Defect Suppression in AlN Epilayer Using Hierarchical Growth Units

Article pubs.acs.org/JPCC

Defect Suppression in AlN Epilayer Using Hierarchical Growth Units Qinqin Zhuang,†,‡ Wei Lin,*,† Weihuang Yang,† Wencao Yang,† ChengCheng Huang,§ Jinchai Li,† HangYang Chen,† Shuping Li,† and Junyong Kang*,† †

Fujian Key Laboratory of Semiconductor Materials and Applications, Department of Physics, Xiamen University, Xiamen 361005, P. R. China ‡ Key Laboratory of Optoelectronic Technology, Fujian Province University, Xiamen University of Technology, Xiamen 361024, P. R. China § State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: Growing AlN layers remains a significant challenge because it is subject to a large volume fraction of grain boundaries. In this study, the nature and formation of the AlN growth mechanism is examined by ab initio simulations and experimental demonstration. The calculated formation enthalpies of the constituent elements, including the Al/N atom, Al−N molecule, and Al−N3 cluster, vary with growth conditions in N-rich and Alrich environments. Using the calculation results as bases, we develop a three-step metalorganic vapor-phase epitaxy, which involves the periodic growth sequence of (i) trimethylaluminum (TMAl), (ii) ammonia (NH3), and (iii) TMAl+NH3 supply, bringing in hierarchical growth units to improve AlN layer compactness. A series of AlN samples were grown, and their morphological and luminescent evolutions were evaluated by atomic force microscopy and cathodoluminescence, respectively. The proposed technique is advantageous because the boundaries and defect-related luminescence derived are highly depressed, serving as a productive platform from which to further optimize the properties of AlGaN semiconductors.

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INTRODUCTION AlN films have recently attracted considerable attention because of their remarkable thermal, electronic, photonic, chemical, and mechanical properties. Advancements in this area will likely enable potential applications operated at the UV region, such as water and air purification, food sterilization, and biochemistry. Although the atomically abrupt, lattice-matched interface in high Al-content AlGaN is expected to be extensively used in electronic devices, practical application remains limited in the preliminary stages because of the difficulty encountered in growing high Al-content AlGaN with low surface roughness and high crystallinity. Atomic layer epitaxy and the production of smooth layers are hindered by the low surface migration of Al atoms and the gas-phase prereaction between trimethylaluminum (TMAl) and ammonia (NH3). A growth temperature higher than 1300 °C is believed to effectively improve crystalline quality;1−3 such temperatures constitute stringent requirements for the operating temperature of growth equipment. In recent years, researchers have developed another method for enhancing the crystallinity of AlN films to alternatively supply groups III and V sources with TMA1 and NH3 instead of simultaneously supplying sources with these reagents by metalorganic vapor phase epitaxy (MOVPE).4 This method avoids the strong vapor-phase © 2013 American Chemical Society

prereaction and enhances the diffusion of Al atoms at relatively lower growth temperatures.5−7 However, AlN films grown by either method are susceptibly epitaxial in forms of fractal-like extensions with a large volume fraction of grain boundaries, causing arbitrary tilts and twists among misaligned grains.8 Observations indicate that the twists in randomly distributed grains form threading dislocation (TD) in grain boundaries after coalescence. The presence of TDs is generally detrimental to radiative recombination and carrier transport. Previous attempts induced a hetero AlN/AlxGa1−xN superlattice (SL), which enables interisland voids to be partially filled, thereby blocking TDs and achieving lateral coalescence.9,10 Nonetheless, an additional heteroepitaxial layer causes complications, such as variations in band structure, lattice, and thermal expansion mismatch and differences in surface energy for various growth units during the growth process. Both experimental knowledge and previous theoretical studies agree that the choice of growth parameters can strongly influence primary growth units and finally as-grown island shapes, such as single atoms, molecules, and clusters.11 By Received: February 19, 2013 Revised: May 12, 2013 Published: June 18, 2013 14158

