GaN Surface Research Articles

Interface engineering of transition metal dichalcogenide/GaN heterostructures: Modified broadband for photoelectronic performance

The GaN-based heterostructures are widely used in optoelectronic devices, but the complex surface reconstructions and lattice mismatch greatly limit the applications. The stacking of two-dimensional transition metal dichalcogenide (TMD = MoS2, MoSSe and MoSe2) monolayers on reconstructed GaN surface not only effectively overcomes the larger mismatch, but also brings about novel electronic and optical properties. By adopting the reconstructed GaN (0001) surface with adatoms (N-ter GaN and Ga-ter GaN), the influences of complicated surface conditions on the electronic properties of heterostructures have been investigated. The passivated N-ter and Ga-ter GaN surfaces push the mid-gap states to the valence bands, giving rise to small bandgaps in heterostructures. The charge transfer between Ga-ter GaN surface and TMD monolayers occurs much easier than that across the TMD/N-ter GaN interfaces, which induces stronger interfacial interaction and larger valence band offset (VBO). The band alignment can be switched between type-I and type-II by assembling different TMD monolayers, that is, MoS2/N-ter GaN and MoS2/Ga-ter GaN are type-II, and the others are type-I. The absorption of visible light is enhanced in all considered TMD/reconstructed GaN heterostructures. Additionally, MoSe2/Ga-ter GaN and MoSSe/N-ter GaN have larger conductor band offset (CBO) of 1.32 eV and 1.29 eV, respectively, extending the range from deep ultraviolet to infrared regime. Our results revel that the TMD/reconstructed GaN heterostructures may be used for high-performance broadband photoelectronic devices.

External charge compensation in etched gallium nitride measured by x-ray photoelectron spectroscopy

Electronic states at GaN surfaces and at regrowth and heteroepitaxy interfaces inhibit electronic device performance. Understanding electronic state configuration at the GaN surface is, therefore, crucial for the development of GaN-based devices, which are currently of considerable interest in power electronic applications. GaN and other wurtzite III-nitrides possess large spontaneous polarization along the c-axis, producing a bound sheet charge at the surface, which affects the electronic state configuration through the formation of internal and external compensation charges. Defects induced by conventional plasma-based dry etching methods may inhibit the internal screening of this bound charge and thus increase the concentration of external charged states. The surface band bending of n-type Ga-face GaN (0001) was measured with x-ray photoelectron spectroscopy after inductively coupled plasma etching to investigate the impact of dry etching on external charge compensation. GaN samples were etched using inductively coupled plasma with varying rf power and a novel plasma-enhanced atomic layer etching method using an oxidation, fluorination, and ligand-exchange mechanism. The band bending varied from 0.0 to 0.8 ± 0.1 eV for the samples measured.

Structural and electronic properties of NbN/GaN junctions grown by molecular beam epitaxy

We report the structural and electronic properties of NbN/GaN junctions grown by plasma-assisted molecular beam epitaxy. High crystal-quality NbN films grown on GaN exhibit superconducting critical temperatures in excess of 10 K for thicknesses as low as 3 nm. We observe that the NbN lattice adopts the stacking sequence of the underlying GaN and that domain boundaries in the NbN thereby occur at the site of atomic steps in the GaN surface. The electronic properties of the NbN/GaN junction are characterized using Schottky barrier diodes. Current–voltage–temperature and capacitance–voltage measurements are used to determine the Schottky barrier height of the NbN/GaN junction, which we conclude is ∼1.3 eV.

A Novel Strategy Achieving GaN Ultra-Smooth Surface via a Square, Flat Top Beam Shaped Femtosecond Laser Processing Combined with Chemical Mechanical Polishing

We propose a strategy for GaN ultra-smooth manufacture by combing of femtosecond laser processing and chemical mechanical polishing. The optimized flat top beam is used to redistribute the laser fluence from Gaussian distribution to square, flat top profile. Surface with high flatness is achieved after femtosecond laser processing through optimization of the technical parameters. The optimum process to control the damages on surface and subsurface reduces negative influence of femtosecond laser processing on chemical mechanical polishing. Roughness (Ra) of processed surface is decreased to less than 0.1 nm when it is be polished for 6 h. In the background of traditional GaN polishing efficiency facing bottleneck, the characteristic of present study is to use a square, flat top beam shaped femtosecond laser to induce surface of GaN. Structures and composition on induced surface are simultaneous removed in chemical mechanical polishing process. It reduces the burden of GaN chemical mechanical polishing. The present method has advantage of high efficiency and easy realization, which has a wide application in GaN ultra-smooth processing.

