PIN Photodetector Research Articles

A self-powered and broadband UV PIN photodiode employing a NiOx layer and a β-Ga2O3 heterojunction

Crucial commercial and space applications require the detection of broadband ultraviolet (UV) rays spanning from UV-A to UV-C. In this study, the authors demonstrate a broadband UV photodetector employing a p-type NiOx layer and an n-type β-Ga2O3 heterostructure in PIN configuration for the first time. Simulations are conducted to optimize the doping concentration and thickness of the NiOx layer, ensuring that (a) a reasonable depletion width is maintained within the NiOx layer for UV-A and UV-B light absorption; (b) anode ohmic contacts are formed on the nondepleted NiOx film, and (c) >70% of the UV-C light is absorbed by β-Ga2O3. The optimized NiOx/β-Ga2O3 PIN photodiode exhibits good responsivity to incident light wavelengths in the UV-A, UV-B, and UV-C regions. While the NiOx layer is considered to be responsible for providing good photoresponsivity in the UV-A and UV-B regions, a highly resistive (near-intrinsic) β-Ga2O3 layer is required for the absorption of incident UV-C light. A record detectivity of >1011 cm Hz0.5 W−1 for the UV-B and UV-C regions and >1010 cm Hz0.5 W−1 for the UV-A region is observed in the NiOx/β-Ga2O3 heterostructure PIN photodiode during the self-powered operation. The results presented in this study are promising and instigate device design strategies for (ultra)wide bandgap semiconductor-based broadband UV PIN photodetectors.

Dark current in monolithic extended-SWIR GeSn PIN photodetectors

Monolithic integration of extended short-wave infrared photodetectors (PDs) on silicon is highly sought-after to implement manufacturable, cost-effective sensing and imaging technologies. With this perspective, GeSn PIN PDs have been the subject of extensive investigations because of their bandgap tunability and silicon compatibility. However, due to growth defects, these PDs suffer a relatively high dark current density as compared to commercial III–V PDs. Herein, we elucidate the mechanisms governing the dark current in 2.6 μm GeSn PDs at a Sn content of 10 at. %. It was found that in the temperature range of 293–363 K and at low bias, the diffusion and Shockley–Read–Hall (SRH) leakage mechanisms dominate the dark current in small diameter (20 μm) devices, while combined SRH and trap assisted tunneling (TAT) leakage mechanisms are prominent in larger diameter (160 μm) devices. However, at high reverse bias, the TAT leakage mechanism becomes dominant regardless of the operating temperature and device size. The effective non-radiative carrier lifetime in these devices was found to reach ∼100–150 ps at low bias. Owing to TAT leakage current, however, this lifetime reduces progressively as the bias increases.

PIC-integrable high-responsivity germanium waveguide photodetector in the C + L band.

We report the demonstration of a germanium waveguide p-i-n photodetector (PD) for the C + L band light detection. Tensile strain is transferred into the germanium layer using a SiN stressor on top surface of the germanium. The simulation and experimental results show that the trenches must be formed around the device, so that the strain can be transferred effectively. The device exhibits an almost flat responsivity with respect to the wavelength range from 1510 nm to 1630 nm, and high responsivity of over 1.1 A/W is achieved at 1625 nm. The frequency response measurement reveals that a high 3 dB bandwidth (f3dB) of over 50 GHz can be obtained. The realization of the photonic-integrated circuits (PIC)-integrable waveguide Ge PDs paves the way for future telecom applications in the C + L band.

Effect of Different Nanoparticles SPP Grating on GaAs PIN Photodetector Performance

Effect of Different Nanoparticles SPP Grating on GaAs PIN Photodetector Performance

Study the effect of temperature variation and intrinsic layer thickness on the linear response of a PIN photodetector: A finite element method approach