dx.doi.org/10.1021/jp401745v | J. Phys. Chem. C 2013, 117, 14158−14164

The Journal of Physical Chemistry C

Article

sequentially inducing the appropriate growth units, epilayers, which can be constructed with different levels of homolayer compactness, are expected to evolve toward the smooth and compact film. In addressing this subject, understanding the basic atomic processes that occur on the growth surface is important. Directly observing the migration behavior of adsorbates beyond the experimental scale is impractical. Theoretically, many investigations regarding migration mechanism of adsorbate−surface systems are conducted by ab initio simulations on an atomic scale,12,13 helping provide a clear insight into the mechanism of layer-by-layer epitaxy. In the present work, we advance the understanding of the nature and formation of growth mechanisms in producing compact AlN layers by epitaxy. A stable cluster is identified by calculating the formation enthalpy of Al−Nn. Understanding the AlN growth mechanism necessitates that the formation enthalpies of the Al/N atom, Al−N molecule, or Al−N3 cluster adsorbed onto AlN surfaces be evaluated by performing firstprinciples total-energy calculations within density functional theory (DFT). The different migration behaviors of the adsorbates are represented by analyzing the calculation results, facilitating AlN epitaxy with improved properties via the appropriate control and choice of precursors in the growth process. With this principle in mind, the hierarchical growth units processed via a three-step epitaxy are introduced to grow AlN layers by MOVPE. The surface morphologies and crystal qualities of the AlN epilayers are characterized by atomic force microscopy (AFM) and cathodoluminescence (CL).

Figure 1. AlN slab model with an (a) Al atom, (b) N atom, (c) Al−N molecule, or (d) Al-3N cluster adsorbed onto the surface.

The most obvious approach to choosing among a variety of Al−Nn clusters is to identify an energetically favorable configuration by evaluating the formation enthalpy of Al−Nn (n > 1) cluster structures; the selection is based on criteria described elsewhere.18 The calculated formation enthalpies are −3.335 eV Al−N2, −4.107 eV Al−N3, and 7.371 eV Al−N4. The stable configuration is triply bonded geometry, that is, one Al surrounded by three N atoms. This configuration was adopted as the typical cluster model in the succeeding simulation. To compare the relative stabilities of the systems with different stoichiometries, we determined the formation enthalpies of the AlN surfaces with an Al atom, N atom, Al− N molecule, and Al−N3 cluster adsorption as a function of Al chemical potential by calculating the chemical potentialdependent formation enthalpy:

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THEORY SIMULATION The DFT calculations were performed using the Vienna ab initio simulation package,14,15 which is based on the Perdew and Wang version of generalized gradient approximation16 and projector-augmented wave representation.17 The Al−N2, Al− N3, and Al−N4 in a cubic cell of side 25 Å were calculated to identify the stable cluster. The individual growth surfaces of AlN layers were modeled by a 3 × 3 unit cell, with the lattice constants of bulk AlN at a = 3.13 Å and c = 5.04 Å. A slab model constructed with four AlN bilayers under periodic boundary conditions was used to represent the full crystal. The dangling bonds on the back side of the slab were passivated by fractionally charged hydrogen atoms. A vacuum region was optimized, and an overall approximate length of 11 Å was determined to be sufficiently large to avoid interaction between neighboring supercells. To simulate different growth modes, we calculated the system of the Al atom, N atom, Al−N molecule, or stable Al−N3 cluster adsorbed onto the AlN surface. Considering the natural tendency of III-nitrides, we added Al and N atoms on the top of the surface N and Al atoms, respectively. The adsorbed structures are shown in Figure 1. A default plane-wave cutoff energy of 520 eV was used, and the Brillouin Zone was sampled by using a 4 × 4 × 1 k-point mesh. At each step of the simulation, the AlN underlayers were fixed in the appropriate bulk-optimized configuration to simulate the growth surface. The additional adsorbates were allowed to relax to minimize the total energy of the system with a convergence criterion of 1 meV.

