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Abstract
This letter reports the influence of material quality and device processing on the performance of AlGaN-based Schottky barrier deep ultraviolet photodetectors grown on Si substrates. The thermal annealing can significantly improve Schottky barrier height and wet chemical etching can effectively remove etching damage. Meanwhile, the decrease of threading dislocation density and the pit size, especially the later, can substantially suppress reverse leakage. As a result, the reverse leakage current density of the as-fabricated deep UV photodetector was reduced down to 3×10−8 A/cm2. Furthermore, the responsivity of the deep UV photodetectors was greatly improved by reducing the point defect concentration.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
AlGaN-based semiconductors (bandgap ranges from 3.4 to 6.2 eV) are ideal materials to make ultraviolet (UV) photodetectors (PDs) [1]. AlGaN with a high Al content of 50% and above can be used to fabricate solar-blind (cutoff wavelength ≤ 280 nm) deep UV PDs, which have a wide range of applications, such as flame detection, biological effects detection, early missile plume detection and pollution monitoring [2–4].
AlGaN-based deep UV PDs are usually grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) [4–6]. However, the coefficient of thermal expansion (CTE) mismatch between sapphire and Si read-out integrated circuit (ROIC) often induces a large thermal stress during the thermal cycling, which often causes reliability issues [6,7]. In contrast, large diameter and low costs Si substrate has no CTE mismatch with the Si-based ROIC, and can be used for the epitaxial growth of AlGaN-based deep UV PDs [6].
There are few studies on AlGaN-based deep UV PDs grown on Si substrate in the previous reports, because the huge CTE mismatch and the large lattice mismatch between AlGaN and Si often cause a micro-crack network and a high density of threading dislocations (TDs) in the AlGaN film, respectively [8–11]. Additionally, the epitaxial growth of AlGaN by MOCVD is often far away from the equilibrium state, which usually induce point defects in the AlGaN epitaxial materials [12–14]. TD density and point defect concentration are assumed to be two vital parameters about crystal quality that affect the leakage current and responsivity of UV PDs [12]. Moreover, the device fabrication processing, including the etching damages of mesa sidewall induced by inductively coupled plasma (ICP) etching and the Schottky contact with AlGaN, often induce interface states [15,16], and hence also affect the device performance [17,18].
In this letter, Al0.5Ga0.5N-based Schottky barrier deep UV PDs grown on Si substrate were successfully fabricated. In particular, the wet chemical etching of the mesa sidewalls by tetramethylammonium hydroxide (TMAH) and the thermal annealing of the Schottky contact were studied to reduce the reverse leakage current. Furthermore, the influence of TDs and point defects in the AlGaN material on the device performance of the as-fabricated deep UV PDs was also studied in detail.
2. Material and methods
The Al0.5Ga0.5N-based Schottky barrier deep UV PDs were all grown on Si(111) substrates by MOCVD. Ammonia (NH3), trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as N, Ga and Al precursors, respectively. Sample A was the reference sample and consisted of an AlN/AlGaN multilayer buffer, a 1000-nm-thick unintentionally doped Al0.5Ga0.5N (u-Al0.5Ga0.5N) layer, a 700-nm-thick n-Al0.5Ga0.5N layer with a Si doping of 3×1018 cm−3, and a 300-nm-thick u-Al0.5Ga0.5N layer. For sample B, an AlN/AlGaN superlattice transitional buffer, replacing the Al-composition step down graded AlGaN multilayer buffer, was inserted between AlN and Al0.5Ga0.5N to reduce the TD density. For sample C, the V/III ratio for the growth of Al0.5Ga0.5N layer was increased from 4000 to 8000 and the growth pressure was increased from 60 to 100 mbar compared with sample B, in order to reduce the point defects. The Al-composition of AlGaN thick layer was 50%, 50%, and 46% for samples A, B and C, respectively. The growth details can be found elsewhere [19].
All the samples were fabricated by using photolithography technology and ICP etching. Ti/Al/Ti/Au (20/150/50/100 nm) metal stack was deposited on the n-Al0.5Ga0.5N layer and annealed under N2 at 1000 °C for 60 s to form ohmic contact. Afterwards, the translucent Ni/Au (5/5 nm) metal stack was deposited on the mesa to form Schottky contact with the underlying u-Al0.5Ga0.5N. The ohmic and Schottky contact-metals were both deposited by magnetron sputtering. To remove the etching damage of the mesa sidewall, TMAH (25 wt.% solution) wet etching was implemented. And rapid thermal annealing (RTA) of the Schottky contact under N2 was applied to reduce the leakage current. Figures 1(a) and 1(b) show the buffer layers structure, the device’s schematic structure and the top-view image of the circular Schottky barrier deep UV PDs with a diameter of 1000 µm, respectively.
