (7) (2018), 075104. [9] M.H. Wong, K. Goto, Y. Morikawa, A. Kuramata, S. Yamakoshi, H. Murakami, Y. Kumagai, M. Higashiwaki, All-ion-implanted planar-gate current aperture vertical Ga2O3 MOSFETs with Mg-doped blocking layer, APEX 11 (6) (2018), 064102. [10] A.J. Green, K.D. Chabak, E.R. Heller, R.C. Fitch Jr., M. Baldini Andreas Fiedler, Klaus Irmscher, Günter Wagner, Zbigniew Galazka, Stephen E. Tetlak, Antonio Crespo, Leedy Kevin, Gregg H. Jessen, 3.8 MV/cm breakdown strength of MOVPE-grown Sn-doped b-Ga2O3 MOSFETs, IEEE Electron. Device Lett. 37 (7) (2016) 902e905. [11] K. Chabak, A. Green, N. Moser, Gate-recessed, laterally-scaled b-Ga2O3 MOSFETs with high-voltage enhancement-mode operation, in: Device Research Conference (DRC), 2017 75th Annual, IEEE, 2017, pp. 1e2.
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The further investigation of N-doped b-Ga2O3 thin films with native defects for Schottky-barrier diode Suzhen Luan a, Linpeng Dong b, Xiaofan Ma b, Renxu Jia b, * a b
Xi'an University of Science and Technology, Xi'an, 710054, China Xidian University, Xi'an, 710071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2019 Received in revised form 24 August 2019 Accepted 26 August 2019 Available online 26 August 2019
The structural, electronic and optical properties of b-Ga2O3 thin films with different N ion concentrations are investigated. With N concentrations increasing, the crystallization degrades and even polycrystalline appears. After Ar annealing treatment, the crystallization of N-doped b-Ga2O3 thin films improved owing to recovery of the implantation-damaged crystals. The N-doped Ga2O3 thin films were used to construct vertical Schottky diodes comprising Ga2O3:N/n-Ga2O3. The net carrier concentrations of these devices degrade one-two orders due to the compensation effect of more N implantation, which suggests that hole could be mobile although N is a deep acceptor dopant. The phenomenon could be explained by the decrease of the ionization energy of N dopant due to the complex defects NGa2O3VO (N-doped b-Ga2O3 with O vacancy) and NGa2O3VGa (N-doped b-Ga2O3 with Ga vacancy). Thus, the development of N ion implantation for b-Ga2O3 has its potential opportunity for b-Ga2O3 power devices. © 2019 Elsevier B.V. All rights reserved.
Keywords: Gallium oxide N-doped Native defects Electronic properties
1. Introduction There have been strong interests in developing high power device based on wide-bandgap compound semiconductors since Si technology is approaching fundamental performance limitations. Recently, b-Ga2O3 has shown its great promise for next-generation power device applications due to its ultra-wide bandgap (4.9eV), higher breakdown field (8 MV/cm), higher Baliga's figures of merit (over 3000) and lower cost for mass production [1e7]. Metal-oxide-semiconductor field effect transistor (MOSFET) is one of the most widely used power devices because of its low driving power, switching loss and on-resistance. The research of Ga2O3-based MOSFET devices is mainly focused on the depletion mode (D-mode) device and the enhancement mode (E-mode) devices are rarely reported [8e19]. However, for high-voltage and low-dissipation power devices, the E-mode MOSFET is the optical choice. Due to the shallow donor level (Ec-0.04eV) formed by oxygen vacancies, the unintentionally doped b-Ga2O3 exhibits n-type conductivity [20]. At the same time, the achievement of p-type bGa2O3 thin films remains challenge largely due to the compensation with oxygen vacancies, the larger effective mass and self-
* Corresponding author. E-mail address: [email protected] (R. Jia). https://doi.org/10.1016/j.jallcom.2019.152026 0925-8388/© 2019 Elsevier B.V. All rights reserved.
