Microdisk integrated with a bus waveguide is fabricated on silicon-on-insulator substrate containing Ge self-assembled quantum dots as active medium. The device is demonstrated to be operated as both light-emitting diode and photodetector. At forward bias, carriers are injected into the microdisk and light emission at 1.45–1.6 μm is extracted through the waveguide via microdisk-waveguide coupling. Sharp resonant peaks with Q-factor as high as 1350 are obtained in the electroluminescence spectra, corresponding to whispering gallery modes of the microdisk. At reverse bias, the device functions as a resonant cavity enhanced photodetector with wavelength-selective photo-response. The photo-current at resonant wavelength of 1533.65 nm is 50 times larger than that at non-resonant wavelength. The dark current density of the photodetector is as low as 0.29 mA/cm2 up to −10 V bias and the peak responsivity is 5.645 mA/W.
© 2014 Optical Society of America
Due to its indirect band gap property, silicon (Si) is thought to be a less efficient material for active photonic devices such as light sources and photodetectors. This has been the last obstacle for complementary metal-oxide-semiconductor (CMOS) monolithic Si-based photonic integrated circuits [1, 2]. Recently, germanium (Ge), which is also a group-IV semiconductor, has attracted great attention as an optical material due to its light emission and absorption wavelength in the telecommunication band and CMOS process compatibility with Si. Although Ge is also with indirect band gap, its light emission efficiency can be enhanced by material engineering. By applying tensile strain and n-type doping on Ge, the small difference between its direct and indirect conduction band valleys can be reduced and it is potential to convert Ge into a quasi-direct band gap material . Direct gap photoluminescence (PL) , electroluminescence (EL) [5, 6], optical gain , as well as pulsed lasing actions from Ge [8, 9] have been demonstrated through these methods.
Another efficient way is to use SiGe nanostructures, in which carriers can be locally confined [10, 11]. Self-assembled Ge quantum dots (QDs) can be easily grown on Si without defects through Stranski-Krastanov mode. Ge QDs have been demonstrated to emit light in the infrared at room-temperature . By embedding them into optical microcavities, the spontaneous emission rate can be significantly enhanced due to the Purcell effect [13, 14]. Strong PL and EL with sharp resonant peaks from photonic crystal cavities and microdisks with Ge QDs have been demonstrated recently [15–19]. However, light emission in all of these devices was extracted from the top surface via free-space optics. In order for monolithic integration with other photonic devices, extracting the light emission from a coupled waveguide on chip is desirable .
On the other hand, Ge is also an excellent platform for optical detection in the telecommunication band. Photodetectors based on bulk Ge films grown on Si have been realized with large responsivity and high speed [21, 22]. However, the dark currents of these devices are usually rather high due to the limited crystal quality of Ge. High quality Ge QDs are attractive materials for photodetectors with low dark current, but the absorption efficiency is rather low due to very small absorption volume [23, 24]. Optical microcavities are also efficient tools to enhance the absorption since light can be trapped in the cavities for a long time for efficient absorption [25, 26]. Ge QDs resonant-cavity-enhanced photodetectors with enhanced absorption has been demonstrated with high reflectivity top and bottom dielectric stacking mirrors . However, the responsivity is still very low due to the small absorption length limited by epitaxy growth. By using a waveguide configuration, the absorption length can be easily controlled via lithography and the responsivity is expected to be significantly increased [28, 29].
In this paper, we report the fabrication and characterization of waveguide-coupled microdisk with Ge QDs. Microdisk is preferred since radiation from the cavity modes is mainly along in-plane directions and is easily coupled with the bus waveguides. A vertical PIN diode is integrated with the microdisk serving as electrical structure. Light-emitting diode (LED) and photodetector are demonstrated on the same device at forward and reverse biases, respectively.
2. Device structure and fabrication
Figure 1(a) shows the three-dimensional schematic diagram of the waveguide-integrated microdisk and Fig. 1(b) shows the cross-section view along the center of the microdisk. The bus waveguide is located adjacently to the microdisk so that light emission can be coupled out from the microdisk, or input light can be launched into the microdisk from the waveguide. A vertical PIN diode is fabricated on the the microdisk, with P+ doped region on the surrounding slab and N+ doped region on the top of the microdisk, to inject carriers into or extract carriers from the microdisk.
