Patent Application
20080224171
The invention concerns the field of light emitting devices such as those disclosed for example in the document FR-2 833 757.
This document describes a process for manufacturing a light emitting device in which a stop layer is inserted in the structure.
The stop layer 12 is in CdHgTe, with an alloy composition equal to that of the photon emitter well 10. The advantage of such a device is that the stop layer 12 no longer induces optical loss and can even participate in the emission of light. In practice, the participation of this layer in the emission of light is not optimal since the maximum of the electrical field, in other words the point of maximum coupling between the photons and the microcavity, is situated at the level of the well layer 10.
This microcavity of
Microcavities of this type have a defect, which is to offer a non guaranteed spectral selectivity. The spectral width of the photoluminescence spectrum is too high to allow a complete selectivity via coupling to the microcavity.
The final emission of the emitting device is the result of the filtering by the mirrors of the optical emission (photoluminescence). This can be seen in
The presence of this stray radiation 26 drastically reduces the spectral selectivity of such a device. Indeed, even if the stray intensity is considerably less than that of the principal peak, the first may however (which is frequently the case) spectrally coincide with the absorption rays of a compound present in much higher proportion than the gas to be analysed. This may be a particular hindrance if the stray emission corresponds to the absorption domain of water molecules.
The emission spectrum of the active layer of the emitter is re-centred by the presence of the resonant cavity. Said cavity allows other wavelengths to pass through outside of its reflectivity strip, in particular shorter wavelengths. The superimposition of two spectra constitutes the emission spectrum of the emitter: stray wavelengths then appear, the presence of which is detrimental to the precision of the device.
According to the invention, in order to filter these hindering wavelengths, the etch stop layer already present in the stack is used as filtering layer. This layer therefore plays two roles: that of stop layer and that of filtering layer.
As a stop layer, it is chemically different to the substrate to be eliminated; moreover, it has a composition that enables the undesirable wavelengths to be cut off.
For this filtering layer, a material similar to that used in the active layer is preferably chosen, which facilitates the manufacturing process.
For instance, for a layer in CdHgTe, its composition is chosen so as to absorb the stray radiation.
The invention therefore concerns a light emitting device comprising, between an input mirror and an output mirror forming a cavity, the latter having a reflectivity strip, a stack itself comprising an etch stop layer and an active layer characterised in that the stop layer is adapted to filter at least the wavelengths lower than the lower limit of the reflectivity strip.
The active layer is for example in CdxHg1-xTe, and the stop layer in CdyHg1-yTe, where x<y.
If the stop layer is in CdyHg1-yTe, the cavity may comprise two stop layers of composition CdzHg1-zTe, where y<z.
The active layer is preferably in a material adapted to emitting in the infrared domain.
A device according to the invention may further comprise two Bragg mirrors, the stop layer being situated in the neighbourhood of the Bragg mirrors.
The stop layer is for example of a thickness between 50 nm and 200 nm.
A device according to the invention is for example adapted to emit at least one wavelength absorbed by methane or an oxide of carbon or an oxide of nitrogen.
A first embodiment of the invention is illustrated in
In this structure of an emitting device according to the invention, the stop layer is also a filtering layer 20, for example a layer in CdHgTe, the composition of which is chosen so that this layer absorbs the stray radiation. In the case where the layer 10 is also in CdHgTe, this layer 20 has a Cd content higher than that of the well 10. The barrier layers 6, 8 may also have compositions of type CdHgTe, in which case the Cd content of the layer 20 is between that of the well 10 and that of the barriers 6, 8.
The layer 20 therefore does not hinder the emission at the wavelength of the device and does not participate, either, in the emission at this wavelength.
This layer 20 is inserted directly in the structure, it may therefore be formed by epitaxy at the same time as the cavity (barriers 6,8+well 10).
This layer 20 is situated directly in the neighbourhood of the Bragg mirrors 2 or 4, which may be deposited on this same layer.
The photoluminescence spectrum (curve I), which has a tail 20 on the high energies side may be recognised; superimposed on this spectrum is also represented curve II, or reflectivity spectrum typical of a pair of Bragg mirrors (Fabry-Perrot resonator).
The resulting emission spectrum is, due to the fact of the filter layer, purged of the stray emission 26 visible in
The filter layer 20 may not be a thick layer (it has for example a thickness of between 50 nm and 200 nm). It absorption efficiency may therefore be less than that shown in
According to one embodiment of a device according to the invention, with stop layer accumulating the function of filter, the stack is developed according to a process comprising the following steps. On a substrate, is grown, for example by epitaxy:
A first mirror is then deposited. By etching down to the stop layer, the substrate is eliminated. On the freed surface is deposited the second mirror.
The filter/stop layer has a composition in cadmium greater than that of the emitting layer and less than that of the barrier layers. Its thickness may be low because it is situated in the cavity. It thereby benefits from backward and forward movements of photons in this cavity.
For example, for an emitter that is intended to detect CH4 (at 3.31 μm) and for a microcavity known as “λ” (microcavity in which the thickness is exactly λ/n, where λ is the wavelength of the photons (here: 3.31 μm) and where n is the index of the material), the structure of a cavity according to the invention is as follows:
The composition x in cadmium of the filter/barrier layer may take all values between the composition y in Cd of the emitting layer and that (z) of the barriers. The thickness of this layer can typically vary between 50 nm and 200 nm. The thicknesses of the other layers, particularly those of the barrier layers, will then be recalculated to conserve a cavity “λ”.
Other gases may also be detected by adapting the composition of the material, for example oxides of carbon (CO, and/or CO2) and/or oxides of nitrogen (NO).
Moreover, other materials may be used such as for example GaInAsSb compounds with all possible compositions such as the ternaries GaAsSb and InAsSb.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05 52680 | Sep 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP06/65886 | 8/31/2006 | WO | 00 | 2/26/2008 |