trapped effect of holes which hampers E-mode MOSFET power devices application. Up to now, theoretical and experimental studies were carried out [21e25], but failed to obtain high quality p-type Ga2O3 films. Nitrogen (N) element is considered to be an effective p-type dopant [26e28]. Besides, the ionic radius and electronic structure of N3 are close to O2, which make it a promising p-type dopant in bGa2O3. However, only N-doped b-Ga2O3 nanowires with a p-type electric conductivity have been reported [29]. We had reported the structural, electronic and optical properties of N-doped b-Ga2O3 with first-principles calculations based on density functional theory (DFT) [30]. It was found that the compensation mechanism and interaction of N dopants with native defects in N-doped b-Ga2O3. The introduction of N atoms formed a variety of complexes with native defects, in particular oxygen vacancies and Ga interstitials. Thus, the band structure calculations found that N dopants act as deep acceptors (Evþ1.33eV) but cannot be effective p-type dopants. Therefore, it is worth further exploring experimentally whether Ndoped Ga2O3 films can be obtained with p-type Ga2O3 films, in order to be better applied in the field of MOSFET power devices. In this paper, we make experimental studies of b-Ga2O3 thin films with different N-implanted concentrations. The structure, surface morphology, electrical and optical properties of these films were characterized by Atom force microscope (AFM), Raman, X-ray diffraction (XRD) and photo-luminescence spectroscopy (PL).
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Finally, current-voltage (IV), capacitance-voltage (CV) and X-ray photo-electron spectroscopy (XPS) characteristics of vertical SBD devices comprising Ga2O3: N/n-Ga2O3 films were investigated. 2. Experiments The n-type b-Ga2O3 substrates with crystal orientation (201), thickness of 680 mm and Si-doping concentration of 3.9 1017cm3 are plotted into eight pieces of 1 cm 1 cm. Then Nitrogen ions were implanted into six n-type b-Ga2O3 substrates of these. In order to get uniform distribution, each target concentration of N ion is adopted three energies and doses. The energies of N ion implantation are 30/150/400 keV and the implantation doses are 3.1 1013cm2, 3.1 1014cm2, 3.1 1015 cm2, resulting in the N concentrations of 1 1018 cm3, 1 1019 cm3 and 1 1020 cm3, respectively. These three samples are denoted as N1Ga2O3, N2Ga2O3 and N3Ga2O3, respectively. Then these samples were annealed at 900 C for 50 min in Ar atmosphere to recover the damaged crystal structure and activate the implanted dopants. The annealed samples were denoted as N1Ga2O3eAr, N2Ga2O3eAr and N3Ga2O3eAr. As a comparison, the undoped b-Ga2O3 with and without annealing are denoted as UndopedGa2O3 and UndopedGa2O3eAr, respectively. The structure, surface morphology and optical properties of these films were characterized by AFM, Raman, XRD and PL spectra. Then, SBDs comprising Ga2O3: N/n-Ga2O3 films are fabricated that Ti/Au electrodes are patterned-top and blanked-bottom to form Schottky and ohmic contacts, respectively. Finally, currentvoltage (IV), capacitance-voltage (CV) and XPS characteristics of vertical SBD devices comprising Ga2O3: N/n-Ga2O3 films were investigated. 3. Results and discussion 3.1. Electrical structures Cross-session AFM analysis of the as-implanted with Si and N3Ga2O3 b-Ga2O3 samples were performed to exam the microstructures of the N-doped b-Ga2O3. The image of N-undoped Ga2O3 before annealing are shown in Fig. 1(a), from which no clear grainlike structures are found expect for the defect dots due to Si implantation. With N ions implanting, the bright defect dots are heavier in Fig. 1(c). After annealing at 900 C, the crystallization of these samples are improved well, the details in that the RMS decreases and the grain size increase. In comparison, the significantly higher crystal damage with N-rich cluster in the Nþþ-doped films
