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In this letter, the photoluminescence spectra of n-type doped GaAs nanowires, grown by the metal organic chemical vapor deposition method, are measured at 4K and 77 K. Our measurements indicate that an increase in carrier concentration leads to an increase in the complexity of the doping mechanism, which we attribute to the formation of different recombination centers. At high carrier concentrations, we observe a blueshift of the effective band gap energies by up to 25 meV due to the Burstein-Moss shift. Based on the full width at half maximum (FWHM) of the photoluminescence peaks, we estimate the carrier concentrations for these nanowires, which varies from 6 × 1017 cm–3 (lightly doped), to 1.5 × 1018 cm–3 (moderately doped), to 3.5 × 1018 cm–3 (heavily doped) as the partial pressure of the disilane is varied from 0.01 sccm to 1 sccm during the growth process. We find that the growth temperature variation does not affect the radiative recombination mechanism; however, it does lead to a slight enhancement in the optical emission intensities. For GaAs nanowire arrays measured at room temperature, we observe the same general dependence of band gap, FWHM, and carrier concentration on doping. [ABSTRACT FROM AUTHOR]

We present a low-temperature, hydrothermal synthesis method for Ta-doped TiO2. Here, alkoxide-based precursors are mixed at low temperatures to suppress differential hydrolysis and phase separation. This method ensures homogeneous, molecular mixing of the Ta dopant with the native oxide up to a concentration of ∼ 2.5 at.%. X-ray diffraction and energy dispersive spectrometer analyses confirm a uniformly doped rutile TiO2. Scanning electron microscopy and transmission electron microscopy analyses reveal a highly branched structure. Optoelectronic properties of these structures were investigated using ultraviolet-visible spectroscopy and low-temperature photoluminescence. [ABSTRACT FROM AUTHOR]

A stack design for carbon-doped GaAs single junction solar microcells grown in triple-layer epitaxial assemblies is presented. As-grown materials exhibit improved uniformity of photovoltaic performance compared to zinc-doped systems due to the lack of mobile dopants while a slight degradation exists in middle and bottom devices. Detailed electrical and optical characterizations of devices together with systematic studies of acceptor reactivation reveal carbon-related defects accompanied by carrier compensation, and associated scattering and recombination centers are primarily responsible for the degraded contact properties and photovoltaic performance, resulting from prolonged thermal treatments of early-grown materials during the multilayer epitaxial growth. [ABSTRACT FROM AUTHOR]

Multijunction solar cells provide us a viable approach to achieve efficiencies higher than the Shockley-Queisser limit. Due to their unique optical, electrical, and crystallographic features, semiconductor nanowires are good candidates to achieve monolithic integration of solar cell materials that are not lattice-matched. Here, we report the first realization of nanowire-on-Si tandem cells with the observation of voltage addition of the GaAs nanowire top cell and the Si bottom cell with an open circuit voltage of 0.956 V and an efficiency of 11.4%. Our simulation showed that the current-matching condition plays an important role in the overall efficiency. Furthermore, we characterized GaAs nanowire arrays grown on lattice-mismatched Si substrates and estimated the carrier density using photoluminescence. A low-resistance connecting junction was obtained using n(+)-GaAs/p(+)-Si heterojunction. Finally, we demonstrated tandem solar cells based on top GaAs nanowire array solar cells grown on bottom planar Si solar cells. The reported nanowire-on-Si tandem cell opens up great opportunities for high-efficiency, low-cost multijunction solar cells.

Unlike nanowires, GaAs nanosheets exhibit no twin defects, stacking faults, or dislocations even when grown on lattice mismatched substrates. As such, they are excellent candidates for optoelectronic applications, including LEDs and solar cells. We report substantial enhancements in the photoluminescence efficiency and the lifetime of passivated GaAs nanosheets produced using the selected area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). Measurements are performed on individual GaAs nanosheets with and without an AlGaAs passivation layer. Both steady-state photoluminescence and time-resolved photoluminescence spectroscopy are performed to study the optoelectronic performance of these nanostructures. Our results show that AlGaAs passivation of GaAs nanosheets leads to a 30- to 40-fold enhancement in the photoluminescence intensity. The photoluminescence lifetime increases from less than 30 to 300 ps with passivation, indicating an order of magnitude improvement in the minority carrier lifetime. We attribute these enhancements to the reduction of nonradiative recombination due to the compensation of surface states after passivation. The surface recombination velocity decreases from an initial value of 2.5 × 10(5) to 2.7 × 10(4) cm/s with passivation.