A. Munkholm

In situ, real-time measurement of wing tilt during lateral epitaxial overgrowth of GaN

By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO2 mask. This was done by repeatedly performing line scans through the 1013 peak in the direction perpendicular to the [1010]GaN stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of ∼1.19° after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to ∼1.36°, indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and stress state during early lateral overgrowth must be considered as possible origins of wing tilt.

Observation of growth modes during metal-organic chemical vapor deposition of GaN

We present real-time surface x-ray scattering measurements during homoepitaxial growth of GaN by metal-organic chemical vapor deposition. We observed intensity oscillations corresponding to the completion of each monolayer during layer-by-layer growth. The growth rate was found to be temperature independent and Ga-transport limited. Transitions between step-flow, layer-by-layer, and three-dimensional growth modes were determined as a function of temperature and growth rate.

Epitaxial Pb(Mg1/3Nb2/3)O3 thin films synthesized by metal-organic chemical vapor deposition

Metal-organic chemical vapor deposition was used to prepare Pb(Mg1/3Nb2/3)O3 (PMN) thin films on (001) SrTiO3 and SrRuO3/SrTiO3 substrates, using solid Mg β-diketonate as the Mg precursor. Parameters including the precursor ratio in the vapor phase, growth temperature, growth rate, and reaction pressure in the reactor chamber were varied in order to determine suitable growth conditions for producing phase-pure, epitaxial PMN films. A cube-on-cube orientation relationship between the thin film and the SrTiO3 substrate was found, with a (001) rocking curve width of 0.1°, and in-plane rocking-curve width of 0.8°. The root-mean-square surface roughness of a 200-nm-thick film on SrTiO3 was 2 to 3 nm as measured by scanning probe microscopy. The zero-bias dielectric constant and loss measured at room temperature and 10 kHz for a 200-nm-thick film on SrRuO3/SrTiO3 were approximately 1100 and 2%, respectively. The remnant polarization for this film was 16 μC/cm2.

Determination of the cubic to hexagonal fraction in GaN nucleation layers using grazing incidence x-ray scattering

Recent electron diffraction and microscopy studies of GaN nucleation layers have shown that faults in the stacking of the close-packed planes result in the coexistence of cubic and hexagonal phases within the layers. Using grazing incidence x-ray scattering, we have quantified the proportion of the cubic and hexagonal phases throughout the nucleation layer. We compare the structure of a 20 nm nucleation layer grown on sapphire by atmospheric pressure metal-organic chemical vapor deposition at 525 °C to that of an identical layer heated to 1060 °C. The fractions of cubic and hexagonal phases in the layers are determined by a comparison of the scattering data with a Hendricks–Teller model. High temperature exposure results in a decrease of the cubic fraction from 0.56 to 0.17. The good agreement with the Hendricks–Teller model indicates that the positions of the stacking faults are uncorrelated.

A search for strain gradients in gold thin films on substrates using x-ray diffraction

The high strengths of gold thin films on silicon substrates have been studied with particular reference to the possible effect of strain gradients. Wafer curvature/thermal cycling measurements have been used to study the strengths of unpassivated, oxide-free gold films ranging in thickness from 0.1 to 2.5 μm. Films thinner than about 1 μm in thickness appear to be weakened by diffusional relaxation effects near the free surface and are not good candidates for the study of strain gradient plasticity. Our search for plastically induced strain gradients was thus limited to thicker films with correspondingly larger grain sizes. Three related x-ray diffraction techniques have been used to investigate the elastic strains in these films. The standard dhkl vs sin2 Ψ technique has been used to find the average strain through the thickness of the films. The results are consistent with wafer curvature measurements. We have also measured a number of dhkl’s as a function of penetration depth to construct depth-depende...

Layer-by-layer growth of GaN induced by silicon

We present in situ x-ray scattering studies of surface morphology evolution during metal–organic chemical vapor deposition of GaN. Dosing the GaN(0001) surface with Si is shown to change the growth mode from step-flow to layer-by-layer over a wide temperature range. Annealing of highly doped layers causes Si to segregate to the surface, which also induces layer-by-layer growth.

In situ x-ray scattering study of PbTiO3 chemical-vapor deposition

We present in situ surface x-ray scattering measurements of PbTiO3 epitaxy by metal–organic chemical-vapor deposition. Oscillations in crystal truncation rod intensity corresponding to layer-by-layer growth are observed under a variety of growth conditions. At lower PbO overpressures, we observe a transition to step-flow growth and an increased rate of recovery after growth, indicating a higher surface mobility.

Transition between the 1×1 and surface structures of GaN in the vapor-phase environment

Abstract Out-of-plane structures of the GaN (0 0 0 1) surface in the metal-organic chemical vapor deposition (MOCVD) environment have been determined using in situ grazing-incidence X-ray scattering. We measured 11 2 l crystal truncation rod intensities at a variety of temperatures and ammonia partial pressures on both sides of the 1×1 to ( 3 ×2 3 ) R 30° surface phase transition. The out-of-plane structure of the ( 3 ×2 3 ) R 30° phase appears to be nearly independent of temperature below the transition, while the structure of the 1×1 phase changes increasingly rapidly as the phase transition is approached from above. A model for the structure of the 1×1 phase with a partially occupied top Ga layer agrees well with the data. The observed temperature dependence is consistent with a simple model of the equilibrium between the vapor phase and the surface coverage of Ga and N. In addition, we present results on the kinetics of reconstruction domain coarsening following a “quench” into the ( 3 ×2 3 ) R 30° phase field.

In situ studies of the effect of silicon on GaN growth modes.

Abstract We present real-time X-ray scattering studies of the influence of silicon on the homoepitaxial growth mode of GaN grown by metal-organic vapor-phase epitaxy. Both annealing of Si-doped GaN and surface dosing of GaN with disilane are shown to change the mode of subsequent growth from step-flow to layer-by-layer. By comparing the growth behavior induced by doped layers which have been annealed to that induced by surface dosing, we extract an approximate diffusion coefficient for Si in GaN of 3.5 ×10 −18 cm 2 / s at 810°C.

Real-time x-ray studies of crystal growth modes during metal-organic vapor phase epitaxy of GaN on c- and m-plane single crystals

Non-polar orientations of III-nitride semiconductors have attracted significant interest due to their potential application in optoelectronic devices with enhanced efficiency. Using in situ surface x-ray scattering during metal-organic vapor phase epitaxy (MOVPE) of GaN on non-polar (m-plane) and polar (c-plane) orientations of single crystal substrates, we have observed the homoepitaxial growth modes as a function of temperature and growth rate. On the m-plane surface, we observe all three growth modes (step-flow, layer-by-layer, and three-dimensional) as conditions are varied. In contrast, the +c-plane surface exhibits a direct crossover between step-flow and 3D growth, with no layer-by-layer regime. The apparent activation energy of 2.8 ± 0.2 eV observed for the growth rate at the layer-by-layer to step-flow boundary on the m-plane surface is consistent with those observed for MOVPE growth of other III-V compounds, indicating a large critical nucleus size for islands.