Ef = (Etot − Eclean) − ΔnN1μAlN − (ΔnAl − ΔnN)μAl

where Etot represents the total energies of the adsorbed surfaces, Eclean denotes the total energies of the clean AlN surfaces, and Δn is the difference between the number of atoms in each atomic species and in the clean AlN surface. The chemical potential of N was determined through the use of the typical equilibrium conditions for bulk AlN: μAl + μN = μAlN(bulk). The Al chemical potential, ΔμAl = μAl − μAl(bulk), varies from ΔμAl = −ΔHf = −2.953 (N-rich conditions) to ΔμAl = 0 (Al-rich conditions). Figure 2 shows that when the Al chemical potential rises, that is, the atmosphere of the reactor varies from a N-rich atmosphere to an Al-rich atmosphere, the formation enthalpy of the N-adatom linearly increase and exceeds zero at a μAl higher than −2.1 eV. This behavior indicates that the N-adatom structure is more stable than the clean surface in Al-rich conditions. By contrast, the formation enthalpy of the Al-adatom linearly decreases and appears to be energetically favored at a μAl higher than −0.4 eV (under Alrich conditions). The Al and N atoms reach the upper/lower bound in the opposite limit of Δμ, suggesting that Al and N atoms tend to adsorb onto a film surface under Al-rich and Nrich growth atmospheres. Under an Al-rich atmosphere, the incorporation of the Al atoms on the surface is more thermodynamically stable, whereas that of the N-adatom is relatively active. The left side of Figure 2, which corresponds to N-rich conditions, indicates a contrasting behavior. In the Al− N molecule, the energy appears to be constant over the entire

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RESULTS AND DISCUSSION In considering the layer epitaxy of AlN films, nucleation is generally initiated with the elemental building blocks including a single Al/N atom, Al−N molecule, and Al−Nn (n > 1) cluster. 14159



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In the conventional MOVPE method, the gas sources with a high V/III ratio are simultaneously opened. The Al−N3 clusters are the dominant growth units because of the strong formation enthalpy. These clusters are stably positioned at a random site with slow surface migration; they also nucleate an island, which develops in a 3D manner at a rapid growth rate. However, the subsequent coalescence between the grown islands is difficult to accommodate, resulting in a rough surface morphology. The calculations show that neither of the growth units is sufficiently universal to form a well-defined film. This situation, however, is reversed upon the appropriate growth sequence of the supplied sources. Initiated under an Al-rich atmosphere, the surface is covered densely with the diffusive Al-adatom instead of the thermally stable Al−N molecule and Al−N3 clusters. When the atmosphere is shifted to N-rich conditions, the Al-adatom shows a high diffusion coefficient, enabling the Al-adatom to reach the edge of an island, seek a stable site before being exhausted, and induce 2D growth for the grains. In addition to 2D growth, inducing the initial nucleation of islands is desirable to enable coalescence and the consequent formation of a continuous layer. Nevertheless, the inequalities in the in-plane strain affect growth kinetics;19−21 thus, the individual grains are inevitably misaligned with their neighbors. In particular, when grain boundaries come into contact, the coalescence of the misaligned facets of the adjacent islands should be misoriented by tilting the c axis with respect to the growth direction or coupled to the twist of the orientation around the c axis.22 Previous experimental and theoretical studies have shown that tilting induces screw dislocations parallel to the growth direction and edge-type dislocations parallel to the basal plane, and twisting generates edge dislocations parallel to the growth direction, indicating that the grain boundary areas contain high-density dislocations that are considered to be unstable.23,24 Moreover, an important point specific to the wurtzite structure is that growth rate differs depending on crystal orientation. The growth velocities of the III-nitrides follow the order V(11−22) > V(11−20) > V(10−10) > V(10−11) > V(0001).25 The volume of the island grows in an anisotropic mode, driving the fast-growing facets to extinction and the slow-growing facets to expand and ultimately define the island shape. Despite the 2D growth, the islands are bordered by the slow-growing facets, particularly the inclined sidewall, which is frequently observed as a trench. If this trend continues, then the trenches would grow in size and depth or form TDs and V-shaped defects. Coalescence toward a smooth surface morphology is inaccessible until the adsorbates grow over the persevered trench. Among the growth units considered in this work, the three Al−N bonds of the Al−N3 clusters have more opportunities to form highly stable binding with the nearest surface atoms because their formation enthalpies are suitable for coalescing bordered grains. This coalescence is achieved when the atoms hang over the trench void areas. Along these views, the hierarchical growth units manipulated by the appropriate choice of precursors offer a realistic prospect of AlN epitaxy with a smooth surface morphology and a compact crystal quality. For a more quantitative explanation of this idea, the hierarchical growth units processed via a three-step epitaxy are induced to improve the surface morphology of the MOVPE samples characterized by AFM. AlN film was grown on c-plane sapphire substrates with an MOVPE (Thomas Swan CCS 3 × 2 in) system. TMAl and NH3 were used as the source gases for Al and N, respectively. These gases were carried by hydrogen

