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arxiv: 1907.05591 · v1 · pith:OIHKI2SInew · submitted 2019-07-12 · ❄️ cond-mat.mtrl-sci

Strain distribution and thermal strain relaxation in MOVPE grown hBN films on sapphire substrates

Kousik Bera , D. Chugh , Atanu Patra , H. Hoe Tan , C. Jagadish Anushree Roy This is my paper

Pith reviewed 2026-05-24 22:43 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords hBN filmsstrain distributionRaman imagingMOVPE growththermal relaxationsapphire substrateswrinklingdelamination
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The pith

Compressive strain in hBN films on sapphire increases with film thickness and can be estimated from their wrinkle morphology.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines strain buildup in hexagonal boron nitride films grown on sapphire using metalorganic vapour phase epitaxy with a flow modulation scheme. Raman imaging maps the residual strain distribution tied to surface wrinkling and shows that overall compressive strain rises as layer thickness increases. Temperature-dependent Raman measurements indicate that thinner films experience a higher rate of strain evolution during heating, which helps explain the thickness dependence in as-grown samples. An empirical relation connects wrinkle morphology to residual strain values, and delamination is demonstrated to release part of the strain. Readers interested in 2D device substrates would care because strain control affects film quality and usability in stacked structures.

Core claim

The authors establish that the overall compressive strain in MOVPE-grown hBN films on sapphire increases with increasing layer thickness. They employ Raman imaging to study the residual strain distribution associated with wrinkling. Temperature-dependent Raman measurements demonstrate that the thermal rate of strain evolution is higher in films of lower thickness. An empirical relation is proposed for estimating the residual strain from the morphology of the films, and partial release of residual strain is achieved by delamination of the films.

What carries the argument

Raman imaging to map the residual strain distribution in wrinkled hBN films from observed peak shifts.

If this is right

  • Thicker films accumulate higher compressive strain and therefore need stronger interventions for relaxation.
  • The empirical relation allows residual strain to be estimated directly from visible film morphology without full spectroscopic mapping.
  • Thinner films relax strain more readily under thermal treatment, accounting for observed differences in as-grown samples.
  • Delamination offers a practical route to partial strain relief after growth.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Growth recipes could be tuned using the thickness-strain trend to limit wrinkling when thicker films are required for device layers.
  • Similar morphology-based strain estimates might be tested on hBN or related 2D films grown by other techniques on sapphire.
  • Combining Raman data with direct lattice measurements could test whether the empirical relation holds beyond the studied samples.

Load-bearing premise

Raman peak shifts correspond directly and exclusively to biaxial compressive strain without contributions from defects, doping or substrate interactions.

What would settle it

X-ray diffraction measurements of lattice constants across films of different thicknesses that show no increase in compressive strain with thickness.

Figures

Figures reproduced from arXiv: 1907.05591 by Atanu Patra, C. Jagadish Anushree Roy, D. Chugh, H. Hoe Tan, Kousik Bera.

Figure 1
Figure 1. Figure 1: Surface morphology of (a) 2 nm, (b) 20 nm and (c) 40 nm thick hBN films on sapphire substrates at room temperature. (d) The variation of average value of the wavelength, right scaleand amplitude, A, (left scale) with the film thickness, h. The solid line is a guide to the eyes to follow the change in with h. Inset of (d) schematically defines the wrinkle wavelength and amplitude A and thickness (… view at source ↗
Figure 3
Figure 3. Figure 3: (a)-(d) present characteristic map of Raman shift of the E2g high vibrational mode of hBN films of thicknesses 52 nm, 102 nm, 203 nm and 403 nm, respectively. The maps of full width at half-maximum (FWHM) of the same mode of these films are shown in [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The variation of the mean value of (a) Raman shift and (b) FWHM of the E2g high mode with the thickness of the film, as obtained by analysing each spectrum of the mapped area. The vertical error bar in each panel is the standard deviation of all values obtained from the image frame. The dashed line in (a) marks the Raman shift of bulk hBN. The same in (b) marks the FWHM of bulk hBN. The solid line in each … view at source ↗
Figure 10
Figure 10. Figure 10: Mapping of the residual strain for the 20 nm delaminated film. The same for the substrate supported film is shown in the inset using the same scale bar [PITH_FULL_IMAGE:figures/full_fig_p024_10.png] view at source ↗
read the original abstract

Recently, hexagonal boron nitride (hBN) layers have generated a lot of interest as ideal substrates for 2D stacked devices. Sapphire-supported thin hBN films of different thicknesses are grown using metalorganic vapour phase epitaxy technique by following a flow modulation scheme. Though these films of relatively large size are potential candidates to be employed in designing real devices, they exhibit wrinkling. The formation of wrinkles is a key signature of strain distribution in a film. Raman imaging has been utilized to study the residual strain distribution in these wrinkled hBN films. An increase in the overall compressive strain in the films with an increase in the layer thickness has been observed. To find whether the residual lattice strain in the films can be removed by a thermal treatment, temperature dependent Raman measurements of these films are carried out. The study demonstrates that the thermal rate of strain evolution is higher in the films of lower thickness than in the thicker films. This observation further provides a possible explanation for the variation of strain in the as-grown films. An empirical relation has been proposed for estimating the residual strain from the morphology of the films. We have also shown that the residual strain can be partially released by the delamination of the films.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript examines MOVPE-grown hBN films on sapphire using a flow-modulation scheme. Raman imaging is used to map residual strain in wrinkled films of varying thickness, revealing an increase in overall compressive strain with thickness. Temperature-dependent Raman measurements assess thermal relaxation, showing faster strain evolution in thinner films. An empirical relation linking residual strain to film morphology is proposed, and partial strain release via delamination is demonstrated.

