Silicon-based Quantum Computing:

 

Target Applications: Quantum Computing, Quantum Sensing, Quantum Information, Nanoscopy  

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. Understanding quantum logic operations to be performed on donor based silicon devices has required the construction of an effective spin dynamics model coupled to, and informed by, empirical-based atomistic models.  We have established a comprehensive atomistic tight-binding model to investigate the donor physics in silicon. This theoretical framework has been successful in the determination of exact positions of phosphorous dopants in silicon after fabrication and over-growth processes based on STM imaging data obtained by UNSW researchers. This result is important in the design and optimisation of highly precise quantum logic gates for scalable quantum computer architecture. Our team applied the atomically precise spatial metrology technique to the STM images of correlated wave functions measured for phosphorous dopant pairs. Based on a systematic analysis of the symmetries of images, supported by a quantitative comparison between experiment and multi-million-atom theory, we identified the exact depths and in-plane locations of the two phosphorous atoms in the measured pairs. This high-precision spatial metrology of dopant pairs allowed a quantitative understanding of the valley interference processes and their relation with the exchange interaction energies as a function of phosphorous pair separations, confirmed through a direct comparison with the STM measurements.



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) JPCM 27, 154207, (2015)

Bismide Alloys and Heterostructures:



Targets Applications: Photonic Devices, Photovoltaics, Spintronics
 
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., Phys. Rev. Applied 10, 044024, (2018)

M. Usman, Phy. Rev. Materials 2, 044602, (2018)

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:



Target Applications: Telecomm-wavelength optoelectronics, photovoltaics, spintronics

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, 081302R, (2014)

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

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