A. Claudel

CFD modeling of the high-temperature HVPE growth of aluminum nitride layers on c-plane sapphire: from theoretical chemistry to process evaluation

Abstract This study presents numerical modeling based on a relatively limited number of gas-phase and surface reactions to simulate the growth rate of aluminum nitride layers on AlN templates and c-plane sapphire in a broad range of deposition parameters. Modeling results have been used to design particular experiments in order to understand the influence of the process parameters on the crystal quality of AlN layers grown in a high-temperature hydride vapor-phase epitaxy process fed with NH3, AlCl3, and H2. Modeling results allow to access to very interesting local quantities such as the surface site ratio and local supersaturation. The developed universal model starting from local parameters might be easily transferred to other reactor geometry and process conditions. Among the investigated parameters (growth rate, temperature, local supersaturation, gas-phase N/Al ratio, and local surface site N/Al ratio), only the growth rate/supersaturation or growth rate/temperature relationships exhibit a clear process window to use in order to succeed in growing epitaxial AlN layers on c-plane sapphire or AlN templates. Gas-phase N/Al ratio and local surface site N/Al ratio seem to play only a secondary role in AlN epitaxial growth.

Growth and Characterization of Thick Polycrystalline AlN Layers by HTCVD

Thick polycrystalline AlN layers were grown at low pressure using high temperature chemical vapor deposition (HTCVD). The experimental setup consists of a graphite susceptor heated by an induction coil surrounding a vertical cold wall reactor. The reactants used were ammonia (NH(3)) and aluminum chloride (AlCl(x)) species formed in situ via chlorine (Cl(2)) reaction with high purity aluminum wire. AlN films were deposited on a 55 mm diameter graphite susceptor between 1200 and 1600 degrees C. AlN layers have been characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and electron backscattered diffraction. The influence of temperature on growth rate, surface morphology, grain size, and crystalline structure is presented. Growth rates of up to 230 mu m/h have been reached. A nonpolar preferred orientation of AlN films is stabilized at a higher temperature. The potential of investigation in this new range of experimental conditions, i.e., high temperature and high growth rate, as well as deposition of nonpolar AlN crystals, is very promising for epitaxial growth and extends the field of applications

High-speed Growth and Characterization of Polycrystalline AlN Layers by High Temperature Chemical Vapor Deposition (HTCVD)

The HTCVD set-up consists of a graphite susceptor heated by induction in a home-built vertical cold-wall reactor working at low pressure [1-3]. The precursors used are ammonia NH3 and aluminium chlorides AlClx species formed in situ via Cl2 reaction with high purity Al wire. AlN films were deposited on 55 mm diameter graphite susceptor between 1200 and 1600°C (Figure 1). The influence of deposition temperature on growth rate, surface morphology, grain size and crystalline state is presented. Growth rates of up to 250 μm.h were reached at 1600°C.

Growth of Thick AlN Layers by High Temperature CVD (HTCVD)

To achieve AlN bulk growth, HTCVD chlorinated process is investigated. High growth rate and high crystalline quality are targeted for AlN films grown on (0001) α-Al2O3 and (0001) 4H or 6H SiC substrates between 1100 °C and 1750 °C. The precursors used are ammonia NH3 and aluminium chlorides AlClx species formed in situ by action of Cl2 on high purity Al wire. Both influences of temperature and carrier gas on microstructure, crystalline state and growth rate are presented. Growth rates higher than 190 μm.h-1 have been reached. Thermodynamic calculations were carried out to understand the chemistry of AlN deposition. AlN layers were characterized by SEM and θ/2θ X-Ray Diffraction. Their epitaxial relationships with substrates were deduced from pole figures obtained by X-Ray diffraction on a texture goniometer.

Fabrication of a 4.4 GHz oscillator using SAW excited on epitaxial AlN grown on a Sapphire substrate

Surface Acoustic Wave (SAW) devices are still the preferred solution for the stabilization of on-board frequency source for radar control. The possibility for developing an oscillator delivering a frequency very close to the usual operating band of these devices is considered in this paper. Double-port SAW resonators are built on epitaxial Aluminum Nitride grown onto Sapphire to take advantage of one of the lowest visco-elastic loss material and a high structural quality piezoelectric layer to optimize the resonance of the acoustic wave device. Experimental test vehicles are built using electron-beam lithography, yielding devices operating near 4.5 GHz with Q factor in excess of 3000 and moderate insertion losses. These resonators are used to stabilized feedback loop oscillators yielding noise floor better than -150 dBc/Hz. Among the other possible application of these devices, high temperature sensors may be considered as the growth temperature of the layer is in the range 1000°C - 1600°C.

Influence of the N/Al Ratio in the Gas Phase on the Growth of AlN by High Temperature Chemical Vapor Deposition (HTCVD)

In order to achieve AlN bulk growth, HTCVD chlorinated process is investigated. High growth rate and high crystalline quality are targeted for AlN films grown on (0001) 4H SiC at 1750°C. The precursors used are ammonia NH3 and aluminium chlorides AlClx species formed in situ by action of Cl2 on high purity Al wire. Influences of N/Al ratio in the gas phase on growth rate, crystalline state and microstructure are presented. Growth rates of up to 200 µm/h have been reached for polycrystalline layers. Thermodynamic calculations were carried out and correlated to the experimental results. As-grown AlN layers were characterized by SEM and X-ray Diffraction. Surface morphology is studied by SEM and FEG-SEM and crystallographic orientations were obtained by X-ray diffraction on θ/2θ.