Nonradiative recombination on Si surfaces during anodic oxidation in fluoride solution
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Hahn-Meitner-institut, Abt. AP, Rudower Chaussee 5, D-12489 Berlin, Germany Moscow State University, Physics Department, 119899 Moscow, Russia Technische Universitat Munchen, Physik-Department E16, D-85747 Garching, Germany
Abstract The anodic oxidation of p-Si(100) in aqueous NH4F solution is investigated in-situ by photoluminescence (PL) with short N2-laser pulses as a function of pH at constant potential. The PL intensity depends sensitively on the modification of the Si surface and on the change of the oxidation rate during current oscillations. The interruption of the anodic oxidation at the maximum of a current oscillation peak leads to a current transient which shows typical features of oxide removal and hydrogenation for two different oxide thicknesses. This makes it possible to realize an intermediate state of the Si surface with a coexisting hydrophilic (oxide) and hydrophobic (hydrogenated) part. Introduction The silicon/fluoride electrolyte system is of great interest due to the possibility of the formation of structured or smooth surfaces as used for micromachining and electronic devices. There exist some surprising effects like a current transient at the end of the dissolution of an oxide covered silicon electrode in fluoride solutions [1-4]. The hydrogenation of the Si surface is obtained when the current transient tends to decay and is completed when the current levels out [4,5]. We use in-situ photoluminescence (PL) measurements to investigate the creation and passivation of nonradiative recombination centers at the silicon / electrolyte interface during anodic oxidation reactions at p-type Si(100) in the oscillating regime. The results are discussed from the point of view of non-radiative defect generation and passivation, which is kinetically controlled by the charge transfer and the oxidation and etching rate, respectively. The stroboscopic probing of the PL was chosen to suppress light induced electrochemical reactions. Experimental P-type Si(100) samples with a specific resistivity of about 1 Qcm are used in the experiments. The Si sample is placed at the center of a quartz tube and serves as working electrode. Pt wire and 1 M KCI/AgCI/Ag are used as counter and reference 51 Mat. Res. Soc. Symp. Proc. Vol. 448 0 1997 Materials Research Society
electrodes, respectively. The electrolyte is continuously pumped through the quartz tube during the experiments. The electrode current or potential is controlled by a galvanostat/potentiostat (Jaissle IMP 88 PC). The electrolytes are made from p.a. NH4F, triply distilled water and H2SO4 for pH adjustment. The PL is excited by a N2-laser (LTB-MSG200, wavelength 337 nm, pulse width 0.5 ns). The light intensity is about 0.5 mJ/cm 2. The PL transients are detected at a wavelength of 1.05 pm using a prism monochromator, a Si-photodiode with a high impedance preamplifier (EMM) and a digital oscilloscope (HP 54510 A). The experimental setup is described elsewhere [6]. Results Fig.la shows a typical current voltage curve of p-type Si(100) in the acidic
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