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Abstract
Dry-etching is often utilized to shape GaN-based materials. However, it inevitably causes plenty of sidewall defects as non-radiative recombination centers and charge traps that deteriorate GaN-based device performance. In this study, the effects of dielectric films deposited by plasma-enhanced atomic layer deposition (PEALD) and plasma-enhanced chemical vapor deposition (PECVD) on GaN-based microdisk laser performance were both investigated. The results demonstrated that the PEALD-SiO2 passivation layer largely reduced the trap-state density and increased the non-radiative recombination lifetime, thus leading to the significantly decreased threshold current, notably enhanced luminescence efficiency and smaller size dependence of GaN-based microdisk lasers as compared with the PECVD-Si3N4 passivation layer.
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1. Introduction
GaN-based microdisk laser is a promising on-chip light source, combined with both advantages of highly-efficient III-nitride semiconductor materials with a direct bandgap ranging from deep ultraviolet to near-infrared and features of whispering gallery mode (WGM) microcavities with compact footprint, simple fabrication, low loss and low power consumption [1–3]. It has been widely studied in various applications, such as nonlinear optics [4], biomedical sensing [5], nanoparticle detection [6], wearable sensors [7,8], visible light communication [9], and cavity quantum electrodynamics [10]. In addition, GaN-on-Si microdisk lasers with III-nitride waveguide or conversion technology offer the potential in large-scale Si-based photonic integrated circuits and high-speed optical modulation in chip-to-chip interconnects [11–13].
For GaN-based microdisk lasers, WGMs are mainly confined at the rim of microdisk lasers through the total internal reflection of light, therefore, the quality of microcavity sidewalls has a profound effect on the microdisk laser performance, including the lasing threshold, leakage current and et al. [14–16]. However, due to the inertness of III-nitride materials, there is nearly no chemical solution that can effectively and accurately etch III-nitride materials. Thus, inductively coupled plasma (ICP) dry-etching is usually employed to pattern GaN-based microdisks, which inevitably leads to severe etching damage [17,18]. The etched surface of microdisk sidewalls is full of mass defects, such as crystallographic defects, impurities and dangling bonds, which often serve as non-radiative recombination centers and charge traps, and thus severely deteriorate the device performance of GaN-based microdisk lasers [14,19]. Such degradation effect becomes more serious with the decrease of device dimension [20]. Therefore, efficacious sidewall passivation techniques are urgently required to reduce the sidewall defects and hence enhance the device performance.
Various sidewall passivation methods, such as wet chemical treatment, sulfur passivation, dielectric passivation with various materials and deposition techniques, have been extensively studied for GaN-based light-emitting diodes (LEDs) to diminish sidewall damage and improve device efficiency [19–24]. By contrast, few literatures have reported the effects of sidewall passivation on the performance of GaN-based microdisk lasers. In this letter, we studied that the impact of SiO2 deposited by PEALD and Si3N4 deposited by PECVD as sidewall passivation layers on the device performance of GaN-based microdisk lasers, respectively. The results showed that the GaN-based microdisk laser with PEALD-SiO2 passivation layer had a lower lasing threshold, higher luminous efficiency, and smaller size dependence. Frequency-dependent capacitance-voltage (C-V) and time-resolved photoluminescence (TRPL) measurements demonstrated fewer trap states and longer non-radiative recombination lifetimes of GaN-based microdisk lasers passivated by PEALD-SiO2.
2. Experiment
GaN-based microdisk laser structure was grown on a Si (111) substrate via metal-organic chemical vapor deposition. 1-µm-thick AlN/AlGaN buffer layer was firstly deposited on the Si substrate to compensate the tensile stress and reduce threading dislocations due to the mismatches of thermal expansion coefficient and lattice constant between Si and GaN [25–27]. Subsequently, high-quality epitaxial layers were grown, including a 1.3-µm-thick undoped GaN layer, a 1.3-µm-thick n-GaN contact layer, a 1.2-µm-thick n-Al0.085Ga0.915N lower cladding layer, a 100-nm-thick In0.01Ga0.99N/GaN lower waveguide layer, three periods of In0.12Ga0.88N/In0.02Ga0.98N multiple quantum wells (MQWs), a 90-nm-thick In0.01Ga0.99N/GaN upper waveguide layer, a 20-nm-thick p-Al0.18Ga0.82N electron blocking layer (EBL), 600-nm-thick p-Al0.15Ga0.85N/GaN superlattice (SL) upper cladding layers, and a 30-nm-thick p-GaN contact layer.
