In this paper, InAs0.81Sb0.19-based hetero-junction photovoltaic detector (HJPD) with an In0.2Al0.8Sb barrier layer was grown on GaAs substrates. By using technology computer aided design (TCAD), a design of a barrier layer that can achieve nearly zero valance band offsets was accomplished. A high quality InAs0.81Sb0.19 epitaxial layer was obtained with relatively low threading dislocation density (TDD), calculated from a high-resolution X-ray diffraction (XRD) measurement. This layer showed a Hall mobility of 15,000 cm2/V⋅s, which is the highest mobility among InAsSb layers with an Sb composition of around 20% grown on GaAs substrates. Temperature dependence of dark current, photocurrent response and responsivity were measured and analyzed for fabricated HJPD. HJPD showed the clear photocurrent response having a long cutoff wavelength of 5.35 μm at room temperature. It was observed that the dark current of HJPDs is dominated by the diffusion limited current at temperatures ranging from 200K to room temperature from the dark current analysis. Peak responsivity of HJPDs exhibited the 1.18 A/W and 15 mA/W for 83K and a room temperature under zero bias condition even without anti-reflection coating (ARC). From these results, we believe that HJPDs could be an appropriate PD device for future compact and low power dissipation mid-infrared on-chip sensors and imaging devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Sensing and imaging in mid-infrared (MIR) wavelength region are significantly crucial for gas detection, medical diagnostics and defense due to its coverage of the absorption band of gases, molecules and its low-loss transmission window (3-5μm) in the atmosphere [1,2]. Emerging IoT era demands MIR photodetectors (PDs) to require low power and to integrate compact on-chip sensors, which consist of sources, detectors and read-out circuits. Thus, various studies have been done to integrate various MIR components on a single chip through different semiconductor technology platforms such as all-Si optical platform, Si-on-nitride (SoN) and germanium (Ge)-on-insulator (GOI) for a possible volume manufacturing [3–5].
In order to obta7in high-resolution and portable imaging devices, many researchers have investigated a high performance MIR focal plane arrays (FPAs) using a hybrid integration with read-out integrated circuits (ROICs) and monolithic integration on the same substrate with metal-semiconductor field effect transistors (MESFETs) [2,6]. However, in the MIR spectral regime, a large dark current always becomes problematic in PDs based on semiconductors due to its small band gap. This makes high temperature operation extremely difficult and results in a relatively bulky and high power consuming devices, because it typically requires an extra cooling system. Therefore, to achieve the desired purpose of on-chip MIR sensors and portable MIR imaging devices, increasing operating temperature of MIR PDs is highly required.
Recently, hetero-junction PDs based on ternary InAsSb material have become one of the promising candidates thanks to its proper band gap to cover IR range, low electron effective mass and low Auger recombination rate [7–15]. Among various types of PDs, photovoltaic detectors attract a lot of attention, owing to their high response speed and zero bias operation, which mitigate self-heating and power dissipation issues present in photoconductive detectors [11–15]. However, to fully utilize the potential of InAsSb photovoltaic detectors with advantages described above, increasing the operating temperature is significantly important, and is essential to realize low power consumption on-chip sensors.
To increase the operating temperature, inserting a wide band gap material as a barrier layer between contact layer and absorption layer has been typically used in many previous studies [7–9,13,14]. As a result, they reduced the excessive dark current induced by the diffusion of majority carriers while allowing transfer of minority carriers, which leads to the increased operating temperature. To achieve this in p-contact/barrier/absorption layer/n-contact structure, the ideal barrier would be an unipolar barrier, which does not have the undesirable valance band offset (VBO), because it hinders the photo-generated hole transport to p-contact. At the same time, the barrier should have a large conduction band offset (CBO) to block dark current by electron diffusion. However, AlSb layer, which is typical barrier has the VBO for InAsSb layer with Sb composition above 15% [16–18]. With this barrier, while photo-generated electron can be collected to contact region, the small VBO can be act as diffusion barrier for photo-generated hole in absorption region. Even though this VBO is quite small, it decreases the quantum efficiency (QE) of the PD. Additional reduction of VBO could be still required for achieving an optimum barrier layer.
