Negative absolute conductance device and method

A quantum wire embedded in another material or a quantum wire which is free standing. Specifically, the quantum wire structure is fabricated such that a quantum well semiconductor material, for example Gallium Arsenide (GaAS), is embedded in a quantum barrier semiconductor material, for example Aluminum Arsenide (AlAs). Preferably, the entire quantum wire structure is engineered to form multiple subbands and is limited to a low dimensional quantum structure. The dimensions of the quantum wire structure are preferably around 150.times.250 .ANG.. This structure has a negative absolute conductance at a predetermined voltage and temperature. As a result of the resonant behavior of the density of states, the rates of electron scattering in the passive region (acoustic phonon and ionized impurity scattering as well as absorption of optical phonons) decrease dramatically as the electron kinetic energy increases.

FIELD OF THE INVENTION 
This invention relates generally to semiconductor devices for electronic 
and optoelectronic devices and particularly to low dimensional, 
semiconductor quantum structures which maybe incorporated in ultrafast, 
super high frequency oscillators, amplifiers and self-regulating switches. 
BACKGROUND OF THE INVENTION 
Low dimensional structures having quantum confinement of one to three 
dimensions, such as quantum wells, quantum wires and quantum boxes, have 
attracted much attention not only for their potential in uncovering new 
phenomena in solid-state physics, but also for their potential device 
applications. The reason for this attention is based on the extremely high 
electron mobilities of the structures themselves and the high performance 
of devices, such as lasers and modulators, incorporating these structures. 
This enhanced electron carrier mobility is further achieved by engineering 
the sub or mini-bands of the structure so that the longitudinal-optical 
(LO), or surface-optical (SO) phonon transitions are nullified. Examples 
of such structures are described in articles, such as, "Quantum wire 
Superlattices and Coupled Box Arrays: A Novel Method to Suppress Optical 
Phonon Scattering in Semiconductors," Sakaki, Japanese Journal of Applied 
Physics, Vol. 28, No. 2, Pages L314-L316, February, 1989; "Optical 
Anisotropy in a Quantum-Well-Wire Array with Two-Dimensional Quantum 
Confinement," Tsuchiya et al, Physical Review Letters, Vol. 62, Number 4, 
pages 466-469, January, 1989; and "Electron-Optical-Phonon Interaction In 
Single and Double Heterostructures," Mori et al, Physical Review B, Volume 
40, Number 9, pages 6175-6188, September, 1989. 
As was predicted in Sakaki and demonstrated by Ismail et al at the 1988 
International Symposium on GaAs and Related Compounds, Atlanta, Ga., 
September, 1988, breaks in the density of states of quantum structures 
lead to negative differential conductance and negative transconductance in 
field effect transistor (FET) configurations. Examples of such devices are 
described in U.S. Pat. No. 4,704,622, entitled, "Negative Transconductance 
Device", and issued to Capasso et al on Nov. 3, 1987 and in U.S. Pat. No. 
4,645,707, entitled, "Semiconductor Devices", and issued to Davies et al 
on Feb. 24, 1987, both of which are incorporated herein by reference 
hereto. The Capasso et al device is a three-terminal resonant-tunneling 
structure based on resonant tunneling of a two-dimensional electron gas 
which is gated into a one-dimensional quantum wire to produce a negative 
transconductance. Quantum wire arrays have also been considered as 
potential low-current-threshold semiconductor lasers as reported in 
Tsuchiya et al. The devices described thus far, however, only provide a 
minimal variation of conductance, even though far greater variations have 
been predicted by authors such as H. Kromer in Physical Review 109, page 
1856 (1958). Therefore, in addition to the relatively commonly described 
negative differential conductivity in semiconductors, a more pronounced 
reversal of a current relative to an electrical field is also realizable. 
