Koichiro Okamoto

Epitaxial growth of GaN on copper substrates

We have grown GaN films on Cu(111) substrates using pulsed laser deposition. We have found that GaN(0001) grows epitaxially on Cu(111) when employing low temperature grown AlN buffer layers with an in-plane epitaxial relationship of AlN[112¯0]‖Cu[11¯0] Reflection high-energy electron diffraction images have exhibited sharp streaky patterns, indicating that GaN grows with a flat surface. Electron backscattering diffraction observations have revealed that neither 30° rotational domains nor cubic phase domains exist in the GaN films. Spectroscopic ellipsometry measurements have shown that the heterointerfaces in the GaN∕AlN∕Cu structure are abrupt. The epitaxial growth of GaN on Cu substrates is likely to raise the power limit for future light emitting and electron devices due to the high thermal conductivity of Cu.

Programmable cell array using rewritable solid-electrolyte switch integrated in 90nm CMOS

Programmable devices such as SRAM-based FPGAs have the major challenges of power consumption and circuit area due to the excessive standby leakage current and the threshold voltage variation in highly scaled SRAM. Back-end-of-line (BEOL) device, which is integrated in the interconnect layers, is attractive for reducing the performance gap between FPGA and cell-based ASIC [1–4]. In this paper, we demonstrate the fundamental operations of a programmable cell array and a 32×32 crossbar switch using a nonvolatile and rewritable solid-electrolyte switch (nanobridge or NB). A 72% reduction in chip-area compared with that of a standard-cell-based design is achieved on a 90nm CMOS platform.

Epitaxial growth of single-crystalline AlN films on tungsten substrates

The authors have grown single-crystalline AlN(0001) films on W(110) substrates with an in-plane alignment AlN[11–20]‖W[001] at temperatures ranging from 450to600°C by pulsed laser deposition. These AlN films have a clear sixfold symmetry without 30° rotational domains. When AlN films are grown at 450°C, the interfacial reaction between AlN and W(110) is fully suppressed and a flat surface with a root-mean-square value as low as 0.20nm for AlN films is obtained. These single-crystalline AlN films grown on W(110) open a good opportunity for the high performance film bulk acoustic wave resonators of the next generation.

Growth of single crystalline GaN on silver mirrors

GaN films have been grown on highly thermally conducting Ag(111) substrates by using pulsed laser deposition. GaN(0001) grows epitaxially on Ag(111) with an in-plane epitaxial relationship of GaN[112¯0]‖Ag[11¯0] when low temperature AlN buffer layers are used. The heterointerfaces in the AlN∕Ag structure are atomically abrupt, and that the abruptness remains unchanged during annealing up to 700°C for the GaN growth. Neither 30° rotational domains nor cubic phase domains exist in the GaN films. It has been also confirmed that the GaN films grown on the Ag substrate exhibit strong near band edge photoluminescence emission at 3.51eV. These results indicate that the epitaxial growth of GaN on mirror-polished single crystalline Ag substrates possibly improve the power limits and the light extraction efficiency of future light-emitting devices.

Improved Off-State Reliability of Nonvolatile Resistive Switch With Low Programming Voltage

A complementary atom switch (CAS) is proposed to realize low programming voltage and high off-state reliability for crossbar switch application. Two atom switches with bipolar operation are connected in series with opposite direction, in which the two atom switches work as a single element. The two off-state atom switches in the CAS complementarily divide voltage stress, greatly enlarging the off-state lifetime. The CAS is embedded in Cu BEOL on a 65-nm-node CMOS platform without degrading the CMOS and interconnect performances. The CAS using two atom switches is one of the candidates for realizing energy-efficient nonvolatile programmable switches.

Epitaxial growth of GaN films grown on single crystal Fe substrates

GaN films have been grown on single crystalline Fe(110), Fe(100), and Fe(111) substrates with low-temperature AlN buffer layers by the use of pulsed laser deposition. AlN films with 30° rotated domains and polycrystalline AlN films grow on Fe(100) and Fe(111), respectively. On the other hand, AlN(0001) films grow epitaxially on Fe(110) substrates. X-ray reflectivity measurements have revealed that the heterointerface of AlN∕Fe(110) is quite abrupt and that its abruptness remains unchanged, even after annealing at 700°C. GaN epitaxial films have been grown on AlN∕Fe(110) structures with an in-plane relationship of GaN[11−20]∥AlN[11−20]∥Fe[001]. The first-principles calculations have revealed that this in-plane epitaxial relationship is determined by the adsorption process of nitrogen atoms on the Fe surfaces at the initial stage of the growth.

Improved ON-State Reliability of Atom Switch Using Alloy Electrodes

Cu-alloy active-electrode coupled with Ru-alloy inert electrode is proposed to improve the ON-state reliability of nonvolatile Cu atom switch. The high rupture temperature (Tr > 400 °C) of the nanometer-scale Cu bridge with high thermal resistance (Rth) is realized by the Cu(AlTi)-alloy without increasing programming current. The anti-Cu diffusive Ru(Ta) alloy contributes to improve the retention of the ON-state at 150 °C and endurance to > 104 cycles. The metallurgical prescription on the electrodes is a key to improve the stability of the nanometer-scale conducting bridge while keeping the switching power low.

Electronic Conduction Mechanism in Atom Switch Using Polymer Solid Electrolyte

Low-temperature characterization of electrical transport on atom switches has been performed to clarify operation and conducting mechanisms. The low resistive (ON) state (<; 400 Ω) shows metallic conduction accompanied with a high residual resistance. In the high resistive (OFF) state(>;108), the resistance exponentially increases with decreasing temperature due to Poole-Frenkel conduction. In the intermediate range (105 - 107 Ω), the resistance has small temperature dependence since the electron tunneling via Cu residues in a solid electrolyte is dominant. The polymer solid electrolyte enables the complete collection of the Cu residues without degrading the electrolyte, resulting in forming-free operation and a high on/off conductance ratio.

Room-Temperature Epitaxial Growth of GaN on Atomically Flat MgAl2O4 Substrates by Pulsed-Laser Deposition

We have grown GaN films on MgAl2O4(111) substrates at room temperature (RT) and investigated the structural properties of the films. The atomically flat surfaces of MgAl2O4 with a step and terrace structure are obtained by annealing at 1000 °C in a box made of ceramic MgAl2O4. It is found that GaN(0001) grows epitaxially on atomically flat MgAl2O4(111) surfaces even at RT. The epitaxial relationship between the GaN epilayer and the MgAl2O4 substrate is GaN[1120]∥MgAl2O4[011]. GaN films have a clear six fold symmetry without 30° rotational domains. The interfacial reaction between GaN and MgAl2O4 is completely suppressed in the case of RT growth.

Low-power programmable-logic cell arrays using nonvolatile complementary atom switch

Programmable-logic cell that utilizes complementary atom switch (CAS) is fabricated using 65-nm node CMOS process. A 16-bit ALU is implemented and demonstrated on a 24×24 programmable-logic cell array including 645kbit CAS for both routing switches and configuration memories. Comparing the conventional cell design using CMOS routing multiplexer (MUX), the proposed programmable-logic cell array performs 60% active power saving and 3 times faster operation.