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Abstract
Photonic structures have been attracting more attention due to their ability to capture, concentrate and propagate optical energy. In this work, we propose a photon-trapping hole-array structure integrated in a nip InAsSb-GaSb heterostructure for the enhancement of the photoresponse in both near- and mid-infrared regions. The proposed symmetrical hole array can increase the photon lifetime inside the absorption layer and reduce reflection without polarization dependence. Significant enhancements in absorption and photoelectric conversion efficiency are demonstrated in dual bands for unpolarized incidence. The enhancement factors of responsivity at room temperature under zero-bias are 1.12 and 1.33 for the near- and mid-infrared, respectively, and they are increased to 1.71 and 1.79 when temperature drops to the thermoelectric cooling temperature of 220 K. Besides, such an integrated hole array also slightly improves working frequency bandwidth and response speed. This work provides a promising way for high-efficiency polarization-independent photoelectric conversion in different electromagnetic wave ranges.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical engineering on photonic structure design provides a key solution to better control the interaction between electromagnetic waves and semiconductors. It has been explored in numerous applications for different photonic structures. Currently, improving the photoelectric conversion efficiency becomes a hot research topic as it plays an important role in realizing high-performance photodetection at high operating temperature. However, the trade-off between light absorption and response speed is always the main obstacle to overcome. Using a thin absorption layer is beneficial to high speed response, but at the expense of decreased light path, which results in less absorption. To this end, various photonic structures have been studied in recent years. which include plasmonic structure [1–5], Fabry–Pérot resonant cavity [6], and heterostructure waveguide [7,8]. In particular, the photonic crystal structure is regarded as a reliable solution to enhance the photoelectric conversion efficiency without compromising speed [9], as there are optical modes with group velocity close to zero, referred to as ‘slow Bloch modes’ [10,11]. Such an exceptional phenomenon of slow light propagation can prolong light dwell time in the absorption layer. It serves as a light trap, leading to significantly enhanced absorption at slow mode resonance [12]. Besides, with the cavity structure incorporated in the optoelectronic devices, the reflection at interface between air and dielectric is reduced due to altered light-matter interaction. Such dual effects of light trapping have been proved effective in applications of mode-locked lasers [13], waveguide modulators [14,15], and solar cells [16]. As efficient light capture can significantly contribute to the improvement of photoelectric conversion efficiency, photon-trapping structures have been successfully integrated with the detectors in different operating wavelength ranges, such as visible [17,18], near infrared (NIR) [19], mid infrared (MIR) [20], and terahertz [21,22]. Up to now, most applications of photon-trapping effect is limited to one band absorption enhancement. The research on multi-band detection enhancement is still lacking although it is much useful for multiband spectroscopy.
In this work, we report an InAsSb-GaSb heterostructure with a photon-trapping hole array (PTHA) for polarization-independent dual-band photoresponse enhancement. By incorporating a circular hole-array structure in hexagonal lattice with optimal topological parameters to the heterostructure, the absorption and photoresponse in the wavelength range of interest can be significantly enhanced. The achieved room-temperature responsivity enhancement rates in NIR and MIR under zero bias are 12% and 33%, respectively. Besides, the amplitude frequency response and speed are also slightly improved.
2. Results and discussion
2.1 Device design
For the purpose of photon trapping, we consider a hexagonal lattice of circular hole array patterned in top dielectric layer, as depicted in Fig. 1(a). The basic heterostructure for photodetection comprises an 850-nm thick nip InAsSb structure grown on a GaSb substrate by molecular beam epitaxy (MBE). The holes are formed by etching through the top n-InAsSb contact layer and part of the i-InAsSb layer, whose cross-sectional diagram is shown in Fig. 1(b) and basic information is detailed in Table 1.
Fig. 1. The InAsSb-GaSb heterostructure with PTHA structure. (a) Schematic of heterostructure with circular hole array in hexagonal lattice. (b) Cross-sectional diagram of the heterostructure with the holes etched into InAsSb layer.
