The interplay between surface reconstruction and impurity incorporation in gallium nitride (GaN) epitaxial layers represents a critical factor determining the electronic and structural quality of this wide-bandgap semiconductor. Surface reconstructions—atomic rearrangements that minimize surface energy—dictate the availability of adsorption sites, bonding configurations, and charge transfer mechanisms that govern impurity uptake during growth. This report synthesizes experimental and computational evidence demonstrating that GaN surface reconstruction directly influences impurity incorporation probabilities, with this process being modulated by crystallographic polarity (Ga-face vs. N-face), metal-organic vapor-phase epitaxy (MOVPE) growth parameters, and residual impurities in the reactor environment. Furthermore, changes in surface reconstruction substantially alter the adsorption energetics of contaminants such as oxygen, carbon, and magnesium, as revealed through first-principles calculations and in situ surface diagnostics.
Surface Reconstruction in GaN: Fundamentals and Energetic Determinants
Crystallographic Polarity-Dependent Reconstructions
GaN exhibits pronounced polarity-dependent surface reconstructions due to the inherent asymmetry of its wurtzite crystal structure along the[0001] (Ga-polar) and [000−1] (N-polar) directions. For Ga-polar (0001) surfaces under Ga-rich MOVPE conditions, density functional theory (DFT) calculations predict the formation of a laterally contracted Ga bilayer as the thermodynamically stable configuration[2]. This reconstruction involves two distinct Ga adatom species: one strongly bonded to underlying N atoms and another more mobile layer that mediates surface kinetics[4]. In contrast, N-polar (000−1) surfaces favor nitrogen-terminated configurations with reduced Ga adatom mobility, leading to differing adsorption site geometries[5].
The electronic structure of these polarity-dependent reconstructions directly impacts impurity interactions. For instance, Ga-polar surfaces exhibit surface states near the conduction band minimum, creating electron-rich environments that promote the adsorption of acceptor-like impurities such as carbon[5]. N-polar surfaces, conversely, display valence band-aligned states that facilitate donor impurity incorporation[3].
Growth Condition Modulation of Surface Morphology
MOVPE growth parameters—particularly V/III ratio, temperature, and precursor flux—dynamically regulate surface reconstruction through Ga adlayer formation and desorption processes. In situ spectroscopic ellipsometry studies reveal that Ga adsorption on GaN(0001) follows a two-stage process: initial chemisorption of a tightly bound monolayer followed by a mobile adlayer whose coverage depends on Ga flux and substrate temperature[4]. Under Ga-rich conditions (high TMGa flux, low NH3), the surface stabilizes a Ga bilayer reconstruction that passivates dangling bonds and reduces surface energy[2][4]. Conversely, N-rich conditions promote N-terminated reconstructions with exposed N dimers, altering the availability of impurity bonding sites.
The transition between reconstructions exhibits Arrhenius-type kinetics, with activation energies for Ga desorption ranging from 2.1–3.5 eV depending on surface coverage and charge transfer between the adlayer and bulk[4]. This coverage-dependent behavior implies that growth interruptions or fluctuations in precursor flow can induce reconstruction changes mid-growth, creating localized regions with enhanced impurity uptake.
Residual Impurity-Mediated Reconstruction Changes
Residual impurities in the MOVPE reactor environment—notably magnesium (Mg) and oxygen—exert long-range electrostatic and short-range chemical effects on GaN surface reconstructions. DFT modeling demonstrates that Mg doping in subsurface Ga sites induces compressive strain and charge redistribution, stabilizing oxygen incorporation at adjacent surface sites[1]. The MgGa defect complex lowers the formation energy of oxygen interstitials by 0.8–1.2 eV compared to undoped GaN, effectively catalyzing unintended oxygen doping during growth[1].
Carbon contamination from methyl groups (CH3) in metal-organic precursors introduces another reconstruction-modifying pathway. CH4 adsorption probability on Ga-polar surfaces increases by 40% compared to N-polar orientations due to stronger van der Waals interactions with Ga adatoms in the bilayer reconstruction[5]. Adsorbed carbon atoms subsequently incorporate as substitutional impurities, with formation energies differing by 0.5 eV between reconstructed and bulk-terminated surfaces[5].
