Enhanced Light Emission from N-Doped Ge Microdisks by Thermal Oxidation
Authors: Hashimoto Hideaki , Xu Xuejun , Sawano Kentarou , Maruizumi Takuya 
Published: February 2020

Germanium (Ge) has been considered a promising material candidate for Optoelectronic Integrated Circuit (OEIC) on silicon (Si) substrates [1, 2, 3]. Although Ge is an indirect-band-gap semiconductor, the energy difference between its direct and indirect band gaps is rather small. Tensile strain and heavy n-type doping have been proposed and demonstrated to be able to compensate for this difference and significantly enhance the direct-gap light emission [4]. To fabricate device structures for optical resonators, lithography and dry etching are desirable. However, defects on the sidewall induced by dry etching would have deleterious effects on the devices performance. In this study, we fabricate circular mesa structures (microdisks) from Ge grown on Si substrate and observe enhanced light emission from device by surface passivation via thermal oxidation.
A 740-nm-thick Ge film with a tensile strain of about 0.22% was grown on a p-type Si(100) substrate by solid source molecular beam epitaxy (SSMBE) using two-step growth method [5]. The samples were heavily n-type doped to a concentration up to about 4×1019 cm-3 by phosphorus diffusion from a spin-on-dopant (SOD) source [6]. Electron-beam lithography was used to define circular patterns with different radii on resist. The patterns were then transferred to the Ge by inductively coupled plasma reaction ion etching. After resist removal and cleaning, the samples were oxidized at atmospheric pressure in a furnace. The light emission was then measured by micro-photoluminescence (PL) under a normal-incidence backscattering configuration, immediately after oxidation, to avoid degradation of oxide film in atmosphere. The PL spectra of a blank Ge film and microdisks with radii of 0.8 µm, before and after thermal oxidation at 550 °C for 1 hour, are shown in figure 1(a) and (b). The pump laser power was fixed at 5 mW in all cases. For blank Ge film, the light emission intensity is enhanced by a factor of 1.31 (with respect to the integrated PL intensity) after oxidation. And the emission peak is slightly blue-shifted, which might be attributed to the reduced doping concentration due to out diffusion of phosphorus during oxidation under such a high temperature. The PL intensity of the microdisks with radii of 0.8 µm after thermal oxidation is significantly enhanced compared with that without thermal oxidation. The enhancement factor is about 3.42. We thus attribute this to reduced surface recombination or defects induced by dry etching. Compared with blank Ge film, the enhancement factor for microdisk is much larger. This indicates that the effect of surface passivation of the etched sidewall is dominant over the top surface. We also investigated the light emission properties of microdisks with different radii, as shown in figure 1(c) and 1(d). The enhancement factors are 1.42 and 1.27 for disk with radii of 1.8 and 3.0 µm, respectively, e.g, enhancement factor decreases with increasing disk size. During PL measurement, the pump laser spot was fixed at the same size (a diameter of about 2 µm) and at the same position (the center of the microdisks). For lager microdisks, the sidewall is further from the pumped area where carriers are generated. The contribution of nonradiative recombination induced by sidewall is less dominant for lager microdisks. Therefore, under the same surface passivation condition, the enhancement of the light emission intensity is more pronounced for smaller microdisks.
In summary, the effect of thermal oxidation on the light emission properties of dry-etched n-type doped Ge-on-Si microdisks was investigated. The direct-gap light emission was significantly enhanced owing to surface passivation, up to 3.42 times for microdisks with radii of 0.8 µm. The enhancement factor increased as the disk size decreased, indicating that a sidewall-related nonradiative recombination process was an important limiting factor achieving a high injected carrier density and it could be mitigated by thermal oxidation. With this simple method, it is thus highly promising to realize low-threshold Ge lasers monolithically integrated on Si substrates by a mature and low-cost complementary metal-oxide-semiconductor-compatible process.
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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] K. Nishida et al, Thin Solid Films 557, 66 (2014)
[6] X. Xu et al, Appl. Phys. Express 8, 092101 (2015)

Figure 1