Method and apparatus for selecting the resistivity of epitaxial layers in III-V devices

A novel heterostructure acoustic charge transport (HACT) device is disclosed having an optimized charge density. The device includes a transducer fabricated on a substrate structure that launches surface acoustic waves. A reflector is formed in the substrate structure at an end portion adjacent to the transducer for reflecting the surface acoustic Waves. Also included is an electrode configured with the transport channel at an end thereof distal to the transducer for generating electrical signal equivalents of the propagating electrode charge. During fabrication, the resistivity of the layers initially configured at a lower than desired value. The layer is subsequently etched to raise the resistivity to the desired value.

TECHNICAL FIELD 
This invention relates to devices fabricated with III-V materials and more 
particularly to devices having a cap layer whose thickness can be varied 
to effect charge density. 
CROSS-REFERENCE TO RELATED APPLICATIONS 
Some of the subject matter hereof is disclosed and claimed in the copending 
U.S. patent applications entitled "Trimming Technique For Acoustic Change 
Transport Device"; Ser. No. 399,207; "Optically Modulated Acoustic Charge 
Transport Device", Ser. No. 283,624; "Acoustic Charge Transport Device 
Having Direct Optical Input", Ser. No. 283,618 and "A Monolithic 
Electro-Acoustic Device Having An Acoustic Charge Transport Device 
Integrated With A Transistor", Ser. No. 283,625, each of which is 
incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
Acoustic charge transport (ACT) phenomena in III-V semiconductor material 
has only recently been demonstrated. Such devices have applications as 
high speed analog signal processors. Delay lines have been fabricated in 
gallium arsenide (GaAs) structures comprising a surface acoustic wave 
(SAW) transducer that launch a surface acoustic Wave along upper layers of 
GaAs or (AlGa)As substrate having transport channel formed therein. An 
input electrode sources charge to be transported by the propagating 
potential wells. There is also an electrode receiving a signal for 
modulating that charge. Spaced down the transport channel are one or more 
nondestructive sensing (NDS) electrodes for sensing the propagating 
charge. There is also an ohmic output electrode for removing the charge. 
Initial acoustic charge transport devices were comprised of a thick 
epilayer (TE-ACT), With vertical charge confinement accomplished by means 
of an electro-static DC potential applied to metal field plates on the top 
and bottom surfaces of the GaAs substrate. The field plate potentials are 
adjusted to fully deplete the epilayer and produce a potential maximum 
near the midpoint thereof. Consequently, any charge injected into the 
channel is confined to the region of maximum DC potential. 
Lateral charge confinement (Y direction) has been achieved in several Ways 
Typically, a mesa is formed to define a charge transport channel. However, 
for thick epilayer acoustic transport devices, the mesa must be several 
microns in height, a fact which presents problems in fabrication and is a 
major impediment to the propagating surface acoustic Wave. Blocking 
potentials extending down both sides of the delay line have also been used 
to define the transverse extent of the channel, as has proton bombardment 
to render the material surrounding the channel semi-insulating. 
A heterostructure acoustic charge transport (HACT) device (HACT) has been 
fabricated using a GaAs/AlGaAs heterostructure that is similar to that of 
quantum well lasers and heterostructure field effect transistors FET (e.g. 
HFET, MODFET, HEMT and TEGFET devices). A HACT device is comprised of a 
sequence of epitaxial layers and vertically confines mobile carriers 
through the placement of potential steps that result from band structure 
discontinuities. Besides providing for inherent vertical charge 
confinement, the HACT devices are thin film devices whose layers have a 
total thickness of approximately 0.25 microns, excluding a buffer layer. 
A cap layer is provided With a HACT device both to protect an upper 
(AlGa)As layer and to permit fabrication of low resistance ohmic contacts 
and low leakage Schottky metalization. However, it is not possible to 
systematically and repeatedly produce a layered structure with the 
required sheet resistivity. Consequently, wafers must often be discarded 
because the grown resistivity of the device is not in an acceptable range. 
