2005 Conference on Lasers and Eiectro-Optics Europe

Multilayers ofPbTe Quantum Dots embedded in SiO2E. Rodriguez, E Jimenez, L. A. Padilha, W.L. Moreira, A. A. R. Neves, EF. Chillcce, C L. Cesar, and L. C. BarbosaUNICAM/IFGW/DEQ,, CP 6165, CEP 13084-971,Bardo Geraldo, Campinas, SP, Brail.C. B. de ArauijoUFPEICCEN/Depto. Fisica, Professor Luiz Freire, s/n, CEP 50670-901, Recife, PE, Brazil.

Semiconductor quantum dots (QD) have attracted attention because they exhibit large optical nonlinearities aswell as fast response times (less than I ps) potentially useful for optical devices [1]. It is well known that semiconductorQD materials consisting of clusters or crystal of nanometric dimensions embedded in dielectric host exhibit propertiesthat differ from those of the corresponding bulk material. These materials have opened many possibilities for their usein various technological applications because of their optical, electrical and magnetic properties.

In the present work, PbTe quantum dots embedded in a dielectric host (SiO2) were fabricated. The structuraland optical properties of the multilayers were studied.

The semiconducting quantum dots were grown by laser ablation of a PbTe target using the second harmonicof a Q-Switched Quantel Nd:YAG laser under high purity argon atmosphere. The glass matrix was fabricated by aplasma chemical vapor deposition method using vapor of tetramethoxysilane (TMOS) as precursor. The samples werefabricated by alternating between PLD of the PbTe target during a time tLASER and PECVD ofTMOS during a time tRF.This alternating growth was achieved with a computer controlled interface using a LabView code. The reason forchoosing PbTe was the band absorptions this material exhibits in the region of interest for optical communications 1.3-1.5,.tm making this material an excellent candidate for development of optical devices[2].

For the TEM measurements on the multilayer, a Si (100) wafer was used as the substrate. Figure l(a) shows alow magnification TEM image of a multilayer. This micrograph shows approximately 9 nm thick QD layers separatedby 20 nm thick SiO2 layers. The bright 5 nm amorphous layer just over the substrate is composed of native oxide ofsilicon as shown by an Energy Dispersive Spectroscopy (EDS) measurement. The equidistance between the 11 layersshows the good repetitiveness ofthe whole process.

For optical characterization a BK7 Corning glass was used as the substrate. Figure l(b) shows the absorptionspectra for three samples where the PbTe ablation time was varied (tLASER=20, 40 and 60 seg) an the glass matrixdeposition time was held constant (tRF=50 seg). The total thickness for all the three multilayers was about 1.5-2.0 Alm.

sample 1 300sample 32 ->m7.2 r.m

2.4 .sale 1.24. * a= .lnm......4̂e-1.20 .... I aI___ 2Lf1.2 *.c ...0...*000

C *

0.0 ..I600 900 1200 1500 5E6 7 8 9 10 111

Wavelength (nm) (b) Particles Diameter (nm) (c)

Figure I (a) Low magnification cross-section image of a multilayer PbTe/SiO2 grown on a Si (100) substrate. (b) Absorption spectraof a SiO2/PbTe quantum dots multilayer grown by altemately PLD of PbTe nanoparticles and PECVD of SiO2. The samples werefabricated with different PbTe ablation time sample 1(.) tLASER= 20 seg; sample 2 (A)tLASER=40 seg and sample 3 (0)tLASER=60 seg.The deposition time for the glass matrix was held constant for all three samples (c) Size distribution for sample grown with ablationtime tLASER =60 seg.

In order to study the in plane morphology of the nanoparticles, samples with simple sandwich structure ofSiO2/PbTe/SiO2 were grown. The nanoparticles were deposited on copper grids with previously deposited carbonfilm. Size distribution studies were carried out by manually outlining the nanoparticles from several dozens of low andhigh resolution TEM images. Once digitized and saved in proper format the image was processed using the GatanDigital Micrograph program which provided data on the size, area and separation of the nanoparticles. Figure l(c)shows the size distribution for a samples grown with ablation time tLASER =60 seg, corresponding to sample 3 in figure1(b).

[1] A. P. Alivisatos; Sci. 271, 933 (1996).[2] G. J. Jacob, C. L. Cesar and L. C. Barbosa; Chem. Phys. Glass, 43C, 250-252 (2002).

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