Abstract:
This invention pertains to more sensitive and more stable electronic devices which can sense electrical and magnetic fields. The devices are characterized by InAs channels confined on both sides thereof by a wide band gap AlSb material; protective layers above the AlSb material; modulation doping above the AlSb material; and layers of the InAs channel material containing 1 to 99 mol percent antimony, with the channel material being deposited in the form of alternating monolayers of InSb and InAs, of a ternary mixture of InAsSb.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to a Hall element electronic device with an InAs channel which has high sensitivity and enhanced stability over a large temperature range and which can sense electrical and magnetic fields. 
     2. Description of Related Art 
     Hall elements using III-V semiconductor materials have widespread use for a variety of military and commercial applications. They are mostly commonly used as magnetic field sensors in small DC brushless motors found in electronic consumer equipment such as video cassette recorders, personal computers, and compact disk drives. They also are used for position, tilt-level, pressure and thickness sensing as well as in tachometers, compasses, magnetizers, and electronic measurement equipment used to detect current, voltage, power, frequency, and magnetic field. They are also used in the non-destructive evaluation of materials to detect hair line cracks in metals for such applications as wing and fuselage inspection. 
     In 1995, approximately one billion Hall sensors were manufactured worldwide. The market for such sensors is growing rapidly, and many new applications for the contactless sensors are expected in the future because they can detect static as well as variable magnetic and electric fields. 
     Hall sensors using InSb material are the most prevalent Hall elements used today compared to those which are composed of GaAs or InAs material. The attractive features of the InSb Hall sensors are high sensitivity, low power consumption, and small offset voltage. Due to the narrow band gap of InSb, however, these Hall sensors have a large temperature dependance which limits their use to near room temperature applications. 
     InAs Hall sensors are more stable over a wider temperature range, and thus are needed for many present and future applications which have more severe operating conditions. For example, automotive sensors located in the engine compartment or outside the body frame are required to operate over a temperature range from −40° C. to 150° C. Compared to InSb Hall sensors, InAs Hall sensors also have a better stability against pulse voltage noise, lower offset voltage drift, and lower noise properties which enable sensing of smaller magnetic fields. 
     To meet the demand for improved performance, considerable effort has focused on the development of advanced material growth and device fabrication technology using the AlSb/InAs materials system. AlSb/InAs Hall sensors use AlSb/InAs heterojunctions to form an InAs deep quantum well. This approach is preferred over InAs material obtained using conventional thin film technology due to the attractive features of this heterojunction material system, which include high electron mobility and velocity, high sheet charge density, good carrier confinement, and enhanced design flexibility. 
     Accurately controlled “band-gap engineered” layer designs with feature sizes on the atomic scale can be used to exploit desirable quantum confinement effects within the structure. As a result of these unique and substantially improved material properties, AlSb/InAs-based quantum well Hall elements are particularly suitable for present and future sensor applications. 
     To realize high sensitivity in a Hall element, a high electron mobility is required for the InAs layer. Compared to GaAs Hall elements, the lower electron effective mass of InAs gives this material system a significant advantage in the room-temperature mobility which can be achieved for a given sheet charge density. 
     Compared to other III-V semiconductor material systems, such as GaAs and InP, AlSb/InAs material growth and device fabrication technology is relatively immature. However, recent progress in these areas has enabled antimonide-based devices to be produced with higher mobilities, higher sheet carrier concentrations, lower contact resistance, and improved overall performance. As an example, AlSb/InAs high-electron mobility transistors (HEMTs) have recently been fabricated which exhibit high frequency performance which constitutes the state-of-the-art at low drain voltage. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     An object of this invention is an electronic device with high sensitivity and enhanced stability over a large temperature range. 
     Another object of this invention is an electronic device with high electron mobility and increased sheet charge density in the channel. 
     Another object of this invention is an electronic device with a modified InAs channel which can be tuned to optimize sensitivity and operating temperature. 
     These and other objects of this invention can be achieved by an electronic device Hall element which is characterized by increased electron mobility in the InAs channel or in the 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the first embodiment of an AlSb/InAs Hall element in cross-section showing the InAlAs/AlSb barrier layer without the ohmic contacts on the top layer. 
