Electronic Transport and Low Frequency Noise Characterization in Si Nanowire and 2-dimensional MoS2 based Field-effect Transistors




Sharma, Deepak K.

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The dimensional scaling of Complementary Metal-Oxide-Semiconductor devices for better performance cannot continue indefinitely. One of the predominant factors is the increasing statistical fluctuation with the down-scaling dimensions. As the dimension shrinks, number of free charge carriers within the device decreases. Consequently, statistical fluctuation in the number becomes an increasing fraction of the total free charge carriers, limiting device performance by adversely affecting its electrical transport properties and eventually makes circuit design more challenging. Low frequency noise (LFN) is one of the major sources of statistical fluctuation in materials. The goal of my work is to study the nature of electrical transport, and the cause/effect of low frequency noise in these reduced-dimension devices. Following is the arrangement of my research project to reach the goal. First, I designed and installed an efficient and precise noise measurement system based on cross correlation technique. This is crucial because device noise, especially when the current signal is low (which is the case with small dimension devices) can be very difficult to accurately measure since it is affected by many other sources, such as amplifiers, cables, connectors, AC power supplies, mechanical vibrations, etc. Second, I fabricated and measured Silicon Nanowire field-effect transistors (SiNW FETs). Temperature-dependent (77 K – 300 K) LFN measurement revealed the presence of generation-recombination (G-R) related Lorentzian-type peaks along with 1/f -type noise in these SiNW FETs. I have successfully detected the presence of electrically active deep-levels associated with Gold and Nickel metals in the SiNWs by using LFN measurements. Third, I performed temperature dependent (77 K – 300 K) electrical transport and LFN measurement on monolayer layer Molybdenum disulfide (MoS2) FETs grown by chemical vapor deposition method. The effect of high-κ dielectric passivation on the electrical transport properties revealed key aspects related to activation energy. The observed channel current noise revealed different trapping states in passivated devices when compared to the devices without high-κ dielectric passivation. For both passivated and etched devices, the bias-dependent LFN at 300 K was explained by carrier number fluctuation and correlated mobility fluctuation; both related to surface effects. Fourth, I studied electrical transport LFN in FETs consisting of different number of MoS2 layers. Free carrier transport in the channel was explained using a model incorporating Thomas-Fermi charge screening and inter-layer coupling. Based on my comparative analysis of both electrical transport and LFN on the MoS2 devices with different number of layers, I found that devices containing 4 to 7 layers may provide the optimum FET performance. Devices with thick MoS2 suffers from low mobility values, weak dependence of channel current on gate voltage and increase in normalized LFN value, while conduction in mono- and few-layer devices is affected by surface states at oxide-semiconductor interface and therefore, exhibiting much higher LFN and lower mobility values. In summary, my thesis has presented a study on the electrical transport and low frequency noise of FETs based on Si NW and two-dimensional MoS2 with focus on understanding the role of defects and surface states on the overall carrier transport.



Statistical fluctuation