Fabrication of Thick Sige/Ge Multiple Quantum Wells on Patterned Ge-on-Si and Their Optical Properties

Kanesawa, Rena

Recently, silicon photonics has been attracting increased attention toward the realization of on-chip optical interconnection, which requires high-efficiency light-emitting devices to be integrated on Si substrates. Although Ge is an indirect-band-gap semiconductor like Si, it is expected that strong direct-transition luminescence can be obtained from tensile-strain induced Ge-on-Si. We have previously reported that highly efficient room temperature EL emission can be obtained from strained Ge-on-Si p-i-n diodes[1]. Further increase in luminescence intensity and control of emission wavelengths can be made possible by introducing a quantum well structure in the active layer. Although the fabrication of high-quality strained SiGe layers on Ge is essential for the fabrication of quantum well structures, critical thickness limits the total growth thickness and numbers of strained layers, which makes device applications difficult. We have succeeded in fabricating a strained SiGe layer that is much thicker than the critical thickness by using a patterning method, and this result is very promising for the fabrication of SiGe/Ge multi-quantum well structure[2]. It also makes it possible to increase the number of layers of MQW structure. In this work, we fabricate relatively thick strained SiGe/Ge MQW structures on Ge-on-Si by utilizing the patterning method, and evaluate their crystallinity and optical properties.

In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si was fabricated using the so-called two-step method. 40 and 500 nm thick Ge layers were subsequently grown on a Si(100) or Si(111) substrate at 350 and 600℃, respectively, followed by annealing at 800℃ for 10 min. Subsequently we carried out the patterning of the Ge-on-Si(111) substrate by photolithography process. 50nm thick Ge buffer layer ware grown on a Ge-on-Si(100) or Ge-on-Si(111), 6nm thick Si_0.1Ge_0.9 barrier layer and 10nm thick Ge well layer ware grown with 10-20 cycles at 350℃.

Laser microscopic measurements showed no clear roughness on the surface. X ray diffraction measurements show periodic peaks originated from the multi layers of SiGe/Ge, which indicates that the sample has high crystallinity and abrupt interface between the Ge and SiGe layers. TEM observation indicated that no defect was found inside the crystal in the MQW structure. Photoluminescence (PL) measurements were carried out at room temperature and a PL peak originated from the quantum-confinement in the SiGe/Ge MQW was obtained. This PL peak wavelength was also found to shift depending on the Ge concentration of the SiGe. PL intensity of the peak was increased compared with Ge-on-Si without the SiGe/Ge MQW overgrowth. From these results, it is demonstrated that high quality SiGe/Ge multiple quantum structures can be fabricated on Ge-on-Si by utilizing the patterning method and strong room temperature PL was obtained from the MQW, indicating that SiGe/Ge MQW is promising for the applications to light-emitting devices integrated on the Si platform.

[1] K.Yamada et al. Appl. Phys. Express 14 045504(2021) [2] Y.Wagatsuma et al. Appl. Phys. Express 14 025502(2021)