Step Bunching Research Articles

Effect of Growth Pressure on Epitaxial Graphene Grown on 4H-SiC Substrates by Using Ethene Chemical Vapor Deposition.

The Si(0001) face and C(000-1) face dependences on growth pressure of epitaxial graphene (EG) grown on 4H-SiC substrates by ethene chemical vapor deposition (CVD) was studied using atomic force microscopy (AFM) and micro-Raman spectroscopy (μ-Raman). AFM revealed that EGs on Si-faced substrates had clear stepped morphologies due to surface step bunching. However, This EG formation did not occur on C-faced substrates. It was shown by μ-Raman that the properties of EG on both polar faces were different. EGs on Si-faced substrates were relatively thinner and more uniform than on C-faced substrates at low growth pressure. On the other hand, D band related defects always appeared in EGs on Si-faced substrates, but they did not appear in EG on C-faced substrate at an appropriate growth pressure. This was due to the μ-Raman covering the step edges when measurements were performed on Si-faced substrates. The results of this study are useful for optimized growth of EG on polar surfaces of SiC substrates.

Synchrotron X-ray topography analysis of local damage occurring during polishing of 4H-SiC wafers

Surface defects with a scratch-like appearance are often observed locally on 4H-SiC wafers after epitaxial film (epi-film) growth. Since optical microscopy has only revealed a very flat surface after chemo-mechanical polishing (CMP) and before epi-film growth, the origins of these scratch-like surface defects have remained unknown. Therefore, we investigated the generation mechanism of such surface defects by synchrotron Berg–Barrett X-ray topography. Although only highly flat surfaces are observed by optical microscopy, we found that damaged regions comprising lattice defects are often introduced locally during polishing in the subsurface region. These lattice defects are almost completely removed by hydrogen etching, a pre-process for epi-film growth; however, surface defects associated with step bunching are formed in such damaged areas of the wafer surface. It is clear that local surface defects develop into surface defects with a scratch-like appearance during epi-film growth.

In situ atomic force microscopy studies of growth mechanisms and surface morphology of zinc thiourea sulfate crystals

In situ atomic force microscopy (AFM) has been utilized in studies of the growth mechanism on the (100) face of zinc tris (thiourea) sulphate (ZTS) crystals growing from solution. The growth on the (100) face of pure ZTS crystal is mainly controlled by two dimensional (2D) nucleation mechanisms, under which the hillock is formed through layer‐by‐layer growth. It is easier to form 2D nuclei at edge dislocation and the apex of steps. The growth of 2D nucleus is in accord with nucleation‐spreading mode. The growth rate along the 〈010〉 direction is faster than that along 〈001〉 direction, both of which increase firstly and then decrease with the spread of nucleus. The kinetic coefficients of one nucleus have been roughly estimated to be 3.6 × 10−4 cm/s and 1.8 × 10−4 cm/s in two directions, while the activation energy E was calculated to be 53.7 kJ/mol and 55.4 kJ/mol, respectively. The 2D nuclei can be generated under lower supersaturation with the addition of EDTA. If there are several hillocks growing together, step bunches will form when the steps moving in the same direction meet each other, while the meeting of steps that move in the inverse direction will result in the separation of steps. The ability of nucleation of edge dislocation outcrops are different even they are close to each other on the same surface. When the nucleus was generated at the edge dislocation sites, it cannot spread speedily until finishes an “incubation period”. Moreover, the detour of microsteps was observed due to the existence of pits. If the microcrystals attached on the surface block the step advancement, or leave the surface or are covered by the macrosteps, the pits are formed. If the macrosteps advanced across the pits, the pits will be covered and the liquid inclusions may form. However, if the microcrystal forming in the pit grow up and expose on the surface, the pit will not be covered by macrosteps. The formation of solid inclusions may be caused by the microcrystals being embedded into the single steps which move layer‐by‐layer.

