Silicon-based Quantum Computing:

       

Ever since the invention of transistor in 1947, impurities in semiconductors have played a crucial role in microelectronics industry. By the end of the twentieth century, as the transistor size shrunk from micrometer range to nanometer regime, scientists and engineers have been increasingly interested in the understanding of physics of devices with countable discrete impurity atoms. Going beyond classical regime of transistor operation, a radical new approach is quantum computing, where the fundamental building block of devices is a qubit stored in nuclear or electron spin of a single impurity atom. Such devices promise to harness the power of the quantum nature of materials for the development of superfast machines capable of solving currently intractable problems, which are inaccessible with conventional computers.  

 We study group V impurities (such as P, As, Bi) in silicon which are promising for spin qubit devices and quantum computing architectures due to the associated long coherence times.  



Selected Publications: 

M. Usman et al, Nanoscale 9, 17013, (2017) 

M. Usman et al, Nature Nanotechnology 11, 763, (2016) 

M. Usman et al, (Invited Article) J. Phys. Cond. Matt. 27, 154207, (2015)

Bismide Alloys and Heterostructures:










Designing new materials with engineered band structure properties is a topic of intense research interest in material science and condensed-matter physics communities. While traditionally Arsenides, Phosphides, and Nitrides have been the focus of research for photonic and optoelectronic devices, recently a new class of materials known as Bismides has emerged as promising medium for the design of devices. Bismides, which are typically formed by replacing a small fraction of As atoms in GaAs or InAs with Bi atoms, offer unique properties at the band-structure level which can be exploited to overcome a number of challenges present in today's devices. For example, Auger loss mechanism that severely degrades the efficiency of today's InP-based devices is expected to be suppressed in Bismide based devices due to crossover between band gap and spin split-off energies. A large tuning of the band gap energy as a function of Bi fraction of alloy offer opportunities for targeting wavelengths in telecommunication and infrared range. Other potential applications for Bismide alloys are in the field of photovoltaics and thermoelectric devices.


We have developed a comprehensive atomistic tight-binding framework to investigate the electronic and optical properties of Bismide alloys and quantum well. Our results have shown that by increasing Bi fraction above 10-11%, band gap energy reduces below spin split-off energy, a proof-of-concept for Auger-loss free photonic devices. Atomistic resolution studies have predicted a crucial role of alloy disorder related effects, with important implications towards understanding device characteristics and designing future devices with tailored functionalities.   



Selected Publications:

M. Usman et al, APL 104, 071103, (2014)

M. Usman et al, PRB 87, 115104, (2013)

M. Usman et al, PRB 84, 245202, (2011)

III-V Quantum Dots:

 

Self-assembled In(Ga)As/GaAs quantum dots are a promising solid-state system, and are widely employed for the design of a variety of optoelectronic devices and quantum information applications. Based on multi-million-atom simulations, we provide an in-depth understanding of their electronic and optical properties, and perform engineering of related geometry parameters for implementation of devices with tailored functionalities. We have investigated both single quantum dots, as well as large stacks of strongly-coupled quantum dots.     



Selected Publications:

M. Usman, Nanoscale 7, 16516, (2015)

M. Usman, (Rapid Comm) PRB 89, 081302, (2014)

M. Usman et al, Nanotechnology 23, 165202, (2012)

M. Usman et al, IEEE Trans. Nanotech., 8, 3, (2009)