Terrace Size Distribution Research Articles

Enhanced indium adsorption and surface evolution of semi-polar (11–22) LED via a strain periodic alternating superlattice (SPAS-L)

To date, considerable research has been devoted to semipolar (11–22) III-N based-light-emitting diodes due to the high demand for the future generation of high indium adsorption materials. However, higher indium content can generate stacking fault defects and partial dislocation that deteriorate the crystal, morphological, and optical quality. This study describes the effect of indium flux and two-dimensional nanoscale strain periodic alternating superlattices on the surface evolution during the growth of semi-polar (11–22) InGaN light-emitting diodes. Semi-polar (11–22) light-emitting diodes were grown on different templates with and without AlN/GaN strain periodic alternating superlattices. A direct correlation between the surface evolution, crystal quality, and optical properties was demonstrated, suggesting higher indium adsorption with increased crystallinity. Moderate indium flux induced lower surface undulation with intensified photoluminescence emission in the green range. Substantial improvement in the crystal quality of semi-polar (11–22) InGaN light-emitting diode was achieved using AlN/GaN strain periodic alternating superlattice. A sample with an AlN/GaN strain periodic alternating superlattice had the lowest terrace size density of ≤ 100 nm2 with no terrace sizes > 400 nm2, indicating a homogeneous terrace size distribution. In addition, the AlN/GaN template improved the optical properties, red shifting the photoluminescence to almost 15 nm from 495 nm (without superlattices). These findings demonstrate the potential of obtaining high indium adsorption semi-polar light-emitting diodes on large-scale sapphires with inexpensive feasibility.

Nanopatterning of GaAs(110) vicinal surfaces by hydrogen-assisted MBE

ABSTRACTThe effect of atomic hydrogen on the growth mode and surface morphology of GaAs(110) thin films grown by molecular beam epitaxy (H-MBE) has been studied for different kinetic regimes using atomic force microscopy (AFM). Growth in the Ga supply-limited regime after H-assisted oxide removal leads to the formation of multi-atomic step arrays by step bunching with a very uniform terrace size distribution in the 80 nm range. Growth under As-deficient conditions after H-assisted oxide removal induces a rapid self-organization of the GaAs(110) surface into a ridge pattern along the <001> tilt direction, which is broken down into a 3D mound morphology when H is also present during growth. A chacteristic nanofacetting of the surface with very straight <1–10> -type steps is observed at high temperatures regardless of atomic hydrogen being used during oxide desorption and/or epitaxial growth.

Layer-by-layer growth of GaAs( [formula omitted]) studied by in situ synchrotron X-ray diffraction

We investigate the time-dependent surface evolution during molecular beam epitaxy of GaAs(0 0 1) using synchrotron X-ray diffraction at a newly-built dedicated beamline at the synchrotron BESSY II. The crystal truncation rods analyzed at growth temperature agree with the room-temperature β(2×4) reconstruction published in the literature. The layer coverage evolution during growth is analyzed by fitting the oscillating intensity along crystal truncation rods. Our results show that the structure of the reconstructed surface unit cell does not change during growth. Using numerical simulations, we determine the terrace size distribution on the surface at growth temperature and verify the validity of our analysis for multi-level initial surfaces, as long as the mean terrace size is larger than the nucleation distance during growth.

Step and kink correlations on vicinal Ge(100) surfaces investigated by electron diffraction

Using spot profile analysis in low-energy electron diffraction, we have investigated vicinal Ge(100) surfaces, which were miscut by 2.7° and 5.4°, respectively, in [011] direction with respect to the surface normal. Within the kinematic approximation the morphology was evaluated quantitatively both perpendicular and parallel to the step edge direction. In contrast to vicinal Si(100) surfaces with similar miscut angles, the Ge(100) surfaces still show an alternating configuration of (2×1) and (1×2) reconstructed (100) terraces, which are separated by steps of single atomic height. From the spot profiles and their energy dependence we extracted the morphological parameters such as the average terrace width, the variance of the terrace size distribution, and the average kink separation. Furthermore, step energies on the vicinal Ge(100) surfaces were estimated. These turn out to be significantly lower than for Si(100) and lead to the formation of the observed double domain structure.

