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Abstract
InAs/GaSb type-II superlattice materials have attracted in the field of infrared detection due to their high quality, uniformity and stability. The performance of InAs/GaSb type-II superlattice detector is limited by dark noise and light response. This work reports a gradual funnel photon trapping (GFPT) structure enabling the light trapping in the T2SL detector absorption area. The GFPT detector exhibits an efficient broadband responsivity enhancement of 30% and a darker current noise reduction of 3 times. It has excellent passivated by atomic layer deposition and achieves a high detectivity of 1.51 × 1011 cm Hz1/2 at 78 K.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Infrared photodetectors have been widely applied in object identification, cancer detection and so on [1]. At present, Mid-infrared photosensitive materials mainly include HgCdTe alloys and III-V type-II superlattice (T2SL). The HgCdTe alloys have some advantages in device performance, but the low yield and high costs limit their availability. The InAs/GaSb T2SL materials are a possible alternative to the HgCdTe because of their smaller tunneling currents and suppressed Auger recombination mechanism but their optical absorption coefficient is lower than the HgCdTe alloys [2–6]. Recently, employing microstructure for light absorption enhancement of materials has been studied, including dielectric, surface metallic and three-dimensional plasma cavity structures [7–16]. The surface metallic microstructure increases the material optical loss and the three-dimensional plasma cavity microstructure requires modification of the material epitaxial structure.
In contrast, the dielectric structure is a reliable solution to have enhanced the material’s effective light absorption and photoelectric conversion [17–22]. J. Budhu et al. coupled the T2SL detector into a Si dielectric antenna to enhanced material light absorption. C. Y. ilGuo et al. used mid-infrared T2SL materials to design a dielectric resonant structure that achieved response enhancement in the visible spectrum. Subsequently, C. Guo et al. proposed a dielectric photon trap T2SL detector with M barrier that showed a responsivity of 0.86 A/W in the near-infrared. However, the photon trapping structure has formed crystalline defects and dangling bonds in the detector, which increases the leakage dark current of the device. Furthermore, only photons near the depletion region have converted to photocurrent in T2SL materials such as PIN and PMIN. It is a challenge that fabricates a suitable photon trapping structure on the detector. Therefore, a new photon trapping structure in the mid-infrared T2SL detector is required to have better light trapping performance and more convenient passivation to reduce the dark current.
In this work, a gradual funnel photon trapping (GFPT) structure in a mid-infrared T2SL detector was presented. The GFPT detector had two-dimensional periodic gradual funnel holes for efficient light trapping. By using atomic layer deposition surface passivation in a GFPT detector, the detectivity reached 1.51 × 1011 cm Hz1/2 at 78 K. It is demonstrated that the detector performance increased due to the GFPT structure, which achieved a broadband light absorption enhancement of 30% and enables a darker current noise reduction of 3 times.
2. Materials and methods
The InAs/GaSb T2SL materials were grown in a solid source Gen II molecular beam epitaxy (MBE) reactor on an n-type GaSb (001) substrate, with an n-i-p structure where the i-region thickness of 1.081 µm. The T2SL materials were characterized by high-resolution X-ray diffraction (HXRD), Scanning Transmission Electron Microscopy (STEM) and Atomic Force Microscopy (AFM). The devices comprise an n-i-p mesa structure (400 µm×400 µm) on top of an n-type GaSb substrate as show in Fig. 1. To compare GFPT structured devices with reference devices, a portion of the devices on the same wafer have GFPT structures etched into the mesa surface. The GFPT structure (average diameters of 2.6 µm and periods of 4 µm) was etched through the mesa to a depth sufficient to reach the i-layer. And the GFPT structure has gradual funnel sidewalls (∼45°), as shown in Fig. 1(d). All devices used ALD deposition of 70 nm thick Al2O3 as a passivation layer. The details of the wafer epitaxial structure and the GFPT detector fabrication process are in Supplement 1 (S1).
Fig. 1. (a) Schematic of T2SL photodetector with the GFPT structure. (b) Optical micrographs picture of the detector. (c) Scanning electron microscope picture of the GFPT structures. (d) AFM characterization of the GFPT structures sidewalls.