Table 1. Basic information of the InAsSb-GaSb heterostructure
The basic heterostructure material, after epitaxial growth, was characterized by High-Resolution X-ray Diffraction (HRXRD) scan with 1.54056 Å radiation, where the corresponding XRD spectra is shown in Fig. 2(a). It is found that the lattice mismatch of InAsSb layer and GaSb substrate is about 300 ppm. According to Vegard’s law, the derived component of Sb in InAsSb is 0.085. The optical property of InAsSb layer was then characterized by photoluminescence (PL) from 7 K to 280 K, as shown in Fig. 2(b). The full width at half maximum (FWHM) at 7 K is around 10 meV, demonstrating good optical quality of the InAsSb layer. The observed clear ‘red shift’ of peak energy with the temperature increasing follows well with the Varshni law Eg(T) = Eg(0) - αT2/(β+T). The lower energy peak (∼0.315 eV) shown in low-temperature PL spectra may be due to the combination effects of non-uniform Sb distribution within the epitaxially grown InAsSb layers and the interband transition from the conduction band of p-GaSb to valence band of p-InAsSb at the InAsSb-GaSb interface [23–25]. Figure 2(c) shows the corresponding fitting with α and β of 0.18 meV/K and 127 K, which are within the reasonable range for InAs0.915Sb0.085.
Fig. 2. Characterization of InAsSb-GaSb heterostructure. (a) HRXRD spectrum. (b) PL spectra from 7 K to 280 K. (c) PL peak energy positions and the corresponding Varshni fitting line.
Figure 3 demonstrates the scanning electron micrographs (SEMs) of the hole array fabricated on the InAsSb-GaSb heterostructure. The smooth and straight sidewalls of the fabricated holes demonstrate the effectiveness of our etching processes. An enlarged view of the unit cell for PTHA with the lattice angle of 60° is also shown, parameterized by the lattice constant (a), hole diameter (d), hole depth (D).
Fig. 3. The SEMs of the fabricated InAsSb-GaSb photodetector with PTHA structure, with the unit cell parameterized by the lattice constant (a), hole diameter (d), hole depth (D) and lattice angle 60°.
To get a better PTHA structure, we firstly use finite-difference time-domain (FDTD) method to perform simulations on absorption maximization. By solving Maxwell equations using this method, the reflection and transmission spectra of the PTHA structure in response to incident light as well as the electromagnetic field distribution in the heterostructure were calculated. The absorption spectra were extracted through subtracting transmission and reflection from one. The calculation included all layers of the heterostructure and the hole array was also taken into account. Periodic boundary conditions were used in the lateral dimensions of the FDTD region, and perfectly matched layers were set in vertical boundary. The structure was normally illuminated from the plane wave source, with the mesh size set in 5 nm. The dielectric constants of heterostructure materials were referred from the measured data and material database in Lumerical software.
Since the wavelength of 2.7 µm is roughly in the middle of peak photoresponses for GaSb and InAsSb, we select it as the center wavelength to simulate for realizing optimal hole-array parameters. Through simultaneously scanning the lattice constant a and rd (the ratio of diameter d over a), the maximum absorption at λ=2.7 µm is achieved at the a and rd of 3 µm and 0.65, respectively, with the hole depth D initially set at 500 nm, as indicated in Fig. 4(a). Then with the optimized a and rd, the hole depth is scanned from 100 to 1200 nm in the wavelengths (λ) from 1.5 to 5 µm, and the results are shown in Fig. 4(b). It is found that when the holes are deeper than 550 nm, there is only less than 1% absorption enhancement for an additional 100-nm depth. To save enough material for photogenerated carriers, the optimal D is selected as 550 nm. In this way, a set of geometrical parameters leading to maximum absorption with PTHA structure are obtained. Another key figure of merit for strong photodetection is the selectivity of photoresponse sensitivity over a wide range of polarization angles. As indicated in Fig. 4(c), the absorption spectra of the PTHA is almost independent of polarization angles due to the structure symmetry. The corresponding absorption of the InAsSb-GaSb heterostructure with/without (w/o) the PTHA are illustrated in Fig. 4(d). Obvious enhancement in absorption is observed in the spectra from 1.5 to 5 µm for the heterostructure with PTHA. The maximum enhancement occurs at about 2.7 µm, and corresponding enhancement rate is about 1.51.
