A detailed study is presented of the changes in photophysical and photovoltaic properties of thin films of perylene bis(phenethylimide), PPEI, as the film structure evolved from amorphous to polycrystalline under the influence of methylene chloride vapor. A strong absorption band grew in on the low-energy side of the original spectrum as the film order increased, and the peak of the emission spectrum shifted to higher energy, decreasing the Stokes shift by 283 meV. The singlet exciton diffusion length increased by 76%, leading to enhanced quenching of the PPEI fluorescence by a surface film of titanyl phthalocyanine, TiOPc. The heterojunction barrier height in TiOPc/PPEI photovoltaic cells decreased by 88 mV after 12 min exposure to methylene chloride vapor and subsequently remained constant. The series resistance of the cells increased monotonically as the films crystallized. Photocurrents increased by a factor of 10 after brief exposure and decreased thereafter. The active depth from which photocurrent ...

Evolution of Photophysical and Photovoltaic Properties of Perylene Bis(phenethylimide) Films upon Solvent Vapor Annealing

Two‐dimensional, self‐consistent solutions of the Schrodinger and Poisson equations are used to find the electron states in GaAs/AlGaAs quantum well wires. Both deep and shallow mesa structures are simulated. Our results show that while these structures are capable of providing the single occupied subband and wide energy separations needed for a true quantum wire, the process tolerances allowed are very small, on the order of 200 A of width variation. Cutoff widths calculated are 1000 A for the shallow mesa and 2100 A for the deep mesa. The agreement with experimental results is good for the shallow mesa, but poor for the deep mesa. This suggests additional process‐induced sidewall depletion mechanisms contributing to the cutoff of the deep mesa structures.

Electron states in mesa‐etched one‐dimensional quantum well wires

Near‐surface (∼1000 A) modification in the net carrier concentration in n‐type InP (n=6×1015–1.5×1017 cm−3) was observed after reactive ion etching (RIE) in Cl‐based (CCl2F2/O2) or organic‐based (C2H6/H2) discharges. The carrier loss is slightly more pronounced in the latter case, due possibly to the creation of deep level, compensating acceptors at greater depths as a result of implantation of the light hydrogen ions. Near‐complete recovery of the initial carrier density occurs after annealing at 500 °C for 30 s. Structural disorder is detected by ion channeling to depths of ∼400 A after C2H6/H2 RIE with a self‐bias of 380 V. This disorder shows significant recovery after 400 °C, 30 s annealing. Current‐voltage measurements on Au Schottky diodes showed ohmic behavior after etching of the InP in a C2H6/H2 discharge, due to the nonstoichiometric surface remaining after RIE. Diodes fabricated on CCl2F2/O2 etched material show only a slight increase in reverse current compared to unetched control samples.

Electrical and structural changes in the near surface of reactively ion etched InP

We experimentally demonstrate single-peak narrow-bandwidth thermal emission with a quality factor (Q factor) of more than 100 at a wavelength of 9.1 μm. The emission is significantly suppressed at all other wavelengths. Our emitter is based on an intersubband transition in a multiple quantum well structure combined with a single high-Q resonant mode in a two-dimensional photonic crystal slab, which allows strong light-matter interaction only at a specific wavelength. Strong thermal emission is exhibited only in a limited angular range (∼20°) from the normal direction. Our results have potential applications in bio- and environmental sensors.

Single-peak narrow-bandwidth mid-infrared thermal emitters based on quantum wells and photonic crystals

We present a novel configuration for the implementation of subwavelength-based graded-index devices. The proposed concept is based on the etching of one-dimensional subwavelength gratings into a high-index slab waveguide to achieve the desired effective index distribution. A graded-index profile can be achieved by gradually modifying the duty ratio of the grating along the horizontal axis, while the beam is confined in the vertical direction by the slab waveguide. On the basis of this concept, novel graded-index lenses and waveguides are both proposed and characterized numerically by use of finite-difference time-domain and finite-element analysis. The proposed devices can be used for guiding, imaging, optical signal processing, mode matching, coupling, and other applications while offering the intrinsic advantages of on-chip integration such as miniaturization, eliminating the need to align each component separately, and compatibility with standard microfabrication techniques for manufacturability.

Implementation of a graded-index medium by use of subwavelength structures with graded fill factor

A dry etching technique which is capable of producing lowdamage, high-aspect-ratio structures on a nanometric scale in GaAs is described. The reactive ion etching employs a mixture of methane and hydrogen; details of the optimum conditions are given.

Reactive ion etching of GaAs using a mixture of methane and hydrogen

We show that while the methane/hydrogen gas mixture is capable of etching GaAs and AlGaAs controllably and with little residual damange, it leads to the passivation of donors in both semiconductors. The passivation can, however, be annealed out with a short thermal cycle to 300 °C. The effects of passivation are illustrated and characterized by processing uniform epilayers and quantum wires in n+‐GaAs and modulation doped AlGaAs/GaAs heterostructures.

Passivation of donors in electron beam lithographically defined nanostructures after methane/hydrogen reactive ion etching

Conducting wires of widths down to 0.1 μm have been fabricated in modulation‐doped GaAs/AlGaAs material using SiCl4 reactive ion etching with a negative resist mask. The resist used was high‐resolution negative (HNR) and its applicability to microstructure fabrication is discussed. A novel dry etch for GaAs, CH4/H2 reactive ion etching, produces low damage and shows much promise for wire fabrication.

Fabrication of quantum wires in GaAs/AlGaAs heterolayers

We have used Raman scattering to investigate the extent and nature of the damage caused by dry etching of GaAs samples. Damage has been seen to be confined to the first few hundred angstroms and has a correlation length larger than 250 A. A sample on which ‘quantum dots’ had been etched has also been studied and a new feature observed in the spectrum. This is discussed in terms of a surface phonon mode.

C. D. W. Wilkinson

Papers

Reactive ion etching of GaAs using a mixture of methane and hydrogen

Passivation of donors in electron beam lithographically defined nanostructures after methane/hydrogen reactive ion etching

Fabrication of quantum wires in GaAs/AlGaAs heterolayers

Raman Scattering of Reactive-ion Etched GaAs

Raman scattering investigations of the damage caused by reactive ion etching of GaAs