CA1239466A - Optical modulation having semiconductor quantum well structures - Google Patents

Abstract

Abstract:

A semiconductor apparatus is provided. The apparatus includes a multiple layer heterostructure having a first material layer, a second material layer and a semiconductor layer positioned between the first and second material layers. Comparatively, the bottom of the conduction band of the semiconductor layer lies below the bottom of the conduction band for each of the material layers. Also, the top of the valence band of the semi-conductor layer lies above the top of the valence band for each of the material layers. The semiconductor layer is controlled to have sufficient thickness for achieving carrier confinement effects within the semiconductor layer to influence the optical properties of the material layer heterostructure. The semiconductor apparatus also includes components for applying an electric field to the multiple layer heterostructure and varying an optical absorption coefficient and an index of refraction for the multiple layer heterostructure in response to the applied electric field. The apparatus is adapted for use in the following applications: an optical absorption modulator, an optical phase modulator, and electrically tuned Fabry-Perot cavity, a polarization modulator, and a nonlinear or bistable apparatus in which the operating point is varied in response to a changing applied electric field.

,

Description

background of the Invention This invention relates to semiconductor devices for controlling light such as optical modulators and nonlinear optical devices.
Conventional semiconductor optical modulators have traditionally made use of the Franæ~Keldysh effect to modulate an incident light beam. According to the Franz-Keldysh effect, the band structure of the semiconductor material is shifted by application of an electric field however, the Franz-Keldysh effect is characterized by small shifts in optical properties such as absorption and index of refraction thereby limiting the modulation to be correspondingly small In order to achieve the deep modulation required for a practical device it is necessary to use either a high electric field or a device naming long optical path length or both. For example, a modulator described by Honda in V. S. Patent No. 3,791,717 issued February 12, 1g74 uses a high electric field l105 to 106 volts per centimeter) in a semiconductor having a crystal of long optical path length (10 microns.
An optical modulator which uses a heterojunction semiconductor device in which optical absorption and no-emission are controlled by an electric field is described by Clang et at in I. S. Patent 4,208,667. Clang et at uses a heterojunction super lattice having two different material arrangements alternatively to form a semiconductor heterojunction in which the bottom of the conduction band of a first layer is lower than the conduction band in the second layer, and also the top of the valence band is lower than the corresponding band in the second layer. This device and super lattice allows electrons and holes to be excited by photo absorption wherein the electrons collect in one layer and holes in the adjacent layer. Charge carriers recombine by making a transition into the adjacent layer with the subsequent emission of light. The rate of recomb bination is controlled by an electric field applied to the super lattice. It is a property of the Clang et. at. device that the emitted light is of a different frequency from the incident light and that the light is emitted in all direct lions. It should also be noted that the incident and emitted light beams are not collinear through the devices.
As a result, the Clang et at device is impractical as an optical modulator.
Nonlinear optical devices have been made using heterojunction semiconductor materials. These devices are characterized by an operating point determined by optical cavity gain or finesse. These nonlinear optical devices exhibit particular aspects such as bistability, amplifica-lion, photonic modulation or the like. A problem with this type of nonlinear optical device is that the operating point is selected by the choice of materials and other design parameters during fabrication of the device. There fore, the operating point cannot be conveniently controlled at the time the device is in use.
Summary of toe Invention In accordance with an aspect of the invention there is provided a semiconductor apparatus comprising: a multiple layer heterostructure having first and second material layers having first and second band gaps, respect lively, and a semiconductor layer having a third band gap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semi-conductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure;
and means for applying an electric field to said multiple ' 'I, 6`
So - pa -layer heterostructure in order to vary an optical absorb-lion coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field wherein said means for applying an electric field to said multiple layer heterostructure comprises: a p doped semiconductor layer and; a n doped semiconductor layer and said semiconductor layer having a third band gap is located between said p doped semiconductor layer and said n doped semiconductor layer.
In accordance with the present invention, the foregoing problems are solved by employing a semiconductor apparatus comprising a multiple layer heterostructure and means for applying an electric field to the multiple layer heterostructure in order to vary optical absorption Coffey-clients and an index refraction of the multiple layer heterostructure in response to the electric field. The multiple layer heterostructure includes first and second material layers having first and second band gaps, respect lively, and a semiconductor layer having a third band gap and being positioned between the wider band gap layers. The bottom of the conduction band of the first semiconductor layer is below the bottom of the conduction bands of the first and second material layers. The top of the valence band of the first semiconductor layer is above the top of the valence band of the first and second Jo I

material layers The thickness of the first semiconductor layer is 1000 Angstroms or less. The semiconductor apparatus is adapted for use as an optical absorption modulator, or an optical phase modulator or an optical polarization modulator. By interposing the apparatus between mirrors, the resulting apparatus is an electrically tuned Fairy Pert cavity. The apparatus is also adapted for other nonlinear or bistable applications in which modification of the optical properties of a semiconductor by application of an electric field is useful. In one embodiment the electric field is conveniently applied by fabricating the multiple layer heterostructure as the intrinsic region of a PIN semiconductor structure.
iffy Description of the Drawing FIG. 1 is a cross-sectional drawing of a MOW with an electric potential applied perpendicular to the layer planes;
FIG. 2 is a eross-sectional drawing of a MOW with an electric field applied substantially parallel to the layer planes;
FAKE. 3 is representative optical absorption for different applied electric fields for a MOW;
FIG. 4 is a graph showing the shift in excitor peak with applied voltage;
FIG. 5 is a graph showing variation in optical absorption with applied voltage for a MOW;
FIG. 6 is a graph showing operation of an absorption modulator;
FOG. 7 is a cross section of a MOW structure;
FIG. 3 is an energy band diagram for a semiconductor, FIG 9 is an energy band diagram for a heterojunction;
FIG. 10 is an energy band diagram for a MOW
structure;
FIG. 11 is a graph showing saturation of optical absorption versus intensity of illumination for a MOW

