Vertical p-GaN/n-Ga₂O₃ heterojunction diode with high switching performance

Abstract

Although the forward current–voltage characteristics and breakdown voltage performance of GaN/Ga₂O₃ p–n diodes have been extensively studied, there have been no reports on their reverse recovery characteristics. This paper investigates the reverse recovery behavior of p-GaN/n-Ga₂O₃ diodes and examines the effects of temperature, forward current magnitude, and doping concentration on their performance. In the simulations of the switching process, a reverse voltage (V_R) of −10 V and a forward current (I_F) of 3 A were applied. The p-GaN/n-Ga₂O₃ diode demonstrated fast reverse recovery, achieving a reverse recovery time (t_rr) of 72 ns, outperforming 6H-SiC p–n diodes under identical dI/dt conditions. Notably, the reverse recovery characteristics of the p-GaN/n-Ga₂O₃ diode remained stable across varying temperatures and forward current levels. Additionally, a high breakdown voltage of 703 V was achieved at 300 K, maintaining stability across different temperatures. These findings highlight the potential of p-GaN/n-Ga₂O₃ heterojunction diodes for high-voltage, fast-switching applications.

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1. Introduction

In recent years, beta-gallium oxide (β-Ga₂O₃) has gained significant attention as a promising candidate for next-generation power electronic applications due to its wide-bandgap semiconductor properties. With a bandgap of approximately 4.7–4.9 eV, a high critical electric field of around 8 MV cm⁻¹, and an exceptional Baliga's figure of merit (BFOM), β-Ga₂O₃ exhibits superior characteristics compared to conventional materials such as silicon (Si) and has an expected electric field that is several times higher than that of silicon carbide (SiC) and gallium nitride (GaN).^1–7 Additionally, β-Ga₂O₃ offers the advantage of cost-effective, large-scale production, made possible by the availability of large, defect-reduced wafers manufactured using techniques such as edge-defined film-fed growth (EFG).⁸

However, one of the main challenges facing Ga₂O₃ is the lack of p-type doping. As a result, research on Ga₂O₃-based devices has primarily focused on high-electron-mobility transistors (HEMTs), Schottky barrier diodes (SBDs), and metal-oxide–semiconductor field-effect transistors (MOSFETs).^9–22 To overcome this limitation, alternative materials such as NiO, Cu₂O, and GaN have been investigated as potential substitutes for p-type Ga₂O₃.^23–27 Among these, GaN/β-Ga₂O₃ p–n heterojunctions have shown promising rectifying behavior and improved breakdown voltages when created through mechanical exfoliation.^28,29 GaN, in particular, stands out due to its wide bandgap and its ability to be epitaxially grown on β-Ga₂O₃ using metal–organic chemical vapor deposition (MOCVD), taking advantage of their well-defined crystallographic alignment.^30–32

The reverse recovery characteristics of power diodes have been extensively studied in other representative wide-bandgap semiconductors, but the reverse recovery behavior of GaN/β-Ga₂O₃ heterojunction p–n diodes has not been explored. This is particularly significant because the combination of GaN and β-Ga₂O₃ introduces unique physical mechanisms due to their distinct material properties and interface characteristics. These mechanisms could greatly impact the reverse recovery characteristics, yet this aspect has not been thoroughly investigated in the literature. In this study, we examine the reverse recovery characteristics of GaN/β-Ga₂O₃ heterojunction diodes, specifically focusing on the effects of temperature, forward current magnitude and doping concentration. This research provides a novel contribution to understanding the reverse recovery dynamics in ultra-wide bandgap semiconductor devices.

