Quantum well optical device

A quantum well optical device including a layer of semiconducting material of large forbidden band width and, situated in this layer, at least one quantum well, such as a quantum line or quantum hole, of a material having a narrower forbidden band than that of the layer. This quantum well has two permitted electron levels. Means exist of assuring the electron population of the first permitted energy level.

BACKGROUND OF THE INVENTION 
The invention concerns a quantum well optical device, in particular a 
device including quantum boxes or quantum wires applicable to optical and 
opto-electronic techniques, and methods of constructing this device. 
In the field of long wavelength electromagnetic waves, and in particular 
wavelengths exceeding 2 .mu.m, few optical and opto-electronic devices 
have been made from semiconductors. The reason is that there are few 
energy levels which are not strongly coupled to excitations of the 
structure. The only exceptions are semiconductors having a narrow 
forbidden band. 
We propose here a new family of optical and opto-electronic devices in 
which energy levels are determined freely by the dimension of quantum 
holes and lines constituted of semiconducting materials inserted in 
matrices of other semiconductors. 
The quantification of the energy levels of electrons and holes in 
ultra-thin semiconductor films (.gtoreq.20 nm) is now well known (see the 
article "Fundamental Properties of III-V Semiconductor Two-Dimensional 
Quantized Structures: The Basis for Optical and Electronic Device 
Applications" by C. Weisbach in Semiconductors and Semimetals, Vol 24, R. 
Dingle Ed., Academic, New York 1987). This is shown in FIG. 1a in which an 
ultra-thin layer of semiconductor SC2 is placed between two semiconductors 
SC1 and SC3. The structure of the energy levels for the specially chosen 
semiconductors is shown in FIG. 1b; the quantified energy levels for the 
electrons (E.sub.1, E.sub.2) and the holes (E'.sub.1, E'.sub.2) in layer 
SC2 are also shown. 
If we take the approximation of an infinitely deep potential well, the 
successive quantified energy levels are situated at a so-called 
confinement energy lying above the bottom of the conduction band and given 
by the formula: 
EQU E.sub.n =n.sup.2 .pi.2h.sup.2 /2m*L.sup.2 
where 
n is the order of the level, 
h is the Planck constant, 
m* is the effective mass of the particle (electron or hole), 
L is the thickness of the film SC2. 
We see that we can control freely the difference between the energy levels 
by the thickness of the film. This possibility is used in detectors where 
the range of sensitivity in wavelength is adjusted at the transistion 
E1-E2 by the film thickness. 
If the thickness is sufficiently small there is only a single energy level 
within the well, the level E2 being situated above .DELTA.Ec in the 
continuum of the states of materials SC1 and SC3. 
The extension to three dimensions of the concept of quantification of the 
energy levels is immediate. It is sufficient to consider a semiconductor 
box inserted in another semiconductor: the wave function of the electrons 
and holes will be quantified in the box if, as we explained in connection 
with FIG. 1b, the electron and hole energy levels are lower in the 
material constituting the box compared with the surrounding material (see 
FIG. 2). 
What is unique about this system is that, in the infinite well 
approximation, the successive energy levels are given by the formula: 
EQU E.sub.nx,ny,nz =.pi..sup.2 h.sup.-2 /2m* (nx.sup.2 /Lx.sup.2 +ny.sup.2 
/Ly.sup.2 +nz.sup.2 /Lz.sup.2) 
where positive integers nx, ny and nz are the orders of the levels along 
the x, y and z axes, 
Lx, Ly and Lz are the dimensions of the box along the x, y and z axes 
The energy levels E.sub.nx,ny,nz can be separated by an energy which is 
either incommensurable with any phonon of the crystal, or greater than 
that of any phonon of the crystal. In this case, energy relaxation by 
transitions producing a single phonon in the crystal can not occur and the 
lifetime of the excited states becomes extremely long, of the order of the 
radiative lifetime. This situation is different from that in quantum films 
where the kinetic energy of the electrons along the film gives continuous 
energy levels of the type: 
EQU E.sub.nzk =nz.sup.2 .pi..sup.2 h.sup.2 /2m*Lz.sup.2 +h.sup.2 k.sup.2 /2m* 
where k is the wave vector of the electrons describing their free 
transmission in the plane. 
