Sphalerite Research Articles

Structural and optical properties of Zn-implanted silica: effect of fluence and annealing

The phase-structural composition of silica films grown on Si substrates implanted with different fluences of Zn ions has been studied using transmission electron microscopy (TEM) and electron diffraction. Small clusters (2–3 nm) and larger clusters (5–7 nm) were formed in the as-implanted silica films with Zn concentration of 6–8 at % and 16%–18%, respectively. Furnace annealing at 750 °С for two hours in air resulted in the formation of an orthorhombic Zn2SiO4 phase (space group R-3) in the case of low fluence (5 × 1016 cm−2) and in the formation of a zinc blended ZnO phase (space group F-43m) in the case of high fluence (1 × 1017 cm−2). Based on the Rutherford backscattering spectrometry (RBS) data, it has been shown that impurity losses during implantation and subsequent annealing increase with the fluence of the implanted ions. The photoluminescence data were consistent with the TEM and RBS.

Dispersive response of polar optical phonon modes on the dielectric and size effects in a nitride nanostructure

Within the framework of the macroscopic dielectric continuum model, the dispersive spectra of polar optical phonon modes of III-nitride based nanostructures with different inner and outer dielectric media are investigated in this paper. The dielectric and quantum size effects on the dispersive frequencies of phonon modes are focused and emphasized. In the case of GaN quantum rings (QRs), we found that the dielectric constant of the inner/outer medium could significantly influence the dispersive spectra of the surface optical (SO) phonon mode as the high/low-frequency SO phonon modes are mainly localized at the inner/outer surface of QRs, respectively. Moreover, such effects associated with different crystallographic structures of GaN, i.e., the wurtzite (WZ) and zinc blende (ZB) crystal structures, are also studied in detail, showing the different degenerating behavior and electron–phonon coupling strength. The results obtained in this study provide a beneficial opportunity to independently adjust dispersive spectra of the two branches of SO modes in GaN nanostructures; thus, it could be used to design and develop phonon-based detectors and sensors.

Structural, electronic, and elastic properties of RbI using the FP-LAPW method

The structural, mechanical, and electronic properties of rubidium iodide (RbI) have been extensively investigated utilizing the generalized gradient approximation (GGA) and the full-potential linearized augmented plane wave (FP-LAPW) approach. The potential was roughly calculated using a modified Becke–Johnson (mBJ) approximation, which increased the precision of the electronic properties. In this study, RbI is analyzed in a wide range of crystal structures, including topologies like rock salt (RS), CsCl, zinc blende (ZB), NiAs, and wurtzite (WZ), among others. Our research shows a strong relationship between the material’s physical properties and the conclusions drawn from both theoretical and experimental studies. Significantly, our results show that the RS form corresponds to RbI’s ground state. All the aforementioned topologies display wide-bandgap semiconductor capabilities, according to further examination of their electronic band structures. Notwithstanding these findings, it was discovered that RbI has a poor fracture resistance due to its low bulk modulus. Born’s stability analysis has shown that RbI is stable in the RS, CsCl, ZB, NiAs, and WZ structures. All RbI structures were discovered to have ionic bonding and to be ductile, and every stabilized system displayed anisotropic stability. Using the Cauchy pressure and Poisson’s ratio, the stiffness of the systems was evaluated, with the RS structure proving to be the stiffest. Overall, the findings illuminate the physical properties of RbI, providing valuable insights that could facilitate the creation and refinement of novel materials possessing desirable characteristics.

Theoretical investigation of the structural, electronic, optical, and elastic properties of the zinc blende SiGe1 − xSnx ternary alloy

Theoretical investigation of the structural, electronic, optical, and elastic properties of the zinc blende SiGe1 − xSnx ternary alloy

S-scheme Co3(PO4)2/Twinned-Cd0.5Zn0.5S homo-heterojunction for enhanced photocatalytic H2 evolution