2. Structure and mechanism

2.1. Device structure

Fig. 1 depicts a schematic representation of a simulated GaN/β-Ga₂O₃ heterojunction p–n diode. This study focuses on simulating the diode and evaluating the reliability of its reverse recovery characteristics using technology computer-aided design (TCAD) tools. The n-Ga₂O₃ epitaxial layer has a thickness of 3.8 μm and a doping concentration of 4.5 × 10¹⁶ cm⁻³, while the n-Ga₂O₃ substrate has a thickness of 8.0 μm and a doping concentration of 2.1 × 10¹⁸ cm⁻³.^29,31 The p-GaN layer has a thickness of 3.2 μm and a doping concentration of 2.0 × 10¹⁹ cm⁻³. The material properties of β-Ga₂O₃ and GaN, such as bandgap, electron mobility, electron affinity, dielectric constant, and thermal conductivity, were obtained from previous research^{2,21,22,32–34,35–40} and are summarized in Tables 1 and 2, respectively. To calibrate the simulation model, experimental results from β-Ga₂O₃ Schottky barrier diode (SBD)²⁵ and GaN p–n diode⁴¹ were utilized. The simulated I–V characteristics closely match the reported experimental data, as shown in Fig. 2(a) for the Ga₂O₃ SBD and Fig. 2(b) for the GaN p–n diode.

Fig. 1 Schematic representation of the simulated GaN/β-Ga₂O₃ heterojunction p–n diode.

Table 1 Fundamental material properties of β-Ga₂O₃ used in the simulation

Parameter	Value
Bandgap (eV)	4.8 (ref. 21)
Conduction band offset (eV)	0.165 (ref. 32)
Valence band offset (eV)	1.625 (ref. 32)
Electron mobility (cm^2 V^−1 s^−1)	118 (ref. 22)
Hole mobility (cm^2 V^−1 s^−1)	10 (ref. 22)
Thermal conductivity (W cm^−1 K^−1)	0.13 (ref. 2)
Dielectric constant	10.2 (ref. 33)
Electron affinity (eV)	4.0 (ref. 21)
Effective density of states in the conduction band/valence band (cm^−3)	3.72 × 10^18 (ref. 21)
Effective density of states in the valence band (cm^−3)	1.16 × 10^19 (ref. 21)
Impact inonization coefficients an (cm^−1)	0.79 × 10^6 (ref. 34)
Impact inonization coefficients bn (V cm^−1)	2.92 × 10^6 (ref. 34)
Electron lifetime (s)	2.0 × 10^−10 (ref. 22)
Hole lifetime (s)	2.1 × 10^−8 (ref. 22)

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Table 2 Fundamental material properties of GaN used in the simulation

Parameter	Value
Bandgap (eV)	3.53 (ref. 35)
Dielectric constant	8.9 (ref. 36)
Electron affinity (eV)	4.1 (ref. 37)
Effective density of states in the conduction band (cm^−3)	2.3 × 10^18 (ref. 38)
Effective density of states in the valence band (cm^−3)	3.5 × 10^19 (ref. 38)
Impact inonization coefficients an (cm^−1)	1.56 × 10^5 (ref. 39)
Impact inonization coefficients bn (V cm^−1)	1.41 × 10^7 (ref. 39)
Electron lifetime (s)	1.0 × 10^−9 (ref. 40)
Hole lifetime (s)	1.0 × 10^−9 (ref. 40)

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Fig. 2 Experimental data (blue) and simulated data (red) for the forward current–voltage (I–V) characteristics of (a) β-Ga₂O₃ SBD and (b) GaN p–n diode. Good agreement confirms the accuracy of the DC device model.

2.2. Models and parameters used in simulations

The performance of the device was evaluated using 2D device simulators that solve Poisson's equation, along with the continuity and drift-diffusion equations.^42–48 These simulations, which also included mixed-mode analyses, were conducted using TCAD tools. During the forward turn-on phase, the p–n junction is positively biased, allowing for carrier conduction. When the device transitions to the off state, the transverse electric field drives the non-equilibrium carriers within the device back to equilibrium. The simulation of the GaN/β-Ga₂O₃ heterojunction p–n diode incorporates key physical models such as Shockley–Read–Hall and Auger recombination to account for carrier recombination mechanisms, concentration-dependent mobility, and lateral electric field-dependent mobility to capture variations in carrier transport behavior. Additionally, bandgap narrowing is included to model changes in the energy band structure under specific conditions.

