Method of making indium arsenide magnetoresistor

COPENDING PATENT APPLICATIONS
This patent application is related to the following concurrently filed United States patent applications, which are assigned to the same assignee to which this patent application is assigned: Ser. No. 289,646, now abandoned entitled "Improved Magnetoresistor," and filed Dec. 23, 1988 in the names of Dale L. Partin, Joseph P. Heremans and Donald T. Morelli; and Ser. No. 289,641, now abandoned, entitled, "Improved Position Sensor," and filed in the names of Donald T. Morelli, Joseph P. Heremans, Dale L. Partin, Christopher M. Thrush and Louis Green. Continuations-in-part of both of the foregoing concurrently filed applications are being filed with this application.
This patent application is also related to the following earlier filed United States patent this invention; Ser. No. 229,396 entitled, "Position Sensor," and filed in the names of Thaddeus Schroeder and Bruno P. B. Lequesne on Aug. 8, 1988, now U.S. Pat. No. 4,926,122.
While not believed to relate specifically to the invention claimed herein, the following copending United States patent applications are mentioned because they also relate to magnetic field sensors and are assigned to the assignee of this patent application: Ser. No. 181,758 entitled, "Magnetic Field Sensor," and filed Apr. 14, 1988 in the names of Dale L. Partin and Joseph P. Heremans, now U.S. Pat. No. 4,843,444; and Ser. No. 240,778 entitled, "Magnetic Field Sensors," and filed Sep. 6, 1988 in the names of Joseph P. Heremans and Dale L. Partin, now U.S. Pat. No. 4,926,226.
FIELD OF THE INVENTION
This invention relates to magnetic field sensors and, more particularly, to improved thin film magnetoresistors and to methods of making such a magnetoresistor.
BACKGROUND OF THE INVENTION
In the past, magnetoresistors were believed to be best formed from high carrier mobility semiconductive material in order to get the highest magnetic sensitivity. Hence, the focus was on making magnetoresistors from bulk materials that were thinned down or on films having sufficient thickness to exhibit a high average mobility.
We have found a new way to approach making magnetoresistors. We have found that if a thin enough film of indium arsenide is formed, and if an accumulation layer-like characteristic is provided in the film, an extremely enhanced magnetoresistor can be formed with such a material. We believe that an accumulation layer is inherently provided in an indium arsenide film when it is exposed to air. Tests indicate that the properties of the accumulation layer relevant to magnetic sensitivity can dominate over those of the remainder of the film, if the remainder of the film is thin enough.
SUMMARY OF THE INVENTION
The present invention is directed to a magnetoresistor formed in a film of indium arsenide having an inherently formed accumulation layer of current carriers, and in which the magnetic change in conductivity of the accumulation layer is not masked by the conductivity of the balance of the film. The accumulation layer can be of the same conductivity type as that of the indium arsenide film or of opposite conductivity type. An opposite conductivity type accumulation layer is also referred to as an inversion layer but is considered to be within the scope of the phrase "accumulation layer" as used in this patent application.
The present invention is also directed to new methods of making indium arsenide magnetoresistors.
The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic view of a magnetoresistor, showing its electrical current flow lines when no magnetic field is applied to it.
FIG. 1B is a schematic view of a magnetoresistor, showing how the electrical current flow lines are redirected in the plane of its major surface when a magnetic field is applied perpendicular to that surface.
FIG. 2 is an isometric view showing a magnetoresistor having two integral sensing areas electrically in parallel.
FIG. 3 is a three-dimensional or contour plot showing the change of electrical resistance in a single element larger band gap semiconductor magnetoresistor with changes in temperature and magnetic field strength.
FIG. 4 is a two-dimensional plot of the fractional magnetoresistance over a wider temperature range than shown in FIG. 3.
FIG. 5 is a two-dimensional plot showing change in resistance with no magnetic field applied over a wider temperature range than shown in FIG. 3.
FIG. 6 is an elevational view showing a semiconductor film in a pattern for providing a series connected plurality of sensing areas integrated in a single magnetoresistor.
FIG. 7A is an elevational view showing a metallization pattern for superposition on the pattern of FIG. 6.
