Perspectives from research on metal-semiconductor contacts: Examples from Ga2O3, SiC, (nano)diamond, and SnS

As part of a Special Issue in Honor of 30 Years of the American Vacuum Society’s Nellie Yeoh Whetten Award, this Invited Perspective discusses results and trends from the authors’ and other published research on metal contacts to -Ga2O3, (4H and 6H)-SiC, nanocrystalline diamond (NCD), and nanocrystalline thin films and single-crystalline nanoribbons of -SnS. The paper is not a comprehensive review of research on contacts to each of these semiconductors; it is instead a perspective that focuses on Schottky barrier height (b) measurements and factors that affect b, such as the metal work function (m) and crystallographic surface plane. Metals and the associated processing conditions that form ohmic or Schottky contacts to each of these semiconductors are also described. Estimates of the index of interface behavior, S, which measures the dependence of b on m, show large variations both among different semiconductors (e.g., S ~ 0.3 for NCD and S ~ 1.0 for SnS nanoribbons) and between different surface planes of the same semiconductor (e.g., (2̅01) vs. (100) Ga2O3). The results indicate that b is strongly affected by the nature of the semiconductor surface and near-surface region Th is is the au tho r’s pe er re vie we d, ac ce pte d m an us cri pt. H ow ev er , th e o nli ne ve rsi on of re co rd w ill be di ffe re nt fro m thi s v er sio n o nc e i t h as be en co py ed ite d a nd ty pe se t.

The formation of a Schottky barrier can be understood by referring to the electron energy diagrams of a typical metal and an n-type semiconductor (Fig. 1a). When the metal and semiconductor contact each other, the Fermi levels in each material align as a result of electron transport from the semiconductor to the metal, resulting in an upward band bending at the semiconductor surface (Fig. 1b). The Schottky-Mott relationship 1 follows from this described charge transport: b = m -s, (Eqn. 1) where m is the metal work function and s is the electron affinity of the semiconductor.
This ideal relationship therefore predicts that one can control b by choosing a metal with a proper m.
However, because of surface states 2 , metal-induced gap states 3,4 , or other factors, it is often found that b is either independent of or weakly dependent on m. The measure of correlation between b and m is called the index of interface behavior: This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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Schottky metal contacts to (100), (010), , and (001) Ga2O3 surfaces, the reader is referred to Lyle et al. 10 Ga2O3 has been reported to have an upward band bending at the surface, [11][12][13] unlike some other n-type transparent conducting oxides such as In2O3. 14 Because of this upward band bending, Schottky contacts tend to form naturally on Ga2O3. However, research to date indicates that properties of the contacts, such as the Schottky barrier heights, are dependent upon the particular Ga2O3 surface on which the contacts are deposited.
Schottky contacts also tend to be dependent on the surface preparation. Prior to contact deposition, Ga2O3  We found that Schottky barrier heights of W, Cu, Ni, Ir, and Pt contacts on the surface of -Ga2O3 showed little dependence on the metal work function 17 (Fig.   2). The results indicate significant Fermi level pinning for Schottky contacts to Ga2O3. This result was attributed to near-surface defects and/or unpassivated surface states.
In a different study by Hou et al., 11 Schottky contacts of metal-oxides were reported to have higher b's and better thermal stability on -Ga2O3 than their unoxidized metal counterparts (Fig. 3). These results also show a narrow range (1.  Although the reactivity between Ti and Ga2O3 appears to be beneficial to forming an ohmic contact, the reaction is not self-limiting. Annealing for longer times at the same temperature or at higher temperatures should increase the thickness of Ti-oxide. The degradation in the electrical behavior of Ti/Au contacts annealed at T > 500 °C is attributed to the formation of a thicker TiOx non-conductive/low-conductivity layer. For stable operation of Ga2O3 devices at elevated temperatures over extended time periods, it will be important to develop contact metallization schemes that are both electrically and thermally stable. A recent study reports that Mg/Au (820 nm / 600 nm) contacts on Sn-doped -Ga2O3 were ohmic after annealing for 2 min. in Ar at temperatures between 300 and 500 °C. 25 A minimum contact resistance of 2.1 x 10 -5  cm 2 on the 4 x 10 17 cm -3 substrate was calculated after a 500 °C anneal. It is perhaps encouraging that this additional metallization scheme has demonstrated ohmic behavior on Ga2O3. However, a caveat is that Mg has an even higher driving force for oxidation than Ti and therefore is also unstable on Ga2O3. The authors of the study found that the electrical characteristics degraded when annealed at 600 °C.
