S. P. Ahrenkiel

Bio: S. P. Ahrenkiel is an academic researcher from National Renewable Energy Laboratory. The author has contributed to research in topics: Superlattice & Quantum dot. The author has an hindex of 16, co-authored 44 publications receiving 957 citations.

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

Abstract: 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.

Synthesis, structure, and optical properties of colloidal GaN quantum dots

Abstract: Colloidal chemistry was used to synthesize GaN quantum dots. A GaN precursor, polymeric gallium imide, {Ga(NH)3/2}n, which was prepared by the reaction of dimeric amidogallium with ammonia at room temperature, was heated in trioctylamine at 360 °C for one day to produce GaN nanocrystals. The GaN particles were separated, purified, and partially dispersed in a nonpolar solvent to yield transparent colloidal solutions that consisted of individual GaN particles. The GaN nanocrystals have a spherical shape and mean diameter of about 30±12 A. The spectroscopic behavior of colloidal transparent dispersion has been investigated and shows that the band gap of the GaN nanocrystals shifts to slightly higher energy due to quantum confinement. The photoluminescence spectrum at 10 K (excited at 310 nm) shows band edge emission with several emission peaks in the range between 3.2 and 3.8 eV, while the photoluminescence excitation spectrum shows two excited-state transitions at higher energies.

Epitaxial growth and characterization of CuInSe2 crystallographic polytypes

Abstract: Migration-enhanced epitaxy (MEE) has been successfully employed to grow epitaxial films of the ternary compound CuInSe2 on (001) GaAs that exhibit distinct coexisting domains of both a nonequilibrium crystallographic structure characterized by CuAu (CA) cation ordering, and the compound’s equilibrium chalcopyrite structure. X-ray diffraction, transmission electron diffraction, and Raman scattering data provide evidence for this structural polytype. Distinctive signatures of the CA polytype are found in the data from each of these methods, and their analyses are consistent with assignment of this crystallographic structure to the P4m2 space group. This structure is found to preferentially segregate into domains that constitute a distinct metastable phase, which may be stabilized by surface kinetic effects favored by the MEE growth process.

Lateral composition modulation in AlAs/InAs short period superlattices grown on InP(001)

Abstract: Spontaneous lateral composition modulation as a consequence of the deposition of a (AlAs)n/(InAs)m short period superlattice on an InP(001) substrate is examined. Transmission electron microscopy images show distinct composition modulation appearing as vertical regions of In- and Al-rich materials alternating in the [110] projection. The periodicity of the modulation is 130 A, and is asymmetric. The transmission electron and x-ray diffraction patterns from the structure exhibit distinct satellite spots which correspond to the lateral periodicity. Transmission electron microscopy images show that the individual superlattice layers possess cusplike undulations, which directly correlate with the composition modulation. Composition modulation in this sample appears to be coupled to morphological and compositional instabilities at the surface due to strain.

Spontaneous Lateral Composition Modulation in III-V Semiconductor Alloys

Abstract: 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.

Quantum dot solar cells

Abstract: Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to third-generation photovoltaic solar cells.

Abstract: Here, we will first briefly summarize the general principles of QD synthesis using our previous work on InP as an example. Then we will focus on QDs of the IV-VI Pb chalcogenides (PbSe, PbS, and PbTe) and Si QDs because these were among the first QDs that were reported to produce multiple excitons upon absorbing single photons of appropriate energy (a process we call multiple exciton generation (MEG)). We note that in addition to Si and the Pb-VI QDs, two other semiconductor systems (III-V InP QDs(56) and II-VI core-shell CdTe/CdSe QDs(57)) were very recently reported to also produce MEG. Then we will discuss photogenerated carrier dynamics in QDs, including the issues and controversies related to the cooling of hot carriers and the magnitude and significance of MEG in QDs. Finally, we will discuss applications of QDs and QD arrays in novel quantum dot PV cells, where multiple exciton generation from single photons could yield significantly higher PV conversion efficiencies.

Gallium Nitride Nanowire Nanodevices

Abstract: Field effect transistors (FETs) based on individual GaN nanowires (NWs) have been fabricated. Gate-dependent electrical transport measurements show that the GaN NWs are n-type and that the conductance of NW−FETs can be modulated by more than 3 orders of magnitude. Electron mobilities determined for the GaN NW FETs, which were estimated from the transconductance, were as high as 650 cm2/V·s. These mobilities are comparable to or larger than thin film materials with similar carrier concentration and thus demonstrate the high quality of these NW building blocks and their potential for nanoscale electronics. In addition, p−n junctions have been assembled in high yield from p-type Si, and these n-type GaN NWs and their potential applications are discussed.

Laser-Assisted Catalytic Growth of Single Crystal GaN Nanowires

Abstract: Xiangfeng Duan and Charles M. Lieber*Department of Chemistry and Chemical BiologyHarVard UniVersity, Cambridge, Massachusetts 02138ReceiVed October 18, 1999Herein we report the bulk synthesis of single crystalline GaNnanowires. Laser ablation of a composite target of GaN and acatalytic metal generates liquid nanoclusters that serve as reactivesites confining and directing the growth of crystalline nanowires.Field emission scanning electron microscopy (FE-SEM) showsthat the product primarily consists of wire-like structures. PowderX-ray diffraction (PXRD) analyses of a bulk nanowire samplecan be indexed to the GaN wurtzite structure, and indicate >95%phase purity. Transmission electron microscopy (TEM), conver-gent beam electron diffraction (CBED), and energy-dispersiveX-ray fluorescence (EDX) analyses of individual nanowires showthat they are GaN single crystals with a [100] growth direction.Nanostructured GaN materials have attracted extensive interestover the past decade due to their significant potential foroptoelectronics.

Structural properties of self-organized semiconductor nanostructures

Abstract: Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.