It would be advantageous to have a method and apparatus for fabricating 
lII-V devices capable of adjusting the resistivity thereon after layer 
growth is complete, thereby reducing the number of unsuitable wafers and 
increasing device yield. The present invention is drawn towards such a 
device. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus 
that selects the resistivity of an electronic device comprised of III-V 
material. 
According to the present invention an acoustic charge transport device is 
formed on a piezoelectric semiconducting structure and includes a 
transducer fabricated on a first surface thereof for launching surface 
acoustic waves along a propagation axis. The surface acoustic waves are 
characterized by maxima and minima of electrical potential and provide 
transport for electric charge provided thereto. A reflector is formed on 
the surface at an end portion there adjacent to the transducer for 
reflecting the surface acoustic Waves. The device has a transport channel 
that is characterized by an intrinsic vertical electrical potential such 
that charge provided thereto is presented to the surface acoustic waves 
for transport. The channel is further formed to provide lateral and 
vertical confinement of the propagating charge. The device also includes 
an electrode configured With the transport channel at an end thereof 
distal to the transducer for generating an electrical signal equivalent of 
the propagating electrical charge. The device is characterized by the 
transport channel having a first layer of aluminum gallium arsenide grown 
on a gallium arsenide substrate; a first layer of gallium arsenide grown 
on the aluminum gallium arsenide layer; a second layer of doped aluminum 
gallium arsenide grown on the first layer of gallium arsenide and a second 
layer of gallium arsenide grown on the second layer of aluminum gallium 
arsenide, and wherein a selected amount of the second layer of gallium 
arsenide is removed, thereby increasing the sheet resistivity of the 
device.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 there is a schematic illustration of an acoustic 
charge transport device provided according to the present invention. The 
device 10 is preferably comprised of a III-V material, such as GaAs and 
AlGaAs which is both piezoelectric and semiconducting. As is known, these 
materials are very closely lattice matched, having lattice parameters that 
differ by less than 0.04%. As a result, their ternary solutions are nearly 
ideal for preparation by epitaxial growth. In addition, the energy band 
gap of an AlGaAs compound (Al.sub.x, Ga.sub.1-x)As increases monotonically 
with the parameter x up to x approximately equal to 0.4, at which point 
the band gap of the ternary becomes indirect. Potential steps as large as 
0.3 ev can be obtained in a heterostructure device. Moreover, the 
heterojunction band structure potential is a property of the composite 
material alone and is not diminished by the transport charge load. 
On the surface of the device 10 there is formed a surface acoustic wave 
transducer 11 and reflector 12. The transducer is fabricated in a known 
manner and preferably comprises an interdigitated (IDT) transducer of 
aluminum copper alloy deposited on device top surface 13. A surface 
acoustic wave is launched along surface 13 via the transducer by means of 
signals presented by transducer driver 14. Similarly, the reflector 12 is 
of a known type and reflects the surface acoustic wave along the surface 
16. Charge is provided via input ohmic contact 15 received by potential 
wells of the surface acoustic wave. The charge is modulated by means of 
signals from source 16 presented to input Schottky contact 17 and is 
propagated along a transport channel 18. Output Schottky electrode 19 
provides signals on line 20 corresponding to the modulated charge 
presented thereto. Finally, the charge is extracted from the device at the 
output ohmic electrode 22. 
The device 10 provides vertical charge confinement through formation of a 
potential well within a GaAs/AlGaAs layered structure using the 
differences in the conduction band energies of the contiguous layers. No 
external applied potentials are required for charge confinement in the 
vertical direction in the device 10. While lateral confinement of the 
propagating charge in the transport channel can be accomplished by 
conventional proton implant to produce a semi-insulating area surrounding 
the channel 18 on the surface, it is preferred to use mesa isolation. 
