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Abstract
We demonstrate flexible GaAs photodetector arrays that were hetero-epitaxially grown on a Si wafer for a new cost-effective and reliable wearable optoelectronics platform. A high crystalline quality GaAs layer was transferred onto a flexible foreign substrate and excellent retention of device performance was demonstrated by measuring the optical responsivities and dark currents. Optical simulation proves that the metal stacks used for wafer bonding serve as a back-reflector and enhance GaAs photodetector responsivity via a resonant-cavity effect. Device durability was also tested by bending 1000 times and no performance degradation was observed. This work paves a way for a cost-effective and flexible III-V optoelectronics technology with high durability.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Flexible optoelectronics are next-generation devices that represent the growing trend to make devices flexible, stretchable, wearable, and implantable [1–3]. Using flexible substrates, optoelectronic devices are made to conform to irregular surfaces and find novel applications in the scientific, medical, military and industrial settings, particularly in energy [4], imaging [5], computing [6], robotics [7], display [8], and biointegration [9]. The use of a flexible substrate puts constraints on the choice of active material and processing technology [10]. Organic-based semiconductors are gaining traction due to the relative ease of processing them into flexible devices, but these materials have inherently inferior performance and stability compared with inorganic semiconductors [11]. On the other hand, III-V semiconductors, having direct bandgaps, high carrier mobility, and high quantum efficiencies, still perform better than organic semiconductors in many aspects [12–15]. Unfortunately, because III-V semiconductors require a rigid substrate for growth, subsequent integration with a foreign flexible substrate is still challenging [11]. The complications that arise from the additional fabrication steps and their high costs greatly hamper their viability in the flexible electronic market.
Over the past few decades, wafer bonding techniques have enabled a variety of III-V semiconductors to be transferred from III-V native substrates to a foreign substrate, but in many cases, the original substrates have to be etched away [16]. Instead, epitaxial lift-off (ELO) was developed and is continuously being refined to allow high throughput transfer of III-V semiconductors, enabling reuse of the parent substrate through the selective etching of a sacrificial layer [4,17,18]. Another advantage of ELO is that by removing the parent substrate and the thick buffer layers that usually accompany epitaxial growth, the final device is thinner than what it originally was, making the final product more suitable for flexible applications. To date, ELO has proven successful in the transfer of photodetectors (PD), light-emitting diodes, solar cells, and MOSFETs to silicon, glass, and polyimide films [4,18–21]. However, the previous studies still rely on the use of expensive III-V commercial wafers to grow the active materials. The cost of a GaAs wafer can range from ∼120–400× that of a Si wafer on a per area basis [22] and in the case of GaAs solar cells, for example, the substrate is estimated to be 84% of the total cost [23].
A number of studies have demonstrated the growth of high quality III-V materials on Si. To avoid the high density of threading dislocations (TD) that arise from the lattice mismatch between silicon and the epitaxial layers, various groups employ a combination of two-step growth, dislocation filter layers, thermal cyclic annealing, and aspect-ratio trapping [24–28]. To further drive down the cost associated with the substrate and leverage the mature Si CMOS industry in the future, we propose to eliminate the use of III-V wafer altogether and grow the PDs on on-axis Si wafer.
Here, we for the first time demonstrate low-cost, high throughput, flexible photodetectors by growing GaAs on an on-axis Si wafer and transferring them to polyimide films through epitaxial lift-off. We achieved 100% transfer yield of a 20 × 10 GaAs PD array, and based on high-resolution X-ray diffraction and photoluminescence studies, the transfer process maintained the GaAs crystalline quality well. Simulation also revealed light-trapping effects from the Pt/Au wafer bonding metal reflector. The GaAs PD array exhibited dark currents as low as 1.25 × 10−10 A and a peak responsivity of 0.41 A/W at room temperature. Bending tests at different curvatures and 1000 bending cycles of the fabricated flexible PD array showed excellent device durability. We believe that this method suggests a new paradigm for robust and flexible III-V optoelectronic device platforms by removing the expensive III-V wafer cost while keeping excellent performance.
