1. Introduction

Si-based light-emitting devices are one of the important components for hybrid optoelectronic integration on Si platform. Various solutions have been proposed in recent years in order to enhance the light emission from Si-based materials. Quantum structures including quantum wells and dots [1–4], Si nanocrystals [5,6], Er doping [7,8], and defects engineering [9,10] have been proposed for creating light-emitting centers in Si or Si-based materials. Ge self-assembled quantum dots are another possible choice for Si-based light-emitting devices [3,4]. The advantages of Ge self-assembled quantum dots are the compatibility to complimentary metal oxide semiconductor (CMOS) technology and the light emission at telecommunication wavelengths. Ge dots emit light in the wavelength range of 1.3-1.6 µm and Si is transparent for the light in this range, which is important for the future integration of Si waveguide circuits and Si-based light-emitting devices on a single Si wafer. Optoelectronic hybrid integration based on Ge dots-based light source is therefore a potential direction if practical Ge dots-based light-emitting devices are available. However, light emission from Ge self-assembled quantum dots is still lack of efficiency and spectrum purity is poor. In order to enhance the light emission, embedding Ge self-assembled quantum dots into microcavities such as photonic crystal cavities [11,12] and microdisk resonators [13] has been reported and strong room-temperature resonant photoluminescence has been observed from the microcavities. A quality factor as high as 20 000 has been also achieved for the resonant light emission from Ge dots in photonic crystal L3 cavities [14]. These progresses show that Ge dots in optical microcavities is a possible choice for Si-based light sources. To develop practical devices, however, current-injection should be realized in such Ge dots-based light-emitting devices. Although there are some reports on electroluminescence SiGe heterostructures [15,16], there are few reports on the study of electroluminescence from Ge dots in optical microcavities. In this paper, we report the research on the Si-based light-emitting device based on Ge quantum dots in a microdisk resonator with p-i-n junction for current-injection.

2. Device fabrication

Figure 1 shows the schematic structure of the fabricated current-injected Si-based light-emitting device. Ge self-assembled quantum dots are embedded into a Si microdisk resonator on silicon-on-insulator (SOI) with a vertical p-i-n junction for current-injection. Under forward bias, the current flows to the microdisk from the surrounding p + area through the thin Si film on the buried oxide. After going through multiple Ge dots layers, the current is collected by the top n + layer. The thickness of the top Si film and buried oxide of the SOI wafer were 260 nm and 2 µm, respectively. As the internal light-emitting centers, multiple layers of Ge self-assembled quantum dots were grown on p-type SOI by solid-source molecular beam epitaxy (SSMBE) at 600°C. In order to enhance the total number of Ge dots in the devices, ten layers of the Ge self-assembled quantum dots were grown with 15-nm-thick Si spacer layers and an 80-nm-thick Si cap layer. To fabricate the vertical p-i-n junction for current-injection, As⁺ ions were implanted into the top Si film at an energy of 25 keV and with a dose of 1 × 10¹⁴/cm2 for the n⁺ layer. Electron beam lithography (EBL) was used to define silicon microdisk resonators on the sample. The top SiGe layer was etched by reactive ion etching (RIE) to form the microdisk resonator. The etching was controlled to remain a 160-nm-thick p-type Si film which works as the current pathway from the p + area. Selective B⁺ ion-implantation was performed in the surrounding sector windows defined by EBL to form the p + area. The energy and dose of the ions were 25 keV and 1 × 10¹⁵/cm², respectively. The sample was annealed at 600°C for 30 minutes in N₂ ambient to activate implanted dopants. After annealing, the surface of the devices was passivated with SiO₂ films grown by plasma-enhanced chemical vapor deposition (PECVD). The contact window for n + area was located at the centre of the microdisk, while the contact window for p + was located in the surrounding area. Aluminum was used as contact metal. Finally, the sample was annealed at 350°C for 10 minutes in forming gas ambient.

 

Fig. 1 Cross-section view of the structure of the current-injected light-emitting device.

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Figure 2 shows the scanning electronic microscope (SEM) image of the fabricated device. The size of the microdisk resonator is 2.8 µm in diameter and it is seen that the device structure is well defined by the fabrication process. Figure 3 shows the I-V characteristic of the device. A typical curve of pn junctions is seen to be obtained. The reverse current at −5 V is 60 µA. Electroluminescent light at room-temperature was collected by a 100 × objective lens and directed into a monochromator with a 32-cm focus length. Dispersed signal was detected by a liquid nitrogen cooled multichannel InGaAs detector.

 

Fig. 2 Scanning electron microscope image of the fabricated 2.8 µm microdisk device.

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Fig. 3 I-V characteristic of the p-i-n junction.

