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Integral monolayer-scale featured digital-alloyed AlN/ GaN superlattices using hierarchical growth units Na Gao, Xiang Feng, Shiqiang Lu, Wei Lin, Qinqin Zhuang, Hangyang Chen, Kai Huang, Shuping Li, and Junyong Kang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01677 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Crystal Growth & Design

Integral monolayer-scale featured digital-alloyed AlN/GaN superlattices using hierarchical growth units Na Gao1, Xiang Feng1, Shiqiang Lu1, Wei Lin1, Qinqin Zhuang2, Hangyang Chen1, Kai Huang1*, Shuping Li1 and Junyong Kang1* 1Department

of Physics, OSED, Fujian Provincial Key Laboratory of Semiconductors

Materials and Applications, Xiamen University, Xiamen 361005, P. R. China 2Fujian

Key Laboratory of Optoelectronic Technology and Devices, School of

Opto-electronic and Communication Engineering, Xiamen University of Technology, Xiamen 361024, P. R. China Abstract Acquiring AlN/GaN digital alloys with matching coherent lattices, atomically sharp interfaces, and negligible compositional fluctuations remains a challenge. In this work, the nature and formation mechanism of the constituent elements of AlN and GaN atomic layers growth was examined by first-principle calculations and experimental demonstration. Basing on the calculated formation enthalpies, we developed a hierarchical growth method wherein AlN and GaN growth units are digitally stacked layer by layer through metal organic vapor-phase epitaxy, which involves the growth sequence instantaneously to control chemical potentials of the

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hierarchical growth units under different atmospheres. High-resolution X-ray diffraction and transmission electron microscopy confirmed that the hierarchical GaN and AlN growth units of digital-alloyed AlN/GaN structures had coherent lattices, abrupt

interfaces,

and

integral

monolayers

at

the

atomic

scale.

The

cathodoluminescence properties featured with single emission, combining with theoretical results, demonstrated that the capability of electronic energies via the digital-alloyed AlN/GaN superlattices. These results provide a basis toward the realization of other digital-alloyed nitride semiconductors. Keywords: digital-alloyed AlN/GaN superlattices, hierarchical growth units, integral monolayer-scale

1. Introduction Aluminium gallium nitride (AlGaN), an essential compound semiconductor for III-nitrides material systems, has attracted considerable attentions in recent years. These systems hold various promising applications in many fields, such as ultraviolet/deep ultraviolet optoelectronic devices, confidential optical communication in space, and chemical/biological agent sensing, thanks to their wide direct band-gaps with great adjustability and great physical, chemical, and mechanical stability.1-4 Many efforts have been put into obtaining AlGaN films with high Al contents. However it still remains challenging to prepare epitaxial AlGaN with atomically sharp interfaces, and negligible compositional fluctuations, due to the large lattice mismatch and strong vapor-phase pre-reaction during growth.5-8 Some researchers reported


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eliminating vapor-phase pre-reaction during the epitaxy of AlGaN films with high Al contents by optimizing the V/III ratio and reactor pressure.9 Peng et. al. used AlN/AlGaN superlattice interlayer, which has potential for strain relief, to improve crystal quality in AlGaN films.10 However, these methods involve large interfacial roughness and compositional fluctuation of AlGaN ternary alloys with high Al contents. Therefore, new strategies or novel structures with comparable electronic and optical properties to conventional AlGaN ternary alloys are required. Digital alloys, which are short-period superlattices constructed with binary or ternary alloys with layer thicknesses in atomic scale, are widely used when overcoming fundamental issues in ternary alloys, especially for AlGaN ternary alloys with high Al contents.11-13 To constitute AlN/GaN digital alloys, a stack of two-dimensional AlN and GaN nanostructures is critical. Recently, many first-principle calculations for two-dimensional AlN, GaN, and digital alloy AlN/GaN structures have been performed, indicating that band gap engineering on electronic structure can be tailored precisely with their corresponding ternary alloys.14 The valence band crossover issue can be reversed by switching from the dominant transverse-magnetic to transverse-electric polarized emission for materials with high Al contents.15 Besides, coherent growth with atomic sharp interfaces is essential for practical applications of AlN/GaN digital alloys. Numerous methods, including the modulation of the AlN layer by migration-enhanced epitaxy, optimization of the Ga/N ratio for the GaN epilayer, and the alternate supply of groups III and V sources have been developed for the preparation of digital-alloyed AlN/GaN structures.16-17 Despite