2. Material growth and device fabrication
Figure 1(a) illustrates the schematic of the GaAs PD structure grown on on-axis GaP/Si (001) wafer using molecular beam epitaxy. The commercially available substrate comprises a thin 50 nm GaP layer grown by metal organic chemical vapor deposition and designed to terminate antiphase domains within the GaP layer. To prevent a high TD density, we grew an optimized GaAs virtual substrate on Si that utilizes a two-step growth method, dislocation filter layers, single insertion layer, and thermal cyclic annealing. The entire heterostructure was grown using a solid-source Veeco Gen-930 molecular beam epitaxy system. First, 100 nm GaAs was grown at 500 °C, then succeeding layers were grown at 580 °C as measured by an optical pyrometer. Another 1500 nm GaAs buffer was grown, and then thermal cycle annealing was performed four times. The following layers where then grown: 200 nm In0.1Ga0.9As as single insertion layer, 200 nm GaAs, 10 cycles of 10 nm In0.1Ga0.9As/10 nm GaAs as dislocation filter layers, and 800 nm n-GaAs (2 × 1018 cm-3). The entire growth was carried out under As2 overpressure. The sophisticated growth structure and epitaxial condition in the GaAs virtual substrate aim to improve material quality of the active device since the TDs do not terminate by themselves and propagate in the epitaxial growth direction. The following layers were subsequently grown: 10 nm AlAs sacrificial layer; 200 nm n-GaAs (3 × 1018 cm-3); 1000 nm i-GaAs; 50 nm p-Al0.3Ga0.7As barrier (7 × 1018 cm-3); and 200 nm p-GaAs (1 × 1019 cm-3). For comparison, we also grew the same AlAs and p-i-n GaAs structure on a native GaAs (100) wafer.
Fig. 1. Heteroepitaxial growth of GaAs PD on Si a) Schematic diagram of the GaAs PD structure grown on Si with AlAs sacrificial layer. b) Differential interference optical microscope image, c) AFM image, and d) ECCI image of the GaAs PD surface. e) X-SEM image of the entire epilayer structure.
The Nomarski image in Fig. 1(b) shows that the surface is free of hillocks and cracks, which are extremely detrimental to PD performance by increasing shunt current in reverse bias [29]. The atomic force microscopy (AFM) image shown in Fig. 1(c) reveals typical surface cross-hatchings of the metamorphic relaxed GaAs layer and an average root-mean-square (RMS) roughness value of 4.00 nm. The TD density of the full PD structure was calculated from electron channeling contrast imaging (ECCI), revealing only ∼1 × 106 cm-2. We have used electron channeling conditions of <040> and <220> to preclude dislocation invisibility criteria and to accurately measure the defects. Figure 1(d) shows an exemplary ECCI image showing only two TDs reaching the surface. Figure 1(e) is a cross-sectional scanning electron microscope (X-SEM) image of the complete epilayer structure with well-defined interfaces. The total III-V layer thickness on the Si wafer was maintained well below 6 μm so that thermal strain-induced cracks do not form during sample cooling after the heteroepitaxial growth. We have added the p-AlGaAs barrier layer to effectively confine photo-generated carriers in the unintentionally-doped (UID)-GaAs absorption region and to increase PD responsivity [30].
The device fabrication procedure is schematically illustrated in Fig. 2(a). We deposited 10 nm/60 nm of Pt/Au on the epitaxial layer and the polyimide (PI) surface by electron beam evaporation. The Pt/Au film simultaneously serves as the p-type contact and the bonding material for wafer bonding. 20 × 10 square mesas with 400 µm side lengths and a pitch spacing of 500-µm were defined through standard photolithography then etched to expose the sidewalls of the AlAs sacrificial layer. We performed metal wafer bonding by placing the two Pt/Au-coated surfaces into contact and applying 40 kgf uniaxially for 4h at 200 °C. After bonding, ELO was performed by dipping the bonded substrates in a 1 HF: 1 acetone solution until the two substrates detached from each other. Figure 2(b) shows that 100% of the 200 mesas were successfully transferred to the PI substrate. The etched surface was also smooth with average RMS roughness of 4.46 nm from 10 × 10 μm2 scans. (See Supplement 1). A stack of 20 nm/40 nm/200 nm of Pd/Ge/Au was deposited as n-type contacts by electron beam evaporation. Finally, the polyimide film was peeled from the Si mother wafer. Figures 2(c)–2(d) are optical microscope and scanning electron microscope images of the finished ELO PD arrays that are highly uniform and thermal crack-free after metal wafer bonding and ELO. Figure 2(e) shows our device in a bent configuration.
Fig. 2. Processing of flexible GaAs PD via Metal wafer bonding and ELO. a) Fabrication procedure. b) Image of completed GaAs PD arrays. c) Optical microscope image of 6 PDs. d) SEM image of a single PD. e) Image of the flexible PD mounted on a curved surface.
