Apr. 2022 - LED Photoshoot

At the recent SIOE conference, some fantastic images of GaN based LEDs were showcased by Guillem Martinez de Ariba and Jack Haggar, both of Professor Tao Wang's research group at the University of Sheffield. See link for the pictures. The diffusive effect of the translucent sapphire substrates on the emitted light made for some great images.

We wanted to see if we could capture some similar shots of our GaAsBi devices in LED operation for our own conference talks. This is complicated and difficult for a number of reasons:

1. Our devices are not optimised LEDs, having a ~300nm window layer that is much thinner than optimal.

2. Our devices emit at wavelengths (~870-1200nm) that are difficult to see using silicon cameras, which normally have an infrared blocking filter integrated into the lens or sensor.

3. Even using a silicon camera that does not have an infrared blocking filter, the photoresponse of the camera sensor is poor above ~850nm presumably due to absorption in the Bayer filter, and abysmal above 1000nm due to this being close to the band edge of silicon.

Despite these challenges, we managed to take some photos of our device STG38 (90nm bulk GaAsBi, with 1.3% Bi). We used a ZWO ASI 224MC camera as this does not have an infrared blocking filter, can take colour pictures, and has an OK photoresponse at the emission wavelength of the devices (930nm)

Our GaAs substrates do not diffuse the emitted light in the same way as the sapphire substrates used by Professor Tao Wang's group, but the images are OK nonetheless! However, we are seeing some edge emission from the wafer, presumably from light that is emitted downwards from the LED mesa and scattered off the relatively rough gold coated back surface of the LED. The small size of the edge emission spot is likely due to total internal reflection of other light. Using this low cost, uncooled silicon camera we were able to see LED emission with current densities as low as 0.5 A/cm^2, which is on par with the best reported values from the literature!

Apr. 2022 - SIOE

Thomas Rockett and Matthew Carr attended the Semiconductors and Integrated Optoelectronics conference hosted in Cardiff by Peter Smowton. Our group gave two talks:

Thomas Rockett: Carrier Collection Efficiency in GaAsBi Photovoltaics

Matthew Carr: Bi Flux Modification of Self Assembled InAs Quantum Dots Growth on <001> GaAs by MBE

Apr. 2022 - SPIE Photonics Europe

Robert Richards attended SPIE Photonics Europe in Strasbourg, France in early April. He gave a talk titled "GaAsBi light emitting diodes for 1050nm broadband light sources"!

Mar. 2022 - UK MBE Working Party Meeting

Robert Richards, Thomas Rockett, and Matthew Carr recently attended the UK MBE Working Party Meeting hosted by Huiyun Liu and Mingchu Tang at University College London on the 30th of March. Robert gave a talk entitled "Back to bismuth as usual" detailing our MBE woes of the past few years and focusing on our recent Nature Communications paper about GaAsBi avalanche photodiodes!

Oct. 2021 - Thermal imaging camera for MBE

Wafer surface temperature measurement is critical for repeatable and high quality MBE growth. Thermocouples suffer from large errors since they cannot be physically located at the wafer surface, while non-contact temperature measurement techniques such as thermal imaging and band-edge thermometry are extremely expensive when using commercial solutions. However, the components for a thermal imaging system (camera, lens, filter) can be purchased for less than £250 ($340), while the knowledge required to calibrate the camera into a thermal imager is freely available online (see for example paper 1 or paper 2). For calibration, many university departments have a blackbody furnace which could be loaned. Alternatively, the thermal imager could be calibrated using known RHEED transitions e.g. c(4x4) to 2x4 at ~400 oC, As cap desorption at ~300 oC, and oxide removal at ~600 oC.

We have implemented a low cost thermal imaging system on our MBE-STM reactor, based on a monochrome astronomy camera (ZWO ASI 120 MM mini) with a vintage Cosmicar 25mm f/1.4 lens and a Thorlabs 940nm 10nm FWHM bandpass filter.

The thermal imaging system shows a high resolution temperature map of the substrate in our MBE machine. Relative to the set temperature (as calculated using RHEED transitions) the thermal imager seems to underestimate the temperature by an average of 13 oC (in the range 350-620 oC) which could be caused by an overestimate of the glass viewport transmission at 940nm or an underestimate of the wafer emissivity (0.6 was used). See image carousel below for test images.

Oct. 2021 - Lab Events: Water Cooling Servicing

We recently had a slight issue with the water cooling on our arsenic cell which was exhibiting reduced flow rate and increased temperature. Not something to be ignored due to the risk of cell damage and water cooling failure!

After cooling and disconnecting the cell we stripped the pipe network down and found the source of the blockage, a large deposit of black sediment resembling a charcoal filter at the arsenic bulk outlet connector. After a few minutes provocation with an acute apparatus (poking with a sharp tool) the fitting was clear and the arsenic cell back in service. We also now have a substantial amount of sample to perform energy dispersive x-ray spectroscopy (EDX) on to figure out where the material came from.

