What Exactly Is a "Step-Terrace" Structure?

A polished sapphire substrate looks like a perfect mirror to the naked eye. But when magnified to the nanometer scale by AFM, the surface is revealed to be not absolutely flat — it consists of a series of flat "terraces" separated by "steps," like a vast stepped plaza. This structure directly determines the initial growth mode of the epitaxial film and is the most critical microscopic metric in substrate quality control.
The crystal structure of sapphire (α-Al₂O₃) is the key to understanding these steps. Along the c-axis, the hexagonal unit cell is stacked from six oxygen layers, with double layers of aluminum sandwiched between them. The c-axis lattice parameter is c = 1.299 nm; divided by six oxygen layers, this gives the spacing between two adjacent oxygen planes — c/6 ≈ 0.216 nm. This is the fundamental single-step height on a sapphire surface and the lowest-energy step structural unit.
 

TEM Directly "Sees" the Atomic Structure of Steps

AFM can only measure surface height undulation. To truly "see" the atomic-scale structure of a step, high-resolution transmission electron microscopy (HRTEM) is indispensable. In this study, the team prepared cross-sectional TEM samples viewed along the [10-10] direction and directly observed the step-terrace structure on the sapphire surface:
Low-magnification TEM (Figure 1) shows that the α-Al₂O₃ substrate surface exhibits regular step-terrace undulation. The substrate bulk appears as the dark region, while the light region above the surface is the resin used to embed the sample during preparation. The terrace widths measured by TEM agree with the AFM results, cross-validating the reliability of both characterization methods. The scale bar is 50 nm, giving an intuitive sense that terrace widths are on the order of tens of nanometers.
 

Figure 1. Low-magnification cross-sectional TEM image of a sapphire sample viewed along [10-10]. The dark region is the α-Al₂O₃ substrate; the light region above is the embedding resin. Regular step-terrace undulation is visible; scale bar = 50 nm.


 
The high-resolution TEM image (Figure 2) further reveals atomic-scale details of the steps: viewed along [10-10], the linear arrangement of oxygen atoms is clearly visible, and the atomic configuration of the entire crystal is exposed. The most critical finding is that the macroscopically observed ~2 nm-high step is not a single "high wall," but a "step bunch" composed of ten independent 0.2 nm single steps tightly clustered together. Each single step corresponds exactly to the spacing between two adjacent oxygen layers (c/6 ≈ 0.2 nm).
This finding is highly significant — it demonstrates that the "macro-steps" on the surface after high-temperature annealing are a product of dynamic evolution, not something the substrate possessed as received. The single steps on the initially polished surface gradually move, approach one another, and bunch together under thermal activation, ultimately forming the 1–2 nm-high step bunches observed by AFM.
 

Figure 2. High-resolution TEM image of sapphire viewed along [10-10]. The linear arrangement of oxygen atoms is clearly visible; the ~2 nm macro-step is composed of ten 0.2 nm single steps bunched together.


 

How Do Steps Merge? The "Zipper-Type" Mechanism

On the surface of 3° miscut samples, steps do not extend perfectly parallel from end to end. AFM images reveal that at certain positions, step "confluence points" (junctions) appear — two or more adjacent low-level steps meet at a specific location and merge into a single, taller multiple step.
The researchers describe this merging process as a "zipper-type" movement mechanism: imagine two adjacent low steps, like the two rows of teeth on a zipper, gradually "pulling closed" from one point toward one end, ultimately merging into a single taller step. The 3D AFM close-up in Figure 4 illustrates this process intuitively — in (a), multiple steps are seen merging at a confluence point; the step profiles in (b) and (c) quantitatively show two independent steps (red line, b) merging into one taller step (green line, c).
What is even more interesting is the deflection of step direction at the confluence point. Normally, step edges run straight along the [10-10] direction; but near the junction, steps suddenly "turn" toward another low-index direction, [1-100]. AFM measurements show that the angle between the two step-edge segments at the confluence point is approximately 150°, which exactly corresponds to the crystallographic angle between [10-10] and [1-100].
This deflection is not random. The paper explains that step bunching and faceting are two simultaneous processes: domains of different phase orientations expand and collide on the surface, and the collision points form phase-mismatched facet junctions. Near these junctions, step edges "switch" from one low-index direction to another because low-index directions have lower surface energy — step edges always tend to extend along the lowest-surface-energy direction.
 

Figure 3. AFM morphology of a step confluence point. At the junction, steps deflect from the [10-10] direction to the [1-100] direction, with an angle of approximately 150° between the two step-edge segments. Confluence points are the result of collision between different facet domains during step coarsening.
 