FIG. 2 is a sectioned diagrammatic illustration of the device of FIG. 1, 
detailing the epitaxially grown layers thereof. Although the 111-V layers 
detailed herein are preferably grown by molecular beam epitaxy (MBE), 
other epitaxial techniques which provide equivalent structural purity can 
be substituted. As described hereinabove, the device 10 is a thin film 
heterostructure device whose charge confinement layers have a total 
thickness typically less than 0.25 micron, excluding any buffer layer. On 
a semi-insulating GaAs substrate 24 there is formed a first, 
non-intentionally doped (NID) 100 nm thick layer 26 of (AlGa)As, which 
receives a 60 nm thick layer 28 of NID GaAs which forms the transport 
channel. A second, upper layer 30 of (AlGa)As is grown on the layer 28 
with a doping in the range of 2.times.10.sup.17 /cm.sup.3 and a thickness 
of about 70 nm. Finally, there is a 20 nm cap layer 32 of GaAs in order to 
prevent oxidation of the (AlGa)As charge control layer and to aid in 
electrical contact formation. 
As demonstrated by the conduction band potentials shown in FIG. 2 (curve 
34), a potential well 0.25 ev deep is created in the GaAs layer 28 which 
serves to confine the charge vertically in the transport channel. The 
thickness and doping level of the (AlGa)As layer 30 is designed to provide 
a sufficient number of electrons to fill the surface states therein while 
leaving the remainder of the structure essentially free of excess 
carriers. In the device of FIG. 1, a mole fraction of 32% aluminum was 
used. As noted above, the heterostructure structure described with respect 
to FIG. 2 provides for vertical charge confinement and eliminates the need 
for backgating consideration and external biasing that is otherwise 
necessary for conventional acoustic charge transport devices. The GaAs 
transport channel is undoped to provide high electron mobility, and there 
is an increased charge transfer efficiency due to a limited charge packet 
volume and lower bulk charge trapping. The transport channel formed in the 
device 10 differs from a double heterostructure FET devices in that the 
charge in a FET transistor is supplied by donors in the (AlGa)As layers. 
However, in the HACT device 10, the transport channel is ideally initially 
empty and charges are injected into the channel. 
A HACT device must have a structure (i.e., substrate with the epilayers 
grown thereon) which is largely depleted in order to maximize the charge 
transfer efficiency by minimizing the undesired charge load. An acceptable 
level of depletion of the device structure has been attained 
experimentally. Starting with a device having an upper barrier layer 70 nm 
thick and charge control layer doped to about 2.times.10.sup.17 /cm.sup.3 
(parameters whose magnitude has been determined theoretically), the device 
resistivity is as measured in a lighted, clean room laboratory. The 
lighting is typically florescent lamps positioned about 10 feet from the 
contactless conductivity probe used to measure resistivity. It is 
necessary to measure device resistivity in a lighted laboratory because 
the resistivity of the device epitaxial layers is above the range (20,000 
ohms/square) of the instrument when the device is in total darkness. The 
light produces charged carriers in the device epilayers which contribute 
to the conductivity of the material, and these carriers are sufficient to 
bring the resistivity into range of the instrument. The resistivity of the 
device is initially noted and devices are then fabricated. Devices with 
low measured resistivity vales (e.g., between 500 and 4000 ohms/sq.) have 
been used, but the HACT device charge transfer efficiency has been very 
poor due to the presence of excess free electrons. Devices with an initial 
measured resistivity between 4000 and 6000 ohms/sq. provide acceptable 
HACT performance. 
In practice, the target doping level of the upper AlGaAs barrier layer is 
usually adjusted each time the MBE system is opened because the background 
doping and contamination level in the epitaxial layers change with each 
MBE system modification. Thus, the target doping level of the upper 
barrier can range between 1.times.10.sup.17 /cm.sup.3 and 
4.times.10.sup.17 /cm.sup.3, depending on the cleanliness of the MBE 
system and the quality of the material sources therein. This target doping 
is determined experimentally by varying the doping (e.g., changing the 
silicon effusion cell temperature or making small changes in the barrier 
layer thickness) of the device epilayers until a resistivity between 3000 
and 4000 ohms/sq. is achieved for as-grown layers by etching off the GaAs 
cap layer in small increments as set forth herein. 
