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Abstract
We report high-performance Al0.1Ga0.9N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with stable breakdown voltages (VBR) around 113.4 V, low dark current densities (JBR) below 9 × 10−4 A/cm2 and a high avalanche gain over 2 × 106. A two-step deposition method was employed to reduce passivation-induced plasma damage while maintaining high dielectric film quality. Consistent JBR for various mesa sizes at the VBR are demonstrated, which reveals the suppression of the surface leakage current. Uniform electroluminescence (EL) distributions during the avalanche multiplication processes are displayed, which confirms the elimination of edge breakdown. Pure bulk leakage current distributions and uniform body avalanche breakdown behaviors are observed for the first time in AlGaN APDs. The emission spectra of the EL at various current levels are also presented.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) photodetectors based on III-nitride semiconductors have wide applications in military and civilian fields [1–4]. In contrast to photomultiplier tubes, which are bulky, fragile, and require optical filters, AlGaN-based avalanche photodiodes (APDs) provide natural filters with tunable cutoff wavelengths and show great potential for high-sensitivity UV detection.
Compared with GaN APDs [5–7], the development of AlGaN visible-blind UV APDs is relatively backward [8]. The heteroepitaxial growth of AlGaN films on lattice-mismatched sapphire substrates will introduce a high density of defects. The growth and fabrication of ternary alloys are more difficult than those of binary alloys. Electric field crowding effects still exist due to the lack of suitable edge terminations. As a result, AlGaN APDs based on sapphire substrates face three major challenges: low avalanche gains, high leakage currents and edge breakdowns.
In our recent work [9], sapphire-based Al0.1Ga0.9N APDs with a record-high gain over 2 × 106 were presented. However, the dark current densities (JBR) at the breakdown voltages (VBR) are relatively high. The leakage current is still a mixture of surface leakage and bulk leakage, and the surface component is comparable to that of the bulk. Additionally, the avalanche electroluminescence (EL) is focused along the device periphery, which indicates the existence of edge breakdown.
In this work, the plasma-enhanced chemical vapor deposition (PECVD) passivation process was optimized. A two-step deposition method was developed to reduce passivation-induced plasma damage while maintaining high dielectric film quality. The fabricated Al0.1Ga0.9N APDs demonstrate excellent comprehensive performance with suppressed surface leakage current and eliminated edge breakdown. Pure bulk leakage current distributions and uniform body avalanche breakdown behaviors are observed for the first time in AlGaN APDs.
2. Device fabrication with two-step passivation
Figure 1(a) plots the schematic structure of the Al0.1Ga0.9N p-i-n APDs. UV light was illuminated from the front side and absorbed in the Al0.1Ga0.9N layers. Since the surface leakage paths mainly exist at the interface between the device sidewall and the dielectric layer, a high-quality SiO2 film plays a dominant role in passivating the surface states and reducing the leakage paths [10–13]. The electrical properties of the PECVD-SiO2 films are characterized by the platform presented in Fig. 1(b). The leakage current densities and breakdown electric fields as a function of radio frequency (RF) powers during SiO2 deposition are shown in Fig. 1(c). With the increase in RF power from 10 to 50 W, the leakage current density decreases continuously from 3.45 × 10−5 to 1.10 × 10−8 A/cm2 (taken at an electric field strength of 7 MV/cm and averaged by 10 points), while the breakdown electric field improves monotonically from 8.77 to 10.37 MV/cm. With a further increase in RF power from 50 to 60 W, the breakdown characteristics degraded significantly. Consequently, 50 W was adopted as the optimal RF power for high-quality SiO2 deposition.
Fig. 1. (a) Schematic structure of the Al0.1Ga0.9N p-i-n APDs with PECVD-SiO2 dielectric films as passivation layers. (b) The characterization platform for the electrical properties of the SiO2 films. (c) The leakage current densities and breakdown electric fields of SiO2 films with various RF powers from 10 to 60 W. The scans for each RF power consist of 10 points.
