Measurement of the activation barrier for carrier transport at n-gallium arsenide semiconductor/liquid junctions

Abstract

5793same order in the ( E 2 -E , ) values, by which the dication state of l a is shown to be most stable among them.X-ray diffraction analysis of l b " indicates that the molecule belongs to the approximate DZd symmetry (excluding eight carbomethoxy groups with flexible conformations) and has the puckered structure of the central four-membered ring. Figure 1 shows a unique stacking structure of molecules of l b related by the crystallographic 2, symmetry along the c axis resulting in a one-dimensional column in the crystal lattice. Some van der Waals contacts are found between the sulfur atoms of interdonor molecules, the shortest one being 3.608 A (twice the van der Waals radius of sulfur is 3.70 A).These results suggest that the [4]radialenes, in particular l a , should be useful electron-donor candidates for organic metals. In fact, l a forms a 1:2 charge-transfer salt (black needle crystals, mp > 300 "C) with TCNQ, and the room temperature electrical conductivity on the compressed pellet is fairly high (0.032 R-I cm-I). Furthermore, the E, value (0.057 eV) is very small. The preparation and electrical characterization of la-TCNQ single crystals are currently under active investigation.Supplementary Material Aiailable: ORTEP drawing and full tables of fractional atomic coordinates and interatomic bond distances and angles in l b ( 5 pages). Ordering information is given on any current masthead page.(1 1) Crystal data of lb: C,,H 4016S8, FW = 921.1, orthorhombic, space group P2,2,2,, a = 17.9317 (1) 2, b = 25.757 (2) A, c = 8.539 (1) A, U = 3808.7 (4) A', D, = 1.606 g cm-,, Z = 4. X-Ray diffraction data were measured on a Rigaku four-circle diffractometer using nickel-filtered Cu Ka radiation. A total of 3217 reflections were collected up to 2% = 120° by the 8-28 scan technique. The structure was solved by the direct method (MUL-TAN-76) and refined by the full-matrix least squares (xRAY-76) by using the 3073 observed reflections [[POI 2 3u(Fo)] to the R index of 0.052 including non-hydrogen atoms with anisotropic thermal parameters

Theoretical and Experimental Upper Bounds on Interfacial Charge-Transfer Rate Constants between Semiconducting Solids and Outer-Sphere Redox Couples

“…Previous studies have established that V oc for the n-GaAs/CH 3 CN-Fc +/0 system under 1 sun illumination at 300 K is not controlled by direct majority carrier transfer to the Fc + acceptor species . Thus, the inequality of eq 16 applies to this system.…”

“…Previous studies have established that V oc for the n-GaAs/ CH 3 CN-Fc +/0 system under 1 sun illumination at 300 K is not controlled by direct majority carrier transfer to the Fc + acceptor species. 54 Thus, the inequality of eq 16 applies to this system. Using the values of V bi ) 1.053 V, V oc ) 0.677 V, J ph ) 6.9 mA cm -2 , and N d ) 1.4 × 10 17 cm -3 measured in this work and the relationship between k et and the photovoltage given by eq 16 yields k et < 10 -14 cm 4 s -1 .…”

“…The n-type gallium arsenide wafers ((100) orientation, selenium-doped; Laser Diode Co.) had dopant densities of (0.9−1.9) × 10 17 cm -3 . Ohmic contacts were made by evaporation of 1000 Å of an 88:12 alloy of Au:Ge, followed by annealing at 480 °C for 5 min under forming gas . The GaAs electrodes were etched in a solution of 0.05% (v/v) Br 2 in CH 3 OH followed by immersion in 1.0 M KOH(aq).…”

Progress in Understanding Electron-Transfer Reactions at Semiconductor/Liquid Interfaces

“…Laser-induced surface damage has also been identified as a confounding variable in the experimental protocol used in the original rate constant determinations on the p-InP/Fe(CN) 6 3-/4- junction . Similarly, studies of GaAs/CH 3 CN contacts by a number of workers have revealed that the energetics and kinetics of these junctions are dominated by surface recombination processes, so that the direct transfer of carriers in the semiconductor to redox species in the solution phase is generally not rate-limiting. ,− These systems thus serve to underscore the rapid charge carrier trapping that can be produced by pathways involving adsorbed ions and surface defects. Unfortunately, it does not appear that any photoluminescence-based transient measurements performed to date have determined directly the dynamics of charge transfer to outersphere redox reagents at a semiconductor/liquid contact.…”

“…Otherwise, surface state charging would result in some of the applied potential being dropped across the Helmholtz layer and would complicate analysis of the dependence of k et on Δ G °‘ and also would complicate extraction of k et from the steady-state J − E data. The presence of a high density of surface states on semiconductors like Si, InP, and GaAs had previously been postulated to be inherent to such surfaces and to lead to extreme nonidealities, resulting in a situation denoted as Fermi level pinning. ,− ,,− However, Fajardo showed that control over etching and handling of the n-Si surfaces was able to minimize these nonidealities. In fact, the band edge position of Si/CH 3 OH contacts was constant, within experimental error, for the entire series of n-Si/CH 3 OH−viologen 2+/+ contacts .…”