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A surface preparation technique has been developed for non-polar m-plane aluminum nitride (AlN) single crystalline substrates. Chemical surface treatments were conducted on chemomechanically polished m-plane oriented AlN substrate surfaces. X-ray photoelectron spectroscopy was used to characterize the surface chemistry after each processing step. A layer of mixed aluminum oxide-hydroxide was identified on ambient air-exposed surfaces. Analysis of the oxygen core level spectra showed oxygen in three bonding states, identified by the binding energies and relative separation of the fitted peaks as O2−, OH− and H2O. Molar ratios of different components showed the oxide to be dominantly bonded in the OH− state. Wet etching reduced the amount of surface oxides. Ammonia annealing at 1100 °C converted the aluminum oxide-hydroxide layer to AlN. Although the overall amount of the oxide on the surface was reduced by surface treatments, the nature of the oxide remained predominantly of the OH−character. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Herein, Al‐rich AlGaN‐based avalanche photodiodes (APDs) grown on single crystal AlN substrates high ultraviolet‐C sensitivity for λ < 200 nm are fabricated, while exhibiting blindness to λ > 250 nm. A maximum quantum efficiency of 68% and peak gain of 320 000 are estimated resulting in a figure of merit of ≈220 000 in devices with ϕ = 100 μm. As expected, a decrease in gain with increase in device size is observed and a gain of ≈20 000 is estimated in devices with ϕ = 400 μm. Overall, two orders of magnitude higher performance are observed in APDs on single crystal AlN substrates compared to those on sapphire. [ABSTRACT FROM AUTHOR]

We report on the absence of strain relaxation mechanism in Al0.6Ga0.4N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m−1) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics. [ABSTRACT FROM AUTHOR]

The interfaces of m- and c-plane AlN with metals of different work functions and electro-negativities were characterized and the Schottky barrier heights were measured. The Schottky barrier height was determined by measuring the valence band maximum (VBM) with respect to the Fermi level at the surface (interface) before (after) metallization. VBM determination included accurate modeling and curve fitting of density of states at the valence band edge with the XPS data. The experimental behavior of the barrier heights could not be explained by the Schottky-Mott model and was modeled using InterFace-Induced Gap States (IFIGS). A slope parameter (SX) was used to incorporate the density of surface states and is a measure of Fermi level pinning. The experimental barriers followed theoretical predictions with a barrier height at the surface Fermi level (Charge neutrality level (CNL)) of ∼2.1 eV (∼2.7 eV) on m-plane (c-plane) and SX∼0.36 eV/Miedema unit. Slope parameter much lower than 0.86 implied a surface/interface states dominated behavior with significant Fermi level pinning and the measured barrier heights were close to the CNL. Titanium and zirconium provided the lowest barriers (1.6 eV) with gold providing the highest (2.3 eV) among the metals analyzed on m-plane. It was consistently found that barrier heights decreased from metal polar to non-polar surfaces, in general, due to an increasing CNL. The data indicated that charged IFIGS compensate spontaneous polarization charge. These barrier height and slope parameter measurements provided essential information for designing Schottky diodes and other contact-based devices on AlN. [ABSTRACT FROM AUTHOR]

We demonstrate Si-implanted AlN with high conductivity (>1 Ω−1 cm−1) and high carrier concentration (5 × 1018 cm−3). This was enabled by Si implantation into AlN with a low threading dislocation density (TDD) (<103 cm−2), a non-equilibrium damage recovery and dopant activation annealing process, and in situ suppression of self-compensation during the annealing. Low TDD and active suppression of VAl-nSiAl complexes via defect quasi Fermi level control enabled low compensation, while low-temperature, non-equilibrium annealing maintained the desired shallow donor state with an ionization energy of ∼70 meV. The realized n-type conductivity and carrier concentration are over one order of magnitude higher than that reported thus far and present a major technological breakthrough in doping of AlN. [ABSTRACT FROM AUTHOR]

Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm-3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm-3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence. [ABSTRACT FROM AUTHOR]

We demonstrate Si as a shallow donor in aluminum nitride (AlN) with an ionization energy of ∼70 meV. The shallow state was achieved by ion implantation of Si into homoepitaxial AlN and a low thermal budget damage recovery and activation process. These results demonstrate that the DX formation may be a kinetically limited process, though being a non-equilibrium process, preventing the Si donor from relaxing to the deep donor state. The room temperature conductivity was measured to be ∼0.05 Ω−1 cm−1, which is one order of magnitude higher than what has been reported for the epitaxially doped or implanted AlN. [ABSTRACT FROM AUTHOR]

