CMOS-process-compatible silicon (Si) based photodiodes could lead to a cheaper and better integrated fiber-optic communication system^[1]. However, Si photodiodes are currently used predominantly in the visible wavelength regime. At near-infrared (IR) wavelengths, such as 840–860 nm used for short-reach (<300m) data communication, traditional Si photodiodes with a weak absorption coefficient of Si can hardly achieve a high external quantum efficiency (EQE, >50%) and a high data rate (20 Gb/s) simultaneously^[2]. Therefore, a lot of work has been done to enhance the absorption of a thin Si film^[3–10]. One effective approach is to integrate micro- and nanostructures in the Si film with the purpose of tailoring its optical properties and trapping the photons^[11]. Such schemes include the formation of nanowires^[12,13], nanodomes^[14], nanodiscs^[15], nanopencils^[16], nano-cones^[16,17], and surface plasmon resonance (SPR)-based devices^[18,19].

In recent studies, an all-Si pin photodiode fabricated on a silicon-on-insulator (SOI) wafer with periodic funnel-shaped holes is presented for absorption enhancement at 800–1000 nm^[20–22] resulting in an enhanced EQE of 52% at 850 nm with a 2-µm-thick i-Si layer and an ultra-fast impulse response of 30 ps. In our earlier work, we have revealed that a beam of vertically incident light is guided laterally and confined in the nanostructured Si film until it gets absorbed^[23]. For long-reach data communication operating at 1310 nm, at the C band (1530–1560 nm), and at the L band (1560–1620 nm), the commercially available optical receivers often contain photodiodes based on III–V materials such as InGaAs/InP^[24,25]. However, these materials are fabricated in a distinct process rather than with CMOS compatible technology. An expensive and complex packing method would incur additional cost and performance degradations^[26]. Ge-on-Si photodiodes provide a solution for high-speed applications at this band^[27].

Compared to pn/pin photodiodes, metal-semiconductor-metal (MSM) photodiodes with a low capacitance have a faster response speed^[28]. However, the performance is also limited by low absorption since it is a surface device with a small light-sensitive area and a shallow electric field distribution. Integrating photon-trapping hole structures between metal fingers is still an effective way to improve the efficiency while maintaining an ultra-fast response speed^[29]. However, there are still challenges and limitations that need to be addressed for the holes-based photodiodes. (1) How to reduce the hole depth to simplify the process while enhancing the absorption of the photodiodes? In the reported pin photodiodes with micro-holes^[20–23,27], holes that span the n, i, and p layers are integrated with a depth of over 2 µm. The deep reactive ion etching (DRIE) process is needed to form the holes. (2) Why are the photons trapped in photodiodes with holes of limited depth on an SOI substrate and how does the period of the holes influence the light-trapping property? (3) Is it effective to introduce a low bandgap film on Si (such as PbS, PbSe, or SeTe) prepared by a CMOS-compatible evaporation or sputtering process to broaden the spectral response band? And to what extent would the performance be improved by integrating photon-trapping surface structures?

In this work, Si-based MSM photodiodes with high speed and enhanced efficiency are demonstrated by integrating photon-trapping holes with a small depth of 250 nm for Si devices and 150 nm for Si-based PbSe devices. Both finite-difference time-domain (FDTD) and rigorous coupled-wave analysis (RCWA) methods are applied to theoretically study the photon-trapping effect and absorption enhancement of photodiodes with different designs of hole structures. The holes-based photodiode structure is simulated and optimized by FDTD while the propagation path of the light and the interaction between the light and the material are calculated and analyzed based on RCWA. This reveals that light is more trapped and absorbed due to the multiple reflection and diffraction effect for the hole array on an SOI substrate with a smaller period (900–1000 nm) due to more bending of light and a higher back reflection. The Si MSM photodiode exhibits an enhanced EQE of 81% and an ultra-fast impulse speed of 22 ps enabling a 3 dB bandwidth up to 23.9 GHz at 850 nm. In order to improve the response at a longer wavelength, the PbSe film with a thickness of 80 nm is prepared on an SOI wafer by using an evaporation process. With photon-trapping hole structures, the EQE of the PbSe-on-Si MSM photodiode is enhanced by over 450% at 1310 nm and over 500% at 1550 nm. These results make the micro-hole-enabled Si-based MSM photodiodes promising to cover the C, L, and even wider bands for low-cost infrared sensing and communication applications.