Furthermore, due to the lack of substrates for homo-epitaxy, many previous efforts have been made to grow high quality InAsSb materials on various substrates such as GaSb, InSb, GaAs and Si [7–15]. Among them, GaAs substrate has advantages in terms of high quality substrate, high transmittance in IR range and monolithic integration to ROICs without flip-chip technology for image sensor array in spite of relatively high lattice mismatch between InAsSb and GaAs compared to GaSb, InAs and InSb [6,15]. To compensate the lattice mismatch between GaAs and InAsSb and to improve the crystalline quality of InAsSb, many research groups have introduced various buffer layers and growth method [11,12,19,20]. Consequently, the operation temperature of InAsSb based PDs grown on GaAs substrates differs depending on each buffer growth techniques. In spite of such advances, only a few results have been reported on photovoltaic detectors in contrast to the vast number of studies on photoconductive detectors such as N-B-n structure [11–15].
In this paper, we report the room temperature operation of InAs0.81Sb0.19-based hetero-junction photovoltaic detector (HJPD) with In0.2Al0.8Sb barrier layer grown on GaAs substrate using Al0.93Ga0.07Sb metamorphic buffer layer via molecular beam epitaxy (MBE). The material qualities of InAs0.81Sb0.19 were characterized by using X-ray diffraction (XRD) measurement and Hall measurement. The resulting PDs clearly showed the different characteristics depending on the presence of barrier. HJPDs exhibited a clear photocurrent response and peak responsivity of 15 mA/W with cutoff wavelength of 5.35 μm under room temperature, whereas the reference p-i-n PD without barrier layer revealed weaker photocurrent response and responsivity of 4mA/W with cutoff wavelength of 5.15 μm under 230K. To the best of our knowledge, this demonstrates the first room temperature operation of a bulk InAs0.81Sb0.19-based photovoltaic detector grown on GaAs substrate in a spectral range of MIR.
2. Barrier layer design and material characterization for growing HJPDs
In order to design a unipolar barrier for InAsSb absorption layer, simulation of the energy band diagram was done by solving 1D Poisson’s and Schroedinger’s equations in self-consistent manner. The simulation result via the technology computer aided design (TCAD) simulator is shown in Fig. 1(a). Parameters we used are tabulated in Fig. 1(b). The structure consisted of In0.2Al0.8Sb barrier layer between doped p-InAs0.81Sb0.19 layer and unintentionally doped intrinsic InAs0.81Sb0.19 absorption layer under zero bias condition at room temperature. The nominal doping concentrations of p-InAs0.81Sb0.19 and intrinsic (lightly n) absorption layers was assumed to be 1 × 1018 and 5 × 1016 cm−3, respectively. A moderate In0.2Al0.8Sb layer thickness of 50 nm was chosen, considering the critical thickness of approximately 90 nm based on the model proposed by People & Bean  from the lattice mismatch between InAs0.81Sb0.19 and In0.2Al0.8Sb materials (approximately 1.09%). At first, the simulation of a HJPD structure with a typical AlSb barrier layer was done to validate the simulation data and input parameters. Band diagram for AlSb showed the 1.2 and 0.1 eV for CBO and VBO, respectively, which is reasonable compared to previous results when considering different Sb compositions and doping concentrations [17-18]. Second, 20% of indium was added in AlSb to form unipolar barrier, resulting in a reduction of both CBO and VBO in band diagram. As intended, this structure has high CBO of 0.9 eV and nearly zero VBO, which is possible to block the diffused electrons while naturally collecting the photo-generated carriers simultaneously. Additionally, the dark current modeling and quantum efficiency (QE) simulation based on these band diagrams will be further studied to accurately predict the performance of the InAsSb based PDs in the future.
