The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet-visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped CdO that is achieved through accurate 'defect equilibrium engineering'. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm(2) V(-1) s(-1) for carrier densities above 10(20) cm(-3). Our work shows that CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.

Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.

Venture capital (VC) firms spent over $25 billion funding clean energy technology (cleantech) start-ups from 2006 to 2011. Less than half of that capital was returned; as a result, funding has dried up in the cleantech sector. But as the International Energy Agency warns, without new energy technologies, the world cannot cost-effectively confront climate change. In this article, we present the most comprehensive account to date of the cleantech VC boom and bust. Our results aggregate hundreds of investments to calculate the risk and return profile of cleantech, and we compare the outcomes with those of medical and software technology investments. Cleantech posed high risks and yielded low returns to VCs. We conclude that among cleantech investments, "deep technology" investments-in companies developing new hardware, materials, chemistries, or manufacturing processes-consumed the most capital and yielded the lowest returns. We propose that broader support from policymakers, corporations, and investors is needed to underpin new innovation pathways for cleantech.

Controllable n-type doping over wide ranges of carrier concentrations in AlN, or Al-rich AlGaN, is critical to realizing next-generation applications in high-power electronics and deep UV light sources. Silicon is not a hydrogenic donor in AlN as it is in GaN; despite this, the carrier concentration should be controllable, albeit less efficiently, by increasing the donor concentration during growth. At low doping levels, an increase in the Si content leads to a commensurate increase in free electrons. Problematically, this trend does not persist to higher doping levels. In fact, a further increase in the Si concentration leads to a decrease in free electron concentration; this is commonly referred to as the compensation knee. While the nature of this decrease has been attributed to a variety of compensating defects, the mechanism and identity of the predominant defects associated with the knee have not been conclusively determined. Density functional theory calculations using hybrid exchange-correlation functionals have identified VAl+nSiAl complexes as central to mechanistically understanding compensation in the high Si limit in AlN, while secondary impurities and vacancies tend to dominate compensation in the low Si limit. The formation energies and optical signatures of these defects in AlN are calculated and utilized in a grand canonical charge balance solver to identify carrier concentrations as a function of Si content. The results were found to qualitatively reproduce the experimentally observed compensation knee. Furthermore, these calculations predict a shift in the optical emissions present in the high and low doping limits, which is confirmed with detailed photoluminescence measurements.

In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.

A prominent 2.8 eV emission peak is identified in bulk AlN substrates grown by physical vapor transport. This peak is shown to be related to the carbon concentration in the samples. Density functional theory calculations predict that this emission is caused by a donor-acceptor pair (DAP) recombination between substitutional carbon on the nitrogen site and a nitrogen vacancy. Photoluminescence and photoluminescence-excitation spectroscopy are used to confirm the model and indicate the DAP character of the emission. The interaction between defects provides a pathway to creating ultraviolet-transparent AlN substrates for optoelectronics applications.

Co-doping AlN crystals with Si is found to suppress the unwanted 4.7 eV (265 nm) deep ultraviolet absorption associated with isolated carbon acceptors common in materials grown by physical vapor transport. Density functional theory calculations with hybrid functionals demonstrate that silicon forms a stable nearest-neighbor defect complex with carbon. This complex is predicted to absorb at 5.5 eV and emit at or above 4.3 eV. Absorption and photoluminescence measurements of co-doped samples confirm the presence of the predicted C N -Si Al complex absorption and emission peaks and significant reduction of the 4.7 eV absorption. Other sources of deep ultraviolet absorption in AlN are also discussed. V C 2014 AIP Publishing LLC. [http://dx.

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