FIG. 7B is an elevational view showing the metallization pattern of FIG. 7A superimposed on the pattern of FIG. 6 to delineate a plurality of sensing areas.
FIG. 8 is a three-dimensional or contour plot showing the change of electrical resistance of a multiple sensing area magnetoresistor such as shown in FIG. 7A.
FIGS. 9 and 10 are two-dimensional electron energy to depth plots showing how electrons could be confined in an accumulation layer under special layers on the surface of the sensing area of the magnetoresistor.
The drawings are not to scale.

DETAILED DESCRIPTION
As shown in FIG. 1A, a typical magnetoresistor element 10 comprises a slab (substrate, body) 12 of semiconductor, typically rectangular in shape, through which a current is passed. Such a magnetoresistor is described by S. Kataoka in "Recent Development of Magnetoresistive Devices and Applications," Circulars of Electrotechnical Laboratory, No. 182, Agency of Industrial Science and Technology, Tokyo (Dec. 1974).
In the absence of magnetic field, current lines 14 go from one injecting electrode 16 to another electrode 18 in parallel lines (see FIG. 1A). Bonding wires (contacts) 16a and 18a are connected to electrodes 16 and 18, respectively. This flow is between electrodes 16 and 18 along the top and bottom edges of the rectangle in FIG. 1A. The geometry (a rectangle in our example) of slab 32 is chosen so that an applied magnetic field, perpendicular to the slab 12, increases the current line trajectory (see FIG. 1B). The magnetic field perpendicular to the plane of the paper thus lengthens the current flow lines 14. As is indicated by the denotation B=0, there is no magnetic field applied to slab 32 of FIG. 1A. The longer length leads to higher electrical resistance, so long as the resulting lateral voltage difference is electrically shorted, as shown, by the top and bottom edge electrodes 16 and 18.
FIG. 1B shows the electrical current flow lines through slab 32 are redirected when a magnetic field B (shown as a B with a circle having a dot in the center thereof) and coming out of the page is applied perpendicular to slab 32.
The best geometry for this effect to occur is one where the current injecting electrodes 16 and 18 are along the longest side of the rectangle, and the ratio of this dimension ("width") to the shortest dimension ("length") is as large as possible. Preferably, the length of the shortest side is 10% to 60% of the length of the longest side and, more preferably, 20% to 40% of the length of the longest side. Such an optimal device geometry hence leads to a very low resistance. Kataoka teaches that the magnetic field sensitivity of such devices is best when the devices are made out of semiconductors with as large a carrier mobility as possible. The resistivity of such devices is made less temperature-dependent when the semiconductor material contains a large donor concentration, giving a large carrier density. These last two constraints imply that semiconductors with high electrical conductivity are best suited for practical applications.
Combined with the geometrical restrictions described earlier, one can deduce that the final magnetoresistor element will have a low resistance. This has a practical drawback. Under a constant voltage, the power dissipated by the device scales as the inverse of the resistance. To limit ohmic heating (which would limit the operational temperature range of the sensor, if not destroy the sensor itself) while maintaining a large voltage output during sensor interrogation, it is desirable that a magnetoresistive element have a resistance around 1,000 ohms. However, a resistance of from about 300 ohms to 6,000 ohms is acceptable in many applications. A number of ways have been proposed to achieve such resistances. For example, as Kataoka has pointed out, one can put a number of elementary devices in series. Making a plurality of sensing areas as integral parts of a single element is shown in FIG. 2, which shows a plurality of magnetoresistors 20 formed in an epitaxial layer 22 which is on an insulating substrate 24. Spaced-apart electrodes 26 are on a top surface of epitaxial layer 22. While only two sensing areas (i.e., devices) are shown, one could make an element with tens or hundreds of integral sensing areas (i.e., devices).
If the metal-semiconductor (magnetic-field independent) interfacial contact resistance of one such elementary device is an appreciable fraction of the semiconductor resistance of this elementary device, it will lower the sensitivity to a magnetic field. Thus, metals must be deposited which have a very low metal-semiconductor interfacial contact resistance to avoid this sensitivity degradation. In most cases, we would prefer that the interfacial contact resistance between the sensing area and its electrodes be 10-100 times less than the resistance of the sensing area between those electrodes. Another option which alleviates the problem of low magnetoresistor device resistance has been to use active layers that are as thin as possible. This has been done by thinning wafers of indium antimonide (InSb), which were sliced from bulk ingots, down to thicknesses as small as 10 microns. The wafer thinning process is a very difficult process, since any residual damage from the thinning process will lower the electron mobility. Reducing electron mobility will decrease the sensitivity to a magnetic field of devices made from this material.