Perhaps mentioning the place of Silicon Valley semiconductor companies in business and technology is enough to remind the place of semiconductors in today's world. One of the applications of semiconductors is in manufacturing detection devices. This research aims to investigate the linearity of the response of a PIN photodetector to incident light. For this purpose, we will first simulate a PIN photodetector using COMSOL Multiphysics 6.0 software. Then we expose this photodetector to the light emitted from the scintillator and check the linearity of the response in the range of wavelengths irradiated to it. The research was done on photodetector PINs that are made of two semiconductor materials, GaAs and Si. The temperature and thickness of the intrinsic layer of a photodetector greatly affect its outputs, such as Current Terminal, Quantum Efficiency, and Responsivity. Therefore, the effect of these two parameters on the linearity of the output response of a photodetector has been investigated. To monitor the linearity of the photodetector output in each mode, a Linear Fitting has been applied to the output of the photodetectors using the Origin Pro software, and the linearity of the output data has been controlled from the R-Square and Adjusted R-Square of this fitting. The investigation illustrates that the response of GaAs is more linear than that of Si in similar cases. For example, the GaAs photodetector in the thickness of 8 μm and at the temperature of T = 70 °C has Adjusted R-Square = 0.9972. In contrast, the Si photodetector has Adjusted R-Square = 0.9178 under the same conditions.

Surface plasmonic nano grating for improving GaAs PIN photodetectors performance

Surface plasmonic nano grating for improving GaAs PIN photodetectors performance

Nanosecond Laser Annealing of In-Situ Boron-Doped Ge Layers for Dopant Activation

Doped Ge can be employed in the sources and drains of Ge pMOS transistors 1 or as p-type films in Ge p-i-n Photo-Detectors (PDs) 2. If the growth temperature is low enough, they can even be used in GeSn PDs and Light Emitting Devices 3. Those Short and Mid Wavelength Infra-Red devices are rapidly gaining interest especially since the demonstration of electrically pumped lasing in GeSn at temperatures up to 100K by the University of Arkansas 4.The electrically active carrier concentration in binary Si:P was significantly increased thanks to a dissolution with nanosecond laser annealing (NLA) of P clusters 5. Ge:B alloys with metastable, ultra-high substitutional concentrations of B can also be obtained thanks to in-situ doping 6. However, electrical activation in such Ge:B binaries was limited by the formation of electrically inactive clusters 7. In this work, we evaluated whether or not they could be dissolved by NLA.The melt threshold shifted from 0.875 Jcm-2 to 0.85 Jcm-2 as the substitutional B concentration increased (from Time Resolved Reflectivity maps during Ge:B NLA; Figure 1 (a)). This was likely due to differences in terms of crystalline quality, surface roughness and/or B concentration itself. Figure 1 (b) shows normalized ω-2ϴ scans around the (0 0 4) X-Ray Diffraction order. The Ge:B peak did not significantly change below the melt threshold at 0.825 Jcm-2. Above it, the Ge:B peak became less and less intense and vanished at the full melt threshold (1.05 Jcm-2). NLA at even higher Energy Densities (ED, 2.00 Jcm-2) did not result in a recovery of the Ge:B peak.At the melt threshold, localized surface structures melted, as shown by Atomic Force Microscopy in Figure 1 (c) with a 30 nm z-scale. Surface structures were rectangular with sides along the <110> directions, as for the cross-hatch of the as-grown layer. The size of all surface structures was quite constant. Comparable surface structures were previously seen for SiGe 8.A High Resolution Transmission Electron Microscopy image of the Ge:B layer after NLA with an ED of 0.85 Jcm-2 is shown in Figure 1 (d). The Ge:B layer had a thickness of 33 nm, which was slightly lower than that of the as-grown layer, i.e. 39 nm. It might be that material agglomerated in the 12 nm high surface structures 8. Surface structures otherwise had a brighter contrast, which might be due to the accumulation of lighter B atoms in those surface structures. Well defined Fast Fourier Transform features outlined the somewhat good crystalline quality of NLA Ge:B. However, they were less bright than for a superior quality epitaxial layer.Laser annealing resulted in a redistribution of B, with the formation of electrically inactive clusters that did not contribute to strain. Accordingly, the sheet resistance increased by 70%, from 39.82 Ω/□ up to 68.62 Ω/□, when the layer melted. This corresponded to an electrically active carrier loss of around 50%, from 8.1x1020 cm-3 down to 3.8x1020cm-3, shown in Figure 1 (e). This behavior was independent of the as-grown substitutional B concentration. Even multiple shots with various energy densities at the same position, shown in Figure 1 (f), were not able to improve electrical activation. However, there was some slight improvements of the sheet resistance in the sub melt regime, which would need to be confirmed in future experiments.We have seen above that single shot NLA resulted in the formation of (i) surface structures becoming larger for high EDs and (ii) electrically inactive clusters not contributing to strain. With the use of multiple shots, surface structures merged and formed even larger surface structures. This led to more B redistribution and likely to the formation of more electrically inactive boron-interstitial clusters in the melt regime. In the sub melt regime, multi shot NLA might improve contact resistance. NLA in the melt regime therefore seemed not to be able to improve electrical activation in heavily in-situ boron-doped Ge layers as it did for heavily in-situ phosphorous doped Si. Vohra, A. et al. Jpn. J. Appl. Phys. 58, SBBA04 (2019).Osmond, J. et al. Appl. Phys. Lett. 95, 151116 (2009).Casiez, L. et al. IEEE Photonics Conference (IPC) 1–2 (IEEE, 2020).Zhou, Y. et al. Optica 7, 924 (2020).Rosseel, E. et al. ECS Trans. 64, 977–987 (2014).Hartmann, J.-M. et al. ECS Trans. 98, 203–214 (2020).Porret, C. et al. Phys. Status Solidi A 217, 1900628 (2020).Dagault, L. et al. ECS J. Solid State Sci. Technol. 8, P202–P208 (2019). Figure 1

