Optical communication has become one of the most important technologies in modern society due to its excellent performance of speed transmission and information capacity, and is widely operated at wavelengths of 850, 1310, and 1550 nm [1]. Since there is little light absorption in silicon for wavelengths longer than 1100 nm, almost all researches have been aimed at 850 nm photodetectors (PDs) in the complementary metal-oxide semiconductor (CMOS) technologies. For 1310 nm and 1550 nm optical receivers, the PDs with excellent performance are mostly based on compound semiconductors (III–V) and Ge–Si materials [2–4], both of which have a higher materials cost. In addition, the compound semiconductors suffer from not being compatible with the standard Si-based CMOS platform [5–7]. Therefore, the development of Si PDs based on mature CMOS technology without additional material or process steps is an essential step for monolithic integrated optical receivers. To overcome such intrinsic drawbacks and extend the response wavelength of Si-based PDs to telecom wavelengths, many new attempts have been proposed, including employing Si-based quantum dots (QDs) based on quantum confinement effect (QCE) [8], combining Ge (0.67 eV) with Si [9,10], two-photon absorption (TPA) [11–13], plasmonic hot carriers [14], and defect mediated band-to-band photogeneration via mid-bandgap localized states [15–17]. However, some of these approaches require complicated designs that limit their overall application. Recently, the Schottky diode has attracted much attention due to its unique internal photoemission mechanism [18–22]. In this configuration, photoexcited carriers from the metal are emitted to the semiconductor over a potential ΦB, called a Schottky barrier (SB), to generate the photocurrent [23]. Typically, an SB is lower than the intrinsic bandgap Eg of a semiconductor, thus allowing the photodetection of photons with energy hν<Eg, which is the basis for silicon-based Schottky PDs (SPDs) to realize the detection above 1.1 μm [20,24,25]. For example, PtSi/p-Si SPDs are widely used for infrared imaging in the 3–5 μm wavelength range with a very low SB height (SBH) (0.2 eV) [26,27], while for shorter wavelength application, Ti, Co, and Ni can be adopted because their contacts on n-Si have the SBH of 0.5–0.7 eV [28]. However, it is difficult for traditional metal-Si SPDs to achieve considerable performance in responsivity. Fortunately, transparent conductive glass (ITO) with good conductivity and light transmission is a promising alternative to metal for allowing sufficient light to enter the junction [29,30]. One of the challenges for the ITO/Si detector is that the low SBH leads to high dark current, which will severely drag down the sensitivity of the detector. To increase the height of the ITO/Si barrier, inserting other materials including an insulator and metal between the ITO and silicon has been performed [18,25,31]. Among these materials, the insulator materials have a high series resistance, while an Ag layer at least 8 nm thick is needed to form a high barrier with Si due to the poor film forming ability of Ag, resulting in poor light transmittance of the electrode and thus a reduced response. In recent studies, the Au/Si SPDs with different kinds of plasmonic designs have had better responsivity, but the dark current for these structures is still high [32–34].

In this work, we demonstrated the ITO/Au/n-Si PDs with the best record of dark current density on the order of 1×10−7A/cm2 (−0.1V to −2V) at present. The mechanism for such low dark current density is a high SBH, resulting from a high work function of the Au insertion layer into the ITO/Si. Furthermore, for the new structure, the transparency of the electrode has been obviously improved and higher photocurrent is obtained by thinning the Au film. As a silicon-based SPD with a record low dark current density, one can envision its role for operation from visible light to a 1550 nm wavelength without damaging the light responsivity.

