Abstract:
An electromagnetic wave detector comprises a stack of quantum wells included between an ohmic contact and a rectifier junction which may be a barrier (Al y  Ga 1-y  As) with a forbidden band width that is greater than that of the barriers of the quantum wells.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electromagnetic wave detector, and notably to a detector made of semiconductor materials with quantum well structures. 
     There are known quantum well based detectors of infra-red radiation using the transitions (a) between bound levels and free levels and (b) between bound levels. Their mode of operation is briefly recalled here below. 
     A detector of this type is formed by a stack of alternating layers of a small gap semiconductor (SGS) between two large gap semiconductors (LGS) as shown in figure 1a. The energy difference between the bottoms of the conduction bands of the two semiconductors is the &#34;band-offset&#34; ΔE. For example, these two semiconductors may be made of GaAs for SGS and Al y  Ga 1-x  As (where x is between 0 and 1) for LGS. The electrons in such a structure are subjected to a potential well with a depth ΔE and a width d, where d is the width of the layer of SGS. If the width d is small enough, the energy of the electrons corresponding to the motion perpendicular to the layers is quantified in levels E 1 , E 2  . . . In the bound/free photoconductive layers, the level E 1  is bound (E 1  &lt;ΔE) and the level E is unbound or free (E 2  &gt;ΔE). If the level E 1  is occupied because of an electron (by doping for example), a photon with energy hv greater than ΔE-E 1  causes an optical transition. The electron is then free to move and can be detected as a current at the terminals of the multiple well structure (see figure 1b). 
     The detector structure is shown then in FIG. 2. It has a stack of doped LGS/SGS/LGS layers sandwiched between two thick and highly doped SGS layers providing the ohmic contact. The device is at a temperature low enough for all the electrons to be trapped in the quantum wells. This current is given by the relationship: 
     
         J.sub.th =J.sub.tho e.spsp.-[(ΔE-E.sbsp.1.spsp.)-E.sbsp.F.spsp.]/kT(1) 
    
     where 
     T is the temperature 
     k is the Boltzmann constant, 
     E F  is the Fermi level of the electrons in the quantum well, 
     J is the thermionic current given by K. Brennan and Y. Wang in: &#34;Analysis of the Two-Dimensional Dark Currents in Quantum Well Devices&#34;, Applied Physics Letter, 57, 1337 (1990). 
     The response of this photodetector is deduced using the following line of reasoning. The density by volume of carriers is determined by the balance between, firstly, the optical generation: 
     
         G=η·φ/t 
    
     where η is the quantum yield, φ the flux of the photons and t the thickness of the quantum well and, secondly, the recombination: 
     
         R=n/τ 
    
     where n is the density by volume of electrons in the conduction band of LGS and τ is the lifetime of this electron. The density of photo-excited electrons n is given by G=R, i.e. 
     
         n=ητφ/t 
    
     and the current density by: 
     
         j=nqμE 
    
     
         j=qητφμE/t                                  (2) 
    
     where μ is the mobility of the electrons in the LGS and E is the applied electrical field. The response R of the structure is given by: 
     
         R=j/φhν=qηιμE/ιhν 
    
     It is observed that, for a constant electrical field, this response is independent of the number of wells. The invention relates to a device in which the response is a growing response when the number of wells increases. The response is then increased, and this increases the detectivity. 
     SUMMARY OF THE INVENTION 
     The invention therefore relates to an electromagnetic wave detector comprising a stack of layers of different materials having different forbidden gap widths, so as to constitute a stack of quantum wells, wherein the stack of layers is included between an ohmic contact and a rectifier junction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The different objects and features of the invention shall appear from the following description and from the appended drawings, of which: 
     FIGS. 1a, 1b and 2 show detection devices known in the prior art; 
     FIG. 3 shows a general exemplary embodiment of a device according to the invention; 
     FIG. 4 shows a detailed view of an exemplary embodiment of a device according to the invention; 
     FIG. 5 shows an alternative embodiment of a device according to the invention; 
     FIGS. 6a to 6c show energy diagrams explaining the working of the device according to the invention; 
     FIGS. 7 and 8 show an operation of the device according to the invention in bound/bound levels mode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There is thus provided a structure as shown in FIG. 3, comprising a stack of quantum wells 1 constituted by large gap semiconductor (LGS) layers and small gap semiconductor (SGS) layers such that: 
     
         ΔE-E.sub.1 =hν 
    
     where: 
     ΔE is the band offset between the conduction band bottoms of the two materials; 
     E 1  is the value of the energy of the first permitted level of the SGS layers; 
     hν is the energy of a photon to be detected. 
     The stack of quantum wells is provided, on one side, with a current rectifier junction 2 and, on the other side, with an ohmic contact 3. 
     A DC voltage generator is connected to the junction 2 and to the ohmic contact by a reversal system SW enabling the voltage applied to be reversed. 
     As shown in greater detail in FIG. 4, the structure according to the invention can be made, for example, as follows: 
     SGS layers (quantum wells) made of GaAs with a thickness d=approximately 6 nm; 
     LGS layers (barriers) Al y  Ga 1-x  As (with x=0.22 for example) with a thickness L=approximately 30 nm. 
     We then have ΔE-E 1  =120 meV, and it is possible to detect waves at 10.6 μm wavelengths. 
     The ohmic contact is a layer with a nature similar to that of the SGS layers and has n+doping. It is made of GaAs-n+ for example. 
     The rectifier junction may be a Schottky type barrier, which gives the following structure: 
     
         Metal/Al.sub.x Ga.sub.1-x As/GaAs/Al.sub.x Ga.sub.1-x As/ . . . /Al.sub.x Ga.sub.1-x As/GaAs n+ 
    