The devices were inserted into the vacuum chamber with probes (Lakeshore Tabletop Cryogenic Probe Station Model PS 100) and connected to a Keysight semiconductor analyzer (B1500A) for electrical measurements under different ambient temperatures. The background temperature of the detector (78 K-300 K) was adjusted with liquid nitrogen and a hot plate (Lakeshore Tabletop Cryogenic Probe Station). A 600 K blackbody was placed at a distance of 17.5 cm to illuminate the device, with an aperture diameter of 1.59 cm. The spectrally resolved of the device was measured by Spotlight 400 Fourier spectrometer with a beam size of 200 µm×200 µm and scanning speed of 2 cm-1. The transmission and reflection spectrum of the device was measured with air and gold as background, respectively. The absorption spectrum of the devices was calculated by 1-R-T.
The finite element method (FEM) simulation of the GFPT detector by used the RF Module of COMSOL [23]. Periodic boundary conditions in the lateral dimensions and a perfectly matched layer were set in the vertical interface. The effective absorption of the devices was calculated by integrate optical loss in the i-region.
3. Results and discussion
3.1 Material interface characterization
The III-V interface and surface roughness greatly influence T2SL material quality in Fig. 2. Figure 2(b) shows a 10-period T2SL fragment taken by STEM-HAADF along the material growth direction. The InAs layer (2.7 nm) and GaSb layer (2.1 nm) can be distinguished by the contrast that the InAs layer is brighter than the GaSb layer. Figure 2(c) shows the elemental distribution of the section in the material growth direction, showing a period of about 4.9 nm, slightly higher than the theoretical value (4.8 nm). The symmetric three-order satellite peaks of the superlattice by HXRD measurement as shown in Fig. 2(d). Figure 2(e) shows the surface morphologies measured by AFM with 5 × 5 µm2 scan areas and the root means square roughness (RMS) is 0.26 nm. The STEM, XRD and AFM data suggested the superior quality of our T2SL materials.
Fig. 2. (a) Schematic of the InAs/GaSb T2SL atomic interface. (b) STEM picture of the T2SL multilayer structure. (c) Energy dispersive spectrometer element distribution at the interface. (d) HXRD ω-2θ scan. (e) 5 × 5 µm2 AFM picture.
3.2 Simulation results
Figure 3 (a) shows the light modes in the T2SL detector with reference and GFPT structure. The GFPT detector has stronger light absorption because of multiple light modes. There are usually three light modes to determine the GFPT detector optical properties: degenerate fundamental Bloch mode, guided resonance mode and channel Bloch mode [24–28]. The fundamental Bloch mode exists in the entire spectrum and predominates in the long-wavelength range; the guided resonance mode exists in the air and dielectric composite cavities, propagating parallel to the layer; the channel Bloch mode is concentrated in the trap structure, predominates in the short wavelength. In the GFPT detector, a collective light mode is formed by the superposition of the above light modes. Figure 3(a) shows the simulated electric field of the detectors with reference and GFPT. The GFPT detector i-region had a significant light absorption effect than the reference detector.
Fig. 3. (a) Resonance maps of the electric field of the detectors with reference and GFPT, in a plane parallel to the incident light at the center of the unit cell. The schematics show the scatter of incident light. (b) Simulated absorption in the GFPT detector i-region as a function of duty cycle (%). (c)Sidewall tilt angle dependent simulated absorption within the detector i-region of GFPT. (d) The simulated absorption spectrum for the detectors i-region with reference and GFPT. The GFPT structure duty cycle and tilt angles are 18% and 45°, respectively.
To investigate the relationship between the GFPT structure parameter and the light modes, we introduced the parameter of duty cycle (the ratio of the GFPT hole and the T2SL material). In the Fig. 3(b), the light absorption enhancement in the full spectrum has proved that the fundamental Bloch mode is the main reason for the light absorption enhancement of the GFPT structure. A larger duty cycle will result less efficiency of the GFPT structure due to the reduced material volume. The tapered sidewall parameter affects the GFPT device light modes, and we analyzed the GFPT sidewalls angle result shown in Fig. 3(c). When the sidewall tilt angle of the GFPT structure was less than 50°, it achieves above 0.6 absorptions in the detector i-region at 2∼3.5 µm. The channel Bloch mode can further improve the light absorption of the structure in the short wavelength. Moreover, the vertical sidewall detector had lower light absorption than the gradient funnel sidewall detector. This is because the gradient funnel sidewall will generate more leaked resonance modes and made greater efficiently light coupled to the detector. The light mode of the GFPT device can be modulated by designing the structural parameters. Figure 3(d) show the GFPT structure that can improve device light absorption, a 20% duty cycle and a below 50° angle.
