The understanding, control, and application of polycrystalline compound semiconductor thin films
University College Cork
In this thesis, low-temperature grown polycrystalline III-V materials are investigated for their potential as channel materials for devices located in the back-end-of-line (BEOL) of integrated circuits. The key aims of this thesis are to demonstrate high carrier mobility III-V materials grown at low temperatures on silicon/amorphous substrates, and to fabricate functional transistor devices using these polycrystalline/amorphous III-V films as channel materials. There are very few reports of low temperature grown polycrystalline III-V materials available in the literature and so this thesis is well placed to add substantial knowledge to the field. Both an n-type (InAs) and a p-type (GaSb) material are investigated. The poly III-V films are grown on Si/SiO2, glass, or Semi-Insulating (SI) GaAs substrates. For the intended implementation in the BEOL, it is necessary to stick to a restrained thermal budget, with maximum temperatures generally being <500◦C. Growth of III-V materials on silicon substrates is an important step towards their integration with silicon-based electronics. Growth at these reduced temperatures and on amorphous substrates such as SiO2 are large challenges to overcome while maintaining a reasonable carrier mobility. It is also important to develop n- and p-type options so that these materials can be implemented in low-power Complementary Metal Oxide Semiconductor (CMOS) type architectures. Material properties of the films as grown are explored through Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and Cross-sectional Transmission Electron Microscopy (XTEM), and Hall Effect measurements. The Hall Effect measurements indicate that both of these materials have high carrier concentrations independent of intentional doping or a high-temperature dopant activation step and display high carrier mobilities despite their polycrystalline or amorphous nature. Electron mobility values in polycrystalline InAs are found to be 100 cm2/Vs at room temperature for InAs grown directly on a glass substrate, and values reaching 155 cm2/Vs for a heterostructure including the polycrystalline InAs film. Hole mobilities in the polycrystalline GaSb films are found to be up to 66 cm2/Vs for a film grown directly on SiO2 and 299 cm2/Vs for a film grown directly on SI GaAs. An amorphous GaSb layer which is part of a heterostructure on SiO2 is found to have a hole mobility of 9.1 cm2/Vs. These results are compared against those found in the literature and found to exceed many, while being competitive with those that are grown at higher temperatures, on crystalline substrates, or to greater thicknesses. All of these are self-imposed limits to keep the material growth suitable for devices in the BEOL, so to be competitive with mobilities obtained without these limitations is a very important and positive result. Development of both n- and p-type semiconductor options is a vital component of this field of research and the p-type mobilities reported in this thesis compare particularly well with those reported in the literature. Junctionless transistor devices are fabricated, with full processing steps described. Both n- and p-type functional devices are demonstrated, and analysis of the device characteristics is performed to determine limiting factors and mechanisms at play. Analysis is performed on the devices at different stages of the device processing to understand the effect on the semiconducting material. The highest ION/IOFF values are achieved at -50◦C for both types of devices, with 104 achieved for n-type devices, and 550 achieved for p-type devices. These results are compared against those found in the literature. Recommendations for future implementation of these materials into high-performance devices are made based on the results generated in this thesis. This includes the recommendation to grow thicker films and develop a Chemical Mechanical Polishing (CMP) process to thin them back down, ideally creating a thinner yet more continuous film, and to conduct some systematic studies of the gate stack process for the transistor devices. Low-temperature grown polycrystalline and amorphous materials show great potential as high carrier mobility materials which can be implemented into silicon-based electronics in the BEOL where a low thermal budget is required for device processing.
Semiconductor , III-V , Polycrystalline , Amorphous , 3D integration , Low-temperature
Curran, A. 2023. The understanding, control, and application of polycrystalline compound semiconductor thin films. PhD Thesis, University College Cork.