Model of step propagation and step bunching at the sidewalls of nanowires

Radial growth of vertically aligned nanowires involves formation and propagation of monoatomic steps at atomically smooth nanowire sidewalls. Here we study the step dynamics with a step flow model taking into account the presence of a strong sink for adatoms at top of the nanowire and adatom exchange between the nanowire sidewall and surrounding substrate surface. Analytical expressions for velocities of steps propagating from the nanowire base to the nanowire top are obtained. It is shown that the step approaching the nanowire top will slow down if the top nanowire facet is a stronger sink for adatoms than the sidewall step. This might trigger bunching of the steps at the sidewall resulting in development of the pencil-like shape of nanowires such as observed in, e.g., the Au-assisted MBE growth of InAs.

Dramatic reduction of dislocations on a GaN point seed crystal by coalescence of bunched steps during Na-flux growth

In our study, we found that threading dislocation density (TDD) in GaN crystals naturally reduced from ~109cm−2 in a seed to less than ~103cm−2, just by using the small-sized seed called a “point seed”. However, the mechanism of the dramatic reduction was unclear. In order to reveal the mechanism of this substantial reduction of TDD, we investigated the relationship between TDD and the crystal habit during the growth. Cathodoluminescence (CL) and scanning electron microscopy (SEM) images showed that TDD was dramatically reduced after the c face became small (<50×50μm2) in the habit-change process caused by changes of supersaturation during growth, in which bunched steps growing from the edge of the c face coalesced at the center. It is thought that the shrinking of the c face in the growth process enabled the coalescence of bunched steps, which led to the gathering of threading dislocations (TDs), and resulted in the dramatic reduction of TDD. We concluded that the natural reduction of TDs was caused by coalescence of bunched steps, which easily occurs in during the Na-flux growth on small-sized “point seeds”, and which allowed fabrication of low-dislocation-density GaN wafers.

Diffusion-controlled growth rate of stepped interfaces.

For many materials, the structure of crystalline surfaces or solid-solid interphase boundaries is characterized by an array of mobile steps separated by immobile terraces. Despite the prevalence of step-terraced interfaces a theoretical description of the growth rate has not been completely solved. In this work the boundary element method (BEM) has been utilized to numerically compute the concentration profile in a fluid phase in contact with an infinite array of equally spaced surface steps and, under the assumption that step motion is controlled by diffusion through the fluid phase, the growth rate is computed. It is also assumed that a boundary layer exists between the growing surface and a point in the liquid where complete convective mixing occurs. The BEM results are presented for varying step spacing, supersaturation, and boundary layer width. BEM calculations were also used to study the phenomenon of step bunching during crystal growth, and it is found that, in the absence of elastic strain energy, a sufficiently large perturbation in the position of a step from its regular spacing will lead to a step bunching instability. Finally, an approximate analytic solution using a matched asymptotic expansion technique is presented for the case of a stagnant liquid or equivalently a solid-solid stepped interface.

Evaluation of F-N Tunneling Emission Current in MOS Capacitor Fabricated on Step Bunching

This work reports about influence of step bunching of SiC epitaxial-wafer surface on Fowler-Nordheim (F-N) tunneling emission current of SiC-MOS capacitor. We have measured the effective barrier height (ΦB) of SiO2/SiC interface, and estimated the deterioration factor of the effective ΦB on step bunching surface by calculating the local tunneling emission currents. Step bunching fluctuates the gate oxide thickness. The effective ΦB value can be successively derived using our proposed partitioned model in which constant ΦB value of flat surface is used. The fluctuation of the oxide film thickness results in the convergence of F-N tunneling emission currents at the thinner oxide in the MOS capacitor.

Effects of Crystalline Polarity and Temperature Gradient on Step Bunching Behavior of Off-Axis 4H-SiC Solution Growth

The off-axis solution growth of 4H-SiC was studied focusing on the morphological instabilities by using conventional TSSG technique. The morphology depends strongly on the crystalline polarity, and that on Si surface can be characterized by wandering while that on C surface is characterized by strong step-bunching. By raising the temperature gradient, step bunching on Si surface is considerably suppressed which can be consistent to the constitutional super cooling scheme. However, C surface exhibits strong step bunching as the temperature gradient increase. These behaviors can be explained by the difference in Ehrlich-Schwoebel barrier and diffusion behavior of adatoms.