In summary, metal contacts to Ga2O3 tend to form Schottky contacts in the as- that contact metal schemes with enhanced stability will be needed for long-term device operation at elevated temperatures.

III. SiC
Silicon carbide (4H-SiC) is being increasingly used as a semiconductor platform in commercial high power devices and is expected to continue to replace silicon in a broad range of high power applications, for which reliability testing of the SiC devices is an ongoing concern of paramount importance. 26  Our early metal contact studies on SiC were conducted mostly on (0001) 6H-SiC.
The nature of the semiconductor surface prior to metal contact deposition is critically important for determining the behavior of metal-semiconductor contacts, especially for covalently bonded semiconductors like SiC. We developed a chemical and thermal cleaning process, 29 which consisted of oxidizing the surface to remove excess C (present on as-received epitaxial films), etching in a 10% HF aqueous solution to remove the oxide layer, and heating in ultra-high vacuum at 700 °C to remove hydrcarbons from the surface. This temperature was chosen to prevent graphitization of the SiC surface, which can begin to occur at 800 °C in vacuum. 30 It's important to note that characterization using XPS showed residual O and a trace amount of F were still present on the SiC This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. Metal contact studies on n-type (0001) 4H-SiC, cleaned using the procedure described above, also indicate the ideal Schottky barrier height increases with metal work function 37 . The index of interface behavior (Eqn. 2) for Ti, Ni, and Pt contacts was estimated to be S = 0.45 (Fig. 8), which is similar to S values extracted for 6H-SiC.
However, a significant fraction of diodes on 4H-SiC epilayers showed inhomogeneous behavior that was modeled as two (low and high) Schottky barriers in parallel (Fig. 9b). Fermi level pinning by defects, such as 3C-SiC stacking faults, in the 4H-SiC epilayers. 37,38 In cases where the local concentration of defects within the diode area was high, both the work function and near-surface dipoles induced by subsurface defects contribute to Schottky barrier formation. 22 Ohmic contacts to SiC with low contact resistance are needed to keep device onresistances low. 41 Nickel is most commonly used as the ohmic contact to n-type SiC.
Annealing at 900-1000 °C produces Ni2Si and c ~10 -5 -10 -6  cm 2 . It is difficult to obtain ohmic contacts with low c to p-type SiC, primarily because of its large band gap and work function. It's interesting to note a prediction we made 25 years ago, that "the ability to create ohmic contacts with low contact resistivities (≤10 -6  cm 2 ) will be one of the major challenges facing the SiC community in the foreseeable future," 33

IV. NANOCRYSTALLINE DIAMOND
Nanocrystalline diamond (NCD) is generally described as comprising crystalline grains (typically 10's of nanometers in size) and grain boundaries that contain predominantly sp 3 -bonded and sp 2 -bonded carbon atoms, respectively. 46 Conductivity in NCD films is ascribed to conduction within grain boundaries, probably through hopping and impurity band conduction. 47 The p-p* states, associated with sp 2 bonding in the grain boundaries, strongly affect the optical and electronic properties of NCD. 47 Preferential incorporation of impurities into the grain boundaries coincides with n-type (e.g., N or S) doping, whereas conventional doping with boron can lead to p-type conductivity in these films. 46 The broader control over the conductivity and carrier type in NCD is considered an advantage relative to conventional diamond films, although the small grain size yields  (Fig. 11). Another conclusion from this study is that the ohmic behavior is likely due to carrier transport through low-b grain boundary regions.
