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Abstract
In this paper, germanium metal-semiconductor-metal photodetectors (MSM PDs) are fabricated on Si using a low-temperature two-step deposition technique by RF-PECVD. The photodetectors are optimized to effectively suppress the dark current through the insertion of n-type a-Si:H interlayer between the metal/Ge interface. Tuning the Schottky Barrier Height (SBH) by inserting different thickness of the interlayer is investigated. Results revealed that SBH for electrons and holes can effectively be enhanced by 0.3eV and 0.54eV, respectively. Furthermore, the dark-current (IDark) is suppressed significantly by more than four orders of magnitude. The measured IDark is ∼76 nA for an applied reverse bias of 1.0 V. Additionally, the Ge MSMs structure exhibited a photo responsivity of 0.8A/W at that bias. The proposed low-temperature (<550°C) Ge-on-Si MSM PD demonstrates a great potential for high-performance Ge-based photodetectors in monolithically integrated CMOS platform.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Monolithic integration of photonic devices in silicon platform has attracted a tremendous research interest in the last decade [1–5]. The primary reason is the bottleneck in the data exchange rate caused by the limitations of the metal interconnects for data rates beyond 10 GB/s. Nowadays, optical interconnects operating in the low-loss window (1.3-1.5µm) of silica fibers have replaced metal interconnects. However, silicon is transparent at those wavelengths, thus it cannot be used at the receiver frontend to convert the optical signal back to the electrical domain.
III-V semiconductor compounds work effectively for high-speed photo-detection, but these compounds are not compatible with silicon IC technology [6]. The use of a Ge based photodetector is very advantageous in terms of its potential compatibility with CMOS technology and it has superior intrinsic electrical and optical properties [7, 8]. Several techniques for fabricating and optimizing the growth of Ge on Si have been adopted [9–11]. For instance, Ge quantum well (Ge QW) based detector is used to help lower down the dark current [12]. However, temperatures as high as 650 °C were used in performing the deposition. The high temperature processes used for direct growth of Ge on Si are not compatible with standard Si technology. Moreover, being able to achieve high quality Ge-on Si layers at low cost and with low thermal budget is a main concern in Ge based devices.
Earlier, we performed a comprehensive study of the structural, optical and electrical properties of the low-temperature PECVD growth (< 550 °C) of Ge on Si [13]. The growth was used to fabricate a MOSCAPs devices [14]. In this work we demonstrate an MSM based photodetector fabricated using the low-temperature Ge layer grown directly on Si [13].
Amongst the photodetector structures, the Metal-Semiconductor-Metal (MSM) design is promising due to the ease of integration and fabrication. Additionally, it offers low detector capacitance, and large device bandwidth [15–17]. However, Ge based MSM PDs still suffer from large dark current which results in poor sensitivity, low signal to noise ratio and large power consumption. The primary causes of the dark current are the low Schottky Barrier Height (SBH) formed by severe Fermi-level pinning at the metal/Ge junction and the metal-induced gap states (MIGS). Figures 1(a) and 1(b) show a schematic representation of MSM based photodetector along with its energy band diagram.
Fig. 1 MSM basic structure (a) schematic presentation of the device (b) band diagram showing the metal-semiconductor (MS) junction and the back-back diode behavior.
Several techniques for suppressing the dark current in Ge MSM detectors have been reported in literature. For instance, insertion of a large band-gap material between the metal and the Ge like SiC [18], and a-Ge [19], dopant segregation process modulating the SBH [20], introduction of asymmetric electrode metal contacts [21] and metal-interlayer-semiconductor-metal structure using TiO2 as an interlayer [22]. Among those techniques, the insertion of larger band gap material does show an effective reduction of dark current but the photo generated current is also reduced. Furthermore, the employment of asymmetric electrode metal or the replacement of the metal cannot reduce the dark current effectively due to the severe fermi pinning between the metal/Ge junctions. In this work, the photodetectors are optimized to effectively suppress the dark current through the insertion of n-type a-Si:H interlayer between the metal/Ge interface. In order to evaluate the improvement in the detector performance, the IV characteristics of the Ge MSM detector with different interlayer thicknesses are tested in dark and under 1.31µm wavelength illumination.