was likely due to the higher implantation energy and dose used for N. Raman measurements of N-doped b-Ga2O3 samples before and after annealing are shown in Fig. 2. Raman activation modes of bGa2O3 are as follows: stretching and bending modes of GaO4 tetrahedron at high frequency (770-500 cm1); deformation modes of GaO6 octahedron at medium frequency (480-310 cm1); vibration and translation modes of tetrahedron and octahedron chains at low frequency (200 cm1) [31]. Fig. 2 (a) shows that the Raman peaks change mainly in the high frequency region before and after ion implantation and annealing. Therefore, N ion implantation mainly affects GaO4 tetrahedron. Fig. 2 (b) shows that after N ion implantation, the Raman peaks near 631.9 cm1 in UndopedGa2O3 film disappear, while the intensity of the Raman peaks at 658.9 cm1 increases and blue shifts to the high frequency region. It is explained that the Raman bands of 653 cm1 and 767 cm1 are attributed to the symmetrical stretching bands of GaO4 tetrahedron, while the Raman bands near 631.9 cm1 are determined to be O-Ga-O bending dies [32]. The peak at 631.9 cm1 decrease due to more simple defects and less complex defects on Ga2O3 surface with N ions concentration increasing. In addition, the enhancement of Raman band near 658.9 cm1 and the appearance of new Raman band near 767 cm1 may be due to the disorderly activation of Raman scattering. After annealing, the crystallization quality of Ndoped Ga2O3 thin films improves. Fig. 3 shows the XRD results of b-Ga2O3 with different N concentration before and after annealing. The spectrum of the asimplanted sample after annealing manifested several peaks, indicating the presence of tensile strain relative to the normal substrate due to Si redistribution while the sample growing preferentially with monocrystal structure along the (201) plane [33]. With the increase of N implantation dose, it can be found that when the implantation dose is maximum, new diffraction peaks of 2q appear at 35.2 and 45.86 , corresponding to GaN (101) plane and Ga2O3 (600) plane [34] respectively. After Ar annealing, it can be found that the crystallization quality of all samples improve due to atoms migrating to normal lattice positions and strain relief. The XPS spectrum of N-doped b-Ga2O3 samples in the bonding energy range of 0e1200eV is shown in Fig. 4 (a). The large image in Fig. 4(b) shows that the O1s signal peaks of UndopedGa2O3 sample do not meet the Gauss distribution. we decomposed them into two fitting peaks for 531.3eV and 532.45eV, that the signal peak at 531.3eV mainly from oxygen in GaeO bond and the signal peak at 532.45eV are from adsorbed oxygen on the surface of b-Ga2O3 [35e37], which disappears under annealing atmosphere. The
Fig. 1. Cross-session AFM images of b-Ga2O3 (a) UndopedGa2O3, (b) UndopedGa2O3eAr, (c) N3Ga2O3, (d) N3
Ga2O3-
Ar and (e) the RMS of all samples before and after annealing.
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Fig. 2. Raman spectra of N-doped Ga2O3 samples w/wo annealing (a) all region of Raman shift, (b)high frequency region of Raman shift.
recombination of electrons by donor (Si) level and holes by acceptor (N) level. It was conceivable that the donor (Si) and the acceptor (N) paired up by Coulombic interactions to form immobile ionic complexes such as Siþ-N-, which deduced the role of the N ion as compensating active charge-carrying acceptor impurities in the Ga2O3 film. Extra emission peaks arise at 476 nm and 524 nm corresponds to blue-green light maybe due to NGa2O3Gai and NGa2O3VO related with N-2p states and the defective states caused by intrinsic defects [30,38,39]. After annealing, the PL peak of N-doped Ga2O3 films tends to be the same as that of UndopedGa2O3 samples, which indicates that defects caused by ion implantation decrease and crystallization quality improves. 3.3. Electric properties
Fig. 3. XRD patterns of N-doped b-Ga2O3.
energy peak for Ga3d of UndopedGa2O3 is located at 20.75eV [in Fig. 4(c)]. The energy peak for Ga2p is located at 1119eV and 1146eV, with a difference of 27eV [in Fig. 4 (d)], in accordance with the literature reported. With the increase of N concentrations, the peak for O1s at 531.3eV shifts to the high binding energy side while the peaks for Ga3d and Ga2p shift right and become weaker, which may be due to the formation of GaN.