     FIG. 2 is the top view of FIG. 1 showing the Pd/Pt/Au ohmic contacts. 
     FIG. 3 is a graph of Resistance v. Contact Spacing showing transmission line model measurements of the Pd/Pt/Au ohmic contacts. 
     FIG. 4 is a graph of Concentration v. Sputter Time showing Auger profile of a Pd/Pt/Au ohmic contact after a hot plate heat treatment of 175° C. for 3 hours. 
     FIG. 5 is a schematic illustration of the second embodiment of an AlSb/InAs Hall element in cross-section with InAs (Si) modulation doping disposed above the AlSb layer and above the InAs channel. 
     FIG. 6 is a schematic illustration of the third embodiment of an AlSb/InAsSb Hall element in cross-section. 
     FIG. 7 is an infrared photoluminescence spectrum from a 150 Å thick AlSb/InAs 0.8  Sb 0.2  layer of the third embodiment shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention pertains to electronic devices having increased sensitivity and stability over a wide temperature range. These devices are characterized by an InAs channel or modified on InAs channels that have higher electron mobility than conventional electron devices with InAs channels. 
     The invention herein is illustrated by three embodiments each of which contains an InAs channel or a modified InAs channel. Due to the heterostructure of the electronic devices in the three embodiments, the channels in the embodiments have increased electron mobility which translates into better sensitivity over a wide temperature range and other advantages. 
     The first embodiment contains the AlSb/InAs heterojunction which increases electron mobility of the InAs channel. Due to the high aluminum content, the AlSb layer of the device is reactive in air and is capped with a second more stable barrier layer to prevent oxidation. The InAlAs/AlSb/InAs material of the first embodiment was grown by molecular beam epitaxy at 510° C. on an undoped (100) GaAs substrate. 
     The lack of an AlSb barrier layer with sufficient chemical stability has been a considerable drawback which has limited the performance and usefulness of AlSb/InAs Hall elements. To improve chemical stability, design of the electronic devices of the first embodiment contains an InAlAs/AlSb composite barrier layer above the InAs channel. Since the InAlAs second barrier layer is chemically stable, Hall elements or electronic devices of the first embodiment can be more easily fabricated and are more reliable. 
     Electron mobility in the channel of the second embodiment is higher than that of the first embodiment. In addition to the AlSb/InAs heterojunction which increases electron mobility in the InAs channel, the second embodiment also includes modulation doping which further increases electron mobility in the channel, as well as sheet charge density. These benefits are imparted to the device by a silicon doped InAs layer or InAs(Si) layer disposed above the InAs channel. Amount of the silicon atoms doped into the InAs monolayers can be varied to meet the desired demand—the more of the silicon atoms introduced into the InAs material, the higher will be the electron mobility. 
     Modulation doping is used to achieve higher electron mobility by spatially separating electrons in the narrow band-gap InAs channel from their parent donor atoms in the AlSb wide band-gap barrier material. The electrons in the channel can travel faster since they are not impeded by the ionized impurity scattering which would otherwise occur. The considerably larger conduction band discontinuity (ΔE c =1.35 eV) at the barrier/channel interface enables formation of a deeper quantum well and the associated benefits of a larger  2 DEG sheet charge density and superior carrier confinement. 
     In the third embodiment, antimony is deposited into the InAs channel in order to improve sensitivity although there is a trade-off of sensitivity and operating temperature range. If sensitivity is improved, what is sacrificed to obtain the improved sensitivity is the operating temperature range. InAs has a narrow band gap of about 0.35 eV and a high electron mobility of about 30,000 cm 2 /Vs whereas InSb has narrow band gap of about 0.17 eV and a high electron mobility of about 50,000 cm 2 /Vs. The sensitivity is increased with higher electron mobility and sheet charge density and the operating temperature range is decreased with higher antimony content in the channel. 