I

I,, structure;
FIG. 12 is a graph showing the relationship between optical absorption and index of refraction as predicted by the Kramers-Rronig relationship as the optical absorption saturates under a high intensity;
FIG 13 is a cross-sectional drawing showing an alternate arrangement of electrodes for applying an electric field to a MOW structure;
FIG. 14 is a top view showing an alternate arrangement of electrodes for applying an electric field to a MOW structure;
FUGUE is a perspective drawing showing an alternate arrangement of electrodes for applying an electric field to a MOW structure;
FIG. 16 is a cross-sectional drawing showing an alternate method of attaching contacts to a MOW structure;
FIG. 17 is a cross-sectional Figure showing a PIN
semiconductor structure for applying an electric field to a MOW;
FIG. 18 is an alternate arrangement of a PIN
structure for applying an electric field to a MOW
structure;
FIG. 19 is an alternate PIN structure for applying an electric field to a MOW;
FIG. 20 is a graph giving optical transmission versus photon energy for various voltages applied to a MOW
using a PIN structure;
FIG. 21 is a cross section showing a controlled Fabry-Perot cavity;
FIG. 22 is a graph giving the optical transmission of a controlled Fabry-Perot cavity versus applied voltage;
FIG. 23 is a cross sectional view showing a controlled Fabry-Perot cavity;
FIG. 24 is a cross sectional view showing a polarization modulator and FIG. 25 shows an array of electric field controlled MOW devices.

I

Description of the Preferred Embodiment In FIG. 1 there is shown an exemplary light modulator 100. A source snot shown) produces unmodulated light 110. The source may be, for example, a laser. Also, for example, the source may be a light emitting diode.
Other light sources such as light transmitted by an optical fiber may conveniently be employed to produce unmodulated input light 110. Unmodulated input light 110 passes through multiple quantum well (MOW) structure 120, after which it is output modulated light 130. A MOW structure 120 includes both a structure with one quantum well or structures with many quantum wells. First electrical contact 140 and second electrical contact 150 are connected by electrical conductors 152, 154 to source 156 of electric potential. An electric field is applied to MOW structure 120 by the potential provided by source 156 through electrical contacts 140 and 150. Source 156 may, for example, be a direct current source, such a a battery.
Or, for example, source 156 may be a klystron oscillator operating at a multiple gigahertz frequency. For example, source 156 may provide electric potential in the 100 gigahertz frequency range. Application of an electric field to MOW structure 120 cause the optical transmission of MOW structure 120 to vary with the applied potential The optical transmission of MOW 120 may either increase or decrease with applied electric field, depending upon the light frequency at which the transmission is considered, as will be more fully explained hereinafter. In the alternative embodiment shown in FIG. 1, electrical contacts 140, 150 are attached to transparent or partially transparent electrodes 160, 162 which, or example, can be semiconductor layers doped to be conductive, or, for example, can be thin metallic layers. Contacts 140, 150 are attached so that an electrical potential applied between them is conducted by electrodes 160~ 162 and therefore produces an electric field which is substantially perpendicular to layer planes 164 of MOW 120. For I

example, electrodes made of Agues are satisfactory for a MOW 120 made of Agues and Gays layers. For example, electrodes 160, 162 may be made of Al/ Or, A, A or alloys of these metals or combinations of layers of these metals, and with thicknesses between 0.05 micron and 0.1 micron.
Also for example, indium tin oxide electrodes may be used.
In an embodiment which uses partially transparent electrodes, it is convenient to choose their thickness to achieve anti reflection properties. Such a choice is given by the equation T = on (up - I
where T is the electrode thickness, n is the index of refraction of the electrodes, is the wavelength of the light being modulated, and p is an integer, p = 1, 2, 3,....etc. For example for electrodes made of indium tin oxide which has an index of refraction n of approximately 1.8, and the use of GaAs/A1GaAs MOW structure operating at = 0.85 micron, the thickness is given as:
P T
micron 1 0.12

2 0.36 The use of the thicker, 0.36 micron layer is more convenient as the thicker layer provides lower contact resistance.
In FIG. 2 the MOW modulator 170 used in an exemplary embodiment was fabricated by molecular beam epitaxy on a Gays substrate (not shown and which was removed during fabrication. A 1.45 em thick Alto guy assay cap layer 172 is attached to 65 periods of alternate AYE Gays and AYE
Aye Gwen assay layers to form the 1.26 em thick MOW 174, and capped by a further 1.45 em thick Aye guy assay layer 176. The cap layers 172, 176 are transparent in the MOW band gap region. All the I

