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.
We have measured the gain spectrum of an optically pumped 40 angstrom ZnCdSe-ZnSe multiple quantum well. Our calculation, which includes many body effects such as Coulomb enhancement and spectral broadening due to carrier scattering, gives excellent agreement with the experimental gain measurements. We then show the importance of the inclusion of the Coulomb enhancement for the calculation of optical gain when predicting laser threshold currents. This is emphasized by using our gain calculation as a basis to theoretically optimize a simple ZnCdSe-ZnSe quantum well laser structure incorporating the leakage current over the p-type cladding.
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