The material quality of these samples was characterized by atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and cathodoluminescence (CL), respectively. The leakage current and responsivity of the as-fabricated deep UV PDs were tested at room temperature by using the Keithley 4200-SCS/F parameter analyzer, and a home-made spectral response measurement system composed of a Xenon lamp, monochromator, chopper and lock-in amplifier.
Fig. 1. (a) Cross-sectional sketch and (b) top-view image of the as-fabricated Schottky barrier deep UV PD.
3. Results and discussion
It is known that Schottky barrier PDs operate under a reverse bias, and the reverse leakage current is an important parameter related to the UV/visible rejection ratio and noise equivalent power [17,18]. In order to reduce the reverse leakage current, thermal annealing was usually used to increase the Schottky barrier height (SBH), which is quite effective according to thermionic emission (TE) and thermionic-field emission (TFE) models [22,23].
Sample B was annealed under N2 at various temperatures for 5 min and at 500 °C for various durations, as shown in Figs. 2(a) and 2(b). The reverse leakage current density of the as-fabricated devices was measured and the SBH were extracted from the current-voltage (I-V) curves for each annealing condition. The colored dots are experimental data, and the hollow square dots with error bars represent the mean value and standard deviation of the experimental data, respectively. It can be seen clearly that the SBH increases with increasing annealing temperature from 450 to 600 °C, and the reverse leakage current under the bias of −5 V decreases accordingly. It can be concluded that annealing at 600 °C for 5 min could significantly reduce the reverse leakage current by about two orders of magnitude, as compared with the as-deposited Schottky contact, and the SBH was increased from around 0.83 to 1.4 eV. The reverse leakage current could be reduced slightly as the annealing duration increased from 1 to 5 min as shown in Fig. 2(b).
Fig. 2. The leakage current density (Jleak) under a reverse bias of −5 V and the SBH (ϕb) of sample B annealed at various temperatures for 5 min (a), and at 500 °C for various durations (b). The I-V curves of the sample B after various treatments (c).
It is known that the interface states and chemical reactions between the contact-metal and semiconductor play important roles in the electrical properties [16]. Ni/Au metal stack may react with AlGaN, and new compounds with higher work function would be formed at the interface. Besides, some point defects like nitrogen and gallium vacancies (VN, VGa) would be removed after thermal annealing, which could also increase the SBH [24]. In addition, according to previous reports, oxygen impurities may act as donors exist in AlGaN layer and form trap levels. During thermal annealing, the oxygen atoms near the surface might diffuse into the contact-metal and form metal-oxide compound, which can reduce the concentration of trap states in the AlGaN layer [25]. All these effects may contribute to the increasement of SBH and the reduction of leakage current.
Figure 2(c) shows that TMAH wet etching of the mesa sidewalls could reduce the leakage current density by about one order of magnitude at the reverse bias of −5 V. ICP dry etching usually induces roughness and etching damage at the sidewall of the devices, which may induce large quantities of defects and then form leakage paths, and hence induce large leakage current under a reverse bias [15]. TMAH wet etching can effectively remove the etching damage at the mesa sidewalls [20,21], and can improve the performance of GaN-based devices [26,27]. With the combination of thermal annealing of Schottky contact and TMAH wet etching of the mesa sidewalls, the leakage current density of the as-fabricated AlGaN deep UV PDs could be reduced down to 3×10−8 A/cm2 under a reverse bias of −5 V.
HRXRD was used to evaluate crystal quality of the AlGaN material grown on Si. As shown in Table 1, by using an AlN/AlGaN superlattice buffer instead of a multilayer buffer, the full width at half maximum (FWHM) of the double crystal X-ray rocking curve (DCXRC) of the (10$\bar{1}$2) plane was decreased from 699 to 628 arcsec, and the estimated dislocation density was reduced from 4.8×109 to 3.6×109 cm−2. Figure 3 shows the AFM images of samples A, B and C, respectively. All the samples showed a quite smooth surface with a root-mean-square roughness of about 0.13 nm. By counting the dark pits on the AFM images, the TD densities (TDDs) of samples A, B and C was estimated and they were consistent with the results obtained from the DCXRCs, as shown in Table 1.
Table 1. The FWHMs of DCXRCs of samples A, B and C
The room temperature CL spectra were used to characterize the concentration and energy level of the point defects for these three samples. As shown in Fig. 4, the defect-related luminescence peaks of all the samples were around 420 nm, which is ascribed to the optical transition from native donor to the carbon related deep acceptors [28–30]. Compared with sample A, sample B with a reduced TD density had a lower defect-related emission. For sample C with an increased V/III ratio for the AlGaN growth, the defect-related emission was significantly suppressed, and the ratio of the integral intensity of defect-related luminescence (DL) to that of the near bandgap emission (NBE) was also greatly decreased. This indicated that the concentration of point defects in sample C was dramatically reduced, as compared with those in samples A and B.