Heterojunction photocatalysts have been demonstrated to be effective for the interfacial charge transfer and utilization, if the charge transfer in bulk phase can be promoted, more photoinduced charges can be utilized to achieve a higher efficiency of light energy conversion. Herein, twinned-Cd0.5Zn0.5S (T-CZS) homojunction formed by zinc blende Cd0.5Zn0.5S (ZB-CZS) and wurtzite Cd0.5Zn0.5S (WZ-CZS) is obtained via a facile hydrothermal method, and then Co3(PO4)2 is introduced to the surface of T-CZS to form homo-heterojunction using a solvent evaporation strategy. The photocatalytic H2 evolution tests show that the activity of T-CZS is obviously higher than ZB-CZS or WZ-CZS, and Co3(PO4)2 can increase the H2 releasing rate of T-CZS from 13.0 mmol·h−1·g−1 to 64.8 mmol·h−1·g−1 over 5.7 wt% Co3(PO4)2/T-CZS. The apparent quantum efficiency (AQE) can reach 6.4% under 400 nm light irradiation. Under 800 nm near-infrared light (NIR), the H2 releasing rate can reach 0.23 mmol·h−1·g−1. The investigation shows that the carrier transfer between T-CZS and Co3(PO4)2 follows the S-scheme transfer path according to the electron paramagnetic resonance spectroscopy analysis and calculation by density flooding theory (DFT) The enhanced activity of Co3(PO4)2/T-CZS can be mainly attributed to the homo-heterojunction synergistic effect between ZB-CZS/WZ-CZS homojunction in T-CZS and the S-scheme heterojunction between T-CZS and Co3(PO4)2, which well avoids the rapid recombination of bulk and surface electron-hole pairs by accelerating charge transfer. Meanwhile, the improved light-harvesting ability, more active sites and lower H2 production overpotential induced by Co3(PO4)2, jointly contribute to a quick kinetics of H2 evolution. In addition, other metal phosphates MPO4 (M = Ba, Bi, Ag and Zn) are also employed to enhance the activity of T-CZS, they are all effective candidates for construction homo-heterojunction with T-CZS.

The effect of surfactants and precursors on the structure and properties of ZnS:Cu nanocrystalline particles

Doped ZnS materials exhibit excellent optical and electrical properties, making them a superior material for alternating current (AC) thin film electroluminescent devices, solar panels, light-emitting diodes, laser diodes and other luminescent device designs and fabrications. This study involves the hydrothermal synthesis of nanocrystalline Cu-doped ZnS materials without a surfactant and using two surfactants – hexadecyltrimethylammonium bromide (HTAB) and sodium dodecyl sulfate (SDS). The structure, morphology and elemental content of the synthesized nanopowders were characterized by X-ray powder diffraction, X-ray absorption spectroscopy (XAS), scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) techniques, while the optical properties were studied by photoluminescence (PL) spectroscopy. The sizes of the nanocrystallites in all the synthesized samples were below 20 nm. XAS results suggest that Cu ions substitute Zn ions in cubic zinc blende ZnS structure and are located in a locally distorted environment. The addition of a surfactant allows control of the morphology of the particles and their photoluminescent properties.

Effect of the tri-sodium citrate as a complexing agent in the deposition of ZnS by SILAR

Using the SILAR method Zinc sulfide coatings were deposited on glass slices. The physical properties and the chemical mechanism throughout the variation in concentration of tri-sodium citrate (TSC) as a chelating agent in the synthesis of thin films were investigated. Results shows that ZnS thin films exhibit an average transmittance of 16% in visible light spectra region and a zinc blende structure. The ZnS films synthesized using TSC as a complexing agent, present a smaller average particle size, an average transmittance of 85%, and an adsorption edge at 300–340 nm. Based on our experimental data and analysis, we conclude that the contribution of the oxychloride species, a subproduct in the chemical deposition, is suggested to be related as an impurity level former in the synthesis of ZnS thin films. TSC as a complexing agent in the SILAR technique is a non-toxic option to reduce the generation of the oxychloride species and synthesize a wide band gap semiconductor. Moreover, the use of complexing agents could be extended to other types of semiconductors deposited by SILAR.

Fabrication and investigation of zinc telluride thin films

Zinc Telluride ZnTe alloys and thin film have been fabricated and deposited on glass substrates by thermal evaporation method which may be a suitable window layer of zinc telluride with different annealing temperatures (373 and473) K for 60 minutes in vacuum. Deposited thin films with thickness 100 nm was characteristic by using X-ray diffraction XRD to know structures, Atomic Force Microscopy (AFM) to evaluate surface topology, morphology. It was found out that the vacuum annealing improves on thin ZnTe films structure and surface morphology. Structural analysis reveals that ZnTe films have zinc blende structure of cubic phase with (111) preferred reflection and grain size is enhanced from 8.6 to 16.7 nm with annealing. Optical properties where optical bandgap energy values slightly decreased (2.3-2.2) eV as the annealing temperatures.