After device fabrication, conventional contactless conductivity probe 
measurements using contactless inductive probes are used to determine 
whether the substrate is underdoped, doped correctly or overdoped. The top 
layer is exposed to a dilute citric acid for several seconds prior to 
these measurements. In accordance with the present invention, devices 
which are overdoped have the sheet carrier density altered or "trimmed" to 
the correct value thereof by removing a portion of the top layer of the 
device. It is preferable that the top layer be removed by a chemical 
etchant consisting of 50% water and 50% hydrogen peroxchemide and ammonium 
peroxide as is required to bring the ph of the solution to 8.2. The 
preferred etchant removes gallium arsenide from the substrate at a rate of 
between 10 and 20 Angstroms per second and removes aluminum gallium 
arsenide having an aluminum/gallium ratio of 0.3 at a rate of 36 Angstroms 
per second. 
Those skilled in the art will note that the sheet carrier density can be 
trimmed regardless of the doping or type of the layers which the device 
may possess. For example, in FIG. 2, cap layer 32 can be undoped or the 
cap layer 36 can be eliminated entirely, in which case "trimming" of the 
sheet carrier density is accomplished by removing a selected amount of the 
charge control layer 30. 
With a device provided in accordance With the present invention, thinning 
the non-intentionally doped (NID) cap layer to 100 Angstroms produces an 
order of magnitude reduction in charge density as compared to an initial 
cap layer thickness of 200 Angstroms, i.e. 10.sup.16 /cm.sup.3 as compared 
with 10.sup.17 /cm.sup.3. 
At the surface of GaAs cap layer, the Fermi level is known to be fixed, or 
pinned, at a level of about 0.8 volts below the conduction band minimum. 
This pinning is affected by the trapping of electrons, removed from the 
bulk of the material, at defect sites at the GaAs surface. In the 
structure of the device 10, the top (or cap) layer has been grown without 
intentional doping (i.e. the addition of electron donors or acceptors) and 
is generally only 20 to 40 nm thick. Below this cap layer is the upper 
trapping layer of (Al,Ga)As which is doped (on the order of 
2.times.10.sup.17 /cm.sup.3, donor atoms) in order to supply the electrons 
necessary to affect the pinning potential. 
It has been found experimentally that optimum device performance is 
obtained when the sheet resistivity of the device (substrate and all 
epitaxial layers) is between 4000 and 6000 ohms/square. Resistivity in 
this range indicates that, in addition to the charge carriers present due 
to the resistivity measurement being made under room lighting, there are 
enough residual charges present to ensure that deep depletion is avoided. 
Deep depletion causes poor charge transfer efficiency because the injected 
electrons are absorbed by the epitaxial layer structure to satisfy native 
trap sites. The doping in the upper barrier layer must therefore supply 
charges to effect Fermi level pinning, as well as a small amount of charge 
to prevent deep depletion. 
In the process of supplying the charges to the surface and the well, the 
upper barrier layer is depleted, and a capacitance is formed between the 
surface (negatively charged with electrons) and the barrier layer which is 
positively charged. The magnitude of the capacitance is directly 
proportional to the thickness of the GaAs cap layer. The amount of charge, 
Q, depleted to the surface is determined by the surface potential and the 
capacitance by the defining equation for capacitance, i.e. 
EQU C=Q/V 
A more instructive form of the equation is Q=C V, since this form shows 
that, given a fixed Fermi level potential (V=0.8 V), the amount of charge, 
Q, depleted to the surface will increase as the capacitance, C, increases. 
Since the amount of charge available is fixed (the upper barrier of 
(AlGa)As being fixed in thickness and doping level) the amount of charge 
available to deplete into the transport well, thereby contributing to the 
resistivity, is decreased as the cap is thinned and the capacitance 
thereby increased. As the amount of charge in the well decreases, the 
sheet resistivity increases. To be useful, it is necessary to ensure that 
the grown layers have a sheet resistivity lower than that desired because 
the process of partial cap removal can only be used to increase the sheet 
resistivity. 
Similarly, although the invention has been shown and described with respect 
to a preferred embodiment thereof, it should be understood by those 
skilled in the art that various other changes, omissions and additions 
thereto may be made therein without departing from the spirit and scope of 
the present invention.