Observation of Damaged Layers in 4H-SiC Substrates by Mirror Projection Electron Microscope

Surface defects with scratch-like appearances are often observed locally on 4H-SiC wafers after epitaxial growth. We evaluated such damaged layer which is the cause of local step bunching using Mirror Projection Microscope (MPJ). As a result, MPJ can be detected l damaged layer which could not be detected using Synchrotron X-ray topography, even if these defects are extremely flat surface, no morphology, damaged layer is used to exist on the subsurface region. Thus, MPJ can be detected dislocation loops on the subsurface region of damage, it is effective to elucidate damaged layer of polishing process, MPJ is to be one of the candidates for inspection techniques of the damaged layer at substrate surface.

Change in Surface Morphology by Addition of Impurity Elements in 4H-SiC Solution Growth with Si Solvent

In solution growth of 4H-SiC, we have investigated changes in macrostep height with addition of the Group III (B, Al, Ga), Group IV (Ge, Sn), Group V (N) elements, and transition metals (Ti, V, Cr, Ni) to Si solvents, in order to find additives improving severe step bunching which often occurs during growth. The addition of Al, B, Sn, N, and V decreased the average macrostep height compared with the crystal grown with Si solvents. The addition of Al, B, Sn, N, and V suppressed the generation of trench-shaped surface defects in long-term growth of 10 hours. This result demonstrated that the addition of Al, B, Sn, N, and V has an advantage to achieve high quality bulk crystal growth from solution.

A scanning tunnelling microscopy study of C and N adsorption phases on the vicinal Ni(100) surfaces Ni(810) and Ni(911)

The influence of N and C chemisorption on the morphology and local structure of nominal Ni(810) and Ni(911) surfaces, both vicinal to (100) but with [001] and 011¯ step directions, respectively, has been investigated using scanning tunnelling microscopy (STM) and low energy electron diffraction. Ni(911) undergoes substantial step bunching in the presence of both adsorbates, with the (911)/N surface showing (411) facets, whereas for Ni(810), multiple steps 2–4 layers high are more typical. STM atomic-scale images show the (2×2)pg ‘clock’ reconstruction on the (100) terraces of the (810) surfaces with both C and N, although a second c(2×2) structure, most readily reconciled with a ‘rumpling’ reconstruction, is also seen on Ni(810)/N. On Ni(911), the clock reconstruction is not seen on the (100) terraces with either adsorbate, and these images are typified by protrusions on a (1×1) mesh. This absence of clock reconstruction is attributed to the different constraints imposed on the lateral movements of the surface Ni atoms adjacent to the up-step edge of the terraces with a 011¯ step direction.

Investigation of giant step bunching in 4H-SiC homoepitaxial growth: Proposal of cluster effect model

We have investigated the H2 etching and growth conditions causing giant step bunching (GSB) in 4H-SiC homoepitaxial growth on 8° off-axis substrates by a chemical vapor deposition method and found that GSB does not occur during H2 etching under a wide range of experimental conditions, whereas GSB occurs during epitaxial growth at extremely low or high C/Si ratios, i.e., an excessive supply of SiH4 or C3H8. To explain these results, we have proposed a model taking into account the effect of Si or C cluster formation called “the cluster effect” model. We have shown that the cluster effect model can explain well our experimental result of GSB occurrence or nonoccurrence during etching and homoepitaxial growth of 4H-SiC.

Effect of aluminum addition on the surface step morphology of 4H–SiC grown from Si–Cr–C solution

For the solution growth of 4H–SiC with Si1−xCrx solvents, the change in surface step structure by 4at% Al addition to the solvent was investigated. Without Al addition, step bunching resulted in the formation of giant macrosteps with height greater than several micrometers, and trench-like surface defects formed with solvent inclusion. The edge of the giant macrosteps composing trench-like defects was faceted into low-index facet planes, (11¯0m) (m=1–4). The formation of the trench-like surface defects is considered to originate from the self-pinning of macrosteps owing to the step-faceting phenomenon, which facilitates further development of macrosteps. On the other hand, the addition of Al to the solvents significantly improved the surface roughening, and suppressed the formation of the trench-like surface defects. In this case, smaller bunched steps with heights from several nanometers to about 10nm formed on the terraces of the macrosteps, while regularly arranged trains of unit-cell-size steps formed on the terrace of macrosteps in growth without Al addition. The decrease in step stiffness might be a possible cause for the formation of the smaller bunched steps on the terrace in growth with Al addition.