Fig. 4. Simulated spectra results of InAsSb-GaSb heterostructure w/o PTHA structure. (a) Simulated absorption at λ=2.7 µm by simultaneously scanning lattice constant (a) and ratio of hole diameter to lattice constant (rd). (b) Simulated absorption by simultaneously scanning hole depth (D) and wavelength (λ). (c) Simulated absorption by simultaneously scanning polarization angle (θpol) and λ. (d) and (e) Simulated absorption and reflection spectra with the optimal geometrical parameters.
The reflection spectra of the InAsSb-GaSb heterostructure w/o the PTHA are plotted in Fig. 4(e). In the wavelength range of interest, the heterostructure with PTHA has a significant reduction in reflection compared to the reference material. There is a reflection valley around the position of λ=2.7 µm, where the reflection reduction is about 81%. It is due primarily to the reduced effective refractive index caused by the hole array. As the filling ratio of air hole (f) is represented by the volume fraction of air to dielectric, $\frac{{\; {\pi}{d^2}}}{{2{a^2}tan({60^\circ } )}}$, a lower effective refractive index (neff) can be obtained from Eq. (1) [26],
To find the reason for absorption enhancement around 2.7 µm, we simulate the electric field distribution during light propagating in the PTHA, and the results are shown in Fig. 5. It is observed there is a transverse optical propagation mode between the holes, which does not exist in the reference, as indicated in Fig. 6. It is referred to as lateral optical slow bloch mode [9]. The resonance can effectively couple the incident waves to the absorption medium, leading to a low-group-velocity mode with large in-plane wave vector (k||) [28]. Thus, a longer optical absorption path is generated in the PTHA, where the corresponding photon dwell time in the structure is obviously longer at 2.7 µm (8.2 fs), compared to those at 1.7 µm (4.6 fs) and 3.9 µm (5.4 fs). It is because such a resonant mode arises from coupling of the incident plane wave to the modes of hole array and occurs at the wavelength on a scale comparable to the lattice period of the structure [12]. The electric field distribution map demonstrates that the interferences at PTHA-air interface can simultaneously prolong the photon lifetime in the dielectric and maintain the shape of electric field. As, such, the combination of prolonged photon lifetime and reduced effective refractive index facilitates the effective light confinement in the dielectric around the resonant wavelength, giving rise to prominent enhancement of absorption.
Fig. 5. Simulated electric field (EX) distributions with light propagating in the PTHA structure at (a) λ=2.7 µm, (b) λ=1.7 µm and (c) λ=3.9 µm, respectively.
Fig. 6. Simulated electric field (EX) distributions with light propagating in the reference design at λ=2.7 µm.
2.2 Experimental verification
The InAsSb-GaSb heterostructures with the PTHA were fabricated based on the optimal geometrical parameters, defined by Electron-beam lithography (EBL) and transferred pattern to InAsSb layers by Ar/Cl2/BCl3 based high-temperature inductively coupled plasma-reactive ion etching (ICP-RIE). Fourier Transform Infrared Spectroscopy (FTIR) integrated with a 36x objective, an infrared polarizer and a KBr beam splitter was utilized to measure the reflection and transmission spectra. The absorption spectra presented were normalized with the results of gold pad. Figure 7(a) shows the measured absorption spectra, which are in good agreement with the simulation ones, except for a more broadband character in measured results due to the wide incident angle distribution in the measurement setup of FTIR [29]. The integrated absorption enhancement is around 130% of the heterostructure with a PTHA in the wavelength range from 1.5 to 5 µm. The maximum enhancement at the resonance wavelength of 2.7 µm is about 150% which agrees with the simulation result.