Figure 1 illustrates the process flow of making ITO/Au/n-Si SPDs. The PDs were fabricated with a commercial epi-ready n-type (0.1–1 Ω·cm, 400±10μm) silicon substrate. First, the PD areas were defined by ultraviolet lithography using a negative resist (AR-U 4030) in leaf patterns with an area of 0.0007cm2. Before being sent to the electron beam evaporation (Ohmiker-50B) chamber for Au deposition at room temperature, the patterned substrates were dipped in a dilute HF solution (H2O:HF=10:1 for 1 min) for oxide removing. During the evaporation process, the deposition pressure was maintained below 2×10−6Pa and the deposition rate was 0.17 Å/s (1 Å = 0.1 nm) controlled by the current with a voltage at 8 kV. In addition, the plate where the substrates were placed rotated at a rate of 8 r/min to ensure the uniformity of the film. After that, 100 nm ITO was immediately deposited on the Au film by a double chamber magnetron sputtering system (Shenyang Defeng Technology) at room temperature under Ar/O2 atmosphere. During the sputtering process, the thickness was controlled by the sputtering time with a deposition rate of 0.93 Å/s. Finally, the ohmic contact on the backside was realized by a 300 nm Al film using Ohmiker-50B with a deposition rate of 1.7 Å/s at room temperature after removing the oxide by a dilute HF solution (H2O:HF=10:1 for 1 min). To protect the ITO/Au electrode from the HF solution, the positive resist (AZ 6130) was used as the protective layer during the removing process. The source of Au and Al in the Ohmiker-50B was the molten metal that came from the particles (purity, 99%; particles diameter, 0.5 cm) by heating an electron beam in the crucible before evaporating; the source of the ITO was a bulk target (purity, 99.99%; In₂O₃:SnO₂ = 9:1, mass ratio).

The surface morphology of the ITO/Au/n-Si PDs was analyzed using a field-emission scanning electron microscope (SEM) (SUS5500) monitored with accelerating voltage (30 kV) and an atomic force microscope (AFM) (Bruker, Multimode8) in a ScanAsyst mode. While the interfaces between the ITO, Au, and n-Si substrate were analyzed by the SEM (SUS5500) and high resolution transmission electron microscope (HRTEM) (JEM-2200FS). The transmission spectra were acquired by a UV-VIS-NIR light spectrophotometer (Shimadzu, UV 3600 Plus). Since the Si substrate is opaque in nature, the ITO layer (100 nm) and the ITO (100 nm)/Au (2 nm, 3 nm, 4 nm, 6 nm) multilayers, processed in the same manner as their Si counterparts, were deposited on glass substrates to obtain the transmission of the electrodes. In addition, the transmission of the glass had been removed during the transmission measurement. At the same time, the resistivity of the ITO/Au/glass was analyzed by a Hall effect testing instrument.

The I–V measurements were performed by the use of a Keithley 4200 in the forward and reverse regions at ±2V by 10 mV per step, while the temperature dependent I–V characteristics were acquired by a standard electrical probe station (Lakeshore Cryotronics) and a Keithley 4200 semiconductor parameter analyzer from −1V to 1 V by a step of 10 mV. In addition, the photocurrent at the certain wavelength was obtained with a Keithley 4200 from −2V to 2 V when the detector was normally incident by 1064 nm (2 mW) or 1310 nm laser (1 mW). In order to study the response of the detector to a wide range of wavelengths, the Fourier photocurrent spectrum of the device was measured with the Bruker V70. When the photocurrent was measured, the distance between the light source and the PDs was kept constant, and the position of the PDs was adjusted on the plane until the maximum photocurrent occurred to ensure consistency of test conditions.

In this study, silicon-based SPDs with two design structures are firstly prepared: (1) ITO (100 nm)/Au (6 nm)/n-Si SPD (our target detector) and (2) reference ITO (100 nm)/n-Si PD, referred to as 6AuSPD and 0AuSPD, respectively. The left side of Fig. 2(a) shows the cross-sectional SEM image of the 6AuSPD. The ITO thickness is about 100 nm, while the top-view SEM images of the two AuSPDs on the right suggest that the surface of the ITO becomes a little mottled and rough after Au insertion; this will be demonstrated later in an AFM test. The HRTEM is used to study the Au insertion layer in detail; the Au thickness is about 6 nm, as clearly seen in Fig. 2(b). Furthermore, it can also be clearly seen that a clear and smooth interface between the ITO and the Au is obtained. Figure 2(c) shows that in the AFM images (area: 5μm×5μm) the root-mean-square (RMS) roughness increases from 0.624 nm of bare ITO (almost the same as with the Si substrate) to 1.32 nm, indicating that the insertion of the Au slightly decreases the surface flatness of the electrode. The main reason for the roughness increment may be the poor film compactness of Au caused by the low atomic energy and the low growth rate during deposition [35]. In order to study the optoelectronic performance of the detectors, the current–voltage characteristics were analyzed under dark (room and low temperature) and light (room temperature) conditions according to the schematic diagram displayed in Fig. 2(d).

