Funnels are efficient tools for channelling liquids into containers with narrow openings. Now, researchers in Exeter have demonstrated the first funnel for electrical charges on a chip. The discovery builds on the ability to oxidize the atomically thin semiconductor, hafnium disulphide (HfS2), with a high-intensity UV laser. The non-uniform strain between oxidized and non-oxidized regions, and the subsequent band-gap modulation, generates electric fields, which effectively funnel the charges in the semiconductor flakes to areas where they can be more easily collected. This concept could enable a new generation of solar cells with 60% efficiency (currently around 21%), thanks to the increased efficiency in collecting photo-excited charges and the potential for hot-carrier extraction.
Intense laser light means oxidation, oxidation means strain
In general, bulk semiconductors can only sustain strains up to 0.4% before breaking. However, a layer of semiconductor that is only a few atoms thick can support strains of up to 25%. This amount of strain changes the band gap in the energy dispersion by up to 1 eV.
In this work, Saverio Russo and his group at the University of Exeter, induce the strain in the HfS2 using a 375 nm laser to remove sulphur atoms, which are then replaced by oxygen atoms. According to calculations performed using density functional theory, the hafnium atoms have different separations in HfS2 and HfO2. This produces a 2.7% strain at the boundary between the oxidized and non-oxidized regions. Electrical contacts anchor the material to a substrate, so a strain gradient is present across the whole flake, shifting asymmetrically the conduction and valence bands to higher energies, and opening the band gap by 30 meV.
Collecting the funnelled charges
Another key to making a useful charge funnel is ensuring that the charges will move towards an area where they are more easily extracted. In most direct semiconductors (such as GaAs), the funnelling effect pushes charges towards the strained region. By using HfS2, which is an indirect semiconductor, the charges were pushed away from the strained region, towards the electrodes at the edge of the flake, as observed in inverse charge funnelling.
To test the inverse charge funnelling, a field effect transistor was made using a flake of patterned HfS2/HfO2. The researchers used a laser to excite charges in the flake and scanning photocurrent microscopy to measure the current from these charges. Comparing the same device before and after a small area was oxidized, they measured a 350% increase in response in a region near the interface between HfS2 and HfO2. Their theoretical calculations of the system supported the results, and also revealed that the charge carriers had an increased lifetime of around four orders of magnitude – another indicator of the charge funnelling effect.
Looking to the future
So how can this new phenomenon be implemented in future technologies, and what benefits will this lead to? Lead author, Adolfo De Sanctis, had this to say: “The electrical measurement of the charge funnelling effect could be revolutionary for photovoltaic devices. Two-dimensional semiconductors are truly unique when it comes to strain, and they offer the best platform for future ‘straintronic’ devices. The next steps will be to implement this technique on a larger scale, optimize the strain gradients, and study other materials in order to demonstrate the feasibility for photovoltaic solar energy conversion.”
Read the full article in Nature Communications.