The evolution of defects due to high‐pressure annealing of magnesium ion‐implanted epitaxial GaN grown on high‐quality GaN substrates is investigated. Changes in the implant‐induced strain are quantified as a function of annealing temperature and time. After annealing at 1300 °C for 10 min, the implant‐induced strain is fully relieved and accompanied by the presence of extended defects such as basal plane stacking faults and prismatic loops. Approximately one‐third of the original implant‐induced strain remains after annealing at 700 °C, and 5% of the original strain remains at 1000 °C for 100 min. In all cases, nearly all of the recovered strain occurs within first few minutes of annealing. A prominent increase in the asymmetric (101¯4) triple axis X‐ray rocking curve full width at 0.01 maximum (FW0.01M) is observed after annealing at 1300 °C for 10 min. After annealing at 1300 °C for 100 min, a subsequent decrease in FW0.01M is correlated with a reduction of the extended defect density from 4 × 108 to 3 × 107 cm−2, determined through transmission electron microscope (TEM) measurements. Further reduction in the density of the extended defects by optimizing annealing temperature and time is expected to improve the performance of GaN‐based vertical power devices. [ABSTRACT FROM AUTHOR]

We demonstrate large area (25 000 μm2) Al-rich AlGaN-based avalanche photodiodes (APDs) grown on single crystal AlN substrates operating with differential (the difference in photocurrent and dark current) signal gain of 100 000 at 90 pW (<1 μW cm−2) illumination with very low dark currents <0.1 pA at room temperature under ambient light. The high gain in large area AlGaN APDs is attributed to a high breakdown voltage at 340 V, corresponding to very high breakdown fields ∼9 MV cm−1 as a consequence of low threading and screw dislocation densities < 103 cm−2. The maximum charge collection efficiency of 30% was determined at 255 nm, corresponding to the bandgap of Al0.65Ga0.35N, with a response of 0.06 A/W. No response was detected for λ > 280 nm, establishing solar blindness of the device. [ABSTRACT FROM AUTHOR]

In this work, we determine the dependence of the defect transition energies, electronic bands, and surface charge neutrality levels in AlGaN. With Vacuum level as reference, we show that energy transitions of localized defects and the surface Fermi level are independent of the alloy composition as electronic bands diverge with the increase in the bandgap as a function of alloy composition. The invariance of localized states on the alloy composition creates a convenient internal reference energy with respect to which other energy states may be measured. We demonstrate a higher generality to the universality rule with the independence of deep transition states of otherwise shallow donor type defects [(+1/+3) transition for VN] and defect complexes (CN+SiIII) in addition to the earlier predicted independent nature of mid-gap states when they are either the antibonding state between cationic impurities and host anion or acceptors at anion sites. [ABSTRACT FROM AUTHOR]

We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices. [ABSTRACT FROM AUTHOR]

Highlights • First report on the microstructure of m-plane AlN single crystals and films. • The effects of bulk mosaicity and film roughness on X-ray diffraction are discussed. • Extensive characterization confirmed the lack of film strain and extended defects. Abstract Homoepitaxial non-polar AlN films were realized on m-plane (10 1 - 0) -oriented AlN single crystals by metalorganic chemical vapor deposition (MOCVD). The microstructural properties of m-plane AlN substrates and homoepitaxial films were assessed by means of atomic force microscopy and high resolution x-ray diffraction characterization. Results indicated that both m-plane AlN substrates and films possessed exceptional structural quality, with some anisotropic mosaic distributions due to the quasi-bulk nature of the non-polar single crystals. An increase in the MOCVD growth temperature was noted to minimize the degree of inherited mosaic anisotropy without altering the m-plane AlN film growth rate, indicating that high temperature growth is critical to produce optimal film crystallinity. A dramatic change in the film surface morphology from heavily faceted "slate-like" features to monolayer steps was observed as the growth temperature was increased. The "slate-like" surface morphology produced low intensity cross-streaks in symmetric (10 1 - 0) reciprocal space maps, tilted about 18° away from the (10 1 - 0) crystal truncation rod. The orientation of these diffuse streaks corresponds to the physical alignment of the slates with respect to the substrate surface normal. X-ray line scans and defect-selective reciprocal space mapping confirmed that these low intensity streaks are solely dependent on this peculiar surface structure produced at low MOCVD growth temperatures and unrelated to basal plane stacking faults or other extended defects. All observations confirm that high quality III-nitride epitaxial structures on m-plane AlN substrates are attainable with controllable MOCVD growth processes, as demanded for future high performing AlN-based non-polar devices. [ABSTRACT FROM AUTHOR]

