Recovery Performance of Ge-Doped Vertical GaN Schottky Barrier Diodes
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Vertical GaN Schottky barrier diodes (SBDs) were fabricated on Ge-doped free-standing GaN substrates. The crystal quality of the SBDs was characterized by cathode luminescence measurement, and the dislocation density was determined to be ~ 1.3 × 106 cm− 2. With the electrical performance measurements conducted, the SBDs show a low turn-on voltage Von (0.70~0.78 V) and high current Ion/Ioff ratio (9.9 × 107~1.3 × 1010). The reverse recovery characteristics were investigated. The reverse recovery time was obtained to be 15.8, 16.2, 18.1, 21.22, and 24.5 ns for the 100-, 200-, 300-, 400-, and 500-μm-diameter SBDs, respectively. Meanwhile, the reverse recovery time and reverse recovery charge both show a significant positive correlation with the electrode area.
KeywordsVertical SBDs Ge-doped GaN substrates Reverse recovery time HVPE
Device under test
Hydride vapor phase epitaxy
Schottky barrier diodes
Scanning electron microscope
Recently, a wide band gap semiconductor—such as GaN—with the inherent advantages, has attracted tremendous research attention for the next-generation electronics devices, particularly in the field of high frequency, high power, and high performance [1, 2, 3, 4, 5, 6]. Meanwhile, thanks to the developments of hydride vapor phase epitaxy (HVPE), low dislocation density (≤ 106 cm− 2) GaN substrates are now commercially available [7, 8, 9, 10]. Compared with lateral devices, vertical-type devices fabricated with these substrates are considered to be a more advanced structure which is conducive to achieving a larger current, less leakage path, and better reliability for the system [11, 12]. Among them, GaN-based Schottky barrier diode (SBD) is a vital component in the switching devices. Differed from a bipolar diode, the SBD with its unipolar nature greatly reduces the minority carrier storage effect and correspondingly offers a high switching speed with low reverse recovery loss. However, few groups have conducted a systematic study of the reverse recovery characteristics for vertical GaN SBDs [13, 14, 15, 16, 17], of which studies focused more on the comparison of the switching time in different structures devices. Thus, there is still an urgent need of a deep investigation into the mechanism of recovery performance for GaN SBDs, especially for the vertical ones.
Meanwhile, since the ohmic contact technique has been continuously explored to improve device performance in many published papers , heavily doped n-type GaN is a key link for fabricating nitride devices. Lately, Ge is proposed as an alternative to Si dopant in GaN, because both of them share a similar characteristic of shallow level impurity (the activation energy is reported to be 20 and 17 meV for Ge and Si, respectively) and the lattice distortion caused by Ge atoms substituting into Ga sites would be smaller owing to their closer ionic radii [19, 20]. The Ge doping is believed to form a smoother surface with fewer defects [21, 22]. Moreover, with the lower lattice distortion and film stress, this doping also shows a promise in high-temperature electronic devices that put more emphasis on the thermal stability. Although the Ge-doped GaN has been studied theoretically, it is essential to investigate the real impact on the relevant device. In this paper, the novel vertical GaN SBDs fabricated on Ge-doped free-standing (FS) GaN substrate are proposed. The vertical GaN SBDs exhibit a superior crystal quality and electronic property. Meanwhile, the recovery performance of vertical SBDs is systematically investigated. The reverse recovery time and reverse recovery charge finally show a significant positive correlation with the electrode area.
Methods and Experiments
The cathodoluminescence (CL) images were obtained using a Quanta 400 FEG scanning electron microscope (SEM) with a 10-kV accelerating voltage to study the spatial distribution of dislocation density for the epitaxial layer. Capacitance-voltage (C-V) and current-voltage (I-V) measurements were performed using a Keithley 4200 semiconductor parameter analyzer to evaluate electronic properties of the SBDs. And temperature-dependent measurements were conducted in the range of 300 to 500 K with a customized experimental setup.
Results and Discussion
The CL result of the epitaxial layer is presented in Fig. 1c. As the dislocation is believed to be a nonradiative recombination center, it appears on the CL image in the form of a dark spot. Since no noticeable spatial distribution difference is observed, the average value of dislocation density was calculated to be ~ 1.3 × 106 cm− 2, with the CL measurements performed at several different regions. This result indicates a high-quality epitaxial layer was obtained for vertical SBDs.
In fact, it is reported that the reverse recovery effect should be mainly from the parasitic inductance and interface trapping of SBDs [25, 26]. Considering that the contribution of parasitic inductance is characterized in the form of oscillation current which is not obviously observed in these reverse recovery curves, thus, the changing of reverse recovery time and reverse recovery charge should have resulted from the traps [27, 28]. Since the concentration of traps is uniform in vertical SBDs, the Qrr would depend on the contact area of the device and finally increase with the electrode area as shown in Fig. 6. Thus, the traps act as an electric field stopper in the interface. During the ta period, the delay was strongly influenced by carrier trapping in the Schottky junction, while in the tb period, reverse recovery speed is also slowed by the time for sweeping the stored charge out of the junction. These results are consistent with our previous report , which suggested the RC time constant increases with the increase of device diameter and shows a good dependency with the reverse recovery time. And a further improvement of reverse recovery characteristics could be expected from a smaller electrode or thinner drift layer in these devices.
In summary, we fabricated vertical GaN SBDs on Ge-doped FS GaN substrates grown by HVPE. With the material characterization and current-voltage measurements performed, it indicates that an excellent crystal quality and electronic property was obtained for the vertical SBDs. The reverse recovery characteristics were systematically investigated. The reverse recovery time was obtained to be 15.8, 16.2, 18.1, 21.22, and 24.5 ns for the 100-, 200-, 300-, 400-, and 500-μm-diameter diodes, respectively. Meanwhile, the reverse recovery time and reverse recovery charge both show a significant positive correlation with the electrode area. Our results may serve as a reference for further improving the recovery performance of GaN-based SBDs.
We acknowledge Photonics Center of Shenzhen University for technical support.
National Key Research and Development Plan (2017YFB0404100).
Availability of Data and Materials
All data generated or analyzed during this study are included in this published article.
HG, FFT, CYZ, JLW, YC, and XHD carried out the related experiments and data analysis. HG drafted the manuscript. XKL supervised the experiments and the writing of the manuscript. KX provided suggestions and guidance for the experiments and data analysis. All authors read and approved the final manuscript.
KX is a professor in materials science. XKL is an associate professor in materials physics. HG, FFT and CYZ are associate professors in material characterization. JLW, YC, and XHD are students in fabrication nano-device.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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