Abstract:
A Schottky diode, and a method of making the same, which is fabricated on InP material and a Schottky layer including In x Al 1-x As with x&gt;0.6, or else including a chirped graded superlattice in which successive periods of the superlattice contain progressively less GaInAs and progressively more AlInAs, the increase in AlInAs being terminated before the proportion of AlInAs within the last period (adjacent the anode metal) exceeds 80%. Such fabrication creates an InP-based Schottky diode having a low turn-on voltage which may be predictably set within a range by adjusting the fabrication parameters.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains to Schottky diodes, and more particularly to Schottky diodes fabricated using InP semiconductor material.  
         BACKGROUND  
         [0002]    Schottky diodes are well-known in GaAs and Si technologies. Some Si-based Schottky diodes have fairly low turn-on voltages but suffer from high series resistance and relatively poor frequency response. Si-based Schottky diodes are not easily integrated into InP-based semiconductor fabrication.  
           [0003]    Low turn-on voltages are often desirable for Schottky diodes, particularly for mixer applications. U.S. Pat. No. 5,665,999 issued Sep. 9, 1997 to H. Brugger, teaches that low turn-on voltage Schottky diodes permit a low conversion loss without requiring a high pumping capacity in the local oscillator, and without using a bias voltage which may lead to Townsend current-hum disturbances. A GaAs Schottky device with reduced turn-on voltage is taught as fabricated using a graded InGa (1-x) As layer over a GaAs substrate, where the In content, x, increases continuously in the direction of the metal contact. Compatibility with InP is not taught, and the spatially varying lattice mismatch created by this process is thought potentially unreliable with InP technologies.  
           [0004]    Schottky diodes compatible with InP semiconductor technology are known. For example, U.S. Pat. No. 5,652,435, issued Jul. 29, 1997 to E. Martin et al., describes a Schottky diode optical detector wherein the light-sensitive InGaAs layer is cladded front and back with InAlAs current-blocking layers. This device has a relatively high turn-on voltage similar to that of conventional GaAs Schottky diodes.  
           [0005]    Some investigation into the relationship between Schottky barrier height and the proportion of In in In x Al 1-x As material on an InP substrate has been undertaken by C. L. Lin, et al. in “Composition dependence of Au/In x A 1-x As Schottky barrier heights,”  Applied Physics Letters  49 (23), Dec. 8, 1986, pp. 1593-1595. Lin performs tests on the noted compound for 0.45≦x≦0.55, a range producing relatively high Schottky barriers.  
           [0006]    InP-compatible Schottky diodes using a graded superlattice Schottky layer are also known. For example, U.S. Pat. No. 5,198,682 issued Mar. 30, 1993 to C. -S. Wu et al. describes an infrared photodetector using a superlattice having a graded dopant concentration, thereby developing an internal field which aids in the collection of photoexcited carriers. Lee et al., in Appl. Phys. Lett. Vol. 54, May 1989, pp. 1863-1865, teaches use of a graded superlattice of InGaAs/InAlAs to obtain enhanced Schottky barrier height. U.S. Pat. No. 5,572,043, issued Nov. 5, 1996 to Shimizu et al. describes another SL-based Schottky diode, using sublayers of variously lattice-mismatched compounds and alternating compressively and tensilely strained layers to balance the mismatches. Each of these three references describe Schottky diodes having a relatively high turn-on voltage of 0.6V -0.7V.  
           [0007]    InP-compatible Schottky diodes having high turn-on voltages are known. For example, U.S. Pat. No. 4,471,367 issued Sep. 11, 1984 to C. Chen et al. discloses a MESFET-gate Schottky structure composed of a thin, heavily doped InGaAs layer overlying a layer of low-doped InGaAs having an increased barrier height. Another example is U.S. Pat. No. 4,954,851, issued Sep. 4, 1990 to W. Chan, which describes a Schottky diode with a cadmium-containing layer overlying an InAlAs layer. The Cd-containing layer enhances the barrier height of the diode.  
           [0008]    Thus, Schottky diodes with reduced turn-on voltages exist but are not known to be compatible with InP-based fabrication techniques. Schottky diodes compatible witn InP are known, but all have conventional or high turn-on voltages. Yet, InP-based fabrication technologies are desirable, and low turn-on voltage Schottky diodes are particularly useful in certain applications, such as high-frequency mixers. A need therefore exists for low turn-on voltage Schottky diodes, and a method of making the same, which are compatible with InP fabrication technologies.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention addresses the need for low turn-on voltage Schottky diodes compatible with InP-based fabrication by employing an InP lattice-compatible Schottky layer which interfaces with the Schottky metal layer and provides a relatively low Schottky barrier height resulting in a low turn-on voltage.  
           [0010]    The Schottky metal layer is preferably Ti/Pt/Au. The Schottky layer is preferably grown over a low-doped GaInAs layer, which in turn is preferably grown over a highly doped GaInAs layer. The highly-doped GaInAs layer forms the Schottky cathode contact. The cathode contact is grown over a buffer layer, such as 2500 A of AlInAs or 100 A of undoped GaInAs, preferably grown directly on the InP substrate.  
