Clean GaN Research Articles

Epitaxial Molybdenum Disulfide/Gallium Nitride Junctions: Low-Knee-Voltage Schottky-Diode Behavior at Optimized Interfaces.

Low turn-on (knee) voltage (∼0.3 V) Schottky-diode behavior of a four-layer (4L) MoS2/GaN junction is achieved by optimizing the in situ interface preparation of the GaN substrate prior to MoS2 overlayer growth in a vacuum system using metallic molybdenum and hydrogen sulfide gas as precursors. The process leads to a clean nitrogen-terminated GaN surface that bonds well to the MoS2 film revealing a 2 × 2 reconstruction at the interface observed in low-energy electron diffraction (LEED). Atomic force microscopy and X-ray photoelectron spectroscopy provide clear images of the GaN terraces through the MoS2 overlayer confirming close adhesion and absence of oxygen and other contaminants. Density functional theory calculations predict the formation of the 2 × 2 superstructure at a clean interface. Transport measurements show diode behavior at an on/off ratio of ∼105 for ±1 V with a forward direction for the positive voltage applied to the MoS2 layer. Combining transport and photoelectron spectroscopy measurements with theory, we deduce a Fermi-level position in the MoS2 gap consistent with interface charge transfer from MoS2 to the substrate. The high performance of the MoS2/Gan diode highlights the technological potential of devices based on GaN/MoS2 interfaces.

Interplay of intrinsic and extrinsic states in pinning and passivation of m-plane facets of GaN n-p-n junctions

Intrinsic and extrinsic pinning and passivation of m-plane cleavage facets of GaN n-p-n junctions were investigated by cross-sectional scanning tunneling microscopy and spectroscopy. On freshly cleaved and clean p-type GaN(101¯0) surfaces, the Fermi level is found to be extrinsically pinned by defect states, whereas n-type surfaces are intrinsically pinned by the empty surface state. For both types of doping, air exposure reduces the density of pinning states and shifts the pinning levels toward the band edges. These effects are assigned to water adsorption and dissociation, passivating intrinsic and extrinsic gap states. The revealed delicate interplay of intrinsic and extrinsic surface states at GaN(101¯0) surfaces is a critical factor for realizing flatband conditions at sidewall facets of nanowires exhibiting complex doping structures.

Bistable Fermi level pinning and surface photovoltage in GaN

Band bending and Fermi level pinning at GaN(0001) surfaces have proved controversial despite their fundamental importance. By combining Kelvin probe, surface photovoltage and X-ray photoemission measurements, we clarify how the Fermi level pinning affects the band bending for n- and p-type GaN(0001). The presence of two different mid-gap pinning levels is confirmed on native oxide-covered samples, and they lie at remarkably similar energies to those previously found for atomically clean GaN(0001). The population of these bistable pinning levels and their influence on the band bending both depend on the bulk doping and level of illumination by above-gap photons.

Electron microscopy and ultraviolet photoemission spectroscopy studies of native oxides on GaN(0001)

We investigated native oxides on GaN(0001) using high-resolution scanning transmission electron microscopy (STEM) and ultraviolet photoemission spectroscopy (UPS). As a result, it was confirmed that the native oxides on both n-type and p-type GaN(0001) with a thickness of ∼1 nm were not amorphous but crystalline with lattice-matched structures to GaN, as we reported previously [Y. Irokawa et al., Jpn J. Appl. Phys. 56, 128004 (2017)]. Moreover, the UPS spectra of native oxides on n-type GaN(0001), which were similar to the reported clean GaN(0001) spectra, showed the existence of surface states at the valence band maximum (VBM), reflecting the defective quality of the native oxides.