Our fastest ELO time of 8 min is among the fastest in reported literature, which range from a few hours [4,31] to as fast as 5 min [32]. We attribute this speed to a few factors. First, the small mesa size increases the area of the etch front and promotes release of H2 bubbles produced during the etching reaction [33]. Second, the hydrophilic nature of acetone in the etchant (1 HF : 1 acetone) prevents the formation of large H2 bubbles [34]. We also note that despite having a relatively high RMS roughness value of 4.00 nm, we still observed 100% transfer yield. The authors speculate that the softness of PI film adapts to the roughened surface morphology. The choice of metal for the metal wafer bonding and ELO process is also important since HF is used as the sacrificial layer etchant. We have chosen Pt as an adhesive layer on p-type GaAs contact layer that can yield low contact resistance and that can be an HF-resistant metal [35].
3. Results
The ELO process should preserve the properties of the as-grown layers after transferring to a foreign substrate. We investigated the retention of structural and optical properties through high-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) spectroscopy. The black curve in Fig. 3(a) shows the entire epitaxial structure from the Si wafer to the GaAs PD layer, including the In0.1Ga0.9As single insertion layer and In0.1Ga0.9As/GaAs dislocation filter layers. After ELO process, however, the flexible PD on PI sample shows the GaAs layer only as shown in the red curve. The Si peak is from the Si carrying piece that we used to mount the flexible PD sample for HRXRD measurement. Note the slight shift of the GaAs peak after ELO compared to as-grown sample, and we attribute this to release of thermal tensile strain in the GaAs layer heteroepitaxially grown on Si [36]. Omega rocking curves in Fig. 3(b) further verify that metal bonding and ELO process essentially maintains the structural quality. The full-width at half-max of the rocking curves were 227 and 271 arcseconds for before and after the ELO process, respectively. HF etching in the AlAs sacrificial layer can roughen the surface and leave etching residues at the interface, which can possibly increase the Omega rocking curve measurement. We have also performed PL measurements to assess retention of optical properties. Figure 3(c) shows similar PL spectra shapes while the black curve is off by ∼4 nm (∼7 meV) compared to the red curve. This result is due to release of the thermal tension [36], well consistent with the HRXRD result above. However, the PL intensity was lowered approximately by 10 times as seen in the inset. We think that the absence of the AlAs layer after ELO increased non-radiative recombination processes of the photo-generated carriers at the etched surface. Also, the ELO PD sample has a much smaller III-V measurement area as shown in Fig. 2, and this should have led to the much weaker PL intensity, not because of degraded optical property. The excellent retention in XRD peaks and PL spectra imply a high quality film and that our ELO process well maintains the crystal structure, not further introducing additional strain and defects that could deleteriously affect the optoelectronic properties of the transferred device.
Fig. 3. Material characterization comparison before and after GaAs PD ELO a) High-resolution x-ray diffraction (004) 2Theta-Omega plots, b) Omega rocking curve, and c) PL spectra of as-grown GaAs PD on Si (black) and GaAs PD transferred to PI and temporarily mounted on a silicon wafer (red). Inset of c) shows PL spectra with absolute values.
We investigated the IV characteristics of the fabricated flexible PDs using a Keithley 4200 probe station/Agilent 4156C system at room temperature. The photo responses were obtained using an 808-nm laser diode coupled to the probe station and calibrated using a Thorlabs S401C sensor and PM100D console. As a comparison, flexible PDs grown on a native GaAs wafer were also prepared via the same fabrication procedure. Figures 4(a)–4(b) show the dark IV curves of 20 PDs grown on either substrate. At a reverse bias of -1 V, the average dark current of GaAs-grown PDs is 3.78 × 10−11 A while that of Si-grown PDs is 1.25 × 10−10 A, which is only ∼3.3 times larger even with the ∼1 × 106 cm-2 TD density in the PD. The optical response of our flexible PDs at different excitation power was measured at room temperature as shown in Figs. 4(c)–4(d). At a reverse bias of -1 V the responsivity is ∼0.31 A/W for both PDs grown on GaAs and on Si. Thus, the Si-grown PDs perform just as well as those grown on a native GaAs wafer. External and internal quantum efficiency of the flexible PD grown on Si are 0.48 and 0.70.
Fig. 4. Comparison of flexible GaAs PD performance. Dark current-voltage curves of 20 photodetector devices grown on a) GaAs and b) Si. Photo current-voltage curves of an exemplary PD grown on c) GaAs and d) Si. e) Relationship between the laser power density and photo current.
