65 Figure 3.13 SXPS data for the Ga 3d region of chloride and phosphine functionalized surfaces after 12 hours of exposure to ambient air. 85 Figure 4.6 Detailed area XPS data for the As and Ga 3d regions of Cl-terminated GaAs.
An Introduction to Gallium Arsenide Surface Chemistry
Semiconductor Surface Chemistry and Charge Carrier Dynamics
The forbidden gap energy between these bands is known as the band gap, EG. Illumination of the semiconductor with light of energy greater than EG leads to the excitation of valence electrons in the conduction band, leaving behind holes in the valence band.
Surface Passivation Techniques: Silicon vs. Gallium Arsenide
These bonds can be formed directly from the H-terminated surfaces10,11 or through a two-step halogenation/alkylation procedure.12-14 These surfaces are resistant to chemical oxidation and exhibit long lifetimes, even after prolonged exposure to ambient air.15 In addition, this chemistry can be used to introduce a diverse set of functional groups, 16-19, enabling a wide range of potential applications. The nature of dangling bonds on these surfaces can be predicted from simple electron counting arguments.
Chemical Passivation of GaAs Surfaces
This process leaves a negative charge in the solution at the interface, and an equally positively charged depletion region of width W within the semiconductor. In the presence of surface carrier traps, electron transfer can occur from both bulk dopants and the surface trap states.
New Strategies for GaAs Passivation
The (111)A plane at the top of the image has Ga dangling bonds perpendicular to the surface plane, while (111)B plane at the bottom of the image has As dangling bonds perpendicular to the surface plane. Together, these data suggest that the surfaces are well ordered over large areas and end with normal Ga-Cl bonds at the surface.
High-Resolution Photoelectron Spectroscopy of Chlorine-Terminated GaAs(111)A Surfaces
- Introduction—Analytical Chemistry of the GaAs Surface
- Experimental
- Materials and Methods
- Instrumentation .1. XPS Measurements
- Results and Discussion
- Conclusions
- Acknowledgements
The magnitude of the observed change necessarily depends on the energy of the chemical state at the surface relative to the bulk. The lineshapes of the native As oxide peaks were ∼90% Gaussian, while the As lineshapes for the Cl (111)A-terminated surface were >99% Gaussian.
Phosphine Functionalization of GaAs(111)A
Introduction
Both standard Al Kα and soft synchrotron X-ray sources were used to provide a complete picture of surface phosphorus and oxidation states of Ga and As surface sites, as described in the previous chapter. In addition to these chemical data, steady-state photoluminescence (PL) intensity measurements were used to est. These experiments do not provide values for the surface recombination rate (S) or the surface trap density (NTS), but provide qualitative information on the electronic effects of the surface chemistry.
A quantitative value for the surface recombination rate (S) in the undoped sample can be obtained from the decay in excess carriers by Eq:9. In particular, Cl-termination of the GaAs(111)A face was reported to result in a factor of 2 enhancement of the PL intensity relative to the native oxide,3 while passivation of the (100) surface with N2H4 followed by annealing gave an increase of a factor of 8.8 These enhancements did not. Recombination through surface traps competes with bulk recombination and reduces the intensity of emission photons with energy, e.g.
Experimental Procedures .1 Materials and Methods
- Instrumentation .1 XPS Measurements
Phosphine-functionalized samples at Caltech were introduced directly from the inert atmosphere glovebox to the front chamber of the XPS without any. Samples for SXPS measurements were functionalized in a glove box, then transported in vials sealed under inert atmosphere to beamline U4A and loaded into the SXPS antechamber, leaving them exposed to ambient air for several minutes after functionalization. Samples were mounted vertically and front-side illuminated with the 442 nm line of a continuous wavelength HeCd laser, operating at 30 mW.
