GB2201835A - A photoconductive infrared detector employing periodic doping of epitaxially growth CdxHg1-xTe - Google Patents

Abstract

A photoconductive I.R. detector comprising a plurality of superimposed thin epitaxial layers 2, 3 extending between a pair of contacts 6, 7 arranged, in use, to generate an electric field substantially in the plane of the layers, wherein the layers are composed of cadmium mercury telluride, one set of alternate layers is isolated from another set of alternate layers and the sets differ in their conductivity characteristics so as to tend, in use, to separate photoexcited electrons from photoexcited holes. One set of layers may be of n- type conductivity and the other of said sets is of p- type conductivity. Alternatively, both sets may be of n- type conductivity or p- type conductivity but differ in their degree of n or p- type characteristics. <IMAGE>

Description

Infra-red detector.

The present invention relates to infra-red photodetectors of the type incorporating cadmium mercury telluride (CdX Hgl x Te) as the photosensitive material and relates particularly to photoconductive detectors of this type.

The minority carrier lifetime is an important material parameter in cadmium mercury telluride photoconductive detectors,particularly in applications where high responsivity or long minority carrier drift length is required.

The effective minority carrier lifetime is limited by bulk recomination processes and by the surface recombination velocity. The surface recombination velocity is highly sensitive to the processing techniques used in the man- ufacture of the photodetector.

An object of the present invention is to provide a high performance cadmium mercury telluride infra-red photodetector by means of a structure which enhances the mean lifetime of the minority carriers.

According to the present invention, a photodetector comprises a plurality of superimposed thin epitaxial layers extending between a pair of contact arranged in use to generate an electric field substantially in the plane said layers, characterised in that said layers are composed of cadmium mercury telluride, one set of alternate layers is isolated from the other set of alternate layers, and said'sets differ in their conductivity characteristics so as to tend, in use, to separate photoexcited electrons from photoexcited holes.

Preferably the photodetector incorporates a multiplicity of said layers - typically 10 or more. Preferably said layers are less thanlysm thick.

The two sets of layers may be doped differently so that one set of layers has p-type conductivity and the other set of layers has n-type conductivity. Alternatively both sets of layers may be p-type (or both n-type) but differ in their degree of doping.

The layers may be grown by molecular beam epitaxy, metal organic vapour phase epitaxy, or by other suitable known epitaxial growth methods-on a suitable substrate.

During deposition of the layers, the type of doping (p-type or n-type) and/or the degree of doping may be varied continuously or discontinuously, depending on the conduction and valence band profiles required. The composition of the cadmium mercury telluride may be varied during deposition by periodically varying the cadmium: mercury ratio, thereby defining a stack of layers in which the value of x in the formula Cd Hgl Te varies periodically.

One embodiment of the invention will now be described, by way of example only, with reference to Figures 1 to 3 of the accompanying drawings, of which: Figure 1 is a diagrammatic cross-section of a photoconductive I.R. photodetector in accordance with the.invention, Figure 2 is an energy level diagram relating to the epitaxial layers of Figure 1, and Figure 3 is a sketch perspective view of the detector of Figure 1.

Figure 1 shows a stack of epitaxially deposited cadmium mercury telluride layers on a substrate 1. One set of alternate layers 2 is doped n-type and the other set 3 is doped p-type. The p-type layers are isolated from the n-type layers by n-type regions 4 and 5. Deposited metal contacts 6 and 7 are arranged to generate a field gradient in the plane of the layers 2, 3.

The band extrema as a function of distance along the growth axis are shown in Figure 2. It will be seen that the energy levels E of the conduction band C and the valence band V vary sinusoidally through the thickness of the detector.

The principle of operation is also indicated in Figure 2. Absorption of radiation hut by by direct interband transitions is followed by relaxation of photoexcited carrtiers to adjacent band extrema (electrons e in conduction band, holes h in valence band). The periodic doping of the structure causes the photoexcited electron and hole carriers to occupy statially discrete planes #e and Ph and recombination rates (shown by dashed arrows) may be lowered compared with uniformly doped material, with consequent enhancement of the effective minority carrier lifetime.

The mobility of electrons and holes in the direction perpendicular to the doping planes is lowered compared with uniformly doped material, and drift or diffusion of photoexcited carriers to the detector surface lying parallel to the doping planes is inhibited by the periodic internal electric fields in the detector. Low surface recombination rates are achieved by this method.

Electron and hole mobility in the two dimensions parallel to the doping planes is not inhibited by periodic electric fields and long minority carrier drift lengths result in these directions.

The detector geometry shown as an example in Figure 3 makes optimum use of this periodically doped structure when L,'W > > tand L can be long compared with conventional detectors and can take-advantage of the highly uniform large area samples characteristic of epitaxial growth. As an example, this structure could be very advantageous in the type of detector known as a 'TED' or 'SPRITE' (Dr.C.T.Elliott, Conference on Advanced Infrared Detectors and Systems, October,1981). This disclosure is hereby incorporated by reference. As such it would provide a significant improvement over the prior art devices.

In general specialised contacts will be required to operate the device illustrated in Figure 3. The p-type layers should be isolated from the metal contact in order to preserve the enhancement of minority carrier lifetime in n-type material, made possible by the use of the multilayer structure. The CMT surface immediately below the metal contacts should be n-type, which can be achieved for example by ion implantation.

The device is essentially an n-type photoconductor with buried isolated p-type layers.

Claims (6)

Claims.

1. A photodetector comprising a plurality of superimposed thin epitaxial layers extending between a pair of contacts arranged in use to generate an electric field substantially in the plane of said layers, characterised in that said layers are composed of cadmium mercury telluride, one set of alternate layers is isolated from the other set of alternate layers and said sets differ in their conductivity characteristics so as to tend, in use, to separate photoexcited electrons from photoexcited holes.

2. A photodetector as claimed in Claim 1 incorporating ten or more of said layers.

3. A photodetector according to Claim 1 or Claim 2 wherein said layers~ are up to 1 pm thick.

A A photodetector according to any preceding Claim wherein one of said sets of layers is of n-type conductivity and the other of said sets is of p-type conductivity.

5. A photodetector according to any of Claims 1 to 3 wherein both of said sets of layers are of n-type conductivity or both of said sets of layers are of p-type conductivity but differ in their degree of n or p-type characteristics.

6. A photodetector substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.

6. A photodetector according to any preceding Claim incorporating means for scanning detected radiation over said layers in the same direction as and at substantially the same speed as the drift of holes within said layers.

7. A photodetector substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.

Claims 1. A photodetector comprising a plurality of superimposed thin epitaxial layers of cadmium mercury telluride extending between a pair of contacts arranged in use to generate an electric field substantially in the plane of said layers, wherein one set of alternate layers exhibits n-type conductivity and the other set exhibits p-type conductivity so that homojunctions are formed periodically in the direction perpendicular to the plane of the layers, thus tending, in use, to separate photoexcited electrons from photoexcited holes, and wherein the p-type layers are isolated from said contacts by n-type material interposed between said contacts and the edges of said layers.

2. A photodetector as claimed in Claim 1 incorporating ten or more of said layers.

3. A photodetector according to Claim 1 or Claim 2 wherein said layers are up to loum thick.

4. A photodetector according to any preceding claim wherein the ratio of cadmium to mercury differs between sets of alternate layers.

5. A photodetector according to any preceding claim incorporating means for scanning detected radiation over said layers in the same direction and at substantially the same speed as the drift of holes within said layers.