Contents I. Introduction
FLASH memory with quantum-dot floating gate is getting more and more attention because it is advantageous in terms of high-speed write/erase, long retention time, and small device size [1], [2]. Achieving longer retention time requires thicker tunnel oxide; however, that causes slower program speed and requires high programming voltage. A lot of work has been done to improve the performance of quantum-dot flash memory, most of them focused on investigating quantum-dot floating gate with different materials, such as Si [1]–[4], Ge [5], [6], SiGe [7], [8], and metals such as Ni [9]–[11]. Double-stacked quantum-dot floating-gate structure was also reported [12]. But, the tunnel-oxide engineering has not received as much attention to improve the memory performance. Some high- materials, such as HfO2 or ZrO2 [7], [8] were used as tunnel oxide to decrease the programming voltage and improve the retention time because of the smaller conduction band offset between Si substrate and high- dielectric and the thicker physical thicknesses with the same equivalent-oxide-thickness (EOT). Other pioneering work from Baik et al. [13] discussed nitride-oxide-nitride (NON) sandwiched structure tunnel oxide to improve the program speed. Recently, a new variable oxide thickness (VARIOT) structured tunnel oxide was reported by Govoreanu et al. [14]. Simulations showed that larger injected gate current density is possible for the memory devices with VARIOT structure tunnel barrier compare to the memories with only single-layered tunnel oxide [14], [15]. In this paper, we demonstrate improved memory performance with VARIOT structured tunnel barrier with quantum-dot floating gates.
Fabricated quantum-dot memory structure.
