Hot carrier solar cells have the promise to increase photovoltaic conversion efficiency beyond the Shockley-Quiesser limit and towards the thermodynamic maximum of 85%. The concept relies on the ability to extract photo-generated electrons from an absorber region faster than they can lose energy to the lattice in a process termed thermalisation. We have previously presented a realization of such a cell under limited operating conditions, in particular at low temperature, for narrowband illumination and with low total absorption of light. In this work we present the idea of a metallic absorber to address some of these limitations and show how such an absorber is a promising candidate to realize the hot carrier solar cell. In addition to a theoretical justification of the metallic hot carrier solar cell, we show device fabrication and experimental current-voltage characteristics of an initial cell, showing absorption of light in a thin-film metal region and a photo-current driven by this absorption.
The growth and fabrication of 405 nm InGaN laser diodes by molecular beam epitaxy (MBE) has made rapid progress
over the last three years. In 2004, the authors reported the first MBE-grown nitride laser diodes. In mid-2005 the authors
then demonstrated room-temperature continuous-wave (cw) operation. This was achieved by significantly reducing the
threshold current density to 5.6 kA/cm2 for facet-coated LDs. The lifetime of these first MBE-grown cw lasers was up to
3 minutes, limited by power dissipation. In this paper we report on the progress we have made in reducing operating voltage
and power dissipation, enabling a significant increase in laser lifetime. Uncoated 2x1000 &mgr;m ridge waveguide lasers
fabricated on freestanding GaN substrates have a continuous-wave (cw) threshold current of 110 mA, corresponding to a
threshold current density of 5.5 kA/cm2. For 2x600 &mgr;m laser diodes the minimum threshold current is 70 mA. Cw laser
lifetime vs. power dissipation data is presented, with a maximum lifetime of 2.6 hours for the best laser. The lifetime
versus power dissipation data shows that the MBE-grown lasers follow a similar trend as lasers grown by metalorganic
chemical vapor deposition (MOCVD). We also report length dependence measurements of these long lifetime lasers,
with a gain G0 of 2000-2200 cm-1 and an internal loss &agr;i=30-45 cm-1.
In this paper we report on progress in the development of nitride laser diodes by molecular beam epitaxy (MBE). We review the steps taken to achieve continuous wave (CW) operation of 405nm lasers grown by MBE and evaluate the performance of such devices. The future potential of the growth method for lasers depends on the demonstration of long lived lasers with good operating characteristics such as high power output and low threshold current. We assess the challenges to achieving such performance in MBE-grown lasers and the progress in evaluating the key laser parameters in our devices.
Semiconductor nitrides have many applications for optoelectronic devices; particularly, blue-violet laser diodes (LDs) are required for blu-ray optical disc systems. Molecular beam epitaxy (MBE) with its fine control of growth parameters and capability for in-situ growth monitoring is a well-established technique for depositing III-V heterostructures. Indeed, many commercial infrared LDs are grown very successfully by MBE. However, MBE-growth of nitrides is much more difficult, because providing enough nitrogen atoms at the growth surface, sustaining the high growth temperatures as well as finding the right growth parameters have proved to be very challenging. We recently reported the first InGaN LDs by MBE, showing that those problems can be solved in practice as well as demonstrating the capability of MBE to produce high-quality optoelectronic devices. As the efficiency of nitrides depends strongly on the growth process, structural differences of MBE over metal-organic vapor phase epitaxy (MOVPE)-material may also lead to device advantages. Our first InGaN LDs were grown on sapphire templates, with a pulsed room-temperature threshold current-density of 30 kA/cm2 and a threshold voltage of 33 V. Here, we report on MBE-grown 405 nm InGaN LDs on freestanding GaN substrates, with threshold current-densities <10 kA/cm2 and threshold voltages <10 V, approaching state-of-the-art values. We will report on details of the material quality and LD structure; and will discuss the advantages of MBE-grown LDs over MOVPE-LDs, resulting from fine growth control and no requirement for p-dopant activation. Therefore, the MBE-growth of nitrides has opened a new approach to efficient optoelectronic devices.