Compensating point defect reduction in wide bandgap semiconductors is possible by above bandgap illumination based defect quasi Fermi level (dQFL) control. The point defect control technique employs excess minority carriers that influence the dQFL of the compensator, increase the corresponding defect formation energy, and consequently are responsible for point defect reduction. Previous studies on various defects in GaN and AlGaN have shown good agreement with the theoretical model, but no direct evidence for the role of minority carriers was provided. In this work, we provide direct evidence for the role of minority carriers in reducing point defects by studying the predicted increase in work done against defect (CN-1) formation with the decrease in the Fermi level (free carrier concentration) in Si doped GaN at a constant illumination intensity. Comparative defect photoluminescence measurements on illuminated and dark regions of GaN show an excellent quantitative agreement with the theory by exhibiting a greater reduction in yellow luminescence attributed to CN-1 at lower doping, thereby providing conclusive evidence for the role of the minority carriers in Fermi level control-based point defect reduction. [ABSTRACT FROM AUTHOR]

A high free hole concentration in III-nitrides is important for next generation optoelectronic and high power electronic devices. The free hole concentration exceeding 1018 cm-3 and resistivity as low as 0.7 Ωcm are reported for p-GaN layers grown by metalorganic vapor phase epitaxy on single crystal AlN substrates. Temperature dependent Hall measurements confirmed a much lower activation energy, 60–80mV, for p-GaN grown on AlN as compared to sapphire substrates; the lowering of the activation energy was due to screening of Coulomb potential by free carriers. It is also shown that a higher doping density (more than 5×1019 cm-3) can be achieved in p-GaN/AlN without the onset of self-compensation. [ABSTRACT FROM AUTHOR]

Metal–polar, Al-rich AlGaN films were grown on both single crystalline AlN and sapphire substrates. The role of surface morphology and surface kinetics on AlGaN composition is presented. With the reduced dislocation density of the films grown on AlN substrates, atomically smooth bilayer stepped surfaces are achieved with RMS roughness of less than 50 pm for a 5×5 µm 2 AFM scan area. By controlling the surface supersaturation through adjusting the growth rate, a transition from 2D nucleation to step flow was observed. The critical misorientation angle for step-bunching in nominal Al 0.70 Ga 0.30 N grown with a growth rate of 600 nm/h on AlN substrates was found to be 0.4°. The composition of bilayer stepped AlGaN was strongly dependent on substrate misorientation angle, where a compositional variation by a factor of two for a change in misorientation angle from 0.05 to 0.40° was observed; this is explained by the different surface diffusion lengths of Ga and Al. Step-bunching resulted in strong compositional inhomogeneity as observed by photoluminescence and scanning transmission electron microscopy studies. [ABSTRACT FROM AUTHOR]

AlN epitaxial thin films were grown on both vicinal (0001)-oriented native single crystal AlN substrates and AlN templates grown on vicinal (0001)-oriented sapphire to develop a surface kinetic framework for the control of surface morphology. A Burton, Cabrera, and Frank (BCF) theory-based model is formulated and utilized to understand the dependence of the surface kinetics on the vapor supersaturation, σ , and substrate misorientation angle, α . The surface energy of the Al-polar surface of AlN was experimentally determined using BCF theory to be 149±8 meV/Å 2 . The critical misorientation angle for the onset of step-bunching was determined to be ~0.25° for a growth rate of 500 nm/h and temperature of 1250 °C. Transitioning from a surface with 2D nuclei to one with bilayer steps required a decrease in σ or an increase in α , whereas the suppression of step-bunching required an increase in σ or a decrease in α . [ABSTRACT FROM AUTHOR]

