“We decided to study a new system in our lab: InAs QDs buried within AlAsSb barrier layers,” said team member Meng Sun. “This semiconductor nanostructure could be ideal for making intermediate-band solar cells (IBSCs) because it is highly efficient at converting solar energy into electricity, according to theoretical calculations. The fact that we are able to fine-tune the optical properties of the QDs within these solar cells by simply adding GaAs(Sb) cladding layers means that we now have a whole new level of control over these materials.”

These QDs should also have a type-II band structure (where only one of the electronic charge carriers is actually confined within the QD) – a prediction that the UCLA has now confirmed using power-dependent and time-resolved photoluminescence measurements. Such a band structure is especially exciting for IBSCs, says Sun, because the longer carrier lifetimes therein can further help increase solar efficiency.

Best cladding layers

The researchers obtained their results using atomic force and tunnelling electron microscopy to study the structural properties of the QDs with differing configurations of cladding layers. They then characterized the optical properties of the dots using spectroscopic techniques. The measurements revealed that the ground state transition energy of the QDs increases linearly with the cube root of the light excitation intensity – behaviour that is typical of a type-II band structure. The experiments allowed the UCLA team to identify what types of GaAs(Sb) cladding layers are best for optimizing the InAs quantum dots’ light-emitting properties.

The semiconductor nanostructures the team created were all grown using molecular beam epitaxy, a technique that allows for incredible control over the thickness and composition of the growing layers. “We are able to interrupt the usual growth of our InAs/AlAsSb structures just before and after we grow the InAs QDs and insert the GaAs(Sb) cladding layers,” explained Sun. “The GaAs(Sb) layers are just 1.4 nm thick, but we found that this is big enough to induce large changes in the behaviour of the QDs.

“The original motivation behind this study was our desire to create next-generation solar cells in the form of IBSCs,” he told nanotechweb.org. We have now not only confirmed that it is possible to actually create InAs/AlAsSb QDs, but that we can enhance their optical performance using GaAs(Sb) cladding layers.”

Sun says that he and his colleagues will now start building IBSC devices using their new material system and look at how these perform.

The current work is detailed in Applied Physics Letters.