Both SPDs exhibit typical rectifying characteristics, as shown in Fig. 3(a). The current is limited by the barrier in the junction under the reverse bias while rising quickly and becoming limited by the series resistance under the forward bias since the barrier is gradually compensated by the applied voltage. In addition, by linear extrapolation, the turn-on voltages for the PDs were extracted, as seen from the inset in Fig. 3(a), as 0.1 V and 0.33 V, respectively, which means the larger SB for the SPD with the Au insertion layer. In the J–V semi-log plot [see Fig. 3(b)], it can be clearly seen that the dark current density of the SPD with the Au insertion layer decreases significantly (7876 times). The dark current density is about 3.7×10−7A/cm2 at −1V, which is only one-sixth of the reported data, while the rectifying ratio (1.5×108 at −1V) is 300 times higher than that reported by other literature [16]. Furthermore, the temperature dependent I–V measurement was carried out to extract the SBH. As shown in Fig. 3(c), the current density grows with increasing temperature at reverse bias, consistent with what is expected for thermionic emission in an SPD [23]. Figure 3(d) shows the good linear correlation for extracted ln(J/T2) versus 1000/T Richardson plots, where the SBH of 6AuSPD ∼0.795eV together with 0.434 eV of 0AuSPD is extracted, respectively, following the procedure described in the literature [36]. The potential barrier formed by Au and silicon in our work is consistent with the value reported in other research [37]. As a matter of fact, the difference of work functions between ITO and n-Si (0.1–1 Ω·cm) is about 0.57 eV, while the difference between Au and n-Si (0.1–1 Ω·cm) is about 0.87 eV. These results suggest that such a low dark current density must be attributed to the SBH increment, which results from the thin Au insertion layer.

The photocurrents of the detectors measured at the wavelength of 1310 nm and 1064 nm are shown in Figs. 3(e) and 3(f), respectively. Because of the high dark current density of the 0AuSPD, it is difficult to distinguish the photocurrent from the dark current under an illumination of 1310 nm. Instead, the dark current and the photocurrent of 6AuSPD are obviously separated with a contrast ratio of more than 1000:1, which benefits from the sharp decline of dark current. Furthermore, stronger absorption occurs under the illumination of 1064 nm due to the addition of inter-band absorption. The dark current and photocurrent can be distinguished in both PDs, but the stronger contrast in the PD with the Au insertion layer indicates the higher sensitivity.

Generally, the proportion of light entering the junction is important to the responsivity of the metal/semiconductor detector, so it is necessary to reduce the Au thickness to improve the transmission of the top electrode with no other performance recession. Therefore, another three SPDs with 4 nm, 3 nm, and 2 nm Au insertion layers were prepared; they are referred to as 4AuSPD, 3AuSPD, and 2AuSPD, respectively. More details about the transmission, uniformity, and resistivity of the top electrodes and morphology related to the Au thickness can be found in Appendix A. Overall, all ITO/Au films exhibit a considerable uniformity and good conductivity [see Appendix A, Figs. 5(b) and 6]. And with the decrease of Au thickness, the transmission as well as the surface smoothness becomes better [see Appendix A, Fig. 7(a)]. When the thickness of Au is reduced to 2 nm, the average transmission increases to about 80% from 850 nm to 1550 nm and the RMS also decreases to 0.683 nm. This indicates that the reduction of Au is of great significance to improve the light transmittance and surface smoothness without damaging the conductivity of the electrodes.

The J–V relationship of AuSPDs with different Au thicknesses is plotted in Fig. 4(a). Overall, each device clearly established rectifying behavior. What excited us is that these detectors had a similar low dark current density of approximately 4×10−7A/cm2 at −1V, which resulted from almost the same SBHs (0.797, 0.801, 0.803, 0.795 eV for Au thickness 2–6 nm) extracted from the ln (J/T2) versus 1000/T curves in Fig. 4(b). Furthermore, the low dark current and high SBH of 2AuSPD indicate that a continuous Au plane has been formed to effectively prevent the diffusion of ITO to Si due to the good film-forming ability of Au [38], which can also be verified in Appendix A [see Fig. 7(b)]. In a word, reducing the Au thickness can greatly improve the electrode transmission without damaging the dark current performance of the device.