Owing to the continuous energy levels, there are always energy levels 
between which transitions can be induced by phonons of the structure, as 
shown in FIG. 3. 
The invention makes use of this situation to allow the construction of 
optical devices including quantum wells of limited dimensions. 
SUMMARY OF THE INVENTION 
The invention concerns a quantum well optical device characterized by the 
fact that it includes: 
A layer of semiconductor material transparent to light waves having a given 
potential energy corresponding to the bottom of the conduction band for 
the electrons; 
At least one quantum box limited in two dimensions (quantum line) or in 
three dimensions (quantum hole or quantum point), the material 
constituting this quantum well having a potential energy value at the 
bottom of the conduction band which is less than that of the layer of 
transparent semiconductor material, and possessing at least a first atomic 
energy level (ground state) and a second allowed level (excited level) 
whose corresponding energies lie between the potential energies of the 
bottoms of the preceding conduction bands; 
Means of assuring the electron population of the first allowed energy 
level. 
According to the invention, a number of wells of limited dimensions 
(quantum boxes or wires) may be involved. 
The invention can be applied in the construction of a laser. The quantum 
well optical device is therefore characterized by the fact that: 
The layer is oriented with its principal faces parallel to the xy plane of 
an xyz trihedron; the quantum holes are oriented in the x direction; 
The faces of the layer perpendicular to the x axis are cleaved or treated 
optically to form an optical cavity; 
An auxiliary source supplies a pump wave approximately perpendicular to the 
principal faces. 
We also have an optical device characterized by the fact that the layer of 
transparent semiconductor material has two principal faces parallel to an 
xy plane of an xyz trihedron, each equipped with an electrode of the same 
conductivity type, a voltage generator being connected to these electrodes 
to apply a potential difference across the two faces of the structure. The 
faces of the layer perpendicular to the x axis are cleaved or treated 
optically to form an optical cavity. 
The invention also concerns a detector, such as an optical device 
characterized by the fact that it includes electrodes of the same 
conductivity type on the two sides of the layer of transparent 
semiconductor material and a detector of potential difference connected to 
these electrodes. 
The invention also concerns a light modulator, such as an optical device 
characterized by the fact that it includes electrodes of the same 
conductivity type on the two sides of the layer of transparent 
semiconductor material and a source applying a potential difference to the 
electrodes, a light beam being transmitted in the layer and being 
modulated by the device. 
Finally, the invention concerns a method of construction of an optical 
device characterized by the fact that it includes: 
A first stage of construction involving the deposit on a substrate of 
alternated layers of two materials having forbidden bands of different 
width; 
A second step involving etching of individual parts in these layers.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1a, 1b, 2 and 3 show structural and electronic properties of quantum 
layers in quantum boxes, as discussed in the background section of the 
application. 
A first embodiment of the invention concerns a laser using transitions 
between two energy levels E111 and E112 (denoted E1 and E2 in the text 
below) of a quantum hole. 
FIGS. 4a and 4b are energy diagrams of a quantum hole according to the 
invention. 
FIG. 4a represents the conduction band of such a quantum hole and its 
intra-band operation. 
At the bottom of FIG. 4a a quantum hole 2 is shown incorporated in a 
material 1. 
As we can see in FIG. 4a, the energy corresponding to the bottom of the 
conduction band of the quantum hole 2 is lower than the bottom of the 
conduction band of material 1. 
The dimensions of hole 2 are such that two allowed quantum energy levels 
are included in the quantum hole, i.e. between the energies of the bottom 
of the conduction bands of the quantum hole 2 and the material 1. 
