Superlattice ultrasonic wave generator

An ultrasonic wave generator comprising a semiconductor superlattice with a periodic variation in its space charge and a far infrared laser for applying a transient electric field to the superlattice transverse to the direction of its periodic variation. The ultrasonic wave produced has a wavelength of the period of the superlattice which can result in 100 gigahertz ultrasonic waves. Structure is included for guiding these waves into an acoustic system.

BACKGROUND OF THE INVENTION 
The present invention relates generally to devices for generating very high 
frequency acoustic waves, and more particularly to a method of converting 
far infrared laser radiation into ultrasonic acoustic waves of the same 
frequency, in the range of 100 GHz to 1000 GHz. 
DESCRIPTION OF THE PRIOR ART 
At the present time there are many acoustic systems which are operating at 
frequencies of less than 1 GHz, such as surface acoustic wave devices used 
for signal processing. One class of these devices can be described as 
surface phonon optics because it involves the interaction of a surface 
acoustic wave and a light wave. The acoustic waves in such devices are 
usually generated by piezoelectric couplers in a periodic structure 
matched to the wavelength of the surface acoustic wave. Such couplers are 
described in U.S. Pat. Nos. 3,399,314 (Phillips) and 2,716,708 
(Bradfield). However, these techniques require individual electrical 
contacts to be made to each of the electrodes of the periodic coupler. The 
separate electrode requirement coupled with limitations of the fabrication 
techniques in piezoelectric materials have imposed a 1 GHz limit on the 
acoustic waves produced. 
High frequency acoustic waves are used in the acoustic microscope. However 
the limitation of 1 GHz imposed by present generators limits the 
resolution of present acoustic microscopes to no better than 10.sup.-4 cm. 
If a source of 100 GHz phonons were available, the resolution of the 
microscope would improve to 10.sup.-6 cm. 
Another use of acoustic waves is for signal processing or for 
acousto-optical data systems. If the frequency of bulk acoustic waves 
could be raised from 1 GHZ to 100 or 1000 GHz, ultrahigh speed phonon 
systems could be developed which would operate at correspondingly higher 
data rates. 
Acoustic waves of 100 to 1000 GHz are matched in frequency to far-infrared 
electromagnetic radiation although the acoustic wavelength is much larger. 
Far infrared light sources are readily available but transducers are 
presently unavailable which easily couple the electromagnetic wave energy 
into acoustic waves. Such transducers would facilitate the fabrication of 
the aforementioned acousto-optical data system. 
Presently available sources of acoustic waves in the 100 to 1000 GHz range 
involve black-body phonon emission of heaters and superconducting 
tunnel-junctions. However black-body sources are broad band and do not 
provide the capability of a monochromatic phonon source. Furthermore they 
need to operate at 4.2K to yield 100 to 1000 GHz phonon generation. The 
superconducting tunnel junction does generate monochromatic waves but is 
inherently disadvantaged by the requirement of cryogenic temperatures. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to provide for the 
generation of acoustic waves in the 100 to 1000 GHz and above frequency 
range. 
It is a further object to provide a transducer from far infrared radiation 
to acoustic waves. 
It is a yet a further object to provide an electrode-free acoustic 
generator. 
It is still another object to provide an acoustic generator of 
monochromatic phonons. 
It is a yet another object to provide a room temperature generator of 
acoustic waves. 
SUMMARY OF THE INVENTION 
Briefly, the present invention is a generator of ultrasonic acoustic waves. 
The core of the invention is a semiconductor superlattice of a type in 
which there is a net space charge which varies periodically with the 
superlattice. For example, a superlattice of InAs-GaSb of appropriate 
period has free excess carriers of opposite charge in the alternate 
layers. If a sinusoidally time varying electric field is applied in the 
plane of the layers, the electric field will transfer to the crystal 
momenta of opposite directions in the alternate layers. The sinusoidally 
varying momentum in the crystal will induce an acoustic wave of the same 
frequency as the electric field. The acoustic wave can be coupled into 
other structures and used therein. 
The invention can also be used as a transducer between electric fields or 
between electromagnetic waves and acoustic waves. 
