Electromagnetic wave modulator with quantum well structure

The invention pertains to an electromagnetic wave modulator comprising a quantum well semiconductor structure having two adjacent wells, the coupling of which is modified under the effect of an electrical field by the use of the phenomenon of anti-crossing of resonating levels. With this type of modulator, it is possible to attain 20% of modulation (as in the prior art) but with heightened sensitivity to the electrical field and a very short response time.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an electromagnetic wave modulator comprising a 
quantum well semiconductor structure, working notably in the 8-13 .mu.m 
range of wavelengths. 
2. Description of the Prior Art 
For many years now, the development of the techniques for growing 
semiconductor structures has enabled the making of artificial materials 
with novel quantum properties. Indeed, the association of different 
semiconductors in thin layers results in discontinuities of the conduction 
band and of the valence band all along the axis of growth. These 
discontinuities of the potential energy of the electrons in the conduction 
band and of the holes in the valence band have been extensively studied 
and exploited in high mobility transistors, for example. By associating 
two semiconductor materials, it is thus possible to form two steps of 
potential energy having opposite directions, this result being obtained on 
a distance that is shorter than the de Broglie wavelength of the electron 
or of the hole. In such a system, which is also called a quantum well 
system, the electron states are no longer distributed in a continuum but 
in subbands, each being associated with a quantum number and an energy 
level. When the well is doped, it is possible to induce electron 
transitions from one subband to another by means of a properly chosen 
electromagnetic wave with a wavelength .lambda.: these are the 
inter-subband transitions illustrated in FIG. 1. The dipole associated 
with these transitions has a size comparable to the width L of the quantum 
well (giving about 0.18 L) and the corresponding state density has the 
shape of a Dirac function. The absorption from the fundamental level, 
referenced .vertline.0&gt; towards the first excited level referenced 
1.vertline.1&gt; is therefore resonant and relatively intense in a 
symmetrical well. 
In the GaAs/GaAlAs type III/V semiconductor systems, this absorption is 
about 0.3% when these semiconductors are doped with 10.sup.12 electrons 
per cm.sup.2 and illuminated at the Brewster angle. Many approaches have 
already been considered to obtain the modulation of this electromagnetic 
wave by using the inter-subband transitions within doped multiple quantum 
well structures. 
A first approach pertains to the modification of the states and energies 
E.sub.i of a multiple quantum well structure by the application of an 
electrical field. The energy difference between levels is thus disturbed 
and, consequently, the resonance photon energies: E.sub.j -E.sub.i 
=hc/.lambda..sub.ij (with h being the Planck's constant, c the velocity of 
light, .lambda..sub.ij the wavelength) are modified. This electrooptical 
effect is low in the symmetrical structures for it is a third-order 
non-linear susceptibility that is brought into play. In an asymmetrical 
structure, this effect is linear and more effective for it is then 
essentially a second-order susceptibility that comes into play. The 
absorption peaks get shifted under the effect of the electrical field 
applied. This is the Clark effect, which was clearly demonstrated in 1990 
("Tunable Infrared Modulator And Switch Using Stark Shift In Step Quantum 
Wells", R. P. G. Karunasini, Y. J. Mii, K. L. Wang, IEEE Electron Device 
Letters, Vol. 11, No. 5). Taking a given wavelength, and amplitude, a 
phase modulator is thus defined by simple translation of the dispersal of 
real and imaginary indices associated with the inter-subband transition, 
this being achieved under the effect of an electrical field. 
A second approach consists in the making of a charge transfer modulator 
("Tunnelling Assisted Modulation of the Inter-Subband Absorption in Double 
Quantum Wells", M. Vodjdani, D. Vinter, V. Berger, in Applied Physics 
Letters, vol. 59, No. 5). This type of device seeks the modification, by 
electrical means, of the concentration N.sub.S of carriers per unit of 
area. To do this, two weakly coupled wells are used. At negative voltage, 
only the large well is populated and the inter-subband transition is 
relative to this well. By applying a positive voltage, the carriers are 
transferred, by tunnel passage, into the narrowest well as illustrated in 
FIG. 2. Since the fundamental level of this well is populated, it is 
possible to measure an inter-subband absorption associated with this well, 
hence one with a shorter wavelength. Thus, a two-color modulator is 
obtained. 
