Carrier Transport in Porous Silicon Light-Emitting Diodes
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required in the anodization process, light assistance is essential to create n-type LEPSI. The LEPSI pn junction devices were formed using a p-type wafer rather than a n-type wafer, so that pore formation could be made possible in the deeper substrate region. For pn junction devices, a high dose phosphorous implant was followed by a thermal anneal to yield junction depths varying from 0.2 to 1 pm. LEPSI layers were formed by electrochemical etching in an HF(48%):C 2H5 OH volume ratio 1:1 solution with a constant current density of 15 mA/cm 2 for 5 to 30 minutes. This anodization process produces LEPSI at a rate of approximately 1 ltm/min. To improve the homogeneity of the LEPSI layer, the backside of the wafers was heavily doped with boron, and a sintered Al film was used to create an intimate backside ohmic contact. After anodization, an optional 1 min. HNO 3 post-anodization chemical treatment was implemented to investigate possible effects on EL. Following LEPSI layer formation, a semitransparent IOOA gold film was deposited to form patterned 0.2 cm 2 contacts. The PL was excited by a He-Cd laser at a wavelength of 445 nm, and was recorded by a grating spectrometer attached to an optical multichannel analyzer. Strong PL was observed from LEPSI at room temperature. Optical transmission measurements taken on LEPSI thin films created from the described anodization process conditions exhibited a band gap of approximately 2 eV. Time resolved PL measurements were conducted using a frequency doubled Q-switched Nd:YAG laser which produced 7 ns pulses at 532 nm. A photomultiplier attached to a monochromator was used to record the PL decay. LEPSI samples with and without the top diffusion layer were tested, and corresponding PL lifetimes were extracted. Comparisons were made with results from frequency modulation measurements of LEPSI LEDs. CARRIER TRANSPORT IN LEDs The band structure of a Metal/LEPSI device at zero bias is shown in Fig. 1. It is based on the work function approximation and assumes that between LEPSI and c-Si, the ratio of valence-band offset to the conduction-band offset is 2.3 . The band gap of LEPSI is taken to be 2 eV, resulting in an estimated band-bending between Au and LEPSI of less than 0.1 eV. The measured I-V characteristics of the device are rectifying but are quite different from that of a Schottky diode. As shown in Fig. 1, the forward bias current-voltage relationship exhibits a relationship (I = KV m, 110__ - --power-law where K is a proportional factor ... --Evac 110 .... and m = 2 to 4) which is typical of 4.0eV 4.8eV 3.6eV 0 a space charge limited current." This is caused by the high 70 •" Eresistivity LEPSI layer. For devices with a LEPSI layer - V1.12eV B----- F1= 50 thickness greater than 5 g.m, high frequency C-V measurements E 30 Au Porous Si c-Si demonstrate that the capacitance is 10 0 Aapproximately independent of the bias voltage (up to reverse applied 1 -10 10 V), suggesting a parallel plate 10 5 0 -5 -10 capacitance model. The value of the capacitance per unit area (C) V(
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