A Near-Infrared pin Photodetector of Strain-Enhanced Ge Layer Epitaxially Grown on a Bonded Si-on-Quartz Wafer

In Si photonics, photodetectors (PDs) of a Ge epitaxial layer on Si have been widely used, which show a high photodetection efficiency, or a high responsivity in the O (1.260–1.360 µm) and C (1.530–1.565 µm) bands. However, an issue lies in a low responsivity in the longer wavelength range of L band (1.565–1.625 µm). In this regard, we have proposed a method using a Ge layer epitaxially grown on a Si-on-quartz (SOQ) wafer [1]. The tensile in-plane lattice strain, derived from the thermal expansion mismatch between the Ge layer and base substrate, is enhanced more than 0.3% in Ge on SOQ, despite ~0.2% on Si (or Si-on-insulator). The enhanced strain in Ge on SOQ extends the fundamental optical absorption edge to about 1.65 µm, leading to a potential increase in the L-band responsivity. In this study, a vertical pin PD is fabricated using a Ge layer on SOQ, showing an increased responsivity in the L band.A bonded SQQ wafer was used as the starting substrate, fabricated by Shin-Etsu Chemical Co. Ltd. The top Si (001) layer was 200 nm in thickness, and was heavily doped p-type by B implantation. An undoped Ge epitaxial layer (500 nm) was grown by chemical vapor deposition using GeH4 as the source gas, followed by a growth of Si cap layer (50 nm) using Si2H6. In the top region of Ge layer as well as in the Si cap layer, a heavily n-type doping was carried out by P implantation, forming a vertical pin junction. Finally, a Ti/TiN electrode was formed. Responsivity spectra were measured in the wavelength range of 1.46–1.64 µm at a normal incidence. A reference pin PD was also fabricated using a Ge layer on a standard bulk Si wafer. According to x-ray diffraction measurements, the Ge layer on SOQ showed a tensile strain of 0.33%, despite 0.14% for the reference Ge on Si.Figure 1(a) shows typical current–voltage curves for a pin diode of Ge on SOQ measured under dark and under an illumination of 1.55-µm light (1.1 mW). The diode showed a good rectifying property under dark, while under the illumination, the current in the reverse bias significantly increased, showing a reasonable PD operation. Figure 1(b) shows typical responsivity spectra at a reverse bias of 1 V for PDs of Ge on SOQ and reference Ge on Si. Circles and lines correspond to experimental data points and theoretical curves, respectively. The responsivity for Ge on SOQ was larger in the measured wavelength range of 1.46–1.64 µm, including the L band. The increased responsivity is ascribed to the enhanced tensile strain. However, the increase in the L band was found to be suppressed by a high reflectance because of the multiple reflections at the interfaces. An optimized Si cap thickness as well as an anti-reflection coating with SiO2 can increase the normal-incidence responsivity as high as 0.3 (0.1) A/W up to 1.60 (1.64) µm without an increase in the Ge thickness.[1] Noguchi et al., J. Appl. Phys. 128, 133107 (2020). Figure 1