SUMMARY OF THE INVENTION 
The two modulators mentioned here above have sensitivities that enable 20% 
of modulation to be attained. The present invention proposes a modulator 
with increased sensitivity that relies on another mode of operation. 
Indeed, in the invention, it is no longer sought, between two adjacent 
wells under the action of an electrical field, to modify the energy 
difference between two levels or even to modulate the concentration of 
electrons at the fundamental level but to change the coupling between two 
excited levels. For, it is sought to modify the states of two coupled 
wells by applying the Pauli exclusion principle or, more specifically, by 
using the phenomenon of the anti-crossing of resonant levels. To this end 
first of all, two independent quantum wells (the barrier between them has 
a substantial width) are considered. For these quantum wells, the first 
level .vertline.1&gt; of energy .epsilon. excited by a first well 1 is 
aligned with the level .vertline.2&gt; of a second well 2. These levels are 
said to be in resonance. FIG. 3 illustrates a possible configuration in 
which the excited state .vertline.1&gt; is aligned with a state .vertline.2&gt; 
which is the fundamental state of the well 2. On the discrete states, 
there are represented the wave functions whose square describes the 
density of probability of the presence of an electron on said states. In 
reducing the thickness of the barrier between the two wells, the states 
.vertline.1&gt; and .vertline.2&gt; are disturbed by the proximity of the 
neighboring well. The resolution of Schrodinger's equation of the total 
structure with the two coupled wells reveals two states: 
##EQU1## 
The energy difference between these two states is no longer zero 
(anti-crossing phenomenon), depends notably on the energy of confinement 
E.sub.c of the undisturbed levels and is equal to V.sub.b -.epsilon., 
V.sub.b representing the height of the barrier between two wells. 
It may be noted that the bonding level always has lower energy than the 
anti-bonding level. 
Under the effect of an electrical field, it is possible to uncouple the 
wells; indeed, under the effect of an electrical field F, the energy 
difference between the two levels .vertline.L&gt; and .vertline.A&gt; tends 
towards the value: eF(Z.sub.22 -Z.sub.11) where Z.sub.11 and Z.sub.22 
represent the barycenters of the wave functions and, by the same token, 
densities of probability of the presence of an electron associated with 
the states .vertline.1&gt; and .vertline.2&gt;. By applying a positive 
electrical field to the structure, the probability of presence in the well 
1 associated with the bonding state increases, and the probability of 
presence associated with the anti-bonding state gets "concentrated" in the 
well 2. 
Under the effect of a negative electrical field, the probability of 
presence associated with the bonding state diminishes in the well 1 while 
the probability of presence associated with the anti-bonding state 
increases to the level of said well. FIG. 4 illustrates the behavior, 
under positive voltage and under negative voltage, of the probabilities of 
the presence of electrons at the different energy levels. 
When there is no electrical field, there are observed two inter-subband 
transitions .vertline.0------&gt;.vertline.L&gt; and 
.vertline.0------.fwdarw..vertline.A&gt; of equal intensity since the states 
.vertline.L&gt; and .vertline.A&gt; are linear combinations with equal weights 
of the states .vertline.1&gt; and .vertline.2&gt; as illustrated in FIG. 3. It 
is possible to favor one of the two transitions by applying an electrical 
field. Indeed, a positive electrical field favors the transition 
.vertline.0------&gt;.vertline.L&gt; and a negative electrical field favors the 
transition .vertline.0------&gt;.vertline.A&gt;. The object of the invention 
therefore is a modulator having two inter-subband absorptions, the 
intensity of which can be modified by an electrical field. 
