Stacked Silicide/Silicon Mid- To Long-Wavelength Infrared Detector

FIG. 1, on coordinates of energy and distance, is a plot depicting the operation of prior art Schottky-barrier detectors;

FIG. 2, on coordinates of energy and distance, is a plot depicting the operation of the Schottky-barrier detector of the invention and illustrating the concept of barrier engineering to achieve longer wavelength response;

FIG. 3 is a cross-sectional view of a particular embodiment of one structure of a stack of such structures which comprise the detector of the invention;

FIG. 4a is a cross-sectional view of a plurality of the structures depicted in FIG. 3;

FIG. 4b, on coordinates of energy and distance, is a plot depicting the energy profile of a portion of the device shown in FIG. 4a; and

FIG. 5 is a cross-sectional view of a means of contacting each detector element, or pixel, of a stack.

Normally, Schottky-barrier detectors work on the principle shown in FIG. 1. The metal contacts 10 in this illustration are usually either PtSi or Pd.sub.2 Si silicides (used by industry). These contacts form relatively low Schottky barriers to p-type silicon 12 providing near- to medium-wavelength infrared response. The barrier height is shown as .phi.. E.sub.V and E.sub.C represent the valence band and the conduction band energy levels, respectively.

Photons (represented by wave 14) absorbed in the silicide 10 near the silicon interface 16 produce "hot" holes 18 with a maximum energy h.nu. (measured downward below the Fermi energy E.sub.F). Holes with energy greater than .phi. (below E.sub.F) can cross the barrier 14 (as indicated by arrow 20) and be collected in the silicon 12 as a photocurrent.

Since the maximum energy the hot holes can acquire is h.nu., the cut-off wavelength .lambda..sub.c is determined by h.nu.>.phi., or .lambda..sub.c =1.24/.phi. (eV) .mu.m. In this case, the barrier height .phi. (e.g., 0.25 eV) provides medium-wavelength cut-off (e.g., 5 .mu.m). This limitation is overcome in the following embodiment.

FIG. 2 illustrates the concept of barrier engineering in accordance with the invention to achieve longer wavelength response. A dopant spike 22 is introduced adjacent the silicon/silicide interface 16. Since a low Schottky barrier height occurs at the valence band (as opposed to the larger barrier at the conduction band), the dopant is an acceptor impurity, typically boron. This spike may be introduced during growth of the stacked structure described below. By selecting the appropriate density per unit area N.sub.s and position .DELTA.x for the dopant spike, the top portion of the Schottky barrier can be made sufficiently narrow so that holes can tunnel into the silicon at energies less than .phi., but above an effective value .phi..sub.eff. Thus, the effective barrier is reduced from .phi. to .phi..sub.eff, and the cut-off wavelength increases from .lambda..sub.c =1.24/.phi. to .lambda..sub.c =1.24/.phi..sub.eff. The value of .phi..sub.eff is determined by Gauss's law

and

Thus, by appropriate choice of N.sub.s and .DELTA.x, the value of .phi..sub.eff can be determined. For example, if the layers are grown by molecular beam epitaxy (MBE), typical values might be .phi.-.phi..sub.eff .apprxeq.0.15 eV, .DELTA. x .apprxeq.10 .ANG., and N.sub.s .apprxeq.10.sup.13 cm.sup.-2. The p doping of the bulk silicon is kept sufficiently low (e.g. 10.sup.15 to 10.sup.16 cm.sup.-3) so that tunneling is not appreciable through the residual barrier beyond .DELTA.x.

High quantum efficiency, on the order of 10% or more, is achieved by using very thin silicide layers and stacking layers of silicon and metal silicide to form repeated Schottky barriers. Such stacked structures could be epitaxially grown, for example, by molecular beam epitaxy (MBE) using silicides which are closely lattice-matched to silicon. CoSi.sub.2 is an example of such a silicide in which Si/CoSi.sub.2 /Si structures have been epitaxially grown by MBE. Another familiar example is Si/NiSi.sub.2 /Si, and still others are being investigated.