     FIG. 5 shows an alternative embodiment in which the rectifier junction has a barrier, the height DE of which is substantially greater than ΔE. For example, this barrier may be made of Al y  Ga 1-x  As with y greater than the value of x provided in the composition of the Al y  Ga 1-x  As barriers. The structure therefore has the following configuration: 
     
         GaAs-n+/Al.sub.y Ga.sub.1-y As/Al.sub.x Ga.sub.1-x As/GaAs/ . . . /GaAs-n+ 
    
     Hereinafter, we shall explain the latter type of structure and shall consider, by way of an example, that y=0.4. 
     In this structure, the static current J is dominated by the GaAs-n+/Al 0 .40 Ga 0 .6 As junction which has a barrier height DE&gt;&gt;E. This current is given by the relationship: 
     
         J.sub.f =A*T.sup.2 e.spsp.-q·DE/kT 
    
     where A*=Richardson&#39;s Constant. Since DE&gt;ΔE, it is seen that, for equal temperature, the static current is far smaller in this structure than in the known structures. This structure is then subjected to potential pulses as shown in FIG. 6a. During a period of time t R , a bias V R  is applied which is such that the quantum wells are filled by the ohmic contact. During a period of time t v , a reverse bias is applied. If no photon is incident, the structure is dominated by the thermal emission of the electrons. This rate of emission e n  is given by 5. 
     
         e.sub.n =σ·v.sub.th ·N.sub.c e-.spsp.[(ΔE-E.spsb.1.sbsb.)-E.sbsp.F.spsp.]/kT 
    
     where: 
     σ is the effective capture cross-section. 
     v th  is the thermal speed of the carriers, 
     N c  is the effective density in the conduction band of Al x  Ga 1-x  As. 
     The term e n  of this equation can be approximately related to the current J th  of equation (1) by the relationship: 
     
         J.sub.th =n.sub.o ·q·e.sub.n 
    
     where n o  is the doping of the wells. 
     It is therefore important to note that the conditions needed to have a weak dark current in a photoconductor are very close to those needed to have a high time constant for the thermal emission. For example, if it is sought to use a structure according to the invention for a video detection (e&lt;24 s -1 ), then: 
     
         J.sub.th =&lt;·10.sup.12 ×10.sup.-19 ×24×1.6 
    
     
         giving J.sub.th &lt;3.6×10.sup.-6 A/cm.sup.2 
    
     The number of electrons n(t) in each well under illumination is given by the equation: 
     
         dn(t)/dt=-σ.sub.op ·n(t)φ 
    
     where σ op  is the effective optical cross-section of the well. Then 
     
         n(t)=n.sub.o e.spsp.σ.sbsp.op·φt 
    
     and the quantity of charges detected is then, assuming that all the photo-emitted electrons are detected: 
     
         Q(t)=qNn.sub.o (1-e.sup.-σ.spsb.op φt) 
    
     N is the number of quantum wells. The detected current is then: 
     
         i.sub.T (t)=qNn.sub.o σ.sub.op φe.sup.-σ.spsb.op φt 
    
     The detected signal is therefore proportional to the number of quantum wells unlike in the prior art photodetector case. 
     For example the typical effective optical cross-section values are 5×10 cm -15  cm 2 . For reading times of 1/24th second, the response remains linear for 
     
         σ.sub.op ·φ&lt;24 that is, for φ&gt;1.10.sup.15 
    
     which is a very high φ (about 50 μw for 100 meV photons). In this case 
     
         i.sub.T =qNn.sub.o σopφ                          (3) 
    
     It is recalled that, in the known devices, the photoconductor current is given by the equation (2). 
     If we compare the current i T  to the current J of the equation (2), it is shown that: 
     
         i.sub.T /J=t/τνE=t/λ 
    
     This is the reverse of the photoconduction gain. For a thickness t=5 μm, a product μτ of 1.20 -9  V.cm -2  and E=10 4  V/cm, this reverse gain has a value of 25.  A gain such as this will enable the device to be made to work at higher temperatures. 
     Another configuration of this blocking electrode entails its use for bound-bound transitions, i.e. when the level E 2  is inside the well (FIG. 7). It is known that it then becomes necessary, in the quantum well photoconductors, to bring the wells sufficiently close to one another to enable the tunnel conduction from one well to another, thus increasing the dark current and hence causing deterioration in the detectivity. In the transient operation using a blocking electrode, the conduction by tunnel effect between wells can be avoided. This time, it is enough for the probability of emission by tunnel effect to be high enough to short-circuit the recombination between sub-bands. This period of time is of the order of ten picoseconds. It is therefore enough to apply an electrical field such that the tunnel time at the second level is lower than 10 ps. 
     Two conditions should therefore be met: 
     
         Condition 1: e.sub.FN (ΔE)&lt;24 s.sup.-1 
    
     
         Condition 2: e.sub.FN (ΔE-hν)&gt;10.sup.11 s.sup.-1 
    
     e FN  is the rate of emission by tunnel effect assisted by the electrical field, or the Fowler-Nordheim effect. 
     Condition 1 means that the rate of emission is far lower than the video frequency and condition 2 ensures that, nonetheless, the electron on the excited state will come out of the well before getting recombined at the fundamental level. An example fulfilling both these conditions is given in FIG. 8. 
     In this example, the aluminium percentage in the barrier is about 25%. 
     Size of well: 7.5 nm. 
     This well absorbs at 10.6 μm and can work up to 100 K. 
     The bias and reading circuits of such pulse devices would be equivalent to the circuits used in the prior art in Si:Ga detectors with blocked impurities band. 
     It is quite clear that the above description has been given purely by way of a non-restrictive example and that other variants may be contemplated without going beyond the scope of the invention. Notably, the compositions of the layers of materials and their thicknesses may be different from those indicated here above.