Oral Exam Announcement: Waqas Mustafeez

Waqas Mustafeez mustafeez27 at stanford.edu
Tue Jul 17 11:05:14 PDT 2012

From: Student Services 
Sent: Monday, July 16, 2012 10:17 AM
To: ee-students at lists.stanford.edu 
Subject: [ee-doctorate] Oral Exam Announcement: Waqas Mustafeez



Waqas Mustafeez

Date and time: 2PM (1:45PM Refreshments) Friday, 20th July 2012

Location: CISX Auditorum 

Advisor: Alberto Salleo



Silicon Nanocrystals, Dust to Gold: Material, Devices and Synthesis


Over the past half century silicon has become a dominant material for electronics but it has remained a very poor optical material. Bandwidth limitations in electronics are currently being seen as a problem. A similar situation to what happened on the long haul transport where we switched from the copper pair to optical fiber communication, may occur at the chip scale through integration of cheap on-chip optical devices. We consider two ways forward for silicon to remain a relevant material for lightwave communication in this era despite challenges posed by the indirect gap and free carrier absorption. In specific, we look at silicon nanocrystals (Si-NC) as a light emitting material to show progress in turning this indirect gap material into an efficient emitter. First we show through a highly sensitive optical absorption spectroscopy technique (photothermal deflection spectroscopy) that the negative effective mass electron states in Si-NC shift down in energy thereby reducing the energy gap at the direct transition. This effect is responsible for low amounts of visible range emission reported in small crystals; as opposed to the emission from band gap minima at the X valley attributed with majority of the total emission. Thus we confirm through a direct measurement the minima in density of states at the Γ valley from splitting of the negative and positive effective mass states. We compare the shift in a few of these states with a model for quantum confinement in the nanocrystals surrounded by oxide using a simple parabolic potential well model.


We then show how resonant nanocavities should be optimized for low gain broad emitters like Si-NC. These cavities offer reduced mode volumes at the cost of quality factors such that "bad emitters" like Si-NC may couple with the cavity and ultimately offer a higher likelihood of observing Purcell enhancement. FDTD simulations show Q factors above 10^3 and mode volumes below 1(l/n)^3 in silicon rich oxide, which has a refractive index of only 1.7.


Finally we show a novel technique to make Si-NCs only on selected areas of the wafer: we direct-write tracks of silicon nanocrystals using a ps UV laser. It is possible to obtain 4 orders of magnitude throughput increase using this process as an alternative to oven annealing while also making localized nanocrystal precipitation a possibility. The tracks show emission enhancement over bare silicon rich oxide films and emission intensity comparable to that observed in conventionally-processed Si-NCs. We study variations in emission spectra, nanocluster phase and film stress across these tracks showing that ultimate limitation in throughput is film densification leading to compressive stress on the clusters that increases interfacial defects leading to emission quenching.


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