This more pronounced effect, termed negative absolute conductance, was 
first predicted by H. Kromer. Since 1958, this effect has been further 
developed and observed experimentally by G. C. Dousmanis, Physical Review 
Letters 1, Vol. 55 (1958) and by E. M. Gershenzon et al, Ukr. Fiz. Zh 9, 
page 948 (1964). If negative absolute conductance was obtainable in low 
dimensional quantum structures, then the performance of semiconductor 
devices utilizing such structures would be greatly enhanced. The present 
invention addresses such a device. 
SUMMARY OF THE INVENTION 
Accordingly, it is an objective of the present invention to provide a 
device which produces picosecond control of electrical functions in 
ultrafast semiconductor devices. 
Another objective of the present invention is to provide for a device that 
produces such control by utilizing negative absolute conductance of a 
quantum wire structure. 
These and other objects of the present invention are accomplished by 
fabricating a quantum wire which is either embedded in another material or 
is free standing. Specifically, the quantum wire structure may be 
fabricated such that a quantum well semiconductor material, for example 
Gallium Arsenide (GaAS), forms a quantum wire structure embedded in a 
quantum barrier semiconductor material, for example Aluminum Arsenide 
(AlAs). Preferably, the entire quantum wire structure forms multiple 
subbands and is limited to a one dimensional quantum structure. The 
dimensions of the quantum wire structure are preferably around 
150.times.250 .ANG. or less. This structure has a negative absolute 
conductance at a predetermined voltage and temperature. Quantum wires have 
two primary advantages for realization of negative absolute conductance 
over bulk materials because: 1) electrons in such a structure have no 
transverse components of velocity, thus these structures provide a unique 
carrier scattering and 2) the longitudinal-optical (LO) phonon emission 
rate is very high in the active region threshold so that electrons are 
prevented from penetrating deeply into the active region above the optical 
phonon energy. As a result of the resonant behavior of the density of 
states, the rates of electron scattering in the passive region below the 
optical phonon energy decrease dramatically as the electron kinetic energy 
increases. 
In operation, a predetermined electric field and/or laser pulses are 
applied to the quantum structure. The electric field and/or laser pulses 
inject electrons into the quantum structure; the electrons, then, exhibit 
transient negative drift velocities. 
If electrons are injected at energies multiple to the optical phonon 
energy, then the electric field forces the electrons from the negative 
part of the .kappa.-space to the lower energy subbands, thus excluding 
them from re-emission. Electrons from the positive part of the 
.kappa.-space, however, emit optical phonons and are forced to bottom of 
the subband. As a result, electrons with high negative velocity dominate 
over those electrons with high positive velocity and cause the current to 
flow against the applied electric field, otherwise known as negative 
absolute conductance. This transient process lasts about 1 to 3 
picoseconds in the field range of 100 to 500 v/cm. However, negative 
absolute conductivity can also be realized in a steady state if 
recombination of electrons is sufficiently intensive. Therefore, it may be 
readily appreciated by those skilled in the art that such a structure may 
have many uses in ultra fast, super high frequency oscillators, amplifiers 
and/or self regulating switches.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, there is shown a quantum structure 1 which is 
comprised of a rectangular quantum wire 5 embedded in an exterior material 
4. The quantum wire 5 may be fabricated from any material which would 
normally be used as a quantum well material in a superlattice 
heterostructure, such as GaAs. Likewise, exterior material 4 may be 
fabricated from any material which would normally be used as a quantum 
barrier, for example AlAs. As noted previously though, quantum wire 5 may 
also be free standing. The ends of quantum wire 5 should be free of any 
material so as to allow for clean electrical contacts to quantum wire 5. 
The quantum wire 5 has a predetermined height 2, and width 3. In a 
preferred example, quantum wire 5 has a width and a height equal to or 
less than 150 .ANG. and 250 .ANG., respectively. Due to its size, shape 
and composition, this structure will have multiple energy subbands and 
will exhibit one dimensional intra- and intersubband scattering by 
confined longitudinal-optical (LO) and localized surface-optical (SO) 
phonons. 
