HACT structure with reduced surface state effects

A heterojunction acoustic charge transport (HACT) device having a charge transport layer 16 surrounded by upper and lower charge confinement layers 14,30, respectively, and having a cap layer 36 at the outer surface, above the upper confinement layer 30, is provided with a P-N junction to minimize the effects of surface states. An intermediate layer 34 is disposed between the cap layer 36 and upper charge confinement layer 30. The upper confinement layer 30 and intermediate layer 34 are doped with opposite polarities to provide a P-N junction which creates a built-in electric field having sufficient strength to keep mobile charge carriers, transported by a SAW along the charge transport channel, from being trapped by or recombined with surface states at the external interface of the cap layer 36. Alternatively, the intermediate layer is not present and a cap layer 42 is doped to provide one side of the P-N junction.

TECHNICAL FIELD 
This invention relates to heterojunction acoustic charge transport (HACT) 
devices, and more particularly, to HACT devices having mobile charge 
carriers near an exterior surface of the device. 
BACKGROUND ART 
It is known in the art to create a heterojunction acoustic charge transport 
device (HACT) having a plurality of layers typically comprising a charge 
transport layer surrounded on its upper and lower surfaces by charge 
confinement layers. Above the upper charge confinement layer (at the 
external air interface), is typically a cap layer. All of the 
aforementioned layers may be grown above a substrate, such as gallium 
arsenide (GaAs). The lower charge confinement layer is typically made of 
undoped (or not intentionally doped) aluminum gallium arsenide (AlGaAs), 
and the upper charge confinement layer is typically made of N-doped AlGaAs 
(i.e., AlGaAs doped with an N-type dopant). The charge transport layer is 
typically made of undoped GaAs. However, other semiconductors having 
piezoelectric properties known to those skilled in the art may also be 
substituted for these materials. It is also known that a surface acoustic 
wave (SAW) may be launched into the HACT structure by known means, such as 
an interdigital SAW transducer. Further, charge may be injected into the 
structure at one end and be carried by the SAW (in groups called "charge 
packets") along the charge transport layer to another where it is removed. 
The charge carried by the SAW stays confined to the charge transport layer 
because the charge transport layer material has a conduction band energy 
lower than that of the surrounding charge confinement layers. Such a HACT 
device is described in commonly-owned U.S. Pat. No. 4,893,161 to Tanski et 
al, which is incorporated herein by reference. 
It is also known in the art that HACT epitaxial layer structures have 
"surface states" at the air/cap layer interface (i.e., the external 
surface of the cap layer). Surface states are a well known phenomena which 
exhibit trapping and recombination sites for mobile charge carriers. It is 
speculated by those skilled in the art that surface states are created due 
to imperfections (defects) in the crystalline structure at the external 
surface of the device which cause loose molecular bonds. However, it is 
known that surface states "trap" (attract and hold) electrons from, or 
supply electrons to "recombine" with, the charge packets propagating 
within the charge transport layer, thereby distorting the information 
carried thereby. 
Prior attempts to reduce the effects of the surface states have included an 
N-doped GaAs cap layer, whereby the dopant electrons are intended to fill 
the surface states so that electrons transported by the SAW do not get 
trapped by the surface states. The precise doping concentration for 
satisfying surface state traps depends on the number of traps at the 
surface, which can vary depending upon material processing. However, even 
if all the traps are satisfied by donor electrons, the surface states will 
still cause carrier recombinations because the bonds to the surface states 
are not strong. Similarly, running an initial group of charge packets 
through the system at power-up in an attempt to fill the surface states 
suffers the same results (i.e., electrons would be attracted to the 
surface states and subsequently leave the surface states and recombine 
with the charge packets). 
Therefore, it is desirable to reduce the effects of surface states in a 
predictable and reproducible manner in order to improve the charge 
transport efficiency along the charge transport layer. 
DISCLOSURE OF INVENTION 
Objects of the invention include provision of a heterojunction acoustic 
charge transport (HACT) device which has improved charge transport 
efficiency along a charge transport channel by reducing effects of surface 
states in a reproducible manner. 
According to a first aspect of the present invention, a HACT device 
comprises a charge transport layer (or channel) surrounded by an upper and 
a lower charge confinement layer, an intermediate layer above the upper 
confinement layer, and a cap layer above the intermediate layer, which 
forms an outer surface of the device. A P-N junction is employed 
comprising the upper confinement layer doped with a first dopant polarity 
and the intermediate layer doped with a second dopant polarity opposite to 
the first dopant polarity. The P-N junction provides a built-in electric 
field having sufficient strength to keep mobile charge carriers, which are 
transported by a SAW along the charge transport channel, from being 
trapped by or recombined with surface states at the air interface 
(external surface) of the cap layer. 