H2 based etching of GaN for re-grown gallium-free InAlN barriers giving very low 2DEG sheet resistance of 185 Ω/sq

We have studied two approaches to GaN surface preparation before regrowth of InAlN layers, using Cl2 or H2 based etch conditions to prepare the wafer for barrier regrowth. We have shown that the Cl2 based etching induces layer degradation and higher sheet resistance values, while an H2 based etch, or desorption, does not degrade the surface morphology and yields improved sheet resistance. With additional optimization of the InAlN layers using this approach, increasing the growth temperature to 830 °C, we have achieved excellent sheet resistance values down to 185 Ohm/sq while maintaining gallium free growth. This regrowth optimization could equally be applied to AlGaN regrowth structures for normally off GaN based transistors.

High reactivity of H2O vapor on GaN surfaces

ABSTRACT Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H2O, O2, N2O, and NO gases. It was found that H2O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H2O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H2O molecules were present on the (0001) (+c) surface: non-dissociatively adsorbed H2O (physisorption), and dissociatively adsorbed H2O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H2O molecules attacked the back side of three-fold Ga atoms on the (0001̅) (−c) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (101̅0) m-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al2O3 layer using H2O vapor, the reactions caused at the interface were more remarkable for p-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H2O and O2 should be used to avoid uncontrollable oxidation on GaN surfaces.

Room temperature bonding of GaN and diamond substrates via atomic layer

GaN and diamond substrates were bonded at room temperature with an atomically thin bonding layer. This entailed sputter etching the surface of the GaN substrate in the bonding machine to expose the active nascent GaN surface. At the same time, rather than using sputter etching, a ∼1 nm-thick Si nano-adhesion layer was deposited on the diamond surface. The advantage of this newly developed approach is that it suppresses the physical damage typically inflicted upon the diamond crystals by sputtering. This restricts the thickness of the amorphous layer at the GaN/diamond bonding interface to less than 2 nm. The proposed GaN/diamond integration is expected to contribute to efficient heat dissipation and facilitate the high-power and high-frequency operation of GaN devices.

Study and characterization of GaN MOS capacitors: Planar vs trench topographies

Developing high quality GaN/dielectric interfaces is a fundamental step for manufacturing GaN vertical power transistors. In this paper, we quantitatively investigate the effect of planar etching treatment and trench formation on the performance of GaN-based MOS (metal oxide semiconductor) stacks. The results demonstrate that (i) blanket etching the GaN surface does not degrade the robustness of the deposited dielectric layer; (ii) the addition of the trench etch, while improving reproducibility, results in a decrease in the breakdown performance compared to the planar structures. (iii) For trench structures, the voltage for a 10 year lifetime is still above 20 V, indicating a good robustness. (iv) To review the trapping performance across the metal-dielectric-GaN stack, forward-reverse capacitance–voltage measurements with and without stress and photo-assistance are performed. Overall, as-grown planar capacitors devoid of prior etching steps show the lowest trapping, while trench capacitors have higher interface trapping and bulk trapping comparable to the blanket etched capacitors. (v) The nanostructure of the GaN/dielectric interface was characterized by high resolution scanning transmission electron microscopy. An increased roughness of 2–3 monolayers at the GaN surface was observed after blanket etching, which was correlated with the higher density of interface traps. The results presented in this paper give fundamental insight on how the etch and trench processing affects the trapping and robustness of trench-gate GaN-metal-oxide-semiconductor field effect transistors and provide guidance for the optimization of device performance.

Demonstration of atmospheric plasma activated direct bonding of N-polar GaN and β-Ga2O3 (001) substrates

Direct wafer bonding of β-Ga2O3 and N-polar GaN at a low temperature was achieved by acid treatment and atmospheric plasma activation. The β-Ga2O3/GaN surfaces were atomically bonded without any loss in crystalline quality at the interface. The impact of post-annealing temperature on the quality of bonding interfaces was investigated. Post-annealing at temperatures higher than 700 °C increases the area of voids at bonded interfaces probably due to the difference in the coefficient of thermal expansion. The integration of β-Ga2O3 on the GaN substrate achieved in this work is one of the promising approaches to combine the material merits of both GaN and Ga2O3 targeting the fabrication of novel GaN/β-Ga2O3 high-frequency and high-power electronics as well as optoelectronic devices.