Inversion of the population can occur between levels E.sub.2 and E.sub.1 by 
the selective capture of electrons in the level E.sub.2 from continuum 
states of the material forming the crystal matrix (material 2), and these 
electrons will slowly relax to level E.sub.1. 
The excitation to the continuum of the states of the matrix can be provoked 
in a different way depending on the implementation chosen. 
Thus this excitation can occur with the aid of a wave of an optical pump FP 
whose energy h.nu. is greater than, or at least equal to, the difference 
E.sub.2 -E.sub.1. 
If the quantum hole 1 contains electrons by doping of one of the materials 
(1 or 2), these electrons can therefore be excited to the continuum states 
by optical pumping using any suitable light source. The electrons will 
relax rapidly to level 2 by emission of phonons (see FIG. 4a). 
FIG. 4b represents the valence band (hole levels) and the conduction band 
(electron levels) of the device shown at the bottom of FIG. 4b. 
One can also excite electrons from valence states by interband transition, 
creating holes in the quantum hole and/or in the material of the 
continuum. These holes will allow the recombination of electrons present 
in the ground state E.sub.1 of the quantum holes, and therefore lead to a 
population inversion as soon as an electron is captured in the state 
E.sub.2 of a quantum hole thus emptied (FIG. 4b). If there are no 
electrons initially in the quantum hole, inversion occurs following 
capture in the state E.sub.2. 
To achieve this, the structure is pumped using a wave FP of energy h.nu. 
equal to or greater than: 
EQU EG+E'+1.sym.EC 
FIG. 5 shows an example of a laser pumped by a diode laser according to the 
invention. 
This laser is made of semiconducting material transparent at the wavelength 
of a pump wave. It includes a layer 1 of semiconducting material in the 
form of a trihedron xyz with the principal faces lying in the xy plane. 
The layer 1 includes quantum lines 20.l to 20.m parallel to the x 
direction. These lines are spaced such that there is no electronic 
coupling between them. 
The material of the quantum lines is chosen such that the energy of the 
bottom of the condition band is less than that of the material of layer 1. 
The faces 12 and 13 are cleaved or treated to be reflecting at the emission 
frequency of the laser and constitute an optical cavity. 
The orientation of the quantum lines relative to the faces 12 and 13 is 
such that they are perpendicular to the planes of faces 12 and 13. 
The luminous emission FE therefore occurs in a direction perpendicular to 
faces 12 and 13. 
According to a preferred embodiment of the invention, the quantum wires 
20.1 to 20m lie parallel to faces 10 and 11. The beam of the pump FP 
emitted by a diode laser 9 is received perpendicularly to the plane of 
these quantum lines. 
Several parallel planes of quantum wires can be used. In FIG. 5, two planes 
of wires 20.1 to 20.m and 20.n to 20.p are shown, but other planes could 
be included. 
In addition, according to a preferred embodiment of the invention, the 
quantum wires 20.n to 20.p and 20.1 to 20.m are staggered relative to the 
direction of the pump wave FP, which allows excitation of a maximum number 
of the quantum wires. 
FIG. 6 shows a laser, according to the invention, in which the layer of 
semiconducting material includes quantum boxes. The example shown in this 
figure is derived from that in FIG. 5 by replacing the quantum wires such 
as 20.1 by rows of quantum boxes such as 2b.1 to 2b.t. 
Another version of the invention concerns an electrically excited laser. 
For this, as shown in FIG. 7, a structure includes lines of quantum boxes, 
such as 2b.1 to 2b.s and 2b.2 to 2b.t, in a layer 1 of semiconducting 
material. These lines of boxes lie between the electrodes 4 and 5, and 
approximately perpendicular to them. 
The electrodes 4 and 5 have the same type of conductivity. 
As shown in FIG. 8, the lateral faces 12 and 13 of the device are cleaved 
or treated to form mirrors and thus form an optical cavity. 
FIG. 9 shows an energy diagram of this laser when a potential difference is 
applied by the generator across the electrodes 4 and 5. 