According further to the first aspect of the invention, the lower 
confinement layer is made of AlGaAs, the charge transport layer is made of 
GaAs, and the upper charge confinement layer is made of AIGaAs. According 
further still to the first aspect of the invention, the cap layer is made 
of GaAs. In still further accord to the first aspect of the invention, the 
intermediate layer is made of AlGaAs. According still further to the first 
aspect of the invention, the mobile charge carriers are electrons, the 
upper charge confinement layer is doped with an N-type dopant, and the 
intermediate layer is doped with a P-type dopant. 
According to a second aspect of the present invention, a HACT device 
comprises a charge transport layer surrounded by an upper and a lower 
charge confinement layer, and a cap layer above the upper confinement 
layer, which forms an outer surface of the device. A P-N junction is 
employed by having the upper confinement layer doped with a first dopant 
polarity and by having the cap layer doped with a second dopant polarity 
opposite to the first dopant polarity. The P-N junction provides a 
built-in electric field having sufficient strength to keep mobile charge 
carriers, which are transported by a SAW along the charge transport layer, 
from being trapped by or recombined with surface states at the air 
interface (external surface) of the cap layer. 
According further to the second aspect of the invention, the lower 
confinement layer is made of AlGaAs, the charge transport layer is made of 
GaAs, and the upper charge confinement layer is made of AlGaAs. According 
further still to the second aspect of the invention, the cap layer is made 
of GaAs. According still further to the second aspect of the invention the 
mobile charge carriers are electrons, upper charge confinement layer is 
doped with an N-type dopant, and the cap layer is doped with a P-type 
dopant. 
The invention represents a significant improvement over previous HACT 
devices by inhibiting disruption of charge packets carried by the SAW 
along the charge transport channel caused by the surface states. 
Furthermore, this technique may be used with mobile charge carriers of 
either electrons or holes. 
The foregoing and other objects, features and advantages of the present 
invention will become apparent in light of the following detailed 
exemplary embodiments thereof, as illustrated in the accompanying drawings 
.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, a typical prior art HACT device similar to that 
described in the aforementioned U.S. Pat. No. 4,893,161, comprises a 
substrate 10 made of GaAs having a thickness of approximately 500 microns. 
Grown above the substrate 10 is a buffer layer 12 of GaAs having a 
thickness of approximately 1000 to 9000 .ANG., which is grown, as is 
known, to provide clean GaAs material to grow the remaining layers from. 
Grown above the buffer layer 2 is a lower charge confinement layer 14 made 
of AlGaAs having a 32% concentration of aluminum (Al) not intentionally 
doped (NID) and having a thickness of approximately 1100 .ANG.. Above the 
lower charge confinement layer 14, is a charge transport layer 16 made of 
NID GaAs, having a thickness of approximately 400 .ANG.. The charge 
transport layer 16 is also called a charge transport channel because the 
layer 16 acts as a conduit (or channel) for propagating electrons without 
having electrons leak into other layers of the device (due to the 
difference in conduction band energy, as discussed hereinbefore). 
A surface acoustic wave (SAW) 18 is launched and propagates through the 
HACT device, as disclosed hereinbefore, and carries charge packets 19 (a 
group of electrons) along the charge transport 16. Above the charge 
transport layer 16 is an upper charge confinement layer 20 made of AlGaAs 
having an Al concentration of 32% and being N-doped with a concentration 
of approximately 2.times.10.sup.17 /cm.sup.3 and having a thickness of 
approximately 700 .ANG.. Above the upper charge confinement layer 20, is a 
cap layer 22 made of NID GaAs having a thickness of approximately 200 
.ANG.. 
The outer surface 24 of the cap layer 22 of the prior art HACT device 
exhibits surface states, as described hereinbefore, which attract or 
recombine with (as shown by lines 26) electrons travelling along the SAW 
18 through the charge transport layer 16. The surface states may be caused 
by dangling bonds, dislocations in the crystal, vacancies within the 
crystal, or by other reasons, and exist at the surface 22 or within an 
atomic layer thereof. As the SAW 18 propagates along the charge transport 
channel 16, electrons in the charge packets 19 are attracted to or are 
recombined with charges from the surface states, thereby altering the 
amount of charge in the charge packets and disrupting the transportation 
of electronic information along the channel (as discussed hereinbefore). 
Referring now to FIG. 2, layers below the line 9 represent the prior art. 
Above the charge transport layer 16, is an upper charge confinement layer 
30 of N-doped AlGaAs having a concentration of Al of 2.times.10.sup.17 
/cm.sup.3 and a thickness of approximately 700 .ANG., similar to that of 
the prior art. Above the upper charge confinement layer 30, is an 
intermediate layer 34 of P-doped AlGaAs having a dopant concentration of 
approximately 10.sup.17 to 10.sup.18 /cm.sup.3 and a thickness of about 50 
to 100 .ANG.. Above the layer 34 is a cap layer 36 of NID GaAs having a 
thickness of 50 to 100 .ANG.. 