their benefits in improving AlN/GaN heteroepitaxy by coherent lattice match and atomically abrupt interfaces, preparing digital alloying structures through accurately stacking AlN and GaN monolayers down to one atomic layer thin in integral scale without compositional fluctuations, remains challenging. In general, by precisely controlling growing units of AlN and GaN monolayer (ML)-scale epilayers below their equilibrium critical thickness, AlN/GaN structures can be coherently grown and the introduced strain in the alloyed substitution can be stabilized without relaxation.18 However, the difficulties for digital alloying AlN/GaN heteroepitaxial growth exist because the nonequilibrium nature of metal organic vapor-phase epitaxy (MOVPE) technique and the limits of operating temperature lead to inevitable vapor-phase pre-reaction and surface roughness for single atomic layer growth. It turns out that preparing digital-alloyed AlN/GaN structures by controlling temperature and pressure of MOVPE process is unlikely to be realized. Thus, controlling chemical potentials of different environments is an attractive route and a deeper theoretical and experimental insight into the underlying mechanism of growing units per layer on atomic scale is essential. The formation mechanism for AlN growth is based on hierarchical growth principle.19 Here, we examined the nature and formation mechanism by calculating growth units for Al/Ga/N atoms adsorbed onto GaN or AlN surfaces. By varying the growth conditions under different environments, a series of AlN/GaN digital alloys with tunable integral scale monolayers was prepared with the MOVPE technique. The results indicate that the hierarchical growth method can provide a new strategy for


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producing the coherent growth units of AlN/GaN digital alloys with abrupt interfaces in the atomic scale and is favorable for precise control of electronic energy in deep-ultraviolet wavelength.

2. Theory and experimental 2.1 Simulation details The formation enthalpies were examined through first-principle calculations, the Vienna ab initio simulation package based on the density functional theory (DFT). A slab of ultrathin alloyed (AlN)m/(GaN)n wurtzite structure was constructed under periodic boundary conditions, where m and n are the number of monolayers for AlN and GaN, respectively, which are both equal to 3. Assuming the coherent lattices of AlN and GaN domains are separated by interfaces, the electron exchange and correlation pseudopotentials were described by the projector augmented wave (PAW) method within the generalized gradient approximation (GGA), with d electrons.20 The interaction between the adjacent supercells was prevented by introducing approximately 11 Å vacuum domain above the surface. Hydrogen atoms were then added to the back side of the slab for the saturation of the dangling bonds. In our calculations, different systems of N atoms adsorbed onto the Al-terminated AlN clean surface and the Ga and N atoms adsorbed on top of the N-terminated AlN surface were constructed. The N atoms adsorbed onto the Ga-terminated and the Al and N atoms adsorbed onto the N-terminated GaN clean surface were modeled. Samples were obtained from the Brillouin Zone by using a 4 × 4 × 1 k-points mesh and a



