R. Loganathan

Controlled nucleation and growth of nanostructures by employing surface modified GaN based layers/heterostructures as bottom layer

Controlled nucleation and growth of Zinc oxide nanorods is achieved on GaN, etched GaN and AlGaN/GaN heterostructure bottom layers grown by a metal organic chemical vapour deposition technique. The effects of the bottom crystalline layers on the structural, morphological and optical properties of the as grown ZnO nanorods have been investigated by high-resolution X-ray diffraction, scanning electron microscopy, photoluminescence and Raman measurements. HRXRD (0002) reciprocal-space mapping (RSMs) studies were performed on GaN and AlGaN/GaN layers before and after the growth of ZnO nanostructures to investigate the impact of strain upon the ZnO layer grown on GaN layers and AlGaN/GaN heterostructures. Raman intensity mapping shows the densely nucleated hexagonal pit like structures for the etched GaN layer, providing an enhanced surface area for primary nucleation suggesting that the growth species prefer to condense on locations with maximum binding energy. The increase in nucleation density for etched GaN layers also result in dense nanorods which exhibit better excitonic emission. Our studies suggest that ZnO nanostructures with improved optical and structural properties can be grown on etched-GaN as well as AlGaN/GaN heterostructures as the bottom layer. It is interesting to observe that the bottom GaN layer can be easily employed to determine the optical quality of ZnO layer.

Effect of Al-mole fraction in AlxGa1−xN grown by MOCVD

AlGaN/AlN layers were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The AlxGa1−xN layer composition was varied from 15% to 25%. The crystalline quality, thickness and aluminum (Al) composition of AlGaN were determined using high resolution X-ray diffraction (HRXRD). The growth rate decreases on increasing Al composition. Reciprocal space mapping (RSM) was used to estimate the strain and relaxation between AlGaN and AlN. The optical properties of AlGaN layers were investigated by room temperature Photoluminescence (PL). The AlGaN peak shifts towards lower wavelength with Al composition. The surface morphology of AlGaN was studied by atomic force microscopy (AFM). Root mean square (RMS) roughness values were found to be increased in AlGaN layers with composition.

Growth of AlN nanostructure on GaN using MOCVD

Aluminum nitride (AlN) nanowalls have been epitaxially grown on dislocation assisted GaN/Al2O3 template by metal organic chemical vapor deposition (MOCVD) without any help of metal catalysts. A large number of nanowalls with thicknesses of 1.5-2.0 µm and height 400 nm have been deposited. The AlN nanowalls were found to have a preferred c-axis oriented with a hexagonal crystal structure. The AlN nanowalls and GaN/Al2O3 template have been characterize at room temperature photoluminescence (PL) and high resolution X-ray diffraction (HRXRD).

Growth and Characterization of AlInGaN/AlN/GaN Grown by MOCVD

AlxInyGa1-x-yN epilayers have been grown by metal organic chemical vapor deposition (MOCVD) at different temperatures from 740 to 940 °C. The incorporation of indium increases with decreasing growth temperature, while the incorporation of Al composition was 30–40 %. The optical properties of the samples have been investigated by photoluminescence (PL). The results show that the sample grown at 890 °C exhibits the best crystalline and optical quality.

Photon decay time studies on Al0.45Ga1-0.55N/Al0.18Ga1-0.82N/Al0.45Ga1-0.55N double heterostructures grown on sapphire substrate by MOCVD

In the present investigation, AlxGa1-xN/AlyGa1-yN/AlxGa1-xN double heterostructures have been grown on sapphire substrate using MOCVD. The active layer Al0.18Ga0.82N thickness has been changed as 14.8 nm and 23.4 nm by keeping the thickness of the Al0.45Ga0.55N barrier layer as constant. It has been found that full width halfmaxium (FWHM) of (002) undoped GaN without double heterostructures as 352 arc-secs where as for the double heterostructures it is found to be 523 and 483 arc-sec for the active layer thickness of 14.8nm and 23.4nm respectively. The photon decay time was found to be 125, 85 and 87 ps for undoped GaN, Al0.18Ga0.82N of thickness 14.8 nm and 23.4 nm respectively using Time Resolved Photoluminescence (TRPL).It has been observed that the well width has no effect on the radiative decay time which has been reported for the first time.

Studies on the dislocation densities of gallium nitride grown by MOCVD

Group III-V nitrides have become versatile semiconducting materials for short wavelength LEDs, high temperature transistors. The growth and device processing of these materials are significant due to unusually high bond energies of nitrides. Inspite of high dislocations densities in the order of 109cm-2 the optical and electronic devices based on nitrides show high performance compared to conventional semiconductor devices. Understanding of the behavior of dislocations in these materials structures are very important for the fabrication of devices. In the present study, GaN was grown on sapphire substrates using MOCVD. The dislocation density of GaN has been estimated by wet etching and HRXRD. The results have been correlated with the growth conditions. The dislocation density of the samples was found to be between 3.5x109cm-2 and 5.0x108cm-2.

Growth and Characterization of AlxGa1-xN on GaN/Al2O3

AlGaN is a promising material to develop UVLEDs and HEMT devices due to the direct wide-band gap material. In the present investigation, AlxGa1-xN alloys were grown on c-plane sapphire substrate by MOCVD. Al content x was varied in the composition range 0≤×≤0.6. The thickness and Al composition of the AlGaN was determined by HRXRD. The growth rate decreases on increasing the composition of Al. The critical thickness of pseudomorphic AlGaN layer decreases on increasing the composition. Thick layers resulted in cracks and it is important to grow thick layers with high aluminum content free from crack for deep UV LEDs.