A Near-Infrared pin Photodetector of Strain-Enhanced Ge Layer Epitaxially Grown on a Bonded Si-on-Quartz Wafer

A strain-enhanced Ge layer is epitaxially grown on a bonded Si-on-quartz (SOQ) wafer and is applied to a near-infrared pin photodetector (PD) at a normal incidence. X-ray diffraction measurement reveals a tensile in-plane lattice strain of 0.33 ± 0.01% in Ge on SOQ, which is generated by the mismatch of the thermal expansion coefficients between the Ge layer and quartz substrate of SOQ. The strain is approximately two times higher than 0.14 ± 0.01% in Ge on an ordinary Si wafer. Resulting from a direct bandgap narrowing under the tensile strain, the fabricated pin PD of Ge on SOQ exhibits a responsivity enhanced in the C+L band (1.530 – 1.625 µm).

Non return to zero line coding with suppressed carrier in FSO transceiver systems under light rain conditions

Abstract This paper aims to simulate performance efficiency of carrier suppressed non return to zero line coding based FSO transceiver systems under light rain conditions with amplification units at 40 Gbps. The max. Q, BER and total optical power are simulated and demonstrated after FSO channel and PIN Photodetector Receiver under light rain weather conditions at maximum reach of 1.2 km at 10 Gbps. As well as the max. Q Factor variations versus max reach variations are clarified after PIN photodetector receiver under light rain weather conditions at 10, 40 Gbps with/without amplification units. Besides the total optical power variations versus max reach variations are assured after FSO channel under light rain weather conditions at 10, 40 Gbps with/without amplification units.

Ultra-low dark current back-illuminated AlGaN-based solar-blind ultraviolet photodetectors with broad spectral response

A high performance AlGaN-based back-illuminated solar-blind ultraviolet (UV) p-i-n photodetectors (PDs) are fabricated on sapphire substrates. The fabricated PD exhibits ultra-low dark current of less than 0.15 pA under -5 V bias, which corresponds to a dark current density of <1.5×10-11 A/cm2. In particular, the PD shows broad spectral response from 240 nm to 285 nm with an excellent solar-blind/UV rejection ratio of more than 103. The peak responsivity at the wavelength of 275 nm reaches 0.19 A/W at -5 V, corresponding to a maximum quantum efficiency of approximately 88%. Based on the absence of any anti-reflection coating, this corresponds to nearly 100% internal quantum efficiency. In addition, the PD shows a quite fast response of 0.62 ms. To the best of our knowledge, this is the record low dark current density and broadest response band reported for the back-illuminated AlGaN-based solar-blind UV detectors.

Nonlinearity Reduction in a Fiber Fabry-Perot Interferometer Interrogated by a Wavelength Scanning Optical Source

A novel method to reduce the nonlinearity effect of a wavelength scanning diode laser interrogating a low-reflectivity Fabry-Perot interferometer is presented. The interferometer is used for refractive index measurement. The proposed method allows improving the resolution of the system from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> ; and generates a temperature insensitive sensing system. Using the resolution improvement and the temperature insensitivity, the proposed system was used to measure the thermo-optic coefficient of two liquid samples with well-known refractive index. To demonstrate the reliability of the proposed system, experimental results of the long-term performance of a six-month interval, using the same samples but different sensing heads, are presented. The proposed method allows the implementation of a standard four-pin diode laser as an optical source and a standard PIN photodetector for signal detection; permitting the implementation of a high-performance fiber sensor system based on small, light, and standard optoelectronic components. These characteristics are important for different applications such as medical and aerospace, for example, to monitor strain, curvature, pressure, and refractive index using fiber interferometers.