The layers 30,34 comprise a P-N junction thereby creating a depletion 
region 35 having an inherent electric field E (or potential voltage 
barrier) which repels mobile electrons travelling along the SAW in the 
charge packets 19 within the charge transport channel 16. The 
concentration of the P-dopant in the intermediate layer 34 is chosen to be 
high enough to form a suitable potential barrier to overcome the 
attraction from the surface states discussed hereinbefore. However, the 
dopant concentration must not be so high as to short-out the SAW surface 
potential. 
The higher the P-doping concentration of the layer 34, the greater the 
built-in field E, the greater the electron repulsion, and the thinner the 
layer 34 needs to be. Conversely, the reverse is true for lower doping 
levels. Furthermore, the depletion region 35 must be designed so as not to 
extend into the charge transport layer 16. Also, as is known, the charge 
transport layer 16 should be kept as close to the surface 40 as possible, 
e.g., 0.01 to 0.1 times the SAW wavelength, to maximize the time response 
of charge sensing electrodes (not shown) typically mounted on the surface 
40. 
The N-type dopant for AlGaAs may be silicon (Si) and the P-type dopant for 
AlGaAs may be beryllium (Be) or carbon (C). However, other dopants may be 
used depending on the type of growing technique used. The deposition 
(growth) technique for the invention is molecular beam epitaxy, where the 
vapor pressure and the sticking coefficient of the dopant material are 
important parameters in selecting a dopant. However, other techniques may 
be used if desired, as is known. Dopants used with other growth techniques 
include sulfur (S) for an N-type dopant and zinc (Zn) for a P-type dopant. 
It should be understood by those skilled in the art of P-N junctions and 
heterojunctions that other doping concentrations and thickness ranges of 
the P-doped layer 34 which satisfy the conditions of electron repulsion, 
depletion layer thickness, and electric field strengths which do not short 
out the SAW fields at the surface 40, may be used. 
With the P-N junction in place, the cap layer 36 still exhibits surface 
states of the upper surface 40; however, they are no longer satisfied by 
mobile charge carriers from the charges carried along the charge transport 
layer due to the electric field E. 
Referring to FIG. 3, a graph of the conduction band energy level shows the 
built-in electric field 80 (potential barrier or potential difference) due 
to the P-N junction of the invention as compared with the potential 
barrier 82 without the P-N junction. The potential barrier 80 between the 
cap layer 36 and just prior to the quantum well of the charge transport 
layer is much larger than the potential barrier 82 of the prior art, 
thereby providing a sufficient repelling electric field E (FIG. 2) to 
allow charge packets to propagate along the charge transport channel 
undisturbed by the surface states at the surface 40 of the cap layer 36. 
Although the invention has been described as using electrons as the mobile 
charge carriers making up the charge packets 19, it should be understood 
by those skilled in the art that holes may be used instead of, or in 
addition to, electrons. In that case, the upper confinement layer 30 would 
be P-doped and intermediate layer 34 would be N-doped, so as to direct the 
electric field E in the opposite direction to that shown in FIG. 2. 
Also, instead of having an NID cap layer, the cap layer may be doped with 
the same polarity as the intermediate layer 34. 
Further, although the intermediate layer is shown as being made of the same 
material as the upper charge confinement layer 30 (i.e., AlGaAs), it 
should be understood that the intermediate layer may instead be made of 
the same material of the cap layer (i.e., a material that properly 
interfaces with the given HACT device design). 
Referring now to FIG. 4, the invention will work equally well in an 
alternative embodiment having a P-doped GaAs cap layer 42 and not having 
the intermediate layer 34 (FIG. 2). In that case, the P-doped cap layer 42 
is the layer directly above the upper charge confinement layer 30, and has 
the same thickness as described hereinbefore for the cap layer 36 (FIG. 2) 
and the same dopant concentration as described hereinbefore for the 
intermediate layer 34 (FIG. 2). 
Furthermore, the charge injection electrodes, nondestructive electrode taps 
for monitoring charges, and charge extraction electrodes, such as those 
described in the aforementioned Tanski et al patent are unaffected by the 
present invention. 
Still further, instead of using the GaAs and AlGaAs as the substrate and 
charge transport layers, respectively, it should be understood that the 
invention will work equally well with any HACT design, i.e., piezoelectric 
semiconductor substrate and semiconductor (or piezoelectric semiconductor) 
charge transport layer, charge confinement layers, and cap layer, provided 
a P-N junction is employed between the charge transport layer and the 
surface states. 
For example, the charge transport layer could be made of InGaAs as long as 
the concentration is not so high as to cause excessive lattice mismatches, 
as is known. Alternatively, the substrate could be made of Indium 
Phosphide (InP) with the charge confinement layers made of In.sub.x 
Al.sub.1-x As (with x=0.52), with the upper charge confinement layer being 
N-doped, and the charge transport layer made of In.sub.x Ga.sub.1-x As 
(with x=0.53). 
Although the invention has been described and illustrated with respect to 
exemplary embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions and 
additions may be made without departing from the spirit and scope of the 
invention.