More specifically, the invention proposes an electromagnetic wave modulator 
comprising a doped semiconductor structure having a stack of layers 
constituted by semiconductor materials enabling the creation of potential 
wells and comprising means for placing the semiconductor structure under 
voltage, wherein this structure comprises a potential well P.sub.1 
possessing at least one discrete energy level e.sub.0 populated with 
electrons and one discrete energy level e.sub.1 such that e.sub.1 is 
greater than e.sub.0, a potential well P.sub.2 possessing at least one 
discrete energy level e.sub.2 such that e.sub.2 and e.sub.1 are equal. The 
wells P.sub.1 and P.sub.2 are designed so as to equal e.sub.1 and e.sub.2. 
The resonance lifts the degeneracy between these two levels and therefore 
creates the states e.sub.L and e.sub.A with e.sub.L and e.sub.A greater 
than e.sub.0 and e.sub.1 =e.sub.2 ranging from e.sub.L to e.sub.A. The 
modulation of an incident electromagnetic wave results from the absorption 
variations related to transitions between, firstly, the levels e.sub. 0 
and e.sub.L and, secondly, the levels e.sub.0 and e.sub.A under the effect 
of a voltage applied to the structure. 
The modulator according to the invention can advantageously be designed on 
the basis of a structure having two quantum wells P.sub.1 and P.sub.2, the 
well P.sub.2 having a fundamental energy state e.sub.2 that is equal to 
the energy e.sub.1 of the excited level .vertline.1&gt; of the well P.sub.1. 
To have sufficient lifting of degeneracy of the states .vertline.1&gt; and 
.vertline.2&gt;, the wells P.sub.1 and P.sub.2 should be close enough to 
provide for the desired coupling. For this purpose, there should be an 
overlapping of the wave functions and, hence, these wells should be 
separated by a distance smaller than the de Broglie wavelength 
.lambda..sub.B with .lambda..sub.B proportional to E.sub.c.sup.-1/2 if 
E.sub.c represents the energy between the potential V.sub.B of the barrier 
between the two wells and the energy of the excited state .vertline.1&gt; of 
the well 1. Preferably, the well P.sub.1 used in the invention has a small 
energy E.sub.c that generates a fairly low confinement thus enabling an 
increase in the length .lambda..sub.B necessary for the coupling of the 
two wells. 
The structure used in the invention can advantageously be prepared on the 
basis of two asymmetrical wells P.sub.1 and P.sub.2 such that the 
potential energy V.sub.1 of the well P.sub.1 is smaller than the potential 
energy V.sub.2 of the well P.sub.2. With a configuration such as this, it 
is possible to use a control voltage that is lower than that needed in the 
case of a double well configuration with the same depth to obtain the same 
electro-absorption effect. A system with two symmetrical wells would not 
work efficiently. 
The modulator according to the invention may be made, for example, out of 
Ga.sub.1-x Al.sub.x A.sub.s materials, where the composition x is variable 
.

MORE DETAILED DESCRIPTION 
The electromagnetic wave modulator with quantum well structure according to 
the invention can advantageously work in the 8-13 .mu.m range in being 
designed with standard semiconductor materials such as those using 
intermediate concentrations of aluminium in the GaAlAs system. A multiple 
quantum well structure can be made by molecular beam epitaxy on a GaAs 
substrate. In the quantum structure used in the modulator according to the 
invention, inter-subband transitions are created from a level populated 
with electrons. This populating can be achieved by an n type doping of the 
material, carried out by the insertion of a plane of silicon atoms within 
the well: the concentration in electron carriers may be typically of the 
order of 10.sup.+12 cm.sup.-2. The absorption of an electromagnetic wave 
is preferably recorded at the Brewster angle (73.degree. angle). This 
configuration makes it possible to provide an infrared wave with a 
polarization perpendicular to the plane of the well, this polarization 
being the only one that is active with respect to the electron intraband 
transitions. In the modulator according to the invention, the electric 
field is applied between two electrodes, it being possible for these 
electrodes to be constituted by n.sup.+ doped layers (10.sup.18 electrons 
per cm.sup.3) on which ohmic contacts are made by the diffusion of an 
AuGeNi alloy. 