materials were unhoped with residual carrier concentration less than ~1015cm 3. A 3 x 3mm2 sample was leaved along the [110] and ~110] directions and glued by the epitaxial cap layer 172~ with a transparent epoxy (not shown) to a sapphire support 180. The entire Gays substrate (not shown) was selectively etched off Electrical contacts 182, 184 owe 100~ Or 186 followed by AYE A 188 were evaporated on the sample to give an electrode spacing 190 d = 300~m. A ow tunable oxazine 750 dye laser (not shown) was used as a light.
source to measure the absorption spectra of MOW structure 170. The light beam 192 was focused to a Siam diameter spot 194 at the center of the interlectrode gap, with a polarization parallel to the electric field. The transmission, as corrected for surface reflection, was measured as a function of the laser frequency for voltages applied between electrical contacts 182 and 184, with the voltage varying from 0 to 650V. The laser power was kept as low as YO-YO to avoid carrier generation The current passing through the sample between electrodes 182 and 184 was 10~ at 150V.
In FIG. 3 there are shown exemplary curves of the absorption coefficient spectra for 0 volt, 400 volts, and 600 volts, as marked. The curves for 400 volts and 600 volts are shifted vertically upward to avoid crossing in FIG. 3. Shifts of the heavy hole excitor shown as peak 200 A at 0 volts, to 200 B at 400 volts, and to 200 C at 600 volts are evident. Also peak 200 A, 200 B, 200 C show a broadening as the voltage is increased. The light hole 30 pea 210 A at 0 volt, 210 B at 400 volts, and 210 C at 600 volts show both a shift and a broadening. The shifts are difficult to measure up to V-200V because they are small compared to the line width; above this value they show a super linear dependence on V.
In FIG. 4 the shift of the heavy hole peak ~200 A, 2G0 B, 200 C in FIG. I is shown as curve 212. Roy shift of the light hole peak (210 A, 210 B, 210 C in FIX. 3) is ~3~3~6~

shown as curve 214~
The light hole-exciton peak was found to shift more than the heavy hole one. The line width could be measured only for the heavy hole-exciton; it was taken as the half width at half-maximum on the low energy side of the peak. The width variation is monotonic and approximately linear with the applied field; varying from 2~8 me at applied voltage = OVA to 4.3 me at applied voltage =
600V.
The precise value of the field applied to the MCKEE
is difficult to evaluate because of possible space charge effects and contact resistances. Assuming that the sample behaves like a simple resistor, at the center of the inter electrode spacing one can take E = yV/d, where the correction factor for the present exemplary geometry is yo-yo, as discussed more fully by M. Cordon in the article "Electric Field Modulation," published in Solid State Physics 11, Academic Press, New York, pages 165-275, 196g.
I the absorption spectra shown in FIG. 3 for 400 volts corresponds therefore to E~104V/cm, and for 600 volts therefore corresponds to Ea1.6x104V/cm. To evaluate the winding energy of the excitors in MOW AYE Y
thick, we use the experimental and theoretical results of R. C. Miller et. at. as disclosed more fully in their article "Observation of the Excited Level of Excitors in Gays Quantum Wells," published in Review, Volt B24, p. 1134, in July 1981. Miller et. at. give En (Huh e) = 9 me and Exile = t0.5 me. The radius can be calculated by the relation; axe constant, which gives axe and axle AYE. The corresponding ionization fields, HI 1.4 x 104V~cm and HI = 1-9 x 104V/cm~ are quite consistent with our measurements. Note that any simple perturbation analysis of our results is invalid because of the high value of the fields, relative to HI, which we utilize. The ~.~23~
_ g observation of the larger binding energy of the light hole excitor and its greater sensitivity to static fields should be interpreted with care, as additional complication may occur due to its proximity to the heavy hole inter band continuum.
Referring to FIG. 5, the variations of the absorption coefficient are shown as a function of the applied potential at the heavy hole-exciton peak in curve 220, and 5 me below in curve 222. Positive or negative changes larger than ~a~+4x103cm 1 are obtained in the absorption at given photon energies as the field is increased from 0 to 1.6 x 104V/cm. This result compares most favorably to the case of bulk Gays where fields up to (5 + 1) x 104V/cm are necessary to induce changes of the absorption coefficient 2 x 102cm 1, as well as to the case of other III-V compounds where changes pa 2 x 103cm 1 are obtainer but only with fields as large as 4 x 105V/cm. Bulk Gays is discussed more fully by Still man et. at. in the article "Electroabsorption in Gays and its Application to Wave guide Detectors and Modulators," published in Applied Physics Letters Vol. 28, page 544 r in May 1976, and other group III-V compounds are discussed more fully by Kingston in the article "Electroabsorption in GaInAsP," published in Physics Letters, Vol. 34, page 744, in June 1979.
It is important to understand that the speed at which the absorption changes is not determined by the excitor lifetime; rather it is related to how fast the energy levels of the crystal can be shifted, which is a very fast process. The speed of a modulator based on this effect will most likely be limited by the speed at which the "static" field can be applied. We showed that the present device produces large variations of the absorption coeffic-tent I I+ 4 x 103cm 1) for applied fields of threadier of 104V/cm~ this effect is usable in very high-speed optical modulation schemes because of the fast I