3.3 Experimental results
Figure 4(a) shows the Fourier light absorption spectrum of detectors with reference and GFPT. The GFPT device produced a collective light mode which increased 20% ∼ 40% device light absorption at 2∼5 µm. A 600K blackbody source was used to characterize the responsivity of the detectors reference and GFPT. Figure 4(b) shows the detector responsivity with reference and GFPT as a function of applied bias. The GFPT devices had greater responsivity and exhibited saturation at low bias. Figure 4(c) shows the detector photoresponse at -0.1 V to the periodic chopping of the blackbody source. The responsivity of the reference and GFPT devices were 1.01 A W−1 and 1.36 A W−1, respectively.
Fig. 4. (a) The room-temperature absorption spectrum of the detectors with reference and GFPT. (b) The responsivity of the detectors with reference and GFPT as a function of voltage. (c) Time-dependent photoresponse of the detectors with reference and GFPT when bias was -0.1 V. A 600 K blackbody is placed at a distance of 17.5 cm to illuminate the device, with an aperture diameter of 1.59 cm.
Detector noise can be described by dark current characteristics. The dark current is dominated by four mechanisms: diffusion current, generation-recombination (G-R) current, tunneling current and leakage current [29–31]. The G-R and the diffusion current are background currents affected by temperature. And the tunneling current and the surface leakage current are related to the defect. Figure 5(a) shows temperature-dependent dark current density of the detectors with GFPT structure and Al2O3 passivation. From 78 K to 300 K, the device’s dark current density increased by four orders of magnitude at -0.1 V because hot carrier generation-recombination and diffusion behaviors enhance by temperature rise.
Fig. 5. (a) Temperature-dependent dark current density with GFPT detector. (b) Dark current density, as a function of applied bias of the detectors with reference and GFPT. The device’s background temperature is 78 K. (c) The dark current density-temperatures (J-K) characteristics of the detectors with reference and GFPT.
This dark current density of the GFPT device and reference device was 2.63 × 10^-4 A/cm2 and 9.63 × 10^-4 A/cm2 at -0.1 V, respectively in Fig. 5(b). About three times reduced current for the GFPT devices compared to the reference detectors. It confirms the effectiveness of ALD deposition for passivation of the GFPT structure. The bulk reduction brought by the GFPT array can reduce the background current of the detector as show in Fig. 5(c). At 100 K, the dark current was reduced by 20% compared to the reference device. As the temperature rises, the dark current could rapidly reduce by 81% at 140 K. This phenomenon may prove that the GFPT structure is more effective in suppressing hot carrier generation-recombination.
Device detectivity D* was calculated as follows.
where RA is the responsivity, Jd is the dark current density, Rd is the dynamic resistance, and Ad is the mesa area. The GFPT detector (D*= 1.51 × 1011 cm Hz1/2) was better than the reference detector (D*= 5.41 × 1010 cm Hz1/2) at 78 K.4. Conclusion
We have demonstrated a T2SL detector with the GFPT microstructure that can significantly improve detector performance by a broad-spectrum absorption enhanced of 30% and a current suppressed of 3 times. The blackbody detectivity of this GHPT detector reached 1.51 × 1011 cm Hz1/2at 78K. This work reveals that the GHPT microstructure has collective light modes to enhance the light-detector interaction. Dark current noise generated by fabricating the GFPT structures on the device surface using wet etching can be effectively suppressed by atomic layer deposition passivation. It provides a microstructure to improve light absorption and reduces device dark current noise, further enhancing the detectivity.
Funding
CAS “Light of West China” Program (2020); National Natural Science Foundation of China (62074021); Natural Science Foundation Project of CQ CSTC (cstc2020jcyj-msxmX0822); Chongqing Talents Innovation and Entrepreneur Leaders Project (CQYC201903020); Key Technologies Research and Development Program (2018YFA0209102, 2018YFA0209104, 2019YFA070104, 2019YFA0705203); Major Program of the National Natural Science Foundation of China (61274013, 61790581, 62004189); Chinese Aeronautical Establishment (20182436004); Key Research Program of the Chinese Academy of Sciences (XDPB22); Research Foundation for Advanced Talents of the Chinese Academy of Sciences (E27RBB03).
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.
Supplemental document
See Supplement 1 for supporting content.
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