AlGaN/GaN heterostructures with an AlGaN layer grown directly on reactive-ion-etched GaN showing a high electron mobility (>1300 cm2 V−1 s−1)

In this study, the metal–organic-vapor-phase-epitaxial growth behavior and electrical properties of AlGaN/GaN structures prepared by the growth of an AlGaN layer on a reactive-ion-etched (RIE) GaN surface without regrown GaN layers were investigated. The annealing of RIE-GaN surfaces in NH3 + H2 atmosphere, employed immediately before AlGaN growth, was a key process in obtaining a clean GaN surface for AlGaN growth, that is, in obtaining an electron mobility as high as 1350 cm2 V−1 s−1 in a fabricated AlGaN/RIE-GaN structure. High-electron-mobility transistors (HEMTs) were successfully fabricated with AlGaN/RIE-GaN wafers. With decreasing density of dotlike defects observed on the surfaces of AlGaN/RIE-GaN wafers, both two-dimensional electron gas properties of AlGaN/RIE-GaN structures and DC characteristics of HEMTs were markedly improved. Since dotlike defect density was markedly dependent on RIE lot, rather than on growth lot, surface contaminations of GaN during RIE were believed to be responsible for the formation of dotlike defects and, therefore, for the inferior electrical properties.

Synchrotron-based photoemission study of electronic structure of the Cs/GaN ultrathin interface

Electronic structure of the Cs/n-GaN nano-interface has been studied in situ via synchrotron-based photoelectron spectroscopy by excitation in the energy range of 70–400 eV. The GaN sample was grown by an original method of epitaxy of low-defect unstressed nanoscaled films on AlGaN/SiC/Si substrate. Changes in the surface state spectra and in the Ga 3d, Cs 4d, Cs 5p, N 1s core level spectra have been revealed under different cesium coverages. The intrinsic surface states for the clean GaN surface at binding energies of ∼5.0 eV and ∼7.0 eV are attenuated during Cs adsorption. Simultaneously three Cs induced surface states are found to arise. Drastic changes in the surface state spectrum were ascertained and shown to be originated from the local interacting Ga dangling bonds and adsorbed Cs atoms initiating the electron redistribution effect with formation of the semiconductor-like Cs/n-GaN interface.

Fuchs–Kliewer phonons of H-covered and clean GaN[formula omitted

Inelastic electron scattering is used to study surface phonon polaritons on H-covered and clean GaN(11¯00) surfaces. The Fuchs–Kliewer phonon of GaN(11¯00)–H gives rise to characteristic signatures of its single and multiple excitation in specular electron energy loss spectra. The loss intensities for multi-phonon scattering processes decrease according to a Poisson distribution. Vibrational spectra of this surface are invariant on the time scale of days reflecting its chemical passivation by the H layer. In contrast, vibrational spectra of pristine GaN(11¯00) are subject to a pronounced temporal evolution where spectroscopic weight is gradually shifted towards the multiple excitation of the Fuchs–Kliewer phonon. As a consequence, the monotonous decrease of the cross section for multiple quantum excitation as observed for the H-covered surface is not applicable. This remarkable effect is particularly strong in spectra acquired at low primary energies of incident electrons, which hints at processes occurring in the very surface region. Scenarios that may contribute to these observations are discussed.

Photo-assisted Kelvin probe force microscopy investigation of three dimensional GaN structures with various crystal facets, doping types, and wavelengths of illumination

Three dimensional GaN structures with different crystal facets and doping types have been investigated employing the surface photo-voltage (SPV) method to monitor illumination-induced surface charge behavior using Kelvin probe force microscopy. Various photon energies near and below the GaN bandgap were used to modify the generation of electron–hole pairs and their motion under the influence of the electric field near the GaN surface. Fast and slow processes for Ga-polar c-planes on both Si-doped n-type as well as Mg-doped p-type GaN truncated pyramid micro-structures were found and their origin is discussed. The immediate positive (for n-type) and negative (for p-type) SPV response dominates at band-to-band and near-bandgap excitation, while only the slow process is present at sub-bandgap excitation. The SPV behavior for the semi-polar facets of the p-type GaN truncated pyramids has a similar characteristic to that on its c-plane, which indicates that it has a comparable band bending and no strong influence of the polarity-induced charges is detectable. The SPV behavior of the non-polar m-facets of the Si-doped n-type part of a transferred GaN column is similar to that of a clean c-plane GaN surface during illumination. However, the SPV is smaller in magnitude, which is attributed to intrinsic surface states of m-plane surfaces and their influence on the band bending. The SPV behavior of the non-polar m-facet of the slightly Mg-doped part of this GaN column is found to behave differently. Compared to c- and r-facets of p-type surfaces of GaN-light–emitting diode micro-structures, the m-plane is more chemically stable.

