Ga-polar Surface Research Articles

Real-time x-ray studies of gallium adsorption and desorption

Real-time grazing-incidence small-angle x-ray scattering has been employed to study the adsorption and desorption of Ga on c-plane sapphire and Ga-polar GaN surfaces. Formation of self-organized liquid Ga nanodroplets has been observed on sapphire during Ga exposure from an effusion cell at high flux. Subsequent to the Ga deposition, the nanodroplets were nitridated in situ by a nitrogen plasma source, which converted the droplets into GaN nanodots. In addition to the droplet studies, at lower Ga flux, the adsorption and desorption of Ga have been studied in the predroplet regime. For identical processing conditions, significantly different Ga adsorption∕desorption rates were observed on sapphire and GaN surfaces.

Oxidized GaN(0001) surfaces studied by scanning tunneling microscopy and spectroscopy and by first-principles theory

Oxidized Ga-polar GaN surfaces have been studied both experimentally and theoretically. For in situ oxidization at 550°C using molecular oxygen, Auger electron spectroscopy indicates a saturation oxygen coverage of 2.1±0.5 ML (monolayer). For these surfaces scanning tunneling microscopy reveals two surface phases, one with 33×33−R30° periodicity and the other with disordered two times periodicity. Scanning tunneling spectroscopy revealed a surface band gap with size close to that of GaN, indicating that any states of the oxide lie predominantly outside of the GaN gap. From first-principles total energy calculations of surface formation energies two models of energetically favorable surfaces structures are developed, with oxygen coverages of 1.25 and 2 ML, respectively. Both structures satisfy electron counting and have surface band gaps close in size to that of GaN, in agreement with experiment.

Effects of KOH etching on the properties of Ga-polar n-GaN surfaces

The effects of a KOH treatment on the properties of n-type GaN surfaces and associated Au/n-GaN contacts have been investigated by X-ray photoelectron spectroscopy, atomic force microscopy, reflection high-energy-electron diffraction, current–voltage and electron-beam-induced current characterization. Ga-polar surfaces grown by molecular beam epitaxy and metal–organic chemical vapour deposition were compared. A decrease in electron barrier height and an increase in non-radiative recombination properties of Au/n-GaN contacts were found with KOH treatment, correlated with an increase of surface Ga vacancies, an increase in surface N–H2 content and a decrease in surface C contamination. A 0.3-eV shift in the Ga3d peak position towards the valence band and a reduction in the dislocation contrast were observed for the case of molecular-beam-epitaxy-grown GaN only, demonstrating that surface Ga vacancies and threading dislocations play only a limited role in defining the resultant metal/GaN contact properties. Accordingly, the surface atomic content and the resulting surface states, following KOH treatment, should be taken into consideration when appraising the electrical properties of n-GaN surfaces and the performance of associated metallic contacts.

Energetics of Mg incorporation at GaN(0001) andGaN(0001¯)surfaces

By using density functional calculations in the generalized gradient approximation, we investigate the energetics of Mg adsorption and incorporation at GaN(0001) and $\mathrm{Ga}\mathrm{N}(000\overline{1})$ surfaces under various Ga and Mg coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find significant differences in Mg incorporation between Ga- and N-polar surfaces. Mg incorporation is easier at the Ga-polar surface, but high Mg coverages are found to cause important distortions which locally change the polarity from Ga to N polar. At the N-rich and moderately Ga-rich GaN(0001) surface, 0.25 ML of Mg substituting Ga in the top bilayer strongly reduce the surface diffusion barriers of Ga and N adatoms, in agreement with the surfactant effect observed in experiments. As the Mg coverage exceeds 0.5 ML, partial incorporation in the subsurface region (second bilayer) becomes favorable. A surface structure with 0.5 ML of incorporated Mg in the top bilayer and 0.25 ML in the second bilayer is found to be stable over a wide range of Ga chemical potential. At the Ga bilayer-terminated GaN(0001) surface, corresponding to Ga-rich conditions, configurations where Mg is incorporated in the interface region between the metallic Ga bilayer and the underlying GaN bilayer appear to be favored. At the N-polar surface, Mg is not incorporated under N-rich or moderately Ga-rich conditions, whereas incorporation in the adlayer may take place under Ga-rich conditions. In the presence of light or electron beam induced excitation, energy differences between Mg incorporated at the surface and in deeper layers are reduced so that the tendency toward surface segregation is also reduced.