Another approach has been to deposit films of InSb onto an insulating substrate. On the other hand, in this latter case, the electron mobility of the resulting films is reduced to a fraction of that of bulk InSb. This reduction occurs because of defects in the film. With typical mobilities of 20,000 cm.sup.2 V-sec.sup.-1, these films produce devices with greatly reduced sensitivity to a magnetic field compared to devices made from bulk InSb. As shown in FIG. 2, usual magnetoresistors 20 made from a film includes an epitaxial layer 22 of the semiconductor material on the surface of an insulating substrate 24. Spaced-apart metal electrodes 26 are on the semiconductor layer 22 and extend thereacross to form rectangular active regions 28 of the semiconductor material 22 therebetween. As shown, the magnetoresistor 20 includes two active regions 28, but may contain many more of the active regions.
The great majority of the prior work until now has focused on InSb. This can be understood from the data in the following Table I.
TABLE I______________________________________Potential Magnetoresistor Materials at 300K Maximum Crystal Energy Electron Lattice BandSemiconductive Mobility Constant GapMaterial (cm.sup.2 V.sup.-1 sec.sup.-1) (A) (eV)______________________________________InSb 78,000 6.478 0.17Bi.sub.1-x Sb.sub.x (x < 0.2) 32,000 6.429(Bi) 0-0.02InAs 32,000 6.058 0.36InO.sub..53 Ga.sub.0.47 As 14,000 5.869 0.75(on InP)GaAs 8,000 5.654 1.4GaSb 5,000 6.095 0.68InP 4,500 5.869 1.27______________________________________
For these III-V compounds, e.g., indium arsenide, the energy band gap decreases with increasing temperature.
Since the magnetoresistance effect is proportional to electron mobility squared for small magnetic fields, InSb is highly preferable. However, the difficulty of growing compound semiconductors in general, and the fact that there is no suitable, lattice-matched, insulating substrate upon which it may be grown, led us to try growing Bi films. Such work has been previously reported by Partin et al. in Physical Reviews B, 38, 3818-3824 (1988) and by Heremans et al. in Physical Reviews B, 38, 10280-10284 (1988). Although we succeeded in growing the first epitaxial Bi thin films, with mobilities as high as 25,000 cm.sup.2 V.sup.-1 sec.sup.-1 at 300K and 27,000 cm.sup.2 V.sup.-1 sec.sup.-1 for Bi.sub.1-x Sb.sub.x at 300K, magnetoresistors made from these films had very low sensitivities. Modeling studies which we have just completed indicate that this is, to our knowledge, an unrecognized effect of the fact that the energy band structure of Bi has several degenerate conduction band minima. Other high mobility materials shown in Table I have a single, non-degenerate conduction band minimum. We then began growing InSb thin films (on semi-insulating GaAs substrates) using the metal organic chemical vapor deposition (MOCVD) growth technique. After many months of effort, we could only produce films with electron mobilities of 5,000 cm.sup.2 V.sup.-1 sec.sup.-1.
However, we have found that good magnetoresistors can be formed of thin films of undoped indium arsenide (InAs) on a semi-insulating substrate. By "semi-insulating," it is meant that the substrate is of such high resistivity as to be substantially insulating. Preferably, the indium arsenide film should be as thin as can be obtained while still retaining reasonably high carrier mobility. A thickness of less than about 3 micrometers is preferred, although films of a thickness of about 5 micrometers will form satisfactory magnetoresistors. The film should have an accumulation layer along its surface with the areal density of the surface accumulation layer being substantially larger, at least an order of magnitude larger, than the areal density in the bulk of the film. The bulk density of the film is generally moderate, of the order of 10.sup.16 electrons per cubic centimeter, or lower. The film should be good crystalline quality so as to have a high average electron mobility, 10,000 to 32,000 centimeters per volt per second. The electron accumulation layer is effective to provide a magnetic sensitivity and range of operating temperature as if the indium arsenide thin film was apparently much thinner and had a much higher electron density and electron mobility.