Figure 1 shows the schematic diagram and scanning electron microscope (SEM) image of the GaN-based microring laser integrated with a direct-connected waveguide to realize efficient light-coupling output [13,28]. The device size is labeled as R-r µm, where R and r represent the outer and inner circle radii, respectively. In this work, microring lasers with four dimensions (50-30, 40-20, 30-10, and 20-0 µm) were fabricated together to investigate the effect of sidewall passivation on devices with various dimensions. The mesas of microring lasers were processed through standard metal deposition, photolithography, ICP etching, and inner ring isolation, which have been demonstrated detailly in our previous studies [29–31]. Then, all devices were treated in a 25% tetramethyl ammonium hydroxide solution at 85 °C for 20 min to reduce the etching damage and shape the device sidewall [32]. Subsequently, the samples were divided into two groups, one with 20-nm-thick SiO2 deposited by PEALD at 250 °C and the other with 20-nm-thick Si3N4 deposited by PECVD at 350 °C, which were labeled as PEALD-SiO2 and PECVD-Si3N4, respectively. SiO2 and Si3N4 were chosen because they have nearly no absorption of visible light and are commonly used as dielectric films for photonic and electronic devices [23,33]. Finally, n-type ohmic contact was formed by evaporating Ti/Pt/Au (50/100/100 nm) and then the GaN-based microdisk lasers with various passivation layers were formed completely.
Fig. 1. (a) Schematic diagram and (b) SEM image of the GaN-based microring laser integrated with a direct-connected waveguide.
A series of optical and electrical measurements for all samples were performed at room temperature. A spectrometer (Ocean Optics HR 4000) and an optical power meter (Thorlabs PM121D) were applied to collect electroluminescence (EL) spectra and light output power under pulse currents which were supplied by a pulse generator (Agilent 8114A) with a pulse width of 400 ns and a repetition rate of 10 kHz. And the sampling port was parallel to the waveguide and about 2 mm away from the waveguide end facet. A continuous-wave power supply (Keithley 2400) was utilized to measure the current versus voltage (I-V) curves to characterize the leakage current and the series resistance. Frequency-dependent C-V characteristics were performed to analyze the trap states by using a semiconductor parameter meter (Keithley 4200) with a frequency range from 1 MHz to 10 kHz. TRPL spectra were collected by a self-built spectra measurement system based on time-correlated single photon counting (TCSPC) technology, where the excitation source is a 375 nm pulse laser with a pulse width of 60 ps.
3. Results and discussion
The effects of PEALD-SiO2 and PECVD-Si3N4 passivation layers on optical characteristics of GaN-based microdisk lasers are demonstrated under various currents at room temperature in Fig. 2. As shown in Fig. 2(a) for the GaN-based microdisk laser passivated by PEALD-SiO2, the spontaneous emission was dominant under a low current injection level (less than 100 mA). As the current increased to 120 mA, a small peak appeared on the spontaneous emission spectrum, and the full width at half maxima (FWHM) of the spectrum decreased from 17.3 to 13.7 nm. When the injection current further raised to 150 mA, the peak intensity rapidly strengthened and the FWHM narrowed down to 1.7 nm, suggesting that the stimulated emission became predominant. It could be seen in Fig. 2(b), EL spectra of the microdisk laser with PECVD-Si3N4 passivation layer were similar to those of the microdisk laser with PEALD-SiO2 passivation layer, but showed a higher threshold current (300 mA). The FWHMs of EL spectra for both devices were plotted in Fig. 2(c), the turning points, which indicate the threshold current, could be seen clearly. At the same time, it could obviously find that the PECVD-Si3N4 passivated microdisk laser had a larger FWHM before reaching the threshold current as compared with the PEALD-SiO2 passivated one, which manifested that more defect-related energy levels were involved in the device. Figure 2(d) shows the light output power versus current curves for both devices. For the PEALD-SiO2 passivated microdisk laser, when the injection current increased to 150 mA, the light output power raised slowly with the current to 1.4 mW. As the current further increased, the device achieved lasing, and the light output power increased sharply and linearly, and was up to 8.9 mW at 444 mA. There was a similar variation trend for the PECVD-Si3N4 passivated device, the light output power varied slowly with the current at the spontaneous emission stage, and it increased more rapidly after the current reached 300 mA. All these observations demonstrated that the room-temperature electrically pumped lasing for both devices was achieved.
Fig. 2. EL spectra of the microdisk laser with (a) PEALD-SiO2 and (b) PECVD-Si3N4 passivation layers under various pulse injection currents, respectively. (c) FWHM of the EL spectrum and (d) light output power of the above microdisk lasers as a function of currents. The dimension of both devices is 20-0 µm.