3.2. Optical properties The optical properties can be described by the PL spectra which can be explained by band-to-band transition. The roomtemperature photoluminescence (PL) spectra of as-implanted and N-doped b-Ga2O3 samples with different N ions concentrations excited with 260 nm laser are shown in Fig. 5 (a).It is noted that a luminescence peak of the UndopedGa2O3 sample exhibits at 369 nm, which is mainly caused by the electron transition from the donor level formed by the Si ion, intrinsic defect oxygen vacancy and the Ga interstitial atom to the acceptor level formed by the Ga vacancy [29,30]. After N ion implantation, four distinct photo-luminescence peaks appear in the visible range and slight red shifts manifest.The broad peaks can be decomposed into four single peaks with Gauss distribution [in Fig. 5(b)]. The peak of 386 nm corresponds to ultraviolet light and that of 430 nm corresponds to purple light [38,39], which are in main due to photons emitted by the
Electrical properties of the vertical Schottky diodes comprising Ga2O3:N/n-Ga2O3 were characterized by current-voltage (IeV) and capacitance-voltage (CeV) analysis. Firstly, IeV curves of all samples are confirmed to satisfy the characteristics of Schottky diodes as follows:
qU 1 1 I ¼ IS exp nkT 2
(1)
where Is is the reverse saturated current, q is the electron charge, U is the applied voltage, n is the correction factor, k is the constant parameter of motion and T is the temperature. Then, the CeV curves of each sample were tested and converted to 1/C2eV curves. According to the formula:
2 1 2ðVbi þ VR Þ ¼ C eεs Nd
(2)
where Vbi is built-in voltage, VR is the reverse voltage, εs is the dielectric constant of b-Ga2O3 (εs ¼ 10 is used here [40]) and Nd is the net carrier concentration. The 1/C2eV and IeV curves of N3Ga2O3 device are shown in Fig. 6 (a). All the samples satisfy the IeV and 1/C2eV characteristics of Schottky diodes. The carrier concentration of each sample is extracted from the 1/C2eV curve as shown in Fig. 6 (b). The UndopedGa2O3 device has a net donor concentration Nd ~2.24 1017cm3 as determined by CeV measurements. The Ti barrier height derived from the voltage intercept in the 1/C2 versus voltage plot was ~1.1eV. With the increase of N injection dose, the net carrier concentration decreases 1e2 orders of magnitude gradually, which suggest that hole could be mobile with N implantation. The phenomenon could be explained by the decrease of
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Fig. 4. XPS spectrum of annealed N-doped Ga2O3 samples of (a) full spectrum, (b)O1s peak, (c) Ga3d peak, and (d) Ga2p peak.(these binding energies corrected by the binding energy of 284.8eV in C1s state).
Fig. 5. (a)PL spectra of N-doped Ga2O3 samples,and(b)decomposition spectra of N1Ga2O3 sample.
the ionization energy of N dopant due to the complex defects NGa2O3VO (N-doped b-Ga2O3 with O vacancy) and NGa2O3VGa (Ndoped b-Ga2O3 with Ga vacancy) in our reported work [30]. In other words, N atoms become easily ionization with complex native defects thus the compensation effect enhances. The net carrier concentrations of the samples decrease after annealing, probably due to more N ions activated by the high temperature annealing, which enhances the compensation effect. 4. Conclusion We investigated the electronics structures, electrical and
optical properties of N-doped b-Ga2O3 thin films by AFM, XRD, XRS, PL, CeV and IeV measurements. The crystallization of bGa2O3 thin films with N concentration increasing degrades and new defect levels appear. Under Ar annealing treatment, the crystal qualities of all samples improve due to recovery of damaged crystal structure. The electrical characteristics of Ndoped Schottky diodes comprising Ga2O3:N/n-Ga2O3 are investigated by means of Capacitance and currents plot voltage. The net carrier concentrations of these devices degrade one-two orders due to the compensation effect of more N implantation, which suggests that hole could be mobile although N is a deep acceptor. In other words, N ion is a weak p-type acceptor and has a
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Fig. 6. (a) 1/C2eV curves of N3Ga2O3 SBD (the IeV curve insected), and (b)Carrier concentration of N-doped Ga2O3 samples.