We grew indium arsenide (InAs) on semi-insulating GaAs, and also on semi-insulating InP substrates. These latter substrates were made semi-insulating by doping them with Fe. They were tried in addition to GaAs because there is less lattice mismatch with InAs (see Table I). After some time, we were able to produce InAs films with a room temperature mobility of 13,000 cm.sup.2 V.sup.-1 sec.sup.-1 on InP substrates, and of lower mobility on GaAs substrates. The better InAs films were formed by the following process.
An MOCVD reactor manufactured by Emcore Corporation was used. InP substrates were heated to the growth temperature in an atmosphere of 40 torr of high purity (palladium diffused) hydrogen to which a moderate quantity of arsine was added (80 SCCM, or standard cubic centimeters per minute). This produced about 0.02 mole fraction of arsine. The arsine was used to retard thermal decomposition of the InP surface caused by loss of the more volatile phosphorus. The way in which arsine reduces the surface roughening during this process is not well understood. Phosphine would have been preferred, but was not available at the time in our reactor. After reaching a temperature of 600.degree. C., the arsine flow was reduced to 7 SCCM, and ethyl-dimethyl indium (EDMIn) was introduced to the growth chamber by bubbling high purity hydrogen (100 SCCM) through EDMIn which was held at 40.degree. C. Higher or lower arsine flows during growth gave lower mobilities and worse surface morphologies. After 2.5 hours of InAs growth time, the EDMIn flow to the growth chamber was stopped and the samples were cooled to room temperature in an arsine-rich atmosphere (as during heat-up).
The thickness of the resulting InAs film was 2.3 micrometers. From conventional Hall effect measurements at 300K, the electron density was a moderate 1.4.times.10.sup.16 cm.sup.-3 and the electron mobility was 13,000 cm.sup.2 V.sup.-1 sec.sup.-1. These are effectively averages since the electron density and mobility may vary within a film. The film was not intentionally doped. Even though this is a very disappointing mobility, a crude magnetoresistor was made, since this required very little effort. A rectangular sample was cleaved from the growth and In metal was hand-soldered along two opposing edges of the sample, and leads were connected to the In. The length, which is the vertical dimension in FIGS. 1A and 1B, was 2 mm, and the width, which was the horizontal dimension in FIGS. 1A and 1B, was 5 mm.
FIG. 3 graphically shows a three-dimensional or contour plot showing the change of electrical resistance in a single element larger band gap semiconductor magnetoresistor with changes in temperature and magnetic field strength.
As expected, the resistance of the device was low (about 50W) since we did not have many elements in series. However, the magnetoresistance effect was large. It is shown in FIG. 3. Furthermore, the device resistance and magnetoresistance were surprisingly stable with temperatures in the range shown in FIG. 3, which is -50.degree. C. to +100.degree. C. A second, similar device was tested less thoroughly at temperatures as high as +230.degree. C. FIG. 4 graphically shows a two-dimensional plot of the fractional magnetoresistance over a wider temperature range than shown in FIG. 3. FIG. 5 graphically shows a two-dimensional plot showing change in resistance with no magnetic field applied over a wider temperature range than shown in FIG. 3. The results of this latter testing are shown in FIGS. 4 and 5. In FIG. 4, the applied magnetic field was 0.4 Tesla. The fractional magnetoresistance is plotted as a function of temperature between B=0.4 Tesla and B=0. Despite the fact that the indium metal used for contacts has a melting point of 156.degree. C., the magnetoresistor still functioned very surprisingly well at 230.degree. C., with the fractional increase in resistance for a given magnetic field (0.4 Tesla) reduced by less than one-half compared to the response near room temperature (as shown in FIG. 4).