           [0011]    According to the present invention, the Schottky layer is processed to produce a low turn-on voltage.  
           [0012]    According to a first embodiment of the invention, the Schottky layer includes strained Al (1-x) In x  As, and for x=0.70 produces a turn-on voltage of 0.25V @ImA. The corresponding 0.3 Al content is lower than that which lattice-matches InP, and accordingly the thickness of the layer is preferably around 100 Å to limit effects from the strain.  
           [0013]    According to second, third and fourth embodiments of the invention, the Schottky layer includes a chirped superlattice composed of a plurality of periods, each period containing sublayers of Ga 0.47  In 0.53 As and Al 0.48  In 0.52 As, which are lattice-matched to InP. The proportional thickness of the two compounds is varied from period to period to provide a step-wise, or chirped, grade in composition of the superlattice from GaInAs toward AlInAs. The turn-on voltage can be precisely controlled by truncating the grading steps before completing a transition to predominantly AlInAs.  
           [0014]    Throughout this patent, GaInAs refers to Ga 0.47 In 0.53 As, and AlInAs refers to Al 0.48 In 0.52 As, unless a different composition is specified.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is the layer structure of a diode according to the present invention after a first etch.  
         [0016]    [0016]FIG. 2 shows the structure of the diode after a second etch step.  
         [0017]    [0017]FIG. 3 shows the structure of the diode after a third etch step.  
         [0018]    [0018]FIG. 4 shows the structure of the diode after passivation and etching of the passivation layer.  
         [0019]    [0019]FIG. 5 shows the structure of the completed diode.  
         [0020]    [0020]FIG. 6 depicts a chirped graded superlattice having nine periods.  
         [0021]    [0021]FIG. 7 depicts the Schottky layer structure for the second embodiment of the present invention.  
         [0022]    [0022]FIG. 8 depicts the Schottky layer structure for the third embodiment of the present invention.  
         [0023]    [0023]FIG. 9 depicts the Schottky layer structure for the fourth embodiment of the present invention.  
         [0024]    [0024]FIG. 10 shows I-V curves for three embodiments of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    While discrete Schottky diodes can be made according to the present invention, the present invention is very well suited to integrated fabrication. Such fabrication, as shown in FIG. 1, preferably uses Molecular Beam Epitaxy (MBE) to grow each of the layers on InP substrate  1 , though any other compatible process, such as Metal Organic Vapor Phase Epitaxy(MOVPE), may be used. Buffer layer  3  is grown directly on InP substrate  1 . Buffer layer  3  is about 2500 Å of undoped AlInAs, or may alternatively be about 100 Å of undoped GaInAs. Cathode contact layer  5  is grown on the buffer layer, preferably highly doped, preferably with Si, to n=1×10 19  cm −3 . Cathode contact layer  5  is preferably grown to about 3000 Å of GaInAs, the thickness helping to reduce bulk resistance of the cathode and also simplifying the later etch step down to cathode contact layer  5  which permits deposition of cathode contact  21  (FIG. 2). Depletion layer  7  is about 1000 Å of undoped GaInAs grown atop cathode contact layer, and helps to minimize the diode capacitance by increasing the depletion width of the diode.  
         [0026]    Schottky layer  9  is grown next. In a first embodiment of the invention the Schottky layer is undoped, strained Al 1-x In x As, preferably Al 0.3 In 0.7 As. In this first embodiment, Schottky layer  9  is preferably grown to about 100 Å thick to reduce crystal anomalies due to the strain from the lattice mismatch. Schottky layer  9  is described further below in accordance with second, third and fourth alternative embodiments of the invention.  
         [0027]    After Schottky layer  9 , a cap layer of about 50 Å of undoped GaInAs (not shown) is grown to protect AlInAs in the Schottky layer from ambient oxygen. FIG. 1 shows the device after the first etch step has removed the cap, and Schottky metal layer  10  has been deposited by any standard technique. Schottky metal layer  10  is preferably about 250 Å Ti, 1000 Å Pt, and 6000 Å Au.  
         [0028]    An etch step, preferably using Schottky metal  10  as an etch stop, mesa etches Schottky layer  9  and depletion layer  7  to expose cathode contact layer  5  as shown in FIG. 2. Due to the thickness of cathode contact layer  5 , a timed wet etch may be used, or any compatible etch process. Next, cathode ohmic contact  21  is deposited on cathode contact layer  5 , preferably with 900 Å AuGe, 100 Å Ni, and 2000 Å Au.  
         [0029]    [0029]FIG. 3 shows the result of the next two steps. Cathode contact layer  5  and depletion layer  3  are both mesa-etched using any compatible etch technique, to isolate the diode structure on substrate  1 . Thereafter, first level interconnect metal  31  is deposited, using any compatible metal, to provide means for connecting external circuits to the device.  
         [0030]    [0030]FIG. 4 shows the device after the step of adding passivation layer  41  of preferably silicon nitride, and after the subsequent step of etching passivation layer  41  to expose Schottky metal  10  and first layer metallization  31 .  