Due to the hopping and impurity band conduction in NCD films, it is plausible that trapassisted tunneling is a parallel current transport mechanism at the metal-NCD interfaces.
contact resistance to NCD films depends on the annealing conditions, film thickness, and film morphology in addition to the particular metal. UV photodetectors based on NCD employed Au 53 or W 54 ohmic contacts. Fig. 12 shows I-V characteristics of metalsemiconductor-metal (MSM) NCD photodetectors using Mo and W, respectively. Both metals were ohmic in the as-deposited condition, but the W contacts were annealed up to 600 °C to reduce the contact resistance.
Tadjer et al. 55 also found that different metals (Al, Ti/Al, Ti/Au, and Ni/Au) formed ohmic contacts in the as-deposited condition to B-doped, p-type NCD films. It is interesting that the contact resistances were lower for the low-work function metals even though the films were p-type. The boron doping level had a much larger effect on lowering the contact resistance than did the particular metal.
One study reports that contacts changed from ohmic to "near Schottky" after hydrogen plasma treatment 56 of nitrogen-incorporated NCD. Au Schottky diodes to boron-doped NCD films with low-doped cap layers have also been reported 57 .
In summary, most metal contacts on NCD (n-type, p-type, or unintentionallydoped) films are reported to be ohmic. The ohmic behavior of metals on NCD films is contrary to the typical Schottky behavior observed on conventional p-type diamond films, which require annealing or other processing steps to form ohmic contacts. 51 Measurements of NCD films reported in the literature indicate that their electrical properties are largely governed by conduction within the nanocrystalline grain boundaries, which likely contributes to the different behavior of contacts to NCD vs.
conventional diamond films. It is important to note, however, that even with grain boundary dominant conduction, the contact resistances to n-type and undoped NCD films This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. undoped hole concentration (10 15 -10 18 cm -1 ) of α-SnS has motivated its study as a promising candidate for thin film solar cells. [60][61][62] SnS has also recently attracted attention for its high thermoelectric performance 63 and for applications in battery anode 64 and photodetector devices. 65 The crystal structure of α-SnS consists of two-atom thick, This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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buckled layers of strongly bonded Sn-S atoms separated by weaker interactions (Fig. 13 a-c). 66 α-SnS is an analogue of black phosphorous/phosphorene with lower symmetry due to the presence of two different elements. 67  Contacts to the (100) surface of individual, solution synthesized, p-type α-SnS semiconductor nanocrystals (Fig. 13 d, e) showed ohmic or semi-ohmic behavior for high work function metals (Ni and Pd) and rectifying behavior for lower work function metals (Cr and Ti) (Fig. 14 a, b). 81 The behavior follows closely that predicted by Schottky-Mott theory (Fig. 15 a, b): specifically, the calculated b's were 0. 39  In contrast, contacts to electron-beam-evaporated, p-type nanocrystalline α-SnS thin films (Fig. 17) did not display the range of electrical behavior observed for contacts to the (100) p-type SnS nanocrystals. 85 Based on the reported electron affinity = 3.8 eV for (100) α-SnS 80 and a bandgap of 1.1 eV, one would predict that metals with work functions ~5 eV or higher would be ohmic and those with lower work functions would be rectifying. Furthermore, additional crystallographic surfaces exposed in nanocrystalline SnS thin films have reported electron affinities even greater than that of the (100) surface. 80 However, all of the contacts (Ti/Au, Ru/Au, Ni/Au, and Au) were ohmic in the as-deposited condition, despite the moderate hole concentration (~5 x 10 15 cm -3 ) of the SnS films. 85 The average specific contact resistances decreased with increasing metal work function, suggesting a work function dependent b and indicating at least partial adherence to Schottky-Mott theory. In the literature, there is more variability in the electrical behavior of contacts to SnS polycrystalline films, whereas some studies report ohmic behavior for low work function metals [86][87][88][89] and others report Schottky behavior. 90,91 This variability of results among different studies is likely due to the variability in properties (e. g. stoichiometry, surface morphology, carrier concentration) of SnS thin films, which have been deposited by a multitude of techniques (e. g. electrochemical deposition, thermal evaporation, atomic layer deposition). Furthermore, SnS is known to form a thin oxide layer at its surface 92 and may be sensitive to differences in surface preparation methods (e. g. immersion in ethanol, 93 dip in dilute HF, 81 UV-ozone treatment followed by dilute (NH4)2S rinse, 94 O2 plasma followed by dilute HF dip, 85 or no reported surface preparation). In the case of our study, we attribute the ohmic behavior of low work function metals on SnS thin films to defect-assisted carrier transport across a nonuniform, nanocrystalline interface.