This structure may be fabricated utilizing known molecular beam epitaxy 
techniques or other comparable techniques utilized in fabricating like 
devices. These techniques are well known in the art and therefore, a 
skilled artisan would be able to fabricate such a device without further 
disclosure. FIGS. 3 and 4 simply depict alternative embodiments of the 
quantum wire 5 utilized in the present invention. As shown, the 
cross-section of the quantum wire 5 may be any configuration including a 
wire with a random cross-section, as shown in FIG. 3, or a wire with 
circular cross-section, as shown in FIG. 4. 
In operation, a narrow range of injected electron energy, preferably less 
than 0.001 eV above the bottom of the first subband energy level or close 
to the optical phonon energy, is introduced into the quantum wire thereby 
inducing current J. The only requirements being that the electrons must be 
injected within a narrow energy range close to the subband bottom or 
optical phonon energy. The electric field must be high enough, however, to 
avoid severe restrictions on the injected energy and the electric field 
must also be low enough to prevent rapid heating of low energy electrons 
and deep electron penetration in the active energy region. 
In principle, the injected electrons exhibit transient negative drift 
velocities at moderate electric fields at the beginning of electron 
heating. This reaction is related to the rapid absorption of LO phonons by 
cold injected electrons and their re-emission. This applied electric 
field, then, forces the electrons from the negative part of the 
.kappa.-space to lower energy, thus excluding these electrons from the 
re-emission process. Electrons from the positive part of the 
.kappa.-space, however, emit optical phonons and are forced to the bottom 
subband. As a result, electrons with high negative velocities dominate 
over those with high positive velocities and cause current J to flow 
against the applied electric field, otherwise known as negative absolute 
conductivity. 
This entire transient process lasts about 1 to 3 picoseconds in the 
electric field range of 100 to 500 V/cm at 300 K. The length of this 
process will vary given different temperatures, quantum wire structures 
and electric fields. In fact, at low lattice temperatures negative 
absolute conductance occurs in a much wider range of conditions including 
varied injection energies, electric fields and recombination rates. Thus, 
in order to get a greater range of conditions it would be preferrable to 
keep the structure at a lower temperature, for example 70 K. 
FIGS. 5a and 5b are schematic illustrations of the formation of the 
electron distribution function which is responsible for the negative 
absolute conductivity wherein FIG. 5a represents such a distribution at 
high lattice temperatures and the electron injection energies rise to the 
first subband bottom; and wherein FIG. 5b represents such a distribution 
at low lattice temperatures and the electron injection is set at Just 
below the optical phonon energy. The arrows depict the electron transfer 
channels due to the absorption (+h.omega.) or emission (-h.omega.) of 
optical phonons. Area 1 represents the electron distribution just after 
injection; area 2 represents the electron distribution after the shift 
caused by the electric field and emission of optical phonons; and area 3 
represents the electron peak above the optical phonon energy subsequently 
scattered down by the emission of optical phonons. When an electrical 
field is applied to the quantum wire, electrons are injected in to the 
first subband bottom (FIG. 5a) or just below the optical phonon energy 
(FIG. 5b). After this injection to the subband bottom, the electrons 
almost instantaneously absorb the optical phonons and enter the active 
region above the optical phonon energy (FIG. 5a). The electric field, 
then, forces the electrons out of (or away from) the active region on the 
negative side of the .kappa.-axis and into the active region of the 
positive side of the .kappa.-axis (FIG. 5a). Due to the fast optical 
phonon emission, electrons from the positive side of .kappa.-axis scatter 
down to the subband bottom; these electrons stay in the negative part for 
a period of time until they are decelerated by the electric field or their 
momentum is randomized by multiple scattering events. As depicted, 
electrons with high negative velocity will dominate over those with 
positive velocity, thereby making the electron drift negative. 
