C. A. Wolden

High rate and selective etching of GaN, AlGaN, and AlN using an inductively coupled plasma

The etching behavior of gallium nitride (GaN), aluminum gallium nitride (AlxGa1−xN), and aluminum nitride (AlN) has been systematically examined in an inductively coupled plasma (ICP) using Cl2 and Ar as the reagents. Etch rates were strongly influenced by ICP power and dc bias, while relatively insensitive to pressure, flow rate, and gas composition. Maximum etch rates of 9800 A/min for GaN, 9060 A/min for Al0.28Ga0.72N, and 7490 A/min for AlN were attained. The etch profiles were highly anisotropic over the range of conditions studied. The dc bias had to exceed certain voltages before significant etch rates were obtained. These values were −50 V for AlN. As such, increasing selectivity for GaN over Al0.28Ga0.72N and AlN was achieved at dc biases below −40 V. At −20 V, the GaN etch rates were 38 times greater than AlN and a factor of 10 greater than Al0.28Ga0.72N. These results demonstrate the importance of ion bombardment in the etching of these materials.

HETEROEPITAXIAL NUCLEATION OF DIAMOND ON NICKEL

Abstract Highly oriented diamond has been grown on (100) nickel substrates by the hot filament chemical vapor deposition method. Epitaxial nuclei were obtained by a diamond powder seeding and high temperature annealing process. Since the timing of the process was crucial for the achievement of a high degree of orientation and high density of diamond nuclei, a real-time, in-situ laser reflectometry system was developed to monitor changes in surface morphology observed during the high temperature annealing stage. Characteristic features observed in the intensities of reflected and scattered light were interpreted by comparison with scanning electron micrographs of the samples quenched at sequential stages of the process. It was concluded that the scattered light signal can be effectively used as a process steering parameter. Using this technique, oriented nucleation and growth of diamond on Ni was reproducibly achieved. Auger spectroscopy showed that up to 6 at% of carbon was dissolved in the nickel surface layer. The investigation of interfacial microstructures and phases involved by transmission electron microscopy revealed the formation of Ni4C already in the early stages of nucleation. This phase was manifested as coherent precipitates and is believed to have been the precursor for diamond nucleation. Perfectly epitaxial diamond was grown by this process. The epitaxial relationship was determined by cross-sectional transmission electron microscopy and selected area diffraction analysis.

Control of diamond heteroepitaxy on nickel by optical reflectance

Real time in situ laser reflectometry was used to investigate changes in surface morphology observed during the nucleation of oriented diamond on Ni in a hot filament chemical vapor deposition reactor. Characteristic features observed in the intensities of reflected and scattered light were interpreted by comparison with scanning electron micrographs of the diamond seeded substrates quenched at sequential stages of the process. Based on this analysis, a process was developed in which the scattered light signal was used as a steering parameter. Using this process, oriented nucleation and growth of diamond on Ni can be repeatedly achieved.

Textured diamond growth by low pressure flat flame chemical vapor deposition

A laminar, flat‐flame burner has been constructed for the deposition of diamond films using an acetylene/oxygen flame at reduced pressures. The stagnation flow results in uniform deposition conditions across the diameter of the burner. Under certain operating conditions the growth rate and morphology are primarily controlled by the substrate temperature. Conditions for the deposition of <100≳ textured films are reported for a flat‐flame system. From growth rate measurements an apparent activation energy of 22 kcal/mol was observed. The maximum growth rate of 5.5 μm/h is the highest reported to date for a low pressure combustion system.

In situ mass spectrometry during diamond chemical vapor deposition using a low pressure flat flame

A combination of experiments and detailed kinetic modeling was used to investigate diamond deposition chemistry in low pressure combustion synthesis. Microprobe sampling was employed to provide in situ , quantitative measurements of the stable gas-phase species impinging the growth surface. The reactant gas ratio was found to be the most critical experimental variable. A detailed kinetic model was developed for the stagnation flow system. Comparison of experimental measurements showed very good agreement with model predictions. The model was then used to predict the concentration of radical species and analyze the sensitivity of predictions to γH, the probability of atomic hydrogen recombination on the surface. It was shown that γH dramatically affects the distribution of radical species near the diamond surface. The analysis also indicates that atomic carbon may be an important gas-phase precursor in this system. Comparison of mole fraction measurements and observations of film morphology were used to draw conclusions on the growth mechanism.