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3. Results and discussion

Figure 4 (a) shows the electroluminescence spectrum recorded at room-temperature under an injected current of 0.1 mA (~8 × 10² Acm⁻²). As seen in the figure, clear electroluminescence is observed in the wavelength range of 1 to 1.4 µm. Several peaks are seen in the spectrum, and they may be assigned to the optical resonance in the microdisk. There are three major peaks locating at 1.185, 1.238, and 1.295 µm, respectively. In order to identify the type of these resonant peaks, three dimensional finite-difference-time-domain (FDTD) method was used to simulate the optical resonance in the disk. Figure 4 (b) shows the calculated TM-polarized-like resonant peaks of the device. Three peaks locating at 1.191, 1.241 and 1.296 µm are well corresponded to the three major peaks in the electroluminescence spectrum. The mode profiles of these peaks are shown by the insets in Fig. 4 (b). They are whispering-gallery modes (WGMs) with the order of TM0_{2, 27}, TM0_{3, 12}, and TM0_{3, 11}. All these WGMs are TM-polarized-like, and no clear TE-polarized-like modes are observed in the spectrum. Based on calculation of the fundamental slab mode indexes in the 0.5-µm-thick disk and the 0.16-µm-thick surrounding slab, it is found that the difference of TM mode indexes in these two areas is larger than that of TE modes, suggesting that the energy loss of TM modes due to coupling to the surrounding slab is smaller than that of TE modes. This leads to the domination of the TM modes in the spectrum. The calculated Q factor of these WGMs is in the range of 400 to 800 which is much smaller than calculated Q factors (~4800) of a typical microdisk resonator with the surrounding area completely etched to the BOX. It is reasonable since the energy loss induced by the coupling to the remained surrounding Si slab is much larger than that of a completely etched microdisk. The quality factors of WGM peaks in the EL spectrum are in the range of 50 to 90 and they are smaller than the calculated value of 400 to 800. The reason for the decrease of quality factors is attributable to the absorption loss induced by the Al contact on the microdisk and free carrier absorption in the high doping layer. The scattering loss induced by the Al contact arm located on the top of the disk may also decrease the quality factor. It is expected that replacing the microdisk resonator with a photonic crystal microcavity will improve the quality factors of the resonant peaks since the metal contact will be located outside the cavity and the current will flow into the cavity through the photonic crystal lattice. No obvious Fabry-Pérot modes [13], which are induced by the reflection at the edges of the disk, are observed in the electroluminescence spectra. Since a 160-nm-thick Si layer is remained as current pathway in the surrounding area of the disk, the reflection at the edge is smaller than that of a fully etched microdisk resonator. It means that most of the optical power escapes from the disk when light is reflected at the edge, which makes the resonance weak and difficult to be observed.

 

Fig. 4 (a). Electroluminescence spectrum of the device under current of 0.1 mA recorded at room-temperature. (b). Calculated wavelengths of TM-polarized-like WGMs supported by the microdisk. The insets show the mode profiles of the WGMs locating at 1.191, 1.241 and 1.296 µm calculated by three-dimensional FDTD simulations.

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It is noted that the wavelength range of the electroluminescence is blue shifted compared with photoluminescence spectra recorded from optical microcavities with Ge dots [11–13]. The possible reason for this blue shift is the interdiffusion of Ge atoms during high temperature processes, such as dopant activation anneal and PECVD deposition of SiO₂. The interdiffusion of Ge atoms in Ge dots with surrounding Si atoms causes the increase of bandgap energy of the dots. The light emission around 1.1 µm can be assigned to Si bulk bandgap transition.

Figure 5 shows the EL spectra at different currents. As well seen in the inset of the figure, the red shift of the resonant peaks is noticed against the injected current. This can be attributed to the increase of the refractive index of Si due to the thermo-optic effect induced by the current heating. Figure 6 shows the current dependence of the integrated EL intensity. The observed nonlinear increase of the integrated intensity is very similar to the PL result, i.e., the PL intensity of photonic crystal microcavities with Ge self-assembled quantum dots nonlinearly increases against the excitation power [11]. The possible reason for this nonlinear increase is the confining of carriers inside the microdisk. When the current is increased, local electron-hole plasma with high density tends to appear inside the disk, which leads to an additional scattering mechanism contributing to the nonlinear increase of radiative recombination.

 

Fig. 5 Electroluminescence spectra at different currents. The inset shows the red shift of the resonant peak against injected current.

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Fig. 6 Dependence of integrated electroluminescence intensity on the injected current.

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4. Conclusion

In summary, a current-injected light-emitting device based on microdisk resonator with Ge quantum dots was fabricated on SOI and room-temperature electroluminescence was successfully observed under DC currents. Resonant peaks corresponding to the WGMs were well identified with the aid of FDTD simulations. The result shows a possible means to develop Si-based light-emitting devices for the optoelectronic integration on Si platform.