plane-wave cutoff energy of 520 eV. In the total energy optimization, full atomic relaxation was allowed for the minimization of the energy of the system with a convergence criterion of 1 meV. Subsequently, considering the electronic structures applied, two sets of (AlN)m/(GaN)n digital alloys were constructed along the [0001] direction. One set increased the AlN layer numbers at constant GaN layer numbers, such as 2/4, 2/6, and 2/8. The other set varied n at 2, 3, and 4 ML, whereas m = 4 ML, which is approximately constant. The plane-wave basis was set with an energy cutoff of 500 eV at special k points generated by an 8 × 8 × 4 Monkhorst–Pack scheme.21 After the size of AlN/GaN supercell was fixed, the conjugate gradient algorithm was employed for the optimization of the atomic positions.22 The total energy of the system with a convergence criterion of 0.1 meV was minimized by enabling the atoms within the cells to relax all the degrees of freedom. The scissors correction of 1.75 eV was applied for the calculation of the underestimated band gap values.23 2.2 MOVPE growth of the digital-alloyed (AlN)m/(GaN)n structures Digital-alloyed (AlN)m/(GaN)n structures were grown on c-plane AlN/sapphire substrates in a vertical Thomas Swan system (3 × 2 inch CCS Aixtron) through the MOVPE technology. Trimethylaluminum (TMA), trimethylgallium (TMG), and ammonia (NH3) were used as Al, Ga and N precursors, respectively. Hydrogen (H2) was used as carrier gas. Prior to the growth, all sapphire substrates were thermally cleaned at 1100 C and 100 Torr under H2 atmosphere, followed by nitridation at 570 C for 200 s at NH3 atmosphere. Subsequently, approximately 1 μm-thick AlN


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templates were grown at 1090 C under 75 Torr. Based on the AlN templates, two sets of digital-alloyed (AlN)m/(GaN)n structures were epitaxial grown: one with AlN barrier thickness (m = 4, 6, 8 ML) but with fixed GaN well thickness n = 2 ML, and one with a fixed AlN thickness at 4 ML but with varying well thickness (n = 2, 3, 4 ML). During the digital alloying (AlN)m/(GaN)n growth, approximately 200–400 periods were grown at 1090 °C under 75 Torr. The hierarchical growth approach was introduced for the separation of the GaN and AlN layers in their specified thicknesses by controlling the flux and time of TMG and TMA and NH3 precursors. 2.3 Characterization The epitaxial samples were monitored with an in-situ optical system (Thomas Swan), including a 635 nm laser as the incident excitation source and an optical sensor for detecting the reflected beam. A high-resolution X-ray diffractometer (HRXRD, PANalytical X’ Pert PRO) with an X-ray wavelength of 0.154056 nm using Cu Kα radiation was used for characterizing crystallinity of the digital-alloyed (AlN)m/(GaN)n structures. The interfacial microstructures between the well and barrier layers was inspected by cross-sectional high-resolution transmission electron microscope (HRTEM) (FEI Tecnai G2 F20 S-Twin). The specimens were prepared by Ar ion milling, and the acceleration voltage for the TEM observation was 200 kV. The optical properties were examined by obtaining cathodoluminescence (CL) spectra with an electron gun (Orsay Physics ‘‘Eclipse’’ FEB Column) at an acceleration voltage of 10 kV. The spectra were obtained at room temperature by using a Horiba



Jobin Yvon model iHR320 spectrometer system at a spectral resolution of 0.06 nm.

3. Results and discussion To understand the underlying formation mechanism that determines the set of growth behavior, we determined the formation enthalpies of different systems of the AlN and GaN surfaces with the adsorption of Al, Ga, and N atoms, using the following formula: 𝐸𝑓 = (𝐸𝑡𝑜𝑡 - 𝐸𝑐𝑙𝑒𝑎𝑛) -∆𝑛𝑖𝜇𝑖 = (𝐸𝑡𝑜𝑡 - 𝐸𝑐𝑙𝑒𝑎𝑛) -∆𝑛𝐺𝑎𝜇𝐺𝑎 -∆𝑛𝐴𝑙𝜇𝐴𝑙 -∆𝑛𝑁𝜇𝑁 (1) where Etot and Eclean represent the total energies of the absorbed and clean surface, respectively, and Δni denotes the difference between the number of atoms in each atomic species compared with the clean surface. We assumed that the chemical potential of the surface was in equilibrium with the internal system of the bulk. Thus, the chemical potentials of Ga and N were related by the equilibrium conditions with the formula μGa + μN = μGaN (bulk). The Ga chemical potential, ∆𝜇𝐺𝑎 = μGa − μGa (bulk), varied from ∆𝜇𝐺𝑎 = −ΔHf = −0.96 (N-rich conditions) to ∆𝜇𝐺𝑎= 0 (Ga-rich conditions). Similarly, the chemical potentials of the excess atomic species of Al and N atoms introduced on the GaN surface followed the condition μAl + μN = μAlN (bulk), and the Al chemical potential varied from ∆𝜇𝐴𝑙= −2.89 (N-rich conditions) to ∆𝜇𝐴𝑙= 0 (Al-rich conditions).


