Christine C. Mitchell

Low-dislocation-density GaN from a single growth on a textured substrate

The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 10{sup 9}/cm{sup 2}. Such high dislocation densities degrade both the electronic and photonic properties of the material. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy (CE), which employs prepatterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is prepatterned with narrow lines and etched to a depth that permits coalescence of laterally growing III-N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.

Improved brightness of 380 nm GaN light emitting diodes through intentional delay of the nucleation island coalescence

Ultraviolet light emitting diodes (LEDs) have been grown using metalorganic vapor phase epitaxy, while monitoring the 550 nm reflected light intensity. During nucleation of GaN on sapphire, the transition from three-dimensional (3D) grain growth to two-dimensional (2D) coalesced growth was intentionally delayed in time by lowering the NH3 flow during the initial high temperature growth. Initially, when the reflectance signal is near zero, the GaN film is rough and composed of partly coalesced 3D grains. Eventually, the reflected light intensity recovers as the 2D morphology evolves. For 380 nm LEDs grown on 3D nucleation layers, we observe increased light output. For LEDs fabricated on GaN films with a longer recovery time an output power of 1.3 mW at 20 mA current was achieved.

Plan-view image contrast of dislocations in GaN

We demonstrate that when vertical threading dislocations in (0001) GaN are imaged in plan-view by transmission electron microscopy, a surface-relaxation contrast operates in addition to that due to the strain fields of dislocations passing through the specimen. We show that all three dislocation types (edge, screw, and mixed) can be detected in the same image using g=(1120) and 18° specimen tilt from [0001], allowing total densities to be assessed properly. The type of an individual dislocation can also be readily identified.

Mass transport and kinetic limitations in MOCVD selective-area growth

Abstract The interplay between transport and kinetics in selective-area growth (SAG) of compound semiconductors is discussed. A thin-film model describing transport of reactants across the boundary layer above the growth surface is developed. A dimensionless Damkohler number ( Da ) quantitatively determines whether the planar (blanket) deposition is in a transport-limited, reaction-rate-limited, or intermediate operating regime. Reactant profiles within the rotating-disk reactor and growth rates predicted by the thin-film model agree very well with numerically exact calculations. The efficiency of the SAG was found to be a strong function of both the Da and the pattern fill-factor ( θ ). The thin-film model was extended to take into account the lowering of the “effective rate constant” (averaged over both the exposed and masked zones). It was found that the product θDa governs the transition between transport and kinetic control of the SAG process. Predictions of the analytical SAG thin-film model were compared to both the numerically exact 2-D calculations and to experimental results from InGaAs, InP, and GaN SAG. The simple theory appears to provide an excellent qualitative and quantitative description of kinetic and transport effects in SAG.

Mass transport in the epitaxial lateral overgrowth of gallium nitride

Abstract We have investigated lateral transport mechanisms in epitaxial lateral overgrowth (ELO) of GaN grown by Metal organic chemical vapor deposition (MOCVD). Portions of a pre-grown GaN buffer layer are patterned with a dielectric mask material, silicon nitride. Further growth of GaN occurs selectively on exposed areas of the underlying buffer layer, and not on the dielectric material. Growth-rate enhancement on the exposed GaN is observed due to lateral transport of material from the masked regions. We describe experiments to distinguish whether the lateral transport of material occurs via gas-phase diffusion or surface diffusion, either on the mask itself or on the epitaxial material. Deep trenches were etched into the wafer prior to the ELO growth, designed to interrupt lateral transport if it were occurring by diffusion along the surface. ELO growth rate profiles on exposed line patterns and on larger area blanket growth zones were examined with and without the trenches. Growth profiles were virtually identical independent of the presence of the trench features. These results indicate that gas-phase diffusion dominates the transport of material during GaN ELO.

Minimizing threading dislocations by redirection during cantilever epitaxial growth of GaN

A 40-fold reduction in density of vertical threading dislocations (VTDs) at the surface of GaN is obtained with cantilever epitaxy by using narrow (<1 μm) mesas etched into a sapphire substrate and conditions producing angled {11-22} facets to initiate growth by metalorganic chemical vapor deposition. These two techniques redirect VTDs over the mesas to the horizontal and away from device areas above. Further reductions appear possible if the facets uniformly cover all mesas prior to cantilever growth.

In situ measurements of GaN nucleation layer decompostion

GaN nucleation layer (NL) decomposition was measured using optical reflectance over a wide range of pressure P, temperature T, and H2/NH3 gas mixture. The GaN NLs show measurable decomposition above 800 °C and do not significantly roughen until above 960 °C. The NL decomposition rates increase with increasing P, increasing T, and decreasing NH3 flow. An activation energy EA of 2.68 eV was measured (from 820 to 960 °C) for NL decomposition and an EA of 2.62 eV was measured (from 900 to 1075 °C) for decomposition of thick, high-T bulk GaN films. Depending on P, the pre-exponential factor A0 was four to nine times larger for NL decomposition compared to bulk GaN decomposition. The EA measured for both NL and bulk GaN decomposition in mixed H2 and NH3 flows is similar to the EA for Ga desorption, suggesting that the rate-limiting step for both NL and bulk GaN decomposition is Ga desorption.

GaN to AlN: materials for deep-UV emitters

Optical sources emitting between 280 to 340nm will enable the development of compact chemical and biological sensors. Emitters for these emission wavelengths require AlGaN alloys with Al compositions ranging from around 10% to more than 50%. The growth of high Al containing alloys can be difficult due to parasitic reactions that result in poor Al incorporation efficiency and nonlinear growth rates.

Cantilever Epitaxy of GaN on Sapphire: Further Reductions in Dislocation Density

The density of vertical threading dislocations at the surface of GaN grown on sapphire by cantilever epitaxy has been reduced with two new approaches. First, narrow mesas ( 7 /cm 2 was achieved by combining these two approaches. We also suggest other developments of cantilever epitaxy for reducing dislocations in heteroepitaxial systems.

AlGaN Materials Engineering for Integrated Multi-function Systems

This LDRD is aimed to place Sandia at the forefront of GaN-based technologies. Two important themes of this LDRD are: (1) The demonstration of novel GaN-based devices which have not yet been much explored and yet are coherent with Sandias and DOEs mission objectives. UV optoelectronic and piezoelectric devices are just two examples. (2) To demonstrate front-end monolithic integration of GaN with Si-based microelectronics. Key issues pertinent to the successful completion of this LDRD have been identified to be (1) The growth and defect control of AlGaN and GaN, and (2) strain relief during/after the heteroepitaxy of GaN on Si and the separation/transfer of GaN layers to different wafer templates.