Fig. 7. Measured absorption and photocurrent (IPH) for the devices w/o a PTHA. The device without a PTHA is taken as a reference. (a) Absorption spectra at room temperature. (b) Room-temperature IPH spectra under zero bias. (c) IPH at 2.7 µm at different polarization angles. (d-e) IPH spectra of (d) reference and (e) PTHA detectors at temperatures from 220 K to 293 K under zero bias. (f-g) IPH spectra of (f) reference and (g) PTHA detectors under the bias from −0.15 V to 0.15 V at room temperature (293 K).
We also made the InAsSb-GaSb heterostructures w/o a PTHA into devices for photodetection, where they were experienced same fabrication process. Firstly, standard lithography technology was used to define the square mesa. After that, the InAsSb and GaSb layers were etched with a citric acid based etchant. Ti/Au (20 nm/280 nm) contacts were finally evaporated to form ohmic contacts on the top n-type InAsSb and bottom GaSb buffer layers, with an active absorption area of 250 µm x 250 µm. To ensure the reliability of results, we fabricated multiple devices w/o a PTHA, where the characterization results were essentially the same.
The photocurrent spectra (IPH) were measured using the same experimental setup as in the absorption measurements, except that the fabricated detectors were used to collect photocurrent signals instead of the internal MCT detector. In the measurement, the MIR light source was first introduced into the Michelson interferometer inside the FTIR. After that, it was focused on the fabricated devices. The photocurrent signals collected by the detector were amplified by a low-noise current preamplifier, and then connected to the FTIR external port. The measured photocurrent spectra of the devices with (red curve) and without (blue curve) PTHA at room temperature under zero bias are shown in Fig. 7(b). Remarkable enhancement in the device with the PTHA is clearly seen, especially in the 3 peak wavelengths of 1.7 µm, 2.4 µm and 3.9 µm. The higher photocurrents at 1.7 µm and 3.9 µm are understandable as they correspond to the bandgap of GaSb and InAsSb, respectively. According to the absorption equation of 1-e-αd [30], where the α and d are the absorption coefficient and the thickness of InAsSb [31], respectively, about 55% of 1.7 µm incident photons can pass through the whole InAsSb layers and 76% of such photons can pass the InAsSb layer in the holes to reach the GaSb layer, which leads to significant photocurrent at around 1.7 µm. The appearance of the peak at about 2.4 µm is primarily due to the absorption of atmosphere [29], which causes reduction of photocurrent in the wavelengths between the 2.5 and 3.9 µm. The significant enhancement in photocurrent at about 2.4 µm for the device with PTHA is due to the enhanced photon incorporation and longer dwell time.
To know the polarization dependence of spectra response, we characterized IPH of both devices under different polarization states. As shown in Fig. 7(c), the enhancement factor for IPH at 2.7 µm does not change significantly with polarization angles due to the structure symmetry of the PTHA. The photocurrent spectra measured at different temperatures and biases for both devices are exhibited in Figs. 7(d)-(g). It is demonstrated that the enhancement is always present in NIR and MIR bands, although the enhancement factor varies with temperature and bias. The corresponding mechanism for this phenomenon will be explained in the following section.
2.3 Performance characterization
In order to quantify the photoelectric conversion capability of the PTHA, a 1000 K blackbody radiation source was used to characterize the responsivities of the reference and PTHA detectors, where a 300 Hz chopper and an NIR (1–3 µm) or MIR (2.5–4.8 µm) bandpass filter were set before the radiation reaching the devices under test. The measured photocurrent was then read out through a low-noise current preamplifier and lock-in amplifier. With the values of photocurrent (IPH) and incident power (Pinc) obtained from a standard power meter, the blackbody responsivity (RA) is derived from RA = IPH / Pinc.