     It has been confirmed, on the basis of infrared photoluminescence measurements, that addition of antimony to the InAs channel changes the energy band structure from a staggered type-II heterojunction lineup to a type-I alignment. Type-I heterojunction lineup is preferred to type-II since a type-I lineup functions as an improved barrier to hole flow in the valence band and thereby gives lower leakage current and less of a trapping effect. 
     Schematic illustration of the first embodiment is shown in FIG. 1, which is a cross-sectional view of an electronic device or a Hall element of this invention. FIG. 1 shows a cross-sectional view of an electronic device  110  wherein AlSb buffer layer  114  is disposed on a semi-insulating GaAs substrate  112 , then InAs channel layer  116  is disposed on the buffer layer  114 , AlSb barrier layer  118  is disposed on the channel layer  116 , InAlAs barrier layer  120  is disposed on the barrier layer  118 , and InAs cap layer  122  is disposed on the second barrier layer  120 . The substrate and the various layers thereon of the electronic device disclosed herein can be of any desired thickness, however, typically substrate  112  is 200 to 700 μm, more typically 400 to 500 μm thick; buffer  114  is typically 0.1 to 10 μm, more typically 1 to 5 μm thick; channel  116  is typically 10 to 500 Å, more typically 50-300 Å thick; first barrier  118  is typically 10-500 Å, more typically 50-300 Å thick; second barrier  120  is typically 10-500 Å, more typically 20-200 Å thick; and cap  122  is typically 5-200 Å, more typically 10-50 Å thick. 
     The actual electronic device shown in FIG. 1 had the following thicknesses: substrate  112 , 450 μm; buffer  114 , 2.4 μm; channel  116 , 150 Å; barrier  118 , 125 Å; barrier  120 , 50 Å and its composition was In 0.5 Al 0.5 As; and cap  122 , 20 Å. 
     The second barrier layer  120  is provided on the first AlSb barrier layer  118  due to the high aluminum content in barrier layer  118 , which is reactive in air. Barrier  120  acts as a protective layer from oxidation for AlSb barrier  118 . Maximum amount of aluminum in barrier layer  120  is about 80% mole fraction that would still allow the InAlAs barrier to act as a protective layer for the AlSb barrier  118 . Obviously, lower amounts of aluminum in the barrier layer  120  than about 80% mole fraction will enable the barrier  120  to act as a protective layer for the AlSb barrier layer  118 , however, sufficient amount of aluminum in the barrier  120  should be used so that its band gap is greater than that for the channel  116 . It is estimated on the basis of its band gap, that the minimum amount of aluminum in the barrier  120  should be about 20% on molar basis. 
     The InAlAs/AlSb/InAs material of the first embodiment shown in FIG.  1  and in the other embodiments, is typically grown in a known way by molecular beam epitaxy at an elevated temperature. Device sample of the first embodiment, schematically shown in FIG. 1, was grown by molecular beam epitaxy at 510° C. on an undoped (100) GaAs substrate  112 . The 2.4 μm undoped AlSb buffer layer  114  was grown first followed by the undoped 150 ÅÅ InAs channel layer  116 , the 125 Å undoped AlSb barrier layer  118 , a 50 Å undoped Ino 0.5 Al 0.5  As barrier layer  120 , and finally the 20 Å undoped InAs cap layer  122 . The 2.4 μm thick AlSb buffer layer accommodates the 7% lattice mismatch between the device material and the GaAs substrate. 
     FIG. 2 is a top view of FIG.  1  and shows four ohmic contacts  212 ,  214 ,  216  and  218  which are typically rectangular, as shown, although they can be in any other desired shape. Each of the embodiments disclosed herein has the ohmic contacts on its top or cap layer. Although any suitable ohmic contact material can be used, preferred is a contact containing a palladium layer in contact with the cap layer  122 , a barrier layer above the palladium layer, and a gold layer above the barrier layer. The barrier layer can be platinum , titanium, titanium tungsten, platinum silicide, titanium carbide, tungsten carbide, or a mixture thereof. Preferred barrier material is platinum. Thickness of the ohmic contact before heat treatment is as follows: palladium layer, typically 10-1000 Å, more typically 50-500 Å; barrier layer, typically 50 -1000 Å, more typically 100-500 Å; and gold layer, typically 100-5000 Å, more typically 200-2000 Å. 
