As the size of transistors is reduced to nanometer scale dimensions, for example in the form of ultra-thin body (UTB) silicon-on-insulator (SOI) field effect transistors (FETs), FinFETs and nanowire FETs, the unwanted resistance associated with transistor sources and drains becomes an ever increasing burden on the performance of these devices and of the integrated circuit products manufactured using such transistors. Furthermore, a reduction of dopant activation is predicted theoretically and demonstrated experimentally when the transistor source and drain regions are reduced in size below approximately 10 nm. By dopant activation, we mean desired free carrier (electron or hole) contributions from deliberately introduced impurity species in a semiconductor host. This decrease in nanoscale dopant activation further contributes to undesirably high resistance of doped source/drain (S/D) regions both at the nanoscale metal contacts and within the bulk portion of the nanoscale doped regions. The resistance of metal contacts to a semiconductor increases if effective doping in the semiconductor decreases, the increase being primarily due to the presence of a Schottky barrier at metal-semiconductor contacts.
It is known that a high concentration of doping in a shallow region of semiconductor proximate to a metal-semiconductor interface can reduce the resistance of the metal-semiconductor contact by decreasing the width of the Schottky barrier. Although it is the barrier width that is reduced, from an electrical response point of view (for example current—voltage measurement), it appears that the Schottky barrier height is reduced. An early article that describes this “effective barrier height” reduction by surface doping is by J. M. Shannon, “Control of Schottky barrier height using highly doped surface layers” in Solid-State Electronics, Vol. 19, pp. 537-543 (1976). It is also known that a high concentration of dopant atoms can be introduced into a shallow region of a semiconductor proximate to a metal contact by so-called dopant segregation out of a metal silicide. A. Kikuchi and S. Sugaki reported in J. Appl. Phys., Vol. 53, No. 5, (May 1982) that implanted phosphorus atoms piled up near a PtSi—Si interface during PtSi formation and reduced the measured height of the Schottky barrier to n-type silicon. The reduction of the measured (effective) barrier height of the Schottky diode was attributed to piled up phosphorus atoms in the silicon causing the barrier to be more abrupt. That is, the result was attributed to the effect described by Shannon in 1976.
For the past several decades the silicon microelectronics industry has relied on high doping concentrations in the silicon proximate metal-silicon contacts as a means of obtaining acceptably low contact resistances to transistor sources and drains. The contact metal has for the most part been a metal silicide, most recently nickel silicide or nickel platinum silicide. This approach to minimizing contact resistance is expected to be insufficient in the future as the transistor dimensions continue to shrink and the contact resistance becomes a larger portion of the total resistance between the source and drain (hence becoming a serious performance-limiting factor). The most recent International Technology Roadmap for Semiconductors (ITRS), published in 2011, reports that there is no known solution to the contact resistance problem in bulk MOS transistors when the transistor gate length scales to 18 nm, as expected in year 2014, and a specific contact resistance of no more than 1.0×10−8 Ohm·cm2 is specified. It is becoming increasingly apparent that the Schottky barrier at metal-semiconductor contacts must be reduced in order to reduce the contact resistance to acceptable levels, i.e. well below 1.0×10−8 Ohm·cm2 in the case of MOS transistor doped source/drain contacts. A technology that is capable of reducing the Schottky barrier and hence reducing the resistance of contacts to doped semiconductor regions may also be applied to so-called “metal source/drain transistors” which do not have doped source and drain but rather utilize a direct contact between the metal and the transistor channel (the region of free carriers that are modulated by the electrical potential on a gate and that transport current between the source and the drain).
A body of work published in 1991-1992 reported experimental verification of theoretical predictions by Baroni, Resta, Baldereschi and others that a double intralayer formed by two different elements would create an interface dipole, capable not only of modifying heterojunction band discontinuities, but also of generating band discontinuities in homojunctions. McKinley et al. first reported obtaining 035-0.45 eV band offsets at {111}-oriented Ge homojunctions using Ga—As dipole intralayers in a 1991 article “Control of Ge homojunction band offsets via ultrathin Ga—As dipole layers”, J. Vac. Sci. Technol. A 9 (3), May/June 1991 and in a similar article in 1992 “Control of Ge homojunction band offsets via ultrathin Ga—As dipole layers”, Applied Surface Science Vol. 56-58, pp. 762-765 (1992).
