S. P. Ahrenkiel

Synthesis of extremely small InP quantum dots and electronic coupling in their disordered solid films

Extremely small colloidal InP quantum dots (QDs) with diameters ranging from 15 to 23 A were synthesized, and the optical properties of close-packed arrays of these dots were studied. The isolated QDs in dilute colloidal solution exhibit pronounced discrete absorption spectra, indicating a narrow size distribution. The absorption spectra of close-packed solids of ∼18 A diameter QDs with interdot spacings of 9 and 18 A show that the absorption onsets and excitonic peaks are, respectively, redshifted and broadened in going from dilute solution to close-packed solids. These results can be explained by electron delocalization in disordered close-packed solids; the spacing of electronic levels in the QDs is reduced and produces a redshift in the absorption spectra.

InGaAs/InP double heterostructures on InP/Si templates fabricated by wafer bonding and hydrogen-induced exfoliation

Applications of InP-based materials are numerous, and thus integration of InP on Si may enable realization of powerful integrated III‐V-on-Si systems. InP and its lattice matched quaternary counterpart In12xGaxAsyP12y are direct gap semiconductors, which have high carrier mobilities, therefore finding applications in lasers, multijunction solar cells 1 and high-speed devices. Additionally, they cover the low dispersion and minimum loss wavelengths for optical fiber communication at 1.3 and 1.5mm, respectively, making them attractive materials for fabricating semiconductor lasers and detectors for telecommunications applications. However, InP is mechanically fragile, is not available in large substrates, and is expensive. Integrating InP thin films on Si substrates improves its mechanical strength and may also allow InP integration on large substrates by a process of tiling transferred thin films. Most importantly, a viable approach to InP/Si may enable cost-effective integration of infrared optoelectronic devices with well-established silicon electronics.

Spontaneous Lateral Composition Modulation in III-V Semiconductor Alloys

Introduction The application of III-V semiconductor alloys in device structures is of importance for high-speed microelectronics and optoelectronics. These alloys have allowed the device engineer to tailor material parameters such as the bandgap and carrier mobility to the need of the device by altering the alloy composition. When using ternary or quaternary materials, the device designer presumes that the alloy is completely disordered, without any correlation between the atoms on the cation (anion) sublattice. However the thermodynamics of the alloy system often produce material that has some degree of macroscopic or microscopic ordering. Short-range ordering occurs when atoms adopt correlated neighboring positions over distances of the order of a few lattice spacings. This can be manifested as the preferential association of like atoms, as in clustering, or of unlike atoms, as in chemical ordering (e.g., CuPt ordering). Long-range ordering occurs over many tens of lattice spacings, as in the case of phase separation. In either short-range or long-range ordering, the band structure and the crystal symmetry are greatly altered. Therefore it is absolutely critical that the mechanisms be fully understood to prevent ordering when necessary or to exploit it when possible.

Strain-dependent morphology of spontaneous lateral composition modulations in (AlAs)m(InAs)n short-period superlattices grown by molecular beam epitaxy

The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and (001)InP substrate. For samples grown under tension, transmission electron and atomic force microscopy reveal composition modulations along directions close to 〈310〉 coupled to a surface cusping. For samples grown under compression, we observe composition modulations roughly along the elastically soft 〈100〉 directions coupled to a surface rippling. For high strains (⩾0.7%), with individual InAs layer thicknesses ⩽1.6 monolayers, we observe weak or no composition modulations.

Carrier transport in ordered and disordered In0.53Ga0.47As

Room temperature recombination dynamics have been studied in partially ordered and disordered ternary alloys of In0.53Ga0.47As by correlated measurements of transmission electron diffraction and photoconductive decay. Ultrahigh frequency photoconductive decay measurements show that pulsed yttrium-aluminum-garnet laser-induced excess carriers in disordered films decay by conventional mechanisms such as the Shockley–Read–Hall effect. In highly ordered ternaries, recombination of excess carriers is retarded by some mechanisms such as charge separation. Excess carrier lifetimes exceeding several hundred microseconds have been observed.

