Semiconductor photonic materials and devices

We have a long standing reputation for research on the physics of semiconductor materials for application in optoelectronic devices.

Focus areas

The current research areas are shown as follows:

Research interest and activity in silicon-photonics over the past twenty years has been driven by the increasing demand from applications including telecommunications, data centres and computing combined with the opportunities for low cost, high density, large scale commercial manufacture by the existing complementary metal oxide semiconductor (CMOS) industry. Compared to conventional electronics, silicon-photonics provides a range of benefits for these applications including electronic integration, higher speed, wider bandwidth, lower power consumption and smaller physical scale.

While key components for photonic integration are now available, there is an outstanding requirement for electrically pumped, low threshold current, temperature stable on-chip silicon-based laser devices operating at communications wavelengths. To make the transition to commercial production these laser devices must also be compatible with existing CMOS manufacturing processes. The indirect nature of silicon makes it unsuitable as an active region and the long-term goal is the direct epitaxial growth of III-V lasers on silicon. However, lattice constant, thermal expansion mismatch and the polar/non-polar interface between silicon and traditional III-V laser materials causes large defect densities, leading to inefficient and unreliable lasers.

The aim of this research is to understand more about the key challenges associated with the monolithic integration of III-V lasers on silicon in order to enable improvements in device design and fabrication. This is investigated through experimental characterisation and modelling of novel devices to identify and discriminate between non-radiative recombination and loss mechanisms and their sources in quantum well and quantum dot lasers [1]. Research also includes the application of spectroscopic ellipsometry methods to determine key optical and electronic properties of innovative alloys for waveguide and barrier design [2].

References:

[1] G.W. Read, I. P. Marko, N. Hossain, and S. J. Sweeney, “Physical properties and characteristics of III-V lasers on silicon,” IEEE J. Sel. Top. Quantum Electron., vol.21, no.6, pp. 377-384, 2015, doi: 10.1109/JSTQE.2015.2424923.

[2] C.R. Fitch, P.Ludewig, W.Stolz, and S.J.Sweeney, “BGa(As)P alloys for III-V integration on silicon,” in Photonics West Proceedings Volume 10923, Silicon Photonics XIV, 2019.

If you would like to learn more about this research, please contact Chris Fitch.

Photonic devices play a key role in a wide range of modern technologies, such as solar cells, gas sensors, and the telecommunications lasers that make up the backbone of the internet. These devices often make use of semiconductor quantum well heterostructures and exploit a variety of quantum effects to allow for efficient light emission. However, in the near-infrared, current telecommunications lasers using conventional GaAs- and InP-based alloys suffer from significant inefficiencies due to non-radiative loss processes such as Auger recombination and carrier leakage. These loss processes both limit device performance and result in poor thermal stability, requiring the use of energy-intensive thermoelectric coolers for stable operation.

In order to overcome these issues and improve the efficiency of next-generation near-infrared devices, it is essential to understand these loss processes and how we can mitigate their effects.

The aim of this research is to investigate the potential of tuning material and active region properties for improving device performance, with a focus on two main areas of technical study:

Investigating the optical and electronic properties of dilute bismide-nitride alloys, which offer the potential to intrinsically suppress Auger-related processes in near-infrared lasers [1].
Exploring the use of type-II “W”-active regions, where electrons and holes are spatially confined to different materials – these systems are predicted to show a reduced rate of Auger recombination [2] and exhibit unique emission properties that show promise for developing lasers with improved thermal characteristics.

This project involves a collaborative approach between computational modelling, epitaxial growth, and experimental material and device characterisation using state of the art facilities both at the University of Surrey and our collaborators.

References:

[1] S. J. Sweeney and S. R. Jin. “Bismide-nitride alloys: Promising for efficient light emitting devices in the near- and mid-infrared”. Journal of Applied Physics 113, 043110 (2013).

[2] G. G. Zegrya and A. D. Andreev. “Mechanism of suppression of Auger recombination processes in type-II heterostructures”. Applied Physics Letters 67, 2681 (1995).

If you would like to learn more about this research, please contact Dominic Duffy.

The search to find sustainable and clean energy sources and more efficient lighting has never been more important. A continuous uptrend in greenhouse gas emissions can be correlated to a global climate crisis and an influx of unprecedented weather events. Solar cells have emerged as a key contender in solving this challenge, harnessing arguably the most abundant clean source of energy on the planet surface. Since first being introduced to the commercial market in the 1970’s, photovoltaic devices have undergone a revolution increasing in efficiency and reducing in assembly costs. Key to developing new photovoltaic technologies is finding highly efficient semiconductors that can absorb a high fraction of the suns energy while remaining low cost and practical for a variety of applications. The same semiconductors may also emit light when supplied with energy leading to further interest in low energy consumption LEDs for efficient consumer electronics.

The ideal photovoltaic cell requires a suitable band gap for optimal overlap with the sun’s emission spectra combined with a high absorption coefficient to maximise the fraction of incident light captured. Coupled with this the required semiconductors require good charge transport properties to reduce losses and ensure a high-power output to the external circuit. Recently a new class of materials known as perovskites have emerged showing high promise as a high-quality optoelectronic material for light capturing and light emitting applications. Several key challenges remain however, including poor environmental stability, defect mediated voltage losses and the problem of scale up.