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Fig. 1. Formation enthalpies of different situations in the system of (AlN)3/(GaN)3 as a function on the Al (Ga) chemical potential. (a) N-adatoms on the Ga-terminated GaN surface, (b) Al atoms absorbed onto the N-adlayer GaN structures, (c) N-adatoms on the Al-terminated AlN surface, and (d) Ga atom absorbed onto the N-adlayer AlN structures. To acquire the AlN/GaN and GaN/AlN hetero-interfaces at an atomic scale, a growth atmosphere that is precisely suitable for the growth of N and Al/Ga atoms was used. The formation enthalpies of the N-adatom on the Ga-terminated GaN layers or Al-terminated AlN layers were simulated through first-principles calculations. Then, the formation enthalpies of the Al- or Ga-adatom on the N-adlayer were calculated (Fig. 1). The schematic adsorbed structures are shown as insertions in Fig.1 (a)–(d). Fig. 1 (a) illustrates that, when Al chemical potential increases, that is, the atmosphere of the reactor varies from an N-rich to an Al-rich atmosphere, the formation enthalpy of the N-adatom linearly increases and exceeds zero at μAl higher than −2.56 eV. This



behavior indicates that the N-adatom on the Ga-terminated surface of GaN tends to be thermodynamically stable under an extremely N-rich atmosphere. Fig. 1 (b) shows that an Al-rich condition is essential for the adsorption of Al-adatom onto the N-adlayer on the top of the Ga-terminated surface of GaN. Similar to the results in Fig. 1 (a), Fig. 1 (c) presents that the N-adatom on top of the Al-terminated surface of AlN also requires an N-rich growth atmosphere. However, further adsorption of Ga atoms onto the N-adlayer on the Al-terminated surface of AlN provided a formation enthalpy for Ga-adatom. The enthalpy was lower than that of the clean surface for all Ga chemical potential values (Fig. 1 [d]). Thus, the adsorption of Ga-adatom onto the N-adlayer of the Al-terminated surface of AlN is not affected by chemical potential environment and tends to be thermally stable under a Ga-rich or N-rich growth atmosphere.


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Fig. 2. Schematic sequence of growth for (AlN)m/(GaN)n digital alloys (a) and typical real-time monitoring curves of the complete growth process for samples with m/n = 4/2, 6/2, 8/2, 4/3, and 4/4 atomic layers for samples A, B, C, D and E, respectively, according to MOVPE (b). On the basis of calculations for the AlN and GaN adsorbate units, a hierarchical growth approach through MOVPE for the growth of (AlN)m/(GaN)n digital alloys was proposed as the schematic growth sequence (Fig. 2 [a]). A perfect N-adlayer on the Al-terminated AlN surface or Ga-terminated GaN surface is essential for confirming the smoothness of the AlN/GaN and GaN/AlN hetero-interfaces. Therefore, combined with the abovementioned formation enthalpy, the ammonia flow was individually initiated for approximately three seconds in a single growing period. The purpose was to supply an extreme N-rich atmosphere. Under N-rich conditions, the N-adatom diffused and sought a stable site to induce a densely N-adlayer on the Al-terminated surface of AlN. Then, the TMG source was introduced for the growth of GaN atomic layers with the same ammonia flow because the adsorption of Ga-adatom onto the N-adlayer of the Al-terminated surface of AlN is not sensitive to chemical potential. Subsequently, followed by the introduced ammonia flow individually for only three seconds, the N-adlayer on the Ga-terminated GaN surface was also epitaxied under N-rich growth atmosphere. Furthermore, given that the efficient adsorption of Al atoms occur only under Al-rich environment, we injected the TMAl source into the reactor with a reduced ammonia flow to shift the reactor atmosphere to an Al-rich atmosphere in order to induce the growth of AlN atomic layers. Thus, the AlN/GaN and GaN/AlN hetero-interface structures were grown by the complete growth process. By precisely controlling the flux and time of the precursors, a series of (AlN)m/(GaN)n