The device resistance in zero magnetic field, R(0), decreased over the same temperature range by a factor of 5 (as shown in FIG. 5). We also found this to be surprisingly good, even taking into account the relatively large energy gap of InAs. Our own detailed analysis of transport data from these films suggests that there are current carriers with two different mobilities present. In retrospect, it looks like our results are related to an accumulation layer of electrons at the surface of the sensing layer. We have now found that Wieder has reported in Appl. Phys. Letters, 25, 206 (1974) that such an accumulation layer exists just inside the InAs near the air/InAs interface. There appear to us to be some errors in the Weider report. However, we think that the basic conclusion that an electron accumulation layer exists is correct. These electrons are spatially separated from the positive charge at the air/InAs interface. Thus, they are scattered relatively little by this charge, resulting in a higher mobility than would normally be the case. They also exist in a very high density in such an accumulation layer, so that as the temperature increases, the density of thermally generated carriers is a relatively small fraction of the density in the accumulation layer. This helps stabilize the resistance (at zero magnetic field) with temperature. Thus, it appears that the relatively low measured electron mobility of 13,000 cm.sup.2 V.sup.-1 sec.sup.-1 is an average for electrons in the accumulation layer and for those in the remainder of the thickness of the film.
Thus, normally one would want to grow a relatively thick layer of InAs to make a good magnetoresistor, since crystal quality (and mobility) generally improve with thickness when growing on a lattice-mismatched substrate. However, the thicker the layer becomes, the greater its conductivity becomes and the less apparent the benefits or presence of a surface accumulation layer would be. Thus, our current understanding of our devices suggests that relatively thinner layers are preferable, even if the average film mobility decreases somewhat, since this will make the conductivity of the surface accumulation layer a greater fraction of the total film conductivity. The exact relationships between film thickness, crystal quality and properties of the surface accumulation layer are currently under study.
We have since made multi-element magnetoresistors from this material using Au (or Sn) metallization. First, conventional photolithography techniques were used to etch away unwanted areas of an Indium Arsenide (InAs) film from the surface of the Indium Phosphide (InP) substrate to delineate the pattern shown in FIG. 6. The delineated film 30 was in the form of a dumbbell having an elongated portion 32 with enlarged portions 34 at each end thereof. A dilute solution (0.5%) of bromine in methanol was used to etch the InAs. Then, a blanket layer of Au metallization 1000 Angstroms thick was deposited using conventional vacuum evaporation techniques over the entire surface of the sample, after removing the photoresist. Conventional photolithography was then used to etch away unwanted areas of the Au film to delineate the gold pattern shown in FIG. 7A. The gold pattern includes a plurality of small, spaced-apart electrodes 36 arranged in a row and a large electrode 38 at each end of the row of small electrodes 36. A dilute aqueous solution of KCN was used for this step. We think dissolved oxygen is helpful. It can diffuse into the solution from ambient air or be supplied in the form of a very small addition of hydrogen peroxide. The resultant composite of the two patterns, with the gold pattern overlying the InAs film pattern, is shown in FIG. 7B where the small electrodes 36 extend across the elongated portion 32 of the film 30 and the large electrodes 38 cover the enlarged portions 34 of the film 30. The electrodes 36 delineate the elongated portion 32 into rectangular active regions 40 and the large electrodes 38 serve as bonding pads.
Leads (not shown) were then attached by silver epoxy to the large Au end bonding pads 38. Leads could also be attached by normal and accepted filamentary wire bonding techniques. If so, and especially if a modern wire bonding apparatus were used, the bonding pads could easily be made much smaller. Also, many devices such as shown in FIGS. 6, 7A and 7B could be made simultaneously using conventional integrated circuit technology. The resulting devices typically have a resistance near 1,000 ohms (typically + or -20%) at room temperature in zero magnetic field. Surprisingly, the magnetoresistance effect on the multisensing area device was much larger than the effect on a single sensing area device. FIG. 8 graphically shows a three-dimensional or contour plot showing the change of electrical resistance of a multiple sensing area magnetoresistor such as shown in FIG. 7A. For comparison, of these effects at a given magnetic field, see FIGS. 8 and 3. In the multi-element device (i.e., plural sensing area element), the sensing areas had a length-to-width ratio of 2/5. We do not understand why the multi-element device works better since the length-to-width ratio of each element is 2/5, the same as for the single element device characterized in FIG. 3, which was fabricated using part of the same InAs grown layer. Another multi-element magnetoresistor was made similarly to the one just described, but with a length-to-width ratio of 4/5. It had nearly as large a magnetoresistance as the one made according to the patterns in FIGS. 4 and 5. Again, we do not yet understand this, but the resulting devices work very well. Even a device with a length-to-width ratio of 6/5 works well.
