Abstract:
Heterostructure barrier quantum well device with a super-lattice structure of alternating lightly doped and heavily doped spacer layers having multiple, stable current-voltage curves extending continuously through zero bias at ambient temperature. The device can be repetitively switched between the multiple current-voltage curves. Once placed on a particular curve, the device retains memory of the curve it has been set on, even if held at zero bias for extended periods of time. The device can be switched between current-voltage curve settings at higher positive or negative voltages and can be read at lower voltages. Switching between current-voltage curve settings can also be effected by additional terminal connection(s) to the spacer layer(s).

Description:
LICENSE RIGHTS 
     The United States government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license to others on reasonable terms as provided for by the terms of Grant No. ECS-8552868 awarded by National Science Foundation which partially funded the research which resulted in the invention disclosed herein. The research was also partially funded by Project No. 003658-294 of the Texas Advanced Research Program. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor devices employing heterostructures. More specifically, the invention relates to a multi-state memory device which provides the functionality of a static random access memory (SRAM) cell using only a single component. 
     2. Description of the Prior Art 
     Previously, heterostructures have been combined with super-lattice structures to create single-component, multi-state semiconductor devices. For example, U.S. Pat. Nos. 4,853,753; 5,017, 973; and 4,849,934 all disclose single-component, multi-state semiconductor devices. The voltage-current characteristics of all these prior art devices can be illustrated by a single curve. Since the current-voltage curves of these devices contained multiple peaks, the peaks have been used to represent different memory states in various applications. 
     The reliance of prior art devices on a single current-voltage curve has proven to have disadvantages in applications requiring non-volatile memory. In order for the memory to be maintained in the prior art devices, the bias voltage has to be maintained constant. If the bias voltage shifts then the memory state of the device shifts and the stored memory is lost. 
     The power costs of having to maintain the bias voltage is another disadvantage of prior art multistate semiconductor devices. A device that does not require that the bias voltage be maintained in order to retain memory could have greater power efficiency. 
     The present invention overcomes these disadvantages of the prior art devices by not requiring that a bias voltage be maintained to maintain memory while also providing a new multistate memory capability. 
     SUMMARY OF THE INVENTION 
     The present invention is a semiconductor device with two terminals a heterostructure barrier and a super-lattice structure of alternating lightly and heavily doped spacer layers which can be used as a multi-state memory device that is capable of retaining memory after zero bias voltage conditions. 
     In hole-based semiconductor devices the super-lattice structure is on the anode side of the heterostructure barrier; In electron majority carrier devices the super-lattice structure is on the cathode side of the heterostructure barrier. 
     In other embodiments of the invention the super-lattice structure can be on both the anode and cathode sides of the heterostructure barrier. 
     In still other embodiments of the invention, one or more additional terminals are electrically connected to individual spacer layers in order to provide greater control for setting the memory state of the device. 
     The ability to maintain memory under zero bias voltage conditions offers the advantage of increased power usage efficiency to devices in which the present invention is utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional representation of a preferred embodiment of the invention; 
     FIG. 2 is a schematic cross-sectional representation of a second preferred embodiment of the invention; 
     FIG. 3 is a schematic current-voltage diagram illustrating the current-voltage characteristics of the invention; 
     FIG. 4 is a schematic current-voltage diagram illustrating the current-voltage characteristics of another embodiment of the invention; 
     FIG. 5A-C are schematic illustrations representative of the charge or electron distribution in the device in self-consistent states at zero bias; 
     FIG. 6 is a schematic cross-sectional representation of another preferred embodiment of the invention; 
     FIG. 7 is a schematic cross-sectional representation of a three terminal embodiment of the invention; 
     FIG. 8 is a schematic cross-sectional representation of a four terminal embodiment of the invention; and 
     FIG. 9 is a schematic circuit diagram utilizing three terminal devices. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a cross-sectional illustration of an electron majority carrier embodiment of the semiconductor device D. The illustration includes a anode terminal 10 and a cathode terminal 12. Between the anode 10 and cathode 12 is a heterostructure barrier 14. On the cathode side of the heterostructure is a super-lattice structure 16 of alternating lightly doped and heavily doped spacer layers 18, 20 and 22. A hole-based semiconductor device can be fabricated also. 
     The embodiment of the invention shown in FIG. 1 incorporates a double barrier, heterostructure barrier 14. Other heterostructure barriers would also be suitable in other embodiments of the present invention. The heterostructure barrier 14 shown in FIG. 1 one consists of an approximately (˜)18 monolayer (ML) lightly doped n-type ˜(10 15  cm -3 ) GaAs quantum well 24 sandwiched between nominally symmetric ˜6 ML, unintentionally doped AlAs barriers 26 and 28. These layers, 24, 26 and 28, comprise the heterostructure barrier 14. 0n the cathode side 12 of the heterostructure barrier 14 is a super-lattice structure 16 having: an ˜18 ML n-type ˜(10 15  cm -3 )) GaAs layer 18, ˜47 ML n +  ˜(4×10 18  cm -3 ) GaAs layer 20, and ˜65 ML n- ˜(10 15  cm -3 ) GaAs layer 22. Finally, a 1770 ML layer of GaAs 30 is placed on top of the spacer layers 18, 20 and 22. On the anode 10 side of the heterostructure barrier 14, is an ˜18 ML n-type ˜(10 15  cm -3 ) GaAs layer 32, and a n +  GaAs substrate 34 on which the device is fabricated. 