     The ohmic contacts on the device sample of FIG. 1 were defined first using a PMMA-based resist and deep UV lithography. The Pd/Pt/Au ohmic contacts were 100 Å, 200 Å, 600 Å thick., respectively, and were formed using e-beam evaporation, acetone liftoff, and heat treatment at 175° C. using a hot plate located within a glove box containing a H 2 : N 2  (5%: 95%) gas mixture at ambient conditions. The sample was heat treated for 3 hours to ensure sufficient reaction of the palladium. The device sample isolation was achieved by wet chemical etching. 
     The Pd/Pt/An ohmic contact employs the low temperature reactivity of palladium but uses platinum diffusion barrier to prevent gold from reacting with the semiconductor material. No reaction was observed, as determined by using scanning electron microcopy in the region adjacent to the contacts. The contacts had a smooth featureless surface morphology and good definition. 
     Transmission line model measurements for the Pd/Pt/Au contact is shown in FIG. 3 for two sampled (A &amp; B) of similar material. A contact resistance as low as 0.04 Ω-mm can be obtained. As shown in FIG. 4, auger depth profiling reveals for the device sample of FIG. 2 that palladium had diffused to a depth slightly beyond the InAs channel layer  116 . It is also observed from FIG. 4 that the platinum layer served as an effective diffusion barrier which prevented the gold from reacting with the semiconductor material. 
     For the device sample of FIGS. 1 and 2, at 300K and for an input current of 1 mA, the measured Hall output voltage was 140 mV for a moderate magnetic field of 2 KG. These values correspond to a good open circuit product sensitivity constant, k HOC , of 70 mV/mA-KG and a low sheet carrier density of 9×10 11 /cm 2 . The Hall mobility of the starting material was 29,000 cm 2 /Vs, which is excellent for the InAs but can be about 50,000 cm 2 /Vs for InSb. Despite the 4% lattice mismatch of the InAlAs material with respect to AlSb and InAs, the InAlAs layer was continuous. 
     To test the effects of the InAlAs layer on the transport properties of InAs channels, a set of 5 samples were grown with varying thickness of the barrier layer  120 . The heterostructures were identical except for the thickness of the In 0.5 Al 0.5 As barrier layer  120 , which was 0 Å, 20 Å, 40 Å, 100 Å and 500 Å thick. The InAlAs layers were separated from the InAs channel by 125 Å of AlSb, as shown in FIG.  1 . The room temperature electron mobility and sheet carrier density were not correlated with the InAlAs thickness. Electron mobilities ranged from 20,000 cm 2 /Vs to 240,000 cm 2 /Vs and sheet carrier densities ranged from 1.3×10 12 /cm 2  to 1.6×10 12 /cm 2 . 
     The second embodiment of the electronic device is schematically illustrated in crosssection in FIG. 5, where electronic device  510  was fabricated by electron beam epitaxy by depositing on the GaAs substrate  512 , GaAs layer  514 , followed by AlSb layer  516 , InAs layer  518 , AlSb layer  520 , InAs (Si) layer  522 , InAs layer  524 , AlSb layer  526 , and InAs layer  528 . Although thickness of the various layer can be varied widely to suit individual needs, layer thickness of the specific device sample of FIG. 5 was as follows: GaAs layer  514 —0.3 μm, AlSb layer  516 —2.4 μm, InAs layer  518 —150 Å, AlSb layer  520 —125 Å, InAs (Si) layer  522 —6 Å, InAs layer  524 —3 Å,.AlSb layer  526 —100 Å, and InAs layer  528 —20 Å. The substrate was 450 μm thick. The thin Si-doped InAs layer was inserted 125 Å above the 150 Å undoped InAs channel or quantum well. The Si-doped InAs layer consisted of two monolayers of InAs (Si) followed by a single monolayer of undoped InAs. 
     The undoped InAs channel layer  518  and the adjacent AlSb layers  516  and  520  were grown at above 500° C. and the growth temperature of the InAs (Si) donor layers  522  and  524  and the 125 Å AlSb layer  526  immediately above the donor layers was lowered to 380° C. to minimize silicon segregation. 
