Carsten Funck

The III-Nitride Double Heterostructure Revisited: Benefits for Threshold Voltage Engineering of MIS Devices

GaN-based devices are seen as ideal candidates for power-switching applications. For the acceptance of GaN-based devices by module designers, obtaining enhancement-mode (e-mode) behavior in GaN-based heterostructure field-effect transistors (HFETs) has long been in the focus. Although the gate-injection approach appears to be the most promising one to achieve e-mode devices, using a double heterostructure in conjunction with a gate insulator has still its advantages, such as steeper turn-ON characteristics and lower leakage currents. An analytical expression to predict the threshold voltage Vth for a given double heterostructure device has not yet been derived. Moreover, neither an evaluation of the tradeoff between Vth and ON-state resistance Rds,ON has been performed to date. This paper addresses these two aspects. We will show an analytical expression for a metal-insulator-semiconductor double HFET (MIS-DHFET), which in certain cases is also valid for the gate-injection transistor. On the basis of this, we will discuss the actual influence of the Al concentration in the backbarrier on Vth. We will further employ technology computer aided design (TCAD) device simulations to evaluate the impact on Rds,ON when using MIS-DHFETs. It will be shown that by implementing a double heterostructure in MIS devices, it is possible to suppress the typically observed negative Vth-oxide thickness relationship while maintaining a constant Rds,ON.

Internal Cell Resistance as the Origin of Abrupt Reset Behavior in HfO2-Based Devices Determined from Current Compliance Series

The resistive switching behavior in different HfO2/TiO2 nano crossbar structures of 100 x 100 nm2 size is analyzed by means of DC voltage sweeps. The devices fabricated from 3 nm thin ALD layers of HfO2 and TiO2 sandwiched between Pt and Hf or Ti electrodes show VCM-type bipolar resistive switching after electroforming. For increased compliance current (cc) during set from 50 μA to 800 μA, the set current runs into self- limitation while the reset behavior changes from gradual to abrupt. A model is defined with an internal resistance being in series with the local resistive switch. A recursive algorithm is applied to the cc series for calculation of the series resistor and evaluation of the intrinsic switching characteristic of HfO2-based cells. The intrinsic LRS turns out to be current compliance controlled and to follow the universal switching rule. Supported by compact modelling, we show that an abrupt reset behavior might arise even for materials with a gradual intrinsic reset characteristic in consequence of an internal series resistor.

Simulation of threshold switching based on an electric field induced thermal runaway

We introduce a new explanation for abrupt threshold switching (TS) based on a Poole-Frenkel like field dependent barrier lowering conduction mechanism. As the barrier lowering induces a thermal runaway the mechanism is named field-triggered thermal runaway (FTTR). The FTTR-type threshold switching has the major characteristics of a hysteresis determined by different switching voltages for the turn-on and the turn-off. In this study, we investigate the influence of a series resistance on the FTTR-type threshold switching. We show that the variation of the memristive resistance is able to lead to a transition of the abrupt TS with a hysteresis into a continuous non-linear TS without a hysteresis effect. The transition between the abrupt and continuous TS occurs suddenly with an increase of the serial resistance, which consequently defines a minimum resistance change for the abrupt TS.

In-Gap States and Band-Like Transport in Memristive Devices

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

Understanding the Coexistence of Two Bipolar Resistive Switching Modes with Opposite Polarity in Pt / TiO2 / Ti / Pt Nano-sized ReRAM Devices

Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO2 cells, is still not completely understood. Here, we report on nanosized (<0.01 μm2) asymmetric memristive cells from 3 to 6 nm thick TiO2 films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO2/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO2 cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO2 Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO2/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO2 interface.

Thermal effects on the I-V characteristics of filamentary VCM based ReRAM-cells using a nanometer-sized heater

The physical principle of redox based resistive switching memory cells is based on ionic and electronic transport propertips. Both normally reveal a strong temperature dependence. A novel heating setup enables to study the influence of high temperatures on the switching mechanism of these memories. It consists of a 100 nm-wide Pt heating line and includes a Pt/Ta2O5/Ta-based ReRAM cell. It is heated up by inducing a current through the Pt line and employing the Joule heating effect. From this setup unique features are obtained by I-V measurements of the ReRAM cell. For instance, a negative differential resistance during the SET process is observed, which results from the circuitry of the heating structure. In case of the high resistive state, an increase in the current is observed at higher temperatures. In contrast, the current in the low resistive state is temperature independent. The increasing temperatures also lead to a reduction of the switching voltages of the SET and RESET events.