Low Thermal Budget PMOS for Qualifying the Semiconductor Environment of STM-placed Donor Qubits*

Understanding and control of the solid-state environment of semiconductor spin qubits is critical for their performance. Donor-based qubits are appealing components of a semiconductor spin-based quantum computer because of their long coherence times. Moreover, donor-donor exchange coupling can be controlled precisely with atomically precise placement of donors using scanning tunneling microscopy (STM). However, unlike gate-defined quantum dots, where capacitor and Hall bar measurements have provided a useful proxy for the qubit environment, it has been difficult to evaluate the environment of STM-placed donor qubits using anything other than time-consuming qubit measurements. Here, we describe the fabrication and measurement of low thermal budget p-type metal oxide semiconductor (PMOS) transistors for quick evaluation of the STM-placed donor qubit environment. 

It is well understood that the growth temperature used during encapsulation of STM-placed donors must be kept low to limit the diffusion of the donors, but low temperatures results in an increased density of point defects. Previously, weak localization has been used to quantify the diffusion of the donor layer. By analyzing the current-voltage characteristics of the PMOS made with capping layers grown with different conditions, we can begin to qualify how those growth conditions impact the electrical quality of the capping material and the qubit environment.