M. A. Pate

Photoprocessed and micropatterned conjugated polymer LEDs

Abstract We present a detailed study of the effect of fabricating light-emitting diodes (LEDs) containing conjugated polymers, using photolithographic processing; a technique more conventionally used in inorganic semiconductor device manufacture. It is shown that for the specific poly (2,5-dialkoxy- p -phenylenevinylene) used here, the photoprocessing procedure chemically modifies the polymer, resulting in an increase in the degree of conjugation and a concomitant reduction in photoluminescence quantum efficiency to 35% compared to an unprocessed film. A similar reduction in electroluminescence quantum efficiency indicates that this is also the dominant effect in photoprocessed LEDs. LED device characteristics show an increase in threshold field for the photoprocessed devices suggestive of the formation of a barrier layer at the polymer/cathode interface, resulting in a further partial reduction in device power efficiency. There are however no catastrophic effects on device performance, showing that standard photoprocessing is a viable approach to fabrication of polymer LED structures. It is anticipated that optimization of the procedures will allow much less degradation in device performance. In addition it is shown that photoprocessing can be readily applied to the fabrication of arrays of micron-sized LEDs, demonstrating advanced applications of this combination of technologies.

Strained and strain-balanced quantum well devices for high-efficiency tandem solar cells

Abstract The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.

Tailoring of internal fields in InGaAs/GaAs multiwell structures grown on (111)B GaAs

We present a study of internal field distributions in strained InGaAs/GaAs multiple quantum wells in p‐i‐n structures grown on (111)B‐oriented GaAs. Room temperature photocurrent spectroscopy shows clear blueshifting of the e1‐hh1 transition as the well fields are reduced by external bias. The relative length of total well to total barrier material is shown to be an important factor in determining the well and barrier fields. We demonstrate a photocurrent contrast ratio of 4.5:1 for only 3 V applied bias across a 25 quantum well In0.13Ga0.87As p‐i‐n diode and discuss the implication of our results to the design of high performance electro‐optic modulators and self electro‐optic effect devices in this material system.

Electroluminescence from a conjugated polymer microcavity structure

We report the observation of electroluminescence and photoluminescence from microcavity structures containing poly(2,5‐dialkoxy‐p‐phenylene vinylene) (PDAOPV) conjugated polymer emitting regions. Strong spectral narrowing from 112 nm full width half‐maximum to 34 nm in electroluminescence and from 128 to 16 nm in photoluminescence and clear angular dependence of the peak emission wavelength are observed. These are characteristic signatures of the modification of spontaneous emission properties in microcavity structures.

Recent progress in polymers for electroluminescence: microcavity devices and electron transport polymers

Abstract In this paper we briefly review the status of polymer electroluminescence and then report recent work on microcavity polymer devices and on electron transport polymers with electron-deficient nitrogen-containing aromatic moieties. Results are presented for electroluminescence and photoluminescence from microcavity structures containing a poly (2,5-dialkoxy-p-phenylene vinylene) conjugated polymer. Strong spectral narrowing from 108 nm full width half maximum to 34 nm in electroluminescence and from 128 nm to 16 nm in photoluminescence, marked enhancement of the peak emission intensity and clear angular dependence of the peak emission wavelength are observed. These are all characteristic signatures of the modification of spontaneous emission properties in microcavity structures and open the way to studies of microcavity physics in polymer based devices. A non-ether poly(phenyl quinoxaline) and poly(2,6-pyridine vinylene-co-2,5-diheptoxy-p-phenylene vinylene) have been investigated as new electron transport polymers and we present results for their operation in both single and bilayer polymer devices. Additional target polymers for electron transport functionality are described.

EFFECT OF STRAIN RELAXATION ON FORWARD BIAS DARK CURRENTS IN GAAS/INGAAS MULTIQUANTUM WELL P-I-N DIODES

The effect of the dislocation line density produced by the relaxation of strain in GaAs/InxGa1−xAs multiquantum wells where x=0.155–0.23 has been studied. There is a strong correlation between the dark line density, observed by cathodoluminescence, before processing of the wafers into photodiode devices, and the subsequent low forward bias (<1.5 V) dark current densities of the devices. A comparison is made of the correlation between the reverse bias current density and dark line density and it is found that, in this range of strain, the forward bias current density varies more. Two growth methods, molecular beam epitaxy and metal organic vapor phase epitaxy, have been used to produce the wafers and no difference between the growth methods has been found in dark line or current density variations with strain.

CHARACTERIZATION OF GAAS/INGAAS QUANTUM WELLS USING PHOTOCURRENT SPECTROSCOPY

We report on characterization studies of high quality metal‐organic vapor phase epitaxy and molecular beam epitaxy grown GaAs/InGaAs quantum wells, set within p‐i‐n diodes, to determine the well widths, indium mole fractions, and conduction band offset. We present photocurrent spectra containing a larger number of transitions than revealed in photoluminescence or photoluminescence excitation experiments. The energies of these transitions have been modeled using a theoretical characterization tool known as ‘‘contouring,’’ which is used in this strained system for the first time. This has enabled determination of the conduction band offset in GaAs/InGaAs quantum wells, to a value between 0.62 and 0.64, for a range of indium fractions between 0.155 and 0.23. As a final, additional check on our results, we compare the field dependence of the e1‐hh1 exciton transition energy with our theoretical calculations and find good agreement.

Impact ionization coefficients in GaInP p–i–n diodes

Impact ionization coefficients have been deduced from photomultiplication measurements performed on Ga0.52In0.48P p–i–n diodes with nominal intrinsic region thicknesses of 1 .0, 0.7, and 0.2 μm. The results indicate that β, the hole ionization coefficient, is slightly greater than α, the electron ionization coefficient at low fields, and they both become effectively equal at high fields. α and β are also found to be significantly lower than GaAs across the range of electric fields studied, with correspondingly higher breakdown voltages.

Terahertz response of zero‐dimensional states in resonant tunneling diodes

The photoresponse of zero‐dimensional states in double barrier resonant tunneling diodes has been observed using high intensity THz radiation (photon frequencies ranging from 4 to 10 THz) from a free‐electron laser. The double barrier resonant tunneling diodes have silicon donors in the quantum well which act as individual localized tunneling channels. The high frequency response of these diodes shows additional features due to the presence of these extra channels. The temperature dependence allows us to identify the contribution of the zero‐dimensional states. The absence of wavelength dependence in the observed photoresponse indicates that photoassisted tunneling does not occur under these measurement conditions.

Emission mechanisms and band filling effects in GaAs–AlGaAs V-groove quantum wires

We present a study of emission mechanisms and band filling effects in GaAs–AlGaAs V-groove quantum wires. A comparison of surface emitted photoluminescence (PL) and electroluminescence (EL) spectra from a p-i-n junction structure shows enhanced quantum wire emission in EL compared to PL. This behavior is discussed in terms of enhanced carrier capture by the quantum wire for electrical injection into the structure. With increasing forward bias current the quantum wire EL spectra exhibit subband filling and saturation effects. In addition to the ground state transition, at least three excited state related transitions are observed.