Room Temperature Emission from Erbium Nanoparticles Embedded in a Silicon Matrix
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excitation wavelength, annealing temperature, sample temperature, and power, make this material a starting point for producing room temperature light emitting Si:Er thin films. EXPERIMENTAL DETAILS Er nanoparticles are produced using a pulsed laser ablation supersonic expansion source, as shown in Fig. 1. A frequency-doubled Nd:YAG laser (%= 532 nm, 7 mJ/pulse, 7 nsec pulse width, 20 Hz repetition rate) is focused to approximately 1 mm diameter at the surface of an Er rod of 6.3 mm diameter placed in a vacuum chamber with a base pressure of 10-8 Torr. The rod is attached to a rotating screw and is mounted in a stainless steel block adjacent to a pulsed valve. The valve gates the flow of high pressure helium (550 psi) which passes over the rod, through a 1 mm diameter channel, and is then expanded into the evacuated chamber. The expanding helium carrier gas entrains the plasma which is generated by the laser ablation pulse and carries it into the vacuum system. The helium experiences an adiabatic expansion, and rapidly becomes cold. The plasma therefore PULSED LASER cools down and condenses to form 532 nm Nd:YAG ABLATION small particles. The size of the ablation laser• clusters depend mainly upon two parameters, the length of the 1 mm diameter channel separating the . surface of the source rod from the He valve •expansion into vacuum, and the * :timing of the ablation laser pulse Rotating relative to the helium valve opening Rotating Er-rod in Si-rod Substrate pulse. In this setup, the channel length stainless-steel block is 3 mm. A substrate is placed in the beam path, 15 cm from the Er-rod. The Fig. 1. Pulsedlaserablationwith asupersonicexpansion source.See Er-nanoparticles are incorporated in a text for details. Si-matrix by simultaneous laser deposition of Si onto the substrate. This is accomplished by placing a second rotating rod made of 99.9999% pure polycrystalline Si 10 cm from the substrate, as shown in Fig. 1. The ablation laser beam is split, and 75% of the laser light (21 mJ/pulse) is directed towards the Si-rod. We have deposited films for time periods of I to 16 hours on p-type (P) Cz-Si(001) wafers and microscope cover-slips. Photoluminescence (PL) experiments were carried out using an argon-ion laser tunable to various wavelengths between 455 and 520 nm, a variable temperature cryosystem, a 0.75 Spex monochromator, a liquid nitrogen cooled Ge-detector, a mechanical chopper, and a lockin amplifier for signal detection. Lifetime measurements were carried out using a InGaP detector and a digitizing oscilloscope. The system response was 10 jis. A laser power of 1 W was used in most PL-experiments, corresponding to a power density of 15 W/cm 2 at the sample. All anneals were carried out in a tube furnace for 30 minutes in an Ar atmosphere. To prevent contamination, the samples were placed between pieces of polished Si during anneal. RESULTS AND DISCUSSION Scanning electron microscope (SEM) images of a Si:Er film deposited for 13 hours show a smooth film, approximately 200 nm thick, with larger particles (d
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