Surface Chemistry of Prototypical Bulk II-VI and III-V Semiconductors and Implications for Chemical Sensing.

The use of thiol-terminated ligands has recently been reported to enhance 10-fold the power conversion efficiency (PCE) of colloidal quantum dot (CQD) photovoltaic (PV) devices. We find herein that, in a representative amine-capped PbS colloidal quantum dot materials system, improved mobility following thiol treatment accounts for only a 1.4-fold increase in PCE. We then proceed to investigate the origins of the remainder of the quadrupling in PCE following thiol treatment. We find through measurements of photoluminescence quantum efficiency that exposure to thiols dramatically enhances photoluminescence in colloidal quantum dot films. The same molecules increase open-circuit voltage (Voc) from 0.28 to 0.43 V. Combined, these findings suggest that mid-gap states, which serve as recombination centers (lowering external quantum efficiency (EQE)) and metal−semiconductor junction interface states (lowering Voc), are substantially passivated using thiols. Through exposure to thiols, we improve EQE from 5 to 22...

Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency

Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 ± 1° from the surface normal with a 43 ± 5° twist, a highly oleophobic and hydrophobic ambient surface, and direct S−GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to ∼100 °C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au{111} surfaces. A two-step solution assembly process is observed:  rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densif...

Molecular self-assembly at bare semiconductor surfaces: preparation and characterization of highly organized octadecanethiolate monolayers on GaAs(001).

High-resolution X-ray photoelectron spectroscopy (HRXPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and Raman scattering have been used to characterize the bonding and electronic properties of the interfaces formed by ambient-temperature, solution self-assembly of octadecanethiol and dodecanethiol under oxygen-free conditions on GaAs(001) surfaces. The combination of HRXPS and ToF-SIMS shows that both monolayers form direct S−[GaAs] attachment to the bare substrate with dominant As−S bonding and some fraction of Ga−S bonding, but the densely packed octadecanethiolate self-assembled monolayers (SAMs) are significantly more effective in protecting the interface from oxide regrowth compared to the less dense dodecanethiolate SAMs. Raman scattering measurements of the GaAs LO phonon modes indicate that the formation of these direct, oxide-free S−[GaAs] interfaces does not induce any significant changes in the GaAs surface electronic states, whereas control experiments with inorganic sulfide t...

Molecular Self-Assembly at Bare Semiconductor Surfaces: Investigation of the Chemical and Electronic Properties of the Alkanethiolate−GaAs(001) Interface

A series of p- and n-GaAs−S−CnH2n+1 || Hg junctions are prepared, and the electronic transport through them is measured. From current−voltage measurements, we find that, for n-GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p-GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5−1.6me for the electronic carriers that cross the junctions. We find that transport is well-described by lowest unoccupied and highest occupied states at 1.3−1.4 eV above and 2.0−2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the va...

Energy level and band alignment for GaAs-alkylthiol monolayer-Hg junctions from electrical transport and photoemission experiments.

A low‐temperature (∼250 °C) high‐pressure oxidation technique is used for the thermal oxidation of gallium arsenide in an ambient of oxygen and water vapor. It is shown that a uniform and chemically stable oxide with high band‐gap energy can be grown on GaAs by this process. The role of water vapor and oxygen is studied in detail to obtain information on the oxidation mechanism. The electrical characteristics and the composition of this oxide are presented to demonstrate its suitability for surface passivation and metal‐oxide–semiconductor devices.

High‐pressure thermal oxidation of n‐GaAs in an atmosphere of oxygen and water vapor

Effects of octa decyl thiol (ODT) treatment on the gallium arsenide surface and interface state density

Silicon nitride is deposited by the direct plasma enhanced chemical vapour deposition (PECVD) technique at a low power on an n-type GaAs sample treated with (NH/sub 4/)/sub 2/S/sub x/. Metal-insulator-semiconductor (MIS) structures are fabricated on this sample and annealed at 450/spl deg/C in nitrogen for 5 min. The 1 MHz capacitance-voltage (C-V) characteristics of these devices demonstrate accumulation, depletion and inversion regions. The interface state density (D/sub it/) estimated from the 1 MHz C-V curve using the high frequency method has shown that the minimum D/sub it/ achieved is /spl sim/4/spl times/10/sup 11/ cm/sup -2/ eV/sup -1/.

Silicon nitride/(NH4)2Sx passivation of n-GaAs to unpin the Fermi level

Optimization of PECVD silicon oxynitride for silicon MIS devices with low interface state density

It is shown that the correct-equivalent-box representation of implanted doping profiles, derived in terms of the process parameters by matching the charge-voltage relationships of the actual profile and the box profile, can be used for accurately simulating the electric characteristics of surface-channel MOSFETs and MESFETs having implanted channels. Because the correct box representation is known directly in terms of the doping profile parameters, which can be obtained from process simulation, the need for experimental determination of the box is obviated. The application of the model can substantially reduce the number of experimental trial-and-error iterations involved in the development of devices and circuits. The correct box is also applicable to the CCDs whose physics of operation is similar to that of MOSFETs. >

K.N. Bhat

Papers

High‐pressure thermal oxidation of n‐GaAs in an atmosphere of oxygen and water vapor

Effects of octa decyl thiol (ODT) treatment on the gallium arsenide surface and interface state density

Silicon nitride/(NH4)2Sx passivation of n-GaAs to unpin the Fermi level

Optimization of PECVD silicon oxynitride for silicon MIS devices with low interface state density

A process-parameter-based circuit simulation model for ion-implanted MOSFETs and MESFETs