High speed Si and Si-based PbSe MSM photodiodes are designed with enhanced efficiency by integrating surface photon-trapping hole arrays. Figures 1(a) and 1(b) show the schematics of a Si MSM photodiode with photon-trapping holes. The Si-based MSM photodiodes were fabricated on an SOI wafer, which has a 1.5 µm device (Si) layer and a 3 µm BOX (SiO2) layer, using CMOS compatible processes. 100-nm-thick aluminum (Al) fingers were fabricated on the Si (or PbSe) film by sputtering and lift-off processes. Afterwards, periodic cylinder-shaped holes with different diameters (d), depths (h), and periods (p) were patterned and etched by lithography and reactive ion etching (RIE) to form hole arrays between the Al fingers. The SEM images of the hole arrays with a diameter of 630 nm and a period of 900 nm arranged in square lattice and hexagonal lattice are shown in Figs. 1(c) and 1(d), respectively. The holes have a thickness of 250 nm. Figure 1(e) shows the SEM image of the active region of a Si MSM photodiode with a diameter of 50 µm. The PbSe film is prepared on the SOI wafer with a thickness of 80 nm using an evaporation process for the response at the longer wavelength. The PbSe powder with a purity of 99.99% is purchased from Zhongnuo Advanced Material (Beijing) Technology Co., Ltd. The designed device structure and the fabricated photodiode (50 µm diameter) are shown in Figs. 1(f) and 1(g), respectively. The depth was optimized to be 150 nm for the PbSe-on-Si devices. Therefore, 80 nm PbSe and 70 nm Si were etched to form holes in the PbSe photodiodes.

FDTD simulation is performed to analyze the influence of the hole size, depth, arrangement, and substrate on the absorption properties of the Si and PbSe film. Figure 2(a) shows the absorption of the Si films on the SOI substrates with different hole structures at the wavelengths between 800 and 950 nm. The hole arrays have a depth of 250 nm, a similar diameter/period (d/p) of around 0.7, and different periods (900 nm, 1000 nm, and 2000 nm). Compared to the flat film, the absorption of the Si films with holes is significantly improved at different wavelengths. The hole array with a smaller period (900 nm or 1000 nm) leads to a higher absorption than the one with a longer period (2000 nm). The underlying physical principle for the effect of the period on the absorption property will be discussed by RCWA in the next section. Generally, the Si film with holes in the hexagonal lattice has a slightly higher absorption than its counterpart with holes in the square lattice due to higher porosity and lower surface reflection. When the hole array has a diameter of 630 nm and a period of 900 nm, the optimized absorption of the Si film is achieved at 850 nm, which shows over 1100% improvement compared to the flat film. Figure 2(b) shows the absorption of the Si films with hole arrays in the hexagonal lattice with different hole depths on a SiO2 substrate between 800 and 950 nm. Hole arrays are integrated with d/p of 630/900 nm. Absorption of the flat Si film without holes and with the same thickness is also presented. Figure 2(b) reveals that when the holes span the whole Si film (h=1.5μm), although the absorption is higher than that of the flat film, the nanostructured Si film has the lowest absorption compared to the Si films with hole arrays of other depths (h<1.5μm). The highest absorption is obtained with a hole depth of 250 nm. It can be concluded that it is advantageous for the light absorption to keep a layer of the flat Si film without holes and with a certain thickness between the hole array and the SiO2 substrate, which is helpful for guiding the light in the laterally propagating modes in the Si film. This will also be discussed in detail in later RCWA.

