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The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ<270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.

We demonstrate energy-conversion-efficiency (η) enhancement of silicon (Si) solar cells by the heterogeneous integration of an In(x)Ga(1-x)As nanowire (NW) array on the rear surface. The NWs are grown via a catalyst-free, self-assembled method on Si(111) substrates using metalorganic chemical vapor deposition (MOCVD). Heavily p-doped In(x)Ga(1-x)As (x ≈ 0.7) NW arrays are utilized as not only back-reflectors but also low bandgap rear-point-contacts of the Si solar cells. External quantum efficiency of the hybrid In(x)Ga(1-x)As NW-Si solar cell is increased over the entire solar response wavelength range; and η is enhanced by 36% in comparison to Si solar cells processed under the same condition without the NWs.

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In the fields of solid-state lighting and high efficiency solar photovoltaics ͑PVs͒, a need still exists for a material system that can target the 2.3-2.5 eV energy range. The ZnSe/GaAs system is shown to have great potential. The digital alloy approach can be utilized as a well-ordered design alternative to the disordered alloyed systems. The effective bandgap of the ZnSe/GaAs͑001͒ superlattice has been studied as a function of the constituent monolayers using tight binding. The possibility of engineering a range of bandgaps with the same material system, to achieve the optimum value for solar PV and light emitting diode ͑LED͒ applications, has been proposed. Why ZnSe/GaAs? The Solar Cell StoryCurrently, no material system is well tuned for the conversion of high energy photons in the range of 2.3-2.5 eV, depicted in Fig. 1. In the field of multijunction or tandem stack solar cell design, this high energy range is of crucial importance for reaching a combined cell efficiency greater than 50%. In particular, a material system with an energy gap of 2.4 eV would be ideal for the topmost cell in a vertically integrated multijunction stack. 1,2 Additionally, the need still exists for a highly efficient true green light-emitting diode ͑LED͒ in the wavelength range of 555-560 nm. InGaN is able to achieve high brightness in green/blue-green LEDs ͑around 532 nm͒, whereas GaP and AlGaInP work best in the yellow-green range ͑around 567 nm͒, leaving a gap in between ͑the so-called "green gap"͒. Most work being conducted in this area is with InGaN. However, indium-rich compositions suffer severely from phase separation of InN, 3 resulting in detuning from the desired spectral range. In addition, all existing InGaN growth processes result in defect densities that are too high for efficient solar cells. As an alternative, the system between GaAs and ZnSe is particularly appealing for investigation. Given that they are both direct bandgap and lattice-matched ͑within 0.27%͒ semiconductors, the possibility exists for engineering materials both in physical and digital alloy ͑DA͒ form over the full range of bandgaps from 1.42 eV ͑GaAs͒ to 2.7 eV ͑ZnSe͒. Shen et al. previously showed calculations on this superlattice ͑SL͒ system, although no details were given on our targeted range of bandgaps. 4 Our work seeks to guide the experimentalist to engineer ZnSe/GaAs DAs based on barrier and well thickness. DAs vs Physical AlloysFrom a materials engineering perspective, there are two possible methods for fabricating materials with bandgaps between that of GaAs and ZnSe: ͑i͒ physical alloying and ͑ii͒ digital alloying using an SL. It might be indicated that the DA technique has an advantage because the density of states for a quantum-well-like structure has a staircase form that translates into a nonzero value of density of states even at the minimum ͑maximum͒ energy for the conduction ͑valence͒ band. 5 In this article, we investigate the DA technique 6 to provide a well-ordered design alternative to the disordered physical alloyed systems...