The device was fabricated as the following procedures: Three layers of Ge QDs were firstly grown on silicon-on-insulator (SOI) substrate with 160-nm-thick top Si layer and 2-μm-thick buried-oxide (BOX) layer by solid-source molecular beam epitaxy. The layer stack includes a 40-nm-thick Si buffer, three-layers of Ge QDs separated by two 20-nm-thick Si spacers, and a 200-nm-thick Si cap. The nominal Ge layer thickness is 10 mono-layers (about 1.4 nm). All the layers were grown at a nominal temperature of 700 °C. Atomic force microscope measurement of the test sample with only one layer of Ge QDs grown with the same condition shows that the typical base diameter of the dots is around 80–95 nm, the height is around 9.0–9.7 nm, and the overall dot density is around 9.5×109 cm−2. The total thickness of Si/Ge layers is about 470 nm. The as-grown samples show intense PL peak around 1.5 μm at room-temperature. The band gap energy corresponding to this peak wavelength is much larger than the indirect band gap of Ge, indicating that there is considerable Si/Ge intermixing under this high growth temperature. However, it’s difficult to determine the exact Ge composition due to the very thin Ge layers and few layer numbers. After material growth, selective ion implantations of As was performed to make the N+ doping region. Microdisk and bus waveguide patterns were then defined by electron-beam lithography (EBL) and dry etching. A thin slab of about 120-nm-thick was left for electrical path. Shallow-etched grating couplers on the input and output ports of the waveguide were also fabricated by EBL and dry etching, with a different etching depth. The grating couplers are used to convert the light propagated along the waveguide upward for easy measurement . P+ doping region was then formed by selective ion implantation of BF2. The samples were then experienced a rapid thermal anneal at 750 °C for 10 seconds for dopant activation. After that, SiO2 film was deposited by plasma-enhanced chemical vapor deposition and VIA contact holes were fabricated by EBL and a combination of dry and wet etching of SiO2. At last, AlSi was thermally evaporated and lifted-off to form the electrodes. The scanning electron microscope (SEM) image of a typical fabricated device is shown in Fig. 1(c), with disk radius of 4 μm, bus waveguide width of 550 nm and gap width of 150 nm.
3. Experimental results and discussion
The current-voltage (I–V) curve of the fabricated device characterized by a semiconductor parameter analyzer is shown in Fig. 2. A typical diode characteristic is obtained with very low reverse current, which is a strong sign of excellent crystal quality of the Ge QDs. With these active layers in the device, carriers will be injected and radiative recombination will occur under forward bias. On the other hand, if there is light input, it will be absorbed by the Ge QDs and photo-current will be generated under reverse bias.
3.1. Operation as light-emitting diode
The light emission of the device was characterized by micro-photoluminescence (μ-PL) at room-temperature as same as our previous works [18,19]. The collection objective lens (100×, NA=0.50) was located on top of one of the grating couplers, which is about 300 μm far away from the microdisk. The recorded EL spectra under different injected currents are shown in Fig. 3(a). Several sharp resonant peaks can be observed from 1.45 to 1.6 μm wavelength range. The maximum peak intensity occurs around 1.55 μm, which corresponds to the peak position of coupling efficiency of the grating couplers. Figure 3(b) shows magnified view of one of the resonant peaks near 1.53 μm from the EL spectrum at 2 mA injected current. The Q-factor is fitted to be around 1350. As the injected current increases, the peak intensity increases, indicating increased carrier density in microdisk. The peak wavelengths show red-shift against current, which is due to the thermo-optic effect caused by Joule heating of the injected current. When the objective lens was located on top of the microdisk, we could rarely detect any signal because the radiation angle for such a large microdisk is very small and light emission couldn’t be collected by the objective lens. This confirmed that the EL spectra mentioned above were indeed collected from the grating coupler.
In order to confirm the coupling between microdisk and bus waveguide, optical transmission through the bus waveguide was also measured. Light from a tunable laser was launched into the waveguide through a single-mode fiber and grating coupler. The output light was extracted by the other grating coupler and directed into a powermeter. Figure 4(a) shows the comparison of the optical transmission and EL spectra under the same injected current of 2 mA. The non-flat background in the transmission spectrum is due to the non-uniform coupling efficiency of the grating couplers. Several resonant dips can be also seen in the spectrum, with the deeper and narrower ones corresponding to the stronger peaks in the EL spectrum and the shallower and broader ones corresponding to the weaker peaks. The peak positions in transmission and EL spectra agree very well, except that there is a 2–3 nm blue-shift of EL peaks compared with transmission dips. We have confirmed by launching the tunable laser directly into the EL measurement system that this difference was due to the different wavelength standard used in two measurement apparatuses. Provided that the grating coupler has a minimum coupling loss of 7.4 dB and the light emission is equally coupled to the two directions of the waveguide, the EL intensity in the waveguide is at least 10 times of those shown in Fig. 3(a). By fitting the resonant dips in the transmission spectrum, we obtained that the coupling efficiency at resonant wavelength of 1533 nm is about 36%. It can be further increased by decreasing the gaps between microdisk and bus waveguide, with the expense of reduction of the resonant Q-factor.
