sábado, 24 de julio de 2010

Introduction to Dilute Nitrides, Gerald Soto, CRF 2010-1, (3er Parcial).

Arsenide and phosphide based III-V semiconductors have been extensively researched for many years, resulting in mature technologies that are used for many of today's electronics and optoelectronics. These materials are however limited to medium and narrow bandgap applications, preventing the fabrication of wide gap devices such as high power electronics and short wavelength optoelectronics. The development of wide-gap material has been slow and problematic, nevertheless, recent advances in nitrides, such as the development of blue LEDs and lasers, have seen these alloys catapulted into the limelight. This has lead to an explosion of research interest, and the natural progression to bridge the gap between the wide-gap nitrides and the medium-gap arsenides. However, instead of the addition of small amounts of nitrogen to GaAs resulting in the expected increase of bandgap, it was found to have the opposite effect, resulting in a rapid reduction of the bandgap. This unusual behaviour has sparked considerable interest both from a fundamental physics point of view as well as for potential narrow-gap applications. These new hybrid alloys, which have become known as "dilute nitrides", are the topic of this current work.

In the late eighties interest in wide-gap materials for short wavelength optoelectronics (and electronics) began to grow, most research concentrated on zinc selenide (ZnSe) although some work was also carried out on nitrides and silicon carbide (SiC). Despite the indirect bandgap of SiC, it was used in the late eighties to produce the first commercial blue LEDs; unfortunately, the indirect gap meant low efficiencies (~0.03 % ) and no scope for producing lasers. Instead, ZnSe-based materials were pursued, leading to the first pulsed blue laser diode in 1991. Despite subsequent advances that lead to continuous wave room temperature lasing, ZnSe was problematic. Unlike SiC and nitrides that have wide-gaps resulting from strong chemical bonds, ZnSe's gap is due to its highly ionic bonds that are weak. As a result, defect growth and propagation is common, leading to device degradation. In addition, ZnSe suffers from poor electrical/thermal properties and Fermi level pinning.

Nitride research dates back to the early seventies when many physical properties such as refractive index, bandgap and lattice constant were measured. However, interest soon diminished, as it was discover that the strong background n-type doping prevented the growth of p-type material and no suitable substrate materials were available. Interest in nitrides was rekindled in the early nineties when a small research group led by Nakamura at a virtually unheard of chemical company in Japan reported blue emission from InGaN devices. The application of innovative growth techniques to the nitride system led to rapid advances in material quality and commercially viable devices. The problems associated with substrate mismatch were overcome using buffer layers, and background doping was reduced via optimised growth. The key breakthrough of p-doping was finally achieved using magnesium dopant activated by electron beam irradiation or thermal annealing. This allowed control over both n and p-type doping, hence nitride based p-n junctions could be produced and subsequently advance heterostructures LEDs and lasers.

The first steps towards bridging the gap between arsenides and nitrides was made in 1992 by Weyers et.al., resulting in the unexpected discovery of a rapid reduction in bandgap energy with increasing nitrogen. In most, if not all other, III-V alloys, when a semiconductor element is replaced by one of smaller ionic radius, the band gap energy increases. When small amounts of nitrogen were added to GaAs, instead of the expected blue shift in emission, a considerable red shift was observed. Furthermore, the decrease in energy per atomic percent of nitrogen was more than ten times greater than the typical increase in other semiconductor alloys. While this behaviour meant that the dilute nitrides could not be used for visible light emitters, fundamental physics as well as potential applications were clear incentives to pursue these materials. Surprisingly, the next few years saw very little published work on dilute nitrides.

