We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.

High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion.

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. The fascinating wave phenomenon of ‘bound states in the continuum’ spans different material and wave systems, including electron, electromagnetic and mechanical waves. In this Review, we focus on the common physical mechanisms underlying these bound states, whilst also discussing recent experimental realizations, current applications and future opportunities for research.

Bound states in the continuum

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Infrared detectors: status and trends

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenom...

Prospects of Nanoscience with Nanocrystals

ZnO nanostructures for optoelectronics: Material properties and device applications

The physical properties of detectors based on intraband optical absorption in quantum dots is described and examined in the interest of providing a competitive alternative infrared (IR) detector technology. These quantum dot detectors are an extension of quantum well infrared photodetectors and are expected to have a large performance advantage. A model is developed for quantum dot infrared photodetectors based on fundamental performance limitations enabling a direct comparison between IR materials technologies. A comparison is made among HgCdTe, quantum well, and quantum dot IR detectors, where quantum dots are expected to have the potential to outperform quantum wells by several orders of magnitude and compete with HgCdTe. In this analysis, quantum dots are expected to possess the fundamental ability to achieve the highest IR detector performance if quantum dot arrays with high size uniformity and optimal bandstructure may be achieved.

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Evaluation of the fundamental properties of quantum dot infrared detectors

We report far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy. Through use of a Fourier transform infrared spectrometer, a photoconductivity signal peaked at 17 μm is observed from a n–i–n detector structure with doped InAs quantum dots in the intrinsic region. Comparison of photoluminescence and band-to-band photocurrent absorption spectra suggests the far-infrared response is due to intersubband transitions in the quantum dots.

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Far-infrared photoconductivity in self-organized InAs quantum dots

The use of self-assembled InAs-GaAs quantum dots in photoconductive intersubband detectors in the far-infrared is presented. Far-infrared absorption is observed in self-assembled quantum dots in the 6-18-/spl mu/m range for subband-subband and subband-continuum transitions. Photoconductive quantum-dot intersubband detectors were fabricated and demonstrate tunable operating wavelengths between 6-18 /spl mu/m using subband-subband or subband-continuum transitions. The use of AlAs barriers allows further tuning to shorter wavelengths of 3-7 /spl mu/m. Subband-continuum quantum dot intersubband detectors show encouraging normal incidence performance characteristics at T=40 K, with responsivities of 10-100 mA/W, detectivities of 1-10 /spl times/10/sup 9/ cm/spl middot/Hz/sup 1/2//W and large photoconductive gain up to g=12 for a ten-layer quantum-dot heterostructure. With improvements in device structure, self-assembled quantum dots can be expected to provide intrinsic normal incidence broad-band detectors with advantages over quantum wells.

Self-assembled InAs-GaAs quantum-dot intersubband detectors

Oxygen doping in ZnTe is applied to a junction diode in the aim of utilizing the associated electron states 0.5 eV below the bandedge as an intermediate band for photovoltaic solar cells. The ZnTe:O diodes confirm extended spectral response below the bandedge relative to undoped ZnTe diodes, and demonstrate a 100% increase in short circuit current, 15% decrease in open circuit voltage, and overall 50% increase in power conversion efficiency. Subbandgap excitation at 650 and 1550 nm confirms the response via a two-photon process and illustrates the proposed energy conversion mechanism for an intermediate band solar cell.

/pdf/intermediate-band-photovoltaic-solar-cell-based-on-znte-o-3ifvn5idl8.pdf

Intermediate-band photovoltaic solar cell based on ZnTe:O

We report the room-temperature operating characteristics of InGaAs/GaAs self-organised quantum dot lasers grown by molecular beam epitaxy. The emission wavelength is 1.028 µm and Jth = 650 A/cm2 for a 90 µm × 1 mm broad-area laser. Steady-state and time-resolved photoluminescence measurements confirm that lasing occurs through the e1-hh2 higher-order transition, and the spontaneous recombination time for this transition is ≃ 200 ps.

Jamie Phillips

Papers

Evaluation of the fundamental properties of quantum dot infrared detectors

Far-infrared photoconductivity in self-organized InAs quantum dots

Self-assembled InAs-GaAs quantum-dot intersubband detectors

Intermediate-band photovoltaic solar cell based on ZnTe:O

Room-temperature operation of In0.4Ga0.6As/GaAs self-organised quantum dot lasers