Room Temperature Superlinear Power Dependence of Photoluminescence from Defect-Free Si/Ge Quantum Dot Multilayer Structures

Abstract

Low-dimensional semiconductor structures, in particular quantum dots (QD) have attracted continuously increased interest from the viewpoints of fundamental physics and of device application. The strained SiGe/Si system, expected to play a dominant role in monolithic integration of Si-based opto-and microelectronics, is still a subject of numerous investigations (see, e.g., [1,2]). Optical properties from Ge islands have been widely studied, e.g., to analyse the complex transition and recombination phenomena in multilayer structures [3]. Recently, few papers reported on the room temperature photoluminescence (PL) originating from Si/Ge QD structures [4-6], however, no detailed investigations on the optical properties were presented. In the present work we would like to report on the fabrication of defect-free multilayer structures containing Ge QD layers in a Si matrix exhibiting strong PL at 1.55 mm at room temperature. This intensity is only ten times lower than that measured at low temperatures. PL integrated intensity versus excitation density shows unambiguously super-linear behavior with m = 1.6.Experimentally, multilayer Si/Ge structures were grown by molecular beam epitaxy (MBE). The structure of the samples consisted of 20 periodic Ge layers (0.7 nm Ge in each layer) separated by 5.5 nm Si spacers. The first 2 nm of each Si spacer were intentionally doped with antimony (Sb). The substrate temperature was kept through the whole growth at 600 C. Structural properties were examined by transmission electron microscope (TEM, JEM 4010). The optical properties were characterised by PL measurements. An argon laser (2.54 eV) was used as excitation source. A cooled Ge photodiode (Edinburgh Instruments) was used as a detector. Figure 1 presents a typical cross-section high-resolution TEM (a) and a plan-view TEM (b) image. Despite the small thickness of the Si spacer used, no structural defects were observed through the structure. The lateral size of the Ge islands amounted to about 80 nm, their heights lay in a range of 3 to 5 nm. The larger heights were found to be in the middle of the structure. Surprisingly abrupt interfaces were formed and no significant intermixing was observed even at the relatively high substrate temperature of 600 C. The average density of Ge QD is about 1 Â 10 10 cm --2 (see, e.g., Fig. 1b). The islands are quite regularly arranged and exhibit well pronounced square-based shape.One goal of the MBE growth was to influence the electronic properties of the grown multilayer structure by, e.g., Sb doping. PL spectra of the samples with different doping levels show that there is a significant effect of the Sb atom concentration on the optical properties. For a relatively small doping level estimated from SIMS data (n % 10 17 cm --3 ) a maximum of PL intensity was achieved. Higher or lower Sb concentrations lead to a decrease of the PL intensity by several times. At the highest doping levels (n % (5-7) Â 10 18 cm --3 ) the maximum of the PL peak shifts towards higher energies. This effect might be...

Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices

“…13 are measured. For optical pumping the value of m-factor does not exceed 1.65 at room temperature [41,42]. We have reported [43] such unusually large m-factor for Ge/Si QDSL EL.…”

“…[41,42] and Fig. 13) has the same nature as the temperature dependence-the conversion to the MB regime due to straightening of a band bending.…”

Type-I Heterostructure and Improved Phase Stability of Formamidinium Lead Iodide Perovskite Grown on WS_2

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Super-resolution provided by the arbitrarily strong superlinearity of the blackbody radiation

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