New Results on Electroluminescence from Porous Silicon
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New Results on Electroluminescence from Porous Silicon Peter Steiner, Frank Kozlowski, Hermann Sandmaier and Walter Lang Institue for Solid State Technology, Munich, Germany Abstract First results on light emitting diodes in porous silicon were reported in 1991. They showed a quantum efficiency of 10-7 to 10-5 and an orange spectrum. Over the last year some progress was achieved: -
By applying UV-light during the etching blue and green light emitting diodes in porous silicon are fabricated.
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When a p/n junction is realized within the porous region, a quantum efficiency of 10-4 is obtained. 1. Introduction
In 1991 first light emitting devices in porous silicon were reported (1,2,3,4,5,6). The devices presented by us (1,2) consisted of a nanoporous film covered with a thin metallic layer for contact. This type of devices can be done in n-type silicon, if the wafer is illuminated during the etching process, or in p-type silicon. The photo- and the electroluminescence show an orange colour. If a voltage of some volts is applied to the contacts, light emission is measured. The voltage onset for emission is difficult to measure due to the noise in the photomultiplier tube. Luminescence can be proved with 2.5 V applied. Thus we suppose that the onset is at or slightly above the energy of the emitted photon. When 30 V are applied, the light can be seen with the eye. The devices show a rectifying current/voltage characteristics. This first generation of LEDs in porous silicon shows a quantum efficiency of 10-7 to 10-5 (7). For further work on electroluminescence there are four major tasks to accomplish: 1. Increase of the quantum efficiency. 2.
Increase of stability.
3.
Shorter time constants.
4.
Obtaining other colours than orange.
In this presentation we are reporting on work done on point 4 and point 1. This does not mean that we think the other points are less important, but that we obtained progress mainly in these points within the last year. 2. Green and blue electroluminescence in porous silicon The intention of the process is to tune the wavelength of the electroluminescence. The basic idea is as follows: There is strong evidence that the wavelength of the emitted light is determined by the size of the crystallites. Therefore, we want to get smaller structures. Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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Take a look at the model of etch stop in p-silicon by quantum confinement given by V.Lehmann (8). During the etching process, the structures get smaller until a quantum confinement due to the small size occurs and the band gap is increased. Then, the holes do not enter the small structures any more and the electrochemical etching process stops. This self-aligned process forms the nanocrystals. This theory holds for p-doped silicon. In n-doped silicon there are no carriers present and a depletion layer forms around the pores; this way macroporous material is formed. If we illuminate during etching, we generate carriers, the etching proceeds until a nanoporous film is gener
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