The lines of quantum holes are sufficiently close to each other to 
communicate by the tunnel effect. 
FIG. 9 shows a possibility of transfer where electrons communicate from 
hole to hole via the resonant tunnel effect of the ground state E.sub.1 of 
one and the excited state E.sub.2 of the other. Since the probability of 
excitation from E.sub.2 to the continuum or another state E.sub.2 is much 
greater than for excitations to the E.sub.1 level, a population inversion 
is achieved. 
A variant of this device is indicated in FIG. 10 where electrons are 
injected by tunnel effect from a wide level, such as a wide quantum well 
Q1, which allows collection of a high electron density from the injecting 
electrode before their efficient tunnel transfer to the quantum holes, as 
shown in the energy diagram of FIG. 11. We can also note that if the 
thermionic emission of the electrons in the holes is more rapid than the 
relaxation from level 2 to level 1, the electronic transport arising from 
the thermionic emission also leads to population inversion. 
The invention can also be applied to the construction of an optical 
modulator. 
For this, using known techniques, materials having the highest possible 
electro-optic coefficient are used, which allows very small devices to be 
made and/or control by very small potential differences. 
According to the invention, a modulator is made using quantum lines or 
quantum holes such as the modulator shown in FIG. 12. These lines or holes 
are made from semiconducting material such that two or three of their 
dimensions are of the order of the de Broglie wavelength of the electrons 
(about 20 nm). 
The modulator shown in FIG. 12 includes a substrate S, either with n.sup.+ 
doping or semi-insulating, covered by a doped layer, a layer 1 of 
semiconducting material which is transparent at the wavelength to be 
modulated. 
The layer 1 contains quantum wires 2.1 to 2.2' arranged parallel to the 
plane of layer 1 and orthogonal to the direction of the light beam to be 
modulated. The electrodes 4 and 5, which have the same type of 
conductivity, are situated on each side of the layer 1 and a source of 
electrical tension G is connected across them. The light flux to be 
modulated arrives parallel to the plane of the layer (beam FM), or 
perpendicular to it (beam FM') in which case the electrode 5 is 
transparent. 
To improve the modulation, the layer 1 can be supplemented by optical guide 
layers G1 and G2. 
The expected gain of such structures is due to a number of properties: 
The density of the quantum states characteristic of one- or 
zero-dimensional systems (FIGS. 13a, 13b). Consequently, any change of 
level induced by a field has a large effect on the absorption spectrum 
(FIGS. 13c, 13d); 
The energy levels of the zero-dimensional system (quantum hole) are 
discrete, therefore the energy relaxation effects are very much slowed 
down, or even prevented for any phonon transition as soon as the distance 
between the quantum levels exceeds the optical phonon energy (36 meV for 
GaAs); 
It can be advantageous to use systems which are asymmetric (FIG. 21) or 
allowing charge transfer such as coupled quantum lines or holes (FIGS. 
22a, 22b) since the charge transfer occurs over a larger distance than in 
simple systems, which produces electrical dipoles--and therefore 
polarizations--which are larger when an electrical field is applied. 
This effect can be used when the invention is used in a bistable optical or 
electro-optical device, analogous to the devices made from quantum films 
operating by interband transitions (see for example the document "Quantum 
wells for optical information processing", by D. A. B. Miller, Optical 
Engineering 26-368, 1987). 
The modulator can be used to modulate amplitude (thanks to the absorption 
change produced by the electric field) or phase (thanks to the change of 
index associated with the absorption by the Kramers-Kronig relations). 
FIGS. 14 and 15 show a variant of the invention in which the quantum wells 
(lines or holes) (example given: quantum holes) are made in the form of 
structures of asymmetric quantum wells having enhanced non-linear optical 
properties. These properties have been described in the following 
documents: 
E. Rosencher, P. Bois, J. Nagle, E. Costard and S. Delaitre, Applied 
Physics Letter 25, 1150 (1989); 
E. Rosencher, P. Bois, J. Nagle, E. Costard and S. Delaitre, Electronic 
Letter 25, 1063 (1989). 
