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Abstract
Flexible mid-infrared photodetectors are essential to realize advanced imaging applications, including wearable healthcare monitoring, security, and biomedical applications. Here, we demonstrate high-performance flexible p-i-n InAs thin-film photodetectors with an optimal In0.8Al0.2As barrier layer. This In0.8Al0.2As barrier inserted between p-InAs and UID-InAs layer reduced leakage currents by a factor of 283 by blocking the flow of electrons. The fabricated flexible device exhibited relatively low dark current densities of 1.03×10−5 at 0 V and 0.85 A/cm2 at −0.5 V, comparable to both commercially available and reported homoepitaxially-grown InAs detectors. Also, the high mechanical robustness and excellent reliability of our flexible InAs photodetector were confirmed by bending tests under various curvatures and bending cycles.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Flexible infrared photodetectors (PDs) operating in the mid-infrared spectral range (MIR, 2–5 µm) have attracted attention in various advanced applications within various healthcare, security, and biomedical contexts [1–3]. In recent years, new classes of flexible PDs have emerged based on functional materials like 2D and organic semiconductor materials, but these PDs still suffer lower quantum efficiency, slow operation speed, and inferior reliability due to the nature of their operation and structural disorder in the materials [4–7]. On the other hand, PDs based on conventional III-V compound semiconductors exhibit superior mechanical robustness and better performance in many aspects [8–11]. In particular, InAs (Eg = 0.36 eV) is an ideal material for MIR detection, but it is limited by wafer cost and rigidity.
One approach for mitigating the high wafer cost is by monolithically growing InAs on a cheaper substrate. However, the large lattice mismatch between InAs and a different substrate like GaAs or Si forms a high density of misfit and threading dislocation (TD), which severely deteriorate the performance of InAs PDs [12,13]. To address this, growth techniques such as graded buffers and single insertion layers have been applied to reduce the TD density (TDD) and produce smooth surfaces. Recently, high-quality InAs-based PDs have been grown on GaAs and Si wafers through the careful engineering of the buffer layers [10,13,14]. In addition, the development of wafer bonding and epitaxial lift-off (ELO) has played an important role in adding cost-competitiveness to conventional optoelectronics devices by enabling the reuse of high-cost III–V semiconductor wafers. Since Konagai et al. first reported the method known today as ELO [15], various devices like PDs, MOSFETs, solar cells, laser diodes, and light-emitting diodes have now been successfully transferred [8–11,16–21]. However, since wafer bonding demands high epilayer quality, most studies have focused on homoepitaxially-grown devices (e.g. GaAs on GaAs wafers). As heteroepitaxial growth of III–Vs (e.g. InAs on GaAs wafers) have shown improved crystalline and surface qualities over the years, these materials have become amenable to the wafer bonding process while at the same time offering advantages of scalability, cost-competitiveness, and mechanical robustness.
In this paper, we demonstrate high-performance flexible p-i-n InAs photodetectors with low dark current enabled by an In0.8Al0.2As barrier and optimized InxAl1-xAs/GaAs buffer layers. Metal wafer bonding and an epitaxial lift-off process were used to transfer InAs epilayer to a polyimide film as the flexible substrate. The smooth InAs surface after the ELO process and low threading dislocation density were achieved by an employing optimized InxAl1-xAs abruptly graded buffer layer. The flexible InAs photodetectors showed dramatic decrease of leakage currents by a factor of 283 by blocking the electrons flow and low dark current levels comparable to both commercial and previously-reported homoepitaxially-grown InAs detectors by inserting a 50 nm In0.8Al0.2As carrier blocking layer. In addition, the high mechanical robustness and excellent reliability of our flexible InAs photodetector were confirmed by bending tests under various curvatures and bending cycles.
2. Experiments
Figure 1(a) shows the entire structure of the InAs photodetector grown by molecular beam epitaxy (MBE) machine on a semi-insulating GaAs substrate with an optimized InxAl1-xAs graded buffer layer. The sample, from the bottom to the top, consists of 50 nm thick GaAs buffer layer, 50 nm thick AlAs sacrificial layer for the ELO process, 250 nm Si-doped InxAl1-xAs abruptly graded buffer layer, 600 nm thick Si-doped InAs buffer layer, 300 nm thick Si-doped In0.95Al0.05As defect filter layer, and 3550 nm thick InAs p-i-n active layer. The abruptly graded InxAl1-xAs buffer layer was introduced to achieve smooth surface morphology after the ELO process as well as to bridge the large lattice mismatch (∼7.2%) between GaAs and InAs. Also, the In0.95Al0.05As single defect filter layer prior to the InAs cap layer was inserted to further decrease TDD. The active layer contains a 50 nm unintentionally-doped (UID) In0.8Al0.2As barrier layer to block the electrons. As a reference sample, we also grew a device without the barrier layer. The schematic energy band diagram of the InAs p-i-n photodetector with the In0.8Al0.2As barrier layer was described. As can be seen in Fig. 1(b), the In0.8Al0.2As barrier was positioned between p-InAs and UID-InAs layer to effectively reduce leakage current by blocking the electrons. The inset shows several parameters of the InAs layer for the energy band diagram.