Figure 4. 3D close-up of a confluence region. (a) shows the 'zipper-type' merging of multiple steps; (b) and (c) are step profiles along the marked lines in (a), showing two independent steps (b, red line) merging into a taller step (c, green line).
 

How Impurities "Disrupt" Steps: Why Substrate Purity Matters

The paper contains a discussion of particular interest to substrate procurement personnel regarding the effect of impurities on step behavior. The study points out that the c-plane surface of sapphire is highly sensitive to various impurities:

• Surface segregation of Ca, Mg, Ba, Si: Once these impurity atoms segregate to the surface, they "pin" the step edges, forcing steps to bend around impurity particles and generating mesoscale irregular bunching.

• High K content: The study reports that high K content can initiate step bunching — in other words, with the same annealing recipe, substrates with higher K may exhibit excessive bunching.

• High Ca + low K combination: Interestingly, this impurity combination instead stabilizes the c/6 single step and prevents bunching. This demonstrates that the effect of impurities on steps is not simply "more is worse," but depends on the specific element types and their relative proportions.

• Asymmetric adatom incorporation: In principle, asymmetric incorporation of adatoms from the upper and lower terraces into the step can also cause kinetic instability of the step sequence during growth/evaporation, inducing bunching.

The practical implication is that even with the identical annealing furnace and identical temperature profile, sapphire substrates from different manufacturers or purity grades can yield very different post-annealing surface morphologies. Substrate impurity content affects not only optical transmittance and electrical insulation, but also directly determines the arrangement of surface steps after high-temperature annealing. This is why, after switching to a new batch of incoming substrates, the same annealing recipe may need to be re-optimized.
 

Practical Tips for Substrate Surface Quality Control

Finally, it must be emphasized that the annealing environment, crucible material, and sample placement all affect the final surface morphology. When developing substrate surface processes, it is recommended to fix all auxiliary conditions and change only one variable at a time (temperature, time, or miscut angle), so that the independent contribution of each factor can be clearly isolated.
Key metrics to monitor when characterizing sapphire substrate surfaces by AFM:

AFM Metric	What to Look For
Step height distribution	Ideally, step heights should be integer multiples of 0.21 nm (c/6). Random, non-integer heights indicate residual machining damage or incompletely cleaned contamination.
Terrace width distribution	A uniform, narrow distribution (e.g., 44 ± 14 nm in this study) indicates well-ordered steps. An extremely broad distribution with mixed large and small terraces indicates non-uniform faceting.
Step edge orientation	Steps should run straight along specific low-index directions ([10-10] or [1-210], etc.). Severe jaggedness or irregular bending may indicate impurity pinning or abnormal surface reconstruction.
Confluence point density	A small number of confluence points is a normal product of step coarsening. Excessive junction density with frequent step turns indicates frequent collisions between facet domains and possibly poor surface uniformity.
RMS roughness	Use as a supplementary reference only. RMS cannot replace step-terrace structural analysis — the same RMS value can correspond to entirely different step arrangements.

 

JXT Substrate Product

For substrate selection and preprocessing, the following products are available:

Product Category	Specifications and Description
SiC substrates	2–8 inch, 4H/6H-SiC, conductive / semi-insulating; miscut angles available (0°/3.5°/8°, etc.); for power devices and RF epitaxy.
Sapphire substrates	2–12 inch; c/a/m/r-plane options; PSS/NPSS patterned substrates; miscut angle and direction customizable; for LEDs and semiconductor epitaxy.
Silicon wafers	2–12 inch; single-side / double-side polished; lightly / heavily doped; SOI wafers; for microelectronics and MEMS.
Fused silica	UV-grade / optical grade, JGS1/JGS2; size and thickness customizable; for optics and optoelectronics.
Lithium tantalate / lithium niobate	3–4 inch; Y-cut / X-cut / Z-cut; optical-grade / SAW-grade; for SAW devices and modulators.
Other compound substrates	GaAs, InP, GaN-on-Si, single-crystal quartz, etc.; for optoelectronics and high-frequency devices.
Wafer bonding services	Si-on-SiC, Si-on-Insulator, wafer-level bonding and thinning/polishing; for heterogeneous integration and power packaging.

JXT supplies high-flatness c-plane sapphire substrates (AFM RMS roughness controllable below 0.2 nm; step heights as integer multiples of c/6), available in sizes from 2 to 6 inches. We also supply Si-on-SiC bonded substrates, SOI wafers, and lithium tantalate / lithium niobate substrates, all shipped with outgoing surface quality inspection reports (including AFM morphology images and step-height distribution data). For customized surface morphology or substrate quality consultation, please contact our technical team for samples and inspection reports.
For substrate selection advice or customized dicing / polishing / annealing recipes, please contact the JXT technical team for detailed documentation and quotations.