Fig. 4. Normalized CL spectra of samples A, B and C at room temperature. The inset is the DL/NBE ratio of the samples
To investigate the influences of TDs and point defects in AlGaN material on the device performance, the leakage current and spectral response curves of samples A, B and C were measured. All the samples were treated by TMAH wet etching and Schottky contact annealing at 600 °C for 5 min. As shown in Fig. 5(a), the reverse leakage current of sample A with a higher TDD was much larger than that of samples B and C with a reduced TDD (Table 1), which may indicate that TDs significantly influence the reverse leakage current. The huge leakage current might be attribute to the combined effect of TDD and pit size of the terminal of TDs, especially the later. As shown in Fig. 3, It can be observed that sample A has numbers of larger dark pits with a diameter of around 40∼50 nm and with a density of about 3×108 cm−2, while most of the dark pits in the AFM images of samples B and C were smaller than 20 nm. Some researchers have studied the reverse leakage current through TDs and the effect of pit size [31–33]. They reported that the TDs contribute to electrical leakage, especially the open-core dislocations, which appear as larger and deeper dark pits than other TDs under AFM (Fig. 3), could induce huge leakage current. According to the literature, those open-core dislocations have inner surface consisting of (10$\bar{1}$0) plane, which is more likely to absorb oxygen atoms than (0001) plane. As discussed before, oxygen impurities act as donors in the u-AlGaN layer, and would form trap states that the carriers could leak through [34]. The open-core-like TDs of sample A may result in the segregation of impurities and form traps, and might become the main reason of the lower SBH and huge reverse leakage current, comparing with sample B.
Fig. 5. (a) The typical I-V characteristics curves and the SBHs (inset) of three samples (b) The spectral response curves of samples A, B and C with both Schottky contact annealing and TMAH wet etching.
It should be mentioned that the reverse leakage current of sample C was not much but still higher than that of sample B, which was attributed to that the SBH of sample C was lower than that of sample B, as shown in the inset of Fig. 5(a), the SBH of Samples A, B and C are about 1.03, 1.36 and 1.09 eV respectively. With higher V/III ratio and growth pressure in the AlGaN growth, the gas phase reaction of Al was severer, which results in lower Al content in sample C, and the slightly lowered Al content reduces the bandgap and results in lower SBH.
Figure 5(b) shows the spectral response curves of samples A, B and C. Sample B with a lower TDD had a higher responsivity than sample A, because the photon-generated electron-hole pairs may recombine non-radiatively through TDs before being separated for electrical current [35]. For sample C with a low concentration of point defects in the AlGaN layer, the responsivity was further substantially improved than that of samples A and B. In fact, the point defects, such as unintentionally incorporated carbon impurities, could form deep-level traps in the u-AlGaN layer. And those deep traps may act as recombination centers, and capture the photon-generated electron-hole pairs, resulting in a decrease of responsivity [12]. Given that the point defect concentration of samples A and B was much larger than that of sample C (Fig. 4), it can be inferred that the point defects mainly affect the responsivity of the UV PDs.
4. Conclusion
In summary, the AlGaN-based Schottky barrier deep UV PDs grown on Si substrate have been successfully fabricated, and the effect of Schottky contact annealing and TMAH wet etching were studied. It is found that thermal annealing can significantly improve the Schottky contact and increase the SBH, and wet chemical etching with TMAH solution can effectively remove the mesa sidewall damage induced by the dry etching, both of which greatly reduced the reverse leakage current of the as-fabricated deep UV PDs. By decreasing the TDD and the size of the pits induced by the open-core-like TDs, the reverse leakage current was much suppressed. As a result, the reverse leakage current density of the as-fabricated deep UV photodetector was reduced down to 3×10−8 A/cm2 under a reverse bias of −5 V. Moreover, the responsivity of the deep UV photodetectors was greatly enhanced by reducing the point defect concentration in the Al0.5Ga0.5N material grown on Si. The device performance of the AlGaN-based Schottky barrier deep UV PDs grown on Si can be boosted by further improving the AlGaN material quality, which is currently underway through strain engineering and defect reduction.
Funding
Key-Area R&D Program of GuangDong Province (2019B010130001, 2019B090917005, 2020B010174004); National Natural Science Foundation of China (61534007, 61775230, 61804162, 61874131); Strategic Priority Research Program of CAS (XDB43000000, XDB43020200); Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-JSC014, ZDBS-LY-JSC040); CAS Interdisciplinary Innovation Team, the Key R&D Program of Jiangsu Province (BE2017079); Natural Science Foundation of Jiangsu Province (BK20180253); Natural Science Foundation of Jiangxi Province (20181ACB20002); Suzhou Science and Technology Program (SYG201846, SYG201927); China Postdoctoral Science Foundation (2018M632408); State Key Laboratory of Reliability and Intelligence of Electrical Equipment (EERIKF2018001).
Acknowledgements
The authors are grateful for the technical support from Nano Fabrication Facility, Platform for Characterization & Test, and Nano-X of SINANO, CAS.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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