Near-Infrared Emission from a Porous Silicon Device
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ABSTRACT Visible to near-infrared emission is produced from a porous silicon device under current injection. The porous silicon emitter is fabricated by selectively under-etching a p-n junction. The device is rectifying when biased across the junction, and exhibits a region of negative differential resistance (NDR) at the higher current levels. Bright red-orange emission is observed along the length of the junction in forward bias, but most of the light is emitted in the near-infrared with a peak near 1.3 gim at 30 mA. The intensity of the visible component has an exponential dependence on photon energy. The optical and electrical properties of this device are presented and possible mechanisms are discussed. INTRODUCTION Since the first report of visible room temperature photoluminescence (PL) 1, several porous silicon Schottky barrier and p-n junction devices have demonstrated visible electroluminescence (EL) 2 -6 . More recent investigations have also revealed that a PL peak in the near-infrared is frequently observed in photoluminescing porous silicon samples 7 -9 . However, near-infrared EL spectra has yet to be published. This report presents the results of a porous silicon device which has broad current-induced emission, and peaks in the near-infrared. DEVICE FABRICATION In order to produce efficient carrier injection into porous silicon, we used crystalline-toporous silicon contacts at the positive and negative terminals. The device is essentially a p-i-n structure, where a porous silicon layer is sandwiched between a phosphorus diffusion and the lightly-doped p-type silicon substrate. A schematic of the device structure is given in Figure 1. This structure was fabricated by first diffusing phosphorus into a 9 ohm-cm p(100) silicon wafer to form 15 gim and 55 gim wide by 2.5 mm long p-n junctions. A high aspect ratio was used in order to under-etch the width of the junction, while still allowing a relatively large emitting area. A silicon nitride layer was then deposited over the diffused junction to protect the surface during the porous silicon etch. Aluminum was also deposited on the wafer back to establish an electrical contact to the substrate. Porous silicon was formed by electrochemically etching the wafer surface using 20 wt% Hydrofluoric acid at a constant current of 20 mA/cm 2 . Due to the selective etching properties of the anodization process, the heavily n-doped region can be completely underetched10 . After the anodization, the remaining silicon nitride layer was removed so that electrical contact could be made to the phosphorus diffusion via a needle probe. Static current-voltage measurements were taken with a Tektronix 370 curve tracer. Current-induced emission spectra were measured under constant current using af14 24 cm monochromator (CVI Digikrom 240) with an S I photomultiplier tube and a liquid nitrogen cooled InSb detector. The visible and infrared current-induced spectra were combined by making the appropriate normalization where the two spectra overlapped. Spectra taken under pulsed cu
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