Ge1−xSnx layers with x∼0.25 on InP(001) substrate grown by low-temperature molecular beam epitaxy reaching 70 °C and in-situ Sb doping

Ge1−xSnx with x∼25%, group-IV alloy semiconductor, is highly attracted to mid-infrared (MIR) photodetector applications because of its narrow bandgap with ∼0.25 eV, the compatibility to the Si integrated circuit platform, and lattice matching of Ge0.75Sn0.25 to InP substrate. Although the heteroepitaxial growth of Ge0.75Sn0.25 on InP has been reported, the basic physical properties of Ge0.75Sn0.25 have not yet been clarified. In this study, we discuss three topics: (1) understanding the crystalline growth features of Ge0.75Sn0.25 on InP, (2) revealing the thermal stability of Ge0.75Sn0.25, and (3) developing a carrier-control technique. First, we found that a low-temperature molecular beam epitaxy could achieve a Ge0.75Sn0.25 layer with superior crystallinity without dislocations. However, low Sn-content Ge1−xSnx regions are locally formed during the Ge0.75Sn0.25 heteroepitaxy; the origins of the low Sn-content Ge1−xSnx regions were discussed by transmission electron microscopy analysis. In addition, we found that lowering the growth temperature from 100 °C to 70 °C also effectively reduces the area of the low Sn-content Ge1−xSnx regions. Next, we found that Ge0.75Sn0.25 layers grown at 70 °C sustain thermal stability up to 200 °C without causing crystalline degradations. Finally, we demonstrated the in-situ Sb doping to Ge0.75Sn0.25. We found that the undoped and in-situ Sb-doped Ge0.75Sn0.25 layers exhibited p-type and n-type conduction, respectively, with carrier concentrations of order 1019 cm−3 by Hall effect measurement. This study suggests the MIR photodetector application is practically possible using low-temperature processes with process temperatures lower than 200 °C.