Schottky contact metallization

Metals for Schottl Contacts. Good Schottky contacts on semiconductor surfaces should not have any interaction with the semiconductor as is common in ohmic contacts. Schottky contacts have clean, abmpt metal—semiconductor interfaces that present rectifying contacts to electron or hole conduction. Schottky contacts are usuaHy not intentionaHy annealed, although in some circumstances the contacts need to be able to withstand high temperature processing and maintain good Schottky behavior. 

The most common Schottky contacts for compound semiconductors are gold-based metallizations deposited by thermal or electron beam evaporation. The metal may include a thin titanium layer in direct contact with the semiconductor which acts as an adhesion layer. AdditionaHy, a thin layer... 

Table 11-2 shows the built-in potential in metal/MEH-PPV/metal structures measured by either electroabsorption [15] or photocurrenl techniques [37] for a variety of contact metals. The uncertainty in both the work function differences and the built-in potential measurements is about 0.1 eV. For all of the structures except the Pt-Ca and Al-Sm devices there is good agreement between the metal work function difference, AW, and the built-in potential, Vhi. This indicates that for a wide range of metal contacts the Schottky energy barrier between the metal and MEH-PPV is well approximated by the ideal Schottky model and that state chaiging, which pins the Schottky energy barrier, is not significant. A built-in potential smaller than the difference between the contact work functions implies that... 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

The SiC Schottky diodes and capacitors that have been processed by the authors were processed on either 6H or 4H substrates (n-type, about 1 x 10 cm ) with a 5-10- m n-type epilayer (2-6 x lO cm" ) [123, 124]. A thermal oxide was grown and holes were etched for the metal contacts. In the case of the Schottky sensors, the SiC surface was exposed to ozone for 10 minutes before deposition of the contact metal. This ozone treatment produces a native silicon dioxide of 10 1 A, as measured by ellipsometry [74, 75]. The MISiC-FET sensors (Figure 2.9) were processed on 4H-SiC, as previously described [125]. The catalytic metal contacts consisted of 10-nm TaSiyiOO-nm Pt, porous Pt, or porous Ir deposited by sputtering or by e-gun. 

The surface morphology of the PLD grown ZnO-based films is important for the interface quality of multilayer structures, including quantum wells with thickness of a few nanometer only, for the formation of metal-semiconductor Schottky contacts and for the optical emission properties. Therefore, the control and optimization of surface properties is essential for the successful application of ZnO thin films in related device configurations. 

Besides the classical Schottky contact, various surface mechanisms are known to influence polymer metal contacts. Band bending in metal/PPV interfaces is also discussed in terms of surface states or chemical reactions between the semiconductor and the metal [70-74]. An excellent review on conjugated polymer surfaces and interfaces is given by [129]. 

Schottky contact — Alternative denomination of metal-semiconductor contact presenting a Schottky barrier. Depending on metal - work function, semiconductor electron affinity, doping of the semiconductor, conditions of the surface of the semiconductor before contact preparation, and preparation process, Schottky contacts with high rectification can be prepared. Devices encor-porating such contacts behave like a diode and for this reason, are also denominated Schottky diodes, whose main features are the capability of high frequency operations and low forward-voltage drop. 

Walter Haus Schottky (1886-1976) received his doctorate in physics under Max Planck from the Humboldt University in Berlin in 1912. Although his thesis was on the special theory of relativity, Schottky spent his life s work in the area of semiconductor physics. He alternated between industrial and academic positions in Germany for several years. He was with Siemens AG until 1919 and the University of Wurzburg from 1920 to 1923. From 1923 to 1927, Schottky was professor of theoretical physics at the University of Rostock. He rejoined Siemens in 1927, where he finished out his career. Schottky s inventions include the ribbon microphone, the superheterodyne radio receiver, and the tetrode vacuum tube. In 1929, he published Thermodynamik, a book on the thermodynamics of solids. Schottky and Wagner studied the statistical thermodynamics of point defect formation. The cation/anion vacancy pair in ionic solids is named the Schottky defect. In 1938, he produced a barrier layer theory to explain the rectifying behavior of metal-semiconductor contacts. Metal-semiconductor diodes are now called Schottky barrier diodes. 

Fig. 9.1 shows a schematic diagram of a metal Schottky contact on a semiconductor. In isolation, the metal and the semiconductor generally have different work functions and Og. (The work function is the energy needed to remove an electron from the Fermi energy to the vacuum.) When electrical contact is made between... 

A variety of surface reactions have been observed with other metals on a-Si H (Nemanich 1984). For example, a similar silicide is formed with platinum and nickel at 200 °C and with chromium at 400 °C. Aluminum and gold form intermixed phases at low temperature, but do not form silicides. Instead both metals promote low temperature crystallization of the a-Si H film. Dendritic crystallization occurs at 200 °C at a gold contact, giving a very non-uniform interface, and aluminum causes crystallization at 250 C. The resulting Schottky contact for gold is surprisingly ideal, but is very poor for aluminum. 

Fig. 9.6. Measurements of the Schottky barrier height versus metal work function for various metal contacts to a-Si H and crystalline silicon. The dashed line shows the relation for an ideal Schottky contact (Wronski and Carlson 1977).

Three important elements of inorganic semiconductor device structures are shown in Figure 3. A Schottky contact between a metal and a semiconductor, to inject or collect electrons (or holes) in a semiconductor, is shown in Figure 3a. In this diagram, the Schottky contact is in forward bias, Vj it is easier for electrons to flow from the semiconductor into the metal than vice versa because of the smaller energy barrier that must be surmounted when electrons move in the semiconductor-to-metal direction. In... 

Schottky contacts on ZnO were realized by the thermal evaporation of Ag, Au, Ni, or Pd, respectively. We used different surface preparation techniques prior to the deposition of the contact metal. For the single crystals a front-back contact configuration was used while a front-front configuration has to be used for thin films grown on insulating sapphire substrates. The homogeneity of the Schottky contacts depends on the surface preparation as revealed by electron beam induced current (EBIC) measurements (Fig. 6). 

Fig. 40. Ballistic emission electron microscopy, (a) Electrons are tunneling at a high voltage out of the tip into the thin metal layer. A small fraction of hot electrons ballistically penetrate the sample. They are collected at the rear ohmic contact of the sample. 0 ) STM (top) and BEEM (bottom) images of an Au/GaAs Schottky contact. Dark regions in BEEM correspond to areas with no collector current (after [186]).

Taking into consideration the above mentioned points, the energy diagram of a forward-biased Schottky contact is shown in Fig. 1. The energy is counted from the Fermi level in the metal. It is possible to write the current as following [2] ... 

A semiconductor can be described as a material with a Fermi energy, which typically is located within the energy gap region at any temperature. If a semiconductor is brought into electrical contact with a metal, either an ohmic or a rectifying Schottky contact is formed at the interface. The nature of the contact is determined by the workfunction, 0 (the energetic difference between the Fermi level and the vacuum level), of the semiconductor relative to the metal (if interface effects are neglected - see below) [47]. 

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