By using the GaAs/GaAlAs system, widths of the order of about ten meV are 
typically obtained at mid-height of the inter subband absorption peaks. It 
is therefore necessary to prepare a structure for which the minimum energy 
difference .DELTA.E between the bonding state .vertline.M&gt; and the 
anti-bonding state .vertline.A&gt; is at least of the order of 20 to 30 meV 
to obtain a real separation of the two transitions firstly between the 
state .vertline.0&gt; and the state .vertline.1&gt; and, secondly, between the 
state .vertline.0&gt; and the state .vertline.A&gt;. It is therefore necessary 
to have an inter-well spacing that is small enough for there to be 
coupling so as to ensure the lifting of degeneracy between the states 
.vertline.1&gt; and .vertline.2&gt;. Furthermore, the efficiency of the coupling 
and decoupling of the wells P.sub.1 and P.sub.2 under the effect of an 
electrical field depends greatly on the mean distance between these two 
wells. For all these reasons, it is possible to use an asymmetrical 
structure having a deep well P.sub.1 and a less deep well P.sub.2. 
As an example, we may indicate the parameters of a structure that can be 
used in a modulator according to the invention. This structure comprises: 
a Ga.sub.0.75 Al.sub.0.25 As "barrier" material with a thickness of 30 nm 
and a potential V.sub.0 =242 meV; 
a well P.sub.1 made of a GaAs material with a thickness nm and a potential 
V.sub.1 =0 V; 
a coupling barrier made of Ga.sub.0.75 Al.sub.0.25 As material with a 
thickness of 2.5 nm and a potential V.sub.b =242 meV; 
a well P.sub.2 made of Ga.sub.0.92 Al.sub.0.08 As material with a thickness 
of 4.5 nm and a potential V.sub.2 =242 meV. 
The band diagram of this structure is shown in FIG. 5. 
This structure has parameters such that they enable coupling between the 
two wells and, consequently, the appearance of the states .vertline.L&gt; and 
.vertline.A&gt;. The photon energies corresponding to the inter-subband 
transitions .vertline.0------&gt;.vertline.A&gt; and 
.vertline.0------&gt;.vertline.L&gt; and their intensities are shown in FIG. 6 
which shows the absorption spectrum at zero voltage. The absorption peaks 
appear clearly and are at a distance of about 25 meV from each other. 
FIGS. 7 gives a schematic view, by means of the diameter of the circles, of 
the amplitude of absorption of an electromagnetic wave. The upper curve 7a 
illustrates the variation of the intensity of absorption related to the 
transition .vertline.0------&gt;.vertline.L&gt; as a function of the electrical 
field applied to the structure. The lower curve 7b illustrates the 
variation of the intensity of absorption related to the transition 
.vertline.0------&gt;.vertline.A&gt;. It can clearly be seen that, for the 
positive fields, the transitions .vertline.0------&gt;.vertline.L&gt; 
predominates while the transition .vertline.0------&gt;.vertline.A&gt; gets 
attenuated, all the more so as the field increases. For negative 
electrical fields applied to the structure, the phenomenon is reversed. 
FIG. 8 shows the absorption spectra of the structure placed under voltage 
of +12 V (curve 8a) and under -12 V (curve 8b); the curve 8c illustrates 
the ratio of the foregoing curves. It can clearly be seen that one of the 
two peaks gets intensified at the expense of the other one. An examination 
of the ratio between the two spectra indicates that the total absorption 
is unchanged. For a control voltage of 24 V, 17% of depth of modulation at 
10.0 .mu.m is obtained. When the anti-crossing of the levels .vertline.A&gt; 
and .vertline.L&gt;.DELTA.E is the minimum, the modulation of transmission at 
9.6 .mu.m (resonance of the transition 0------&gt;A) is 0.5% per volt applied 
to the structure. It is seen that this modulator can function between 9 
and 14 .mu.m. It is therefore a wideband device. The modulation can 
furthermore be obtained very swiftly, within the time taken to apply an 
electrical field, giving a period of time of the order of one picosecond, 
hence more swiftly than with modulators necessitating charge transfer 
phenomena.