digital alloys with integral monolayer-scale growth units was fabricated, named as samples A, B, C, D, and E corresponding to (AlN)4/(GaN)2, (AlN)6/(GaN)2, (AlN)8/(GaN)2, (AlN)4/(GaN)3, and (AlN)4/(GaN)4, respectively. Fig. 2 (b) shows the typical real-time monitoring curves recorded during the complete growth process for samples A, B, C, D, and E. The initial steep rise followed the growth of approximately 20 nm-thick nucleation layer, leading to a high reflectivity owing to the increased refractive index of AlN, which corresponded to peak 1. By introducing the hierarchical growth method, a remarkable increased amplitude represented by peak 2 appeared upon the establishment of a compact layer with a smooth surface. The growth rate was determined at 540 nm per hour for AlN epitaxy. Thus, the total thickness of the AlN buffer layer was approximately 1 μm. Subsequently, the intensity of interference oscillation increased at peak 3 because of the increased reflectivity of the digital-alloyed (AlN)m/(GaN)n structures. The growth rate started to decrease, and the oscillation intensity remained steady and uniform until the end of the growth process for samples A, B, C, D, and E. This observation indicates that the hierarchical growth approach led to a two-dimension layer-by-layer growth and a smooth hetero-interface.


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Fig. 3. HRXRD patterns of the ω/2θ (0002) scans, with layer thicknesses of the digital-alloyed AlN/GaN (m/n) at 4/2 (black line), 6/2 (red line), 8/2 (blue line), 4/3 (green line), and 4/4 (purple line) atomic layers for samples A, B, C, D, and E, respectively. To confirm the structural parameters of (AlN)m/(GaN)n digital alloys, results of symmetric HRXRD ω/2θ scans are illustrated in Fig. 3. All samples exhibit the diffraction peaks from the AlN template and sapphire substrate associated with a series of satellite peaks, which refer to the combined thickness of the GaN and AlN constituting layers, demonstrating the formation of sharp interface and pronounced period structures. Using the formula,24 the average lattice constant c, periodic thickness d, and GaN and AlN monolayers were determined in Table 1. The structural parameters are consistent with the theoretical expectations. For samples A, B, and C, the well thickness of GaN was evaluated at 2 ML, and the thickness of the AlN barrier



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thickness was evaluated at 4, 6, and 8 ML. As the barrier thickness increases, the distance of both the zero-order satellite of the digital alloys to the adjacent AlN peak and that between the satellite peaks decreases. The zero-order satellite peak gradually approached AlN because of the increase in average Al content. Moreover, the narrow full-width at half-maximum (FWHM) with high intensity was observed. Samples D and E were evaluated by varying the GaN well thickness from 3 ML to 4 ML and fixing the 4 ML-thick AlN barrier as that of sample A (Table 1). Therefore, by the hierarchical

growth

control,

digital-alloyed

(AlN)m/(GaN)n

structures

with

controllable well and barrier layers on the scale of integral monolayers can be accurately achieved through MOVPE.

