Enhancement of the Spontaneous Emission Rates in all Porous Silicon Optical Microcavities
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where is the interaction hamiltonian matrix element between the initial and final electronic state and p(ho)) is the photon mode density. If one modifies the photon mode density, e. g., by placing the active medium in an optical microcavity, the spontaneous emission rate is modified accordingly. [5] Indeed, in the PSM a narrowing in the emission lineshape of PS down to 3.7 nm, [6] an increase in the peak emission efficiency up to 22 times and a directionality of the emission about a cone of 300 is actually achieved. [3] These facts are attributed to the spatial redistribution of the optical modes due to the planar microcavity. [5] An interest exists in verifying whether the time decay of the luminescence of PSM is changed with respect to that of a simple PS layer. This is the subject of the present paper. 717
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
TABLE I. Summary of the time resolved luminescence measurements. The column labeled nc reports the measured refractive index of the layers at the central wavelength Xc which is shown in the third column. linteg refers to the normalized spectrally integrated luminescence intensity, I(Xc) to the normalized peak emission intensity at the wavelength of the microcavity peak (kc), and 'rc and P4 to the parameters extracted from a stretched exponential fit of the luminescence decay at Xc (for the reference DBR they are reported for the different Xc of the various PSM). The PS 45% sample had a luminescence too weak to be detected by our system. nc % % % (45%) (62%) (75%) PSM 45 % PSM 62 % PSM 75 % PS 45 PS 62 PS 75 DBR
X'c (nm)
Iinteg
I(Xc) -
2.1 1.52 1.30 1.52/2.2
720
0.13 0.26 0.84
2.1 1.52 1.30
692 741 692
1 0.95 0.92
0.03 0.08 0.23 0.20 0.24 0.81 0.66 1
'Tc (gts) 22±2 21±0.5 7.5±0.3 9.2±0.4 7.5±0.3 8.7±0.1 13.3±0.1 15.2±0.1
04c 0.50-0.03 0.72_+0.01 0.47±0.01 0.49±+0.01 0.47±+0.01 0.62±0.01 0.62±0.01 0.62±+0.01
MICROCAVITY FABRICATION AND CHARACTERIZATION An all PSM consists of a Fabry-Perot structure (two mirrors separated by a thin central layer), where the porosity of the different porous silicon layers are changed to obtain the desired refractive index profile. This is realized by an appropriate modulation of the anodization time and the current density during the electrochemical etch. [7,8] A detailed discussion of the procedure to obtain the PSM has been already presented in [8]. In this work, we present data on various PSMs which differ only in the porosities and thicknesses of the central layer in order to obtain different refractive indexes nc in the central layer. All the PSM were obtained by electrochemical anodization of 0.01 Qcm p-type doped Si-wafers with a 15% HF solution. They were formed by two distributed Bragg reflectors as mirrors (constituted by 12 periods of 62%-45% porosity layers, with optical thicknesses of Xc/ 4 ) which sandwiched a central layer of optical thickness Xc/ 2 . Xc is the wavelength of the Fabry-Perot resonance. In the following we label the PSM by adding to "PSM" the porosity of the cent
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