mechanisms involved and the small volume (< 100~m3) necessary to achieve large change of transmission.
Referring to FIG. 6, there is shown an exemplary operating frequency 230 for a laser which has its output light beam modulated by the present invention. Solid curve 232 shows the absorption coefficient (a) plotted versus photon energy for the case that no electric potential is applied between contacts 182 and 184 as shown in FIG. 2.
Curve 234, curve 236 and curve 238 show the contribution of the heavy hole, a light hole, and band gap absorption respectively, to the absorption coefficient curve 232.
pun application of an electric potential between contacts 182 and 184 shown in FIG. 2, the potential results in an electric field within the MOW structure 174, and a consequent shift on the excitor absorption. Curve 240 shows, for example, a shifted absorption coefficient a which gives the optical absorption of the MOW structure 174 after application of electric potential between contacts 182 and 184. Curve 242 and curve 244 show the shifted heavy hole and shifted light hole excitor absorption respectively. The shift in the heavy hole excitor peak is labeled "excitor shift" 2460 The band gap absorption is shifted a Swahili amount through the Fran~-Keldysh effect as is indicated by curve 250. The shift due to the Franz-Keldysh effect is labeled and indicated by reference numeral 252.
At laser operating frequency 230 the optical absorption coefficient has a low value shown by line 260 when zero electric potential it applied between contacts 182 and t84, and in contrast has a high value shown by line 262 upon the application of an electric potential between con teats 182 and 184. The shift in the optical absorption curve, labeled "excitor shift" 246, which occurs as a result of the application of a modest potential between contacts 182, 1~4, causes a very large change in the optical absorption coefficient at laser operating frequency 230, and therefore ~3~i6 "

provides deep modulation of the laser beam.
As an example of the amount of modulation which can be achieved with a micron dimension modulator, the decibels (DUB) of modulation can be calculated from the expressions = eve (V1)~ *
and IVY) where IVY) and ~(V13 are the intensity and absorption coefficient at voltage V1, IVY) and ~(V2) are the intensity and absorption coefficient at voltage V2, and T
is the length of the optical path through the MOW. The example presented above showed a pa where pa = avow (V1 ) of approximately 4 103 cm 1 for applied voltages of less than 600 volts. Using equation I with various thicknesses gives the modulation shown in Table 1.

Thickness T Modulation DUB
Microns Thus the "excitor shift" 246 provides excellent deep modulation of a laser beam with only micron dimension optical path length.
A KIWI structure, as designated in FIG. 1 by reference numeral 120 and in FIG. 2 by reference numeral 174, is a semiconductor multiple layer heterostructure~ An MOW is made of alternate layers of wide band gap material and narrow band gap semiconductor material. FIG. 7 gives a cross-sectional view of a typical MOW structure. ho I

exemplary embodiment of an MOW uses AlxGa1 was as the wide band gap material and Gays as the narrow band gap semiconductor material.
Layers of narrow band gap Gays 268-1 to 268-N are alternated with layers of wide band gap Al XGa1 Was 270-1 to 270-N+l. Convenient choices for the dimensions of the structures in an MOW are, for the thicknesses of the Gays 268-1 to 268-N layers .01 micron, for the thickness ox the Algal was 270~2 to 270-N layers .01 micron, and for the thickness of the Gays substrate 274 approximately 100 microns. The side dimensions of the substrate 274 may be chosen conveniently as approximately 1 to 5 millimeters. The MOW structure then has layer planes of Gays 268-1 to 268-N whose length and width are approximately 1 to 5 millimeters and whose thickness is approximately .01 microns. Also the alternate layers of Algal was 270-2 through 270-N have the same ratio of length and width to thickness, that is, 1 to 5 millimeters in length and width and approximately .01 microns in thickness. Thus, the MOW structure comprises essentially plane layers of Gays 268-1 to 268-N interleaved with plane layers of AlxGa1 was 270-1 to 270-N+1.
The alternate layers of Gays and A1xGa1_xAs may be deposited using, for example molecular beam epitaxy using methods as, for example, taught by Dangle et. at.
in U. S. patents Nos. 3,982,207, 4,205,329 and 4,261,771.
Epitaxial growth of heterostructures is further described in the reference book by Casey and Punish, "Heterostr~cture Lasers Part B; Materials and Operating Characteristics" r at Chap. 6, pp. 71-155, and molecular beam epitaxy is particularly discussed at pp. 132-144, academic Press, New York, 1978.
Further details of an exemplary design for an MOW
multiple layer heterostructure are also given in Fig I
There is shown in FIG. 7 capping layers 270-1 and 270-Nt1.
The capping layers 270-1 and 270-N+1 are the first and last wide band gap layers, and they are made thicker than the it layers 270-2 to 270-N which only separate layers of narrow band gap material. An internal capping layer 270 1 may be epitaxially crown on substrate 274 in order, for example, to cover over any imperfections in the upper surface 278 of S substrate 274. An external capping layer 270-N+1 may serve to protect the underlying thinner layers from mechanical injury. Further, the upper surface 280 of external capping layer 270-N~1 may be shaped or treated to serve as a partially reflecting mirror, or surface 280 may serve to attach the multiple layer heterostructure to an external device (not shown), or surface 280 may serve as the side of an optical wave guide used to direct a beam of light to propagate substantially parallel to the layer planes 268-1 to 268-N and 270-1 to 270-N+1. Surface 280 and external capping layer 270-N~1, or internal capping layer 270-1, may serve additional purposes which will be apparent to those skilled in the art of optical devices. Capping layers 270-1 and 270-N~1 are normally made from the wide band gap material which forms the charge barrier layers, and so additionally serve the function of preventing charges from leaking out of the narrow band gap charge carrier material.
The narrow band gap layers 268-1 to 2G8-M are charge carrier layers and each layer forms a quantum well.
The width of the quantum well is determined by the thickness of the narrow band gap material. The barrier heights for the quantum well are determined by the differences between the conduction bands and between the valence bands of the narrow band gap material 268-1 to 268-N
and the wide band yap material 270-1 to 270-N+1. The on barrier heights at the junction between epitaxially grown narrow band gap and wide band gap materials are shown in FIG.
9 for the GaAs/A1xGa1 was case. In FIG 9 both the conduction band carrier and the valence band barrier are shown.
The wide band gap material used for layers 270-1 to ?70-N+1 need not be a semiconductor. The layers must be epitaxially grown upon the substrate 274, and one upon the

