Continuous scaling of Si transistors has enabled us to sustain Moore’s law for over 30 years. However, the physical gate length has already reached 30nm in the current 65nm technology node and is expected to reach 10nm within the next few years. This is widely believed to be the physical limit, beyond which we have to look at alternatives to scaling in order to continue enjoying the benefits of Moore’s law. III-nitrides offer unique advantages over other materials for high frequency and high power applications. Although, in terms of mean electron velocity, these predictions are much less than those possible with GaAs, GaN has a larger peak electron velocity, larger saturation velocity, higher breakdown voltage and thermal stability, making it ideal for use as a channel material in microwave and high frequency integrated circuits. Successful realisation of these devices requires Schottky contacts with large Schottky barriers and good thermal stability. The wide and direct bandgap (3.4eV) of GaN results in a lower leakage current and consequently, an ability to operate at higher temperatures. These properties make GaN an important material for high frequency and high power applications. Schottky diodes have advantages like high operating frequency, fast switching speed and low forward voltage drop. As a result, Schottky diodes are widely used in a variety of RF and microwave applications like varactors, detectors, mixers, multipliers and low-voltage reference circuits. In the study of semiconductor surfaces, the metal-insulator-semiconductor Schottky diode is an important device. The performance of semiconductor devices is closely related to their surface conditions and an understanding of surface physics with the help of MIS diodes is of great importance to device operation.
GaN (n-type) films were grown using plasma assisted molecular beam epitaxy (PA-MBE). Pt/HfO2/n-GaN metal-insulator-semiconductor Schottky diodes were fabricated using standard lithography techniques. 10nm thick HfO2 was deposited using RF sputtering as the insulator layer. Conventional Pt/n-GaN metal-semiconductor Schottky diodes were also fabricated. The performance of the Pt/n-GaN (MS) structure was compared with the Pt/HfO2/n-GaN (MIS) structure . IV and CV measurements were carried out to confirm the Schottky nature of contacts.
It was found that introduction of the HfO2 layer resulted in a decrease in leakage current, slight increase of barrier height and a reduction in the ideality factor. Mechanism of current transport in MS and MIS structures were studied. Diode parameters like barrier height and ideality factor were extracted and were found to be temperature dependant. This indicates that there are inhomogenities in the barrier height at the interface. The experimental observations were modelled by assuming a Gaussian distribution of barrier heights at the interface. Results were validated using industry standard device simulator Silvaco Atlas.