Jeremy D. Acord

Tensile stress generation and dislocation reduction in Si-doped AlxGa1−xN films

The effects of Si doping on the evolution of stress in AlxGa1−xN:Si thin films (x≈0.4–0.6) grown on 6H-SiC by metal organic chemical vapor deposition were investigated using in situ wafer curvature measurements. The results were correlated with changes in film microstructure as observed by transmission electron microscopy. The incorporation of Si into the films resulted in a compressive-to-tensile transition in the biaxial stress at the surface, and the magnitude of the tensile stress was found to increase in proportion to the Si concentration. The stress gradient was attributed to Si-induced dislocation inclination resulting from an effective climb mechanism. Si doping also resulted in a decrease in the threading dislocation density in the AlxGa1−xN layers, which was attributed to increased dislocation interaction and annihilation. The model describing tensile stress generated by dislocation effective climb was modified to account for the dislocation reduction and was found to yield an improved fit to th...

In situ observation of coalescence-related tensile stresses during metalorganic chemical vapor deposition of GaN on sapphire

Surface roughness and stress evolution were monitored in situ during the growth of GaN on sapphire substrates using low-temperature AlN buffer layers of varying thickness. A reduction in buffer layer thickness decreases the concentration of GaN nucleation sites which in turn increases the time to nuclei coalescence, thus varying the temporal evolution of surface roughness. By monitoring the accompanying changes in stress evolution, it is shown that island coalescence consisting of initial contact followed by subsequent surface roughness reduction is a source of tensile stress during growth of GaN films on sapphire. Such delayed coalescence also leads to an improvement in the structural properties of the material.

In situ measurement of stress generation arising from dislocation inclination in AlxGa1−xN:Si thin films

The effect of Si-doping on the stress and microstructure of AlxGa1−xN (x≈0.39–0.45) films grown by metalorganic chemical vapor deposition on SiC substrates was investigated. In situ measurements revealed a compressive-to-tensile transition of the stress state at the film surface upon the addition of SiH4 during growth, which correlated with a change in the angle of inclination of threading dislocations in the film. The magnitude of the in situ measured stress gradient was comparable to that predicted by the dislocation effective climb model, suggesting that dislocation inclination is the dominant mechanism responsible for tensile stress generation in the films.

Thermal factors influencing the reliability of GaN HEMTs

Gallium Nitride high-electron mobility transistors (HEMT) devices show great promise in their ability to tolerate the high temperature environments of advanced radar systems. This paper examines how GaN HEMT junction temperature determination can vary, owing to factors such as packaging variability, measurement error, and uncertainty in material property data. To demonstrate the impact of these variables, this paper uses practical examples of infrared thermography, micro-Raman thermography, device transient electro-thermal response analysis on GaN HEMT devices, and finite element analysis (FEA). These variations in temperature are combined into a probability model to estimate how life prediction will change as a function of these various factors.

Growth of GaN films on GaAs substrates in an As-free environment

We investigated growth of GaN films on [001] GaAs substrates by plasma-assisted molecular beam epitaxy in an As-free chamber. The crystalline quality and the surface morphology of the films were studied with x-ray diffraction and transmission electron, scanning electron, and atomic force microscopes. We determined that direct GaN deposition on the thermally desorbed substrate resulted in the growth of a polycrystalline film containing misoriented grains and inclusions. We achieved a significant improvement of the film quality by adopting a procedure consisting of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth.

Stress and morphology evolution during the heteroepitaxial growth of group III-nitrides

In this study, in-situ wafer curvature measurements were used to monitor the evolution of film stress during MOCVD growth of GaN and AlGaN. These studies were carried out using a multi- beam optical stress sensor (MOSS) incorporated onto a custom-designed vertical cold-wall MOCVD reactor. The MOSS system provides real-time information on growth rate and changes in substrate curvature which are related to film stress via a modified version of Stoneys equation. Post-growth structural characterization including atomic force microscopy, X-ray diffraction and transmission electron microscopy was used to correlate measured changes in film stress to film morphology evolution.

Observation of Second-Harmonic Generation from Wurzite Al x Ga 1-x N Multilayers in Reflection Geometry

We have observed second-harmonic generation from Al_xGa_1-xN multilayers in reflection geometry and measured the ratio between two elements of the second-order susceptibility tensor.

Evolution of Threading Dislocations in GaN Films Grown on (111) Si Substrates with Various Buffer Layers

Compared to commonly used sapphire and SiC, (111) Si offers several advantages as a substrate for Group-III nitride growth including high quality, low cost, large wafer size, and the potential integration of nitride devices with Si microelectronics. Therefore, growth of GaN on (111) Si is of great interest at the present time. In order to prevent the reaction between Ga and Si and improve the wetting properties of the film, a thin (~100 nm) AlN layer is often used as the buffer between the GaN film and the Si substrate. Due to the tensile coefficient of thermal expansion (CTE) mismatch stress and the additional tensile growth stress arising from island coalescence and lateral grain growth, cracks often form in thick (>~250 nm) GaN films grown on Si using thin AlN buffer layers. For example, a crack is evident in Fig. 1 (a), a bright-field cross-sectional transmission electron microscopy (TEM) image of a GaN/Si heterostructure with a thin AlN buffer layer. By using a compositionally graded Al1-xGaxN buffer layer instead of the AlN buffer, a compressive lattice mismatch stress can be induced to offset the tensile CTE mismatch stress and growth stress, thus suppressing the cracking [1-4]. In this work, the effects of Al1-xGaxN buffer layers and Al1xGaxN/GaN/AlN buffer combinations on the evolution of threading dislocations (TDs) in GaN films are studied.

Preparation and evaluation of damage free surfaces on silicon carbide

A new chemical mechanical polishing process (ACMP) has been developed by the Penn State University Electro-Optics Center for producing damage free surfaces on silicon carbide substrates. This process is applicable to the silicon face of semi-insulating, conductive, 4H, 6H, onaxis and off-axis substrates. The process has been optimized to eliminate polishing induced selectivity and to obtain material removal rates in excess of 150nm/hour. The wafer surfaces and resultant subsurface damage generated by the process were evaluated by white light interferometery, Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and epitaxial layer growth. Residual surface damage induced by the polishing process that propagates into the epitaxial layer has been significantly reduced. Total dislocation densities measured on the ACMP processed wafers are on the order of the densities reported for the best as grown silicon carbide crystals [1]. Characterization of high electron mobility transistors (HEMTs) grown on these substrates indicates that the electrical performance of the substrates met or exceeded current requirements [2].