Figures 8(a) and (b) shows the corresponding room-temperature RA for NIR and MIR measured under biases from −0.5 V to 0.5 V for the device with (red dots) and without (blue dots) a PTHA. The RA values of the PTHA detector for NIR and MIR at zero-bias are enhanced to 0.73 A/W and 0.48 A/W, respectively, compared to the 0.65 A/W and 0.36 A/W for the reference. The RA is increased when the bias becomes negative. It is due to the enlarged depletion region with the reverse bias to −0.8 V, as the width of depletion region (W) is proportional to √(Vbi-Vbias), where Vbias is the applied voltage bias, Vbi is built-in voltage [32]. Light absorption happens within the depletion region, therefore, the wider depletion leads to more photogenerated carriers and larger RA [33]. In addition, the responsivities at different temperatures under zero-bias are also characterized, and the results for NIR and MIR are exhibited in Figs. 8(c) and (d). Obvious enhancement occurs over the whole temperature range studied and the enhancement becomes more significant at lower temperature. The corresponding responsivity map measured at different temperatures and biases are shown in Figs. 8(e)-(h). Since the intrinsic carrier density is reduced when temperature decreases, it leads to the increased width of depletion region [34]. In this case, a larger responsivity enhancement can be expected in PTHA device, as the increased absorbed photons. The zero-bias enhancement factors for NIR and MIR at room temperature are 1.12 and 1.33, respectively, and they are increased to 1.71 and 1.79, respectively, when temperature drops to thermoelectric cooling (TEC) temperature of 220 K.
Fig. 8. Responsivity (RA) of the devices w/o a PTHA. RA for (a) NIR and (b) MIR at room temperature under the bias from −0.5 V to 0.5 V. (c-d) RA for (c) NIR and (d) MIR under zero bias at temperatures from 220 K to 293 K. (e-f) RA for NIR at temperatures from 220 K to 293 K and under biases from −0.5 V to 0.5 V of (e) reference and (f) PTHA detectors. (g-h) RA for MIR at temperatures from 220 K to 293 K and under biases from −0.5 V to 0.5 V of (g) reference and (h) PTHA detectors.
Finally, to gain insights of the PTHA structure’s influence on response speed, the amplitude-frequency response was characterized by using a 2.94 µm laser with 20 ns rise and fall time. As shown in Fig. 9(a), the f−3dB (the frequency when signal amplitude decreases to 70.7%) is slightly broader in the PTHA device [35], with the value around 2.2 × 105 Hz. We also characterize it in time domain, as indicated in Fig. 9(b). The device with a PTHA shows a rise time (tr) of 1.6 µs which is shorter than the 1.9 µs from the reference. This can be attributed to the fast collection of photocarriers associated with etched holes in the PTHA device, where the reduced structure volume may lead to lower junction capacitance and smaller RC delay, thus an enhanced response speed [36].
Fig. 9. Response speed. (a) Amplitude-frequency response of reference and PTHA detectors. The line of −3 dB is indicated by the black dash dot line. (b) Response waveform of the PTHA (left in red) and reference (right in blue) devices. The rise time (tr) is defined as the time duration of amplitude from 10% to 90%.
3. Conclusion
We proposed and demonstrated a dual-band enhanced infrared photodetector which is made by integrating a PTHA on the nip InAsSb-GaSb heterostructure. Significant enhancements of photoresponse, beneficial from the reduced effective refractive index and lateral propagating slow modes, are realized in both NIR and MIR without polarization selectivity. Photoresponse enhancement is observed at different temperatures and voltage biases, where the enhancement factor becomes greater at lower temperature. In addition, improvements in working frequency range and response speed are also observed.
Funding
Agency for Science, Technology and Research (SERC 1720700038, SERC A1883c0002).
Acknowledgment
Jinchao Tong would like to thank the support of Nanyang Technological University Presidential Postdoctoral Fellowship.
Disclosures
The authors declare no conflicts of interest.
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