Highly oriented diamond deposited using a low pressure flat flame

Abstract A multi-step process for the achievement of highly oriented, 〈100〉 textured diamond films on silicon using flat flame deposition has been developed. First, a bias-enhanced technique was used to achieve oriented nuclei on a Si 〈100〉 substrate in a microwave plasma reactor. Substrates were then transferred to the combustion system and rapidly grown into coalesced 〈100〉 films at a growth rate of 4–5 μm/h. X-ray texture analysis was used to characterize the films. It showed a 12 ° misalignment of the crystallites with respect to the surface normal, while the azimuthal misalignment was measured to be 20 °.

Surface melting in the heteroepitaxial nucleation of diamond on Ni

Abstract Surface melting associated with the heteroepitaxial nucleation of diamond on Ni was investigated. Scanning electron microscopy of quenched samples revealed flow patterns and a recrystallized surface morphology. A combination of techniques including in situ optical monitoring, differential thermal analysis, Auger depth profile analysis, and cross-section transmission electron microscopy (TEM) analysis were performed to identify the nature of the molten layer. Data obtained from different experiments were in good mutual agreement. All experimental results strongly indicated that a molten Ni–C–H surface layer was involved in the nucleation process. The presence of both carbon and atomic hydrogen played an important role in the depression of the melting point which was measured to be >300°C less than the melting point of pure Ni.

Low-temperature deposition of optically transparent diamond using a low-pressure flat flame

Abstract The radial uniformity and scaleable nature of flat flames make them an attractive technique for diamond deposition. Due to the high temperatures involved in combustion synthesis, typically molybdenum and silicon have been used as substrates. Here we report low-temperature diamond deposition on glass substrates. Diamond deposition was achieved on ordinary sodium silicate glass at substrate temperatures of ∼500°C; however, film delamination occurred during cooling after deposition. Vycor™ and Pyrex™ are two glasses that have thermal expansion coefficients that are similar to diamond. Continuous, optically transparent films were successfully deposited on both glasses. The diamond films have been characterized by scanning electron microscopy, Raman spectroscopy and secondary ion mass spectroscopy (SIMS). The dependence of hydrogen and sp 2 -bonded carbon incorporation in the films on reactant composition was quantified. These films were optically transparent and showed good adhesion as measured by a simple tape test.

Combustion Flame Deposition of Diamond

Combustion synthesis is one of the competing chemical vapor deposition (CVD) technologies for diamond film growth. It was invented in 1988 by Hirose [3.1], who realized that acetylene flames produce copious amounts of atomic hydrogen and hydrocarbon radicals that are required for diamond growth. The discovery was subsequently confirmed at the Naval Research Laboratory [3.2] and has become the subject of intensive study. It has been argued that combustion synthesis is the most flexible of the CVD alternatives because of its scaleable nature, minimal utility requirements, and significantly reduced capital costs relative to plasmaaided processes [3.3]. In this chapter, the development of combustion CVD will be discussed from its inception, when conventional welding torches were used, to its present implementation with flat-flame burners. The unique features of the combustion CVD system are highlighted. The key experimental parameters, and their impact on deposition, are discussed in detail. The chemistry of combustion diamond CVD and its relation to the deposition mechanism is analyzed through a combination of in-situ diagnostics and detailed reactor modeling. Lastly, examples of the implementation of combustion synthesis for specific applications are provided.

Coalesced oriented diamond films on nickel

The growth of coalesced, highly oriented diamond films has been achieved on nickel substrates using a multistep process that consisted of (i) seeding the Ni surface with 0.5 μm diamond powder, (ii) annealing at 1100 °C in a hydrogen atmosphere, and (iii) growth at 900 °C in a mixture of hydrogen and 0.5% methane. Auger depth profile analysis of a sample quenched after the annealing stage showed the presence of significant amounts of carbon (6 at. %) close to the substrate surface and about 3 at.% deeper in the substrate. The loss of carbon into the substrate resulted in relatively low nucleation density. The addition of methane into the gas phase during the annealing stage proved very effective in compensating for the diffusion. An addition of 0.5% methane in the gas phase produced optimum results, as the nucleation density, orientation of diamond particles, and uniformity were substantially improved. Substrates nucleated under these conditions were grown out into coalesced, 30 μm thick films. Both (100) and (111) oriented films showed a high degree of orientation and Raman spectra obtained from these orientations showed intense and narrow diamond signature peaks with FWHMs of 5 and 8 cm -1 , respectively.