POROUS SILICON: Erbium-doped porous silicon emits at 1.54 µm

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 bandgap 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 reduces 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 group 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 sharp 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 pump 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. 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 bandgap 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.