Epitaxial graphene on SiC: modification of structural and electron transport properties by substrate pretreatment

The electrical transport properties of epitaxial graphene layers are correlated with the SiC surface morphology. In this study we show by atomic force microscopy and Raman measurements that the surface morphology and the structure of the epitaxial graphene layers change significantly when different pretreatment procedures are applied to nearly on-axis 6H-SiC(0 0 0 1) substrates. It turns out that the often used hydrogen etching of the substrate is responsible for undesirable high macro-steps evolving during graphene growth. A more advantageous type of sub-nanometer stepped graphene layers is obtained with a new method: a high-temperature conditioning of the SiC surface in argon atmosphere. The results can be explained by the observed graphene buffer layer domains after the conditioning process which suppress giant step bunching and graphene step flow growth. The superior electronic quality is demonstrated by a less extrinsic resistance anisotropy obtained in nano-probe transport experiments and by the excellent quantization of the Hall resistance in low-temperature magneto-transport measurements. The quantum Hall resistance agrees with the nominal value (half of the von Klitzing constant) within a standard deviation of 4.5 × 10−9 which qualifies this method for the fabrication of electrical quantum standards.

N-face GaN/AlN/GaN/InAlN and GaN/AlN/AlGaN/GaN/InAlN high-electron-mobility transistor structures grown by plasma-assisted molecular beam epitaxy on vicinal substrates

Since on-axis GaN-on-sapphire substrates with low threading dislocation density are not available in the N-face orientation, we explored the growth of InAlN on vicinal (4° miscut along GaN GaN-on-sapphire substrates. The microstructure of In0.18Al0.82N layers grown by plasma-assisted molecular beam epitaxy at different temperatures was studied using scanning transmission electron microscopy (STEM). The cross-sectional and plan-view STEM images revealed lateral variations in the InAlN composition along (perpendicular to the step edges). Also, step bunching was observed in InAlN layers thicker than 10 nm. N-face high-electron-mobility transistor structures with lattice-matched InAlN backbarriers were then grown on these vicinal substrates with different InAlN thicknesses. Transmission line measurements showed that step bunching and lateral variation of InAlN composition degraded the two-dimensional electron gas (2DEG) mobility in the directions parallel and perpendicular to the steps. A 2DEG charge density of 1.1 × 1013 cm−2 and mobility of 1850 cm2 V−1 s−1 were achieved on a GaN/AlN/InAlN/GaN structure with 7.5 nm thick In0.18Al0.82N. By designing a double backbarrier (In0.18Al0.82N(7.5 nm)/Al0.57Ga0.43N(7 nm)), a 2DEG charge density of 2 × 1013 cm−2 and mobility of 1360 cm2 V−1 s−1 were attained, which resulted in a sheet resistance of 230 Ω/□.

Thickness-modulated InGaAs/GaAsP superlattice solar cells on vicinal substrates

InGaAs/GaAsP superlattice (SL) is a promising narrow-gap material for III–V multi-junction solar cells on Ge. In metal-organic vapor phase epitaxy (MOVPE) of SL on vicinal substrates, the component layers tend to be undulated due to step bunching occurring at high temperature. In this paper, the effects of growth temperature and thickness modulation of the SL-region on the photovoltaic performance were investigated. Lowering the growth temperature successfully enabled epitaxy of an extremely uniform SL, from which a clear step-like absorption spectrum including sharp exciton peaks was obtained due to layer-by-layer deposition of the individual layers. Larger layer undulation at higher temperature led to poorer in-plane coverage of the InGaAs region, resulting in the reduction of both light absorption and short circuit current. The open circuit voltage, on the other hand, was higher for the cells grown at higher temperature owing to suppressed dark current as a result of reduced crystal defects. Moreover, the lateral thickness variation of the GaAsP barriers in the undulated SL allowed efficient tunnel transport through the thinner part of the barrier, and improved the carrier collection and the fill factor. By optimizing the growth temperature for SL on vicinal substrates, an N-on-P cell including 100-period SL with a bandgap of 1.21 eV achieved 1.11 times higher efficiency than a GaAs reference cell with 36% current enhancement as middle cell performance.

