porous silicon

Optically pumped porous silicon implanted with trivalent erbium ions (Er3+) generates visible output from the silicon and luminescence at 1540 nm from the Er3+ ions. The bandga¥properties of the porous-silicon host reduce thermal quenching of the Er3+ luminescence to a factor of two for a temperature range from 15 K to room temperature. In contrast, thermal quenching of Er3+ emission from other doped-silicon-based hosts such as polycrystalline and amorphous--or crystalline--silicon re duces

porous silicon

Erbium-doped porous silicon emits at 1.54 µm

Kristin Lewotsky

Optically pumped porous silicon implanted with trivalent erbium ions (Er3+) generates visible output from the silicon and luminescence at 1540 nm from the Er3+ ions. The bandga¥properties of the porous-silicon host reduce thermal quenching of the Er3+ luminescence to a factor of two for a temperature range from 15 K to room temperature. In contrast, thermal quenching of Er3+ emission from other doped-silicon-based hosts such as polycrystalline and amorphous--or crystalline--silicon re duces output by factors of 3.3 and 20, respectively.

With good performance over a broad temperature range and a long upper-level lifetime, the doped porous-silicon material has considerable potential for optoelectronic components such as light-emitting diodes, lasers, and optical amplifiers, but researchers have not clearly understood the excitation mechanism. Now photoluminescence excitation (PLE) studies by Uwe Hömmerich and his grou¥at the Research Center for Optical Physics at Hampton University (Hampton, VA) and collaborators at Spire Corp. (Bedford, MA) have shown that the Er3+ excitation is driven by excitation of the silicon host, which offers a method for controlling Er3+ emission efficiency.1

Photoluminescence excitation studies

Led by F. Namavar, scientists at Spire prepared porous-silicon hosts by etching p-type silicon wafers in a solution containing hydrofluoric acid and ethanol under constant-current conditions, using two different current densities to create one high-porosity sample and one low-porosity sample. Both hosts were implanted with Er3+ at a concentration of 1 ¥ 1015 cm-2, then annealed in a nitrogen atmosphere at 850°C; by annealing above 600°C, the researchers ensured epitaxial regrowth of the implanted layer, making the existence of an amorphous phase in the samples unlikely.

A Nd:YAG-pumped optical parametric oscillator (Continuum, Santa Clara, CA) excited the samples with pulsed output at wavelengths from 400 to 600 nm. The cold finger of a two-stage, closed-cycle refrigerator cooled the samples to 15 K.

Spectra from the two samples showed a broad visible luminescence from the silicon and a shar¥infrared wavelength luminescence centered at 1540 nm from the Er3+ ions. The peak of the visible luminescence shifts from 640 nm in the first sample to 750 nm for the second sample, caused by bandgap-widening in the more porous material. Excitation performed at 400, 450, 500, and 550 nm successively showed that the silicon-based photoluminescence spectra are essentially independent of pum¥wavelength.

To draw a correlation between porous silicon carrier activity and Er3+ output, the researchers examined excitation spectra of the visible luminescence amplitude at 750 nm and Er3+ luminescence at 1540 nm. For excitation wavelengths varying continuously from 400 to 650 nm, the plots of the Er3+ luminescence are essentially the same as those for the silicon luminescence (see figure on p. 42). The excitation of the implanted Er3+ ions follows the generation of photocarriers in the porous silicon.

This experimental evidence led the researchers to conclude that the IR luminescence arises from Er3+ ions located in porous-silicon nanograins and that photogenerated electrical carriers confined in the nanograins excite the ions into luminescence. The bandga¥of porous silicon can be thus controlled by the porosity, an effect that can be used to control the efficiency of the Er3+ emission.

REFERENCE

1. X. Wu, U. Hömmerich, F. Namavar, and A. M. Cremins-Costa, Appl. Phys. Lett., Sept. 23, 1996; in press.

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