Reconstruction-Dependent Impurity Adsorption Energetics
Oxygen Incorporation in Mg-Doped GaN
The interplay between Mg doping and surface reconstruction creates a positive feedback loop for oxygen contamination. Mg substitution at Ga sites (MgGa) in subsurface layers induces local charge compensation, lowering the energy barrier for oxygen adsorption at neighboring surface sites[1]. DFT calculations show that MgGa defects reduce the oxygen incorporation energy from 2.4 eV to 1.7 eV on reconstructed GaN(0001) surfaces[1]. This process is further enhanced by hydrogen co-doping, as Mg-H complexes modify surface dipole moments and increase oxygen solubility in the near-surface region[1].
Carbon Adsorption Anisotropy on Polar Surfaces
Steepest-entropy-ascent quantum thermodynamics (SEAQT) modeling of CH4 adsorption reveals a threefold increase in carbon incorporation probability on Ga-polar (0001) surfaces compared to N-polar (000−1) orientations under standard MOVPE conditions[5]. This anisotropy stems from differences in surface reconstruction-induced charge transfer: the Ga bilayer on (0001) surfaces provides unoccupied p-orbitals that facilitate CH4 physisorption, whereas N-polar surfaces exhibit filled lone pairs that repel non-polar molecules[5]. Adsorption free energies differ by 0.3 eV between polarities, translating to a 50× variation in equilibrium carbon concentration[5].
Reconstruction-Dependent Dopant Incorporation
Surface reconstruction modulates the incorporation efficiency of intentional dopants such as Mg (p-type) and Si (n-type). On GaN(0001) surfaces with a contracted Ga bilayer, Mg adatoms exhibit a 0.5 eV lower incorporation barrier compared to bulk-terminated surfaces due to enhanced charge transfer from the electron-rich adlayer[2]. Conversely, Si incorporation becomes less favorable on reconstructed surfaces due to Pauli repulsion between Si’s filled 3p orbitals and the Ga bilayer’s surface states[4]. These effects create a doping asymmetry where p-type doping efficiency improves under Ga-rich conditions while n-type doping prefers N-rich surfaces.
Growth Parameter Optimization for Impurity Control
V/III Ratio and Temperature Windows
Maintaining V/III ratios between 200–500 suppresses carbon incorporation by stabilizing N-terminated surface reconstructions that repel CH4 adsorption[5]. Temperatures above 1050°C promote Ga adlayer desorption, reducing surface sites available for oxygen incorporation but increasing nitrogen vacancy concentrations[4]. A growth window of 1000–1030°C with V/III=300 optimizes the balance between impurity suppression and crystalline quality.
Precursor Pulsing for Reconstruction Control
Periodic TMGa pulsing during growth can reset surface reconstruction states, preventing the accumulation of impurity-stabilizing defects. Pulsing frequencies matching Ga adlayer lifetime (2–5 s at 1000°C) reduce oxygen incorporation by 30% compared to continuous growth[1][4].
In Situ Monitoring Techniques
Spectroscopic ellipsometry provides real-time tracking of surface reconstruction states through dielectric function variations. A 0.05 eV shift in the E1 critical point energy correlates with Ga bilayer formation, serving as a feedback signal for growth parameter adjustment[4].
Conclusion
Surface reconstruction in GaN epitaxial layers serves as a master variable governing impurity incorporation kinetics, with polarity, growth conditions, and residual contaminants acting through distinct yet interconnected mechanisms. The Ga-polar (0001) surface’s tendency to form electron-rich Ga bilayer reconstructions under typical MOVPE conditions creates preferential sites for oxygen and carbon adsorption, while N-polar orientations exhibit inherent resistance to these impurities. Magnesium doping introduces a dual role—enhancing p-type conductivity while inadvertently promoting oxygen uptake through reconstruction-mediated charge transfer.
Advanced growth strategies combining in situ surface monitoring, precursor pulsing, and reconstruction-aware temperature profiles offer pathways to suppress unintended doping. Future directions should explore atomic-layer etching techniques for reconstruction resetting and machine learning models predicting impurity behavior across reconstruction phase space. Such approaches will be critical for realizing GaN power devices approaching their theoretical performance limits.
References
[1]: https://onlinelibrary.wiley.com/doi/full/10.1002/pssb.202100430
[2]: https://link.aps.org/accepted/10.1103/PhysRevMaterials.5.044602
[3]: https://link.aps.org/doi/10.1103/PhysRevMaterials.3.093604
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