Atomic-resolved structural and electric field analysis of the passivation interface of MIS-HEMTs

The passivation interfaces of GaN-based MIS-HEMTs with Si3N4 and ZrO2/Si3N4 bilayers were investigated through atomic resolution scanning transmission electron microscope–energy dispersive spectroscopy–differential phase contrast microscopy methods. It is found that the Si3N4/GaN interface exhibits atomic disorder fluctuation, and the GaN surface is discontinuous at the depth of 1–2 atomic layers. An oxide layer of ∼2 nm is formed at the ZrO2/GaN interface, and the GaN surface is atomically flat. Furthermore, the local minimum of the potential is located at the Si3N4/GaN interface, while it is distributed in the GaN side at the ZrO2/GaN interface. The electric field or potential distribution is affected by the crystal orientation of the polycrystalline ZrO2 layer. Finally, the difference in passivation mechanism is discussed.

Material removal mechanism and deformation characteristics of GaN surface at the nanoscale

Using atomistic modelling, this paper examines the removal behaviours and wear mechanism of GaN at the nanoscale. The diamond tooltip could slide or roll on the crystalline GaN or amorphous/crystalline GaN surfaces at various machining depths. The results indicate that if the machining depth is lower than 4.0 ​Å, the deformation type of the crystalline GaN is elastic. Machining deeper than this depth, the crystalline GaN started to deform plastically. Moreover, increasing the depth results in improving the structure change, stress, and temperature rates. The stress, deformation, and dislocation occur simultaneously. Notably, the wear mechanisms of the crystalline GaN surface strongly depend on the machining depth. The adhering, ploughing, and cutting mechanisms could happen when the tooltip slides over the crystalline GaN surface. In rolling motion, the adhering and ploughing mechanisms could appear during the machining process. Due to the padding function of the amorphous GaN layer, the amorphous GaN layer reduces rates of induced force, stress, structure change, and temperature when compared to the crystalline GaN surface. At the nanoscale, the removal ability of the amorphous GaN layer is higher than the crystalline GaN.

Comprehensive characterization of low-damaged GaN surface exposed to NH3 plasma toward plasma-induced metalorganic chemical vapor deposition

To examine the application of plasma-induced metalorganic chemical vapor deposition (MOCVD) toward the epitaxial growth of III-V nitride semiconductors, a GaN surface exposed to NH3 plasma at 0.4 kPa from a novel plasma source was comprehensively characterized using physical and chemical techniques. The exposure time (10–40 min) dependence at 700 °C and substrate temperature (200–700 °C) dependence for 20-min exposure were investigated using various methods, such as photoluminescence (PL), vacuum ultra-violet (VUV) spectroscopic ellipsometry, transmission electron microscopy (TEM), atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and current–voltage (I–V) characteristics. The PL intensity decreased linearly with the increase in exposure time and was maintained at ∼ 60% intensity after an exposure of 10 min. Compared to that of the as-grown GaN, the dielectric function (DF) of the exposed GaN was slightly modulated. For temperature dependence, the PL intensity decreased linearly with temperature and remained at ∼ 80% intensity at 700 °C. The I–V characteristics showed ∼ 50% decrease in conductivity for an exposure time of 20 min at 700 °C. TEM and XPS revealed extremely low modifications. In case of long-term exposure, hexagonal pits and particle-like nanostructures appeared at the surface. Based on the TEM results, the pit is attributed to threading dislocations, and the nanostructure is not really a particle but a protrusion with a GaN crystal structure. From this study, it can be concluded that, compared to the XPS and TEM techniques, optical techniques such as PL measurement and VUV-SE have higher sensitivity for detecting the effect of NH3 plasma exposure on the GaN surface. The comprehensive characterizations revealed that no significant damage is induced by the plasma exposure in the time corresponding to the usual growth time of an InGaN quantum well, that is, several minutes.

Direct Protein Detection in Solutions of High Ionic Strength using Polyethylene Glycol‐modified AlGaN/GaN High Electron Mobility Transistors

AbstractHerein, we proposed a simple 3‐(aminopropyl) triethoxysilane/polyethylene glycol (APTES/PEG)‐AlGaN/GaN high electron mobility transistors (HEMT) biosensor that enabled direct detection in high ionic strength solutions. Unlike previous modification strategies, the PEG‐modified on the GaN surface established a stable screening system by forming a porous biopolymer layer, which can overcome Debye shielding. In our work, the APTES/PEG modification enabled immunoglobulin G (IgG) to be easily detected in solutions with concentrations of up to 30 mM (3x) of phosphate buffered saline (PBS), indicating that the APTES/PEG modification strategy had the potential for direct detection of biomolecules in physiological solutions, which provided a new method for bio‐surface modification on GaN and advanced the practical application of AlGaN/GaN HEMT biosensors.