FIG. 3 illustrates the detector configuration 30 of a single back-to-back Schottky element with selective doping of the Schottky barriers. Contact layers 32, conveniently comprising p.sup.+ -doped silicon, are used to provide electrical contact to the device to detect the resulting photo currents. These p.sup.+ layers should be doped to at least about 10.sup.19 cm.sup.-3 acceptors to provide sufficiently narrow Schottky barriers for ohmic contact.

The high quantum efficiency of the device of the invention is achieved by making the silicide layers 10 very thin, much less than the mean-free-path of the photo holes in the silicide. This thickness (e.g., <10 nm) increases the probability that the photo holes can be collected at one of the Schottky barriers, since they can then make many attempts as they are repeatedly reflected elastically back and forth between the contacts before losing their energy by inelastic collisions. According to Dalal, supra, in the limit of thin metal films, the quantum yield increases approximately inversely with the metal thickness w when w is less than the effective scattering distance L. Therefore, if w/L<1, an enhanced quantum efficiency per absorbed photon is gained. The photo holes are also collected at both interfaces, thus further doubling the quantum efficiency per layer.

Since each thin silicide layer 10 absorbs only the fraction 1-exp(-.alpha.w).apprxeq..alpha.w (for .alpha.w<1) of the incident IR radiation, where .alpha. is the metal absorption coefficient, the use of repeated layers increase the overall quantum efficiency. The silicon layers 12 are virtually transparent to the medium- and long-wavelengths. The silicon layers 12 each need only be thick enough to support the total depletion width of the two Schottky barriers (typically, about 1,000 to 2,000 .ANG.). Ideally, the optimum number of layers N occurs when most of the IR radiation is absorbed, that is when N.alpha.w.gtoreq.1. For example, if .alpha..apprxeq.(200 .ANG.).sup.-1, Nw.gtoreq.200 .ANG..

While FIG. 3 depicts a single back-to-back Schottky-barrier structure comprising two silicon layers 12 sandwiching one metal silicide layer 10, many such sandwiched, or stacked, layers may be utilized, as shown in FIG. 4a. FIG. 4b depicts the energy profile of a portion of the device shown in FIG. 4a.

Separate contacts to the metal silicide layers and n-silicon layers need to be provided at the sides of each detector element (pixel). One preferred configuration is schematically depicted in FIG. 5, showing one pixel 34. N.sup.+ -silicon regions 38 make contact to the metal silicide layers 10 and p.sup.+ -silicon regions 36 make contact to intervening p-silicon layers 12. The p-n junction 40 provides a sufficient electrical barrier such that contact regions 36 and 38 effectively connect only through the set of interleafing Schottky barriers between the p-silicon and metal silicide layers. The Schottky barriers are thus all connected in parallel, each contributing to the total photo current and quantum efficiency. These contact regions 36, 38 may in turn be connected to signal processing circuitry (not shown), for example, by metal contacts and wires. In order to achieve good contact to the detector p-silicon and metal silicide layers and to interconnects to the circuitry, the p.sup.+ and n.sup.+ doping should be sufficiently large (e.g., N.sub.A .apprxeq.N.sub.D >10.sup.19 cm.sup.-3.

Various techniques are being developed for achieving configurations such as shown in FIG. 5, using, for example, during MBE growth in situ techniques such as ion beam writing of the dopants in the preferred regions and/or use of shadow masks. Alternately, one may use combinations of ion implantation, patterning, etching, and MBE regrowth.

Since the above structures can be grown by MBE, they could be grown directly on the same silicon wafers that are processed for VLSI circuits. In this case, the p.sup.+ or n.sup.+ contacts 36 and 38 shown in FIG. 5 may also be patterned to serve as internal interconnects to circuitry processed on the same silicon chip. Therefore monolithic silicon detector arrays with on-chip processing are possible.

The stacked silicide/silicon detector of the invention is useful in infrared detectors in which longer wavelength detection and higher quantum yield are required and in which monolithic silicon detectors with on-chip processing is desired.

Thus, a stacked silicide/silicon mid- to long-wavelength infrared detector is disclosed. It will be apparent to one of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention, and all such changes and modifications are intended to fall within the scope of the invention, as defined by the appended claims.