In one embodiment, the alternating electric field may be provided by a far 
infrared laser. The alternating space-charge regions are also present in 
GaAs-GaAlAs superlattices and in modulation doped superlattices, i.e. a 
superlattice composed of the same semiconductor material but with dopants 
varying in density or of opposite signs in the alternate layers.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, a 
superlattice is shown in FIG. 1a. A superlattice is a material structure 
consisting of alternate layers of dissimilar materials. The thicknesses of 
the layers are much less than the lateral dimensions so only one dimension 
need by represented. FIG. 1a shows a superlattice of InAs-GaSb. The InSb 
layers 11 alternate with the GaSb layers 12. Only two complete periods are 
represented in FIG. 1a for ease of display but many more periods are 
required before the effects associated with the periodic variation 
dominate any edge effects. The InSb layers are all of essentially the 
thickness d.sub.1 ; likewise the GaSb layers are of thickness d.sub.2. The 
thickness d.sub.1 and d.sub.2 need not be equal but usually are made so in 
order to maximize periodic effects. The superlattice period d is the sum 
of d.sub.1 and d.sub.2 and is the distance between repeating structure. 
The two materials InAs 11 and GaSb 12 are both semiconductors, the 
electronic energy band structures of which are shown in FIG. 1b. InAs 11 
has a valence band 16 and a conduction band 18 separated by a bandgap 20 
in which there are no possible energy states. Similarly GaSb 12 has a 
valence band 22, a conduction band 24 and bandgap 26. In normal bulk 
semiconductors the valence bands 16 and 22 are filled, there are no 
available states in the bandgaps 20 and 26, and the available states in 
the conduction bands 18 and 24 are unoccupied because of the lack of 
additional charge carriers. When a superlattice of InAs-GaSb is brought 
together as shown in FIG. 1a, the bands of the materials come into 
equilibrium relative to each other as shown in FIG. 1b. The details of the 
bands of the superlattice are complex and are described in the articles 
"Semiconductor Superlattices in High Magnetic Fields" by L. Esaki and L. 
L. Chang, Journal of Magnetism and Magnetic Materials, Volume 11, page 
208, 1979 and "InAs-GaSb Superlattice Energy Structure and its 
Semiconductor-semimetal Transition" by G. A. Sai-Halasz, L. Esaki and W. 
A. Harrison, Physical Review B, Volume 11, page 2812, 1978. The important 
point is that in equilibrium, electronic states are allowed at those 
energies where the InAs conduction band 18 overlaps the GaSb valence band 
22 in InAs-GaSb superlattices with periods greater than 17 nm. For the 
effects to be seen it is required that the superlattice be well made, such 
as those grown by molecular beam epitaxy as described by Cho et al. in 
U.S. Pat. No. 3,929,527. When the normally filled GaSb valence band 22 is 
at higher energy than the normally empty InAs conduction band 18, 
electrons transfer from the GaSb 12 to the InAs 11 creating the space 
charge distribution as shown in FIG. 1c. It can be seen that excess 
negatively charged electrons 28 occupy the InAs layers 11 and positively 
charged holes 30 occupy the GaSb layers, i.e. there results an alternating 
space charge. 
The invention as shown in FIG. 2 requires a semiconducting superlattice 
composed of alternating layers 32 and 34 along a z-direction 36 with space 
charge varying along this same direction. Shown in FIG. 2 is a relatively 
uniform positive charge density 38 in one set of layers 32 and a 
corresponding negative charge density 40 in the other set of layers 34. 
The charge distribution within the layers 32 and 34 need not be uniform in 
the z-direction 36 for the superlattice 31 to be subject to the same type 
of effects. 
If an electric field E 42 is externally applied to the space charge regions 
of the superlattice 31 in a direction 44 perpendicular to the z-direction 
36, it will impart momentum to all charges. The electric field can result 
from electromagnetic radiation or by impressing a voltage between two 
plates. Because of the differing signs of the charges, the momentum 46 
imparted to the positive charge 38 in layer 32 will be in the opposite 
direction from that 48 imparted to the negative charge 40 in layer 34. The 
momenta 46 and 48 on the charges 38 and 40 will be transferred by 
collisional drag to the crystal structure of the layers 32 and 34. The 
transferred momenta produce a structural distortion which is in different 
directions in the alternate layers 32 and 34. When the electric field 42 
is reversed to the direction opposite to the first direction, the crystal 
distortion reverses. There results a distortion wave 50 along the 
z-direction 36 which constitutes a transverse acoustic wave or a wave of 
phonons. The wave 50 is not confined to the superlattice region or the 
alternating layers 32 and 34 but propagates into a substrate 52 that is 
properly matched with the superlattice and properly coupled at the 
substrate interface 54. 