These properties can be used for the detection and optical processing of 
electromagnetic radiation. In these asymmetric quantum wells, in addition 
to the asymmetric potential along the growth axis, a double lateral 
confinement is created. This allows literal asymmetric "superatoms" to be 
obtained in a single direction, with completely discrete energy levels 
whose spacing can be adjusted by the growth parameters, the choice of 
materials and the dimensions L.sub.x, L.sub.y and L.sub.z of the quantum 
hole. 
As shown in FIGS. 14 and 15, each quantum hole is made from a material 2 
and a material 3 such that the structure has the energy profile in the 
conduction band shown in FIG. 16. FIG. 16 shows an energy level diagram 
for layers 2 and 3 as a function of position across the two layers. This 
figure shows energy level E.sub.1 confined to the second layer and energy 
level E.sub.2 confined to the quantum well defined by both layers. 
Material 2 has the lower energy at the bottom of the conduction band 
whereas material 1, with a higher energy, constitutes the barriers of the 
well. 
The quantum well (hole) has a first allowed level E.sub.1 situated in the 
narrowest part of the well and a second allowed level E2 in the widest 
part. 
When illuminated by electromagnetic radiation of a given excitation 
wavelength, the electron in the ground state E.sub.1 of the box is excited 
to level E.sub.2, provided the energy difference corresponds is 
approximately to the wavelength of the photon. These two levels have 
distinct barycenters along the growth axis and an electric dipole will 
develop in the structure, introducing strongly non-linear properties. The 
lifetime of this dipole, to which the extent of the observed effects is 
associated, is equal to the lifetime of the excited state. In simple 
asymmetric wells the lifetime is short (about 1 ps) since the 2D states 
are not quantized in other intermediate directions and only the radiative 
recombination to the ground state is possible. The lifetime in the excited 
state will be longer, about 10 ns, and the non-linear coefficients are 5 
orders of magnitude larger. 
The device can be used for the detection of electromagnetic radiation. In 
this case these dipoles will be detected at the terminals of the device 
containing such quantum holes. The device is fitted with electrodes, of 
the same conductivity type, to which a detector is attached. 
Other non-linear properties of such a structure can also be used, possibly 
with the addition of a third level (which can be virtual) for processes 
involving several photons such as the generation of second harmonics, 
heterodyning or parametric amplification. 
These quantum holes can be made by pairs of holes each constituting a 
dipole with the same applications as for asymmetric boxes. 
This device can be used for the detection of electromagnetic radiation. In 
this case the devices will operate in an optimal way up to wavelengths of 
40 .mu.m. 
Another interest of quantum holes compared with quantum films is the 
following. 
To couple the incident light and the dipole, the light must imperatively be 
polarized parallel to the growth axis in the case of quantum films, i.e. 
the illumination must be at Brewster's angle to maximize the coupling, and 
even in this case it is only 1%. 
In the case of boxes, the excited level is in fact a threefold degenerated 
level. Therefore if the difference between the levels E.sub.1x, E.sub.1y 
and E.sub.1z is less than 36 meV, i.e. the energy of a longitudinal 
optical phonon in GaAs, all polarizations of the incident light will allow 
optical excitation and in all cases some of the electrons will relax to 
the E.sub.1z level by diffusion of acoustic phonons or impurities and the 
dipole will effectively be formed along the z axis. 