Given the above and as may be appreciated by those skilled in the art, a 
steady state negative absolute conductivity may occur if the electron 
heating is compensated by the recombination of the electrons or some other 
mechanism which keeps the electrons close to the bottom subband energy 
level. In other words, in order to get the effect of negative absolute 
conductivity in a steady state, electrons must be eliminated from the low 
energy regions before they are heated by the applied electric field and 
contribute to the positive part of the conductivity. Obviously, then, the 
electrons which are eliminated must be replaced by injecting new 
electrons. This elimination of electrons can be accomplished by either 
linear recombination of the electrons or by extracting the electrons 
through contacts. In considering linear recombination, it will be evident 
to those skilled in the art that the recombination rate must be such that 
electrons recombine primarily during the regime of transient negative 
conductivity; i.e., the rate of recombination must be compatible with the 
rate of the dominant electron scattering mechanism or the electron heating 
rate by the electric field whichever is higher. 
FIG. 6 illustrates the steady-state velocity field dependencies of injected 
electrons under intensive recombination. Curve 1 represents the electron 
drift velocity as a function of applied electric field at a temperature of 
300 K. for a quantum wire engineered to have 7 subbands and an electron 
injection into the first subband bottom. Curve 2 is for the same 
structure, but the electron injection occurred at the LO phonon energy. 
Curve 3 is the dark electron (background) velocity. For this graph, the 
recombination rate, R.sub.1, is a step function of electron energy with a 
cutoff energy .sub.1 of 0.022 eV. As shown, the negative absolute 
conductivity appears in a certain range of electric fields and higher 
recombination rates. However, it should be noted that even when the 
velocity does not reach negative values, it decreases with the increase of 
the electric field, thus exhibiting negative differential conductivity. 
Therefore, even at these positive values the structure as described above 
is also useful in devices such as those described in Davies et al. 
The existence of lower threshold fields for the occurrence of negative 
absolute conductivity is primarily caused by the injection of electrons 
slightly above the subband bottom, for example in FIG. 6, 0.2 meV. As a 
result, the electric field must be large enough to extract electrons from 
the negative part of the active region before the electrons emit optical 
phonons. As shown in FIG. 6, the effect of negative absolute conductivity 
is more pronounced for electron injection close to the optical phonon 
energy than for injection to the subband bottom. Thus, the negative 
absolute conductivity may be induced not only when electrons are injected 
close to the phonon energy, but also when electrons are injected with an 
energy close to a multiple of the optical phonon energy. 
Certain injection energies at which negative absolute conductance occurs 
are illustrated in FIGS. 7a and 7b. In FIGS. 7a and 7b the electron drift 
velocity (FIG. 7a) and the mean electron energy (FIG. 7b) are depicted as 
a function of the injection energy. Like FIG. 6, the recombination rate 
was a step function with a cut-off energy of 0.022 eV and a low energy 
value of R.sub.1 =2.times.10.sup.12 sec.sup.-1. As shown, the amplitudes 
of the oscillations of the electron velocity are larger than those found 
in bulk materials. The lowest minimum on this dependency graph is seen at 
the single energy of the LO phonon, thereby confirming the conditions for 
the occurrence of negative absolute conductivity. At an energy equal to 
three times the LO phonon energy the effect of negative absolute 
conductivity strongly decreases as a result of electron transfer to the 
second subband. The shape of the minimum is asymmetric with a steeper 
increase in conductivity above the multiple phonon energy. As shown in 
FIG. 7b, the mean electron energy is also an oscillatory function of the 
injection energy; i.e. relative cooling of the electron system takes 
place. 
As a result, the frequency of negative absolute conductivity can be 
controlled by controlling the injection of electrons, the applied voltage 
and/or the recombination rate. Thus, those skilled in the art will readily 
recognize the myriad of applications for a structure exhibiting the above 
described qualities. For example, such a structure may be utilized as an 
oscillator by merely engineering the size of the quantum wire to 
experience negative absolute conductivity at a predetermined frequency. 
Moreover, one skilled in the art would also readily appreciate the use of 
such a structure as a self regulating switch. 
Therefore, while the principles of the present invention have been 
described in connection with a specific structure, it is to be understood 
that this description is made only by way of example and not as a 
limitation to the scope of the invention, because those skilled in the art 
will readily recognize that the example structure described herein may be 
further engineered by altering its size, shape and/or composition in order 
to achieve greater or lesser magnitudes of negative absolute conductivity.