The InAs0.81Sb0.19 epitaxial layer quality was analyzed using the structure consisting of i-InAs0.81Sb0.19/Al0.93Ga0.07Sb buffer (2/1 μm) on GaAs substrate. This structure was grown by a Riber compact 21E solid source MBE system with As and Sb cracker cells. During the growth, reflection high energy electron diffraction (RHEED), and BandiT band-edge thermometry system were utilized to monitor the growth condition and temperature. First, surface oxide of GaAs substrate was desorbed by the heating at 620°C under As ambient condition. Then, 0.1 μm-thick GaAs buffer layer and 1 μm-thick Al0.93Ga0.07Sb buffer layer were grown at 580°C and 515°C, respectively. Subsequently, for growing InAs0.81Sb0.19 random alloy, indium flux of 0.8ML/s, As and Sb dimer mode were used at 440°C. To examine the crystalline quality, high-resolution XRD (HRXRD) measurements were performed by using ATXG equipped with Cu Kα1 radiation and double crystals (data not shown). The threading dislocation density (TDD) was calculated by using Ayer’s model regarding full width at half maximum (FWHM) values of InAs0.81Sb0.19 (004) peak . Figure 2(a) shows TDD values of InAsSb layer grown on GaAs substrate versus buffer layer thickness grown by different growth methods including metalorganic vapor phase epitaxy (MOVPE) and MBE. It is ideal for both buffer layer thickness and TDD value decrease, simultaneously. The FWHM value of this work was found to be 396 arcsec at 60.18° from HRXRD data, which corresponds to the TDD of approximately 3.3 × 108 cm−2. Although there were only a few studies to compare the quality of InAsSb materials grown on GaAs, our InAs0.81Sb0.19 quality exhibited quite a low TDD value and comparable or even better crystallinity compared with previous reports as shown Fig. 2(a).
For electrical characterization, Hall measurement was conducted by using van der Pauw method for InAs0.81Sb0.19 layer at room temperature. Figure 2(b) shows that previous results on InAsSb layer on GaAs substrate with different Sb compositions at room temperature [19,23–26]. Mobility of InAs0.81Sb0.19 was approximately 15,000 cm2/V·s, which is the best result among InAsSb with Sb composition around 20% grown on GaAs substrate. These results can be attributed to the optimization of Al0.93Ga0.07Sb buffer layer. The surface morphology and electrical properties of Sb-based materials are sensitive to the modulation of V/III ratio, which could significantly affect the layer quality . These results strongly suggest that current InAs0.81Sb0.19 layer is very high quality enough to grow and fabricate devices.
After examining the energy band alignment and material properties, the final HJPD structure was decided as shown in Fig. 3(a). This structure is composed of Al0.93Ga0.07Sb metamorphic buffer/n-InAs0.81Sb0.19 layer (5 × 1017 cm−3)/i-InAs0.81Sb0.19 absorption layer (unintentionally doped)/In0.2Al0.8Sb barrier/p-InAs0.81Sb0.19 (3 × 1018 cm−3) layer from bottom to top. A reference p-i-n PD without In0.2Al0.8Sb barrier was also grown for comparison. Growth temperatures for each layer were the same as previously mentioned. After the growth of HJPD, HRXRD measurement was carried out to confirm the quality of reference p-i-n PD and HJPD. Figure 3(b) shows the normal θ-2θ scan results for p-i-n PD, HJPD and simulated spectra using EPITAXY simulator at room temperature. Only two peaks were observed for p-i-n PD at 60.24° and 66.05° corresponding to InAs0.81Sb0.19/Al0.93Ga0.07Sb and GaAs, whereas three peaks were observed for HJPD due to the existence of In0.2Al0.8Sb with different lattice constant. The peak at 66.05° completely corresponded to GaAs (004) substrate. InAs0.81Sb0.19 and Al0.93Ga0.07Sb peaks were combined to a single peak. In HJPD, additional In0.2Al0.8Sb peak was at a smaller angle region of 58.86° than the combined peak of InAs0.81Sb0.19 and Al0.93Ga0.07Sb. In EPITAXY simulation, InAs0.81Sb0.19 and Al0.93Ga0.07Sb can be distinguished thanks to its perfect crystallinity. However, due to their similarity of lattice constants and imperfect crystallinity, combined peak in XRD measurements could not be divided. Therefore, a composition of InAs0.81Sb0.19 was further verified through optical characterization of devices. Given this situation, the peak positions of grown samples matched well to the simulation result. These diffraction patterns indicated that MBE grown InAs0.81Sb0.19 photovoltaic detectors are successfully formed on the GaAs substrate with a precise control of target compositions.
