Optical Properties of Free-Standing Ultrahigh Porosity Silicon Films Prepared by Supercritical Drying
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which is comparable to that of direct semiconductors like GaAs and dramatically higher (4-5 orders of magnitude) than that of bulk crystalline silicon. However, the highest reported external efficiencies of porous silicon LED's are in the 0.1 % regime [2] [3]. The efficiency gap between the electroluminescence (EL) and PL is in large part caused by the poor electrical contacts to LEPSi and insufficient carrier transport in the film itself [4]. Increasing the porosity improves the luminescence efficiency but degrades the transport properties and the porous silicon internal network breaks down for P > 75 % after air drying. Supercritical drying of porous silicon on substrate [5] has been shown to avoid this dilemma by completely maintaining the structural integrity even at ultrahigh porosities. As it is the highest porous silicon that has the highest quantum efficiencies we employed supercritical drying to manufacture free-standing porous silicon thin films of porosities up to 95 % and subjected them to transmission measurements from the UV to the IR. The results were compared to the PL response of identically prepared samples on substrate. These highly luminescent ultrahigh porosity films may enable new LED device structures due to direct access to the porous layer in terms of contact and light emission as well. EXPERIMENTAL Free-standing LEPSi layers of ultrahigh porosity were produced by electrochemical etching and subsequent lift-off as reported elsewhere [6][7][8]. Highly Boron doped and polished c-Si substrates (p+ type, 0.007-0.02 Qcm) were anodized at current densities between 30 and 180 mA/cm 2 using ethanoic etching solutions with HF volume concentrations between 10 % and 25 % 565
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
to obtain porosities in the range from 50 % to 95 %. The porosity was determined gravimetrically and layer thinning during etching was accounted for by measuring the physical thickness (d) with an a-step profilometer (Tencor Instruments) and the weight of the LEPSi film. The etching time ranged between 14 sec and 30 min and resulted in thicknesses from 0.7 gim to 47.5 gm depending on the porosity. Moderately Boron doped c-Si substrates of the same orientation (p type, 5-7 Qcm) were also in use and etched at current densities ranging from 15 to 150 mA/cmý in ethanoic etching solutions containing HF volume concentrations between 25 % and 40 % which resulted in porosities from 40 % to 92 %. Depending on the desired thickness (2.8 gtm-54.2 gm) and porosity, the etching time was varied between 45 sec and 24 min. In all cases, the film area was 2 cm2 and the etching solutions were stirred magnetically. The LEPSi films were lifted off the substrate by applying a current pulse in a 10 % ethanoic HF solution. The lift-off of LEPSi made from p+ substrates required a higher current density of 200 mA/cm2 than that of films made from p type c-Si (50 mA/cm 2). Extreme care had to be taken during the lift-off as hydrogen bubbles often broke the fragile layers or removed them
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