Semiconductors are commonly used in imaging sensors and solar cells, as they can directly convert light into an electrical current. The highest band of electron energies that are fully occupied is known as the valence band while the lowest unfilled energy band is the conduction band. The energy difference between the conduction and valence bands is known as the bandgap. When electrons from the valence band are excited into the conduction band by absorbing light with energy equals to or greater than the bandgap, the change of charges induces an electrical current. Consequently the bandgap is the most important parameter in the design of semiconductor photodetectors. While visible wavelength photodetectors are widely available, detectors for infrared wavelengths are significantly less mature and more costly. Progress in infrared detectors has been hindered by the limited choice of bandgaps currently available. In this work we will introduce a novel approach, by incorporating Bismuth (Bi) atoms into existing semiconductors such as InAs and InGaAs, to achieve a wide range of bandgap energies to detect infrared signals across a correspondingly wide wavelength range. Achieving this will lead to a new range of infrared detectors that can have transformative impact on applications including night vision imaging, medical diagnostic sensors, environmental monitors and for accurate temperature measurements in manufacturing processes.

We will also exploit Bi-alloys to engineer a noiseless charge amplification process in photodiodes known as avalanche photodiodes (APDs). When an electron leaves the valence band a vacant state (a hole) is created. Therefore an electron and a hole are created as a pair of charges in semiconductors. Properties of the conduction and valence bands will determine how electrons and holes gain energy from an applied electric field. In materials such as InAs, electrons gain energy at a much faster rate and travel at higher velocity too, when a voltage is applied. Therefore InAs is an excellent material for high speed electronic devices and also for providing internal signal amplification in APDs. When designed appropriately, the energetic electrons in InAs APD ensure that the amplification process, known as impact ionisation, is coherent so that negligible amplification noise is generated. In this work we will incorporate Bi into InAs to alter the valence band such that only electrons will gain significant energy from the electric field. This ability to suppress energetic holes will allow us to design very high gain APD across a wide range of electric field while concomitantly suppressing the noise associated with impact ionisation. By carefully controlling the fraction of Ga and Bi atoms, we will also develop a range of InGaAsBi APDs suitable for detecting a wide range of infrared wavelengths.

The proposed research to introduce a new class of Bi-containing infrared detectors and APDs, will be carried out by a carefully assembled team of world leading researchers from Universities of Sheffield and Surrey, in collaboration with the Tyndall National Institute, as well as partners from LAND Instruments, Laser Components and the UK Quantum Technology Hubs in Enhanced Quantum Imaging. Our work will start with a focus on formulating growth conditions (such as temperature and atomic fluxes) to obtain high quality InGaAsBi crystals. Following an intensive crystal growth programme, we will develop procedures to fabricate the grown InGaAsBi semiconductors into devices for a wide range of measurements to extract key material parameters. A model that accurately describes the bandstructure of InGaAsBi will be developed so that we can use them to design high performance infrared detectors and APDs. These newly engineered devices will be evaluated with our industrial partners for applications ranging from temperature measurements in manufacturing to novel imaging techniques using quantum properties of light.