Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1999-09-24
2001-09-11
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S096000, C257S103000, C257S184000, C438S047000, C438S094000
Reexamination Certificate
active
06288415
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optoelectronic semiconductor devices, for example photoemitters and photodetectors.
BACKGROUND OF INVENTION
In an optoelectronic semiconductor device there are “photoactive” regions in which either emission or detection of photons can take place. Emission of photons takes place when an applied electrical current injects holes and electrons across a junction and the electrons and holes can combine in the photoactive region, the resultant energy being released in the form of photons. Detection of photons takes place when photons incident in the photoactive region create electronhole pairs, causing an electrical current to flow.
Silicon has an indirect band gap and this has hindered the development of acceptable silicon based photoemitters suitable for use in integrated silicon optoelectronic applications. Silicon's band gap is also high, hindering development of photodetectors sensitive to wavelengths of longer than around 1 &mgr;m. Optoelectronic devices which are emissive of, or sensitive to, electromagnetic radiation of about 1.5 &mgr;m, which is the basis of optical fibre systems, would be particularly significant in communications applications and in optical computing systems that are resistant to severe electromagnetic interference (EMI). The device architecture proposed by this invention permits such silicon-based optoelectronic devices to be made.
Several different approaches have already been investigated, with a view to developing a suitable photoemissive device which is capable of producing radiation at a wavelength of about 1.5 &mgr;m from a silicon-based device.
In one approach SiGe superlattice-based structures have been developed making use of zone folding to produce a pseudo direct band gap. In another approach, silicon has been doped with erbium which has an internal transition energy equivalent to 1.5 &mgr;m. However, neither of these approaches has led to a practical device.
BRIEF SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided an optoelectronic semiconductor device having an architecture wherein a junction built in an indirect band gap semiconductor substrate has direct band gap semiconductor material incorporated to form a distinct photoactive region.
The introduction of the direct band gap material to form a distinct photoactive layer related to, but separate from, the junction permits a novel set of semiconductor devices to be created. Semiconductors that do not possess a direct band gap lattice are called “indirect band gap” semiconductors and are generally incapable of efficient electroluminescence.
The inventors have discovered that provision of a direct band gap semiconductor material as hereindefined improves the efficiency with which photons can be absorbed by, or alternatively emitted from the photoactive region.
According to another aspect of the invention there is provided an optoelectronic semiconductor device comprising a junction formed, at least in part, by a layer of an indirect band gap semiconductor material, wherein said layer has a photoactive region containing a direct band gap semiconductor material in which, in operation of the device, electron-hole pairs are either created or combined and which has an energy gap equal to or smaller than the energy gap of the indirect band gap semiconductor material.
Optoelectronic semiconductor devices according to the invention include photoemitters, e.g. a light emitting diode, and photodetectors, e.g. a photodiode.
In the case of a photoemitter, charge carriers are transferred across the junction and are injected into the photoactive region where they may undergo radiative transitions; that is, electrons and holes recombine there, creating photons whose energy is less than or equal to the band gap energy of the direct band gap semiconductor material.
In the case of a photodetector, incident photons having an energy equal to or greater than the band gap energy of the direct band gap semiconductor material create electron-hole pairs in the photoactive region giving rise to a photocurrent.
Preferably, said direct band gap semiconductor material has the form of isolated precipitates or microcrystals. Typically, these are of the order of fifty to several hundred nanometers so that no significant quantum confinement effects will arise.
Alternatively, though less desirably, the direct band gap semiconductor material may form a continuous layer, or a series of continuous layers.
In preferred embodiments of the invention, the direct band gap semiconductor material is beta-iron disilicide (&bgr;-FeSi
2
).
Beta iron-disilicide (&bgr;-Fe Si
2
) a direct band gap semiconductor material having a transition energy corresponding to 1.5 m. Accordingly, embodiments of the invention which incorporate &bgr;-FeSi
2
in their photoactive regions may find application in optical fibre communications. As already explained, it is preferred that the &bgr;-FeSi
2
be in the form of isolated precipitates or microcrystals; however, a continuous layer could alternatively be used. Furthermore, the &bgr;-FeSi
2
may be unalloyed or alloyed, or undoped or doped. &bgr;-FeSi
2
alloyed with cobalt germanium, indium or aluminium, for example, has a slightly lower transition energy than does the undoped material.
In a preferred photoemitter according to the invention, e.g. a light emitting diode, said junction is a p-n junction formed by a layer of p-type indirect band gap semiconductor material and a layer of n-type indirect band gap semiconductor material.
In this specification, the convention is adopted that the layer of n-type semiconductor material is more heavily doped than is the layer of p-type semiconductor material. In such circumstances, the photoactive region is situated in said layer of p-type semiconductor material so that under forward-biassed conditions electrons will be injected across the junction and captured by the photoactive region. Alternatively, the layer of p-type semiconductor material could be more heavily doped than the layer of n-type semiconductor material. In this case, the photoactive region would be situated in said layer of n-type semiconductor material and holes are injected across the junction from the layer of p-type semiconductor material and captured by the photoactive region.
In the case of a p-n junction emitter, the photoactive region is preferably situated as close as possible to, but wholly outside, the relatively narrow depletion layer prevailing when a forward bias voltage is being applied across the junction. This configuration is preferred so as to maximise the efficiency with which charge carriers are captured by the photoactive region where they can undergo radiative transitions. Alternatively, the photoactive region could be spaced apart from the depletion layer; however, capture of carriers by the photoactive region would then be less efficient.
The p-n junction may be a silicon p-n junction; however it is envisaged that a different homojunction or a heterojunction formed from indirect band gap semiconductor materials could alternatively be used.
In another photoemitter according to the invention, said junction is formed by a layer of indirect band gap semiconductor material and a metallic layer defining a Schottky barrier, and said photoactive region is situated in said layer of indirect band gap semiconductor material. Under forward biassed conditions carriers are transferred across the junction and are captured by the photoactive region where they may undergo radiative transitions.
The photoactive region is preferably situated outside the depletion layer to maximise the capture efficiency. In this embodiment, the indirect band gap semiconductor material is either n-type material or p-type material, and is preferably, though not necessarily, silicon.
Photodetectors according to the invention include photodiodes, such as avalanche photodiodes and depletion-layer photodiodes.
In the case of an avalanche photodiode, said junction is a p-n junction formed by a layer of p-type indirect band
Harry Milton Anthony
Homewood Kevin
Leong Daniel
Reeson Kirkby Karen Joy
Leydig , Voit & Mayer, Ltd.
Mintel William
University of Surrey
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