Fig. 1. (a) Schematic diagram of the entire InAs PD structure with abruptly graded InxAl1-xAs buffer on semi-insulating GaAs. (b) Schematic energy band diagram of the InAs p-i-n photodetector with In0.8Al0.2As barrier layer. The table shows InAs parameters for the energy band diagram.
The fabrication of the flexible device was performed through metal wafer bonding and the ELO process, as shown in Fig. 2(a). Since a very thin polyimide film (Kapton, Dupont, 13 µm) is used to transfer the InAs epitaxial layer, the polyimide film was first laminated onto a Si handling substrate using polydimethylsiloxane (PDMS) to prevent wrinkling or curling during the fabrication process. Afterward, standard photolithography and wet chemical etching were performed to define the mesa, window, and metal contact areas. The size of the mesa and window was 400 µm and 300 µm, respectively. The native oxide was removed by dipping the sample in an HCl-based solution for 1 minute. Pt/Au (10/50 nm), which served as a bonding material and p-type ohmic contact, was deposited on both InAs epitaxial layer and polyimide film by electron beam evaporation. The sample was etched down using a 1:1:5 mixture of H3PO4, H2O2, and DI water to expose the AlAs sacrificial layer. Then, metal wafer bonding was conducted by applying a uniaxial pressure of 40 kgf/cm2. After bonding, the InAs epitaxial layer was separated from the GaAs donor substrate by the ELO process in which the AlAs sacrificial layer was etched by immersion in HF: acetone (1:1) solution. The top contact of Ti/Au (20/200 nm) was deposited on the n-InAs contact layers by electron beam evaporation. Then, top and bottom metal pads were separately formed with 200 nm SiNx film as an isolation layer. Finally, the flexible device was peeled off from the Si handling substrate. Figure 2(b) shows an SEM image of the fabricated flexible InAs PD with the metal pad.
Fig. 2. (a) Fabrication procedure of flexible InAs PD using metal wafer bonding and epitaxial lift-off process. (b) SEM image of the flexible device with metal pad line.
3. Results and discussion
Figure 3(a) shows the optical microscope image of the InAs active layer grown on GaAs substrate. No thermal cracks and hillocks were observed throughout the wafer, which increase leakage current in reverse bias [13,16]. Figure 3(b) shows a 10 × 10 µm2 atomic force microscope (AFM) image of the InAs surface, which shows the surface morphology. Although the previous result showed smooth surface features with root-mean-square (RMS) surface roughness of 0.36 nm, this structure exhibits relatively rough surface properties with a surface roughness of 5.62 nm. [10]. The rough surface with visible linear defects is presumed to originate from misfit dislocations generated during the strain relaxation of the In0.8Al0.2As blocking layers, resulting from the small lattice mismatch (ɛm ∼1.33%) between the p-InAs contact and In0.8Al0.2As barrier layer [22–25]. Note that despite such a high surface roughness value, the transfer process of InAs PD was conducted without any material quality degradation. Also, it can be seen that the leakage current is dramatically decreased by inserting the In0.8Al0.2As barrier layer. Further investigation of these surface defects, such as misfit dislocation, will be carried out in the future. The TDD on the InAs surface was measured by electron channeling contrast imaging (ECCI) for non-destructive and accurate characterization. Figure 3(c) shows a representative ECCI image, showing only three TDs. The total device structure has a TDD of 7.6×107 cm−2, after surveying more than 120 µm2. The cross-sectional scanning electron microscope (SEM) image for the entire structure is shown in Fig. 3(d). More details of the optimized InxAl1-xAs buffer for high-quality of InAs on GaAs substrate can be found in our previous work [10].
Fig. 3. (a) Optical microscope image of the InAs active layer on GaAs substrate. (b) AFM image of the InAs surface. The scale bar ranges from 0 to 49.9 nm. (c) ECCI image. (d) Cross-section SEM image of the entire InAs epitaxial layer.
Room-temperature dark current density-bias curves of the flexible InAs detector with and without the In0.8Al0.2As barrier layer were investigated under flat configuration. Figure 4(a) shows a large decrease from reverse bias to 0 V bias with the insertion of a barrier layer, which effectively lowered the dark current density by about two orders of magnitude. At 0 V bias (photovoltaic mode), the dark current density of the flexible InAs PD with a barrier layer was 1.03×10−5 A/cm2, while that of the reference device was 2.83×10−3 A/cm2. We also compared the dark current density at a reverse bias of 0.5 V with commercial and reported homoepitaxially-grown InAs photodetectors [26–30], as shown in Fig. 4(b). Despite the dislocations and defects in our heteroepitaxially-grown sample, our flexible InAs PD still exhibited a dark current density comparable with state-of-the-art InAs PDs. This suggests that our structure with the optimal buffer and barrier layers not only suppressed the formation of TDs, but also block the diffusion of electrons, both critical in low noise, and high-detectivity PDs.