Arsenic, gallium, and germanium depositions were done at room temperature on p-type Ge(111) substrates. Valence band offsets were measured by in situ core level x-ray photoluminescence. The deposited Ge region (overlayer) had a valence band offset to the Ge substrate as manifested by a splitting of the Ge 3d core level into two components; one due to the Ge substrate and the other to the Ge overlayer. Both positive and negative valence band offsets were obtained in Ge homojunctions by introducing Ga—As dipole intralayers with either “Ga-first” or “As-first” growth sequences. The band offset was found to be 0.35-0.45 eV with the Ge valence band edge on the As side of the junction at a lower energy (i.e., more bound). Dipole intralayers were explained on the basis of the Harrison “theoretical alchemy” model described by W. A. Harrison et al. in “Polar Heterojunction Interfaces”, Phys. Rev. B 18, 4402 (1978). Intralayer control of band discontinuities was thus applied to homojunctions, expanding the potential domain of band offset engineering beyond semiconductor heterojunctions.
In 1992, Marsi et al. followed up on the reports by McKinley et al. with the articles “Microscopic manipulation of homojunction band lineups”, J. Appl. Phys., Vol. 71, No. 4, 15 Feb. 1992, “Homojunction band discontinuities induced by dipolar intralayers: Al—As in Ge”, J. Vac. Sci. Technol. A 10(4), July/August 1992 and “Local nature of artificial homojunction band discontinuities”, J. Appl. Phys. 72 (4), 15 Aug. 1992. In the first article, Marsi et al. reported valence-band discontinuities at Si—Si and Ge—Ge homojunctions when III-V double intralayers of atomic thickness were inserted at the interfaces. Valence band discontinuities were again measured by in situ core level x-ray photoluminescence. In Ge samples, a deposited Ge region (overlayer) had a valence band offset to a Ge substrate as evidenced by a splitting of the Ge 3d core level into two components and a deposited Si region had a valence band offset to a Si substrate as evidenced by a splitting of the Si 2p core level. The observed discontinuities with magnitudes in the range 0.4 to 0.5 eV (for example 0.5 eV for Si—P—Ga—Si and 0.4 eV for Si—P—Al—Si) were in qualitative agreement with theoretical predictions although most theories estimate larger valence band discontinuities due to the dipole effect. A III-V intralayer at a group-IV homojunction systematically induced an artificial valence-band discontinuity when the anion was deposited first. It was also reported that in the case of Si—Si homojunctions with Al—P or Ga—P intralayers, a reversal of the interface deposition sequence led to a reversal of the valence-band discontinuity, as expected.
In the second article it was shown, again using x-ray photoemission, that a similar band offset effect can be induced using Al—As as a “dipolar intralayer” between two regions of {111}-oriented germanium. Specifically, an offset of 0.4 eV was obtained for the “anion-first” Ge(substrate)-As—Al—Ge(overlayer) sequence, consistent with the “anion-first” As—Ga sequence reported by McKinley, the overlayer component exhibiting a lower binding energy with respect to the substrate component. In the third article, multiple III-V bilayer (intralayer) stacks were investigated. The measured value of valence band offset remained the same, 0.5 eV, for an individual double layer, for double-stacked bilayers and for triple-stacked bilayers. Experiments performed on 2(Ga—P) and 2(P—Ga) were fully consistent with those on 2(Al—P) and 2(P—Al); no substantial increase was observed on going from the individual bilayers to two bilayers or even to three bilayers. It was therefore concluded that stacked interfacial III-V bilayers do not increase the effect of an individual bilayer, contrary to elementary predictions based on sequential dipoles.
In U.S. Pat. Nos. 7,084,423, 7,176,483, 7,462,860, and 7,884,003 and in pending U.S. patent application 2011/0169124, Grupp and Connelly described metal-semiconductor contacts having an interfacial layer at the interface between a metal and a group IV semiconductor for the purpose of reducing the Schottky barrier at the contact and, hence, reducing the specific resistivity of the contact. A monolayer of arsenic (or nitrogen) was included amongst the possible embodiments/specifications of the interfacial layer.