Results
- Reactions of PEt 3 and PCl 3 on Cl-Terminated GaAs(111)A
- Reactions of PCl 3 on Native Oxide-Terminated GaAs(111)A
- Oxidation of Phosphine-Functionalized GaAs(111)A in Ambient Atmosphere To help understand the stability of these passivation chemistries, SXPS spectra
- Steady-State Photoluminescence of Functionalized GaAs(111)A
A small amount of As2O3 was observed in the spectrum of the PEt3 treated surfaces (figure 3.4b). Peak widths are therefore limited to ±10% of the value observed for the bulk Ga(As) 3d5/2 peak on the Cl-terminated surface. The developed Ga 3d spectra of the PEt3 functionalized surface were well fitted by 4 Voigt function peaks (figure 3.5).
GaAs(111)A, fits into a single spin-orbit pair; Uncorrected SXPS As 3d b-spectrum, showing a small amount of As2O3 contamination between 44 and 45 eV. High-resolution SXPS spectra of the As 3d region of these surfaces confirmed that no As or As0 oxides were present on the surface in a detectable concentration and the observed signal was suitable for singlet pairs (figure 3.7). The Ga 3d3/2 peaks of the Ga 3d spectra of these samples were deconvoluted using the procedure described above and the spectrum fit to 4 peaks (figure 3.8).
Discussion
Despite this greater oxidation, these Cl-terminated samples still show stronger PL than the freshly prepared PCl3/oxide samples.
Conclusions
Acknowledgements
Chemical Functionalization and Passivation of Gallium Arsenide Nanocrystals
Introduction—Surface Passivation and Semiconductor Nanocrystals
Several procedures have been reported for the synthesis of GaAs nanocrystals,6,7 many of which involve the reaction of an As precursor with GaCl3. The reaction is carried out in a coordinating solvent that covers the nanocrystals and controls their growth.8,9 But unlike II-VI nanocrystals, where both organic and inorganic surface passivation techniques significantly reduce the density of interfacial states and yield intense band gap photoluminescence (PL) no such method has been reported for GaAs so far. In this chapter, bulk surface passivation chemistry is applied to GaAs nanocrystals to obtain greatly enhanced bandgap PL emission.
Furthermore, among all low-index GaAs surfaces, the (111)A plane is known to have the slowest etch rate in contact with oxidative etching.13 Thus, it seemed reasonable to treat GaAs nanocrystals with 6 M HCl as synthesized, with oxide capped GaAs nanocrystals14(aq) would cleanly remove the oxide layer and etch the nanocrystals anisotropically, mainly producing surfaces terminated by Ga-Cl bonds. This two-step functionalization procedure is not limited to bulky groups chosen to limit the growth of the nanocrystals, and allows a significant degree of control over the chemistry of the resulting capped GaAs. Importantly, such functionalized GaAs nanocrystals exhibit an intense bandgap PL, indicating that the electrical trap density on such surfaces is significantly reduced and presumably enabling the use of GaAs nanocrystals for spectroscopic research and electronic device applications similar to those hitherto have been developed. for core-shelled II-IV nanoparticles.1,15,16.
Experimental
- Materials and Methods
- Instrumentation
The supernatant solution was removed, ∼300 mL of CH3OH was added, and the solution was allowed to settle for another 24 h. To end the surfaces with Cl, the synthesized GaAs nanoparticles were sonicated in a 6 M HCl solution (aq) for 40 min and the suspension was then centrifuged. The etching solution was removed with a pipette and the collected particles were rinsed with fresh etching solution.
The particles were sonicated for another 5 min in fresh etching solution and then centrifuged again. Once dispersed in acetone, the solids could not all be collected by centrifugation, and the acetone washes retained a reddish-brown color, suggesting significantly higher solubility for the functionalized particles. The particles were then loaded into quartz cuvettes or deposited as films on Si substrates.