Then, device fabrication process was carried out. After native oxide removal with HCl:DI (1:5) for 1 minutes, a top electrode of Ni/Au (30/300 nm) was evaporated by electron beam (EB), followed by lift-off process. Subsequently, mesa etching was conducted by H3PO4 and HF based-solutions for InAs0.81Sb0.19 and In0.2Al0.8Sb etching until n-InAs0.81Sb0.19 contact layer was exposed. A 200 nm-thick SiNx layer was deposited on the sidewall at 120°C by plasma enhanced chemical vapor deposition (PECVD) for the surface passivation and isolation of the electrode. Then, bottom contact for n-InAs0.81Sb0.19 layer was formed by Ni/Au (30/300 nm) evaporation and lift-off process. The final devices had 500x500 μm2 optical window for front illumination. Figure 3(c) shows the schematic image of fabricated HJPD and cross-section scanning electron microscopy (SEM) image. SEM image shows each layer with clear interfaces in between these layers, indicating the In0.2Al0.8Sb layer was well formed between InAs0.81Sb0.19 layers. Thickness of In0.2Al0.8Sb were measured to be approximately 50 nm as expected.
3. Analysis of electrical and optical characteristics for fabricated PDs
To investigate the electrical characteristics and dominant dark current mechanisms in HJPD, the current density (J) - voltage (V) characteristics of the fabricated p-i-n PD and HJPD were measured at room temperature, as shown in Fig. 4(a). Measurements were carried out by a Keithely 4200 in a probe station. While the p-i-n PD exhibited weak rectifying characteristic, it was found that clear improvement of rectifying property was achieved in HJPD over the whole reverse bias range. For p-i-n PD, a weak rectifying characteristic could be due to relatively large leakage current. To investigate the physical origin of large leakage current, we have characterized the surface leakage current of these devices at room temperature according to our previous report . By extracting of surface leakage current from various patterns, HJPD showed the 2mA/cm and 0.2mA/cm of surface leakage current for non passivated and passivated conditions, respectively. The p-i-n PD revealed the 3mA/cm and 1mA/cm for non passivated and passivated PDs. Though SiNx passivation helps the surface leakage current reduced, it is still large leakage current which suggested that our SiNx passivation layer is not suitable for the termination of InAsSb surface defects. Nevertheless, enhancement of rectifying behavior with barrier layer is consistent with previous report of InSb photovoltaic detectors with barrier layer . This significant decrease of dark current can be attributed to the blocking of the electron diffusion induced current and surface leakage current by inserting a wide band gap barrier . For analysis, the 97temperature dependence of J from 280K to 90K was measured as shown in Fig. 4(b). With decreasing the temperature, obvious suppression of dark current was observed in the diode characteristic. Figure 4(c) shows the temperature dependence of the J measured at −10 mV as a function of inverse temperature for p-i-n PD and HJPD. Through extraction of activation energy (Ea) from the slope of the Arrhenius curve, the dominant mechanism for dark current with respect to temperatures was evaluated. The Ea fittings were performed by the following expression for the diffusion limited current (Jd),31]. In contrary to reference p-i-n PD, extracted Ea in HJPD was found to be as high as 0.271 eV above 200K. This value closely matches with the zero temperature band gap (Eg(T = 0) = 0.276 eV) of InAs0.81Sb0.19. The fact that the activation energy is close to Eg(T = 0) implies that the diffusion limited current could be dominant in this range. The small difference between Ea and intrinsic band gap suggests that potential barrier in the valance band is significantly small . Therefore, it can be interpreted that In0.2Al0.8Sb barrier layer does not impede the collection of photo-generated carriers, while it still suppresses the electron diffusion and surface leakage current. Lower Ea smaller than a half of band gap from the second gradient at low temperature indicates that the current is dominated by generation-recombination in absorption region. Figure 4(d) shows the bias voltage dependence of Ea, extracted from the Eq. (1). The dotted line shown in Fig. 4(d) is for the Eg(T = 0), indicating the diffusion limited behavior. In accordance to a small reverse bias range from 0 V to −0.15 V, diffusion limited behavior would be dominant for HJPD. In a larger bias range above −0.15 V, smaller Ea was obtained which implies HJPD could be seriously affected by generation-recombination or tunneling limited behavior. From these results, we believe that the superior performance of HJPD as well as p-i-n PD can be achieved through further passivation technique optimization.