Electron band bending and surface sensitivity: X-ray photoelectron spectroscopy of polar GaN surfaces

The role of electron band bending and surface sensitivity in determining the core level binding energies by X-ray photoelectron spectroscopy is investigated. A dominating contribution of surface atomic layers to photoemission intensity is confirmed for normal photoemission. The energy of the photoelectron core level peak does not deviate from core level peak energies of electrons photoemitted from the surface atomic layers of the crystal. The higher surface sensitivity regime, achieved e.g. at off-normal photoelectron detection angle, can be used to study the surface potential barrier in just a few topmost atomic layers. In addition, it is demonstrated that core level binding energy measured by angle-resolved X-ray photoelectron spectroscopy reflect the electron attenuation anisotropy. In particular, core level binding energy changes with emission angle and correlates with the forward focusing directions in a crystal. This effect is demonstrated by measuring the polar angle dependence of Ga 3d core levels on clean GaN(0001) and GaN(0001¯) surfaces with a higher and a lower band bending, respectively. The effect is explained by variation of emission depth in a crystal for normal and off-normal photoelectron emission angles.

Adsorption and desorption of hydrogen at nonpolar GaN(11¯00) surfaces: Kinetics and impact on surface vibrational and electronic properties

The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of H2 and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab-initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100) surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga-N surface dimer buckling. The signatures of the Ga-H and N-H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean GaN(1-100) surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion.

Adatom Interactions on GaN(0001) Surface I: Coverage-Dependent Adsorption

The adsorption of an adatom on the surface of a crystal is affected by the surface coverage due to adatom–adatom and adatom–substrate interactions. We study the dependence of adsorption energy of Ga and N adatoms on wurtzite GaN(0001) as a function of decreasing surface coverage from 0.25 to 0.04 monolayers through first-principles calculations. The adsorption energies of Ga and N adatoms on the flat, clean GaN(0001) substrate do not converge with decreasing coverage. Further, it appears that the Ga and N adatoms significantly distort the substrate lattice from the flat configuration. We found that these distortions increase with increase in system size (or, equivalently, with reducing coverage). This observation is counterintuitive since it is expected that lattice distortions should decrease with decreasing surface coverage. We separate the different contributions to the adsorption energy and identify the part of adsorption energy that arises due to lattice distortions. This contribution appears to be m...

Ab initiostudy of Ga-GaN system: Transition from adsorbed metal atoms to a metal–semiconductor junction

Ab initio studies of a GaN(0001)-Ga system with various thicknesses of a metallic Ga layer were undertaken. The studied systems extend from a GaN(0001) surface with a fractional coverage of gallium atoms to a Ga-GaN metal–semiconductor (m–s) contact. Electronic properties of the system are simulated using density functional theory calculations for different doping of the bulk semiconductor. It is shown that during transition from a bare GaN(0001) surface to a m–s heterostructure, the Fermi level stays pinned at a Ga-broken bond highly dispersive surface state to Ga–Ga states at the m–s interface. Adsorption of gallium leads to an energy gain of about 4 eV for a clean GaN(0001) surface and the energy decreases to 3.2 eV for a thickly Ga-covered surface. The transition to the m–s interface is observed. For a thick Ga overlayer such interface corresponds to a Schottky contact with a barrier equal to 0.9 and 0.6 eV for n- and p-type, respectively. Bond polarization-related dipole layer occurring due to an electron transfer to the metal leads to a potential energy jump of 1.5 eV, independent on the semiconductor doping. Additionally high electron density in the Ga–Ga bond region leads to an energy barrier about 1.2 eV high and 4 A wide. This feature may adversely affect the conductivity of the n-type m–s system.