Numerical simulation based on finite element method (FEM)  was performed to identify these resonances. The following geometrical parameters were used: microdisk thickness is 470 nm, slab thickness is 120 nm, and the disk radius is 3.96 μm. Since the grating couplers used in measurement were designed only for transverse-electric (TE) polarization, we only considered TE modes of the microdisk. Figure 4(b) shows the calculated resonant wavelengths of whispering gallery modes (WGMs) of the microdisk with different azimuthal mode numbers. As a comparison, the resonant wavelengths extracted from the transmission spectrum without current injection are also shown. We can see that the simulation and experimental results agree very well. The sharper resonances correspond to the fundamental-radical-order TE WGMs and the broader ones correspond to the first-radical-order TE WGMs. By inspecting the mode profile of the two typical WGMs near 1533 nm in Fig. 4(c) and 4(d), we can see that the fundamental mode is confined well in the microdisk, while the first-radical-order mode is somewhat leaked towards the surrounding slab region. This can explain why the fundamental modes have much higher Q-factors than those of first-order modes.
3.2. Operation as photodetector
The photo-response of the device was measured in a similar configuration with transmission measurement, except that the PIN diode was reversely biased and the photo-current was measured by a source-meter. Figure 5 shows the photo-current spectrum of the device with input laser power of 1 mW and bias of −10 V. As a comparison, the transmission spectrum is also shown. Sharp resonant peaks are again clearly observed in the current spectrum. The peak wavelengths agree well with those of resonances in the transmission spectrum, indicating that enhanced absorption occurs at resonances. At the peak wavelength of 1533.65 nm, the photo-current is enhanced by a factor > 50 compared with background current (off-resonance). This large wavelength selectivity makes the device very attractive for wavelength-division multiplexing optical receiver.
The I–V curves of the photodetector at the resonant wavelength of 1533.65 nm are shown in Fig. 6(a). In the case of no laser input, the dark current of the photodetector is as low as 0.15 nA even when the bias is up to −10 V. This corresponds to a dark current density of 0.29 mA/cm2, which is several orders lower than those of photodetectors based on bulk Ge film grown on Si [21,22]. The photo-current can be obviously seen when there is laser input. Figure 6(b) shows the photo-current at 1533.65 nm with different input laser power. The horizontal axis indicates the coupled optical power into microdisk, in which the coupling loss of grating couplers and propagation loss of the waveguide before and after the microdisk has been eliminated from the total input power. At low input power, the current increases linearly, but the slope decreases at high input power. This is because the resonant wavelength slightly red-shifts at high input power due to thermal-optic effect. The wavelength at which the photo-current was measured was no longer the resonant wavelength. The red-shift was confirmed by comparing the transmission spectra at different input laser power.
The responsivity is then simply extracted by the slope of linear fitting of the current-power curve at low input power. The responsivity at −5 and −10 V is obtained to be 3.430 and 5.645 mA/W, respectively. The higher responsivity at higher bias is due to the increased built-in electrical field, so that more photo-generated carriers can be swept towards the electrodes and generate photo-current. These values of responsivity are several orders higher than previous reported one of resonant-cavity enhanced photodetectors based on Ge QDs , which is due to the high trapping efficiency of our cavity. For the resonance at 1533.65 nm, the trapping efficiency is about 88% according to the extinction ratio of the resonant dip in the transmission spectrum. If we consider the quantum efficiency, however, it is only 0.52%. We atributed this to the relatively large loss in the microdisk compared with Ge QDs absorption, including bending loss, absorption of heavy doping regions and metal contacts, and scattering loss due to the sidewall roughness. These losses will not contribute to the photo-current and compete with the Ge QDs absorption. In order to further increase the responsivity, we can either increase Ge QDs absorption by increasing layer number and optimizing the growth conditions to decrease the Si/Ge intermixing, or decrease other losses. For example, the bending loss can be decreased by increasing etching depth; the electrical structure absorption can be decreased by optimizing the layout of the doping regions and contacts to reduce their overlaps with WGMs.
We have demonstrated both light emitting diode and photodetector on the same device with waveguide-coupled microdisk containing Ge QDs. Under forward bias, the carriers were injected into the microdisk, and room-temperature light emission with sharp resonant peaks was obtained from the coupled bus waveguide. Those resonant peaks corresponded to the whispering gallery modes and had high Q-factor around 1350. Under reverse bias, on the other hand, the photo-current was generated when there was light input. The absorption at resonances were significantly enhanced due to the light trapping. We obtained ultra-low dark current density of 0.29 mA/cm2 and large peak responsivity of 5.645 mA/W at resonant wavelength of 1533.65 nm. With two identical such devices connected by a single waveguide, it is potential to realize on-chip optical data link with capability of bidirectional transmission.
This work was partly supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities (2009–2013), the Strategic Information and Communications R&D Promotion Programme (SCOPE) from MIC, Japan, and the Exchange Research Grant from Marubun Research Promotion Foundation.
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