2. Device fabrication
A 6 inch (100) arsenic doped (n-type) Si wafer with resistivity of 0.01-0.02 Ω.cm is used as a starting substrate for the Ge growth. To achieve a high-quality Ge layer, the substrate surface should be free from contamination and native SiO2. Therefore, a pre-growth cleaning process is performed, where the wafer is ultrasonicated in acetone bath and rinsed with isopropanol and deionized water, followed by 60s dip in diluted hydrofluoric acid (HF) 1:10 solution. After being cleaned, the wafer is loaded immediately to a load lock of an Oxford Instrument System 100 PECVD activated by 13.56 MHz radio frequency signal.
High purity germane (GeH4), H2 and Ar gases are used as precursors for Ge growth. The Ge film was grown using the two-step approach. In step one (low temperature (LT), high rate (HR)) the deposition was performed at 350°C with 3 sccm flow of GeH4. This step is followed by an in situ annealing at 350°C using 20 sccm of H2 and 200 sccm of Ar for 10 min. After this, step two (high temperature (HT), low rate (LR)) of the Ge deposition starts immediately without removing the wafer from the system. The Ge growth carried out at 500°C with 1 sccm of GeH4. With these two steps one cycle of Ge growth is done. For thicker films, step two of growth can be used for longer time. The details of the two-step approach has been described in our previous work [13]. It should be noted that no dopant gases are introduced during the growth hence the Ge films are intrinsic, facilitating low-voltage operation. After the growth process, post annealing at 550° C was carried out to enhance the surface mobility and to reduce the Threading dislocation density (TDD) of the Ge film [23, 24].
Prior to MSM fabrication, the Ge/Si wafers were pre-cleaned by dipping the samples into diluted HF (1:10) solution at room ambient, followed by a rinse and drain in deionized water, and subsequently dried by N2. After this, ~5-20nm layer phosphine doped a-Si:H was deposited using PECVD reactor at the following conditions: 100 sccm flow of Ar, 50 sccm flow of H2, 10 sccm flow of SiH4 and 80 sccm flow of 2% PH3/H2. The deposition temperature and the pressure were set at 250 °C and 1 Torr, respectively. In order to study the effect of n-type a-Si:H thickness on the photo-generated current, the deposition time was varied between 30s-120s with a fixed RF power of 10W. Following the growth of a-Si:H interlayer, the wafer is coated with ~80 nm PECVD grown high quality SiO2 at 250°C. This layer acts as an anti-reflection coating, where the deposited thickness is optimized to minimize the reflections from the surface. In addition, it electrically separates different devices from each other. The electrodes are defined by photolithography and patterned by standard photoresist lift-off process.
Prior to metal deposition, the PECVD SiO2 is etched from the electrodes using 1:10 buffered oxide etchant (BOE). Finally, a range of contact areas were fabricated by depositing 200nm Ag film using high purity Ag pellets by thermal evaporator. MSM detectors with circular geometry were chosen to avoid secondary effects such as fringing fields and obtain different electric field distribution along the contacts. Thus the detectors are designed with an active absorption areas of 102-104 μm2.
A schematic diagram, Transmission Electron Microscope (TEM) and Scanning Electron Microscope images, and X-ray diffraction pattern of the fabricated photodetector layers are shown in Fig. 2.
Fig. 2 Ge MSM based photodetector structure (a) schematic representation (b) SEM image of Ge-on-Si layer with an inset showing XRD spectra and TEM image of the photodetector layers (c) SEM top view image of the fabricated detectors showing detector active area of 104 µm2.
3. Results and discussion
3.1 Electrical characterization
There are two configurations for the electrical testing of the photodetector: First, a top connection or lateral configuration, where the two top metal pads are biased. Second, a bottom connection or vertical configuration, where one pad is biased with respect to the substrate body. The two electrical connections with their voltage contour distribution at an applied voltage ranges from 0V-4V are represented in Figs. 3(a) and 3(b).
Fig. 3 Electrical connection showing the voltage contour maps at an applied voltage ranges from 0V-4V of (a) top connection (lateral) (b) bottom connection (vertical).
Prior to the MSM electrical testing, the individual metal-semiconductor (MS) junctions are characterized to verify the Schottky behavior (rectifying diode). In our case, the Ge growth is carried out on a low resistivity Si substrate, thus we avoid using the vertical connection in verifying the MS junction. Under this condition, the substrate can influence the current flow as the Si/Ge interface acts as another rectifying junction. The metal pads from the top are connected by grounding one pad and sweeping the voltage over the other.
