Strong Room-Temperature Electroluminescence from Ge-on-Si by Precise in-situ Doping Control
Authors: Yamada Kodai , Hoshi Yusuke , Sawano Kentarou 
Published: December 2020

Germanium (Ge) has been considered as a promising material for optoelectronic integrated circuit (OEIC) on a silicon (Si) platform [1-3]. Although Ge is an indirect-band-gap semiconductor, the energy difference between its direct and indirect band gaps is rather small and it has been shown that introduction of tensile strain can reduce this difference and significantly enhance the direct-gap light emission [4]. For realization of high efficiency electroluminescence from the tensile strained Ge, high quality diode structures with low leakage current should be formed. In this study, a p-i-n junction is formed within the epitaxial Ge by the precise in-situ doping method and additionally high density P delta doping is performed in the vicinity of the surface to create a low resistivity Ohmic contact. After the growth, a mesa-defined vertical diode is fabricated to reduce the leakage current. As a result, an excellent diode property with very large on-off ratio is observed and strong room temperature electroluminescence is observed owing to the very low leakage current.
Solid-source molecular beam epitaxy was used for the growth of the Ge-on-Si (GOS) structure. First, an undoped Ge buffer layer of 40 nm was grown on a p-type Si (100) substrate at a temperature of 350°C. Subsequently, a B-doped (5×1018 cm-3) Ge layer of 500 nm, an undoped Ge layer of 90 nm and a P-doped (1×1019 cm-3) Ge layer of 500 nm were grown at 300°C. For p and n-type in-situ doping, B and GaP effusion cells were used, respectively. After the growth of the p-i-n structure, a growth of an ultra-thin (UT) Si layer (2 monolayers), P delta doping (2×1014 cm-2) and 7 nm Ge capping were performed to obtain a low resistivity Ohmic contact [5]. As top and back metal contacts, Au and AuGa were deposited, respectively, where the top metal contact area (90μm×40μm) was defined by lift-off process. An optical active area (125μm×75μm) were mesa-defined by the photo-lithography and following reactive ion etching (RIE) process.
SIMS depth profiles of B and P in the Ge epilayer are shown in Fig. 1(a). It is found that uniform and abrupt doping can be realized for both B and P. Current-voltage characteristic shown in Fig. 1(b) reveal a good diode rectifying property with an on/off ratio of over 105. We can say that the leak current is highly suppressed by the mesa etching. A current injected light emission was measured by micro-photoluminescence (PL) system equipped with a cooled InGaAs detector, a detection limit of which was 1600 nm. Room temperature EL spectra at various continuous injected currents are shown in Fig. 1(c). The EL spectrum appears at around 200 mA injected current, and the EL intensity drastically increases with the injection current increase. As shown in the inset of Fig.1 (c), the EL intensity against the injected current shows a behavior with a threshold around 260 mA.
In summary, a GOS mesa structure p-i-n vertical diode was fabricated by the in-situ doping method. We observed excellent I-V characteristic with an on/off ratio of over 105. As a result, strong EL spectra were observed with injection currents over 260 mA, indicating that the Ge-on-Si p-i-n structure has high potential to realize high efficiency Ge lasers monolithically integrated on the Si platform.
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[2] P. Boucaud et al, Photo. Res. 1, 102 (2013)
[3] J. Liu et al, Opt. Express. 15. 11272 (2007)
[4] X. Sun et al, Appl. Phys. Lett. 95. 011911 (2009)
[5] M.Yamada et al, Appl. Phys. Lett. 107. 132101 (2015)

Figure 1