Table 1. Structural parameters of the digital-alloyed (AlN)m/(GaN)n structures. Sample A B C D E

c (Å) 5.030 5.009 5.005 5.032 5.045

d (nm) 1.564 2.200 2.578 1.871 2.033

GaN (ML) 2 2 2 3 4

AlN (ML) 4 6 8 4 4

The cross-sectional TEM images (Fig. 4) were used for characterizations for the AlN/GaN and GaN/AlN hetero-interface structures of the digital alloys. In these figures, the darker fields represent GaN layers, whereas the bright fields represent the AlN layers. Clear and atomically sharp interfaces and 2 ML of GaN layers and 8 ML of AlN were recognized from Fig. 4a, whereas in Fig. 4b 4 ML of GaN well and 4


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ML of AlN can be identified. Thus the structure was grown at the atomic scale. Smooth and sharp interfaces were observed between GaN and AlN, indicating that the intermixing of Al and Ga composition of the ternary AlGaN alloy did not fluctuate throughout the period. Moreover, detailed asymmetric (105) reciprocal space mapping (RSM) of the sample (AlN)8/(GaN)2 shows the growth of coherent lattices between the AlN and GaN constituting layers. There are two coordinates (Qx, Qy) in the map correspond to a pair of lattice constants (a, c). The main peaks are aligned at nearly the same Qx value (denoted by a dashed blue line), while the reciprocal vector perpendicular to the surface is resolved. There is no observation of the independent diffraction patterns of both the well and barrier, which further indicates that the well and barrier layers are coherently grown on the AlN/sapphire templates and therefore fully strained.

Fig. 4. Cross-sectional TEM images showing digital alloying (AlN)m/(GaN)n structures with different well and barrier layers for the typical samples of m/n with (a) 8/2 and (b) 4/4 atomic layers and (c) asymmetric (105) reciprocal space mapping (RSM) of the sample (AlN)8/(GaN)2. The band structures of the digital alloys were investigated by first-principles calculations. By arranging the position-dependent densities-of-states (PDOS) of each atomic layer along the [0001] direction in sequence, the details of the band structures



were reflected at the atomic level in Fig. 5 (a)–(d). Regarding to the sharp interface identified by HRTEM, sharp DOS difference across the AlN/GaN boundary was shown, indicating a good confinement of the carriers in the GaN wells. As shown in Fig. 5 (a)-(c), when the thickness of the AlN barrier layer increases from 4 ML to 8 ML, the band gap of the GaN well remains approximately unchanged. Meanwhile, the band bending is reduced as the thickness of the AlN barrier increases. However, as illustrated in Fig. 5 (d), when the thickness of the GaN well layer is increased to 4 ML at a constant barrier thickness of 4 ML, the band gap of GaN well rapidly decreases. This tendency reveals that the band gap of the GaN well is more sensitive to the thickness of the GaN wells than that of AlN barriers. More detailed information can be illustrated in the band structures as shown in Fig. 5 (e)–(h). By varying the AlN thickness from 4 ML to 8 ML with a constant GaN well thickness of 2 ML, the band gap is enlarged by about 130 meV. On the contrary, the band gap is reduced by 540 meV while increasing the width of the GaN layer from 2 ML to 4 ML. The variation of the band structure is also observed by the energy dispersion along the [0001] direction of the conduction band in the well. The energy dispersion is approximately 253 meV for the (AlN)4/(GaN)2 digital alloys but decreased to 34 meV for (AlN)8/(GaN)2 digital-alloyed superlattices.


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Fig. 5. Position-dependent DOS and band structures of the modeled (AlN)m/(GaN)n digital alloys of the stacked GaN bilayers separated by different AlN atomic layer numbers m = 4 ML (a) (e), m = 6 ML (b) (f) and m = 8 ML (c) (g) and stacked GaN n = 4 ML with AlN m = 4 ML (d) (h). The PDOS for individual atoms are arranged along the [0001] direction. The values are represented by the color bar, and the ordinate represents the energy.

