Development of InSb dry etch for mid-IR applications
We present a new chlorine-free dry etching process which was used to successfully etch indium antimonide grown on gallium arsenide substrates while keeping the substrate temperature below 150°C. By use of a reflowed photoresist mask a sidewall with 60degree positive slope was achieved, whereas a nearly vertical one was obtained when hard masks were used. Long etch tests demonstrated the non-selectivity of the process by etching through the entire multi-layer epitaxial structure. Electrical and optical measurements on devices fabricated both by wet and dry etch techniques provided similar results, proving that the dry etch process does not cause damage to the material. This technique has a great potential to replace the standard wet etching techniques used for fabrication of indium antimonide devices with a non-damaging low temperature plasma process. Display Omitted Chlorine-free low-temperature dry etch of MBE grown InSb on GaAs60° positive sloped or nearly vertical etch depending on maskNon-selective process etching through Insb, GaSb and GaAsLow damage process with potential to replace standard InSb wet etch
A novel monolithic focal plane array for mid-IR imaging
The use of Mid-infrared (mid-IR) imagers has great potential for a number of applications in gas sensing and medical diagnostics, but so far for many of those non-defence fields it has been significantly limited by their high price tag. One of the reasons behind the great cost of mid-IR imagers is that most of them need to operate at cryogenic temperatures. Thanks to more than half a century of research, state-of-the-art mid-IR photodetectors have finally achieved premium detection performance without the need for cryogenic cooling. Some of them have even demonstrated very promising results, suggesting room temperature operation is on the horizon. As a result, the cost associated with cooling equipment has been significantly suppressed. However, most mid-IR imagers are still based on hybrid technologies needing a great number of die-level process steps and being prone to connection failure during thermal cycles. The high manufacturing cost this entails is also preventing a wider diffusion of mid-IR imagers. Currently, there is still a lack of an effective monolithic approach able to achieve low-cost mass production of mid-IR imagers in the same way as monolithic integration has been widely used for imagers working at visible wavelengths. 
This thesis presents a novel monolithic approach for making mid-IR imagers based on co-integration of mid-IR photodetectors with GaAs-based MESFETs on the same chip. The initial focus of the project was the development of the fabrication steps for delivery of prototype devices. In order to achieve monolithic fabrication of pixel devices made in either indium antimonide (InSb) or indium arsenide antimonide (InAsSb) on a gallium arsenide (GaAs) substrate, various highly controllable etch processes, both wet and dry etch based, were established for distinct material layers. Moreover, low temperature annealed Ohmic contacts to both antimonide-based materials and GaAs were used. The processing temperatures used never exceeded 180˚C, preventing degradation of photodetector performance after fabrication of transistors, thus avoiding well-known thermal issues of InSb fabrication. Furthermore, an intermediate step based on polyimide was developed to provide a smoothing section between the lower MESFET and upper photodetector regions of the pixel device. The polyimide planarisation enabled metal interconnects between the fabricated devices regardless of the considerable etch step (> 6 µm) created after multiple mesa etches. Detailed electrical and optical measurements demonstrated that the devices were sensitive to mid-IR radiation in the 3 to 5 µm range at room temperature, and that each pixel could be isolated from its contacts by switching off the co-integrated MESFET. 
Following the newly developed fabrication flow, InSb-based mid-IR imaging arrays (in two sizes, 4×4 and 8×8) are presented here for the first time, with pixel addressing achieved by monolithically integrated GaAs MESFETs. By demonstrating real-time imaging results obtained from these array devices at room temperature, implementation of a new type of monolithic focal plane array for mid-IR imaging has been confirmed. The device is suitable for further scaling (up to 64×64 pixel and beyond) and potential commercialisation. More importantly, the monolithic approach developed in this work is very flexible, as a number of III-V materials with mid-IR detecting capabilities can be grown on GaAs substrates, meaning alternative semiconductor layer structures could also be investigated in the near future.
Effects of Surface Treatments on Interfacial Self-Cleaning in Atomic Layer Deposition of Al2O3 on InSb
The atomic layer deposition (ALD) of Al 2 O 3 using trimethylaluminum (TMA) as a metal precursor on an InSb substrate with or without surface pretreatments was investigated. It was found that both in situ TMA/Ar purging (half-ALD cycle) and ex situ CP4A (HNO 3 HF:CH 3 COOH:H 2 O = 2:1:1:10) chemical etching can remove the native oxides on InSb before ALD of Al 2 O 3, and lead to a native-oxides-free Al 2 O 3 /InSb structure. Characteristics of current density-voltage and capacitance-voltage were also investigated to evaluate the insulative and interface quality in a Pt/Al 2 O 3 /Insb metal-oxide-semiconductor structure.