Recently, Sheen et al. [14] verified that the plasma-enhanced passivation process induces point defects to the device periphery and increases the sidewall leakage current. Additionally, Liu et al. [15] revealed that in a PECVD system, an RF power of 30 W could generate nitrogen radicals for the formation of Ga-N bonds. Simultaneously, Han et al. [16] demonstrated that in a PECVD system, an RF power of 60 W could generate plasma nitridation that is strong enough for ion implantation termination. As a result, the sputtering of high-energy ions during the PECVD passivation process generates plasma damage concentrated on the sidewall and has a critical impact on device performance.
In this article, the deposition of PECVD-SiO2 dielectric films was initiated with an RF power of 20 W for a thickness of 10 nm and then rose to an RF power of 50 W for the remaining thickness. With a low RF power of 20 W in the initial stage, the passivation-induced plasma damage to the device sidewall was minimized. With an optimal RF power of 50 W in the following stage, a high dielectric film quality with low leakage current density and high breakdown electric fields can be maintained for metal pad [10–13] and field plate [17–20] applications. Except for the PECVD passivation process, the epitaxial growth and device fabrication remained the same as in our previous work [9].
3. Results and discussions
Figure 2(a) plots the reverse biased J–V characteristics of a 40-µm-diameter device. In dark conditions, the device maintains a low leakage current density below 1 × 10−8 A/cm2 before 50 V. At a VBR of 113.7 V, the device presents a JBR as low as 8.72 × 10−4 A/cm2. Beyond 116 V, the device reaches a high avalanche current density over 1 × 102 A/cm2. Under 345 nm UV illumination, the photogenerated electron−hole pairs in the depletion region are separated and accelerated in opposite directions. The external quantum efficiency (EQE) is calculated by:
where nph and nelec are the number of incident photons and collected electrons, respectively. At a voltage of 100 V, the EQE reaches 100%, which is determined to be the onset point of carrier impact ionization. The avalanche gain is calculated by: where IML and IMD are the multiplied photocurrent and dark current, respectively, and IL and ID are the unmultiplied photocurrent and dark current taken at 100 V. The photocurrent density increased noticeably over the dark current background during the avalanche multiplication process, as plotted in the enlarged view of Fig. 2(a). The avalanche gain rises sharply after 100 V and exceeds 2 × 106 at 116 V.Fig. 2. (a) Reverse biased J–V characteristics of a 40-µm-diameter device. The wavelength and intensity of the UV illumination were 345 nm and 0.15 mW/cm2, respectively. Inset is an enlarged view of the dark current and photocurrent over the 106 V to 116 V reverse voltage range. (b) The benchmark of JBR versus avalanche gain of GaN-on-GaN and AlGaN-on-sapphire APDs. The 40-µm-diameter device presents a high avalanche gain and the lowest JBR.
The benchmarks of the JBR versus avalanche gain of GaN-on-GaN and AlGaN-on-sapphire APDs are presented in Fig. 2(b). The device achieves a high avalanche gain, which is two orders of magnitude higher than that of the AlGaN-on-Sapphire devices [8,10], and comparable to our previous work [9] and that of the GaN-on-GaN device [7]. Simultaneously, the device presents the lowest JBR, which is more than one order of magnitude lower than that of the AlGaN-on-Sapphire devices [8,10], and only half that of our previous work [9] and that of the GaN-on-GaN device [7].
Figure 3 presents the statistical analysis for the avalanche breakdown behaviors of the devices in dark conditions. The multiple reverse biased J–V scans for various mesa diameters of 20–80 µm are shown in Fig. 3(a)–(d). The box plots of the VBR and JBR for the corresponding devices are displayed in Fig. 3(e) and (f), respectively. The VBR is stable at around 113.4 V, with no premature or soft breakdown observed. The JBR remain constant at approximately 8.8 × 10−4 A/cm2, without noticeable dependency on mesa sizes. The identical JBR distributions indicate that the leakage current at the VBR mainly originates from the bulk of the device. We can conclude that the surface leakage current, which was comparable to the bulk leakage current as in our previous work [9], is effectively suppressed owing to the reduced sidewall damage in the improved passivation process.