Theoretical study of cesium and oxygen activation processes on GaN (0001) surface

The structure properties and work function of reflection-mode GaN photocathodes are investigated by using first-principles calculation within density-function theory (DFT). Seven different GaN (0001) (1×1) surface models are used in this paper to simulate cesium and oxygen activation process. Before starting the surface models calculations, clean bulk GaN surface is first optimized, when compared with the models before optimization, the correctness of the calculations method is verified. Next, change of work function of different surface models caused by adsorption is calculated and analyzed, the results show that over-cesiuminizd atmosphere is not only benefit for work function declining but also conductive to the formation of negative electron affinity (NEA), the optimal ratio of Cs to O for activation is between 3:1 and 4:1, excess Cs or O modules increase the work function and undermine the photocathode. Then a series of experiments are performed to verify the calculation results. First of all, NEA GaN photocathode activation and evaluation system is established, “yo-yo” activation method is verified. Then, an activation experiment is performed on high quality p-type Mg-doped reflection-mode GaN substrate grown by metal organic chemical vapor deposition (MOCVD), the photocurrent is controlled and monitored by multi-information on-line monitoring system, the results are consistent with our calculation models. Finally, combined with the data provided by Stanford University, schematic energy band variation of the GaN photocathode after only-Cs and Cs/O activations is given. Our result opens the possibility to engineer the activation properties of GaN photocathode.

A Comparative Photoelectron Spectroscopic Analysis of MBE and MOCVD Grown Epitaxial GaN Films

The report presents X-Ray Photoelectron Spectroscopic analysis of as-received and cleaned MBE and MOCVD grown undoped epitaxial GaN films. We correlate observed changes in electronic structure and surface properties after employing a newly derived approach to clean GaN surfaces. The adopted approach is a combination of two different standard cleaning procedures consisting of ex-situ wet chemical (HCl) etching followed by an in-situ ultrahigh vacuum anneal (up to 750 °C), and successfully yielded a clean and stoichiometric GaN surface. The method was employed to investigate cleanliness of MBE and MOCVD grown GaN films and it was observed that major contaminants such as carbon and oxygen were significantly removed from the surface. XPS analysis revealed that MBE grown films show higher amount of surface contamination than the MOCVD grown films but can also be cleaned more efficiently. Thermal annealing (≥ 750 °C) resulted in the dissociation of GaN bonds and accumulation of metallic gallium on the surface on MOCVD grown GaN films, which was confirmed by core level as well as the valence band spectra. The n O/n N ratio of the films reduced drastically from 1.79 to 0.04 and 0.73 to 0.09 after cleaning with the stoichiometry (N:Ga) of 1.0 ± 0.04 and 1.1 ± 0.05 for MBE and MOCVD grown GaN films respectively. The VB maximum position of MBE and MOCVD grown films was observed to be shifted from 2.9 ± 0 eV to 2.2 ± 0.1 eV and 2.8 ± 0.1 eV to 1.4 ± 0.1 eV after cleaning of the surface.

Energetics and magnetism of Co-doped GaN(0001) surfaces: A first-principles study

A comprehensive first-principles study of the energetics, electronic, and magnetic properties of Co-doped GaN(0001) thin films are presented and the effect of surface structure on the magnetic coupling between Co atoms is demonstrated. It is found that Co atoms prefer to substitute the surface Ga sites in different growth conditions. In particular, a CoN/GaN interface structure with Co atoms replacing the first Ga layer is preferred under N-rich and moderately Ga-rich conditions, while CoGax/GaN interface is found to be energetically stable under extremely Ga-rich conditions. It is worth noted that the antiferromagnetic coupling between Co atoms is favorable in clean GaN(0001) surface, but the existence of ferromagnetism would be expected to occur as Co concentration increased in Ga-bilayer GaN(0001) surface. Our study provides the theoretical understanding for experimental research on Co-doped GaN films and might promise the Co:GaN system potential applications in spin injection devices.

New Approach to Clean GaN Surfaces

New Approach to Clean GaN Surfaces

Oxidation of GaN(0001) by low-energy ion bombardment

Abstract Oxygen bombardment is used as the oxidation method to form an insulator on GaN substrate at room temperature. The surface of clean substrate and the Ga2O3/GaN interface are characterized in situ by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and low-energy electron diffraction (LEED). XPS spectra of the Ga 2p core level show a peak shift of ∼1 eV from Ga O to Ga N bonding, demonstrating an oxide formation of Ga2O3. Electron affinity of the clean GaN surface amounts to 3.65 eV; the electron affinity of the Ga2O3 on GaN(0001) surface is 3.7 eV. Valence band offset (VBO) of the Ga2O3/GaN interface measured using XPS and UPS is 1.15 eV. After annealing the system at 500 °C the electron affinity and the VBO decrease to the values 3.5 and 0.35 eV, respectively.