We also simulated the absorption of the PbSe films with a thickness of 80 nm on the SOI substrates with different hole structures [shown in Fig. 2(c)] and different depths [shown in Fig. 2(d)] between 800 and 1600 nm. Four different structures of holes are integrated in the PbSe-on-Si film with a depth of 150 nm. Figure 2(c) shows that the integration of the photon-trapping holes also greatly improves the absorption of the PbSe film. Similar with Si devices, higher absorption is obtained when the hole array has a smaller period (1000 nm). A different arrangement of holes (in the square lattice or the hexagonal lattice) does not show much difference. The hole array in the square lattice seems to have better absorption at a longer wavelength. The flat PbSe film has a low absorption (11.21%–41.76%) at 800–1600 nm, which decreases as the wavelength increases. With a hole diameter of 700 nm and a period of 1000 nm, the nanostructured PbSe film exhibits an enhancement absorption of 51.26%–80.05% in the same range. The absorption reaches 58.26% at 1310 nm and 52.41% at 1550 nm. In Fig. 2(d), the optimized depth of the holes is 150 nm. Holes of much smaller depth can be easily formed by an RIE process.

To demonstrate the working principle of the photon-trapping holes, the photon distributions in the Si-based photodiodes with different designs of holes are simulated by FDTD method, as shown in Fig. 2(e). A plane source of 850 nm wavelength is simulated to illuminate upon the surface of three devices with three different structures, namely a device without holes, a device with holes in the hexagonal lattice with d/p of 630/900 nm, and a device with holes in the hexagonal lattice with d/p of 1500/2000 nm. For the flat device, photons travel vertically with minimal photon distribution in the device due to the lack of a bottom reflection. For the devices with holes, photons are more diffracted by the micro-structures and maintain a high back reflection at the Si/SiO2 interface. This increases absorption and greatly broadens the photon distribution in the device layer. The device with holes in the hexagonal lattice with d/p of 630/900 nm exhibits a better photon-trapping performance compared to its counterpart with d/p of 1500/2000 nm. Additionally, the electric field distributions in the PbSe-on-Si devices with d/p of 700/1000 nm and 1400/2000 nm are simulated, as shown in Figs. 2(f) and 2(g), respectively. Similar with Si devices, the field strength in the PbSe device with a smaller period of holes is higher compared to the one with a larger period. Better trapping of photons in the device with holes of a smaller period leads to a higher absorption of light. The influence of the period on photon-trapping properties will be theoretically analyzed in the next section.

In order to investigate the effect of the hole diameter (d) and the period (p) on the light trapping and absorption enhancement of the photodiodes on an SOI substrate, a detailed RCWA is carried out to analyze the light propagation in the Si films with hole arrays with d/p of 630/900 nm and 1500/2000 nm. According to the grating theory, the light propagating through the hole array becomes multi-ordered light with different angles deflecting from the incidence direction for each order. The number of grating orders (N) of reflected light (|N|<pλ) and transmitted light (|N|<pnsλ) from the hole array is limited by the period (p) of holes and wavelength (λ), where ns is the refractive index of the substrate. The deflection angle (θN) of the Nth order transmitted light can be calculated by the equation sinθN=NλpnSi. Figure 3(a) shows the transmission efficiency (TN) from the hole array to the flat Si layer and deflection angles (θN) of the different orders for the hole arrays with d/p of 630/900 nm and 1500/2000 nm at the wavelength of 850 nm. The transmission efficiency is relatively high at low orders (0th and 1st) and is much lower at higher orders for the two designs of holes. Figure 3(a) also reveals that a smaller period leads to a higher deflection angle and more bending of light at each order. Calculation suggests that for a vertically illuminated Si film with holes with d/p of 630/900 nm, 72.17% of the incident light will transmit into the flat Si layer after one diffraction of the hole array, only 16.76% of which is 0th order light that does not experience a bending. In other words, 76.78% of the transmitted light deflects from the incidence direction and causes laterally slow propagating waves. For the hole array with d/p of 1500/2000 nm, 55.41% of the transmitted light deflects from the incidence direction. Some of the transmitted light will be reflected back by the SiO2 film and then become incident to the hole array from the bottom^[23]. Figure 3(b) shows the reflectivity (rN) of the SiO2 film for the transmitted light with different orders from hole arrays with d/p of 630/900 nm and 1500/2000 nm at 850 nm wavelength. Taking the hole array with d/p of 630/900 nm as an example, the critical angle of the total reflection at the Si/SiO2 interface is 23.79°, and the deflection angles of the 0th, 1st, 2nd, and 3rd order light are 0°, 15°, 31.16°, and 50.92°, respectively. Therefore, all the transmitted light with orders N≥2 will be totally reflected by the SiO2 film. Figure 3(c) shows the transmission in the flat Si layer and back reflection of the SiO2 film for the hole arrays with d/p of 630/900 nm and 1500/2000 nm at the wavelengths of 800 nm, 830 nm, 850 nm, 870 nm, and 900 nm. The transmission from the hole array into the flat Si layer is similar for the two designs of holes at each wavelength. However, the back reflection of the SiO2 film shows an obvious difference. A much higher back reflection occurs in the nanostructured Si film with d/p of 630/900 nm. At 850 nm, the back reflection is 71.1 % when d/p=630/900nm and 40.1 % when d/p=1500/2000nm, respectively. For a flat Si film without holes, the back reflection is only 18% at the Si/SiO2 interface. A high back reflection in nanostructured Si film with a smaller period of holes significantly reduces the transmission loss into the substrate.