Structural and Electrical Properties of "Cadmium Sulfide- Cobalt" Compound and Using it as a Gas Sensor

Background: Both the cubic (zinc blend) and hexagonal (wurtzite) phases of cadmium sulphide are semiconductors due to the presence of a row of four equally spaced Cd atoms surrounded by one S atom. Both the cubic and hexagonal crystal forms of Cd-S have very similar nearest neighbor bond lengths. Material and Methods: The cadmium sulfide compound was prepared at a concentration of 1 M by dissolving the percentage by weight of each of cadmium(13.326g) and sulfur (3.806g) in 100 ml of distilled water. Cobalt was prepared at a concentration of 0.5M by dissolving a percentage by weight of Cobalt acetate (14.55g) in 100 ml of distilled water. Films CdS and CdS:Co were prepared using the chemical method, which is the thermal chemical spraying technique, which deposited a solution of cadmium sulfide mixture of cobalt solution with cadmium sulfide on a glass floor.
 Results: The thickness of the prepared films was 100 mm. The surfaces of the prepared films were studied by measuring transmission electron microscopy. The results showed that the prepared films are homogeneous and have crystalline granules of spherical shape, and the addition of cobalt to the cadmium sulfide compound led to an increase in the particle size values. The electrical properties represented by measuring the voltaic- current were studied, and the results showed that the relationship between the voltaic- current is linear, and this indicates that the prepared films behave as ohmic.
 Conclusion: The continuous electrical conductivity was also calculated, and the results showed that the addition of cobalt acetate to the cadmium sulfide compound led to an increase in the electrical conductivity values. The gas sensitivity of the prepared films towards hydrogen sulfide gas was studied, and the results showed that the prepared films had good sensitivity, and adding cobalt acetate to the cadmium sulfide compound led to a decrease in the sensitivity values.

Assessment of tight-binding models for high-harmonic generation in zinc blende materials.

Using a simulator for semiconductor Bloch equations (SBEs) accounting for the entire Brillouin zone, we examine the tight-binding (TB) description for zinc blende structure as a model for high-harmonic generation (HHG). We demonstrate that TB models of GaAs and ZnSe exhibit second-order nonlinear coefficients that compare favorably with measurements. For the higher-order portion of the spectrum, we use the results published by Xia et al. in Opt. Express26, 29393 (2018)10.1364/OE.26.029393 and show that the HHG spectra measured in reflection can be closely reproduced by our simulations free of adjustable parameters. We conclude that despite their relative simplicity, the TB models of GaAs and ZnSe represent useful tools to study both the low- and higher-order harmonic response in realistic simulations.

Cadmium Sulfide Nanoparticles: Synthesis, Characterization, and Antimicrobial Study

Group II–VI cadmium sulfide nanoparticles (CdS NPs), a potential alternative to antibiotics, were synthesized using cadmium acetate [Cd(Ac)2] as a precursor, sodium sulfate as a reducing agent, and starch as a capping agent through chemical precipitation techniques. The CdS NPs were characterized using different characterizing techniques; X-ray diffraction and selected area electron diffraction patterns verify that the nanoparticles had cubic zinc blende structure with average dimension size of 2.43 nm. Scanning electron mictroscope and transmission electron microscope images illustrated that the particles were uniformly distributed in cluster form, and FTIR and EDX confirmed the synthesized nanoparticles were pure. The CdS NPs show relevant antimicrobial sensitivity against Gram-positive and Gram-negative bacterial strains and the zone of inhibition increases with the increase in the concentration of nanoparticles. In comparison to Gram-positive and Gram-negative bacteria, the synthesized nanoparticles were reported to be more effective toward Staphylococcus aureus, Gram-positive bacteria.

Advancement of Physical and Photoelectrochemical Properties of Nanostructured CdS Thin Films toward Optoelectronic Applications.