Figure 2. Formation enthalpies of the AlN surfaces with an Al atom, N atom, Al−N molecule, and Al−N3 cluster adsorption as a function of the Al chemical potential.

range of the Al chemical potential. This energy is lower than those of the Al-adatoms and N-adatoms in almost the entire range of the allowed chemistry potential, except at the N-rich limit. As ΔμAl changes from 0 to −2.953 eV, the formation enthalpy of the Al−N3 cluster dramatically decreases from −0.486 to −6.386 eV, a value remarkably lower than the other growth units considered in the calculation. The lower formation enthalpy, except under Al-rich environments, suggests that the Al−N3 cluster is more likely to be pinned on the surface. In general, single adatoms are more considerably diffusive than their complexes. The distinct migration behaviors of the growth units typically arise from their bonding characteristics. To examine the nature of this bonding, we show the electron localization function (ELF) of the isosurface at a value of 0.7 in Figure 3. An ELF

Figure 3. ELF isosurfaces of (a) Al and (b) Al-3N cluster at a value of 0.7.

value close to 1 at any given point indicates a high degree of electron localization at that point. Considering that the surface atoms are left with unsaturated chemical bonds, the valence electrons hybridize into the sp2 orbital, thereby reducing total energy, as evidenced by the 109° angle tetrahedral bonding (Figure 3). The ELF visualization shows that unlike the Al−N3 cluster, the Al-adatoms and N-adatoms, as well as the Al−N molecule, cannot saturate the Al dangling bonds. For Al, the electrons delocalized in the Al atoms (Figure 3a) typically break the underlying hybridization into a more metallic bond at the cost of extra free energy. The exact opposite of this situation occurs in the cluster (Figure 3b), in which the chemical bonds at the interface are more covalent, thereby forming the tetrahedral bonding of the typical groups III−V solids. For the Al−N3 cluster, the obtained formation enthalpy at the lower bound is −6.386 eV, a value considerably lower than 2.597 eV for the Al-adatom at the higher bound. This enthalpy causes energetic rebonding and highly stable adsorption. 14160