In this work, the Fermi level and band alignment at c-plane surfaces and interfaces of AlGaN thin films grown on sapphire and native single crystalline AlN substrates were analyzed via x-ray photoelectron spectroscopy. The dependence of charge neutrality level (CNL) on Al composition is found to be linear with n-type Schottky barrier heights (Φbn) exhibiting an overall quadratic behavior due to bandgap bowing. A general theoretical expression for Schottky barrier height on AlGaN is determined as a function of Al composition and metal electronegativity utilizing the interface induced gap states (IFIGS) model and is corroborated with current-voltage (I-V) characterization on Ni-based Schottky diodes. The measured CNLs were used to determine the conduction and valence band offsets in AlGaN hetero-junctions according to the IFIGS and were found to be split approximately 2/3 and 1/3 of the bandgap, respectively, at interfaces with AlGaN having comparable Al and Ga concentrations. Nonlinearities in Φbn result in an increase (>2/3) and decrease (<2/3) of the conduction band split at AlN/AlGaN and GaN/AlGaN interfaces, respectively. Characterization of core level binding energies revealed a composition-independent surface work function. Consequently, electron affinity is found to be a linear function of the barrier height at the CNL and band offsets determined via IFIGS and Anderson's rule were found to be identical. The origin of the bandgap bowing arising in the conduction band is proposed. [ABSTRACT FROM AUTHOR]

Since the band ordering in AlGaN has a profound effect on the performance of UVC light emitting diodes (LEDs) and even determines the feasibility of surface emitting lasers, the polarization properties of emitted light from c-oriented AlGaN and AlGaN-based laser structures were studied over the whole composition range, as well as various strain states, quantum confinements, and carrier densities. A quantitative relationship between the theoretical valence band separation, determined using k•p theory, and the experimentally measured degree of polarization is presented. Next to composition, strain was found to have the largest influence on the degree of polarization while all other factors were practically insignificant. The lowest crossover point from the transverse electric to transverse magnetic polarized emission of 245nm was found for structures pseudomorphically grown on AlN substrates. This finding has significant implications toward the efficiency and feasibility of surface emitting devices below this wavelength. [ABSTRACT FROM AUTHOR]

AlGaN lateral polarity structures (LPS) with varying Al composition, have been fabricated by metalorganic chemical vapour deposition on LPS templates containing µm size domains. III-metal and N-polar AlGaN domains have been grown on LT-AlN and c-sapphire domains, respectively. LPS characterization by SEM and AFM reveals two major effects when the Al composition has been varied: (a) A high Al content leads to columnar growth within N-polar domains and (b) a height difference between the polar domains can be observed for low Al compositions towards III-polarity. The surface quality of the III-polar domains is not effected by the Al composition. The surface roughness of N-polar domains decreases for lower Al compositions. The high periodicity of the polar domains and the high quality of the boundary between domains suggest a high potential of AlGaN LPS to be used as lateral wave guides for quasi phase matching. [ABSTRACT FROM AUTHOR]

A Fermi-level control scheme for point defect management using above-bandgap UV illumination during growth is presented. We propose an extension to the analogy between the Fermi level and the electrochemical potential such that the electrochemical potential of a charged defect in a material with steady-state populations of free charge carriers may be expressed in terms of the quasi-Fermi levels. A series of highly Si-doped Al065Ga035N films grown by metalorganic chemical vapor deposition with and without UV illumination showed that samples grown under UV illumination had increased free carrier concentration, free carrier mobility, and reduced midgap photoluminescence all indicating a reduction in compensating point defects. [ABSTRACT FROM AUTHOR]

Growth and characterization of AlN based lateral polarity structures (LPS) are presented. The LPS were grown by MOCVD using patterned low temperature AlN/sapphire substrates: the Al-polar and N-polar AlN grew on AlN buffer layer and nitrided sapphire, respectively. AFM images showed a height difference between the two adjacent domains of different polarity on the order of 30 nm, which was equivalent to the low temperature AlN buffer layer thickness. SEM images provided an insight in the growth mode of the two polarities. It was shown that Al-polar AlN grew two-dimensionally, leading to a smooth well coalesced layer, while N-polar AlN grew primarily three-dimensionally, leading to a columnar structure. This difference in the growth mode led to different properties of domains with opposite polarity, including strain and defect incorporation, as observed by X-ray diffraction, photoluminescence, and Raman spectroscopy measurements. Finally, an outlook on the applicability and future development of AlN-based LPS is given. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The control of the polarity of metalorganic chemical vapor deposition grown AlN on sapphire is demonstrated. Al-polar and N-polar AlN is grown side-by-side yielding a lateral polarity structure. Scanning electron microscopy measurements reveal a smooth surface for the Al-polar and a relatively rough surface for the N-polar AlN domains. Transmission electron microscopy shows mixed edge-screw type dislocations with polarity-dependent dislocation bending. Raman spectroscopy reveals compressively strained Al-polar and relaxed N-polar domains. The near band edge luminescence consists of free and bound excitons which are broadened for the Al-polar AlN. Relaxation, better optical quality, and dislocation bending in the N-polar domains are explained by the columnar growth mode. [ABSTRACT FROM AUTHOR]