The light reflected back from the SiO2 substrate becomes an incident light of the whole array from the bottom. With the flat Si layer modeled as the bottom incident layer and air modeled as the top substrate [see the inset in Fig. 3(d)], the top transmission of the bottom incident light with different incident angles for hole arrays with d/p of 630/900 nm and 1500/2000 nm is calculated by RCWA, as shown in Fig. 3(d). It reveals that as the incident angle increases, the top transmission decreases rapidly. When the incident angle is higher than 20°, less than 5% of the bottom incident light can transmit to the air from the top surface. Taking the hole array with d/p of 630/900 nm as an example, the deflection angle of the 1st order transmitted light is 15°, and the deflection angle increases with the increase of the order. This means that most of the light reflected by the SiO2 film will be trapped in the Si film. The trapped light will be completely absorbed after multiple reflections and diffractions in the Si film on a SiO2 substrate. It should be noted that after each diffraction, the light will be deflected and guided more and more laterally. Now it is clear that the light trapping effect of the Si photodiodes with integrated holes on an SOI wafer is attributed to three reasons: (1) the vertically incident light undergoes a deflection and generates lateral modes subsequent to the diffraction of the hole array; (2) the SiO2 film in the SOI substrate provides high back reflection and guides the light back into the photodiode for further absorption; and (3) the back-reflected light will be trapped and fully absorbed after multiple reflections and diffractions with little transmission loss from the top surface of the Si film.

In order to verify the light bending effect of the hole arrays, electric field distributions in Si films with d/p of 700/1000 nm and 1500/2000 nm are simulated at the wavelength of 850 nm, as shown in Figs. 2(e) and 2(f), respectively. It shows light propagation over time in the Si films with different designs of hole arrays. The holes arranged in the square lattice with a depth of 1 µm are integrated in the surface of the Si films with a thickness of 1.5 µm on a SiO2 substrate. Light is vertically illuminated at one point on the Si surface between two holes with a coordinate value of 0 at the time of t=0fs. The electric field distribution in the nanostructured Si films at t=5fs, t=7fs, t=9fs, t=11fs, and t=20fs shows light propagation over time in the Si films with different designs of hole array. It can be seen that the vertically incident light undergoes a bending and propagates laterally for each nanostructured Si film due to the diffraction of the hole array. It shows a strong electric field distribution in the holes, and the effective optical path of light in the Si film is also increased with an enhanced absorption. In the Si film with a hole array with d/p of 700/1000 nm, a strong electric field distribution can be observed in the holes 2 µm away from the incident point at t=7fs. For the Si film with a hole array with d/p of 1500/2000 nm, more than 11 fs is needed to form a strong electric field in the holes at the same distance of 2 µm. Light has a higher laterally propagating speed in the nanostructured Si film with a smaller period of holes. A stronger electric field can be observed in Si rather than in the holes when the hole array has d/p of 700/1000 nm. More bending of the light and a stronger interaction between the light and the Si result in higher absorption.