Coarsening dynamics at unstable crystal surfaces

In this paper we focus on crystal surfaces led out of equilibrium by a growth or erosion process. As a consequence of that the surface may undergo morphological instabilities and develop a distinct structure: ondulations, mounds or pyramids, bunches of steps, ripples. The typical size of the emergent pattern may be fixed or it may increase in time through a coarsening process which in turn may last forever or it may be interrupted at some relevant length scale. We study dynamics in three different cases, stressing the main physical ingredients and the main features of coarsening: a kinetic instability, an energetic instability, and an athermal instability.

Step-bunched Bi2Te3 and Bi2Se3 layers epitaxially grown on high-index InP substrates

Bi2Te3 and Bi2Se3 layers are grown on InP with various substrate orientations by hot wall epitaxy. The layers develop step bunching on high-index substrates as a consequence of the alignment of the c axis to the InP[111] direction. The bunched Bi2Te3 steps are ragged due to significant reevaporation. Pronounced porosity is caused in Bi2Te3 layers on InP(114) as the c axis is aligned also to the [001] direction of InP as well as to the [111] direction. We compare the Seebeck coefficient in the step-bunched and (0001)-oriented Bi2Te3 layers, where the anisotropy of the thermoelectric property is shown to be small. In addition, we demonstrate for Bi2Se3 layers on InP(115) that a (0001)-oriented minority component is incorporated as submicrometer-size islands in the bunched steps as it is also lattice-matched to the substrates.

Alignment nature of ZnO nanowires grown on polished and nanoscale etched lithium niobate surface through self-seeding thermal evaporation method

High aspect ratio catalyst-free ZnO nanowires were directly synthesized on lithium niobate substrate for the first time through thermal evaporation method without the use of a buffer layer or the conventional pre-deposited ZnO seed layer. As-grown ZnO nanowires exhibited a crisscross aligned growth pattern due to step bunching of the polished lithium niobate surface during the nanowire growth process. On the contrary, scratches on the surface and edges of the substrate produced well-aligned ZnO nanowires in these defect regions due to high surface roughness. Thus, the crisscross aligned nature of high aspect ratio nanowire growth on the lithium niobate surface can be changed to well-aligned growth through controlled etching of the surface, which is further verified through reactive-ion etching of lithium niobate. The investigations and discussion in the present work will provide novel pathway for self-seeded patterned growth of well-aligned ZnO nanowires on lithium niobate based micro devices.

Gettering of Luminescent Point Defects along Step Bunching in 4H-SiC Epitaxial Layers by Ultraviolet Excitation

Luminescent lines, perpendicular to the step flow direction, were observed in SiC epitaxial layers with ultraviolet (UV) excitation of the sample surface. These lines appear at step bunching sites in SiC epilayers with UV illumination of greater than 3 s at 50 W/cm2. They become brighter and wider, reaching a width of ∼5 μm. Zygo profilometry shows that there is a shallow triangular-shaped valley at the surface to the left of the luminescent lines, with reference to the step flow direction. The left apex of the valley starts at a local defect that is created during the epitaxial growth, and the valley fans out during subsequent epitaxial growth. The luminescent lines are at the right-side boundary of the triangle. Atomic force microscopy (AFM) profilometry shows that at this boundary there is a double macrostep at the sample surface, which is between 2.5 nm and 8 nm high. This relates to 5 to 15 bilayer steps. The first step is always observed to be smaller than the second one. There is a valley approximately 4 nm to 8 nm deep between them. The luminescent lines appear due to UV-activated diffusion and gettering of point defects to the double macrostep.