However, the GaN-based microdisk laser with PEALD-SiO2 passivation layer had a lower lasing threshold current (150 mA) as compared with the PECVD-Si3N4 passivated microdisk laser (300 mA). And the light output power of the PEALD-SiO2 passivated microdisk laser was also much higher than that of the PECVD-Si3N4 passivated one, as shown in Fig. 2(d). This indicated that the radiative recombination of the PECVD-Si3N4 passivated microdisk laser was severely suppressed. Through further simulation, the PEALD-SiO2 passivated microdisk laser and the PECVD-Si3N4 passivated one had almost equal reflectivity at the lasing wavelength while varying the incident angle, which indicated that the difference in the refractive index of the two materials was not the main reason for the device performance. Since the other parts except the sidewall passivation are all the same, the difference in the threshold current and light output power could be attributed to the sidewall defects caused by dry-etching damage. These defects as non-radiative recombination centers increase the probability of non-radiative recombination and hence reduce the radiative recombination intensity, thus making the device’s optical efficiency degrade. Therefore, a much lower threshold current and higher luminescence efficiency for the microdisk laser with PEALD-SiO2 passivation layer suggested that PEALD-SiO2 passivated devices had fewer sidewall defects and thence weaker non-radiative recombination.
The effect of sidewall defects on device performance becomes more significant as the dimension decreases. Figure 3 shows the dependence of lasing threshold current density on the dimensions of GaN-based microring lasers with PEALD-SiO2 and PECVD-Si3N4 passivation layers. The lasing threshold current density of microring lasers with both passivation layers increased as the device size decreased. This is mainly due to the increase in the ratio of sidewall area to carrier injection area, which enhances the probability of non-radiative recombination on the sidewall surface, resulting in an increase in lasing threshold. Such a phenomenon has also been reported in GaN-based micro-LEDs [34]. However, it could be observed that the increasing trend of PECVD-Si3N4 passivated devices was more apparent. The lasing threshold current density of 20-0 µm microring lasers passivated by PEALD-SiO2 and PECVD-Si3N4 increased by 178% and 245%, respectively, as compared with that of 50-30 µm microring lasers. This result further manifested that the GaN-based microring laser passivated by PEALD-SiO2 had fewer sidewall defects, and thus had smaller size dependence.
Fig. 3. Dependence of lasing threshold current density on the dimensions of GaN-based microring lasers with both different sidewall passivation layers.
In order to confirm visually the influence of passivation layers on optical performance of GaN-based microdisk lasers, room-temperature TRPL measurements were implemented for both devices, as shown in Fig. 4. The TRPL plots are fitted via a two-exponential decay model with two carrier lifetimes, ${\tau _{PL,initial}}$ and ${\tau _{PL,\; final}}$ [35,36]. Both radiative and non-radiative recombination determine the lifetime of the first stage (${\tau _{PL,initial}}$), of which the former one is dominant. The lifetime at the final stage (${\tau _{PL,\; final}}$) is essentially determined by non-radiative recombination. It could be observed that the attenuation curves of microdisk lasers with PEALD-SiO2 and PECVD-Si3N4 passivation layers almost coincided at the initial stage, but showed an obvious difference at the final stage. The microdisk laser with PEALD-SiO2 passivation layer had a longer ${\tau _{PL,\; final}}$ value of 33.5 ns than that with PECVD-Si3N4 passivation layer of 21.7 ns, which specified that the non-radiative recombination rate of the PEALD-SiO2 passivated device was much lower than that of the PECVD-Si3N4 passivated device.