Low trap density of oxygen-rich HfO2/GaN interface for GaN MIS-HEMT applications

The high-k nature of HfO2 makes it a competitive gate oxide for various GaN-based power devices, but the high trap densities at the HfO2/GaN interface have hindered the application. This work was specifically carried out to explore the interface between GaN and ozone-based atomic-layer-deposited HfO2 gate oxide. Furthermore, the GaN surface is preoxidized before gate oxide deposition to prepare an oxygen-rich HfO2/GaN interface. On the preoxidized GaN surface, a sharper HfO2/GaN interface and amorphous HfO2 bulk form during the subsequent deposition, translating to improved electric performance in metal–insulator–semiconductor (MIS) devices. The ozone-based HfO2 shows a high breakdown electric field (∼7 MV/cm) and a high dielectric constant (∼28). Furthermore, the MIS high electron mobility transistors' negligible VTH hysteresis and parallel conductance measurements reflect the ultralow trap densities of the HfO2/GaN interface (<1012 cm−2 eV−1). Therefore, the proposed HfO2 gate oxide scheme offers a promising solution for developing GaN MIS devices.

Interfacial Polarization of Thin Alq3, Gaq3, and Erq3 Films on GaN(0001).

This report presents results of research on electronic structure of three interfaces composed of organic layers of Alq3, Gaq3, or Erq3 deposited on GaN semiconductor. The formation of the interfaces and their characterization have been performed in situ under ultrahigh vacuum conditions. Thin layers have been vapor-deposited onto p-type GaN(0001) surfaces. Ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS) has been employed to construct the band energy diagrams of the substrate and interfaces. The highest occupied molecular orbitals (HOMOs) are found to be at 1.2, 1.7, and 2.2 eV for Alq3, Gaq3, and Erq3 layers, respectively. Alq3 layer does not change the position of the vacuum level of the substrate, in contrast to the other layers, which lower it by 0.8 eV (Gaq3) and 1.3 eV (Erq3). Interface dipoles at the phase boundaries are found to be −0.2, −0.9, −1.2 eV, respectively, for Alq3, Gaq3, Erq3 layers on GaN(0001) surfaces.

Modulated interfacial band alignment of β-Ga2O3/GaN heterojunction by the polarization charge transfer

Designing β-Ga2O3/GaN heterojunction has been regarded as a promising approach to optimize the application of β-Ga2O3. Here, band characters of β-Ga2O3/GaN heterojunction are deeply understood by the polarization effect of GaN, which is vital to design high-performance β-Ga2O3/GaN heterojunction. Due to the inconsistent band bending and orbital distribution induced by GaN polarization effect, just N-terminated β-Ga2O3/GaN heterojunction theoretically shows the type-II band alignment with two-dimensional electron gases (2DEGs). Such 2DEGs are just actually observed for N-terminated β-Ga2O3/GaN heterojunctions with weak interfacial interaction, but obvious interfacial energy barriers are found for heterojunctions with strong interfacial interactions which can weak the polarization charge transfer. The interfacial energy barriers can be turned into energy grooves upon introducing O-dopant with low concentration into the GaN region of heterojunction. In addition, all Ga-terminated β-Ga2O3/GaN heterojunctions possess 2DEG band alignments, but abundant interfacial gap states are observed due to the lost electron of Ga atom from β-Ga2O3 region and accumulated polarization electron of N atom from GaN region. In addition, band bending characters of β-Ga2O3/GaN heterojunctions are negligibly dependent of the atomic arrangements of β-Ga2O3 and GaN surfaces. These works deeply understand the interfacial performances of β-Ga2O3/GaN heterojunctions and provide useful guidance for realizing high-performance β-Ga2O3/GaN heterojunctions.