Monolithic integrated two-stage cascaded SOA-PIN receiver for high-speed optical wireless communication.

Monolithic integrated receivers are highly desired due to the potential of mass production and the reduction of device size and cost. In this Letter, a monolithic integrated optical wireless communication (OWC) receiver with optical preamplifiers is designed, fabricated, and investigated to achieve high sensitivity based on photonic integration technology. The proposed receiver consists of one waveguide PIN photodetector integrated with two semiconductor optical amplifiers (SOAs). Compared with using a one-stage optical amplifier, using two independent SOAs as a two-stage amplifier offers the advantage of optimizing the noise figure of each amplifier independently by tuning their injection currents, which leads to the reduction of the total noise and an improvement of the receiver sensitivity. The achieved sensitivity for a 10-Gb/s OOK signal with 10-dBm launch power at 1550-nm wavelength by using the designed receiver is up to -27.5 dBm at a bit-error-ratio (BER) level of 3.1×10-3 over a 0.9-m indoor free-space link. The experimental results show the potential to achieve a high-speed OWC link with high sensitivity by using a cascaded SOA/PIN monolithic integrated receiver.

Performance analysis of optical extra-WBAN links based on realistic user mobility modeling

We address the features channel characterization and performance evaluation for wireless body-area networks (WBANs) in medical applications during a walk scenario using optical wireless transmission. More specifically, we focus on optical extra-WBAN uplink communication between a central coordinator node (CN) placed on the patient’s body and an access point (AP) in a typical hospital room. To characterize the optical wireless channel, we use a Monte Carlo ray tracing-based method and take into account the effects of body shadowing and mobility based on realistic models, in contrast to the previous simplistic models considered in the literature. Using this approach, we derive the dynamic behavior of the channel DC gain for different configurations of the CNs and APs. Furthermore, based on the obtained results, we develop a statistical channel model based on kernel density estimation, which we use to investigate the impact of CN and AP placement on the communication link parameters. Also, based on the outage probability criterion, we discuss the link performance and further analyze the improvement in performance achieved through spatial diversity, i.e., by using multiple APs in the room, for different photodetector types, under different background noise conditions. The presented results show that CN placement and user’s local and global mobility significantly impact the performance of extra-WBAN links, which can nevertheless be reduced using spatial diversity. Finally, the presented performance analysis shows that a single AP equipped with an avalanche photodiode photodetector allows an acceptable link performance for low-to-moderate background noise conditions, whereas multiple APs equipped with PIN photodetectors should be used in the case of moderate-to-strong background noise.

Responsibility optimization of a high-speed InP/InGaAs photodetector with a back reflector structure.

Top-illuminated PIN photodetectors (PDs) are widely utilized in telecommunication systems, and more efforts have been focused on optimizing the optical responsibility and bandwidth for high-speed and capacity applications. In this work, we develop an integrated top-illuminated InP/InGaAs PIN PD with a back reflector by using a microtransfer printing (µ-TP) process. An improved µ-TP process, where the tether of silicon nitride instead of photoresist, is selected to support an underetched III-V device on an InP substrate before transfer. According to theoretical simulations and experimental measurements, the seamless integration of the PD with a back reflector through µ-TP process makes full use of the 2nd or even multiple reflecting light in the absorption layer to optimize the maximum responsibility. The integrated device with a 5 µm square p-mesa possesses a high optical responsibility of 0.78 A/W and 3 dB bandwidth of 54 GHz using a 500 nm i-InGaAs absorption layer. The present approach for top-illuminated PIN PDs demonstrates an advanced route in which a thin intrinsic layer is available for application in high-performance systems.