(1) Carrier generation–recombination models: the numerical simulation of carrier generation and recombination is carried out using concentration-dependent models for Shockley–Read–Hall (SRH) and Auger recombination.^46–48

R_Auger = 3 × 10⁻²⁹(pn² − nn_i²) + 3 × 10⁻²⁹(np² − pn_i²) (2)

Here, n_i represents the effective intrinsic carrier concentration, E_t denotes the recombination center energy level, E_i is the intrinsic Fermi level, and τ_n0 and τ_p0 represent the electron and hole lifetimes, respectively.

(2) Impact inonization model: the equation describes the rate of carrier generation during the avalanche breakdown process.⁴⁹

G = α_nJ_n + α_pJ_p (3)

Here, α_n and α_p represent the impact ionization coefficients for electrons and holes, respectively, while J_n and J_p denote the electron and hole current densities. The ionization rate is effectively modeled using the Selberherr model.

Here, E represents the electric field along the direction of current flow, and A_n,p, B_n,p, and C_n,p are the respective fitting coefficients.

(3) Thermal conductivity specification: we performed 2D physics-based electrothermal device simulations through TCAD.⁵⁰ These simulations involved solving coupled drift-diffusion and heat flow equations.

3. Results and discussion

We used TCAD to simulate the band diagram of the heterojunction, as shown in Fig. 3. By selecting the appropriate electron affinity values for GaN and Ga₂O₃, we obtained a conduction band offset (ΔE_c) of 0.165 eV and a valence band offset (ΔE_v) of 1.625 eV. These values were chosen to match the required band offsets, in accordance with experimental data. The resulting band offset values are consistent with those reported by W. Li et al.³² using X-ray photoelectron spectroscopy.

Fig. 3 Simulated band diagram of the p-GaN/n-β-Ga₂O₃ heterojunction.

The simulated forward current–voltage (I–V) characteristics of the GaN/β-Ga₂O₃ p–n diode at 300 K are illustrated in Fig. 4. When a forward bias is applied, the diode exhibits rectifying behavior, and the turn-on voltage is the point at which the current begins to increase. The turn-on voltage (V_on) is observed to be 3.5 V, which is in reasonable agreement with experimental results.²⁸

Fig. 4 Simulated forward current–voltage (I–V) characteristics of the GaN/β-Ga₂O₃ p–n diode at 300 K.

Fig. 5 illustrates the simulated temperature-dependent forward current–voltage characteristics of the GaN/β-Ga₂O₃ p–n diode. It can be observed that the turn-on voltage (V_on) decreases from 3.5 V to 3.25 V, and the specific on-resistance (R_on,sp) in the current range from 0.02 A to 0.03 A decreases from 5.9 mΩ cm² to 5.5 mΩ cm² as the temperature rises from 300 K to 450 K. The decrease in turn-on voltage can be attributed to the increase in diffusion current across the p–n heterojunction and the bandgap narrowing effect at higher temperatures. Similarly, the decrease in R_on,sp with increasing temperature can be explained by the thermally enhanced conductivity modulation effect. As the temperature rises, the injection of minority carriers (holes) from p-GaN into n-β-Ga₂O₃ increases, leading to a decrease in space-charge resistance in the drift region. Additionally, the thermal generation of electron–hole pairs increases at higher temperatures, improving the conductivity of the drift region and resulting in more efficient current conduction.

Fig. 5 Simulated temperature-dependent forward current–voltage (I–V) characteristics of the GaN/β-Ga₂O₃ p–n diode.