It should be noted that the grown Ge film is intrinsic since no intentional dopant gases are introduced during the deposition. It has been reported previously that extended defect, dislocation, and grain boundary introduce acceptor states in the Ge layer. Thus, the film exhibits p-type behavior without any intentional doping [25, 26]. However, in this growth, the Ge layer exhibits n-type doping (p-channel) with ~0.5x1017 cm−2 carrier concentration [14]. This is attributed to the arsenic auto-doping effect during the growth of Ge film on n-type Si substrate [27]. This n-type behavior is depicted in our previous work of Ge/Si MOSCAP using the same growth methodology [14]. It is well known that in metal/Ge interfaces, Schottky junctions are formed with n-type Ge while ohmic ones are formed with p-type Ge due to the strong fermi level pinning to the valence band edge of the Ge [28].
Several studies have adopted Al2O3, or GeOx to shift the SBH. In our fabrication process the strongly pinned metal/germanium interface is tuned by inserting an ultra-thin SiO2 layer between the metal and the Ge film. In order to keep the process simplicity, the SiO2 layer is under-etched to keep a ~2nm film underneath the metal to engineer the SBH and achieve rectifying junction.
3.2 Band diagram
The band diagram of a basic Ge MSM structure is shown in Fig. 4(a) As shown, the SBH for the holes is only 0.16 eV and 0.5 eV for the electrons which is due to the sever Fermi level pinning at Ge/metal interface. Therefore, the device suffers from high leakage current. However, by introducing n-type a-Si:H interlayer, the SHB for both hole and electron is significantly increased as illustrated in Fig. 4(b). The electron affinity and the band gap of the a-Si:H interlayer are assumed to be 3.7 eV and 1.5eV, respectively [29–31]. The increase in the barrier height can effectively reduce the dark current in the device and increase the tunneling resistance.
3.3 Dark current suppression
Figure 5 shows the IV characteristics of the MSM devices with and without the insertion of n-type a-Si:H barrier layer. The Ge MSM structure fabricated without an interlayer shows a large dark current of ~4.1 mA at 1V. The active area of the tested devices is 3.1x104 µm2. Therefore, in order to suppress the dark current, phosphine doped a-Si:H interlayer with various thicknesses is adopted. It is critical to deplete the semiconductor region completely to achieve highest internal photo-detection quantum efficiency. Thus, the interlayer is n-type doped to avoid forming any junction with Ge and to keep the depletion in Ge region. In order to determine the current-voltage (I-V) characteristic of the detectors in dark, the measurements were carried out using Agilent B 1505 A curve tracer + Signatone manual 1160 prober.
Fig. 5 (a) IV dark characteristics of the MSM devices with and without the insertion of n-type a-Si:H barrier layer (b) A histogram showing the reduction in dark current at 1V reverse bias as a function of thickness.
As can be seen in this figure, the interlayer thickness was varied from 5 to 20 nm which is associated with a deposition time of 30s to 2min, respectively. It was reported that dark current at room temperature is dominated by Shockely Read Hall mechanism [32]. This mechanism depicts the generation of electron-hole pair in Ge detector via defect and trap levels in the Ge band gap. The threading dislocation density (TDD) of the grown Ge on Si is ~106 [13]. Compared to PDs fabricated in bulk p-type Ge, this measured dark current is roughly an order of magnitude lower [22]. This could be attributed to the n-type behavior of our Ge layer that reduces the hole contribution to the overall current. By introducing a 20 nm thick a-Si:H interlayer a significant reduction in the dark-current to 76nA at 1V reverse bias is observed. This dark current is ~four orders of magnitude (x 53247) less than the MSM structure fabricated without this interlayer. This means that the insertion of a-Si:H led to an enhancement in Schottky barrier height. This is described in the band diagram of the MS junction barrier height shown in Figs. 4(a), and 4(b) before and after the insertion of n-type a-Si:H, respectively.
In order to estimate the potential barrier at the Ge/metal interface, using the IV dark curves the dark saturation current is extracted through Eq. (1) [33]:
where J0 is the dark saturation current, A* is the effective Richardson constant and is equal to 143 A cm−2 K−2 for n-type Ge [34, 35], and ϕB is the potential barrier. J0 is extrapolated as a y-axis intercept from the linear part of log (J) in forward bias. Calculated potential barrier values, ϕB, for diodes without and with interlayer are 0.48 eV and 0.83 eV, respectively. This is consistent with the values reported in section 3.2 (band diagram) of the paper.3.4 Optical characterization
The optical response of the detectors are tested using monochromatic 1310 nm laser with incident optical power of 2.4mW. The laser is coupled vertically to the sample using a vertically mounted fiber with a far field beam waist diameter of approximately 200 µm. The reflectivity of the PD surface is minimized by optimizing the thickness of the (SiO2) which acts as anti-reflecting coating (ARC). The IV characteristics of the detectors under illumination are measured by reverse biasing the top metal pads.