Fig. 4. (a) Dark current density-bias curve of flexible InAs detector with and without In0.8Al0.2As barrier layer. (b) Comparison of dark current density at −0.5 V with the commercial photovoltaic (PV) detector and reported homoepitaxially grown device. The model number of Hamamatsu InAs detector is P10090-01 [26–30].
In the fabrication of flexible devices through the ELO process, it is essential to maintain the original material quality without any degradation. To confirm the influence of the transfer process, we have investigated the electrical properties of the as-grown and flexible InAs PDs with the barrier layer at room temperature. Figure 5(a) shows the respective dark and photocurrent of both PDs for comparison. The as-grown and flexible device show dark currents of 1.17 × 10−7 and 6.17 × 10−8 A at 0 V bias. Other measured devices in general showed similar dark current densities. This result indicates that the wafer bonding and ELO process did not affect the material quality. The photocurrent measurement was performed using a 2 µm laser with a fiber optic cannula, and the power was calibrated using an optical power meter and a sensor. The responsivity of devices was calculated following the equation:
where R is the responsivity, Iphoto is the photocurrent, Idark is the dark current, and P is the power of the laser. The responsivity at 0 V bias is 0.38 and 0.68 A/W for the as-grown and flexible devices. Despite the surface defects, resulting from lattice mismatch of the barrier layer, the as-grown device shows comparable performance when no barrier is added [10]. Also, flexible PD exhibited a 179% enhancement in responsivity, which is attributed to the micro-cavity effect due to the thin p-i-n active layers with bonding metals [8,10,11]. Note that the responsivity of the commercial InAs PD from Hamamatsu is 0.6 A/W at 2 µm. Figure 5(b) shows the laser power dependence of photocurrent characteristics, which indicates information on the trap state and photocurrent generation mechanism. The photocurrents at 0 V bias were plotted according to the power-law: Iph = APα where A is a constant depending on the responsivity of the device, P is the laser power density, and the fitting parameter of α represents trap states in the grown structure. The parameter α of 1 indicates the ideal trap-free state, which does not affect the photocurrent generation [31]. As can be seen in Fig. 5(b), the photocurrent showed linear characteristics with the laser power density, with an α value of 1.09, which suggests high-quality InAs active layers despite a large lattice mismatch.Fig. 5. (a) Dark and photocurrent of as-grown and flexible InAs detector. (b) Photocurrent characteristics of InAs PD with 2000-nm laser power dependence. The inset shows the relationship between photocurrent and laser power density.
To confirm the mechanical flexibility and durability of flexible InAs PD, the on/off ratio was investigated under various bending stresses. The dark and photocurrent were measured using an HP-4156C semiconductor analyzer with metal chucks. Figure 6(a) shows the on/off ratio of the flexible InAs PD mounted on metal chucks with different curvatures. The on/off ratio was not changed under a high bending radius of 0.5 cm (curvature= 2 cm−1). The slight deviation of the data under various bending curvatures could be attributed to uncertainties of the measurement. In addition, the on/off ratio under successive bending times at a 0.5 cm radius metal chuck was measured. Figure 6(b) shows that our flexible detector remains the on/off ratio even after 1000 × bending times. Both these results indicate high mechanical robustness and excellent reliability of our flexible InAs photodetectors.
Fig. 6. (a) On/off ratio of flexible InAs photodetector at different bending curvatures. (b) Bending test of flexible device. The inset shows measurement configuration.
4. Conclusion
In conclusion, we demonstrate low-cost and high-performance flexible InAs thin-film photodetectors with low dark current enabled by an optimized InxAl1-xAs/GaAs buffer and In0.8Al0.2As barrier layer. The high-quality InAs active layer was grown on a GaAs substrate with a low threading dislocation density of 7.6×107 cm−2 then transferred to a polyimide film via metal wafer bonding and epitaxial lift-off. The flexible InAs detector with a barrier layer exhibits a dramatic decrease of dark current compared to the flexible device without a barrier layer and showed dark current density comparable with commercial and reported homoepitaxially-grown InAs photodetectors. The flexible InAs PD showed the responsivity of 0.68 A/W at 2 µm, which exhibited a 179% enhancement in responsivity attributed to the micro-cavity effect. Also, bending tests under various curvatures and 1000 × bending cycles confirm high mechanical robustness and excellent stability of our flexible detector.
Funding
Korea Institute of Science and Technology (2E31532); National Research Foundation of Korea (2021R1C1C1004620).
Disclosures
The authors declare that there are 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 request.
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