Interfacial oxidation

It has been reported that a thin interfacial oxide layer such as Si02, SiOvN, or Ti02 can improve device performance [39 11], Although the exact mechanism of such thin buffer layers is not clear, the enhanced performance may arise from the improved smoothness of the surface of ITO, which leads to more homogeneous adhesion of the HTL. In addition, the optimized thickness of the buffer layer also helps balance the device charges due to reduced hole injection. 

A hydrophilic surface condition has been related to the presence of a high density of silanol groups (Si-OH) or to a thin interfacial oxide film. Such an oxide can be produced chemically by hot HN03 or by solutions containing H202. The three most common cleaning solutions for silicon are based on the latter compound ... 

It was demonstrated that reproducible gas-sensitive silicon Schottky sensors could be produced after terminating the silicon surface with an oxide layer [71, 72]. This interfacial oxide layer permits the device to function as a sensor, but also as a diode, as the charge carriers can tunnel through the insulating layer. The layer made the Schottky diode behave like a tunneling diode, and the ideality factor could be voltage-dependent [73]. 

Weidemann et al. found that wet etching of a GaN surface before Pd deposition also produced an interfacial oxide, which increased the hydrogen sensitivity by approximately a factor of 50 [14], They concluded that comparing device parameters between different GaN Schottky diode gas sensors requires a defined standard treatment of the GaN surface to introduce a controlled interfacial oxide. 

In contrast to the interfacial oxidative condensation polymerization, where discharge of anions occurs at the growing end through the conductive polymer, a new cathodic process of formation of unstable monomers has been developed, followed by polymerization. 

A second approach to semiconductor stabilization is the utilization of an electrolyte in which semiconductor photodecomposition products cannot form. Thus, in the case of n-Si, Lewis has noted that employment of a rigorously anhydrous nonaqueous electrolyte eliminates the possibility of interfacial oxide formation [18]. However, the fact that subnanomolar concentrations of water are sufficient to generate surface oxides makes the application quite difficult. Semiconductors that undergo decomposition to metal ions can likewise be stabilized by using a low-dielectric-constant nonligating electrolyte. Both organic liquids and solid-state ion conductors have been employed for this purpose. Unfortunately, such electrolytes are at best high resistance, and thus observable photocurrents are minimal. However, a hybrid approach in which a nonaqueous... 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

In summary, Group III V semiconductors have several positive features that make them attractive for water photosplitting applications. The combination of high carrier mobility and an optimal band gap (particularly for many of the alloys, see below) coupled with reasonable photoelectrochemical stability for the p type materi al under HER conditions, should inspire continuing scrutiny of Group III V semi conductors. The control of surface chemistry is also particularly crucial to avoid problems with surface recombination. For example, the studies on p InP photoca thode surfaces have shown that a (controlled) ultra-thin interfacial oxide layer is critical for minimizing carrier recombination at the surface.66,199,201,554... 

Due to the technological importance of metal-insulator-semiconductor (MIS) devices, understanding of the nature of their electrical characteristics such as current-voltage (1-V) and tunnel magnetoresistance (TMR) is of great interest. Unless intentionally fabricated, a silicon Schottky diode possesses a thin interfacial oxide layer between the metal and the semiconductor. Additionally, a density of interface states is always generated at the boundary between the semiconductor and insulator. 

D. R. Lilliongton and W. G. Townsend, Effects of interfacial oxide layers on the performance of sihcon Schottky-barrier solar cells, Appl. Phys. Lett. 28(2), 97, 1976. 

J.-N. Chazalviel, Ionic processes through the interfacial oxide in the anodic dissolution of silicon, Electrochim. Acta37(5), 865, 1992. 

H. J. Lewerenz, J. Stumper, C. Pettenkofer, and R. Greef, Photoelectrochemically synthesised interfacial oxides on silicon Composition and electronic properties, Electrochim. Acta 34(12), 1729, 1989. 

Cation was found to be the key parameter influencing both the nanotube growth rate and length [49], With increasing cation size, the interfacial oxide layer was getting thinner. Ionic transport was facilitated and the nanotube growth was enhanced. The thinnest nanotubes ever reported was 5 nm, which was obtained in an electrolyte containing 0.5 M tetrabutylammonium fluoride in formamide with 5% water (Fig. 8). 

Si to Si02, this number of equivalents corresponds to the oxidation of approximately 5x10 atoms of Si per cm, i.e. an interfacial oxide layer with a thickness of roughly 5000 monolayers, or 15 /an. 

These results for contacts, which do not have an interfacial oxide layer, but do show switching properties, indicate that the oxide layer may assist the switching process, but can not be the main reason for the phenomenon. This is supported by other reports on switching in configurations without oxide interlayers [12, 13]. Remarkably, these reports also relate to structures with very small cross sections, i.e. an STM-tip and nanowires. Therefore, the oxide layer seems to be essential to achieve stable switching in devices with relatively large-area contacts. 

D. C. Card and H. C. Card, Interfacial oxide layer mechanisms in the generation of electricity and hydrogen by solar photoelectrochemical cells, Solar Energy 28 (1982) 451-460. 

In the experiments on nanoscale Pt islands on Si (Section 2.5), the situation is even more complex because, besides the tunnel gap between Pt and the tip, an interfacial oxide film also exists between Si and Pt At this interface, the photocurrent-voltage characteristics indicate the presence of Si surface states details are given together with the experimental data in the appropriate section 2.5.3.2. 

Figure 2.100 Scanning tunneling spectroscopy data of samples as in Figure 2.99 recorded with the tip on top of a Pt island (top) and on top of the interfacial oxide film (bottom).

In most metal-semiconductor contacts, the semiconductor surface before metal deposition is prepared by chemical cleaning and a thin insulating oxide layer is invariably left on the surface of the semiconductor. The thickness of this interfacial layer depends on the method of surface preparation and, for a good Schottky contact, must be less than about 20A. The energy-band diagram of a contact with an interfacial oxide layer is... 

---