In addition to high contact resistivity, instability of contacts to SnS can be detrimental to device performance and reliability, particularly for devices, such as thermoelectrics, operating at elevated temperatures. 63  Au contacts showed greater stability with no significant change in specific contact resistance upon annealing between 300 °C -500 °C in Ar for 5 min. 85,94 Au is not expected to react with SnS based on thermodynamic predictions, and significant intermixing at the Au-SnS interface was not observed. 94 In contrast, annealing Au/SnS back contact structures in H2S at 400 °C for 1 hr resulted in an increase in Au contact resistivity, 89 suggesting the longer annealing time, different annealing ambient, difference in interface geometry, and/or difference in SnS film characteristics permitted diffusion at the interface. Of all contact metals we investigated on SnS thin films, the lowest contact resistivity (1.9 × 10 −3 Ω cm 2 ) occurred for Ru/Au contacts annealed at 350 °C in Ar for 5 min. 85 In summary, the behavior of unannealed contacts to α-SnS appears to be dependent upon the properties of the SnS material itself in addition to the contact metal.
Schottky barrier heights of metals on near-ideal (100) surfaces of α-SnS crystals are very close to that predicted by Schottky Mott model, suggesting a lack of Fermi-level pinning for this surface. In contrast, contacts to polycrystalline α-SnS thin films are typically ohmic regardless of metal work function. As-deposited contact resistivities of many metals on polycrystalline SnS films exhibit a decreasing trend with increasing in metal work function, suggesting some dependence of Schottky barrier height on metal work function for polycrystalline SnS films, and that high work function metals should be considered to form low resistance ohmic contacts to SnS films. Certain annealing conditions have been shown to lower the contact resistance of metals such as Pd, 94 Ru, 85 Al, 88 Mo, 89 Ti 89, 93 and Ag 88 . However, annealing at high temperatures or for long durations may result in an increase in contact resistance or deterioration of the contact for certain metals including Pd, 94 Ti, 85,93 Ni, 85 Al, 88 Au, 89 Sn, 88 and In. 88 For this reason, the identification of a low resistivity contact that is stable over a range of operating conditions or the development of a diffusion barrier may be beneficial for SnS-based devices.  -Ga2O3 surface. SiC is another semiconductor that has shown significant differences for different surfaces: e.g., metals on C-face SiC tend to have higher b's than the same metals on Si-face SiC.

VI. SUMMARY AND CONCLUSIONS
Ohmic contacts to all of these semiconductors have been demonstrated. Ohmic contacts tend to form readily to NCD and nanocrystalline SnS films, whereas few metals have been demonstrated as ohmic contacts to Ga2O3. Although progress has been made to enhance thermal stability of metal-semiconductor contacts, improvements are needed to realize the full potential of semiconductors like Ga2O3 and SiC that are being developed for devices for extreme operating conditions.

ACKNOWLEDGMENTS
Published results from our group would not have been possible without the contributions from many former and current students and colleagues. This work is supported by the Air Force Office of Scientific Research under award number FA9550-18-1-0387.
pertains to fabrication, processing, and characterization of electronically-functional interfaces and has included dielectric-semiconductor (e.g., SiO2/SiC) and semiconductorsemiconductor (e.g., InGaN/GaN multi-quantum wells for LEDs) interfaces, with emphasis on metal-semiconductor contacts. In addition to the semiconductors presented in this paper (Ga2O3, SiC, nanocrystalline diamond, and SnS), Dr. Porter's research has covered a broad range of (semi)conducting materials such as transparent conductors (e.g., Reproducibility and Replicability issues, and to prepare a foundation for a new five-year strategic plan for the Society, while doing her best to keep tabs on countless other This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1116/1.5144502