InP-based high-speed monolithic PIN photodetector integrated with an MQW semiconductor optical amplifier

We demonstrate an InP-based high-speed monolithic PIN photodetector (PD) integrated with a multi-quantum well semiconductor optical amplifier (SOA). A butt-joint scheme is adopted to connect the SOA and evanescent wave PD. The chip allows for a separate design for the SOA and the PD, and needs only two metal-organic chemical vapor deposition growth steps, which promises high yield and reduced manufacturing cost. The fabricated PD shows a low dark current of 300 pA at −3V. The optical gain bandwidth of the SOA is 50 nm, covering the whole c-band. The gain ripple of the SOA is 0.5 dB, indicating that the internal parasitic reflectivity is negligible. For an integrated chip with a 500 μm SOA, the on-chip gain and total chip responsivity at 1545 nm can reach 12.8 dB and 7.8 A W−1, respectively. The insertion loss of the butt-joint interface is estimated to be 1.05 dB/interface. The small signal 3 dB bandwidth at −5V of the integrated chip reaches 20 GHz, showing no deterioration compared to a discrete PD.

Optimal Design and Noise Analysis of High-Performance DBR-Integrated Lateral Germanium (Ge) Photodetectors for SWIR Applications

This work presents the high-performance Si/SiO2 distributed Bragg reflector (DBR)-integrated lateral germanium (Ge) p-i-n photodetectors (PDs) for atmospheric gas sensing and fiber-optic telecommunication networks in the short-wave infrared (SWIR) regime. In addition, this study also proposes an optoelectronic compact small-signal noise equivalent circuit model (SSNECM) of the designed device to compute the noise performance at the detectors&#x2019; output. Various figure-of merits including current under dark and illumination, responsivity, detectivity, bandwidth, and the noise of the proposed device are estimated at the room temperature (RT) for an incident optical power of 0.5 &#x03BC;W. Furthermore, the impact of width and height scaling on dark current, responsivity, and bandwidth are investigated to optimize the proposed device. The validation of the proposed model is done by comparing various parameters including dark current, responsivity, and detectivity of the designed device with other Ge PDs. The estimated results show the reduced trade-off between responsivity and bandwidth of the designed device. At &#x03BB;=1550 nm, the proposed device achieves a high detectivity and SNR of &#x003E;2&#x00D7;1011Jones and 120 dB (at 3 THz), respectively, with the bias voltage of -2V. These encouraging results pave the path for the future development of low-noise and high-speed detectors.

Design and simulation of InAsBi PIN photodetector for Long wavelength Infrared applications

In the quest of the potential and superior LWIR photodetector we have designed and simulated InAsBi based PIN photodiode using COMSOL multiphysics. The device is designed with a radius of 5 µm and total height of 1 µm and compared with a InAs based photodiode. In compare to InAs PIN photodiode, our proposed device outperform in several aspects. The responsivity of the device for optical power of 1 µW at 77 K operating temperature and 2 V reverse bias is 10 times as compared to that of InAs. The operating wavelength is shifted from 2.55 to 5.7 µm in case of InAsBi as compared to InAs. Nonetheless, the dark-current at 77 K for an applied reverse bias of 2 V is obtained as 34nA, which is almost 13 times larger than that of InAs device (2.5nA). Our simulated device can act as a benchmark for experimentally investigating InAsBi PIN photodiode as a prospective candidate for highly efficient thermal image sensors, and LWIR photodetectors.

Strain—Engineered Asymmetrical Si/Si1–xGex IR-Photo-Detector: Theoretical Reliability and Experimental Feasibility Studies