This study evaluates the implementation of Menter's γ-Reθ Transition Model within the CFXI2 solver for turbulent transition prediction on a natural laminar flow nacelle. Some challenges associated with this type of modeling have been identified. The computational fluid dynamics transitional flow simulation results are presented for a series of cruise cases with freestream Mach numbers ranging from 0.8 to 0.88, angles of attack from -2 to 0°, and mass flow ratios from 0.60 to 0.75. These were validated with a series of wind-tunnel tests on the nacelle by comparing the predicted and experimental surface pressure distributions and transition locations. A selection of the validation cases are presented in this paper. In all cases, computational fluid dynamics simulations agreed reasonably well with the experiments. The results indicate that Menter's γ-Rθ Transition Model is capable of predicting laminar boundary-layer transition to turbulence on a nacelle. Nonetheless, some limitations exist in both the Menter's γ-Reθ Transition Model and in the implementation of the computational fluid dynamics model. The implementation of a more comprehensive experimental correlation in Menter's γ-Reθ Transition Model, preferably the ones from nacelle experiments, including the effects of compressibility and streamline curvature, is necessary for an accurate transitional flow simulation on a nacelle. In addition, improvements to the computational fluid dynamics model are also suggested, including the consideration of varying distributed surface roughness and an appropriate empirical correction derived from nacelle experimental transition location data. [ABSTRACT FROM AUTHOR]

It was demonstrated that Ge has the same effect as Si on the strain evolution in n-type GaN as measured by x-ray diffraction. Dislocation inclination, which causes tensile strain in n-type GaN, was clearly observed by transmission electron microscopy where Ge doping was introduced during epitaxial growth. This result is explained by the Fermi level effect model that indicates dislocation inclination due to the climbing process through Ga vacancies. Therefore, there is no dependence of dislocation inclination on dopant species. [ABSTRACT FROM AUTHOR]

Using high resolution x-ray diffraction and Hall effect measurements, we found that the tensile strain caused by dislocation inclination in Si doped GaN became immeasurable when carbon codoping was used to compensate the free carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration. Such an effect could be explained by the Fermi level effect on the surface-mediated dislocation climb governed by Ga vacancies, whose concentration is strongly influenced by the Fermi level position. This phenomenon is possibly similar to the well-known Fermi level effect in GaAs and GaP systems. [ABSTRACT FROM AUTHOR]

• GaN substrates grown by the ammonothermal method are characterized by X-ray topography. • Threading screw, edge and mixed dislocations are imaged. • Dislocations characterized by correlation with ray tracing simulated images. Ammonothermal growth of bulk gallium nitride (GaN) crystals is considered the most suitable method to meet the demand for high quality bulk substrates for power electronics. A non-destructive evaluation of defect content in state-of-the-art ammonothermal substrates has been carried out by synchrotron X-ray topography. Using a monochromatic beam in grazing incidence geometry, high resolution X-ray topographs reveal the various dislocation types present. Ray-tracing simulations that were modified to take both surface relaxation and absorption effects into account allowed improved correlation with observed dislocation contrast so that the Burgers vectors of the dislocations could be determined. The images show the very high quality of the ammonothermal GaN substrate wafers which contain low densities of threading dislocations (TDs) but are free of basal plane dislocations (BPDs). Threading mixed dislocations (TMDs) were found to be dominant among the TDs, and the overall TD density (TDD) of a 1-inch wafer was found to be as low as 5.16 × 103 cm−2. [ABSTRACT FROM AUTHOR]

In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range. [ABSTRACT FROM AUTHOR]

Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type-precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance.