Kulandaivel Jeganathan

Improvement of film quality using Si-doping in AlGaN/GaN heterostructure grown by plasma-assisted molecular beam epitaxy

Silicon(Si)-doping was employed to improve the film quality of AlGaN/GaN heterostructure by radio frequency plasma-assisted molecular beam epitaxy. By Si doping during the buffer-layer growth, the electrical and structural characteristics of the films were improved. The electron mobility of the two-dimensional electron gas increases without changing the sheet carrier density. The full-width at half-maximums of (1012) X-ray rocking curves reduces. By the atomic force microscopy measurements, smooth surface morphology with micro-steps lower than unit cell height is found. We consider that the improvement of the film quality was caused by the reduction of the nano-pipes due to the growth-mode change into step-flow mode.

High-Quality GaN Layers on c-Plane Sapphire Substrates by Plasma-Assisted Molecular-Beam Epitaxy Using Double-Step AlN Buffer Process.

High-structural-quality c-axis GaN layers were grown on (0001) sapphire substrates by N2-plasma-assisted molecular-beam epitaxy (MBE) using a double-step AlN buffer layer process. The introduction of double-step AlN buffer layers grown at two different high temperatures improved the quality of the GaN layers dramatically. The full-width at half maximum (FWHM) (ω scan) of GaN layers along the symmetric (0002) and asymmetric (1011) diffraction planes were 104 and 848 arcsec, respectively, whereas they were 532 and 1335 arcsec for the GaN layers grown by a single-step AlN buffer process. Double-step AlN buffers improve the structural quality of the layers and decrease the X-ray dislocation density. We have shown that the quality of the underlying AlN buffer layer is the determining factor in the successful growth of high-quality GaN epitaxial layers.

High-Quality Growth of AlN Epitaxial Layer by Plasma-Assisted Molecular-Beam Epitaxy

We report the growth and characterization of high-quality h-AlN epitaxial layers on basal c-plane sapphire substrates by plasma-assisted molecular-beam epitaxy (MBE). The c-axis lattice constant has been found to vary with the layer thickness. AlN layers grown in an intermediate Al/N growth regime show a 2D streaky reflection high-energy electron diffraction (RHEED) pattern and the surface morphologies have few surface irregularities, and sufficient to speculate that the layers are free from large Al metal droplets. The full width at half maximum (FWHM) (ω scan) of the AlN layers along the symmetric (0002) and asymmetric (11–24) diffraction planes were 42 and 180 arcsec, respectively. Excitonic transitions (D°,X and A°,X), indicating high quality of the layers, have been found in all the samples by cathodoluminescence measurements, and a violet band has been attributed to either shallow or deep donor pair recombination.

High Quality GaAs Epitaxial Layers Grown from Ga–As–Bi Solutions by Liquid Phase Epitaxy

The liquidus isotherms for the Ga–As–Bi system were determined at 973 and 923 K in the Ga–As–GaAs region. The liquid phase epitaxial growth of GaAs from Ga–As and Ga–As–Bi solutions were investigated. The addition of ten atomic percent of Bi to the Ga–As solution increase the growth rate of the grown epilayers nearly 4.4 times than in the case of GaAs layers from Bi solution. Above 9 at.% of Bi in Ga solution the problems associated with the edge growth were almost eliminated. Optical measurements at 4.2 K revealed that Bi does not alter the band gap energy value of the GaAs epitaxial layers. The photoluminescence (PL) intensity and full width at half maximum (FWHM) revealed the good quality of undoped GaAs epilayers from Bi solvents compared to that of Ga solvents even though the purity of the Bi was relatively less.

Structural Properties of GaN Films with AlN Buffer Layers with Varying Growth Temperatures by Plasma-Assisted Molecular Beam Epitaxy

We report the characterization of GaN films grown on AlN buffer layers with varying growth temperatures on sapphire (0001) substrates by plasma-assisted molecular beam epitaxy (PAMBE). GaN films grown on AlN buffer layers with varying growth temperatures were studied by means of high-resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL). The GaN film on the AlN buffer layer with two-step growth shows extremely narrow full-width at half maximum (FWHM) values at ω scans of XRD, symmetric (0002), and layer thickness interferences in the 2θ/ω direction. In the PL measurement, the FWHM value of the sample with a two-step AlN layer is comparatively narrow. From the SEM observation, the surface morphology has excellent surface flatness free from pits and grooves. The root mean square (RMS) of the sample surface from the AFM observation is 0.6 nm.

Improvement of DC Characteristics in AlGaN/GaN Heterojunction Field-Effect Transistors Employing AlN Spacer Layer

AlGaN/GaN heterojunction field-effect transistors (HJFETs) employing an AlN spacer layer have been fabricated. By employing the AlN spacer layer, the two-dimensional electron gas (2DEG) mobility increased from 650 cm2/(Vs) to 950 cm2/(Vs). With regard to DC operating characteristics, the maximum transconductance gmmax increased from 100 mS/mm to 125 mS/mm. In addition, the Schottky barrier at the gate electrode improved, and the saturation current Is decreased from 1.0×10-9 A to 6×10-11 A.

Optical and Structural Quality of GaAs Epilayers from Gallium, Bismuth Mixed Solvents by Liquid Phase Epitaxy

Liquid phase epitaxy (LPE) of GaAs epilayers grown at 645°C was studied for different Bi compositions in Ga solvent . Mixed solvents such as Ga+Bi were used to improve GaAs epilayer quality. The grown layers were characterized by photoluminescence (PL) and etch pit density (EPD) measurements. The Bi composition was varied from 0 to 100% in Ga. Good quality epilayers were obtained for 100% Bi composition. PL investigations revealed that the epilayer grown using equal atomic percentage of Ga and Bi had good crystallinity, which is comparable with the 100%-Ga-grown epilayer. The PL measurements reveal that carbon inclusion in the epilayer grown using 50% Ga and 50% Bi solution is comparably less than other solvent compositions. The EPD was reduced to more than half order of magnitude when the solvent was changed from Ga to Bi. The EPD results of epilayers grown using mixed solvents are reported for the first time. The EPD of the GaAs epilayer grown using 50% Ga and 50% Bi solution is markedly reduced compared to the other Ga+Bi mixed solvents, and it is almost equal to the EPD of 100%-Bi-grown epilayers.