Sean Joseph Hearne

The Dynamic Competition Between Stress Generation and Relaxation Mechanisms During Coalescence of Volmer-Weber Thin Films

Real-time measurements of stress evolution during the deposition of Volmer–Weber thin films reveal a complex interplay between mechanisms for stress generation and stress relaxation. We observed a generic stress evolution from compressive to tensile, then back to compressive stress as the film thickened, in amorphous and polycrystalline Ge and Si, as well as in polycrystalline Ag, Al, and Ti. Direct measurements of stress relaxation during growth interrupts demonstrate that the generic behavior occurs even in the absence of stress relaxation. When relaxation did occur, the mechanism depended sensitively on whether the film was continuous or discontinuous, on the process conditions, and on the film/substrate interfacial strength. For Ag films, interfacial shear dominated the early relaxation behavior, whereas this mechanism was negligible in Al films due to the much stronger bonding at the Al/SiO2 interface. For amorphous Ge, selective relaxation of tensile stress was observed only at elevated temperatures...

Stress evolution during metalorganic chemical vapor deposition of GaN

The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.

Stress and Defect Control in GaN Using Low Temperature Interlayers

In organometallic vapor phase epitaxial growth of Gail on sapphire, the role of the low- temperature-deposited interlayers inserted between high-temperature-grown GaN layers was investigated by in situ stress measurement, X-ray diffraction, and transmission electron microscopy. Insertion of a series of low temperature GaN interlayers reduces the density of threading dislocations while simultaneously increasing the tensile stress during growth, ultimately resulting in cracking of the GaN film. Low temperature AIN interlayers were found to be effective in suppressing cracking by reducing tensile stress. The intedayer approach permits tailoring of the film stress to optimize film structure and properties.

Tensile stress evolution during deposition of Volmer–Weber thin films

A simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer–Weber mechanism. The generation of a tensile stress was attributed to the impingement and coalescence of growing islands, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism. To model the process of island coalescence, finite element methods were employed and yielded average tensile stresses more consistent with experimental observations than those predicted using previously reported analytical models. A computer simulation was developed that models the process of film growth as the continuous nucleation of isolated islands, which grow at a constant rate to impinge and coalesce to form a continuous polycrystalline film. By incorporating the finite element results for stress generation and a microstructure-dependent stress relaxationmodel, the simulation qualitatively reproduced the complex temperature-dependent trends observed from in situ measurements of stress evolution during the deposition of Ag thin films. The agreement includes simulation of the decreasingstress relaxation rate observed during deposition at increasing temperatures.

Control and Elimination of Cracking of AlGaN Using Low-Temperature AlGaN Interlayers

We demonstrate that the insertion of low-temperature AlGaN interlayers is effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks during growth of high-temperature AlGaN directly upon GaN epilayers. Stress evolution and relaxation is monitored using an in situ optical stress sensor. The combination of in situ and ex situ characterization techniques enables us to determine the degree of pseudomorphism in the interlayers. It is observed that the elastic tensile mismatch between AlGaN and GaN is mediated by the relaxation of interlayers; the use of interlayers offers tunability in the in-plane lattice parameters.

Brittle-ductile relaxation kinetics of strained AlGaN/GaN heterostructures

The authors have directly measured the stress evolution during metal organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to directly determine a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained Al{sub x}Ga{sub 1{minus}x}N on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation.

Misfit dislocation formation in the AlGaN∕GaN heterointerface

Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨1123⟩∕{1122} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mec...

Mechanisms inducing compressive stress during electrodeposition of Ni

The evolution of stress during electrodeposition of Ni films on Au substrates has been investigated as a function of bath chemistry and deposition conditions to examine the microstructural origins of the compressive stress observed during deposition from an additive-free sulfamate bath. Three likely mechanisms for the generation of compressive stress in this system were investigated: interstitial hydrogen/impurity incorporation, capillarity stress, and a chemical-potential gradient driven atom incorporation model. Only the last model, the chemical-potential gradient model, could not be discounted as the active mechanism. However, further study is required to verify that this is the primary compressive stress generation mechanism.

Competition between tensile and compressive stress creation during constrained thin film island coalescence

Various analytical models have been proposed to predict the tensile stress created when discrete islands contact during a Volmer-Weber thin film growth. Past efforts to experimentally validate these models have been hindered by the stochastic nucleation of islands, which results in coalescence over a large distribution of times and length scales. To avoid this we systematically varied island geometries using electrodeposition of Ni islands on lithographically patterned conductive substrates (Au film on Si), which allowed for independent control of island size and growth rate. Using this technique, we previously demonstrated that most of the coalescence stress occurred after the initial contact of the neighboring islands, reaching a steady state when the film surface became nearly planar. In this work, we expand on these initial results to examine the kinetics of the coalescence process and to systematically evaluate the stress transition from discrete islands to a planar film. The steady state stress in p...

Microstructural Origins of Saccharin-Induced Stress Reduction in Electrodeposited Ni

In this work, we provide direct evidence of the fundamental mechanism through which saccharin, a standard industrial Ni electroplating additive, reduces stress in electrodeposited Ni. This was accomplished using real-time in situ stress measurements taken during through-mask electrodeposition of Ni films from a bath where the saccharin concentration was varied. This technique facilitated the direct measure of the effect of saccharin on the stress created at the Ni island boundaries. We demonstrated that increased saccharin concentration in a Ni-sulfamate-based bath resulted in a systematic reduction in the tensile grain-boundary coalescence stress. Based on this and ex situ S concentration measurements of the Ni films, we propose that the reduction in tensile stress was the result of a reduction in the grain-boundary energy due to S incorporation at the island boundaries.