Effect of AlN nucleation layer on the structural properties of bulk GaN grown on sapphire by molecular-beam epitaxy

The properties of GaN, grown on sapphire by molecular-beam epitaxy, have been investigated with respect to the properties of the underlying AlN nucleation layer. We show that the AlN thickness and the Al∕N flux ratio have pronounced effects on the surface morphology and crystal quality of the GaN overlayer. Low Al∕N ratio (<0.4) leads to N-polarity GaN with poor crystal quality. For ratios between 0.4 and 0.7 the surface is smooth with Ga polarity. Higher Al∕N flux ratios result in Ga-polar surfaces having plateaus intersected by holes. Optimum values for the GaN(0002) ω scan (full width at half maximum of 250 arc sec) and the surface roughness (root mean square of 0.7 nm) were found for a 3-nm-thick AlN nucleation layer (Al∕N ratio of 0.6).

Deposition of thick GaN layers by HVPE on the pressure grown GaN substrates

GaN substrates grown from solution under high N 2 pressure without an intentional seeding are almost dislocation free single crystals in the form of thin (about 100 μm) hexagonal platelets with a lateral size up to 1 cm. They were used as substrates for HVPE which allows very fast growth of GaN in the c-direction. Both n-type GaN crystals (lattice mismatch to pure GaN of 10 −4 due to high electron concentration) and high resistivity GaN:Mg crystals (lattice matched to pure GaN at RT) have been used. Most of the layers were grown on the Ga-polar (0 0 0 1) surface of the substrates. It is shown that at the applied growth conditions, near dislocation free GaN can be deposited only up to a critical thickness dependent on the type of the substrate used. The possible mechanisms of the relaxation processes is discussed on the basis of defect selective etching and micro Raman measurements for the cleaved cross-sections of the HVPE-GaN/pressure-GaN samples. It is also shown that stable (in terms of flat crystallization front and material continuity) growth of GaN on GaN substrates is possible at a rate as high as 500 μm/h. In this way, the single crystals of GaN with thickness exceeding 2 mm were grown. These crystals were sliced along non-polar crystallographic planes. Thus, both {1 0 1¯ 0}- and {1 1 2¯ 0}-oriented platelets have been obtained to be used for epitaxial growth of quantum structures by MBE.

Polarity determination for GaN/AlGaN/GaN heterostructures grown on (0001) sapphire by molecular beam epitaxy

In order to study the formation of polarization-induced two-dimensional electron gases (2DEGs), GaN/AlGaN/GaN heterostructures with different polarity were grown on sapphire (0001) substrates by plasma-assisted molecular-beam epitaxy. The polarity of GaN layers can be changed from the normal N-polarity to a Ga-polarity surface by inserting a thin Al metal layer prior to the growth of AlN buffer layer. The surface stability of each lattice polarity film was studied by in situ reflection high-energy electron diffraction. The polarity of the films and the location of 2DEGs in the heterostructure have been evaluated by investigating the carrier concentration profiles in the GaN/AlGaN/GaN heterostructures. A simple model to explain the polarity change by a thin Al metal layer is proposed.

GaN nanotip pyramids formed by anisotropic etching

We experimentally demonstrate the formation of GaN nanotip pyramids by selective and anisotropic etching of N-polar GaN in KOH solution. For samples grown with adjacent Ga- and N-polar regions on the same wafer, the KOH solution was found to selectively etch only the N-polar surface while leaving the Ga-polar surface intact. An aggregation of hexagonal pyramids with well defined {10 1̄ 1̄} facets and very sharp tips with diameters less than ∼20 nm were formed. The density of the pyramids can be controlled by varying the KOH concentration, solution temperature or the etch duration. The GaN etching activation energy is estimated to be Ea≈0.587 eV. Dense GaN pyramids with sharp tips have applications in both electronic and photonic devices.

Directional crystallization of GaN on high-pressure solution grown substrates by growth from solution and HVPE

High-pressure solution grown hexagonal GaN platelets have been used as substrates for subsequent solution growth of GaN in the 〈0 0 0 1〉 directions and also for hydride vapor phase epitaxial (HVPE) growth of GaN. The solution growth was stabilized due to the application of large positive temperature gradient perpendicular to the surface of the GaN substrate. The growth by HVPE was carried out at atmospheric pressure in a horizontal quartz reactor. Relatively rapid growth rates of about 10 μm/h and about 50 μm/h have been achieved on the Ga-polar (0 0 0 1) surface of the substrates for solution growth and HVPE, respectively, which constitutes an increase in the c-direction growth rate for both methods. Both methods produced nearly dislocation-free GaN.