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roughness of AFM scan for 2 × 2 μm2 sample area decreases to 3.2 Å and the full width at half-maximum of the X-ray diffraction rocking curve (0002 reflection) is 117 arcsec, which indicates smooth AlN layers with low density of surface defects. These features can be explained by considering the interplay between the growth units and the surface with regard to migration behavior; this interplay results in the geometrical arrangement of atoms. Given that Al-rich conditions have higher formation enthalpy than do N-rich conditions (Figure 2), the resultant high surface migration enables the Al-adatoms to relax to a more favorable site with an orientation similar to that achieved before they were confined to a distorted position by the arrival of one or more additional atoms. The continuous TMAl supply yields excess Al accumulation on the surface. When uniformity of island distribution is achieved, the reactor atmosphere shifts to a N-rich atmosphere because of NH3 injection into the reactor. Under a N-rich atmosphere, the Aladatoms gain higher surface diffusion lengths and easily migrate to the step edges. Figure 5d shows that the adsorbates preferably attach to the newly persistent island edges on the top layer. However, the islands exhibit a fractal-like shape, similar to the underlying layer. The simultaneous supply of TMAl+NH3 sources causes the bridged lateral growth of the overgrown AlN, suggesting that the clusters are more likely to span the underlying trench. Given the lowest formation enthalpy, the clusters stably attach to the nearest islands rather than detach. Consequently, the fractal configuration cannot be extended to the as-grown layer. The large surface area indicates that over time most of the growth units, particularly the Al−N3 clusters, bind with terraces and incorporate into existing islands, facilitating coalescence and eventually forming a more compact surface. In general, a smooth and compact surface is represented by low-density dislocations down to the nanoscale. These dislocations are usually too small to be directly identified. However, they can be observed as peaks in CL measurement, which was used to examine the crystal quality of the grown samples. As shown in Figure 6, all of the samples exhibit a sharp CL emission at 210 nm, accompanied by a broad emission band centered at 326 nm. The emission peak at 210 nm is assigned to the band-edge emission of AlN,27 and the broadband emission is a signature of a defect-related emission.28,29 This defect-related emission probably originates from the nonstoichiometric surface and TDs. The CL spectra have been normalized to the intensities of the band-edge emission of AlN for comparison. Under Al-rich conditions, the broadband emission increases after the surface of the sample in group i (Figure 6a−c) was covered by Al adatoms. In the samples of group ii (Figure 6c−e), injected with NH3, the integrated CL intensity of the broadband begins to decline. After nucleation was initiated, the highly nonstoichiometric surface formed under Al-rich conditions was the primary driver of the enhanced intensity of the broadband emission given that the growth front considerably deviated from bulk stoichiometry. In the latter stage, the grown island expands with irregular boundaries. A more stoichiometric surface contributes little to broadband emission, accounting for the decline in corresponding intensity. In this stage, the largest contribution is that from the grain boundaries. With the bridged lateral growth of the overgrown AlN film in Figure 6f, the broadband intensity continues to drop, demonstrating that the microscopic TDs are highly depressed. Compared with the weakened broadband luminescence, the relatively higher band edge emission in

(H2). After predeposition heat treatment in H2, surface nitridation was carried out in NH3 flow at 571.2 °C for 400 s. Under a reactor pressure of 150 Torr, a low-temperature AlN buffer layer and a thin high-temperature AlN layer were subsequently grown by a continuous source supply method (conventional growth) at 875 and 1217 °C, respectively. The successive three-step growth process is illustrated in Figure 4,

Figure 4. Schematic of the three-step processing schedule that yields hierarchical growth units. The final flow times of the samples in each period are 0 s TMAl, 1 s TMAl, and 3 s TMAl (Group i); 1 s NH3 and 3 s NH3 (Group ii); and 1 s TMAl+NH3 and 3 s TMAl+NH3 (Group iii) growth times.