Structure and morphology of concave-shaped surfaces on 6H–SiC(0 0 0 1) after H 2 etching

Concave-shaped surfaces have been prepared on 6H–SiC(0 0 0 1) substrates by exposing troughs, prepared by dimple grinding of flat SiC surfaces, to H 2 at high temperature. The morphological changes occurring under this H 2 etching were studied using scanning electron microscopy (SEM), atomic force microscopy (AFM) and ultrahigh vacuum scanning tunnelling microscopy (UHV-STM). Already after heating in H 2 at 1600 °C for 20 min, morphological changes are observed and heating in H 2 at 1700 °C for 15 min leads to the formation of alternating large and small terraces, separated by straight steps of 0.75 nm height on the flat parts of the substrate. A model is proposed which ascribes this bimodal terrace size distribution to the atomic structure of the SiC steps and to H diffusion on the terraces. For the same etching conditions less significant morphological variations were detected on the curved areas of the surface, where dimple grinding had been performed. The beginning of a considerable structural reorganisation of the concave-shaped surfaces was detected only when the etching temperature was increased to 1700 °C and the etching time to 1 h. Then, terrace steps with preferential edge orientation along the family of 1 1 2 ̄ 0 directions are formed in the concave-shaped areas, as observed by SEM.

Simple analysis of spot splitting due to diffraction at surfaces with atomic steps

Probing surface morphology by diffraction techniques is well established, especially for atomic steps on bare surfaces or on adlayer covered surfaces. The diffuse scattering contains information about the terrace size distribution. The sharpness of this distribution determines the shape of the spot profiles. While one observes a single broad shoulder for broad distributions, the shoulder splits into satellites for sharp distributions. For sharp distributions the average terrace size determines mainly the satellite position while the half-width of the satellites is governed by the standard deviation of the distribution. For broad distributions the situation is vice versa. Generally the shoulder can be fitted very well by two satellites with Lorentzian shape so that one can easily obtain the two prominent parameters of the terrace size distribution (average terrace size and standard deviation) from the splitting of the shoulder and the half-widths of the satellites. Additionally, this model is extended to two-dimensional surfaces with step trains. For a single step train with fluctuating steps the diffraction pattern shows a weak diffuse shoulder besides the strong satellites due to the average step-step distance. This effect is also predicted for crossing step trains which mimic mounds on the surface. Therefore equivalent to the one-dimensional case, the average terrace size and the local slope of the surface can be analysed from the satellite position and their half-width.

Properties of kinks in vicinal face-centered cubic (111) surfaces

We study properties of vicinal fcc(111) surfaces by means of kinetic Monte Carlo simulations where both the initial and the final local atomic environments are used to determine the hopping probability. By studying the terrace size distribution as a function of annealing temperature and surface vicinality, we conclude that an effective attractive interaction between steps is present at lower temperatures and smaller terrace lengths but is less important at higher temperatures and larger terrace lengths, as a result of the local pairwise interactions between neighbouring atoms. Kink motion was measured from the time-dependent correlation function G K ( t) ∼ t 1/4 after the number of kinks had reached equilibrium, which confirms that the step relaxation in this regime is one-dimensional. Furthermore, a detailed balance calculation is performed which gives the kink formation energy.

Step structure and dimer row correlations in vicinal Si(100)

The surface structures of 7°, 3.5°, and 2.5° vicinal Si(100) samples tilted about [011̄] have been studied with low‐energy electron diffraction. Double‐atomic‐height steps can be formed on these surfaces after sufficient heat treatment. Angular profile analysis of the integral‐order diffracted beams is used to determine step height, average terrace size, terrace size distribution, and degree of step edge roughness. Angular profile analysis of the fractional‐order diffracted beams is used to determine the relative fraction of single‐atomic‐height steps and to measure the degree of correlation in the (2×1) dimer row shifts. The latter is seen as a splitting in the (2×1) beam that is one‐half the splitting distance in the (00) beam. This splitting is observed to be reversible with temperature, indicating a phase transition in the configuration of (2×1) dimer row shifts.

Determination of terrace size and edge roughness in vicinal Si{100} surfaces by surface-sensitive diffraction

The capabilities of high-resolution surface-sensitive electron diffraction to determine quantitatively the terrace size, terrace size distribution, and terrace-edge roughness are explored and illustrated with measurements on a vicinal near-{100} Si surface. It is shown that terrace edge roughness can be determined with the appropriate measurement. To explain measurements on the Si surface, shadowing, terrace-edge distortion, double-height steps, unequal areas of adjoining terraces, and multiple scattering are considered. The influence of the instrument response on measurement precision is discussed.

Diffraction from surfaces with randomly distributed steps

A closed-form solution is obtained for the angular distribution of intensity in diffraction from a surface on which the terrace size distribution is given by the geometric distribution, i.e. a surface in which the occurrence of steps is random. Several distributions of step heights that are integral multiples of the monatomic step height are considered. It is shown that a random occurrence of monatomic steps will cause some multiatomic steps. If a very broad distribution of step heights is assumed, the beam width no longer oscillates with energy but approaches a constant value except at the characteristic energies of zero width. Comparisons are made with a previous model and with measurements on GaAs(110).