Fig. 6 illustrates the simulated temperature-dependent reverse breakdown characteristics of the GaN/β-Ga₂O₃ heterojunction p–n diode. It is found that the breakdown voltage remains consistent at 703 V across various temperatures, indicating the excellent temperature stability of the diode. The sharp increase in reverse current near the breakdown voltage suggests that the breakdown is dominated by avalanche breakdown. This occurs when high-energy carriers generate additional charge carriers through impact ionization, resulting in an accelerated current flow. Furthermore, Y. Duan et al. reported that the measured breakdown voltage increases with rising temperature, indicating that the diode with a triple-zone junction termination extension structure achieves avalanche-limited breakdown.⁵¹ These findings demonstrate the excellent temperature stability of the p-GaN/n-β-Ga₂O₃ diode and confirm its ability to achieve avalanche-limited breakdown. In addition, Fig. 7 displays the electric field distribution at breakdown for the GaN/β-Ga₂O₃ p–n diode. The analysis clearly reveals a significant concentration of the electric field at the p-GaN and n-Ga₂O₃ junction, which can lead to catastrophic damage at this location. These results suggest that using β-Ga₂O₃ material in heterojunction p–n diodes has the potential to achieve high breakdown voltages by minimizing electric field concentration and leakage current. This highlights the potential application of the p-GaN/n-Ga₂O₃ heterojunction diode as a power device for high-voltage operations.

Fig. 6 Simulated temperature-dependent reverse breakdown characteristics of the GaN/β-Ga₂O₃ p–n diode.

Fig. 7 Simulated electric field distributions of the GaN/β-Ga₂O₃ p–n diode at breakdown: (a) two-dimensional distribution of the electric field; (b) line profile of the electric field along A₁–A₂.

The reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode were investigated and compared to those of the 6H-SiC p–n diode under the same dI/dt conditions. Fig. 8 illustrates the cross-sectional schematic of the 6H-SiC p–n diode used for comparison with the GaN/β-Ga₂O₃ p–n diode.⁵² The 6H-SiC p–n diode consists of a 3.8-μm-thick n-type epitaxial layer on an 8-μm-thick n⁺-type 6H-SiC substrate with a doping concentration of approximately 2.1 × 10¹⁸ cm⁻³. The n-type epitaxial layer has a doping concentration of about 4.5 × 10¹⁶ cm⁻³. The device also includes a 3.2-μm-thick p⁺-type 6H-SiC layer with a doping concentration of approximately 2.0 × 10¹⁹ cm⁻³. The material parameters for 6H-SiC are provided in Table 3.⁵³ For simulating the reverse recovery characteristics, the parameters for the diode components were set as follows: a reverse voltage (V_R) of −10 V and a forward current (I_F) of 3 A. The results of this comparison are presented in Fig. 9, along with a summary of the values for the peak reverse recovery current (I_rr) and recovery time (t_rr) for both the GaN/β-Ga₂O₃ p–n diode and the 6H-SiC p–n diode in Table 4. Here, the peak reverse recovery current (I_rr) is defined as the current that occurs during the removal of stored charges in the drift region when the diode switches from the on-state to the off-state. Reverse recovery time (t_rr) refers to the time it takes for the switching diode to transition from the on-state to the fully off-state.

Fig. 8 Schematic representation of the simulated 6H-SiC p–n diode.⁵²

Table 3 Fundamental material properties of 6H-SiC used in the simulation⁵³

Parameter	Value
Bandgap (eV)	2.9
Electron lifetime (s)	1.0 × 10^−9
Hole lifetime (s)	1.0 × 10^−9
Electron mobility (cm^2 V^−1 s^−1)	330
Thermal conductivity (W cm^−1 K^−1)	5.0
Dielectric constant	9.66
Electron saturation velocity (cm s^−1)	2.0 × 10^7
Effective density of states in the conduction band (cm^−3)	7.68 × 10^18
Effective density of states in the valence band (cm^−3)	4.76 × 10^18

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Fig. 9 Simulated reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode compared to the 6H-SiC p–n diode at 300 K.