Results and Discussion
- Photoluminescence of Functionalized GaAs Nanocrystals
As expected, Cl signals were observed in the examinations of HCl(aq)-etched GaAs nanocrystals. XPS data for the As 3d region of GaAs particles etched with HCl(aq) showed complete removal of As oxides (Figure 4.6). The Ga 3d region showed removal of most of the surface oxides as a result of the 6 M HCl(aq) etching process.
Thus, any Ga-Cl surface species are expected to appear as part of the bulk peak at 19.3 eV. If the particles were deposited directly from the original etching solution onto the Si substrate, another strong peak, centered ~21.1 eV, was observed in the Ga 3d region of the XPS data. During the annealing process, black and pale yellow solids collected on the cold finger of the sublimator.
Conclusions
For particles dispersed on Si substrates, all samples examined exhibited broad light scattering peaks centered between 650 and 700 nm. For the as-etched and N2H4- or NaSH-treated nanocrystals, a weak PL peak centered at 868 nm was observed. Nanocrystals that were coated and annealed exhibited a peak at this wavelength that was more than 40 times more intense than the other samples, suggesting significant.
For particles dispersed in CH3OH, photoluminescence was observed only from particles coated with N2H4 and annealed. The fact that the N2H4-treated and annealed particles still showed significant PL under these conditions makes them a particularly promising candidate for photoelectrochemical applications. The band edge PL of these particles is strongly enhanced after this annealing step, confirming both that elemental As is an important electronic trapping state for GaAs nanocrystals and that this functionalization chemistry effectively reduces the density of surface carrier traps.
Relationships Between Nonadiabatic Bridged Intramolecular, Electrochemical, and Electrical
Introduction
This approach should be useful to experimentalists interested in formulating expectations for the current density through molecular wires, given the measured rate constants and electronic coupling values for analogous binding species in intramolecular electron transfer processes.
Theoretical Approach .1 General Rate Expressions
When the energy distance between the donor and/or acceptor electronic states is small, the total electron transfer probability, WDA, becomes the summation of the rates between all the specific states that contribute to electron transfer. This is similar to the generalization of the rate constant for electron transfer when a number of vibrational modes are relevant, except that a Boltzmann weighting of the states is not assumed and different states may have a different element of the electron coupling matrix .15. 34;p( )Ep are the densities of states (ie, states per unit energy) of the reactants and products in energy.
The probability g(EP) that the product state is unoccupied was introduced because the integral is over all product levels, but only unoccupied product states contribute to the electron transfer process. The experimentally observed rate is the sum of forward (from D to A) and reverse (from A to D) velocities. In any system of interest, the estimation of the net rate of electron transfer therefore involves integrations of all the occupied states of the reactants and the unoccupied electronic states of the product participating in the electron transfer process.
- Intramolecular Donor-Bridge-Acceptor Electron Transfer
- Metal Electrode-Bridge-Molecular Acceptor Electron Transfer
- Metal Electrode-Bridge-Metal Electrode Electron Transfer
- Electron Transfer Between a STM Tip and a Molecularly Coated Metal Electrode
- Rate Relationships Derived from the Above Expressions
- Application to Experimental Systems of Interest 1 Tunneling Through Alkane Linkers
- Tunneling Through Oligonucleotides
- Tunneling Through Conjugated Molecular Wires
- Conclusions
- Acknowledgements
The effective density of states of the metal is evaluated on the left side of the bridge, while the effective density of states on the top is evaluated on the right side of the bridge. The initial states of the electron are the donor states in the continuum of the metal electrode, and Frank-. The velocity is a function of the potential, E, of the electrode with respect to a reference potential, and is given by:16-19.
The integral I(λMBA, E) represents the overlap between the Fermi distribution function and the Gaussian function containing the free-energy dependence of the Franck-Condon weighted density of vibrational states for the electron transfer. Extrapolation of the rate constant measured at these potentials to the value at equilibrium yields an expression for the rate constant at zero driving force. In the model used, none of the observed quench rates were taken to be limited by electronic coupling through the bridge.