CdS thin films were grown on an FTO substrate at different temperatures, employing the low-cost hydrothermal method. All the fabricated CdS thin films were studied using XRD, Raman spectroscopy, SEM, PL spectroscopy, a UV-Vis spectrophotometer, photocurrent, Electrochemical Impedance Spectroscopy (EIS), and Mott-Schottky measurements. According to the XRD results, all the CdS thin films were formed in a cubic (zinc blende) structure with a favorable (111) orientation at various temperatures. The Scherrer equation was used to determine the crystal size of the CdS thin films, which varied from 25 to 40 nm. The SEM results indicated that the morphology of thin films seems to be dense, uniform, and tightly attached to the substrates. PL measurements showed the typical green and red emission peaks of CdS films at 520 nm and 705 nm, and these are attributable to free-carrier recombination and sulfur vacancies or cadmium vacancies, respectively. The optical absorption edge of the thin films was positioned between 500 and 517 nm which related to the CdS band gap. For the fabricated thin films, the estimated Eg was found to be between 2.50 and 2.39 eV. According to the photocurrent measurements, the CdS thin films grown were n-type semiconductors. As indicated by EIS, resistivity to charge transfer (RCT) decreased with temperature, reaching its lowest level at 250 °C. Flat band potential and donor density were found to fluctuate with temperature, from 0.39 to 0.76 V and 4.41 × 1018 to 15.86 × 1018 cm-3, respectively, according to Mott-Schottky measurements. Our results indicate that CdS thin films are promising candidates for optoelectronic applications.

Al–SrTiO3/Au/CdS Z-schemes for the efficient photocatalytic H2 production under visible light

Development of sustainable solar fuels to meet the continued increase in the energy need worldwide is of particular importance. In addition, reducing carbon emissions could be the last hope to solve global challenges such as global warming and climate change. For this endeavor, different Al–SrTiO 3/Au/CdS Z-Scheme samples were developed by changing the weight percentage (wt. %) of gold nanoparticles (Au NPs) and the wt% ratio of Al–SrTiO3 to CdS NPs using post annealing of mixed powders method. The prepared samples were characterized using several physicochemical techniques to reveal their properties, then their photocatalytic H2 evolution performance was evaluated under visible light using photodeposited RhCr2O3 as H2 evolution cocatalyst. Our results showed that, Al–SrTiO3 particles possessed high crystallinity and cubic perovskite phase, Au NPs exhibited face-centered cubic (FCC) structure, and CdS NPs had cubic Zinc Blende phase as confirmed by XRD analysis. Our UV–Vis. diffuse reflectance analysis revealed that all prepared Z-scheme samples obtained a strong absorption in both UV and visible regions. SEM analysis confirmed the cubic morphology of the perovskite Al–SrTiO3 particles which were randomly deposited by Au NPs of relatively smaller size. While CdS NPs showed semi-spherical shape and were conjugated with Al–SrTiO3 directly to form the proposed Z-Schemes as proposed by our STEM mapping analysis. The highest amount of H2 (656.7 μ mole) was evolved by Al–SrTiO3/4 wt% Au/CdS (1:1) Z-scheme sample after 3 h of visible light irradiation. This sample had 4 wt % of Au NPs and equimolar ratio of Al–SrTiO3 and CdS NPs. This relatively higher activity might be because of the efficient light absorption, prolonged lifetime of the photogenerated charge carriers and improved charge separation as revealed by the PL analysis. Our study can promote the ongoing efforts toward more carbon neutral societies.

Highly efficient twinned MnxCd1-xS homojunction photocatalyst modified by noble metal-free Ni12P5 for H2 evolution under visible light

A series of twin-phase MnxCd1-xS solid solutions with zinc blende (ZB) and wurtzite (WZ) structures were prepared by an alkaline hydrothermal method in this work. When using 0.35 M Na2SO3/0.25 M Na2S solution as a sacrificial agent under visible light (λ ≥ 420 nm), the H2 production activity of T-Mn0.5Cd0.5S is 33.4 mmol h−1 g−1, significantly higher than that of ZB-Mn0.5Cd0.5S (15.4 mmol h−1 g−1) or WZ-Mn0.5Cd0.5S (18.2 mmol h−1 g−1). The photocatalytic activity of T-Mn0.5Cd0.5S is further enhanced by introducing Ni12P5 as a cocatalyst, in which the H2 evolution rate of 0.2% Ni12P5/T-Mn0.5Cd0.5S can reach 96.5 mmol h−1 g−1, 2.89 and 1.77 times than that of pure T-Mn0.5Cd0.5S and 0.5% Pt/T-Mn0.5Cd0.5S (54.6 mmol h−1 g−1), respectively. The crystalline phase, microstructure, elemental composition, optical and optoelectronic properties of the samples were characterized by XRD, SEM, TEM, HRTEM, XPS, UV–vis, PL and electrochemical workstation. The results show that the formation of a homojunction with interlaced energy band inhibits carrier recombination in the bulk phase of T-Mn0.5Cd0.5S. In addition, Ni12P5 can substitute for the noble metal as a cocatalyst to reduce the overpotential and improve the separation rate of electrons and holes to enhance H2 evolution. This work provides reference for building homojunction photocatalysts and loading inexpensive transition metal phosphides instead of noble metals.

