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The research program at Kansas State University is to develop innovative approaches for fabricating high quality III- nitride QWs, heterostructures, microstructures, and micro-devices and to study their optical and optoelectronic properties. By optimizing the epilayer mobilities and optical emission properties, we have produced GaN,Al(x)Ga(1-x)N (x up to 1), In(x)Ga(1-x)N (x<0.3), and In(x)Al( y)Ga(1-x-y)N epilayers, III-nitride QWs with device qualities. We have successfully fabricated micro-size blue emitters and developed a bonding scheme that allows us to address micro-size pixels individually in an array comprising many 111-nitride micro-emitters/micro-detectors. We have demonstrated the operation of the first prototype blue microdisplay made from InGaN/GaN QWs. We have achieved highly conductive n-type Al(x)Ga(1-x)N alloys of high Al contents (for x up to 0.8). We have developed Mg-delta-doping for enhanced p-type doping efficiency in GaN and AlGaN. A 2-fold enhancement in lateral and a 5-fold enhancement in vertical p-type conductions have been achieved for AlGaN and AlGaN epilayers. We have also achieved AlN epilayers with very efficient band-edge PL emission the optical quality of AlN is comparable to that of GaN and determined the detailed band structure near the gamma point of wurtzite AlN, including energy gap, optical selection rules, exciton binding energies and lifetimes, and acceptor/donor energy levels., The original document contains color images.

During Phase I duration, we have further improved blue micro-size light emitter output power efficiencies by optimizing the material qualities as well as device structures. During Phase I optional phase duration, we learned how to achieve white light emission from InGaN/GaN QW micro-size emitters. We have employed three-color emitting (red-blue-green) phosphorus coating on near UV micro-size LEDs and conventional LEDs to obtain white light emission. Comparing with coating an yellow-emitting phosphor on blue LEDs, the three-color phosphors approach yielded improved white light color rendering. We have also further developed interconnected microdisk LED technology to enhance the extraction efficiency and improve the current spreading. Large-area LED chips have been successfully fabricated. An output power as high as 50 mW has been achieved in interconnected microdisk blue LEDs with a 3 x 3 mm squared chip area size. Further works dealing with heat management and efficiency improvement are still needed. The technologies developed in the optional phase laid the groundwork for the development of solid-state white lighting, which is a technology with an enormous market interest worldwide for energy savings as well as for pollution reduction. The color down conversion technology developed in the optional phase can be equally applied to the development of full color microdisplays based on III-nitrides., Original contains color plates: All DTIC reproductions will be in black and white.

Consistent with our tasks we have further improved our micro-size light emitter output efficiencies by optimizing device layer structures, including superlattice structures for enhancing the hole concentration, the thickness of the top Mg doped p-type layer to reduce the light absorption, and the structure of the active region. We have also carried out measurements on the size dependence of the micro-size light emitter characteristics. It was found that the micro-LEDs were very efficient and the beating effect was not significant in micro-LEDs that are greater than 10 micrometers in diameter. Our results also revealed that the operating speed increases with decreasing micro-LED size and the response time reduced from 0.21 ns for 15-micrometers LEDs to 0.15 ns for 8-micrometers LEDs. Several integrated photonics devices have been fabricated and Their operation under current injection has been achieved. Sub-micron waveguides have also been fabricated from AlGaN/GaN multiple quantum wells and their optical properties have been measured. Effects related to reduced size have been observed. The ability of 2D array integration with advantages of high speed, high resolution, low-temperature sensitivity, and applicability under versatile conditions, make III-nitride micro-photonics unique for many applications, such as optical communications, chemical- and bio-agents detection., Topic no. N01-045.

During the project period, we have investigated the optical properties of III-nitride heterostructures and quantum wells and provided input for the improvement of materials quality, understanding of impurity properties, and optimization of device design. We were the first group to observe the persistent photoconductivity (PPC) effect in p-type GaN epilayers and in AlGaN/GaN heterostructures and utilized PPC effect to study the impurity properties in these materials. We were also one of the first few research groups to employ picosecond time-resolved photoluminescence (PL) measurement technique to study mechanisms of optical transitions, LED emission, and lasing in GaN and related materials.