Figure 5(a) and 5(b) shows the normalized photocurrent response of p-i-n PD and HJPD measured by a Bruker Vertex 80v Fourier transform infrared spectrometer (FTIR). In this FTIR system, a global source emitting MIR radiation along with a KBr beam splitter was used. Signals with various temperatures were amplified by a low-noise current amplifier (Keithley 428) and then embedded in the spectrometer. PDs were measured at different temperatures with no external bias (0V). Photocurrent responses of p-i-n PD were observed in a range from the 10K to 230K as illustrated in Fig. 5(a). The band edge shift of p-i-n PD was clearly shown in the low energy regime with increasing temperature. Photocurrent response and operating temperature of our p-i-n PD are similar to the result in previous report which has InAs0.8Sb0.2 absorption layer showing maximum operating temperature up to 240K . This suggests that the fabricated p-i-n PD was successfully grown and has the comparable performance with a well-matched transmission window of MIR in a spectral range of 3-5 μm.
On the other hand, the clear photocurrent responses up to room temperature were observed for the HJPD under the same measurement condition as a p-i-n PD. Its band edge was also shifted to low energy region with increasing temperature. The strong photocurrent responses for HJPD indicated that it is possible to operate the device sufficiently at room temperature. With only 50 nm-thick of In0.2Al0.8Sb barrier insertion, the operating temperature was dramatically increased from 230K to 300K. It was verified that a barrier layer can play an important role to increase the operating temperature by reducing the dark current. Interestingly, the PD characteristics show the strong photocurrent response at broad wavelength between 3 and 5 μm. To investigate its physical origin, optical intensity distribution in the PD layer was simulated. Figure 5(c) shows the simulated optical intensity distribution in HJPD structure as a function of the incident light wavelength. Incident light with a short wavelength is considerably absorbed near the surface, whereas long wavelength having above 3 μm is mainly absorbed in absorption region. There is still a significant amount of unabsorbed light, which reaches n-InAs0.81Sb0.19 contact layer and Al0.93Ga0.07Sb buffer layer. It should be noted that there is strong resonance at 3-5 μm in the absorption region, which is definitely consistent with the photocurrent response spectra measured by FTIR. These results strongly suggest that further layer thickness optimization will be very useful to enhance the detector performances.
To accurately determine the material band gap of InAs0.81Sb0.19 through optical measurement, the temperature dependence of the cut-off wavelength (λc) for HJPD was investigated when λc was decided by a half of the peak intensity of photocurrent response. As illustrated in Fig. 5(d), λc is fitted by Varshni expression,33,34]. As a result, the change of λc from 3.7 to 5.35 μm was in a good agreement with the expectation for the Sb composition of 19% as depicted in Fig. 5(d). It is a reasonable value compared with that of previous result because slightly shorter λc was obtained than that of InAs0.8Sb0.2 at the same temperature . From this optical characterization, it was concluded that the composition of Sb in InAsSb alloy is 19%, confirming the result obtained by XRD measurement.