Fig. 3. Multiple reverse biased J–V scans for devices with mesa diameters of (a) 20 µm, (b) 40 µm, (c) 60 µm, and (d) 80 µm in dark conditions. The scans for each mesa diameter consist of 10 devices. Box plots of the (e) VBR and (f) JBR for the corresponding devices.
Figure 4 exhibits the microscope images of EL in forward turn-on and reverse breakdown conditions at current levels of 1–10 mA. A 60-µm-diameter device was employed for clear observation. The edges of the mesa and p-type electrode are illustrated with red and yellow dashed lines, respectively. In forward turn-on condition, EL originates from the recombination of electron-hole pairs during the current injection process [21–23]. The luminescence is uniformly distributed in the device center at various current levels. In reverse breakdown condition, EL originates from the recombination of impact ionized carriers during the avalanche multiplication process [24–26]. At a current level of 1 mA, a bright spot appears in the lower left of the electrode, which marks active avalanche multiplication. As the avalanche current increases to 2 mA, more bright spots appear, which suggests the lateral extension of avalanche multiplication. With a further increase in the avalanche current, the bright spots are uniformly distributed all over the device center. The edge breakdown luminescence that focused along the device periphery, as displayed in our previous work [9], is not observed. We can conclude that the avalanche multiplication process occurs in the body and that the edge breakdown is sufficiently eliminated. This phenomenon further confirms the conclusion from Fig. 3(f) that the avalanche current at the VBR mainly originates from the bulk of the device. Therefore, the bulk EL distributions suggest not only uniform avalanche behaviors but also pure bulk leakage current distributions at the VBR.
Fig. 4. EL images of a 60-µm-diameter device in forward turn-on and reverse breakdown conditions at current levels of 1–10 mA. The edges of the mesa and p-type electrode are illustrated with red and yellow dashed lines, respectively.
Figure 5 plots the corresponding emission spectra of the EL. The luminescence intensity increases overall with increasing current. In forward turn-on conditions, as shown in Fig. 5(a), blue luminescence with a peak intensity of approximately 410 nm was observed in the spectrum. The wavelength corresponds to a bandgap of 3.0 eV, which is below the bandgaps of GaN and Al0.1Ga0.9N. This may be attributed to the Mg-related shallow acceptor levels [27–29], which act as recombination centers. Yellow luminescence with a peak intensity of approximately 560 nm was also observed, which may be related to the carbon-related deep acceptor levels [30–32]. The spectrum characteristics indicate the existence of traps and defects throughout the epitaxial films, which remains a challenge for Al0.1Ga0.9N APDs. Under reverse breakdown conditions, as shown in Fig. 5(b), the luminescence is mainly focused in the blue range. With increasing avalanche current, the peak emission shifts slightly toward larger wavelengths due to the increased junction temperature [33]. The yellow luminescence was not quite obvious, which may be suppressed by the high electric field. The luminescence intensity in the reverse breakdown condition is more than one order of magnitude lower than that in the forward turn-on condition. This indicates that most of the electron-hole pairs generated in the avalanche multiplication process are quickly drawn away by the high electric field before recombination.
Fig. 5. Emission spectra of the EL in (a) forward turn-on and (b) reverse breakdown conditions at current levels of 1–10 mA. The peak intensities of the blue and yellow luminescence are at wavelengths of approximately 410 nm and 560 nm, respectively.
4. Conclusion
In summary, we reported sapphire-based Al0.1Ga0.9N p-i-n UV APDs with the employment of a two-step deposition method to minimize passivation-induced plasma damage while maintaining high dielectric film quality. The fabricated devices demonstrated excellent comprehensive performance with stable VBR around 113.4 V, low JBR below 9 × 10−4 A/cm2 and a high avalanche gain over 2 × 106. Most importantly, the surface leakage current at the VBR is effectively suppressed, and the edge breakdown during the avalanche is sufficiently eliminated. Pure bulk leakage current distributions and uniform body avalanche breakdown behaviors are observed for the first time in AlGaN APDs.
Funding
National Key Research and Development Program of China (2022YFB3604902); National Natural Science Foundation of China (U2141241); Key R&D Project of Jiangsu (BE2021026).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding authors upon reasonable request.
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