The relative stability of these magnetoresistors with temperature also now appears to be increasingly important, since some automotive applications require operation from -50.degree. C. to as high as +170.degree. C. to +200.degree. C., and there are known applications requiring even higher temperatures (to 300.degree. C.). There is reason to believe that our invention will provide magnetoresistors operating at temperatures as high as 300.degree. C. and even higher.
A potential problem with InAs magnetoresistors made in accordance with this invention is the potential importance of the air/InAs interface, which might cause the device characteristics to be sensitive to changes in the composition of ambient air, or cause the characteristics to slowly change with time or thermal history because of continued oxidation of the surface. We have tried coating the surfaces of two devices with a particular epoxy made by Emerson and Cuming, a division of Grace Co. The epoxy we used was "Stycast," number 1267. Parts A and B were mixed, applied to the devices, and cured at 70.degree. C. for two hours. We did not observe any significant changes in the device characteristics at room temperature as a result of this encapsulation process. We have not yet systematically tested these devices at other temperatures, but we are encouraged by this preliminary result. We think other forms of encapsulants need to be explored, such as other epoxies and thin film dielectrics, such as SiO.sub.2 or Si.sub.3 N.sub.4. Since exactly what occurs at the air/InAs interface which causes the accumulation layer is not yet known, one thing we intend to explore is depositing a thin film of dielectric or high energy gap semiconductor (such as GaAs, In.sub.1-x Ga.sub.x As,In.sub.1-x Al.sub.x As, or AlSb) right after growth of the InAs is complete, and before exposure to air. We hope that this will still result in an accumulation layer at the interface between InAs and the dielectric or high energy gap semiconductor.
In order to still have a very low metal-semiconductor contact resistance between the InAs and the contact and shorting bar metallization, it may be necessary to modify the processing sequence previously described in connection with FIGS. 6, 7A and 7B. For example, with an inverse of the mask contemplated in the previous discussion, the photoresist on the surface could then be used as a mask for wet etching (e.g., by wet chemicals or reactive ions, or ion beams) of the dielectric or high energy gap semiconductor layer to expose the InAs. Au or other metals could then be deposited by vacuum evaporation (or by other conventional processes, such as sputtering, electroplating, etc.) and then the photoresist could be removed, resulting in lift-off of the undesired regions of metal. Alternatively, after etching through to the InAs, the photoresist could be removed. Au or other metal could then be deposited uniformly across the surface, and, after deposition of photoresist, the mask pattern in FIG. 7A could be aligned with the pattern etched into the dielectric. Then, the Au could be patterned as before.
As an additional alternative, if a sufficiently thin layer (e.g., 200 Angstroms) of high energy gap semiconductor is present, the original processing sequence described could be modified by deposition of a low melting temperature eutectic alloy, such as Au-Ge, Au-Ge-Ni, Ag-Sn, etc., in place of Au. After patterning similarly to the way Au was (or using the inverse of the mask in FIG. 7A and lift-off), the sample is heated to a moderate temperature, typically to somewhere in the range of 360.degree. C. to 500.degree. C. for Au-Ge based alloys, thus allowing the liquid metal to locally dissolve the thin layer of high energy gap semiconductor, effectively contacting the InAs.
In our most recent work, we have changed our InAs growth procedures somewhat. The procedures are the same as before, but the InP wafer is heated to 460.degree. C. in a larger arsine mole fraction (0.1). After 0.5 minutes at 460.degree. C., during which the native oxide on InP is believed to desorb, the temperature is lowered to 400.degree. C. and 200 Angstroms of InAs thickness is grown. The temperature is then raised to the growth temperature of 625.degree. C. (with the arsine mole fraction still 0.1), and then EDMIn is introduced while the arsine flow is abruptly reduced to 5 SCCM (about 0.001 mole fraction). The EDMIn is kept at 50.degree. C., and the high purity hydrogen is bubbling through it at a rate of 75 SCCM. Again, the arsine flow of 5 SCCM seems near-optimal for these growth conditions. The resulting films have somewhat enhanced sensitivity to a magnetic field relative to those grown earlier.
