Unidirectional Oxide Hetero-Interface Thin-Film Diode with Improved Electrical Current
Authors: Nam Buil , Chung Sung Yun , You Duck-Jae , Kim Youn Sang 
Published: February 2020

Thin film technology is a basic technology that is used various electronic device applications. It is becoming increasingly important in commerce and research such as TFT LCD, Semiconductor, solid state circuitry and chemical experiments. Many company and research institute are increasingly interested in low-voltage device, particularly in the commercial areas of new memory and LEDs. This research has focused on making high performance and low power diodes with oxide hetero-interface structure. We introduce a new method that increases on-current of unidirectional oxide hetero-interface thin-film diode at insulator thickness control. The unidirectional oxide hetero-interface thin-film diode consists of the metal anode, electron-transporting oxide(ETO), electron-injecting oxide(EIO), metal cathode (MEEM) structure. Previously reported MEEM diodes have a vulnerability which is low on-current density. So we reduced electron-transporting oxide (Y2O3) thickness below 10nm, gained high current density. Electrical characteristics of improved MEEM diode are 2X10-2A/cm2 (previous is lower than 3X10-6A/Cm2) on-current density measured at 2V.

This MEEM diode was fabricated on 2.5cmX 2.5cm P++Si substrates. Next, a 3~10 nm thick Y2O3layer was deposited by atomic layer deposition (ALD). . Then, the deposition of the ZnO followed using radio frequency (RF) magnetron sputtering from ZnO target with an Ar working pressure of 10m torr and 25 sccm. The RF power was 91 W. We used Rapid Thermal Annealing (RTA) to anneal under atmospheric pressure at 350 ℃ for 1 minute. After RTA, a 100 nm thick aluminum layer was deposited as a top electrode thermal evaporator.

Principle of MEEM diodes is as follows. Applying positive voltage at metal anode, electrons were injected to EIO from metal cathode, because the junction of metal cathode and EIO is ohmic-like. Then electrons were injected to ETO from EIO, because bandgap of ETO and EIO deceases due to interface dipole. Finally, electrons were passed to metal anode from ETO as space charge limited current (SCLC) modelling. On the contrary to this, applying negative voltage at metal anode, electrons were couldn`t pass to ETO from metal anode due to large band gap between metal and insulator. Figure 1-(b) shows J-V characteristics of diodes by various insulator thicknesses. The more insulator thickness decreases, the more on-current density increase. An equation for the SCLC in an insulator can be developed as shown below, [Equation (1)] where εiis the permittivity of the oxide, μ is the mobility, θ is the ratio of free and shallow trapped charge, and d is thickness of the oxide. An equation for the DT Tunneling in an insulator can be developed as shown below, [Equation (2)] where m* is the electron effective mass in the oxide, h is Planck’s constant, and ΦB is the junction barrier height, κ is the relative dielectric constant of the oxide layer and tox,eqis the equivalent oxide thickness(EOT). The current density characteristics of thicker insulators over to 10 nm and the simulated SCLC current fitting curves are as shown in figure 1-(b)(Blue line), while the simulated DT(Direct Tunneling) current for thinner insulators down to 10 nm are shown in figure 1-(b)(Red line).

To use a MEEM diode in many applications, current density of the diode should be high and operating voltage should be low. However, the current of the existing MEEM diode was absurdly insufficient. In order to increase the current density, the electrical characterization was conducted on different insulator thickness. Especially when the thickness of the insulator is reduced to less than 10 nm, the current increases exponentially. Our fitting model of the results was changed DT model into SCLC model. As a result, current density is increase in a million times. Hence, improved MEEM diodes were confirmed to available as various applications such as crossbar array 1D1R RRAM device.

Figure 1