Burton-Cabrera-Frank theory for surfaces with alternating step types

Burton-Cabrera-Frank (BCF) theory has proven to be a versatile framework to relate surface morphology and dynamics during crystal growth to the underlying mechanisms of adatom diffusion and attachment at steps. For an important class of crystal surfaces, including the basal planes of hexagonal close-packed and related systems, the steps in a sequence on a vicinal surface can exhibit properties that alternate from step to step. Here we develop BCF theory for such surfaces, relating observables such as alternating terrace widths as a function of growth conditions to the kinetic coefficients for adatom attachment at steps. We include the effects of step transparency and step-step repulsion. A general solution is obtained for the dynamics of the terrace widths, assuming quasi-steady-state adatom distributions on the terraces. An explicit simplified analytical solution is obtained under widely applicable approximations. From this we obtain expressions for the full-steady-state terrace fraction as a function of growth rate. Fits of the theoretical predictions to recent experimental determinations of the steady state and dynamics of terrace fractions on GaN (0001) surfaces during organometallic vapor phase epitaxy give values of the kinetic coefficients for this system. In Appendixes, we also connect a model for diffusion between kinks on steps to the model for diffusion between steps on terraces, which quantitatively relates step transparency to the kinetics of atom attachment at kinks, and consider limiting cases of diffusion-limited, attachment-limited, and mixed kinetics.

Enhancement of the magnetic anisotropy in single semiconductor nanowires via surface doping and adatom deposition

The magnetic anisotropy of single semiconductor (ZnO and GaN) nanowires incorporating both a transition metal (Co and Mn, respectively) as a substitutional surface dopant and a heavy metal (Au, Bi, or Pt) adatom is studied by performing density-functional supercell calculations with the Hubbard U correction. It is found that a substantial enhancement in the magnetic anisotropy energy is obtained through the deposition of Bi; the deposition of Au and Pt leads to significant variation in other magnetic properties, but not in the magnetic anisotropy energy. An analysis within a band description shows that the coexistence of Bi adatom and a surface dopant with large spin moment activates a mechanism involving reorientation and readjustment of the spin moments of electrons in occupied bands in response to the change of magnetization direction, which promotes giant magnetic anisotropy. Our results for adsorption energetics indicate that the accommodation of Bi in the neighborhood of the surface dopant is more likely in GaN nanowires, because the Bi adatom does (not) tend to be closer to the Mn (Co) dopant on the surface of GaN (ZnO) nanowire. The stability of GaN nanowire with giant magnetic anisotropy owing to the incorporation of both Mn and Bi is demonstrated by performing ab initio molecular dynamics simulations at temperatures considerably higher than room temperature. These results suggest that adatom deposition and surface doping can be used complementarily to develop single nanowire-based spintronic devices.

Insight into interface electrical properties of metal–oxide–semiconductor structures fabricated on Mg-implanted GaN activated by ultra-high-pressure annealing

GaN-based metal–oxide–semiconductor (MOS) devices, such as n- and p-type capacitors and inversion- and accumulation-type p-channel field effect transistors (MOSFETs), were fabricated by Mg-ion implantation and ultra-high-pressure annealing (UHPA) under 1-GPa nitrogen pressure. Even though UHPA was conducted at 1400 °C without protective layers on GaN surfaces, n-type MOS capacitors with SiO2 gate dielectrics formed on non-ion-implanted regions exhibited well-behaved capacitance–voltage characteristics with negligible hysteresis and frequency dispersion, indicating distinct impact of UHPA in suppressing surface degradation during high-temperature annealing. Efficient activation of the implanted Mg dopants and reasonable hole accumulation at the SiO2/GaN interfaces were also achieved for p-type capacitors by UHPA, but the fabricated inversion- and accumulation-type p-channel GaN MOSFETs were hardly turned on. The findings reveal extremely low hole mobility at GaN MOS interfaces and suggest an intrinsic obstacle for the development of GaN-based MOS devices.

DFT Modeling of Unintentional Oxygen Incorporation Enhanced by Magnesium in GaN(0001) and AlN(0001) Growth Surfaces during Metal‐Organic Vapor‐Phase Epitaxy

Understanding the physics of unintentional doping and defect formation during epitaxial growth of III‐nitride semiconductors is crucial to develop optical and electronic devices. Herein, the impact of magnesium doping on unintentional oxygen incorporation into GaN and AlN during metal‐organic vapor‐phase epitaxy is investigated by first‐principles calculations. It is found that the presence of Mg substituting group‐III atoms (Ga or Al) in subsurface layers energetically promotes unintentional oxygen incorporation. The calculation results also suggest that even when Mg + H complex defects exist in subsurface layers, they promote unintentional oxygen incorporation in a similar manner. The mechanism of unintentional oxygen incorporation enhanced by magnesium doping and complex defect structures is discussed in terms of charge neutrality or electron‐counting model in the growth surface.