Any type of change in the electric field 42 will induce a corresponding 
acoustic wave 50. The field may be pulsed, reversed, varied sinusoidally 
or time varied in any manner so as to be transient rather than time 
invariant. However, the frequency of variation must satisfy 
EQU .omega..multidot..tau.&lt;&lt;1 (1) 
where .omega. is the angular frequency of the propagating acoustic wave and 
.tau. is the lifetime of the charge carriers. 
Furthermore any spatial variation of the electric field along the wave 
propagation direction, i.e. along the z-direction 36, must be slow 
relative to the superlattice period d. 
The preferred embodiment is shown in FIG. 3 wherein a far infrared laser 51 
is aligned with the superlattice 54 substantially parallel to its axis of 
variation. The far infrared radiation wave 56 propagates toward the 
superlattice with an alternating electric field 57 and magnetic field 58 
orthogonal to each other and to the axis of propagation. The far infrared 
radiation 56 is characterized by frequency .omega..sub.IR and wavelength 
.lambda.. The radiation wave 56 penetrates the superlattice 54 wherein its 
wavelength is modified by the dielectric characteristics of the 
superlattice. It should be noted that the modified wavelength .lambda.' of 
the infrared radiation must be much greater than the superlattice period 
d. 
The alternating electric will produce a force, F.sub.c, on a unit volume of 
the superlattice at a frequency .omega..sub.IR. The equation of motion of 
the displacement .xi.(r,t) of the lattice is given by 
EQU .rho..sub.I .xi.(r,t)=-C.sub.t .gradient.x[.gradient.x.xi.(r,t)]+F.sub.c ( 
2) 
where .rho..sub.I is the specific density of the superlattice and C.sub.t 
is the proper elastic constant associated with shear distortion. The space 
and time Fourier transform .xi.(q,.omega.) of the displacement vector. 
.xi.(r,t) will have a resonance for 
EQU .omega.=.omega..sub.IR (3) 
and 
EQU q=2.pi.N/d (4) 
where N is an integer. The acoustic wave 53 resulting from the displacement 
has its frequency and wavenumber related by .omega.=s.sub.t q where 
s.sub.t is the velocity of a transverse acoustic wave in the superlattice. 
The exact form of the acoustic wave 53 set up by the electromagnetic wave 
56 depends on the boundary or loading conditions imposed upon the 
superlattice 54. If one end 59 of the superlattice 54 is left free of any 
further mechanical constraints and if the other end 60 is matched to a 
substrate 62 which in turn is matched to the acoustic system 64 which does 
not reflect waves back into the substrate 62, then the wave 53 generated 
in the superlattice 54 will propagate therefrom through the substrate 62 
and be guided into the acoustic system 64. The acoustic system 64 is the 
system for which the acoustic waves are being generated such as an 
acoustic microscope or a acousto-optical processor or any system requiring 
high frequency acoustic waves. Reflections of the acoustic wave 53 at 
either the superlattice-substrate interface 60 or the substrate-system 
interface 66 can be prevented by impedance matching the various materials. 
This matching can be accomplished by using materials for the superlattice 
54, substrate 62 and acoustic system 64 with similar elastic constants and 
by joining the parts with a rigid mechanical bond at the interfaces 60 and 
66. For instance, the substrate can be grown by the same method of 
molecular beam epitaxy as the superlattice with a uniform composition that 
is a mixture of the compositions of the alternating layers of the 
superlattice 54. 
The frequency .omega. of the acoustic wave 53 generated in the superlattice 
56 and transported into the acoustic system is that of the electromagnetic 
wave 56. The acoustic wave is excited only when the resonance conditions 
of Equations (3) and (4) are satisfied, i.e. when the far infrared 
frequency is matched to the superlattice period d by the relation 
EQU .omega.=2.pi.s.sub.t N/d (5) 
If a non-sinusoidal waveform for electric field is used, such as a pulsed 
electric field supplied by capacitive plates, then that waveform's Fourier 
components will determine the multiple frequencies characterizing the 
forcing waveform. 
The velocity of a transverse acoustic wave 53 is about 3.times.10.sup.5 
cm/s. A far infrared laser 51 of angular frequency 10.sup.11 to 10.sup.12 
/s will coherently excite the acoustic wave 53 characterized by phonons of 
wavenumber q between 3.times.10.sup.5 and 3.times.10.sup.6 cm.sup.-1. 
These wavenumbers correspond to a superlattice period d of between 20 and 
200 nm for the transducer operating in its most efficient mode, i.e. N=1. 
