Ukjin Jung

Influence of extrinsic factors on accuracy of mobility extraction in graphene metal-oxide-semiconductor field effect transistors

Graphene has attracted attention because of its extraordinarily high mobility. However, procedures to extract mobility from graphene metal-oxide semiconductor transistors have not been systematically established because the accuracy of mobility value is affected by many extrinsic parameters. In this work, the influence of extrinsic parameters, such as contact resistance, transient charging effect, measurement temperature, and ambient on mobility are examined in order to provide a protocol capable of accurately assessing the mobility of graphene metal-oxide-semiconductor field effect transistors. Using a well controlled test protocol, the mobility of graphene is found to be temperature independent up to 450 K.

Highly sensitive wide bandwidth photodetectors using chemical vapor deposited graphene

A photodetector generating a nearly constant photocurrent in a very wide spectral range from ultraviolet (UV) to infrared has been demonstrated using chemical vapor deposited (CVD) graphene. Instability due to a photochemical reaction in the UV region has been minimized using an Al2O3 passivation layer, and a responsivity comparable to that of Highly Ordered Pyrolytic Graphite graphene photodetectors of ∼8 mA/W has been achieved at a 0.1 V bias, despite high defect density in the CVD graphene. A highly sensitive multi-band photodetector using graphene has many potential applications including optical interconnects, multi-band imaging sensors, highly sensitive motion detectors, etc.

Process-Dependent N/PBTI Characteristics of TiN Gate FinFETs

A study of the negative and positive bias temperature instability (N/PBTI) reliability of FinFETs with different TiN metal gates deposited by either atomic layer deposition (ALD) or physical vapor deposition (PVD) on HfO2 dielectrics found that the nonuniformity of the interfacial oxide layer is closely related to reliability characteristics. FinFETs with an ALD TiN gate exhibit better NBTI and PBTI lifetimes than those with a PVD TiN gate. In addition, the dependence of fin width on NBTI reliability appeared to be worse with narrower fins, whereas PBTI reliability improves.

Quantitatively estimating defects in graphene devices using discharge current analysis method.

Defects of graphene are the most important concern for the successful applications of graphene since they affect device performance significantly. However, once the graphene is integrated in the device structures, the quality of graphene and surrounding environment could only be assessed using indirect information such as hysteresis, mobility and drive current. Here we develop a discharge current analysis method to measure the quality of graphene integrated in a field effect transistor structure by analyzing the discharge current and examine its validity using various device structures. The density of charging sites affecting the performance of graphene field effect transistor obtained using the discharge current analysis method was on the order of 1014/cm2, which closely correlates with the intensity ratio of the D to G bands in Raman spectroscopy. The graphene FETs fabricated on poly(ethylene naphthalate) (PEN) are found to have a lower density of charging sites than those on SiO2/Si substrate, mainly due to reduced interfacial interaction between the graphene and the PEN. This method can be an indispensable means to improve the stability of devices using a graphene as it provides an accurate and quantitative way to define the quality of graphene after the device fabrication.

Correlation between the hysteresis and the initial defect density of graphene

The role of the initial defects of graphene characterized by Raman spectroscopy is correlated with the physical mechanisms causing the hysteretic device characteristics of graphene field effect transistors (FETs). Fast charging related to the tunneling-induced charge exchange is found to be closely correlated with the initial defect density, while slow charging related to environmental influences such as the water redox reaction showed a weak correlation. It can be concluded that the intrinsic quality of graphene should be improved to minimize the hysteresis of graphene FETs even in an air-tight environment.

Contact resistance reduction using Fermi level de-pinning layer for MoS 2 FETs

Achieving a low contact resistance for 2D materials is a critical challenge for device applications. In this work, the contact resistance of MoS2 FETs has been drastically reduced by five times from the reference data using an optimized TiO2 Fermi level de-pinning layer which reduced the effective Schottky barrier height to 0.1 eV. As a result, a very low contact resistance ~5.4 kΩ·μm was achieved without any doping technique.

Quantitative analysis of interfacial reactions at a graphene/SiO2 interface using the discharge current analysis method

Using the discharge current analysis method, the contribution of charge generation through an interfacial reaction at a graphene /substrate interface is assessed to be on the order of 1014/cm2, which is ∼20% of the total charging sites. The validity of this method, which separately extracts the density of the charging sites related to the initial defect density of the graphene from the contribution of interfacial reactions is examined by measuring the discharge current of graphene field-effect-transistors at different ambient and temperatures. This method will be crucially instrumental in finding an optimal substrate material for graphene devices.

Intrinsic Time Zero Dielectric Breakdown Characteristics of HfAlO Alloys

A thermochemical model describing the relationship between the dielectric breakdown field (EBD) and dielectric constant (k) of high- k dielectric has been calibrated for Hf_xAl_1-xO_y alloys with k values from 7 to 24. Metal-insulator-metal (MIM) capacitors with Hf_xAl_1-xO_y high- k dielectric films were used to extract the intrinsic time zero dielectric breakdown characteristics. Breakdown field values of these Hf_xAl_1-xO_y alloys were found to decrease as a function of k^-0.77 while the electric field acceleration parameter, γ, increases as a function of k^1.37. Using the thermochemical model calibrated with the experimental data, a Hf_xAl_1-xO_y 10-year lifetime was extrapolated as a function of the dielectric constant to provide insight for future dielectric development.

Demonstration of Complementary Ternary Graphene Field-Effect Transistors

Strong demand for power reduction in state-of-the-art semiconductor devices calls for novel devices and architectures. Since ternary logic architecture can perform the same function as binary logic architecture with a much lower device density and higher information density, a switch device suitable for the ternary logic has been pursued for several decades. However, a single device that satisfies all the requirements for ternary logic architecture has not been demonstrated. We demonstrated a ternary graphene field-effect transistor (TGFET), showing three discrete current states in one device. The ternary function was achieved by introducing a metal strip to the middle of graphene channel, which created an N-P-N or P-N-P doping pattern depending on the work function of the metal. In addition, a standard ternary inverter working at room temperature has been achieved by modulating the work function of the metal in a graphene channel. The feasibility of a ternary inverter indicates that a general ternary logic architecture can be realized using complementary TGFETs. This breakthrough will provide a key stepping-stone for an extreme-low-power computing technology.

Capacitance Analysis of Highly Leaky

Characterization of metal-insulator-metal (MIM) capacitors with a scaled dielectric is a challenge using conventional capacitance-voltage (C-V) measurements due to a high leakage current. In this letter, a method to analyze MIM capacitance that is more immune to the leakage current problem has been successfully demonstrated using time domain reflectometry (TDR). The TDR method can be applied to Al2O3 MIM capacitors with a capacitance density up to ~ 11.1 fF/μm2, for which an impedance analyzer has failed to measure capacitance at 1 MHz. Differences in the voltage coefficient of capacitance and dielectric constant (k) were also investigated.