The photo-response with and without a-Si:H interlayer are shown in Fig. 6. The detectors fabricated without a-Si:H barrier interlayer exhibit very low photo-response which is approximately the same as the dark current values. However, devices fabricated with just ~5 nm barrier interlayer demonstrate a considerable difference between the illuminated and dark state. The difference is about two orders of magnitude at 1V.
Figure 7(a) shows the responsivity as a function of the reverse bias voltage for detectors fabricated with and without a-Si:H barrier layer. A responsivity of ~0.8 A/W is obtained for photodetectors with an interlayer thickness of ~9 nm compared to ~0.04A/W for the ones without. A lower responsivity values at 1550nm are expected, where responsivity decreases at longer wavelength. Typically, Ge is highly absorbing at this band (1310nm) due to the large absorption coefficient that increases responsivity. Therefore, with a layer thickness < 2um, most of the light can be absorbed [36]. Previous work in SiGe and Ge reported the effect of layer thickness on photon absorption [37]. The results indicated that at high enough Ge fraction in the epitaxial layer the photon absorption did not show an effective decrease with the reduction in the layer thickness. However, we believe that in case of direct growth of Ge-On-Si thick layers will improve the photodetectors responsivity. This can be explained as the following: most of the misfit dislocation and traps are confined next to Si/Ge interface [13], far away from the Si interface high quality Ge layer (less dislocation) can be achieved [21].
Fig. 7 (a) Responsivity as a function of reverse bias voltage for Ge MSM detector fabricated with and without a-Si:H layer (b) A histogram showing the percentage reduction of the photo-response as a function of interlayer thickness at 1V.
It should be noted that the substantial lattice mismatch between the a-Si:H barrier and the underneath Ge film does not affect the photo-response for thicknesses less than 9nm. However, devices fabricated with interlayer thicknesses of 14 nm and 20 nm exhibited a reduction of the photo response as illustrated in Fig. 7(b). This might be due to the fact that for interlayer thicknesses > 9 nm a heterojunction between the Ge and a-Si:H interlayer is formed. This leads to void creation and interface traps that hinder the photo-generated current. The interface between a-Si/Ge layers can contribute to light degradation and hinder the photo-generated current. This has been reported before in solar cell application [37], where a c-Si layer is deposited prior to a-Si to passivate the Ge surface. Solar cells fabricated with this passivation indicated higher performance, this means that a-Si/Ge interface suffers from traps and dangling bonds. Furthermore, the deposited a-Si layer in this work with the reported process conditions might has some microcrystalline silicon embedded in the amorphous matrix, this behavior has been studied in our previous work [29], where SiH4 flow low and high H2 flow leads to this phenomena. Based on this finding, a tradeoff between the dark and photo generated currents can be achieved by introducing an interlayer thickness between 9 and 14 nm.
Table 1 shows a summary of the dark current characteristic and photo-responsivity of Ge MSM photodetectors reported in the literature using various techniques. The current work demonstrates that low dark current and good responsivity is achievable by introducing a-Si:H interlayer. In addition, the low temperature processing of the fabricated Ge on Si MSM PD has a great potential for high-performance Ge photodetectors in monolithically integrated CMOS platform.
Table 1. Summary of the comparison of dark current and responsivity performance of Ge MSM photodetectors fabricated by different research group
4. Conclusion
In summary, we demonstrated MSM photodetectors fabricated using low-temperature Ge layer grown directly on Si. The Ge layers are grown using a two-step growth technique by RF-PECVD reactor. The possibility to suppress leakage current in Ge MSM detectors have been investigated by inserting a large band gap material between the Ge and the metal. A 5-20nm layer of n-type Si:H is used as an interlayer to engineer the SBH. Results showed that a ~4 order of magnitude reduction of dark current is achieved by inserting >14 nm of a- Si:H. Furthermore, the fabricated photodetectors responsivity was tested under 1310 nm wavelength illumination. A value of 0.8 A/W is achieved at 1V reverse bias compared to 0.04 A/w for the ones without the interlayer. This highlights the fact that the incorporation of a-Si:H barrier layer induces no significant degradation to the illuminated state. Also, the high responsivity indicates the high quality of the grown Ge layers. Therefore, this technique is very promising for the future monolithic integration of Ge and Si optoelectronics.
Funding
Masdar Institute of Science and Technology.
References and links
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