This paper reports the prospect of strained Si/Si1-xGex nano-scale p++-i-n++ photo-detector in the infrared (IR) wavelength region (1000nm-2200nm). The proposed device is capable of offering high photo-responsivity and quantum efficiency at 1600 nm wavelength in comparison to the unstrained conventional Si based pin photo-detector device. The authors have developed a Quantum Modified Drift Diffusion Model to analyze the nano-scale properties of strained Si photo-detector device. The analysis reveals that the strained Si based pin photo-detector offers high photo responsivity 0.67 A/W at 1600nm wavelength and quantum efficiency 0.60 at the same wavelength compared to the conventional unstrained Si based photo-detector. The enhancements of the photo electric characteristics of the Si photo device are obtained by incorporating selective amount of Ge into the Si active region. The electrical properties in the strained Si semiconductor at room temperature (300K), have made this material more attractive for being used as a photodetector. Due to the incorporation of this Ge layer, in-plane biaxial strain is produced in the active region. This phenomenon enhances the out-of-plane mobility of charge carriers. In this current research, in-plane strain is used to boost up the outplane mobility depending on the geometrical structure of the active region. The validity of the proposed model is established by comparing the simulated data with the corresponding experimental results. The authors also report the reliability study of the proposed device in details. Further, the authors have developed a 4x4 array type photo-detector based on the strained Si structure and the results are compared with the corresponding single array type photo-detector. In case of the 4x4 array type photo detector, the photo electric characteristics i.e. photo responsivity and quantum efficiency increase to 0.74 A/W and 0.69 respectively at 1600nm wavelength. In IR wavelength region, the designed photo-sensor depicts a significant improvement of performance compared to the available photo-detector in published research work. The proposed photo-detector can play an important role in Biomedical, Defence and Agro industries. According to the authors’ knowledge, this is the first report on strain engineered nano scale asymmetrical Si/Si1-xGex based photo-detector array.

(Invited) Selective Epitaxy of Submicron Ge Wire Structures for Photodetectors and Optical Modulators in Si Photonics

Ge epitaxial layer on a Si-on-insulator substrate has been applied to waveguide photodetectors (PDs) in Si photonics, which operate at the optical communication wavelengths (1.3–1.6 µm). For enhancing the operating frequency over 50 GHz with keeping a high photodetection efficiency, a sub-micron-wide Ge wire structure with a lateral pin junction is effective because of the RC delay smaller than that for conventional vertical pin PDs. Such a Ge wire structure allows a single-mode light propagation, being also applicable to electro-absorption optical modulators (EA-MODs) based on the Franz-Keldysh effect. Here, in terms of higher-performance PDs and EA-MODs, sub-micron Ge wire structures by selective epitaxial growth on Si are presented. Bandgap engineering is discussed utilizing a control of grown-in tensile strain in sub-micron Ge.Figure 1(a) shows a typical cross-sectional scanning electron microscope image of a Ge wire structure with the width as narrow as 0.6 µm. The Ge wire in the [110] direction was selectively grown on an exposed Si (001) surface in a SiO2-covered Si substrate by ultrahigh-vacuum chemical vapor deposition at 700°C. The wire surface was surrounded mainly with inclined {113} and {111} facet planes, remaining a narrow (001) top plane. For protecting the fragile Ge surface, a Si cap layer was subsequently grown at 600°C, whereas a surface roughening was unfavorably induced on the {113} planes near the boundary with the {111} plane, as in Fig. 1(b). Raman spectra show that a SiGe alloy was accompanied on the wire surfaces, although no such SiGe was detected on the (001) plane of wide mesa top [1]. Based on theoretical analyses, a mass transport together with a Si-Ge interdiffusion would be induced by a stress concentration at the {113}/{111} facet boundaries. It is important that the roughening and alloying were significantly suppressed by decreasing the growth temperature for the Si cap layer to 530°C, as in Fig. 1(c).Typical photoluminescence (PL) spectra are shown in Fig. 1(d). For the Ge wire structure, the PL peak due to the direct transitions was located at the wavelength of 1.55 µm, which is shorter than 1.57 µm for the blanket Ge film on Si. The peak position is similar to that for a bulk Ge wafer. The blue shift for the wire corresponds to a widening of direct bandgap, which results from an elastic relaxation [2] of grown-in tensile strain generated by the thermal expansion mismatch with the Si substrate. The blue shift plays a significant role for Ge EA-MODs. As reported recently [3], in contrast to the operating wavelength range of > 1.60 µm in previous studies, EA-MODs of a sub-micron Ge wire favorably operates in the 1.55 µm range of C band (1.530–1.565 µm).[1] Y. Ishikawa et al., Proc. SPIE 11193, 1119309 (2019).[2] M. Nishimura et al., ECS Trans. 86, 3 (2018).[3] J. Fujikata et al., Opt. Express 28, 33123 (2020). Figure 1