Growth and Characterization of GaN/AlN Resonant Tunneling Diodes for High‐Performance Nonvolatile Memory

Abstract

GaN/AlN resonant tunneling diodes (RTDs) are studied to realize a high‐speed nonvolatile memory based on the intersubband transactions and electron accumulation in the quantum well, which has the potential to operate at picosecond time scales. The crystal quality of GaN/AlN RTDs is improved by changing the growth conditions and structure of the buffer layer. The surface roughness and dislocation density of the GaN/AlN RTDs are successfully suppressed, and clear ON/OFF switching due to intersubband transitions is observed by inputting pulse voltage sequences. However, the voltages for write and erase operations are changed by improving the crystal quality of GaN/AlN RTDs. The theoretical analysis of resonant levels in the GaN/AlN RTDs indicates that the voltages for write and erase operations are very sensitive to the well and barrier widths and the density of electrons accumulating in the quantum well. Based on the results, the design of GaN/AlN RTDs for higher‐performance nonvolatile memory operations is investigated.

Enhancement of nonvolatile memory characteristics caused by GaN/AlN resonant tunneling diodes

“…The density of dislocations is also lowered through control of the growth conditions and structure of the buffer layer between a GaN/AlN RTD and a sapphire (0001) substrate. These effects lead to an extremely high ON/OFF ratio greater than 1300, which is 20 times higher than the values reported in previous studies [5,6]. The results are explained by the suppression of leakage current through the quantum well, by atomic force microscopy (AFM) and transmission electron microscopy (TEM) observations and by theoretical calculations based on Poisson's equation and the Tsu-Esaki tunneling current formula.…”

“…Therefore, high-speed nonvolatile memory operations can be realized using intersubband transitions and quantum-well electron accumulation in GaN/AlN RTDs. In addition, the voltages of the write and erase operations can be controlled by the structural parameters of the GaN/AlN RTDs, such as their well thickness, barrier thicknesses, spacer thicknesses and the Fermi levels in the emitter and collector (E f and E ′ f , respectively) [6]. Furthermore, this memory technique could be applied to other materials-based RTDs when the intersubband transition time is shorter than the resonant tunneling time.…”

“…These unique characteristics of nonvolatile memories are expected to lead to their use in various applications such as normally-off computing systems, neuromorphic computing systems and nonvolatile fieldprogrammable gate arrays (FPGAs) [1][2][3][4]. We are studying a new type of high-speed nonvolatile memory based on intersubband transitions and quantum-well electron accumulation in GaN/AlN resonant tunneling diodes (GaN/AlN RTDs) [5][6][7][8]. A high-speed nonvolatile memory operating at picosecond time scales is expected to be realized using resonant tunneling and intersubband transitions in GaN/AlN RTDs.…”

“…A high-speed nonvolatile memory operating at picosecond time scales is expected to be realized using resonant tunneling and intersubband transitions in GaN/AlN RTDs. Therefore, this nonvolatile memory can potentially be used for logic circuits and low-level caches in computing systems and programmable switches in FPGAs, which presently rely on volatile memories such as static and dynamic random-access memory [5,6]. In addition, advances in the techniques for heteroepitaxial growth of nitride materials on Si substrates [9][10][11] and in three-dimensional (3D) integration between semiconductor devices [12,13] might lead to the hybrid integration of GaN/AlN RTDs with Si devices and other nonvolatile memories and enable their use in computing systems and nonvolatile FPGAs.…”

“…The bistability was initially attributed to the trapping of electrons in the crystal defects of GaN/AlN RTDs [16][17][18]. However, the characteristics of ON/OFF switching using highspeed pulse voltages and resonant analysis based on Poisson's equation indicated that the bistability was caused by intersubband transitions and quantum-well electron accumulation in GaN/AlN RTDs [5,6]. Other nonvolatile memories based on Si/CaF 2 /CdF 2 and CaF 2 /Si/CaF 2 quantum wells support our argument concerning the origin of the bistability of GaN/AlN RTDs [19,20].…”