Table 4 Summary of reverse recovery characteristics

Parameter	GaN/β-Ga2O3 p–n diode	6H-SiC p–n diode
Irr (A)	7.5	19.1
trr (ns)	72	83

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The results presented in Fig. 9 and Table 4 demonstrate that the GaN/β-Ga₂O₃ diode has better recovery characteristics than the 6H-SiC diode, with lower I_rr and faster recovery time. The reverse recovery time (t_rr) is calculated from the moment when the current crosses zero until the reverse recovery current decreases to 10% of I_rr. The presence of electron traps and gallium vacancies in Ga₂O₃ plays a crucial role in enhancing minority carrier recombination, thereby reducing both the reverse recovery time (t_rr) and the reverse recovery current (I_rr). In our simulations, defects such as electron and hole traps have been carefully incorporated to reflect their impact on carrier dynamics and diode performance. Previous studies have reported electron traps located near E_c − 0.6 eV, E_c − 0.75 eV, and E_c − 1.05 eV in epitaxial β-Ga₂O₃ films, as well as the E₁, E₂, and E₃ traps in bulk Ga₂O₃ crystals.^54–59 Additionally, gallium vacancies, which often form in oxygen-rich environments or during annealing processes, act as electron traps and form high-binding-energy gallium–oxygen vacancy complexes that capture returning holes, further improving the reverse recovery characteristics.⁶⁰ Furthermore, we have compared the simulated reverse recovery characteristics of the p-GaN/n-Ga₂O₃ diode with the experimental results reported for the Si ultrafast diode STTH3012D,⁶¹ as shown in Fig. 10. It is evident that the reverse recovery characteristics of the GaN/β-Ga₂O₃ diode are superior to those of the Si ultrafast diode STTH3012D. These findings, which are consistent with our defect-inclusive simulations, demonstrate the superior reverse recovery performance of the p-GaN/n-Ga₂O₃ diode.

Fig. 10 Comparison of reverse recovery current for the GaN/β-Ga₂O₃ p–n diode and the Si ultrafast diode STTH3012D.⁶¹

Fig. 11 and 12 depict the temperature and forward current dependence of the reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode. In standard diodes, the reverse recovery time and peak reverse recovery current typically increase with temperature due to the accumulation of holes in the n-layer when a forward bias is applied. During the reverse recovery phase, the depletion region extends from the p–n junction into the n-layer. If the width of the depletion region is smaller than the n-layer, the remaining holes in the n-layer recombine and disappear. As the temperature rises, the carrier lifetime increases, causing a delay in recombination and resulting in a longer reverse recovery time. Additionally, as the accumulated holes recombine and the anode voltage recovers, the reverse recovery current continues to rise, leading to an increase in the peak reverse recovery current. When the forward current is high, both holes and electrons traverse the junction and enter the opposite region. It takes time for these holes to return, thus the reverse recovery time (t_rr) becomes longer as the forward current increases.

Fig. 11 Simulated temperature-dependent reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode.

Fig. 12 Simulated forward current-dependent reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode.

However, Fig. 11 and 12 demonstrate that the reverse recovery waveforms of the GaN/β-Ga₂O₃ diode are not affected by changes in temperature or the magnitude of the forward current. Both the peak reverse recovery current (I_rr) and recovery time (t_rr) remain nearly constant, regardless of increases in temperature or forward current. This can be attributed to the short carrier lifetimes in both GaN and β-Ga₂O₃. The electron lifetime and hole lifetime of n-Ga₂O₃ are 2.0 × 10⁻¹⁰ s and 2.1 × 10⁻⁸ s,²² respectively, while both the electron and hole lifetimes of p-GaN are 1.0 × 10⁻⁹ s.⁴⁰ Additionally, the excellent property of a large critical electric field in GaN and β-Ga₂O₃ wide-bandgap materials results in a strong electric field in the depletion region, which quickly sweeps out charge carriers and reduces recovery time. Furthermore, the good thermal conductivity of GaN and optimized device design help to minimize the impact of temperature. In contrast, conventional narrow-bandgap semiconductors typically have longer carrier lifetimes and increased reverse recovery time (t_rr) at higher temperatures. The independence of the diode's reverse recovery characteristics from the forward current (I_F) indicates minimal accumulation of minority carriers, which is due to the optimized layer design of the p-GaN/n-Ga₂O₃ diode structure in this study. The n-Ga₂O₃ epitaxial layer has a moderate doping concentration of 4.5 × 10¹⁶ cm⁻³ and a thickness of 3.8 μm, while the n-Ga₂O₃ substrate has a higher doping concentration of 2.1 × 10¹⁸ cm⁻³ and a thickness of 8.0 μm. This design minimizes stored charge during forward conduction, reducing its impact on reverse recovery. In comparison, other structures with lower doping and thicker epitaxial layers were found to store more charge, resulting in increased I_rr and t_rr at higher forward currents. The optimized structure design in our study ensures stable reverse recovery performance, making it suitable for high-speed switching applications. Additionally, F. Zhou et al.⁶² also reported that the reverse recovery waveform is dominated by capacitive ringing rather than minority carrier recombination in the NiO/Ga₂O₃ p–n heterojunction diode.