Superlattice periods of such values are compatible with the period 
required to create space charge in the InAs-GaSb superlattice of FIG. 1a. 
Since such an acoustic wave is of a frequency far higher than the audible 
range, it is also called an ultrasonic wave. 
The foregoing description of the InAs-GaSb superlattice and transducer 
should not imply that only the combination of InAs and GaSb will produce 
an effective acoustic wave generator. Nor is the charge transfer mechanism 
characterized by the band structure of FIG. 1b the only one that can 
create a space charge differing in the two types of layers. 
Another pair of materials which when used as constituents of a superlattice 
can produce acoustic waves are GaAs and GaAlAs where GaAlAs is shorthand 
for Ga.sub.1-x Al.sub.x As where x can assume any of a range of values 
between 0.03 and 1.0. The band structure has been calculated for x=0.65 so 
that this value of x is the preferred one. In FIG. 4a is shown the 
superlattice of alternate layers of GaAs 68 and GaAlAs 70 repeating on a 
period d. The GaAlAs layers 70 are doped with donor atoms which create 
donor energy levels 72 near the top of the band gap, but which are 
spatially localized in the GaAlAs 70, i.e., the quantum mechanical 
electron wave function of the donors does not significantly extend into 
the GaAs 68. The lower edges of the conduction bands of the GaAs 74 and of 
the GaAlAs 76 differ significantly in energy while the valence bands of 
the GaAs 78 and GaAlAs 80 are relatively equal. 
Because the donor levels 72 lie so close to the GaAlAs conduction band 76, 
they will be mostly ionized but the resulting free electrons, instead of 
staying in the GaAlAs conduction band 76, will transfer into the lower 
energy states of the GaAs conduction band 74. There results, as shown in 
FIG. 4c, a space charge distribution of excess negatively charged free 
electrons 82 in the GaAs 68 and uncompensated positively charged donors 84 
in the GaAlAs 70. This space charge distribution can interact with a 
transient electric field in the same way as the space charge in a 
InAs-GaSb superlattice. 
Yet another method of creating periodic space charge requires only a 
periodic variation in the dopant instead of a periodic change in the 
semiconductor composition. The method is often called modulation doping. 
In FIG. 5a is shown a semiconductor dopant superlattice composed of 
alternating layers of n-type silicon 86 created by doping that layer of 
silicon with a donor such as phosphorous and p-type silicon 88 created by 
doping that layer of silicon with an acceptor such as boron. The doping 
repeats on a superlattice period d. 
The resulting superlattice band structure is shown in FIG. 5b wherein the 
relative spatial positions of the conduction band 90 and the valence band 
92 are controlled by the density and energy levels of the positively 
ionized donors 94 and negatively ionized acceptors 96. Under normal 
conditions in bulk material, most of the donors 94 would be ionized, with 
the associated free electrons 98 producing local charge neutrality. 
Likewise the holes 100 freed from the mostly ionized acceptors 96 would 
produce local charge neutrality. However the thermal equilibrium bending 
of the bands 102 and 104 is effected by the ionized dopants 94 and 96 near 
the interface 106 between the differently doped regions not neutralized by 
corresponding free charge. In equilibrium the p-n junction 106 shown in 
FIG. 5c between the p-region 88 and n-region 86 has positive space charge 
region 108 of width w.sub.1 on the n-side 86 of the interface 106 occupied 
by unneutralized donors 94 and a negative space charge region 110 on the 
p-side 88 of the interface 106 of width w.sub.2 occupied by unneutralized 
acceptors 130. The space charge is not necessarily spread throughout the 
superlattice layers 86 and 88. Instead the widths w.sub.1 and w.sub.2 of 
the layers 108 and 110 are controlled by the doping densities and to a 
lesser extent the species of dopant. 
The space charge regions 108 and 110 can interact with a transient electric 
field in much the same way as the space charge regions in the InAs-GaSb 
superlattice. 
The generator of this invention can be implemented as an opto-acoustic 
transducer which is a specialized type of acoustic wave generator. If the 
source of far-infrared radiation or other transient electric field is not 
always active but supplies the radiation to the herein described generator 
at intermittent intervals, then acoustic waves will be generated at those 
same intermittent intervals. Thus a signal impressed upon a far-infrared 
optical link can be transformed to an equivalent signal on an acoustic 
link by a transducer comprising the superlattice of this description. Such 
a transducer would be useful at the input to a phonon data processing 
system or as a coupler in a opto-phonon processor. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.