While all of our recent work has concentrated on magnetoresistors fabricated from InAs films on semi-insulating (i.e., substantially electrically insulating) InP substrates, we think that a more mature growth capability will permit films of InAs with nearly comparable quality to be grown on semi-insulating GaAs substrates as well. In either case, other growth techniques such as molecular beam epitaxy liquid phase epitaxy or chloride-transport vapor phase epitaxy may also prove useful.
In another embodiment, a lightly doped p-type film is grown (typically doped with Zn, Cd, Mg, Be, or C). In the case of InAs, the surface would, we believe, still have a strongly degenerate electron layer, but it would be an inversion layer. Such an inversion layer would have a large electron density near the surface, and then a relatively thick (typically 0.1 micrometer to 1 mm or more, depending on dopant density) region of very low carrier density, similar to the space charge region of an n+/p junction. This might be advantageously used to reduce the conductivity of the film adjacent to the electron strong inversion layer. At very high device operating temperatures, the intrinsic carrier density of narrow energy gap semiconductors like InAs would tend to defeat this strategy somewhat, and other, higher energy gap semiconductors such as In.sub.1-x Ga.sub.x As might be preferred (see Table I). In.sub.0.53 Ga.sub.0.47 As is a special case, since it can be lattice-matched to semi-insulating InP substrates. This makes it easier to grow such films with high crystalline quality.
The acceptor dopants mentioned above (i.e., Zn, Cd, Mg, Be, and C) have small activation energies in the III-V compounds of interest (see Table I). However, there are other acceptor dopants with relatively large activation energies, such as Fe, in In.sub.0.53 Ga.sub.0.47 As. This means that relatively large thermal energy is required to make the iron ionize and contribute a hole to conduction. However, the iron will compensate a concentration of donor impurities frequently present in the material so that they do not contribute electrons to the conduction band. Thus, doping this material with iron will make it tend to have a high resistivity, except in the electron-rich accumulation layer. It would, in this case, be desirable to grow a thin undoped In.sub.0.53 Ga.sub.0.47 As layer (e.g., 0.1 micrometer thick, after correcting for iron diffusion effects) on top of the iron doped layer in order to obtain the highest possible electron mobility and density in the accumulation layer. It is recognized, however, that finding suitable dopants with large activation energies may not be practical for smaller band gap semiconductive materials. Furthermore, the other embodiments discussed above could also be used in conjunction with this one advantageously to reduce the conductivity of the film adjacent to the high electron density region.
The emphasis of the above discussion has been on electron accumulation or inversion layers. Hole accumulation or inversion layers could also be used. However, electrons are usually preferred as current carriers in magnetoresistors since they have higher mobilities in the materials shown in Table I.
We think that these types of magnetoresistors are especially attractive for automotive applications as part of linear or rotary position magnetic measurement systems.
Also, we are convinced that the presence of an accumulation layer in the above-mentioned thin film InAs magnetoresistors is what makes them work so well, and which enabled production of a practical device. We believe that the accumulation layer is naturally occurring in the fabrication process. The fundamental concept of incorporating and using an accumulation layer in a magnetoresistor is believed to be new. It is also believed that this thought can be expanded in a multiplicity of ways, not only with Indium Arsenide magnetoresistor films, but expanded to other semiconductive materials as well. The concept, of specifically inducing an accumulation layer in indium arsenide film magnetoresistors and in magnetoresistors of other semiconductor materials, by a variety of techniques, is described and claimed in the above-mentioned U.S. patent application Ser. No. 289,646, now abandoned, entitled, "Improved Magnetoresistor," that was filed simultaneously with the parent of this patent application in the names of J. P. Heremans and D. L. Partin.
The use of our indium arsenide magnetoresistor in a specific magnetic sensing circuit is being claimed in the above-mentioned U.S. patent application Ser. No. 289,641, now abandoned, entitled "Improved Position Sensor," and simultaneously filed in the names of Donald T. Morelli, Joseph P. Heremans, Dale L. Partin, Christopher M. Thrush and Louis Green.

Method of making indium arsenide magnetoresistor

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