     The device D can be fabricated using a Varian Gen II molecular beam epitaxy (MBE) system. Before metallization, the semiconductor material can be etched in a 2:1 HCl:H 2  O solution to remove surface oxides and to improve ohmic contact adhesion. An array of square front-side contacts (nominal sizes ranging from 10 μm×10 μm to 50 μm×50 μm) can be defined by liftoff, using ˜l.5 nm Ni/˜80 nm AuGe/˜15 nm Ni/˜100 nm Au metallization. The front-side contacts serve as a mask during mesa isolation by an ˜8:1:1 H 2  SO 4  :H 2  O 2  :H 2  O etching process. Finally, the device structures is preferably annealed in forming gas at ˜450° C. for ˜30 sec. The specific contact resistance for this process is preferably between 1×10 -6  Ω-cm 2  and 5×10 -6  Ω-cm 2 . 
     The present invention is not limited to the semiconductor device D disclosed above; other semiconductor material systems may also be employed. For example, SiGe/SiC/Si material systems have exhibited properties indicating that they, too, may be used to fabricate a device embodying the present invention. 
     The operation of the present invention depends on quantum interference effects between the forces caused by the heterostructure barrier 14 and the charge/electron distribution in the super-lattice structure 16. In order to obtain sufficient quantum interference effects, the thickness of the layers in the device must be sufficiently small to allow such interference. In order to get the sufficient charge distribution in the super-lattice structure 16, there should be sufficient disparity in the doping density of the lightly and heavily doped spacer layers 18, 20, and 22 to balance the charge distribution forces against the forces imposed by the heterostructure barrier 14. 
     The room temperature current-voltage characteristics of the device D in FIG. 1 described above are shown in FIG. 3. FIG. 3 illustrates the two most repeatable and stable conduction branches (current-voltage curves) A and B observed during multiple sweeps of the device. 
     A second embodiment of the device D is shown in FIG. 2. This second embodiment of the device D differs from the first in that additional layers have been added to the anode 10 side of the device 42 and 44 so that the both the anode 10 and cathode 12 sides of the heterostructure barrier 14 have super-lattice structures 16 and 46 comprised of alternating light and heavily doped spacer layers. This second embodiment can also be used as a two-conduction branch device. In practice, the specific differential resistance at low bias on the high current density curve A has been found to be approximately 1.8 ×10 -5  Ω-cm 2  and on the low current density curve B the differential resistance has been found to be approximately 4.6 ˜10 -5 Ω-cm 2 . 
     Practically, each current-voltage curve can be used to represents a different memory state. The memory state can be changed by switching between the curves by applying a large enough voltage across the device. When the bias voltage applied across the device exceeds approximately 1.2 V in the third quadrant (i.e., the cathode 12 is negative), the device switches from curve A to curve B. For convenience, the bias voltage at which the switch occurs can be called a switching threshold voltage. 
     As the bias voltage is swept back towards zero, the device follows curve B, which remains separated from curve A all the way through zero bias voltage. Curve B continues through zero bias into the first quadrant. With the device on curve B, when the bias voltage is increased to approximately 1.5 V in the first quadrant, a transition occurs which switches the device back to curve A. The bias voltage at which this switch occurs can also be called a switching threshold voltage. The device remains on curve A until the bias voltage is swept back into the third quadrant, where at a bias voltage of approximately -1.2V the device with switch back to curve B. This cycle can then be repeated. 
     At voltages beyond approximately 1.5 V in the third quadrant, curve B is not stable and the device switches back to curve A. Since curve A has a much lower resistance, when this transition occurs, the device current can increase substantially. If precautions are not taken to limit the current passing through the device, the high current will cause very high dc power dissipation in the device causing &#34;burn out&#34; of the device. 
     The device&#39;s state (which current-voltage curve the device is following) can be determined by applying a test voltage. If the device is set on curve A, then the current will be higher for any given test voltage. If the device is set on curve B, then the current will be lower. Test voltages must be between the switching threshold voltages in order to maintain the device on curve A or B. If a test voltage is either higher than the high threshold voltage or lower than the low threshold voltage then the device may change states, and, in effect, write over the stored memory. 
     Because the device can be set to either curve A or curve B by applying the appropriate high or low switching threshold voltage, the device possesses memory. Moreover, because the device maintains the curve A or curve B setting through zero-bias, the device maintains its memory setting even in zero-bias conditions caused by an open circuit, power loss or short circuit. 
