Dimensions of Luminescent Porous Silicon by Thermal Effusion of Hydrogen
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Garching, Germany
ABSTRACT Using electrochemically prepared porous Si with hydride covered internal surfaces, we determine the H-content by thermal effusion and measurement of the total gas evolution. The amount of Si is obtained by weighing. In this way, we derive a value of the ratio of H- to Si-atoms and from the latter a value of the linear dimension of the luminescing particles
INTRODUCTION In the discussion of model concepts explaining light emission from porous Si the particle size plays an important role. There have been various more or less successfil attempts to correlate the photo-excited luminescence energy (PL) with the dimensions of the Si crystallites in order to confirm the quantum size effect as an essential element of the model. Direct observations via TEM [1], or the more indirect analysis of X-ray diffirachtion data [2] have provided to date most of what is known about sizes. Until recently, most researchers in the field felt comfortable with the available numbers. A size of order - 30 A for particles luminescing inthe typical yellow-red part of the spectrum (- 1.75 eV) was generally accepted. Nevertheless, the need for a more substantial and precise determination has been acutely sensed. This is especially the case, because of the coexistence of clearly size-independent PL emissions alongside with those that vary with the particle dimensions. A recent answer to the much needed size determination is Schuppler et al [3] in a letter publication. In these experiments the X-ray absorption fine structure spectroscopy is utilized to measure the coordination number of the Si atoms in the porous Si sample. The authors provide experimental values of this parameter for various luminescing samples. In this language, fully coordinated Si has X = 4. Since surface Si atoms have a local value less than 4 (depending on the number of surface H-bonds) it follows that an ensemble average value identifies the fraction of surface atoms and a volume to surface ratio. The ratio depends sensitively on the average number of H-bondings per surface Si, a quantity that cannot be derived independently from the measurement. Ref. [3] proceeds via a calibration step in which porous Si PL is compared with the luminescence of spherical particles produced in a gas-phase reaction and whose size are measured independently by TEM. Two conclusions are derived from this calibration - one, good agreement is achieved when the porous Si crystallites are described as asymptotic (100) cubes in which each surface Si is assigned two H-bonds; two, the porous Si crystallites are smaller than previously assumed. The typical, 1.75 eV-luminescing porous Si has size 22 A (- 500 Si atoms) according to [3]. 423 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
THE H-EFFUSION EXPERIMENT AND RESULTS
Instead of measuring as in [3], we have several years earlier proposed and demonstrated an experiment that provides complementary information [4] and should be carefully compared with that letter publication. Thermal effusion of the
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