which shows the time sequence of the source flow patterns for the AlN layers. In examining the surface morphologies of the AlN layers, three sets of samples were prepared with progressively longer purge times for each of the flow modulation sequences. The samples are denoted as i−iii. Figure 5 shows the AFM images of the samples obtained by the hierarchical growth units initiated with the TMAl supply on the AlN template. Before growth was initiated, the surface of the AlN template was characterized by atomic planes separated by open trenches (Figure 5a) with a root-mean-square (RMS) roughness of 5.45 nm. The presence of the trenches between the grown islands suggests that newly deposited atoms preferentially join pre-existing islands and effectively prevent the nucleation of new islands. Injecting only the TMAl into the reactor causes the surface planes to vanish into small islands with fractal or dendritic shapes (Figure 5b). Figure 5c shows that the fractal islands continuously evolve into uniform hexagonal shapes with rounded corners, covered with closely packed Al layers. In particular, the emerging small-sized particles tend to aggregate at the lower concave area. The epilayer morphology is similar to that described in previous studies, in which Al droplets tend to form on the film surface under Al-rich conditions.26 The subsequent nitridation causes the newly formed islands to spread on top of the film surfaces (Figure 5d) and develop these surfaces into step terraces. The island shape evolves adherent to the underlying structure with irregular grain boundaries, which normally have an adverse effect on subsequent coalescence processes. A major change occurs as TMAl and NH3 were induced. The clusters laterally spread on the top layer, bridging the neighboring islands over the trenches (Figure 5f). Figure 5g shows the coalescence among the overgrown islands; this coalescence transfigures the surface morphology with smooth surface topologies, as expected. More impressively, with accumulation through long-range periodicity of hierarchical growth units, the RMS 14161



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Figure 5. AFM images of an AlN microcrystal measured over an area of 2 × 2 μm and grown under different conditions: (i) TMAl (a) 0, (b) 1, and (c) 3 s; (ii) NH3 (d) 1 and (e) 3 s; and (iii) TMAl+NH3 (f) 1 and (g) 3 s.

Figure 6g is consistent with the process of island coalescence; that is, coalescence spans across the underlying trenches and transforms the film into one with a smoother surface morphology and improved properties.

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SUMMARY Microscopic growth mechanisms with basic constituent growth units, such as Al, N, Al−N, and Al−N3, have been theoretically investigated. Under N-rich conditions, Al-adatoms are physically diffusive, suggesting that such adatoms tend to rebond on film surfaces. In general, the Al−N3 cluster has energy lower than those of the other adsorbates because of the high saturation of the dangling bonds of the surface Al atoms. This result indicates that the cluster is more readily incorporated into the lattice. The different migration behaviors enable the control of diffusion during the growth process and, in turn, better control for producing films with more compact surface morphologies and improved properties. In an early stage of growth, Al-rich conditions enable the adatoms to easily diffuse, thereby forming a uniform nucleus and reconstruct the surface geometry. Nonetheless, the inequalities in the in-plane strain prevent sequential nucleus orientation. Switching to N-rich conditions enables the highly active Al-adatoms to relax to a

Figure 6. CL spectra at 5 K of the samples with final flow times of TMAl (a) 0, (b) 1, and (c) 3 s; NH3 (d) 1 and (e) 3 s; and TMAl +NH3 (f) 1 and (g) 3 s. 14162



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lattice site before they are pinned at an unfavorable site. The stable formation of the Al−N3 clusters is appropriate for crystallographically bridging the trenches between the islands on the top layer, leading to the formation of a continuous epilayer. On the basis of these results, we demonstrate using hierarchical growth units processed via a three-step epitaxy with MOVPE samples. Further evidence that favors compact layer epitaxy is provided by the MOVPE samples and three-step growth process. The subsequent growth tends to form a smooth 2D growth morphology characterized by AFM. The amplified band-edge emission and the weakened broadband luminescence observed in CL are indicative of the improvement in AlN film properties with defect suppression. This result demonstrates further property optimization for AlGaN semiconductors.

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ASSOCIATED CONTENT

S Supporting Information *

Complete author list of the references with more than 10 authors. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-592-2185962. Fax: +86-592-2187737. E-mail: [email protected] and [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “973” program (2012CB619301 and 2011CB925600), the “863” program (2011AA03A111), National basic research program (61227009 and 90921002), the Natural Science Foundations of Fujian Province (2012J01024), and the fundamental research funds for the central universities (2012121014 and CXB2011029).

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REFERENCES

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Defect Suppression in AlN Epilayer Using Hierarchical Growth Units

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