The relationship between the illuminated power and the photocurrent can provide insights into the recombination mechanism and the presence of traps. We plotted our data in Fig. 4(e) and found the data to fit well with the power law,
where Iphoto is the photocurrent, A is a fitting constant related to the detectivity, P is the laser power density and α is a fitting parameter related to the trap states. An α value of unity ideally represents a semiconductor without trap states. We obtained α values of 0.98 and 0.96 for the flexible PDs grown on GaAs and Si, respectively, indicating a low density of active trap states [37,38]. The low dark current and responsivity equal to the GaAs-grown PDs and α value very close to unity from the flexible PDs epitaxially grown on Si are a strong testament to the effectiveness of our work using high quality, low-cost, and large-size GaAs/Si template.We also have investigated the resonant cavity effect of the wafer bonding metal layers (Pt/Au//Au/Pt) used for the ELO process on our flexible GaAs PD structure [39,40]. Lumerical finite-difference-time-domain (FDTD) software was used for dispersion calculation. Figures 5(a)–5(b) clearly shows that the incoming light below 700 nm is almost fully absorbed within the 1450 nm-thick p-i-n GaAs PD except for the reflected light at the surface, due to strong light absorption in GaAs material (absorption coefficient > 104 cm-1). In contrast, photons above 700 nm are not fully absorbed. The transmitted photons are reflected by the wafer bonding metal stack and contribute to an enhanced PD responsivity compared to the case of a PD without the metal layers. Figure 5(c) shows that the Fabry-Perot cavity effect can selectively enhance power absorbance in the flexible GaAs PD, revealing a 30% increase in the absorption at the 800 nm wavelength relative to the structure without metal layers. More structural design in the back-reflector with various dielectric materials is going to be further investigated to improve light-trapping.
Fig. 5. Cavity resonance by wafer bonding metal reflector. (a, b) Lumerical FDTD electric field intensity profiles for structure with and without the wafer bonding metal stack. (c) Calculated power absorbance in the GaAs layers over wavelength.
To test for mechanical flexibility and durability, we subjected our flexible GaAs PD arrays under different bending stresses. Specifically, we performed bending tests at different curvatures and bending cycles. The flexible PD was mounted on metal chucks with different curvatures, then the dark and photocurrents were measured as illustrated in the Fig. 6(a) inset. Figure 6(a) shows the changes in dark and photo-currents under bending compared with that without bending, i.e. the current under bending normalized with respect to the current at a flat configuration. Under a high curvature of 2 cm-1 (radius = 0.5 cm), our flexible PDs still performed similarly to that without bending. There are deviations in the data which we attribute to measurement uncertainties brought about by equipment sensitivity and small variations in the position of the sample, light source and probes. We then subjected the flexible PDs to continuous bending around a 0.5 cm radius chuck at a rate of about 1 bend/sec and found the currents to remain at almost the same values after 1000 bending cycles [Fig. 6(b)]. These tests suggest that at a wide range of bending stresses, no cracks were generated and that the integrity of our device’s optoelectronic properties was retained, making our device promising for being mounted on curved and dynamically changing surfaces.
Fig. 6. Device flexibility and durability. Normalized dark current and photocurrents at different (a) bending curvatures. Inset shows bending configuration. (b) PD durability test by repeating device bending up to 1000 times. Inset shows 200 PD arrays when being flat and bent.
4. Conclusion
Several improvements can still be made for the proposed method. With a strategic inclusion of an etch stop layer [41], we envision our virtual substrate to be reusable and thus be more cost-effective. We also believe that the metal wafer bonding and ELO process can be scaled to more than 200 cells by redesigning a photomask. The dark and photo currents can further be improved by applying anti-reflective [42], passivating layers on the finished device surface [43], and implementing resonant cavity with oxide/metal reflector scheme [44]. Also, we believe that our fabrication process can be extended to other optoelectronic devices such LEDs, MOSFETs, capacitors, and solar cells.
In conclusion, we demonstrated a low-cost, high-throughput flexible GaAs photodetector arrays epitaxially grown on Si and transferred to a polyimide film via metal wafer bonding and epitaxial lift-off. Through high-resolution x-ray diffraction, photoluminescence, we showed that the optical and structural properties were retained well after the transfer process. Consistent dark currents across numerous devices were achieved, illustrating the uniform epitaxial growth and device fabrication. The same responsivities were obtained from the Si-grown and GaAs-grown photodetectors. The wafer bonding metals used for epitaxial lift-off also performed as additional metal reflectors for light-trapping enhancement. Lastly, bending tests at small radii of curvatures and for 1000 times demonstrate our device’s flexibility and durability. Therefore, we believe that the presented work is promising for cost-effective and flexible III-V optoelectronic device platform with high durability and scalability.
Funding
Korea Institute of Science and Technology (2E30100); National Research Foundation of Korea (NRF-2017M1A2A2048904).
Acknowledgements
The authors are also thankful to Dr. Gunwu Ju for MBE machine maintenance.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
See Supplement 1 for supporting content.
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