The photoresponse of these AuSPDs is measured at zero bias voltage for wavelength varying from 1100 nm to 1700 nm, as shown in Fig. 4(c). Overall, the response increases with the decrease of Au thickness within the range of the measured wavelength. Two obvious absorption edges determined by silicon intrinsic bandgap (1.12 eV) at 1110 nm and the SBH (0.795 eV) at 1560 nm are clearly observed, suggesting that the detection is realized through both inter-band and internal photoemission. To further confirm the effect of Au thickness on the response of near-infrared, these detectors were also measured at 1310 nm with a low power excitation (∼14mW/cm2), as shown in Fig. 4(d). The photocurrent of 2AuSPD is nearly twice that of 6AuSPD, which also can verify the above conclusion. In addition, the response improvement is more obvious as the Au layer decreases from 6 nm to 4 nm, which is consistent with the results of the electrode transmission in Appendix A [see Figs. 5(a) and 5(b)], fully demonstrating that the raising response mainly benefits from the improvement of the transparency of the electrodes. A more comprehensive display of the performance improvement can be found in Fig. 4(e), including the dark current, the photocurrent at −1V measured under 1310 nm, and the photocurrent-to-dark current ratio (PDR) of AuSPDs. The PDR is defined as where Iphoto and Idark are the photocurrent and dark current, respectively. The PDR values for all samples (Au thickness: 0–6 nm) are 1, 4625, 4440, 4510, and 3833, respectively. It is clearly seen that the PDR of AuSPDs is significantly improved due to the effective suppression of Idark, which contributes to the high SBH from the Au insertion layer. Meanwhile, 2AuSPD has the best PDR due to the best transmission of 2 nm Au. The properties of 2AuSPD are summarized in Table 1, together with those of other silicon-based SPDs used for telecommunication wavelength for comparison. In comparison with the Au/Si SPDs supporting surface plasmon polaritons and other special design SPDs such as the addition of graphene or using pyramidally shaped Si in Table 1, these AuSPDs with simple processing exhibit the lower dark current density, leading to a better detection performance [32–34,40]. We also investigated other designs that use higher SBH to suppress the dark current density of SPDs, such as NiSi/n-Si, ITO/CuO/n-Si, ITO/Ag/n-Si or ITO/AgNWs/ITO/p-Si [25,29,31,41]. Apart from the lower dark current density, the AuSPDs exhibit an unprecedented high rectification ratio, as well as a better normalized photocurrent-to-dark current ratio (NPDR). The value of responsivity for the 2AuSPD was acquired from the method in Appendix A (see Fig. 9). In addition, the responsivity spectra as well as the external quantum efficiency spectra with different biases of all AuSPDs are also observed, as shown in Appendix A (see Fig. 10). The responsivity and quantum efficiency of the AuSPDs are related to the bias voltage, which is consistent with the photocurrents of different biases at 1310 nm in Fig. 4(d). Moreover, the AuSPDs have a high response speed and can reach sub-GHz, as shown in Appendix A (see Fig. 11).

In conclusion, we have demonstrated a silicon-based internal-photoemission AuSPD with a record low dark current density operating in a broad wavelength range. Since implementation of a thin Au insertion layer can effectively increase the SBH and suppress the dark current, the fabricated device showed a dark current density of 3.7×10−7A/cm2 at −1V and a high rectification ratio of 1.5×108 at ±1V. Furthermore, we found that the responsivity was obviously improved without costing the dark current performance of the device by reducing the Au thickness, resulting in an increase of the PDR and thus an overall performance enhancement of silicon-based SPD. We believe that the demonstrated device is a step forward in merging silicon technology and internal photoemission with simple design and high efficiency for optical communication bands.

Acknowledgment. The authors wish to acknowledge the assistance on the Fourier photocurrent spectrum received from the Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Science.