3~23~4~

other, The charge carriers produced by photon absorption within the layers of narrow band gap material 268-1 to 268-N
then may propagate throughout the entire epitaxially grown crystal with their motion limited only by the potential barriers which occur a the boundaries of narrow band gap material and wide band gap material, as is shown in FIG. 9 and FIG. 10 for the GaAs~A1xGa1 was case Referring to FIG. 8, the band structure of Gays is shown in a simplified diagram. Reference to the Gays band structure as shown in FIG. 8 provides insight into excitor absorption in an MOW structure. Energy is plotted along the vertical axis 300. The valence band Eve 302 and the conduction band Be 304 are shown along with the energy gap ERG 306. An excitor level 310 is shown with a binding energy EN 312 measured from the conduction band 304.
A photon absorption transition 306 from the valence band 302 to the excitor level 310 is shown Transition 306 represents an excitor creation transition, and such transitions are thought to be the cause of resonant absorption peaks AYE and OWE as shown in FIG. 3.
After the excitor level 310 is formed as a result of photon absorption, the excitor may break apart and form both a conduction band electron 312 and a valence band hole 314, The excitor is thought to break apart as a result of ionization by a lattice vibration photon which supplies the necessary energy An inter band photon absorption transition 320 in which a conduction band electron 312 and a valence band hole 314 are formed as a result of photon absorption is shown. Inter band photon transitions 320 are thought to account for the band gap absorption as indicated by reference numeral 238 in FIG. 6. The inter band photon absorption transition 320 is a direct transition because of the band structure of Gays.
Referring to FIG. 9, the band gap 330 of Gays and the band gap 332 of A1~Ga1_xAs are shown for an epitaxially grown junction 335. Such junctions occur between the layers of epitaxially grown alternate layers of Gays and AlxGa1 was as shown in Figs 1 2 and 7.
The valence band edge 338 of AlxGa1 was is believed to be lower in energy Han the valence band edge 340 of Gays. The conduction band edge 342 of A1xGa1 was is believed to be higher in energy than the conduction band edge 3~4 of Gays. The total difference between the two gaps, of Gays and AlxGa1 was, is believed to be distributed as approximately 15 percent 346 of the difference appears at a lowered valence band edge 338 of AlxGa1 was, and approximately 85 percent 348 of the difference appears as an increase in the conduction band edge 342 of AlxGa1 was, relative to Gus The excitor binding energy EN for Gays is designated by reverence numeral 345 in Figs 8 and 9, for the case in which the Gays layer is substantially thicker than 1000 Angstroms Referring to FIG. 10, the potentials seen by both a conduction band electron and by a valence band hole within a MOW structure are shown. The conduction band electron energy barrier EKE 354 is shown. The valence band hole energy barrier REV 356 is shown, Conduction electrons within a narrow band gap layer 360 are trapped in a potential well with sides of height EKE 354.
Correspondingly, valence band holes within a narrow band gap layer 360 are trapped by the energy barrier ARC 356.
A conduction band electron or a valence band hole may be produced within the narrow band gap layers 360.
Alternatively if the electron and hole are produced within the wide band gap layers 3~2, they will experience potentials due to he and he which will drive them into the narrow band gap layers 360 where they will be trapped by the potential barriers he and eke.
In an exemplary embodiment in which the narrow band gap material is Gays and the wide band gap material is AlxGa1~xAS~ the magnitude of both rev and ~3~6 EKE depends upon the mole fraction x of Al in the wide band gap layers.
The energy Levels of electrons and holes trapped within the narrow band gap layers 360 are shifted relative to their locations in bulk material because of quantum confinement effects arising from the thinness of the narrow band gap layers 360~ A thickness of 1000 Angstroms or less can cause appreciable shift in the allowed energy levels of a semiconductor layer. Also the electrons and holes interact to form excitor pairs. The excitor pairs occupy energy levels which are shifted from the single particle energy levels. All of these energy levels are affected by an electric field applied to the narrow band gap layers 360.
Referring to FIG. 11, the effective optical thickness of the MOW structure mounted as shown in FIG. 2 is shown plotted versus the intensity of incident light beam l92~
The incident light beam 192 is adjusted to coincide with peak AYE in the optical absorption curve shown in FIG. 3.
The intensity of light beam 192 was varied. The effective optical thickness of the sample measures the total attenuation of the light beam as it traverses the sample.
Curve 370 shows the effective optical thickness of a bulk sample of Gays. Curve 372 shows the effective optical thickness of the MOW device. The effective average I intensity of light beam 192 is plotted along the lower margin 374 of FIG. 11 and is shown to vary from 0 to approximately S0,000 Watts/cm2. The total incident power in beam 119 is plotted along the upper margin 376 of FIG. 11 and is shown to vary from .01 to approximately 50 milliwatts. A comparison of the effective optical thickness of bulk Gays and a GaAs/AlxGa1 was MOW
device at an incident light power of 0.1 milliwatts is shown by lines 380, 382, 384. The effective optical thickness of the bulk baas shown in curve 370 is shown to decrease from a value of approximately 2 to a value of approximately 1.9 at an incident power of approximately 0.1 milliwatts, an approximate change in effective optical thickness of (2.0-1.9)/2 = 5%. In contrast, the effective optical thickness of the MCKEE device is seen to vary from approximately .75 to approximately .63 as the incident power varies from zero to 0.1 milliwatts, or a percentage change of approximately (.75-.63~/.75 = 16%. The decrease in effective optical thickness with increasing beam intensity is attributed to saturation of the optical absorption of the material, and is commonly referred to as a nonlinear absorption.
Referring to FIG. 12, an example of the variation of optical absorption 390 and index of refraction 392 with photon energy is shown as the two are related by the Kramers-Kronig relationship using a Lorentzian absorption line shape. The curves of optical absorption 390 and index of refraction 392 illustrate generally the variation of these quantities for excitor absorption over a photon energy range in the vicinity of the band gap. Roy curves shown in FIG. 12 illustrate the relationship between optical adsorption as shown in FIG. 3 for an MOW structure and the corresponding index of refraction, as that relationship is given by the Kramers-Kronig relationship using a Lorentzian absorption line shape.
Curve 390-A represents a large optical resonant absorption for a low incident light intensity and a corresponding index of refraction is shown in curve 392-A.
A smaller resonant absorption is represented by curve 390~B
for a hither incident light intensity and the correspondingly smaller index of refraction is represented by curve 392-~. A further smaller resonant absorption is represented by curve 390-C for a still higher incident light intensity and the correspondingly smaller index of refraction is represented by curve 390-C.
For a single excitor resonance the Kramers-Kronig model illustrated in FIG. 12 shows that for photon energy below the resonant energy 395 the index of refraction decreases with increasing light intensity, while for photon energies above the resonant energy 395 the index of refraction increases with increasing incident light intensity.
For a multiple quantum well, the variation of index of refraction with light intensity depends upon the S interaction of at least one and possibly several excitor resonances with the processes leading to the background index of refraction. These interactions involve quantum interference effects which further complicate the detailed variation of the index of refraction with both light intensity and photon energy. For example, the s/AlxGal_xAs MOW, whose measured optical absorption coefficient is shown in FIG. 3, is dominated by two resolvable excitor absorption peaks superimposed upon an inter band transition background. The interaction of those absorption processes complicates the variation of index of refraction beyond the simple predictions of the one peak model using the Kramers-Kronig relationship.
Louvre, the model makes a useful connection between the measured optical absorption and the index of refraction for the practice of embodiments of the invention which depend upon a variation of the index of refraction with incident light intensity, or applied electric field.
FIG. 13 shows an alternative attachment of electrical contacts to a MOW structure 403. Contacts 400, 401 provide an electric field substantially parallel with the planes 405 of MOW 403.
JIG. 14 shows a top view of MOW 410. Electrical contacts 412 and 414 may, for example, be deposited upon the upper surface (shown in top view but not indicated by 30 reference numeral) of MOW 410. Contacts 412, 414 provide a substantially uniform electric field between end 416 and end 418 which penetrates MOW 410 and provides an electric field substantially parallel to the layer planes of MOW
410.
Referring Jo FIG. 15, there is shown an alternative arrangement of electrical contacts 420, 422 to MOW structure 424. Contacts 420, 422 may be made, for I