Interest in dilute nitrides really began in the mid nineties after Kondow et.al. published results on the quaternary alloy, GaInNAs. This new alloy allowed independent control over the In:Ga and N:As ratios. Increasing the In:Ga ratio causes a reduction in bandgap and an increase in lattice parameter, while increasing the N:As ratio also causes bandgap reduction, but a decrease in the lattice parameter. GaInNAs, therefore, gives the flexibility of tailoring both bandgap and lattice parameter. Such tailoring potential opens up a wide range of possible applications, however, the 1.3 mm lasers based on GaAs for optical communications were identified as a key application. At this wavelength silica fibre has zero dispersion and relatively low attenuation, making it an attractive communications window. Many of today's medium haul systems operate in the 1.3 mm window, and if transceiver systems could be made economical then they could be used in local area networks. The first laser to operate in the 1.3 mm window was produced in 1976 by J. Hsieh, using InGaAsP/InP, since then most, if not all, 1.3 mm systems have used InP based lasers. Unfortunately, this material system is not ideal for producing cheap lasers as a number of problems ultimately increase costs. While some innovative attempts have been made to solve these problems, many are far from ideal, often causing additional problems or using processing techniques that are not easily integrated into commercial production. Instead of solving the problems, considerable research efforts are currently being put into finding alternative materials that avoid the problems altogether. InGaAs has been extensively researched and investigations have been done to see how far the material can be pushed towards long wavelength emissions. While highly strained InGaAs QWs lasers operating out to around 1.2 mm have been produced with good characteristics using strain-compensated QWs, increasing the wavelength further becomes very difficult. Lasers operating close to 1.3 mm based on GaAsSb QWs have been demonstrated, however, the conduction band discontinuity between GaAsSb and GaAs is believed to be small resulting in high leakage currents and hence poor performance.

At present, the main research drive for alternative 1.3 mm lasers is split between self-organised quantum dots (QD) and dilute nitrides. QD lasers have attracted substantial interest, as the physics of QDs could potentially improve laser performance considerably. They are expected to have reduced temperature dependence, reduced thresholds and higher efficiencies. However, this can only be realised if methods are found to control dot density, distribution and, most importantly, size. Even small size fluctuations can 'smear' the density of states, producing bulk-like behaviour. While considerable progress has been made towards dot uniformity, there appears to be some trade off with dot density, hence gain. In addition, the expected temperature performance of QD lasers has not yet been demonstrated, so far devices have exhibited poor characteristic temperatures no better than InP lasers.

Considerable interest in dilute nitrides for long wavelength lasers was stimulated by the first reported GaInNAs lasers. The materials were expected to have very good temperature characteristics resulting from the predicted large electron confinement. In addition, being based on GaAs, dilute nitrides would be easily integrated with the established GaAs technology, including AlGaAs based Bragg mirrors. However, producing high quality dilute nitride has proved difficult, with increasing nitrogen fraction resulted in reduced optical quality. Post-growth annealing was soon found to improve the optical quality, however, the increase in optical quality was usually accompanied by a blue shift, which partly offset the red shift induced by nitrogen incorporation. Despite these problems, early research lead to the demonstration of a range of devices and a gradual improvement in material quality. Progress in dilute nitride devices has been rapid, 1.3 mm laser emission was realised in 1998, and good characteristic temperatures have been reported. The emission wavelength has even been pushed out to 1.515 mm using GaInNAs with high indium and nitrogen fractions. Unfortunately, many early lasers had high thresholds and low slope efficiencies compared to nitrogen-free lasers. These problems have largely been attributed to poor crystal quality, which is hardly surprising in a new material system. However, considerable improvements have been reported recently, with some 1.3 mm single and triple quantum well lasers having thresholds as low as 400 and 680 A/cm2 respectively. A number of dilute nitride VCSELs have also been successfully fabricated, the first in 1997 operated at 1.22 mm under optical pumping, however, it was not long before an electrically pumped version was demonstrated. Subsequently, VCSELs operating close to 1.3 mm were demonstrated, first optically pumped and then electrically pumped. In addition to lasers, dilute nitrides have been used in a number of other devices including:

Resonant cavity enhanced (RCE) photodetectors.
Electro-absorption (e/a) modulators.
Solar cells.
Heterojunction bipolar transistors (HBTs).

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