compensation effect, suggesting its potential opportunities for bGa2O3 power devices. Acknowledgement Authors would like to acknowledge that this work is supported by the National Natural Science Foundation of China (Grant No. 61974119, 51602241 and 61834005) and Science Foundation of Xi'an University of Science and Technology (Grant No. 2018QDJ036). References [1] L.P. Dong, J.G. Yu, Y.M. Zhang, R.X. Jia, Elements (Si, Sn, and Mg) doped aGa2O3: first-principles investigations and predictions, Comput. Mater. Sci. 156 (2019) 273e279. [2] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal bGa2O3 (010) substrates, Appl. Phys. Lett. 100 (2012), 013504. [3] Z. Galazka, R. Uecker, D. Klimm, K. Irmscher, M. Naumann, M. Pietsch, A. Kwasniewski, R. Bertram, S. Ganschow, M. Bickermann, Method for growing beta phase of gallium oxide (b-Ga2O3) single crystals from the melt contained within a metal crucible, ECS J.Solid State Sci. Technol. 6 (2017) Q3007. [4] Y.J. Lv, X.Y. Zhou, S.B. Long, X.B. Song, Y.G. Wang, S.X. Liang, Z.Z. Zhao, T.T. Han, X. T, Z.H. Feng, H. D, X.Z. Zhou, Y.T. Yu, S.J. Cai, M. Liu, Source-field-Plated bGa2O3 MOSFET with record power figure of merit of 50.4 MW/cm2, IEEE Electron. Device Lett. 40 (1) (2019) 83e86. [5] J.G. Yu, Z.Z. Nie, L.P. Dong, L. Yuan, D.J. Li, Y. Huang, L.C. Zhang, Y.M. Zhang, R.X. Jia, Influence of annealing temperature on structure and photoelectrical performance of b-Ga2O3/4H-SiC heterojunction photodetectors, J. Alloy. Comp. 798 (2019) 458e466. [6] Y. Lei, H.p. Zhang, R.X. Jia, L.X. Guo, Y.M. Zhang, Y.M. Zhang, Energy-band Alignment of (HfO2)X (Al2O3)1-X gate dielectrics deposited by atomic layer deposition on b-Ga2O3 (-201), Appl. Surf. Sci. 433 (2018) 530e534. [7] S.Z. Luan, L.P. Dong, R.X. Jia, L. Yuan, Y.M. Zhang, Investigation of the structural, anisotropic elastic and electronic properties of b-Ga2O3 and a-Ga2O3 under pressures, J. Cryst. Growth 505 (2019) 74e81. [8] H.P. Zhang, R.X. Jia, L. Yuan, X.Y. Tang, Y.M. Zhang, Y.M. Zhang, Leakage current conduction mechanisms and electrical properties of atomic-layer-deposited HfO2/Ga2O3 MOS capacitors, J. Phys. D Appl. Phys. 51 (7) (2018), 075104. [9] M.H. Wong, K. Goto, Y. Morikawa, A. Kuramata, S. Yamakoshi, H. Murakami, Y. Kumagai, M. Higashiwaki, All-ion-implanted planar-gate current aperture vertical Ga2O3 MOSFETs with Mg-doped blocking layer, APEX 11 (6) (2018), 064102. [10] A.J. Green, K.D. Chabak, E.R. Heller, R.C. Fitch Jr., M. Baldini Andreas Fiedler, Klaus Irmscher, Günter Wagner, Zbigniew Galazka, Stephen E. Tetlak, Antonio Crespo, Leedy Kevin, Gregg H. Jessen, 3.8 MV/cm breakdown strength of MOVPE-grown Sn-doped b-Ga2O3 MOSFETs, IEEE Electron. Device Lett. 37 (7) (2016) 902e905. [11] K. Chabak, A. Green, N. Moser, Gate-recessed, laterally-scaled b-Ga2O3 MOSFETs with high-voltage enhancement-mode operation, in: Device Research Conference (DRC), 2017 75th Annual, IEEE, 2017, pp. 1e2.
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