example, by ion implantation of an MCKEE Selective ion implantation produces conductive regions in a semiconductor which result in electrical contacts 420, 422. Also, diffusion may be used to make doped contacts Referring to FIG. 16, there is shown an alternative exemplary method of making electrical contacts to the layer planes of an MOW. As a first step, an MOW 430 is grown epitaxially upon a substrate 432~ As a second slept a mesa 434 of MOW material is made by selectively 1G etching away MOW material As a third step, contacts 436, 438 are regrown on substrate 432 so as to electrically contact the edges 440, 442 of layer planes 444 of the mesa 434 of MOW material. A potential applied between contacts 436, 438 produces an electric field within mesa 434 which is substantially parallel with the MOW layer planes 444.
The narrow spacing, 446, provides a small volume modulator. For example, the active region could be 1 micron square, and the thickness could, for example be 20 microns to provide a modulation of 35 DUB as given in TABLE
1, for a total active volume of 20 cubic microns. Such a small volume provides a small capacitance and operates satisfactorily in the multiple gigahertz frequency range.
Materials other than the GaAs/AlGaAs system are useful in practicing the invention. A MCKEE may be made in which the narrow band gap material is Alga was and the wide band gap material is AlxGa1 was. The band gap of the material is larger as the fraction of Al in the material is larger, and so the wider band gap mole fraction x must be larger than the narrow band gap mole fraction y.
The invention uses a super lattice in which the conduction band of the narrow band gap layer is below the conduction band of the wide band gap layer, and the valence band of the narrow band gap layer is above the valence band of the wider band gap layer, and this type of super lattice band structure is named a Type 1 super lattice. The Type 1 super lattice is distinguished from a Type 2 super lattice in I

which both the conduction band and valence band of one of the materials are below the corresponding conduction and valence bands of the other material. A Type 2 super lattice device is disclosed by Clang et. at. in US. Patent No.