Figure 4(a) plots the I-V curves of the Si MSM photodiodes with a diameter of 30 µm on the SOI wafer with holes in the hexagonal lattice with d/p of 630/900 nm under the illumination of 850 nm laser and dark conditions, showing an over 103Ilight/Idark ratio. The dark current of the photodiodes at −4V bias is below 30 nA. The relatively high dark current is attributed to the defects in the Si surface (vacancies, interstitials, dislocations, etc.) caused by RIE process. These defects can be partially eliminated by a surface passivation process via wet etching or low energy dry etching^[18]. A HNO3/HF mixture in a certain proportion can be employed to remove the damaged layer. However, the process is difficult to control since the etch rate is very sensitive to temperature. A low energy etch in an RIE system is a more controllable and repeatable method for removing the damaged Si surface compared to using a wet etch. When the light power intensity varies, the photocurrent displays a well linearity at −5V bias, as shown in Fig. 4(b). Figure 4(c) shows the measured EQE of the Si photodiodes with hole arrays of different arrangements and d/p of holes at 850 nm when the power intensity is 441.5mW/cm2. It shows that the EQEs of the Si photodiodes are significantly enhanced by integrating the hole arrays. With a similar d/p around 0.7, hole arrays with periods of 900 nm and 1000 nm result in higher EQEs compared to the ones with a period of 2000 nm. This is in accordance with the results of the FDTD simulation and the RCWA that a smaller period leads to a higher back reflection in the SOI substrate. An EQE of 81% is achieved by the Si photodiode with holes in the hexagonal lattice with d/p of 630/900 nm, which is nearly four times higher than the one without holes. For the high speed measurement, we used a mode-locked pulsed fiber laser, which generates light with a pulse width of 31 ps and has a repetition rate of 40 MHz at 850 nm. The laser pulse was focused onto the active region of the photodiodes using a single-mode fiber tip on a probe station. Figure 4(d) shows the resulting impulse response curves of the Si photodiode with holes in the hexagonal lattice with d/p of 630/900 nm at −3V, −5V, and −8V bias (using a 20 GHz bias-T). The measured full-width at half-maximum (FWHM) of the photodiode with holes from the 20 GHz oscilloscope is 51 ps (−3V), 44 ps (−5V), and 42 ps (−8V). Increased bias results in a faster response due to enhanced carrier migration in a high electric field region. A higher bias voltage is advantageous for reducing response time and improving efficiency.

Considering the 22 ps FWHM response for the sampling oscilloscope and the optical laser pulse width of 31 ps, the actual response of the device is estimated to be 22 ps at −5V bias at 850 nm, based on Eq. (1)^[30], τmeans=τactual2+τscope2+τoptical2,(1)where τmeans, τactual, τscope, and τoptical are the measured, actual, oscilloscope, and laser optical pulse widths in the time domain, respectively. This is acceptable for Gaussian pulses and is a valid approximation for our actual measurements. Figure 4(e) shows the Gaussian fitting curve of the impulse response at −5V bias. The ultra-fast response corresponds to a 3 dB bandwidth of 23.9 GHz [as shown in Fig. 4(f)], which is calculated by fast Fourier transform (FFT) method. Additionally, we tested and calculated the 3 dB bandwidths of the Si photodiodes with other hole structures, as shown in Table 1. A Si-based MSM photodiode with photon-trapping hole array is demonstrated with an ultra-fast response and an enhanced efficiency. Additionally, a simple MSM device structure and integrated holes with a small depth of 250 nm are much easier to fabricate compared to the Si-based pin photodiodes with holes in the lattices^[20–23].