Finally, the responsivity measurement was performed as a function of temperatures using 500 K of blackbody source radiation with a 0.3-inch aperture size, a lock-in amplifier and a chopper frequency of 500 Hz. From this measurement, the peak responsivity can be extracted from the Eq. (3) associated with the blackbody geometrical configuration and the λc for both PDs ,Eq. (3), we have used the cutoff wavelength of photocurrent responses shown in Fig. 5. Figure. 6(a) shows that the peak responsivity of p-i-n PD and HJPD as a function of temperature. The peak responsivity was 0.38 and 1.18A/W for p-i-n PD and HJPD at 83K under zero bias. This corresponded to the quantum efficiency of 11% and 34% for p-i-n PD and HJPD. Also, HJPD recorded the 15 mA/W at room temperature, whereas p-i-n PD showed the peak responsivity 4 mA/W at 230K and could not be operated at temperatures higher than 230K. The threefold performance improvement in HJPD than p-i-n PD was achieved by inserting a 50 nm of In0.2Al0.8Sb barrier layer only, which contributed to the decrease of dark current as shown in Fig. 4. These results on operating temperature completely coincide with photocurrent response as depicted in Fig. 5. When temperature is increased above crossover temperature of approximately 200K as described in Fig. 4(c), peak responsivity is rapidly degraded for both PDs due to an increased noise current by a diffusion of holes in n-type region . Furthermore, the resistance-area (RA) products for HJPD at 300K and 100K are illustrated in Fig. 6(b). The HJPD showed the slightly smaller RoA product of 0.013 and 0.75 Ω⋅cm2 at 300K and 100K under zero bias respectively compared to other papers on InAsSb PD showing approximately 0.1 Ω⋅cm2 of RoA product at room temperature . This would be attributed to the leakage current caused by the non-optimized surface passivation for HJPD. Additionally, our HJPD may have defect-assisted leakage current, since our HJPD is grown on lattice-mismatched GaAs substrate rather than lattice-matched GaSb substrate in other works. Nevertheless, it is still very meaningful that we demonstrated room temperature operation of InAsSb PD by barrier engineering even on lattice-mismatched substrates. Moreover, these results strongly suggest that it can be further much improved by the suppression of leakage current.
Consequently, the notable improvements on operating temperature and responsivity were accomplished in the HJPD. Performance of HJPD at cryogenic and room temperature was quite good compared to commercialized photovoltaic detectors. Photovoltaic detectors produced by Hamamatsu  revealed the 4.5 mA/W of peak responsivity at room temperature. The obtained performance was three times better at room temperature. Furthermore, there is still room for the additional improvement of responsivity due to insufficient thickness of absorption layer for collection of light, as shown in Fig. 5(c). In addition, since anti-reflection coating (ARC) and an optimal surface passivation technique were not used, performance can be further improved by employing proper ARC and passivation
We proposed InAs0.81Sb0.19-based photovoltaic detector with In0.2Al0.8Sb barrier layer grown on GaAs which having a spectral range of MIR regime at room temperature. Band simulation showed that In0.2Al0.8Sb barrier layer could minimize the VBO compared to the typical AlSb barrier. In addition, a high quality InAsSb epitaxial layer was confirmed by XRD measurement and Hall measurement, which is significantly crucial for detector performance. From the optical characterization using FTIR and the simulation of electric field distribution, it was shown that absorption of HJPD occurred in the targeting 3-5 μm MIR range. Finally, responsivity measurement demonstrated the 1.18 A/W and 15mW of peak responsivity at 83 K and room temperature under zero bias, respectively. This is a significant improvement by at least three times compared with the reference p-i-n PD. In addition, the room temperature operability became comparable to that of commercialized InAsSb photovoltaic detectors.
KIST Institutional Program of Flag-ship (2E28180); National Research Foundation of Korea; (2015004870, 2016910562, NRF-2017M1A2A2048904); Brain Korea 21 Plus project for SNU Materials Division for Educating Creative Global Leaders (F15SN02D1702).
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