4,208,667 issued June 17, 1980 and entitled "Controlled Absorption in Heterojunction Structures".
Materials which are relieved to exhibit the Type 1 super lattice band structure and are suitable for making an MOW structure of the present invention include InGaAs, InGaAlAs, InGaAsP, and also HgCdTe. These materials are useful for application in the 1.5 micron to 1.3 micron wavelength range.
Lattice-matched growth of these materials and their band structure is discussed more fully in the books by Casey and Punish entitled "Heterostructure Lasers, Part A", and "Heterostructure Lasers, Part B", Academic Press, New York, 1978. Also HgCdTe is believed to exhibit the type 1 super lattice band structure and is therefore suitable for making an MOW of the present invention.
In a structure using InGaAs, the narrow band gap layers use InGaAs and the wide band gap layers use In.
These materials are lattice-matched and so are suitable for epitaxial growth as a multiple quantum well device In a structure using InGaAlAs as the narrow band gap material, the wider band gap material may be In or may be other compositions of InGaAl~s which are chosen to have a wider band gap. Such a choice is possible because the band gap of the material can be varied by varying the composition, while maintain lattice-matched crystal growth.
In a structure using InGaAsP as the narrow band gap material, the wider band gap material may be selected from In or may be another composition of InGaAsP which is lattice-matched for crystal growth.
In a structure using HgCdTe as the narrow band gap material, the wider band gap material may be made using HgCdTe of a different composition or Cute and therefore ~;~3~9L6$~

wider band gap.
Additional materials which are believed to exhibit type 1 super lattice band structure include Gas, AlGaSb, So and Go.
Turning now to FIX&. 17, there is shown an MOW used as the intrinsic, or I, layer of a PIN semiconductor diode structure. An electric field may be advantageously applied to the MOW by reverse biasing the PIN diode. The PIN diode structure is particularly suitable for applying the electric field perpendicular to the layer planes of the MOW. Advantages gained from the use of the PIN structure to apply an electric field to the MOW are that the diode is operated in reverse bias, and this condition gives a high resistance to current slow through the MOW. Also the device may be made small in lateral area in order to minimize the capacitance.
FIG. 18 shows an alternate PIN diode structure for conveniently propagating the light in the direction parallel to the MOW layers. The MOW may have only one narrow band gap layer sandwiched between wide band gap layers to form only one quantum well. Or the MOW may have several narrow band gap layers in order to provide a stronger interaction between the light and the optical properties of the quantum wells, such as absorption, index of refraction bireringence, and other polarization properties. Lateral confinement of the light can be achieved by etching a ridge or by other means Turning to FIG. 19, there is shown an exemplary embodiment of a PIN diode structure used to apply an electric field perpendicular to the layer plane of an MOW.
The sample was grown on a So doped [100] Gays substrate.
The unhoped optically active layer contained 50 Gays wells each 95 Angstroms thick and was surrounded by unhoped buffer layers and doped contact layers. This structure creates a PIN diode which was operated in reverse bias mode. buffer and first contact layers were also made of a super lattice of alternating layers of Galas and Gays.

Introducing the thin layers of Gays into the nominally unhoped buffer regions was found to reduce the background doping level by more than an order of magnitude. This reduction of background doping level ruckuses the field S in homogeneity across the active region and reduces the drive voltage of the device. The device was defined laterally by an etched mesa 600 microns in diameter. A
small hole was etched through the opaque substrate by a selective chemical etch. The capacitance of the device was 20 pi The device was made fairly large or ease of fabrication, but a smaller device will advantageously have smaller capacitance.
The lower portion of FIG. to shows the internal electric field in the various layers at two different applied voltages as calculated within the depletion approximation with a p-type background doping level of 2 x 1015cm 3 in the intrinsic layers. The active layer can be switched from a low field to a high field of approximately 6 x 104V/cm by the application ox 8 volts. Because the device is operated as a reverse biased device, its resistance is high and its capacitance is low, which are desirable electrical properties. The heavily doped contact layers can easily be metallized and contacted.
Turning to FOG. 20, there is shown the optical transmission of an exemplary embodiment of the invention as shown in FIG. 19. The transmission at 0 volts shows the usual excitor peaks. Between 0 volts and 8 volts applied there is almost a factor of 2 reduction in transmission at 30 a photon energy of 1.446 eve (857nm). Greater modulation depth is possible with a thicker sample. The dynamical optical response was determined by bridging the PIN diode structure with a 50 ohm resistor and driving it with a pulse generator with a rise time of 1.8 nanosecond. The optical output was detected with a So avalanche photo diode of roughly 1 nanosecond rise time. The observed 10~ to 90%
rise time was 2.8 nanoseconds. The calculated ARC rise time ~39i~

