Porous Silicon as an Ultraviolet Light Source
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The porous silicon samples are prepared from 3" or 4" n-wafers with a phosphorous doping level of 4x10"5 cm 3 (=3 0icm). Light supported anodization is performed in a double cell that is filled with a mixture of HF:ethanol:DI-water (1:1:2). The samples are etched for 10 minutes at a voltage 2 of 18 V (typical current densities are 60-80 mA/cm ). To achieve electrical contact we evaporate 15 n~m gold onto the porous silicon wafers. The samples resulting from this process show an inhomogeneous depth profile. A layer containing nanosized silicon structures is built on the top of a layer with pores and structures in the micrometer range. Some of the so called macropores extend to the top layer. Additionally to the nanostructures and pores, the top layer contains current leading wires with typical diameters a 100 A and fissures with typical dimensions 1 x 3 jtm. In refs. 4 and 5 the morphology of the samples used is described in more detail.
629 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
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Fig. 1: UV emitting porous silicon can excite dyes so that the luminescence can be observed even under direct illumination of a tungsten lamp. For the green dye (ZnS:Cu,A1) we could obtain an illumination density of 240 cd/m 2; for the red one (YVO4:Eu) a value of 40 cd/m 2 was measured.
2. Experimental Section 2. 1. Experimental setup Luminescence spectroscopy was performed with a 250 mm spectrograph equipped with an optical multichannel detector that allows simultaneous detection of light from 300-850 nm. The limits of the system are 250 nm and 950 nm. Using the 150 1/mm grating the resolution is 2 nm at a slitwidth of 50 gm. The chamber used for our low pressure experiments has a suprasil window that allows transmission of light down to 180 nm. 2. 2. General observations
At this point it seems to be the right place to pronounce that the electroluminescence that we are talking about in other publications"' '- ' must not be confused with the observations reported here. When a voltage is applied to the UV-emitting samples for the first time the UV line emission is observed additionally to an electroluminescence (EL) spectrum. With increasing voltage the line spectrum becomes more and more dominant4 and is able to induce clearly observable flourescence in dyes (Fig. 1).
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pressure [mbar] Fig. 2: At lower pressures the intensity of the line emission increases about two orders of magnitude. The efficiency increases, too.
2. 3. Pressure dependence of the UV-emission
Parallel to the observation of the UV-light the question arose where the light generation takes place. Looking at the samples with the microscope, the first idea is to assume fissures in the top layer as appropriate discharge volumina4 . Thus we expected at first that with decreasing pressure the conditions for light emission should deteriorat
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