Finally, as another application, the structure of the quantum holes 
described here can be used as a device for the detection of 
electromagnetic radiation by photoelectric current. FIG. 17 shows such a 
device: on a doped substrate a buffer layer of the same doped material is 
grown. This is followed by several layers of the quantum hole structure, 
then a doped layer of the material of narrow forbidden band. Using 
standard microlithographic techniques a mesa is formed and the substrate 
is fitted with an ohmic contact. The upper layer is fitted with an ohmic 
contact transparent to the radiation to be detected. These two contacts 
have the same conductivity type. The operation of the device is as 
follows. An electrical tension is applied across the contacts; the photons 
of a given wavelength excite the electrons initially in the lowest level 
of the quantum holes: 
Either to a higher level of the well if the energy difference between the 
excited level and the fundamental level is approximately equal to the 
excitation wavelength; 
Or above the conduction band of the barrier material; the electrons then 
leave the well by tunneling under the influence of the electric field 
created by the applied voltage, which creates a current measurable in the 
external circuit. 
Compared with classical techniques using two-dimensional quantum wells, the 
advantage of the proposed structure is: 
The electrons can be easily excited and the increased localization of the 
elections in the two-dimensional quantum well considerably increases the 
coupling and therefore the absorption; 
The probability of recapture of an ejected electron in another well is 
reduced because the interaction with the vibrations of the crystal 
structure do not allow in first order such a capture in the case of a 
completely discrete level of the well. 
The construction of quantum holes or lines can be achieved in several ways. 
According to the procedure shown in FIG. 18a and 18d, on a substrate S 
alternate layers of the materials corresponding to the material of layer 1 
and the quantum wells in the preceding description are deposited (FIG. 
18a). 
Next, parts of the dimensions of the quantum holes (or lines) are etched by 
masking (see FIG. 18b). It is possible to engrave parts as small as 
80.times.80 nm. From an electronic point of view, such geometrical 
sections correspond to quantum holes of 20 to 30 nm. 
Finally, a deposit of material identical to layer 1 is made (FIG. 18c) and, 
if necessary, the structure is planarized (FIG. 18d). 
According to another procedure shown in FIGS. 19a and 19b, the parts G1 are 
made by etching a substrate (FIG. 19a). 
Next, a selective growth (FIG. 19b) is made on the this etched substrate 
which creates anisotropies in two different ways: in the direction of 
growth, owing to the directionality of the growth; in a perpendicular 
direction to the growth by etching asymmetric forms. This growth procedure 
is described in an article by Y. D. Galeuchet, P. Roentgen and V. Graf, 
Applied Physics Letters, 53, 2628 (1988). The structure is then planed?? 
if necessary as in the procedure described previously. 
Finally, according to the procedure shown in FIG. 20, quantum holes can 
also be made by three-dimensional nucleation on a plane interface I. The 
nucleation procedure is described in an article by L. Goldstein, F. Glas, 
J. Y. Marzin, M. N. Charasse and G. Le Roux, Applied Physics Letters, 47, 
1099 (1985). 
This method allows asymmetries to be obtained easily in the direction of 
growth by varying the fluxes during growth. Coupled quantum holes are 
obtained if the distance between holes is sufficiently small to allow 
delocalization of wave functions between two adjacent quantum holes. After 
having made the quantum holes such as 2.1 in a surface I, a deposit of 
material identical to that of the substrate is made. Another surface II is 
obtained (shown as a dashed line) on which other quantum holes can be made 
such as 2.2, and so on. 
For simplicity, we have represented the quantum holes by cubes everywhere 
in the diagrams. All forms are acceptable as long as the characteristic 
dimensions are of the order of, or less than, the electron de Broglie 
wavelength. 
The active materials can be of very varied nature, for example the families 
IV, III-V, II-VI or IV-VI. It is not indispensable that the localization 
of the holes takes place in the boxes (or the wires) in which the 
electrons are quantified. However the boxes can be localized in the 
material around the boxes. This depends on the materials used. This can be 
achieved with a pair of materials GaSb/InAs. 
The preceding description concerns structures in which the active particles 
are electrons, but similar structures can be conceived in which the active 
particles are holes by using type P doping instead of type N. 
It is clear that the preceding description is a non-restrictive example and 
that other variants can be envisaged within the framework of the 
invention. The numerical examples and the natures of the materials and the 
methods of construction are given only to illustrate the description.