The aim of this research is to produce high quality perovskite opto-electronic devices with low losses while also considering the practical issues such as scaling, and the environmental consequences associated with manufacture. In our previous works we have shown how Cs+ cations can be introduced into a double cation perovskite enabling a pathway to producing high efficiency triple cation perovskites using a scalable two-step method.[1] The focus of the group is to study the emission properties both under a laser excitation (Photoluminescence) and via an electrical excitation (Electroluminescence) to better understand the underlying semiconductor physics.[2]

References:

[1] M. Yavari, X. Liu, T. Webb, K. D. G. I. Jayawardena, Y. Xiang, S. Kern, S. Hinder, T. J. Macdonald, S. R. P. Silva, S. J. Sweeney, W. Zhang, J. Mater. Chem. C 2021.

[2] B. Li, Y. Xiang, K. D. G. Imalka Jayawardena, D. Luo, Z. Wang, X. Yang, J. F. Watts, S. Hinder, M. T. Sajjad, T. Webb, H. Luo, I. Marko, H. Li, S. A. J. Thomson, R. Zhu, G. Shao, S. J. Sweeney, S. R. P. Silva, W. Zhang, Nano Energy 2020, 78, 105249.

If you would like to learn more about this research, please contact Thomas Webb.

Retinal dystrophies affect roughly 1 in 3,500 people, and currently there is no cure. The partial or complete loss of vision from these diseases is largely due to the degradation of retinal photoreceptors whether from a genetic disorder or by a physical condition which causes systematic deterioration. The goal of this project is to design and build a prototype artificial retinal implant to provide an alternative therapy to those suffering from loss of vision.

The active materials used in the device absorb light and generate enough electrical potential to stimulate neurons. These novel organic semiconductors are specifically configured to operate in narrow wavebands in the visible part of the electromagnetic spectrum. Organics rather than silicon devices are used for a few important reasons: their chemical structure make them inherently stable, and because they are fabricated from polymers with a conjugated carbon backbone, they are biocompatible in vivo. [1, 2]These materials in the form of electroactive inks are intended to be mass produced by printing in micro-electrode arrays onto flexible substrates for implantation, but they are implemented as thin films for the purposes of this study. Different bandgaps of each organic semiconductor may offer signal acquisition in different parts of the electromagnetic spectrum, and ideally, three colours, this being an improvement on current work in the field.[3, 4]

The project falls naturally into three stages: the first stage is to optically and electronically characterise a range of organic semiconductors. Once a group of materials is identified, the second stage will be concerned with the construction of the prototype. Deposition of the materials will be carried out through ink jet printing or a similar process to create a mosaic of individual pixels of organic-semiconductor molecular inks onto substrates built from carefully chosen interlayers. The third stage will be to test the prototype in vitro to assess the biocompatibility of the device. We are confident that the conversion of light to electrical signal will be of a sufficient amplitude to activate damaged photoreceptors.

1. Martino, N., et al., Photothermal cellular stimulation in functional bio-polymer interfaces. Sci Rep, 2015. 5: p. 8911.

2. Shaposhnik, P.A., et al., Modern bio and chemical sensors and neuromorphic devices based on organic semiconductors. Russian Chemical Reviews, 2020. 89(12): p. 1483-1506.

3. Ciocca, M., et al., Colour-sensitive conjugated polymer inkjet-printed pixelated artificial retina model studied via a bio-hybrid photovoltaic device. Scientific Reports, 2020. 10(1).

4. Shkunov, M., et al., Pixelated full-colour small molecule semiconductor devices towards artificial retinas. The Journal of Materials Chemistry C, 2021.

If you would like to learn more about this research, please contact Leslie Askew

The recently discovered ion beam implantation induced stress of suspended nanomembranes has shown to generate >3% biaxial and >8% uniaxial strain. Such high built-in strain levels have never been achieved in a silicon (or silicon compatible) structure before, and previous records (4.5% uniaxial strain in Si <110>, [1]) have employed challenging multi-step lithography procedures. The proposed method has the potential to transform the silicon electronics/ photonics industry by:

allowing far higher electron mobility in silicon transistors for much better speed and heat dissipation, and by
producing silicon compatible direct gap materials for lasers and other photonic devices.

Higher mobility for silicon is extremely attractive in its own right, but probably the most important, disruptive implications of this work are in photonics. Moving photonics onto silicon has been a long-held goal, and while there have been many successes in developing passive silicon photonic devices, an active silicon-based laser that is truly CMOS-industry compatible remains elusive. The fundamental problem with silicon (and Germanium) is its indirect band gap. Germanium can be converted to direct band gap more easily through strain engineering, but although this has led to some success, the high strain levels required have only been achieved with a very complex multi-step lithography process [2].

A process that is not only simpler but also offers even higher strain is very desirable. Here, the proposed method allows design flexibility, and points the way to a versatile, fast, generally applicable, and widely available technique for strain control in host of different materials.

This project is funded by EPSRC grant number EP/V048732/1, with further details in this link.

References:

[1] R. A. Minamisawa, M. J. Süess, R. Spolenak, J. Faist, C. David, J. Gobrecht, K. K. Bourdelle, and H. Sigg, Top-down Fabricated Silicon Nanowires under Tensile Elastic Strain up to 4.5%, Nat. Commun. 3, 1 (2012)

[2] F. T. A. Pilon, A. Lyasota, Y.-M. Niquet, V. Reboud, V. Calvo, N. Pauc, J. Widiez, C. Bonzon, J.-M. Hartmann, and A. Chelnokov, Lasing in Strained Germanium Microbridges, Nat. Commun. 10, 1 (2019)

If you would like to learn more about this research, please contact Mateus Masteghin.