Behavior of aluminum adsorption and incorporation at GaN(0001) surface: First-principles study

First-principles calculations are performed to study the energetics and atomic structures of aluminum adsorption and incorporation at clean and Ga-bilayer GaN(0001) surfaces. We find the favorable adsorption site changes from T4 to T1 as Al coverage increased to 1 monolayer on the clean GaN(0001) surface, and a two-dimensional hexagonal structure of Al overlayer appears. It is interesting the Al atoms both prefer to concentrate in one deeper Ga layer of clean and Ga-bilayer GaN(0001) surface, respectively, while different structures could be achieved in above surfaces. For the case of clean GaN(0001) surface, corresponding to N-rich and moderately Ga-rich conditions, a highly regular superlattice structure composed of wurtzite GaN and AlN becomes favorable. For the case of Ga-bilayer GaN(0001) surface, corresponding to extremely Ga-rich conditions, the Ga bilayer is found to be sustained stable in Al incorporating process, leading to an incommensurate structure directly. Furthermore, our calculations provide an explanation for the spontaneous formation of ordered structure and incommensurate structure observed in growing AlGaN films. The calculated results are attractive for further development of growth techniques and excellent AlGaN/GaN heterostructure electronic devices.

Fermi level influence on the adsorption at semiconductor surfaces—ab initio simulations

Chemical adsorption of the species at semiconductor surfaces is analyzed showing the existence of the two contributions to adsorption energy: bond creation and charge transfer. It is shown that the energy of quantum surface states is affected by the electric field at the surface, nevertheless, the potential contribution of electron and nuclei cancels out. The charge transfer contribution is Fermi level independent for pinned surfaces. Thus for Fermi level pinned at the surface, the adsorption energy is independent on the Fermi energy, i.e., the doping in the bulk. The DFT simulations of adsorption of hydrogen at clean GaN(0001) and silicon at SiC(0001) surfaces confirmed independence of adsorption energy on the doping in the bulk. For the Fermi level nonpinned surfaces, the charge contribution depends on the position of Fermi level in the bulk. Thus adsorption energy is sensitive to change of the Fermi energy in the bulk, i.e., the doping. The DFT simulations of adsorption of atomic hydrogen at 0.75 ML hydrogen covered GaN(0001) surface confirmed that the adsorption energy may be changed by about 2 eV by the doping change from n- to p-type.

Electronic properties on GaN(0001) surface – ab initio investigation

Density Functional Theory (DFT) calculations were used to determine physical properties of bulk GaN and GaN(0001), i.e. N-terminated surface. The bulk data are in agreement with the theoretical and experimental data reported by Magnuson et al. [M. Magnuson et al., Phys. Rev. B 81 (2010) 085125], demonstrating that bonding in GaN is of a mixed nature in which Ga 4s, Ga 4p, Ga 3d, and N 2s, N 2p orbitals are involved. Ga 4s and Ga 4p states are sp3 hybridized while the N states are involved independently in bonding. Ga sp3 states overlap with the N 2s states creating the upper subband extending down to 6 eV below the valence band maximum (VBM). The lower valence band (VB) subband is composed of Ga 3d states overlapping with N 2s states whose energy levels range from 13 to 17 eV below VBM. GaN bulk bonding enforces creation of the two surface states at clean GaN(0001) surface: the lower energy state, located 15 eV below VBM is composed of N 2s states. The upper energy surface state, located below VBM, is composed of N 2pz nonsaturated state, i.e. it is of the broken bond type. The Fermi energy is pinned by this dispersionless acceptor state. Ga adsorption leads to emergence of donor state, which pins the Fermi level at 1.1 eV above VBM which is compatible with the Fermi level pinned at the surface for p-type GaN and is inconsistent with n-type bulk. The total energy of slab has the energy lower by about 2 eV for the adsorbed than for the separate Ga atom. The inconsistent data obtained from thermodynamic stability and band diagram analyses suggest further investigations to explain inconsistencies between these two pictures.