Fig. 6. Normalized CL spectra at room temperature for the digital alloying of



AlN/GaN (m/n) structures with (a) 4/2 (black line), 6/2 (red line), 8/2 (blue line), (b) 4/2 (black line), 4/3 (green line), and 4/4 (purple line) atomic layers for samples. The calculated band structures were verified by obtaining the normalized CL spectra from the digital-alloyed (AlN)m/(GaN)n structures at room temperature (Fig. 6). Sharp peaks were observed at 5.22, 5.29, and 5.36 eV for samples A, B, and C, respectively, as the thickness of the AlN layer increased from 4 ML to 8 ML (Fig 6[a]). Noticeably, all the three samples exhibited a single emission, which indicated the uniform distribution of composition and geometry. Hence, the precise control of the electronic energies in the deep-ultraviolet wavelength was favored. The inter-well coupling was reduced with the blue shift in the relatively thick barrier, which reduced the inner stress.25 In Fig. 6 (b), when the width of the GaN well layer increased from 2 ML to 4 ML, the energy peak shifted from 5.22 eV to 4.78 eV. Thus, all the spectra were consistent with the theoretical predictions. However, a shoulder peak appeared in sample D, which broaden the FWHM, possibly due to the inhomogeneous composition distribution in the variation of well and barrier thickness.26-27 From Fig. 5 and Fig. 6, the measured emission wavelength shift are in good accordance with the theoretical calculations, confirming the capability of band structure design via the digital-alloyed AlN/GaN superlattices.

4. Summary The first-principles calculations demonstrate that the chemical potentials in digital alloy (AlN)m/(GaN)n structures can be controlled by varying the well and barrier thickness on integral monolayer-scale. By combining the microscopic growth


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mechanisms for the formation enthalpies of the growth units, such as AlN and GaN, a series of (AlN)m/(GaN)n digital alloys with varying well and barrier thicknesses were accurately prepared with the hierarchical growth method by MOVPE. Coherence and high crystallinity was achieved for the (AlN)m/(GaN)n digital alloys with integral monolayers and atomically sharp interfaces. And the electronic energy of the (AlN)m/(GaN)n digital alloys was favorably controlled for the deep-ultraviolet applications.

Corresponding Authors *E-mail: [email protected]. Phone: +86-592-2187737. *E-mail: [email protected]. Phone: +86-592-2185962. Author Contributions All authors participated in the conception of the project and took part in the discussion of results. NG and XF wrote the first draft of the manuscript. NG, KH and JYK revised the final manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements The authors appreciate Dr. Li Ji at the University of Texas at Austin for assistances of manuscript revision and Dr. Shuai Wang at Huazhong University of Science



Technology and Dayi Liu at Xiamen University for technical assistances. This work was supported by the National Key Research and Development Program (2016YFB0400903), the National Natural Science Foundation of China (61604124, U1405253, 61874090 and 61874091), the Natural Science Foundation of Fujian Province (2017J01121) and the fundamental research funds for the central universities (20720160018, 20720170098 and 20720170012).

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For Table of Contents Use Only Integral monolayer-scale featured digital-alloyed AlN/GaN superlattices using hierarchical growth units

Na Gao1, Xiang Feng1, Shiqiang Lu1, Wei Lin1, Qinqin Zhuang2, Hangyang Chen1, Kai Huang1*, Shuping Li1 and Junyong Kang1*

TOC graphic

Synopsis On the basis of the calculated chemical potentials of different environments for the underlying mechanism of AlN and GaN growing units per layer, we developed a hierarchical growth method wherein AlN and GaN units are digitally stacked layer by layer. High-resolution TEM confirmed that the digital-alloyed AlN/GaN structures had coherent lattices, abrupt interfaces, and integral monolayers at the atomic scale.


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GaN

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