Ligand and solvent effects on the absorption spectra of CdS magic-sized clusters.

The absorption spectra of congenetic wurtzite (WZ) and zincblende (ZB) CdS magic-sized clusters are investigated. We demonstrate that the exciton peak positions can be tuned by up to 500meV by varying the strong coupling between X-type ligands and the semiconductor cores, while the addition of L-type ligands primarily affects cluster midgap states. When Z-type ligands are displaced by L-type ligands, red shifts in the absorption spectra are observed, despite the fact there is a small decrease in cluster size. Density functional theory calculations are used to explain these findings and they reveal the importance of Cd and S dangling bonds on the midgap states during the Z- to L-type ligand exchange process. Overall, ZB CdS clusters show higher chemical stability than WZ clusters but their optical properties exhibit greater sensitivity to the solvent. Conversely, WZ CdS clusters are not stable in a Lewis base-rich environment, resulting in various changes in their spectra. Our findings enable researchers to select capping ligands that modulate the optical properties of semiconductor clusters while maintaining precise control over their solvent interactions.

First-principles calculations to investigate structural, electronic, and optical properties of zinc-blende CdxMg(1-x)TeyS(1- y) matched to CdX (X= S, Te) for Predicting critical thickness and corrective Term's effect

We report in the present paper ab initio calculations of the structural, electronic and optical properties of zinc blende quaternary alloys CdxMg(1-x)TeyS(1-y) lattice matched to CdX (X = S, Te) exploring the density functional method within the generalized gradient approximation (GGA). From the obtained values of the ground state energy, we have determined the structural parameters and the band gap energies. We found an underestimation of the gap energies obtained for both (GGA-PBE) and the Modified Becke-Johnson exchange potential (TB-mbJ) approximations for the CdS, CdTe, MgS, and MgTe. Even though mbJ significantly corrects the energy gaps, it is no longer valid to properly reproduce the exchange-correlation potential. Consequently, we tend to use the corrective term ΔE to exploit it to enhance the electronic band structures.Moreover, we have reported the concentration depending on the refractive index and the dielectric function using (mbJ). The critical thickness, for which the lattice of the CdxMg(1-x)TeyS(1-y) matches CdS and CdTe, respectively, are determined. The results show that for all Cd and Te incorporated amounts, the CdxMg(1-x)TeyS(1-y) lattice matched to CdX (X = S, Te) substrate alloys exhibits a semiconducting behavior with a small lattice mismatching ratio which might make CdxMg(1-x)TeyS(1-y)/CdX materials promising and useful for optoelectronic applications.

Influence of growth time on the properties of CdTe thin films grown by electrodeposition using acetate precursor for solar energy application

Cadmium telluride (CdTe) thin films were deposited using a two–electrode electrodeposition (ED) configuration from an aqueous acidic solution. The electrolyte solution contains 1 M of cadmium acetate dihydrate (Cd (CH3OO) 2.2H2O) as cadmium precursor and 1 ml of tellurium dioxide (TeO2) as tellurium precursor. The thin films were grown for different deposition times of 60, 120, 180, 240, and 300 min to investigate the effect of the deposition period on the structural, optical, electrical, surface morphology, elemental composition, and surface roughness properties of the CdTe thin films in both as–deposited and heat–treated forms. X-ray diffraction (XRD) analysis indicates that the CdTe thin films have polycrystalline cubic zinc blend, orthorhombic and hexagonal structures. The result confirmed that the cubic phase is dominant and the peak for preferred orientation is along the (111) plane. Ultraviolet-visible (UV–vis) spectrophotometry study shows that the band gap of the as-deposited thin films varies from (1.41–1.45) eV, and after heat treatment, the band gap decreased to (1.39–1.42) eV. Photoelectrochemical cell (PEC) measurements show that CdTe thin films haven-type conductivity in both as–deposited and annealed forms. Scanning electron microscopy (SEM) analysis shows that the surface morphology of CdTe thin films changed as the deposition period increases. After heat treatment, increase in grain size was observed. Energy–dispersive x-ray spectroscopy (EDS) analysis shows that the percentage composition of as–deposited and heat-treated CdTe thin films varied with deposition time. After post–deposition treatment (PDT), the concentration of Te decreased, while that of Cd increased due to recrystallization during annealing. For the film deposited for 120 min, stoichiometric composition of CdTe was observed after heat treatment. Scanning probe microscopy (SPM) measurements revealed that the average surface roughness of the thin films varied with deposition time. The maximum average surface roughness was recorded when the film was deposited for 120 min. These results show that the prepared CdTe thin films have potential application as absorber layers in thin film solar cells.