of the device when driven by a 50 ohm load is 2.2 nanoseconds The optical photon energy was 1.454 eve (853 no). Thus modulation occurred with a rise time limited by the capacitance of the device.
Turning now to FIG. 21, there is shown an illustrative embodiment of an electrically tuned Fairy-Pert cavity. The Fabry-Rerot cavity is formed by two substantially parallel mirrors which are partially transparent. The MOW is placed between the two mirrors and therefore within the Fairy Pert cavity. The MOW has an adjustable electric field applied perpendicular to the layer planes. The optical transmission of the Fabry-Perot cavity for light incident substantially perpendicular to the mirrors it high when the optical path length between the mirrors is an integral number of one-half wavelengths of the light The index of refraction of the MOW, and hence the optical path length within the Fabry~Perot cavity can be changed by changing the voltage applied to the MOW.
FIG. 22 is an illustrative graph of the optical transmission of the Fabry-Perot cavity shown in FIG. 21.
The optical transmission is plotted versus the voltage applied to the MOW. The index of refraction of the MOW
varies as the voltage varies, and so at values of the voltage for which the optical path length within the ~abry-Perot cavity is an integral number of one-half wavelengths, the optical transmission is high, and at other values of the voltage the transmission is low.
A losing gain medium may be optionally included within the Fabry-Perot cavity along with the voltage-controlled MCKEE The frequency at which the gain mediumlases may be selected by a choice of the optical path length within the Fabry-Perot cavity, and so the frequency of losing of the gain medium may be selected by adjustment of the voltage applied to the McKee Choosing the voltage applied to the MOW determines the index of refraction of the MCKEE the index of refraction determines the optical path length within the Fahry-Perot cavity, and the losing I
- I -frequency will be limited to those frequencies for which a one-half integral number of wavelengths exist within the Eabry-Perot cavity. Losing occurs at those frequencies for which the output spectrum of the losing medium matches the S frequencies at which a one-half integral number of wavelengths can exist within the Fabry-Perot cavity.
Turning now to FIG. 23, a cross sectional view of a controlled Fabry-Perot cavity is shown. Light propagates parallel to the layers of the MOW. Mirrors in which the plane of the reflective surface lies substantially perpendicular to the propagation direction of the light are shown and define the Fairy Pert cavity. Only one exemplary quantum well is shown because the optical path length is long when the light beam propagates parallel to the layers, and consequently sufficient optical interaction is achieved with only a few quantum wells.
FIG. 24 shows a cross-sectional view of a polarization modulator. Light propagates parallel to the layer planes of the MOW, and it guided by the guiding I layers. The directions of propagation of the incoming light is indicated by the direction k. The polarization of the incoming light is described by reference to the Zeus coordinate axis in which the y axis is shown to lie along the direction of propagation of the incoming light, that is, the direction k coincides with the y axis. The polarization of the incoming light wave is defined by the components of the electric field of the incoming light as they are resolved along the x and z axes. En is the component of the light wave electric field resolved along the x axis and lies parallel to the layer planes of the MCKEE. En is the component of the Lotte electric field resolved along the z axis and lies perpendicular to the layer planes of the MOW. The optical transmission of the MOW along the direction parallel to the layer planes, which is along the y axis, is different for the two polarization components En and En. This difference in transmission is a birefringence of the MOW.

'~L%~3~6 The birefringence of the MOW may be varied by application of an electric field to the MOW. For example, an electric field may be applied to the MOW in a direction substantially perpendicular to the layer planes.
Alternatively the electric field may be applied parallel to the layer planes. For a parallel application of the electric field to the layers of the MOW there are two possibilities, the first being parallel to the propagation direction of the light, that is along the y axis. The second possibility is perpendicular to the propagation direction of the light, that is along the x axis. Each of these possible applications of an electric field may be used to cause variation in the polarization of the outgoing light. A downstream polarization filter (not shown) permits intensity modulation of the light as it emerges from the filter.
The input light may be tuned to a frequency which optimizes the modification ox absorption property of the MOW by the applied electric field.
In FIG. 6, the laser operating frequency 230 indicates an exemplary adjustment of the laser frequency for operation of a device as an absorption modulator. We turn now to optimization of the MOW device as a modulator of optical path lengths. The laser may be operated at a I frequency AYE, which is well below the energy of the large absorption as modified by the electric field. The influence of the electric field on the index of refraction extends below the absorption peak. Because the index of refraction effect extends below the absorption peak it is possible to tune the low frequency much as shown by AYE in FIG. 6 in order to optimize the apparatus to modify the optical path lengths through the MOW. The light frequency for optimum operation of the device will depend upon the polarization of the incident light.
The optical absorption and the index of refraction may be saturated by increasing the intensity of the input light. The saturation may be influenced by application of ~3~i6 - I -an electric field to the MOW. For example, a bistable optical device may be caused by application of an electric field to become not bistable. Also the operating characteristics of a nonlinear optical device may be changed by application of an electric field.
In FIG. 25 an array of electric field controlled MOW devices is shown. An array of electric field controlled type of MOW devices may be grown on a single substrate. These MOW devices may form a logical array of optical switching elements. The logic of the element may be modified by application of electric fields to the individual elements. A different electric field may be applied to each MOW device. An array of MOW devices which are individually controlled by electric fields is a programmed logical array.
It is to be understood that the above-described embodiments are simply illustrative of the principles of the invention. Various other modifications and changes may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

Claims (8)

Claims:

1. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said means for applying an electric field to said multiple layer heterostructure comprises:
a p doped semiconductor layer and;
a n doped semiconductor layer and said semiconductor layer having a third bandgap is located between said p doped semiconductor layer and said n doped semiconductor layer.

2. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said semiconductor layer is made of HgCdTe.

3. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said first and second material layers further comprise:
at least one of said first and second material layer are multiple layer structures.

4. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, further comprising:
a first very lightly doped buffer layer positioned between a p doped contact semiconductor layer and said semiconductor layer having a third bandgap; and a second very lightly doped buffer layer positioned between a n doped contact semiconductor layer and said semiconductor layer having a third bandgap.

5. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said semiconductor layer is made of Si.

6. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said semiconductor layer is made of Ge.

7. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said semiconductor layer is made of GaSb.

8. A semiconductor apparatus comprising:
a multiple layer heterostructure having first and second material layers having first and second bandgaps, respectively, and a semiconductor layer having a third bandgap and being positioned between said material layers, the bottom of the conduction band of said semiconductor layer being below the bottom of the conduction bands of said material layers, and the top of the valence band of said semiconductor layer being above the tops of the valence bands of said material layers, the thickness of said semiconductor layer being sufficient for carrier confinement effects within said semiconductor layer to influence the optical properties of said multiple layer heterostructure; and means for applying an electric field to said multiple layer heterostructure in order to vary an optical absorption coefficient and an index of refraction of said multiple layer heterostructure in response to said electric field, wherein said semiconductor layer is made of AlGaSb.