In addition to the effects of temperature and forward current, the doping concentrations of the n-Ga₂O₃ epitaxial layer and the p-GaN layer significantly influence the reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode. Therefore, our study has been expanded to investigate how variations in doping concentration affect the reverse recovery time (t_rr) and peak reverse recovery current (I_rr). We believe that by optimizing the doping levels in the n-Ga₂O₃ and p-GaN layers, the diode's switching performance and breakdown voltage can be improved. Fig. 13 and 14 demonstrate that the reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode are more affected by changes in the doping concentration of the n-Ga₂O₃ epitaxial layer than those in the p-GaN layer. As the doping concentration of the n-Ga₂O₃ epitaxial layer increases, both the peak reverse recovery current (I_rr) and recovery time (t_rr) of the GaN/β-Ga₂O₃ diode also increase. This is because the n-Ga₂O₃ layer acts as the drift region, storing charge carriers during forward conduction. A higher doping concentration results in a larger amount of stored charge, leading to a higher reverse recovery current and a longer recovery time. This effect is further amplified by the relatively short electron lifetime (2.0 × 10⁻¹⁰ s) and longer hole lifetime (2.1 × 10⁻⁸ s) in the n-Ga₂O₃ layer. The short electron lifetime allows for rapid electron removal, while the longer hole lifetime delays recombination, thereby extending the reverse recovery time. In contrast, the electron and hole lifetimes of approximately 1.0 ns in the p-GaN layer⁴⁰ primarily affect the forward conduction process rather than the reverse recovery. Therefore, changes in the doping concentration of the n-Ga₂O₃ layer have a greater impact on the reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode than those in the p-GaN layer.

Fig. 13 Simulated n-Ga₂O₃ epitaxial layer doping concentration-dependent reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode.

Fig. 14 Simulated p-GaN layer doping concentration-dependent reverse recovery characteristics of the GaN/β-Ga₂O₃ p–n diode.

4. Conclusion

Our simulations of the GaN/β-Ga₂O₃ p–n heterojunction diode showed a turn-on voltage of 3.5 V and a breakdown voltage of 703 V at 300 K, with stable performance across different temperatures. This stability highlights the potential application of the p-GaN/n-Ga₂O₃ heterojunction diode as a reliable power device for high-voltage operations. In the reverse recovery simulation, the GaN/β-Ga₂O₃ diode demonstrated fast switching behavior, surpassing that of 6H-SiC diodes. Furthermore, the reverse recovery characteristics were found to be unaffected by temperature or the magnitude of the forward current, suggesting that the device primarily operates with majority carriers and is not affected by minority carrier recombination. The optimized structure design presented in our study significantly reduces the impact of temperature and forward current magnitude on the reverse recovery characteristics of the p–n heterojunction diode. Our study also revealed that the reverse recovery characteristics are solely dependent on the doping concentration of the n-Ga₂O₃ epitaxial layer, rather than the doping concentration of the p-GaN layer. These findings highlight the significant potential of GaN/β-Ga₂O₃ diodes for applications requiring fast switching, low loss, and high reliability, such as electric vehicles, aerospace, renewable energy systems, and next-generation power devices.

Author contributions

All authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript

Declaration of generative AI in scientific writing

There is no generative AI in scientific writing.

Ethics approval

No experiments on animal or human subjects were used for the preparation of the submitted manuscript.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to express their gratitude to Prof. Cong-Kha Pham of the University of Electro-Communications (UEC), Tokyo, Japan, and the VLSI Design and Education Center (VDEC) at the University of Tokyo, Tokyo, Japan, for their support in providing simulation tools.

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