Sep. 2021 - Conference: 21st International Conference on MBE

Three of our group members have oral/poster presentations at this years international MBE conference. The presentations will be available virtually for several weeks after the conference, using the Whova online platform

Robert Richards: Growth of Thick GaAsBi Diodes for Detectors

Thomas Rockett: GaAsBi Multiple Quantum Well Photovoltaics: Trade-off Between Carrier Collection and Light Absorption

Nicholas Bailey: Control of InAs/GaAs QD properties through the use of a Bi surfactant

Aug. 2021 - Royal Academy of Engineering Research Fellowships anniversary

With this year's announcement of the Royal Academy of Engineering Research Fellowship awardees, the scheme has reached 20 years old. See the Royal Academy's press release here.

Congratulations to Dr Lewis Owen, who is one of this year's awardees, along with Dr Amy Gandy, who won a Leverhulme Trust Research Fellowship from the Royal Academy, both of whom are also at the University of Sheffield.

Aug. 2021 - Sheffield III-V-Bi group in Nature Communications!

Congratulations to Dr Yuchen Liu, who has had his recent work on the impact ionisation properties of GaAsBi published in Nature Communications.

Impact ionisation is a process in which a carrier (electron or hole) absorbed by a device is accellerated in a very large electric field, to the point that it has sufficient energy to excite another electron/hole pair. This process repeats itself over and over again, to produce a very large current from a single absorbed carrier. Of course, the holes that are created can also impact ionise, which creates more electrons and so on...

Dr Liu showed that the incorporation of Bi into GaAs drastically reduces the ability of holes to initiate impact ionisation. In an ideal case, only one carrier type (electrons or holes) would impact ionise, making the final signal from a single photon much more predictable and reducing the "excess noise" of a device.

Dr Liu's work shows that further development could produce a new family of low-noise, dilute bismide based, infrared photodetectors.

Mar. 2021 - Conference: SIOE

Dr Richards and Nick Bailey presented work at this year's Semiconductor and Integrated OptoElectronics conference. Of course the conference was online and it was a shame not to be able to visit Cardiff, but the conference was very interesting in any case.

Dr Richards presented work relating to his recent J Phys D publication and Nick presented preliminary results from a series of quantum dot samples that he grew recently. Dr Richards won one of the conferences SIOE Merit Awards for his presentation.

Feb. 2021 - Publication: diode characteristics

Dr Richards published a report in Journal of Physics D:Applied Physics this month. Dr Richards carefully characterised the bias, temperature and band gap dependent dark currents of a series of diodes grown by Dr Rockett. He was able to show that the bias, temperature and band gap dependent dark currents could be accurately described by a single formulation of the Shockley diode equation, provided that the growth temperature of the devices was not changed. This suggests that Bi itself does not negatively impact on GaAsBi device performance. When the growth temperature was changed, however, the device properties changed significantly, with lower growth temperatures producing devices of poorer quality. This work highlights that the potential of GaAsBi based devices is not limited by absolute Bi contents, but relies heavily on the expertise of the grower, to produce well optimised material.

Jul. 2019 - 10th International Conference on Bismuth-containing Semiconductors

Dr Richards, Nada Adham and Nick Bailey presented their recent work at international bismides conference in Toulouse, France. The weather was excellent, if a little hot!

Oct. 2018 - Lab Events: Turbo Pump Implosion

After much honourable service the Pfeiffer turbomolecular pump fitted to the V80 FEL suddenly imploded during operation, causing a postgraduate student two feet away to nearly jump out of his skin!

After inspecting the interior of the pump, we found the metal salad you see below.

Nov. 2016 - Custom RHEED Software

Since our old RHEED software was missing some features and ran on an old and unreliable windows XP computer, we wrote our own new software in Matlab! The software uses a commercially available webcam to view the RHEED screen.

If any MBE users are interested in this software, contact Thomas Rockett.

Requirements:

• Windows 7 or 10

• Intel i5 4xxx CPU

• Microsoft LifeCam Cinema USB Webcam (1280x720 resolution)

Left image: Growth Rate Oscillations for GaAs (001) growth, with no post processing

Right image: RHEED pattern for GaAs (2 x 4) reconstruction, as viewed with the webcam

The success of this system shows that a cheap mass produced commercial camera is sensitive enough to measure the growth rate via RHEED oscillations in an MBE system. More generally, as camera technology progresses further, it is likely that many scientific imaging applications will become possible using cheap cameras

Aug. 2016 - Delivery of VG V80 MBE System

Gallery showing how our new V80 MBE machine was transported from London to Sheffield. The journey up the M1 motorway shook a lot of arsenic free from the growth chamber walls, an effective method for cleaning MBE machines!