         [0031]    [0031]FIG. 5 shows airbridge  51  placed to connect Schottky anode metal  10  to first layer metallization  31 . Airbridge  51  is preferred for high frequency applications, as it reduces diode capacitance. In many applications, the anode may be connected by other well-known metallization techniques.  
         [0032]    [0032]FIG. 6 shows the layer structure of a superlattice composed of nine periods  61 - 69  which are each compound of first sublayer  60  and second sublayer  70 . Such a superlattice may be used to grade a material in a stepwise or “chirped” fashion, from predominantly the material of second sublayer  70  in first period  61 , to predominantly the material of first sublayer  60 . Between successive periods, the thickness of first sublayer  60  is increased, while the thickness of second sublayer  70  is correspondingly decreased. For convenience, first sublayer  60  is about 10% of the thickness of first period  61  of a nine-period chirped graded superlattice, increasing linearly to 90% of the thickness of ninth period  69 ; while second sublayer  70  is about 90% of the thickness of first period  61 , reducing to about 10% of the thickness of ninth period  69 .  
         [0033]    [0033]FIG. 7 shows Schottky layer  9  according to a second embodiment of the present invention. For convenience, periods  71 - 77  are each about 33 Å thick. First sublayers  78  are AlInAs, and second sublayers  79  are GaInAs, both lattice-matched to InP. In first period  71 , first sublayer  78  is about 3.3 Å thick; subsequent sublayers  78  increase by about 3.3 Å in each successive period so that in seventh period  77 , first sublayer  78  is about 23.1 Å thick. Second sublayer  79  decreases by about 3.3 Å per period from about 29.7 Å in first period  71  to about 9.9 Å in seventh period  77 . Thus, FIG. 7 shows a chirped graded superlattice based on a nine-period grading transition, but truncated at seven periods. A Schottky diode fabricated according to this embodiment results in a room-temperature turn-on voltage of about 0.37V at 1 mA. I-V curve  700  of FIG. 10 is for this embodiment of the invention.  
         [0034]    [0034]FIG. 8 shows the layer structure of Schottky layer  9  according to a third embodiment of the present invention. This embodiment is also based on a nine-period superlattice grading arrangement, but is truncated at  5  layers instead of at  7 . First sublayer  88  of first period  81  is about 3.3 Å of AlInAs, increasing by about 3.3 Å in successive periods  82 ,  83 ,  84  and  85  so that in fifth period  85  it is about 16.5 Å. Second sublayer  89  is GaInAs, and is about 29.7 Å in first period  81 , decreasing to about 16.5 Å in fifth period  85 . A Schottky diode fabricated according to this embodiment results in a room-temperature turn-on voltage of about 0.25V at 1 mA. I-V curve  800  of FIG. 10 is for this embodiment of the invention.  
         [0035]    [0035]FIG. 9 shows the layer structure of Schottky layer  9  according to a fourth embodiment of the present invention. This embodiment is also based on a nine-period superlattice grading arrangement, but is truncated at 3 layers instead of at 5 or 7. First sublayer  98  of first period  91  is about 3.3 Å of AlInAs, increasing by about 3.3 Å in successive periods  92  and  93  so that in third period  93  it is about 10 Å. Second sublayer  99  is GaInAs, and is about 29.7 Å in first period  91 , decreasing to about 23 Å in third period  93 . A Schottky diode fabricated according to this embodiment results in a room-temperature turn-on voltage of about 0.15V at 1 mA. I-V curve  900  of FIG. 10 is for this embodiment of the invention.  
         [0036]    [0036]FIG. 10 shows an I-V curve for each of three described embodiments. Curve  700  is the IV curve for the embodiment of FIG. 7, curve  800  for the embodiment of FIG. 8 (below), and curve  900  for the embodiment of FIG. 9 (below).  
         [0037]    For sufficiently thin periods, the effective bandgap tracks the average composition of the superlaffice. Since the superlattice of the second, third and fourth embodiments of the invention is chirped, the Schottky barrier height changes discontinuously from period to period. Small chirped grading steps produce smaller Schottky barrier height discontinuities, and hence are thought to help reduce carrier pile-up which may occur on Schottky barrier height discontinuities. Since such pile-up may reduce speed, smaller grading steps may thus enhance speed. Accordingly, the present invention could be practiced with a superlattice truncated partway through a GaInAs to AlInAs grading arrangement which requires more than nine periods to complete, and correspondingly has smaller steps from period to period.  
         [0038]    Embodiments of the present invention have been described which truncate a nine-period grading arrangement at three, five and seven periods. Truncating at different numbers of periods permits close control of the turn-on voltage of Schottky diodes fabricated according to the present invention. If truncation is based upon grading arrangements requiring more than nine periods to completion, the on-voltage step size between different truncation points is correspondingly reduced, enhancing the fineness of fabrication control of that parameter.  
         [0039]    Periods of different, and indeed varying, thickness are within contemplation of the present invention. The composition of sublayers need not change in equal-sized steps between successive periods. Periods may have sublayers of alternative materials, and may have additional materials added.  
         [0040]    Fabrication details are merely exemplary; the invention is defined by the following claims.