GaInP2 overgrowth and passivation of colloidal InP nanocrystals using metalorganic chemical vapor deposition

We have used metalorganic chemical vapor deposition to deposit thin GaInP2 passivating films on both isolated and close-packed arrays of colloidal InP/GaInP2 core-shell nanocrystals. Conformal GaInP2 layers of 10–20 nm were grown on the nanocrystals after organic capping molecule removal by a thermal annealing treatment. We show that the InP nanocrystals retain their crystallinity, shape and luminescence efficiency after being exposed to growth temperatures of 600 °C. The GaInP2 nanocrystal composite showed strong photoluminescences indicating effective passivation of surface states. In close-packed nanocrystal arrays, the emission band is redshifted compared to films of isolated nanocrystals indicating electron coupling between dots embedded in GaInP2.

Recent Advances in Low‐Bandgap, InP‐Based GaInAs/InAsP Materials and Devices for Thermophotovoltaic (TPV) Energy Conversion

Salient advances in the development of thermophotovoltaic (TPV) energy converters based on low‐bandgap, InP‐based, GaInAs/InAsP heterostructures are presented and discussed. InP‐based materials are well‐suited and advantageous for TPV converter applications. Substantial improvements in the quality of lattice‐mismatched (LMM) heterostructures have been realized through an enhanced understanding of the relaxation behavior, and associated microstructure, of InAsP compositionally graded layers and GaInAs/InAsP interfaces. Double‐heterostructure, GaInAs/InAsP test structures with bandgaps as low as 0.5 eV (1.6% lattice mismatch) have been demonstrated with exceptional low‐injection, minority‐carrier lifetimes (several μs) and large estimated diffusion lengths — comparable to those for lattice‐matched materials. The advances in material quality have contributed to a number of notable TPV device achievements. A record in‐cavity efficiency of 23.6% was reported for a 0.6‐eV, GaInAs/InAsP monolithic interconnected module. Additionally, 0.52‐eV GaInAs/InAsP TPV converters were demonstrated with near‐unity internal quantum efficiencies and reverse‐saturation current densities nearly equaling the best reported for lattice‐matched, 0.52‐eV GaInAsSb/GaSb devices. Furthermore, InP‐based, 0.74/0.63‐eV, monolithic, series‐connected, tandem TPV converters are also under development and show promising performance; an in‐cavity efficiency of 11% has been reported for preliminary devices.

The nature and origin of lateral composition modulations in short-period strained-layer superlattices

The nature and origin of lateral composition modulations in (AlAs){sub m}(InAs){sub n} SPSs grown by MBE on InP substrates have been investigated by XRD, AFM, and TEM. Strong modulations were observed for growth temperatures between {approx} 540 and 560 C. The maximum strength of modulations was found for SPS samples with InAs mole fraction x (=n/(n+m)) close to {approx} 0.50 and when n {approx} m {approx} 2. The modulations were suppressed at both high and low values of x. For x >0.52 (global compression) the modulations were along the directions in the (001) growth plane. For x directions rotated {approx} {+-} 27{degree} from [110] in the growth plane. The remarkably constant wavelength of the modulations, between {approx} 20--30 nm, and the different modulation directions observed, suggest that the origin of the modulations is due to surface roughening associated with the high misfit between the individual SPS layers and the InP substrate. Highly uniform unidirectional modulations have been grown, by control of the InAs mole fraction and growth on suitably offcut substrates, which show great promise for application in device structures.

ATOMIC ORDERING AND TEMPERATURE-DEPENDENT TRANSIENT PHOTOCONDUCTIVITY IN GA0.47IN0.53AS

Influences of CuPtB atomic ordering on transient photoconductivity in epitaxial Ga0.47In0.53As films grown by metal-organic chemical vapor deposition are examined. Low-injection lifetimes of several ms are measured in double-variant ordered samples at 77 K; these lifetimes decrease rapidly with temperatures above 180 K, giving a thermal activation energy for recombination of 0.19 eV. Single-variant ordered samples exhibit typical lifetimes of 30–60 μs, with no noticeable temperature dependence up to 300 K. Charge separation in double-variant samples may be driven by a type-II band alignment between ordered and disordered regions, or by an alternating internal electrical polarization between ordered variants. Recombination in both double- and single-variant samples may be influenced by inhibited transport across antiphase boundaries.

Recombination lifetime in ordered and disordered InGaAs

The ternary semiconductor InxGa1−xAs has been a key component of current thermophotovoltaic energy converters. These studies indicate that the bandgap of the epitaxial films, that are lattice‐matched to InP, show varying degrees of ordering of the metal sublattice depending upon growth temperature. The bandgap of partially ordered films is lowered by as much as 75 meV. The transport of carriers in ordered films is dominated by domain trapping.