In order to broaden the response range of the Si photodiodes, the PbSe film with a thickness of 80 nm was prepared on an SOI wafer by an evaporation process, and the Si-based PbSe MSM photodiodes were fabricated with integrated holes. The Raman spectrum of the PbSe film exhibits two prominent peaks at 137cm−1 and 263cm−1 as shown in Fig. 5(a), which are consistent with those reported in the literature^[31]. The atomic force microscope (AFM) image [shown in the inset of Fig. 5(a)] shows that the film has a good uniformity and relatively low roughness. Figure 5(b) plots the I-V curves of the PbSe-on-Si photodiodes with different designs of holes under dark conditions. Under the same bias, the dark current of the devices with hole structures is slightly higher than that of the flat device due to the surface roughness, damage, and traps introduced during the etching process. The flat device exhibits a dark current of 0.42 µA at −2V bias. As shown in Fig. 5(c), a broadband response from 405 nm to 1550 nm is observed for the PbSe-on-Si photodiodes with five different designs of hole structures. The photocurrent is relatively low at 1310 nm and 1550 nm compared to the shorter wavelengths at 808 nm, 980 nm, and 1064 nm. A three-folds enhancement of the photocurrent is generated in the photodiode with holes in the square lattice with d/p of 700/1000 nm at 1310 nm and 1550 nm compared to the flat device. Figures 5(d) and 5(e) show the EQE and EQE enhancement of devices with different hole structures, respectively. The EQE of the pure Si-based photodiode is low and decreases to nearly 0% at 1310 nm and 1550 nm. The PbSe film based photodiodes show a significantly improved EQE on the Si and the SOI substrates, and it can be further enhanced by integrating photon-trapping holes in the surface. The PbSe photodiode on an SOI substrate with holes in the square lattice with d/p of 700/1000 nm exhibits an EQE of 68.92%–23% in the same range, which represents an enhancement of 136.68%–505.26%. The EQE enhancement of the device with holes reaches 451.32% at 1310 nm and 505.26% at 1550 nm, respectively. Figure 5(f) shows the noise characteristic curves of the PbSe photodiodes with different hole structures at a bias of −2V. The devices have a small internal noise current, which decreases as the operating frequency increases, and the 1/f noise predominates within the operational frequency range. Specific detectivity (D*) quantifies the ability to detect the signal from the noise and is defined as D*=RInoiseSΔf,(2)where R is the responsivity of the photodiode, S is the effective device area, Δf is the bandwidth, and Inoise is the noise current of the device at the operating frequency. The photodiode with holes in the square lattice with d/p of 700/1000 nm achieves D* of 6.97×109 Jones at 980 nm, 5.27×109 Jones at 1310 nm, and 4.2×109 Jones at 1550 nm, which represents a 3.56 folds enhancement compared to the flat device without holes.

This research provides an effective and CMOS process compatible method to improve the efficiency of high-speed Si-based detectors in different bands from 808 nm to 1550 nm. It is worth mentioning that the PbSe film in this paper is prepared by a simple evaporation process, and the performance of the device can be further improved by higher quality films prepared by sputtering or other methods.

In this paper, we have presented a theoretical and experimental study of Si-based MSM photodiodes with high speed and enhanced efficiency by integrating periodic cylinder-shaped holes at the wavelengths between 800 and 1550 nm. Hole arrays, designed with different diameters, depths, and periods, are arranged in square and hexagonal lattices in Si or PbSe-on-Si films on SOI substrates. Photon distribution and enhanced absorption in the nanostructured Si film are demonstrated by FDTD simulation to obtain an optimized size and depth of holes. Light diffraction of the hole array and back reflection of the SiO2 film in the SOI substrate are analyzed by calculating the diffraction efficiency of the grating orders based on the RCWA method. The diffraction of the hole array guides the vertically incident light in laterally slow propagating modes. Light is more trapped and absorbed due to multiple reflections and diffractions between the nanostructured Si film and SiO2 substrate for hole array with a smaller period (900–1000 nm) due to more bending of light and a higher back reflection. With a small hole depth of 250 nm, the Si MSM photodiode exhibits an enhanced EQE of 81% and an impulse response speed of 22 ps (corresponding to a 3 dB bandwidth of 23.9 GHz). By integrating a PbSe film on the surface of Si by an evaporation process, the response band is extended to 1550 nm. With photon-trapping surface structures of 150 nm depth, the EQE of the PbSe-on-Si device is enhanced by 451.32% at 1310 nm and 505.26% at 1550 nm, resulting in a specific detectivity of 4.2×109 Jones at 1550 nm.