     In addition to the two conduction branches previously described for the first embodiment, the second embodiment of the invention also has two additional conduction branches. The room temperature current-voltage characteristics of the device in FIG. 2 are shown in FIG. 4. This figure shows four distinct curve settings A&#39;,B&#39;,C&#39; and D&#39;. These self-consistent states can best be understood by viewing the conduction band edges and electron densities at zero bias for the self-consistent states of the device. These are shown in FIGS. 5A-C. The main differences between the solutions occur FIGS. 5A-C. The main differences between the solutions occur in the heavily doped spacer layers of the super-lattice structure. FIG. 5A illustrates the conduction band edge and electron concentration at zero bias for solution A&#39; which is illustrated as a current-voltage curve in FIG. 4; FIG. 5B illustrates the conduction band edge and electron concentration at zero bias for solution B&#39; which is illustrated as a current-voltage curve in FIG. 4; and FIG. 5C illustrates the conduction band edge and electron concentration at zero bias for solution C&#39; which is illustrated as a current-voltage curve in FIG. 4. The electron distribution shown in FIG. 5C differs from the distribution shown in FIGS. 5A and 5B in that the distribution is asymmetric. The asymmetric charge distribution shown in FIG. 5C has a mirror image asymmetric charge distribution about the heterostructure barrier. The mirror image of the conduction band edge and electron concentrations at zero bias in FIG. 5C illustrates the conduction band edge and electron concentration at zero bias for solution D&#39; which is illustrated as a current-voltage curve in FIG. 4. 
     Mathematical Model 
     Mathematically, the multi-state behavior of the device can be explained using a Schrodinger/Poisson model of the device. In the model used here, the electron concentration in the device is given by: ##EQU1## where k z  and k.sub.| are the wave vectors parallel and perpendicular to the direction of current flow, f L  (k z ,k.sub.|) and f R  (k z ,k.sub.|) are the Fermi-Dirac distribution functions at the left and right boundaries, and the wave functions Ψ(z) (the superscripts indicate the direction of electron propagation) are solutions to the Schrodinger equation obtained with plane wave boundary conditions. The self-consistent solutions at a given bias are obtained by iteratively solving the Schrodinger and Poisson equations. At zero bias, any bound states that exist in the device are also taken into account, assuming that these states are occupied according to Fermi-Dirac statistics. The equations are solved on a uniform spatial mesh with a mesh spacing of approximately one (1) monolayer (ML). 
     Multiple Terminal Apparatus 
     FIGS. 6 and 7 show additional embodiments of the present invention. In FIG. 6, an electron majority carrier device D has three terminals 70, 72 and 74. One terminal 70 is the anode 10 side of the device and the second terminal 72 is connected to the cathode 12 side of the device. In FIG. 6, the third terminal 74 is attached to one of the n, heavily doped, spacer layers 20 on the cathode 12 side of the heterostructure barrier 14. In FIG. 7 the third terminal 74 is attached to one of the n, heavily doped spacer layers 42 on the anode 10 side of the heterostructure barrier 14. 
     FIG. 8 illustrates another embodiment of the invention in which a third 76 and a fourth 78 terminal are attached to the heavily doped spacer layers 20 and 42 on both the cathode 12 and anode 10 sides of the heterostructure barrier 14. 
     The third 76 and fourth 78 terminals can be used to switch the device from one current-voltage curve (state) to another. Otherwise the third 76 or fourth 78 terminals can be left floating. In other words, the third and fourth terminals 76 and 78 can be used to write to memory and the other terminals 70 and 72 are used to read from memory. 
     Fabrication of Multiple Terminal Apparatus 
     Since the spacer layers are very thin, care must be taken when fabricating a three or more terminal device to avoid shorting the device. Therefore, etching selectivity is important. One possible solution to the problems associated with fabricating devices with electrical connections to the very thin spacer layers has been disclosed in a Broekaert et al. publication in IEEE Transactions on Electron Devices in March 1992. Broekaedt et al. have developed wet chemical etching solutions such as succinic acid that allow for selective etching of InP lattice-matched in AlGaAs layers using thin pseudomorphic AlAs layers as etch stops. If this process is used, the invention could be fabricated using InAlGaAs lattice-matched to InP, where the quantum well heterostructure would consist of thin (&lt;10ML) strained AlAs barriers sandwiching an InGaAs quantum well. Using the succinic acid solution, an etch-back to the first AlAs etch stop barrier should be possible, since the etch selectivity of AlAs to InGaAs is greater than 1:1000. This high selectivity allows large process latitude. After etching back to the first AlAs etch stop, a non-spiking Pd/Ge metallization could be used to form the heavily doped spacer layer contact. Since the InGaAs layer has a very low Schottky barrier height and can be doped to the mid 10 19  cm -3  levels, a very low resistance ohmic contact with negligible surface depletion may be formed. Thus, due to the very large etch selectivities available in the InAlGaAs/AlAs material system, it is possible to fabricate a three or more terminal embodiment of the present invention. 
     FIG. 9 shows a circuit diagram having four (4) three terminal devices. Such a circuit can easily be worked into the fabrication sequence of the semiconductor device. The diagram shows read terminals and write terminals. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics. The described embodiments, including materials used and approximate dimensions and doping densities, are to be considered in all respects only as illustrative and not restrictive, and the scope of the invention is, therefore, indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are embraced within their scope.