Modification of Semiconductor Surfaces

Semiconductor surfaces can be modified by electrochemical deposition of metals. This is of great technological importance because metal-semiconductor contacts such as ohmic contacts and Mott-Schottky diodes are required for many semiconductor devices. For instance, an ideal Mott—Schottky junction was formed by electrochemical deposition of Cu on n-GaAs as discussed in Section 2.2. The electrochemical method is very attractive because the adjustment of the electrode potential offers a unique tool of controlling and structure of the interface. 

During the last decade, various studies were performed in order to understand the initial deposition steps. Most investigations were performed by Kolb and his group [57-60]. These authors studied mainly the metal deposition on the well-defined silicon surfaces with their Si-H bonds. The most interesting results were obtained for the deposition of Pb. This metal could be deposited at electrode po- 

The deposition of various other metals on n-Si(lll) surfaces was also studied. In all cases epitaxial growth of the metal was found [57, 60]. However, the processes were not reversible because the corresponding standard potentials occurred much below the conduction band. Accordingly, the metal deposition was possible but not the reoxidation. In addition, the analysis of the first current transient of Cu or Co deposition was disappointing insofar as the nucleation-growth process did not follow the models discussed earher [57, 60]. 

---

Semiconductor/Liquid Junction Photoelectrochemical Solar Cells 9.4 Chemical modification of semiconductor surfaces... 

Chemically pure semiconducor materials can absorb only those photons, the energy hv of which exceeds the band gap E . Therefore, E. value determines the "red boundary of the light that is used in photocatalytic action of these materials. By way of example. Table 1 presents the values of Eg and the corresponding values of boundary wave length Xg= hc/E (where c is the velocity of light) for some semiconductor and dielectric oxides [2]. However, a semiconductor PC can be sensitized to light with X> by chemical modifications of its surface layer or adsorption of certain molecules on its surface, provided that such treatment creates additional full or empty electron levels in the band gap of the semiconductor material. 

Kohei Uosaki received his B.Eng. and M.Eng. degrees from Osaka University and his Ph.D. in Physical Chemistry from flinders University of South Australia. He vas a Research Chemist at Mitsubishi Petrochemical Co. Ltd. from 1971 to 1978 and a Research Officer at Inorganic Chemistry Laboratory, Oxford University, U.K. bet veen 1978 and 1980 before joining Hokkaido University in 1980 as Assistant Professor in the Department of Chemistry. He vas promoted to Associate Professor in 1981 and Professor in 1990. He is also a Principal Investigator of International Center for Materials Nanoarchitectonics (MANA) Satellite, National Institute for Materials Science (NIMS) since 2008. His scientific interests include photoelectrochemistry of semiconductor electrodes, surface electrochemistry of single crystalline metal electrodes, electrocatalysis, modification of solid surfaces by molecular layers, and non-linear optical spectroscopy at interfaces. 

On the other hand, modification of substrate surfaces, especially semiconductor surfaces, has been an intensively... 

Fig. 59. Molecular modification of semiconductor silicon surfaces. Removal of the oxide generates a hydrogen-terminated layer that reacts with a range of molecular functional groups including alkenes. 

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

The second topic of this chapter is the role of coordination compounds in advancing electrochemical objectives, particularly in the sphere of chemically modified electrodes. This involves the modification of the surface of a metallic or semiconductor electrode, sometimes by chemical reaction with surface groups and sometimes by adsorption. The attached substrate may be able to ligate, or it may be able to accept by exchange some electroactive species. Possibly some poetic licence will be allowed in defining such species since many interesting data have been obtained with ferrocene derivatives thus these organometallic compounds will be considered coordination compounds for the purpose of this chapter. 

Hotchandani, S. Kamat, P. V. Modification of electrode surface with semiconductor colloids and its sensitization with chlorophyll a, Chem. Phys. Lett. 1992, 191, 320. 

Very detailed studies also have appeared on catalytic modification of semiconductor electrode surfaces to improve the HER performance the reader is referred to the many review articles and book chapters on this topic.22,29,88,135,136,1363... 

It has been demonstrated in earlier sections that the catalytic activity of nickel oxide in the room-temperature oxidation of carbon monoxide is related to the number and the nature of the lattice defects on the surface of the catalyst and that any modification of the surface structure influences the activity of the solid. Changes of catalytic activity resulting from the incorporation of altervalent ions in the lattice of nickel oxide may, therefore, be associated not only with the electronic structure of the semiconductor (principle of controlled valency ) (78) but perhaps also with the presence of impurities in the oxide surface or a modification of the surface structure because of this incorporation. In order to determine the influence of dopants on the lattice defects in the surface of the solid and on its catalytic activity, doped nickel oxides were prepared under vacuum at a low temperature (250°). Bulk doping is not achieved and, thence, one of the basic assumptions of the electronic theory of catalysis (79) is not fulfilled. 

Phosgene has been employed in the modification of the surfaces of cellulose-acetate membranes used for water desalination and waste water treatment [1450]. Similarly, phosgene has been used to surface-modify porous diaphragms for electrolytic cells [324]. Aluminium and aluminium-based alloys can be etched at a high rate when COClj is used in a mixed gas plasma [1004], as can semiconductors (see Section 9.12). 

Professor in 1981 and Professor in 1990. He is also a Principal Investigator of International Center for Materials Nanoarchitectonics (MANA) Satellite, National Institute for Materials Science (NIMS) since 2008. His scientific interests include photoelectrochemistry of semiconductor electrodes, surface electrochemistry of single crystalline metal electrodes, electrocatalysis, modification of solid surfaces by molecular layers, and non-linear optical spectroscopy at interfaces. 

Modification of the surface of semiconductor electrodes by catalysts (mediators) attached to it also deserves intensive investigation. 

Develop new electrode materials and the modification of electrode surfaces by investigation of conductive polymers, organometallic conductors and semiconductors, and the phenomena of absorption and covalent attachment... 

It is reasonable to expect that, if methods for quantitative measurement of the transport of ions in surface phases of semiconductors are developed, the way will open to the exploration of chemical and physical modification of these surface phases. The goal is to make these less conductive solid electrolytes—i.e., surface phases in which ion transport is reduced. Such modification is likely to reduce the cost of encapsulation and packaging and increase the reliability of microcircuits. 

---