Design and development of four-layer anti-reflection coating stacks (ZnS and YF3 thin films) for HgCdTe-based mid-wave infrared detectors

The four-layer anti-reflection coating (ARC) film stacks (consisting of four layers of zinc sulphide (ZnS) and yttrium fluoride (YF3) materials with an optimized thickness of 699 ± 2.6 nm) were grown on the backside of the HgCdTe (mercury cadmium telluride)-CdZnTe (cadmium zinc telluride) assembly (i.e., CdZnTe substrate) using the thermal-evaporation deposition technique. Successively, the characterization tools like atomic force microscope, X-ray diffractometer, energy dispersive X-ray spectrometer, and scanning electron microscope were utilized to realize the optimized ARC film stacks. ARC stacks (ZnS/YF3/ZnS/YF3) with cubic zinc blende phase and preferred orientation (111) were confirmed. The desired surface morphology (dense and low roughness), elemental composition (purity), and micro-structural features (reasonable grain size and material continuity) of ARC films were achieved. The optimized films were further examined through their utilization in the fabrication of two similar ARC stacks/structures with a configuration of YF3/ZnS/YF3/ZnS/CdZnTe–HgCdTe and Fourier Transform infrared spectrometer was used to measure their reflectance. ARC film stacks (with remarkably thin film thickness and good repeatability) have reduced the reflection of HgCdTe–CdZnTe assembly to a minimum value from 22% to < 1.0% within the mid-wave infrared spectral region, which further validates theoretically. Thus, the four-layer ARC film stacks are the effective reflection agent that could be used to fabricate high-performance HgCdTe infrared detectors.

Growth of Cd1-xMnxTe crystals by vertical Bridgman method and analysis of composition dependence on the bandgap

This article reports the growth of cadmium manganese telluride (Cd1-xMnxTe, CMT) single crystal by vertical Bridgman method and the property measurements for three different Mn compositions (x = 0.05, 0.10 & 0.15). The single crystalline nature of the grown crystals was confirmed with Laue back reflection method. The lattice constants were calculated using single-crystal X-ray diffraction analysis. Phase formation and zinc blende crystal structure were confirmed with powder X-ray diffraction analysis. The shifting of the diffraction peak position indicates the reduction in lattice parameter of Cd1-xMnxTe crystals with Mn concentration. Raman spectra exhibited two mode behavior having CdTe and MnTe-like structures and their longitudinal optical phonon modes displaying peak shift with Mn concentration. The increase in the Mn concentration in CMT lattice made a blueshift in photoluminescence emission band leading to changes in the bandgap from 1.5776 to 1.7250 eV. The experimentally observed IR transmittance was found to be in line with theoretically observed transmittance. The observed Mn concentration dependent structural and optical properties confirm the formation of high quality CMT single crystals by vertical Bridgman technique.

Substrate Terrace Width Dependence of Direct Growth of GaN on ScAlMgO4 by Radio‐Frequency Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted considerable attention as platforms for GaN growth in recent years because GaN can be grown directly on SAM without any buffer layer. Herein, the effect of the terrace width of SAM substrates on direct GaN growth is investigated using radio‐frequency molecular beam epitaxy (RF‐MBE). A flat and single wurtzite (WZ)‐phase GaN film is grown on a SAM substrate with a large terrace width. In contrast, a rough GaN film with a mixture of WZ and zinc‐blende (ZB) phases is obtained on SAM with a small terrace width. The SAM step height of 0.8 nm corresponds to three GaN molecular layers. Therefore, ZB stacking faults may be generated during the coalescence of islands located on adjacent terraces, although only the WZ‐GaN islands grow in the initial growth stage. This indicates that SAM substrates with a larger terrace width, that is, a smaller off‐cut angle, are preferable for obtaining a flat WZ‐GaN film.