Julie M. Rehm

How methanol affects the surface of blue and red emitting porous silicon

The effects of liquid methanol on the photoluminescence intensity and FTIR spectra of red and blue emitting porous silicon were investigated. Hydrogen passivated red emitting samples exhibit quenching and recovery of photoluminescence intensity and broadening of the Si‐Hx stretch bands upon exposure to liquid methanol. Oxygen passivated red emitting samples exhibit no photoluminescence quenching. The sensitivity of the red emitting sample is due to the microstructure of porous silicon at the surface and the ability of methanol to penetrate the pores. The blue photoluminescence of thermally oxidized samples is quenched upon exposure to methanol. This is attributed to the solvent’s ability to change the surface passivation which modifies existing traps and introduces competitive recombination channels for electrons.

Ion implantation of porous silicon

We have investigated the properties of light‐emitting porous silicon after ion implantation and successive annealing through continuous‐wave photoluminescence (CWPL) and time dependent photoluminescence (TDPL) spectroscopies. Implantation was performed with phosphorus, boron and silicon ions of different doses and energies. Low dose dopant implantation keeps or even increases the CWPL intensity and increases the TDPL decay time. High dose dopant implantation and silicon self‐implantation reduce the CWPL intensity and slightly decrease the TDPL decay time.

Prospects for light-emitting diodes made of porous silicon from the blue to beyond 1.5 μm

Since the 1990 discovery that porous silicon emits bright photoluminescence in the red part of the spectrum, light-emitting devices (LEDs) made of light-emitting porous silicon (LEPSi) have been demonstrated, which could be used for optical displays, sensors or optical interconnects. In this paper, we discuss our work on the optical properties of LEPSi and progress towards commercial devices. LEPSi photoluminesces not only in the red- orange, but also throughout the entire visible spectrum, from the blue to the deep red, and in the infrared, well past 1.5 micrometers . The intense blue and infrared emissions are possible only after treatments such as high temperature oxidation or low temperature vacuum annealing. These new bands have quite different properties form the usual red-orange band and their possible origins are discussed. Different LED structures are then presented and compared and the prospects for commercial devices are examined.

Luminescence Properties of Indirect Bandgap Semiconductors: Nanocrystals of Silver Bromide

Abstract Using a modified reverse micelle protocol with acetyl bromide as the bromide source, AgBr nanocrystals can be grown over a size range from 30 A radius to bulk. These clusters exhibit quantum confinement indicated by an increasing blue shift of the free exciton emission peak as a function of size. As the cluster size decreases, a concomitant decrease occurs in the apparent radiative lifetime. This is predicted as the k selection rules break down at small crystal diameters. However, the effects are quite modest. indicating that even at these smallest sizes, the indirect character of the transition is largely retained.

Luminescence Properties of Silver Bromide: From Nanocrystals to Microcrystals

Using a modified reverse micelle protocol with acetyl bromide as the bromide source, AgBr nanocrystals can be grown over a size range of 30 to 1000 A radius. These clusters display quantum confinement indicated by an increasing blue shift of the luminescence arising from the recombination of the free exciton as a function of size. As the cluster size decreases, the apparent radiative lifetime decreases as well. This is predicted by the expected breakdown of the k selection rules for small crystal sizes. However, the effects are quite modest, indicating that even for extremely small nanocrystals the indirect character of the transition is largely retained. In an attempt to overcome the problem of sample polydispersity and to resolve individual bound and phonon assisted excitonic transitions, a non-aqueous preparation of AgBr was developed, capable of providing more homogeneous size distributions of AgBr nanocrystals in the 100–400 A diameter range. Low temperature luminescence studies show size restriction effects associated with these excitonic transitions. An examination of the dynamics of trapping of excitons at iodide impurities is undertaken, and the radiative lifetimes of some of the excitonic processes are measured in order to obtain a more global perspective on the photophysics of AgBr.

Investigation of Chemical Adsorbate Effects on Blue and Red Emitting Porous Silicon Samples

We have investigated the sensitivity of blue (PLmax = 480 nm) and red (PLmax = 660 nm) emitting porous silicon samples to various chemical adsorbates. Steady-state and time-resolved photoluminescence measurements and FTIR spectroscopy were employed to characterize the photophysical and optical effects induced by chemical exposure. The red samples, which are hydrogen terminated, exhibit quenching and recovery of photoluminescence intensity and broadening of the Si-H x stretch bands upon exposure to liquid methanol. This behavior is attributed to the ability of the Si-H x specie on the surface of the PSI to interact with the solvent molecules which temporarily traps the electrons and causes PL loss and Si-H x broadening. The blue samples, which are oxygen terminated, display similar sensitivity to methanol. This sensitivity is attributed to the solvents ability to change the surface passivation and thereby introduce competitive radiative and nonradiative recombination channels. The origin of the blue PL is discussed.