Significance. If the Raman-to-strain conversion is shown to be free of significant confounders, the thickness-dependent trend and empirical morphology-strain relation would provide a practical, experimentally grounded approach to strain assessment in scalable hBN films for 2D devices. The differential thermal relaxation rates offer a plausible mechanistic link between growth conditions and final strain state.

major comments (2)
  1. [Abstract / Raman imaging results] Abstract and Raman imaging analysis: the central claims of increasing compressive strain with thickness and the empirical morphology-strain relation rest on direct conversion of E2g peak shifts to biaxial strain. No calibration details, reference unstrained frequency, peak-fitting procedure, or controls for confounding shifts (defects, doping, interfacial bonding) are described; if any of these scale with thickness or wrinkle density, both the trend and the empirical formula lose uniqueness.
  2. [Temperature dependent Raman measurements] Temperature-dependent Raman section: the reported higher thermal rate of strain evolution in thinner films is used to explain the as-grown thickness dependence, yet the abstract and described analysis provide neither quantitative rates, error bars, nor the number of films or measurement points per thickness; without these, the differential-relaxation explanation cannot be evaluated.
minor comments (2)
  1. [Empirical relation proposal] The empirical relation is stated to contain free coefficients; the manuscript should explicitly list the fitted parameters and the data used for the fit so that the relation can be reproduced or tested independently.
  2. [Abstract] Abstract lacks any mention of sample statistics, error bars on strain values, or the number of films examined per thickness; these should be added for transparency even if they appear in the main text.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback on our manuscript. We have carefully considered the comments and will revise the manuscript to address the concerns raised regarding the Raman analysis details and the quantitative aspects of the temperature-dependent measurements.

read point-by-point responses

  1. Referee: [Abstract / Raman imaging results] Abstract and Raman imaging analysis: the central claims of increasing compressive strain with thickness and the empirical morphology-strain relation rest on direct conversion of E2g peak shifts to biaxial strain. No calibration details, reference unstrained frequency, peak-fitting procedure, or controls for confounding shifts (defects, doping, interfacial bonding) are described; if any of these scale with thickness or wrinkle density, both the trend and the empirical formula lose uniqueness.

    Authors: We thank the referee for highlighting this important point. The manuscript does rely on the standard Raman shift to strain conversion for hBN, using literature values for the biaxial strain coefficient. However, we agree that explicit details are needed. In the revised manuscript, we will add: (1) the reference unstrained E2g frequency used (typically 1365 cm⁻¹ for bulk hBN), (2) description of the Lorentzian peak fitting procedure, (3) calibration reference if any, and (4) discussion of potential confounders with arguments why they do not dominate the observed thickness trend (e.g., the correlation with morphology which is strain-related). If additional controls are possible from existing data, they will be included. This will strengthen the uniqueness of the claims. revision: yes

  2. Referee: [Temperature dependent Raman measurements] Temperature-dependent Raman section: the reported higher thermal rate of strain evolution in thinner films is used to explain the as-grown thickness dependence, yet the abstract and described analysis provide neither quantitative rates, error bars, nor the number of films or measurement points per thickness; without these, the differential-relaxation explanation cannot be evaluated.

    Authors: We agree that quantitative details are essential for evaluating the claims. The original manuscript presented the observation qualitatively. In the revision, we will include the calculated thermal rates of strain evolution (in cm⁻¹/°C or equivalent strain units) with error bars, specify the number of films studied per thickness category, and the number of measurement points or cycles per film. This will allow proper assessment of the differential relaxation rates and their link to the as-grown strain. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental measurements and data-derived empirical relation

full rationale

The paper reports direct experimental observations via Raman imaging and temperature-dependent measurements on MOVPE-grown hBN films. The claimed increase in compressive strain with thickness and the proposed empirical morphology-strain relation are presented as outcomes of data analysis rather than any derivation, prediction, or first-principles result that reduces to its own inputs by construction. No self-citations, ansatzes, or fitted parameters are invoked as load-bearing steps in a theoretical chain. The work is self-contained against external benchmarks with no evidence of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The paper rests on the standard conversion of Raman E2g peak shift to strain (domain_assumption) and introduces an empirical morphology-strain formula whose coefficients are fitted to the observed data (free parameters). No new particles or forces are postulated.

free parameters (1)
  • coefficients in empirical morphology-strain relation
    Fitted to match observed wrinkle patterns with Raman-derived strain values; exact functional form and fit details not given in abstract.
axioms (1)
  • domain assumption Raman peak position shift is a linear proxy for biaxial compressive strain in hBN
    Invoked when mapping Raman images to strain distribution; standard in the field but requires calibration constants.

pith-pipeline@v0.9.0 · 5767 in / 1332 out tokens · 19136 ms · 2026-05-24T22:43:29.460417+00:00 · methodology

discussion (0)

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Reference graph

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