All the above results demonstrated that PEALD-SiO2 passivated devices had fewer sidewall defects compared to PECVD-Si3N4 passivated devices, thence, the dynamic capacitance dispersion measurement was accomplished for both devices to quantify sidewall trap states after dielectric passivation. These sidewall defects acting as different trap energy levels are distributed in the band gap, and the energy separation from the conduction-band edge determines the trap time constant (τtrap) [37,38]. The measurement signal period (τm) is smaller than τtrap at high frequency and larger than τtrap at low frequency. Accordingly, at high frequency, the traps cannot follow the alternating current (AC) signal and consequently have less contribution to the capacitance at the same voltage [39]. Figure 5 shows the frequency-dependent C-V curves for the microdisk laser with PEALD-SiO2 and PECVD-Si3N4 passivation layers. The significant frequency dispersion of the C-V curves of the microring laser with PECVD-Si3N4 passivation layer was observed, which indicated that there were much more traps responding to the AC signal when the frequency swinging from 1 MHz to 10 kHz, compared to the PEALD-SiO2 passivated microdisk lasers. The trap-state density (Dit) was evaluated by the following equation:
where q is the fundamental electron charge, ${C_{lf}}$ and ${C_{hf}}$ are the capacitance measured at 10 kHz and 100 MHz, respectively. And the densities of the trap states were 2.7 × 1011 and 9.0 × 1011 cm-2 for the microring laser with PEALD-SiO2 and PECVD-Si3N4 passivation layers, respectively. These results suggested that the utilization of PEALD-SiO2 layer could effectively passivate the GaN-based microdisk laser sidewall and reduce sidewall defects.Fig. 5. Frequency-dependent C-V curves of 50-30 µm microring lasers with (a) PEALD-SiO2 and (b) PECVD-Si3N4 passivation layers, respectively.
Sidewall defects caused by etching damage can also act as leakage current paths increasing the current leakage, which can be reduced by dielectric sidewall passivation [40]. Therefore, the effects of PEALD-SiO2 and PECVD-Si3N4 passivation on electrical properties of GaN-based microdisk lasers were characterized by measuring the I-V curves, as shown in Fig. 6. It could be seen that, in the bias voltage range of -5∼0 V, the reverse current of the PECVD-Si3N4 passivated microdisk laser was slightly larger than that of the PEALD-SiO2 passivated one. At the forward bias voltage range of 0∼3 V, especially larger than 1.4 V, the PECVD-Si3N4 passivated device performed a much higher forward leakage current as compared with the PEALD-SiO2 passivated one. This manifested that the PECVD-SiO2 passivated device had a small amount of leakage current resulting from defects. When the forward bias voltage was greater than 3 V, the GaN-based microdisk laser with PEALD-SiO2 passivation layer exhibited better forward characteristics, especially in the voltage of 4∼7 V as shown in the inset of Fig. 6. It implied that the PEALD-SiO2 passivation layer grown on a lower temperature of about 250 °C could not only passivate parasitic current paths, but also avoid the negative effect on ohmic contact as compared with 350 °C PECVD-Si3N4 passivation layer, thus enhancing the electrical efficiency of GaN-based microdisk lasers [23,41].
Fig. 6. I-V characteristics of both microdisk devices (20-0 µm) in semi-log scale, and the inset showed the I-V curves in linear scale.
4. Conclusions
In summary, the effects of PEALD-SiO2 and PECVD-Si3N4 passivation layers on GaN-based microdisk laser performance were characterized by a series of optical and electrical measurements at room temperature. The results showed that GaN-based microdisk lasers with PEALD-SiO2 passivation layer had a lower lasing threshold, higher luminescence efficiency, and smaller size dependence, as compared with PECVD-Si3N4 passivated microdisk lasers. This was mainly because the PEALD-SiO2 layer could effectively passivate sidewall defects, thus resulting in a lower sidewall trap-state density and a lower non-radiative recombination rate, which were confirmed by frequency-dependent C-V and TRPL measurements. In addition, the I-V curves of both devices demonstrated that the PEALD-SiO2 passivated GaN-based microdisk laser had better electrical characteristics, especially in the forward voltage.
Funding
National Key Research and Development Program of China (2021YFB3601600, 2022YFB2802801, 2022YFB3604300, 2022YFB3604802); Guangdong Province Key-Area Research and Development Program (2019B090904002, 2019B090909004, 2019B090917005, 2020B010174004); National Natural Science Foundation of China (62074158, 62174174, 62274177, 62275263); Jiangxi Science and Technology Program (20212BDH80026); Strategic Priority Research Program of CAS (XDB43000000, XDB43020200); Key Research Program of Frontier Science, Chinese Academy of Sciences (ZDBS-LY-JSC040); Bureau of International Cooperation, Chinese Academy of Sciences (121E32KYSB20210002); Key R&D Program of Jiangsu Province (BE2020004-2, BE2021051); Natural Science Foundation of Jiangsu Province (BK20220291); Science and Technology Program of Suzhou (SJC2021002, SYC2022089); Basic and Applied Basic Research Foundation of Guangdong Province (2021A1515110325, 2022A1515110482); Scientific and Technological Research Council of Turkey (2568); CAS Bilateral Cooperation Program (121N784).
Acknowledgment
We are thankful for the technical support from Nano Fabrication Facility, Platform for Characterization and Test, and Nano-X of SINANO, CAS.
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 authors upon reasonable request.
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