     The second embodiment of the electronic device of FIG. 5 is characterized by the thin doped InAs layer in the upper AlSb barrier, the dopant being silicon or any other n-type dopant. 
     In reference to the embodiment of FIG. 5, electron Hall mobility and sheet carrier concentration of the starting material were 17,000 cm 2 /Vs and 2.5×10 12 /cm 2 , respectively. Additional measurements on material grown with a second InAs (Si) donor layer below the well indicated a carrier density of 5.6×10 12 /cm 2  and electron mobility of 1.34×10 4  cm 2 /Vs at 300K. 
     Results with respect to the second embodiment of FIG. 5 indicate the silicon doping in a thin InAs layer located adjacent to the AlSb barrier layer  520  can be used to produce layers with high sheet charge density. Electron Hall mobilities of 100,000 cm 2 /Vs at 77K and 180,000 cm 2 /Vs at 4K have been demonstrated and still higher electron mobilities and sheet charge densities are possible. 
     The third embodiment is shown in FIG. 6 where the electronic device is fabricated with an InAsSb channel containing 1-99%, preferably 2-40% antimony, on molar basis. This embodiment has higher sensitivity than the second embodiment due to the antimony in the InAs channel. The channel can consist of alternating layers of InSb and InAs or it can be a ternary mixture InAsSb. 
     The device  610  shown in FIG. 6 can be fabricated by molecular beam epitaxy by depositing at an elevated temperature AlSb layer  614  on the GaAs substrate  612 ; followed by layer  616  consisting of any number of periods until the desired channel or quantum well thickness is grown, of InSb and InAs; AlSb layer  618 ; doped InAs layer  620 ; AlSb layer  622 ; InAlAs layer  624 ; and undoped InAs layer  626 . 
     The specific device sample of FIG. 6, grown at 400° C. as a digital alloy superlattice, had the following layer thickness; AlSb layer  614 —2.1 μm; InAs 0.8 Sb 0.2  channel layer  616 —about 50 Å consisting of 10 periods of one monolayer of InSb and four monolayers of InAs with each monolayer being about 3 Å thick; AlSb layer  618 —125 Å; InAs (Si) layer  620 —12 Å; AlSb layer  622 —12 Å; In 0.4 Al 0.6 Al layer  624 —40 Å; and InAs layer  626 —20 Å. The substrate  612  was 450 microns. Here, modulation doping was achieved through the use of the thin silicon doped InAs layer  620  located 125 Å above the AlSb barrier layer  618 . 
     Infrared photoluminescence measurements at 5K confirm that the addition of antimony into the InAs channel in the FIG. 6 embodiment, changes the band structure of the device from a staggered type II heterojunction lineup to type I. Type I alignment acts more as a barrier, compared to a type II alignment, thereby giving a lower leakage current and less trapping effect. 
     The sheet carrier density and electron mobility of the starting material of the specific device sample of the third embodiment of FIG. 6 at 300K were 1.4×10 12 /cm 2  and 13,400 cm 2 /Vs, respectively. 
     To confirm the type I alignment between AlSb and InAs 0.8 Sb 0.2 , photoluminescence measurements were carried out at 5K with a Fourier transform infrared spectrometer. Samples were mounted on a cryogenic dewar and excited with an 810 nm laser diode. The room temperature blackbody radiation was eliminated by a double-modulation technique. FIG. 7 shows the photoluminescence spectrum from the InAs 0.8 Sb 0.2  single quantum well. In contrast to the nonluminous AlSb/InAs single quantum wells, this sample exhibited bright luminescence at 272 meV. The arrow in FIG. 7 indicates the measured band gap energy for a thick 880 Å InAs 0.8 Sb 0.2  digital superlattice. The results imply that the lowest subband energy for a 150 Å InAs 0.8 Sb 0.2  quantum well is 62 meV, consistent with k.p calculations. 
     While presently preferred embodiments have been shown of the novel electronic device and of the several modifications thereof, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention as defined differentiated by the following claims.