Defect-selective etching of GaN in a modified molten bases system

The method of defect-selective etching of Ga polar (0 0 0 1) surface of GaN single crystals and epitaxial layers is presented. Etching is performed in molten KOH–NaOH eutectic (E) with an addition of 10 wt% of MgO powder (E+M). At temperatures above the melting point of E, a sticky droplet of etch is formed, which allows revealing of defects on one side of the crystal only, leaving the other side intact. This method is advantageous for defect study on Ga-polar surface of GaN single crystals, because the much higher temperature required for etching of this surface will not result in the fast dissolution of N-polar side. The reliability of the method has been confirmed by: (i) revealing of the indentation-induced dislocations in the GaN single crystals and (ii) comparison of the results of (E+M) etching with photoelectrochemical etching and transmission electron microscopy on hetero-epitaxial layers.

Interface Engineering in Heteroepitaxy

We report the importance of interface engineering in heteroepitaxy with examples of plasma-assisted molecular beam epitaxial ZnO growths on (0001) sapphire substrates and on (0001) GaN/sapphire templates, whose interfacesare engineered to improve and to control properties of ZnO films. The growth of rocksalt structure MgO buffer on Al 2 O 3 (0001) is developed for ZnO epitaxy. By employing the MgO buffer layer, the formation of 30° rotated mixed domains is prohibited and two-dimensional layer-by-layer growth of ZnO on sapphire substrate is achieved. High-resolution X-ray diffraction reveals the superior improvement in a crystal quality of ZnO films with an MgO buffer. Polarity of wurtzite structure ZnO films on Ga-polar GaN/sapphire templates is controlled by changing interface structures. By forming a single crystalline, monoclinic Ga 2 O 3 interfacial layer between GaN and ZnO through O-plasma pre-exposure on the Ga-polar GaN surface, O-polar ZnO films are grown. By forming the ZnO/GaN heterointerface without an interfacial layer through the Zn pre-exposure on the Ga-polar GaN surface, Zn-polar ZnO films are grown.

Conditions for seeded growth of GaN crystals by the Na flux method

Conditions for seeded growth of GaN crystals by the Na flux method were investigated. Seeded growth in the Na flux could be achieved under temperature–N 2 pressure conditions between the condition of GaN formation without seeding and the condition of GaN decomposition. GaN single crystals with a maximum area of 3.0×1.5 mm 2 and thickness of 1.0 mm were grown from seeds at 850 °C and 2 MPa of N 2 for 200 h. The crystal growth occurred selectively on the (0001) Ga-polar surface of the platelet seed crystals. The growth rate in the c direction was higher than those of other directions and increased with growth temperature and N 2 pressure. The maximum growth rate in the c direction was about 4 μm/h at 850 °C and 2 MPa of N 2.

Control of crystal polarity in a wurtzite crystal: ZnO films grown by plasma-assisted molecular-beam epitaxy on GaN

ZnO/GaN heterointerfaces are engineered to control the polarity of ZnO films grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN templates. The polarity of ZnO films is determined both by coaxial impact collision ion scattering spectroscopy (CAICISS) and by convergent beam electron diffraction (CBED). Polarity inversion can be achieved by inserting an interface layer with a center of symmetry, because the polarity comes from a lack of the center of symmetry. An O-polar (anion-polar) ZnO film can be grown on Ga-polar (cation-polar) GaN by inserting a ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layer at the interface, while Zn-polar ZnO is grown on GaN without forming an interface layer. A single-crystalline monoclinic ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layer, which has a center of symmetry, is formed by O-plasma preexposure on the Ga-polar GaN surface prior to ZnO growth, while the ZnO/GaN interface without any extra layer is formed by Zn preexposure. The orientation relationship between ZnO, ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3},$ and GaN is determined as $[2\ensuremath{-}1\ensuremath{-}10{]}_{\mathrm{ZnO}}\ensuremath{\parallel}[010{]}_{{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}}\ensuremath{\parallel}[2\ensuremath{-}1\ensuremath{-}10{]}_{\mathrm{GaN}}$ and ${(0001)}_{\mathrm{ZnO}}\ensuremath{\parallel}{(001)}_{{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}}\ensuremath{\parallel}{(0001)}_{\mathrm{GaN}}.$ The CAICISS results reveal the growth of an O-polar ZnO film on O-plasma-preexposed GaN, while a Zn-polar ZnO film on Zn-preexposed GaN. The origin of the observed features in polar-angle-dependent CAICISS spectra can be analyzed by considering the shadow cones of Zn and O atoms formed by incident ions and shadowing and focusing effects of scattered ions. Azimuthal-angle-dependent CAICISS spectra reveal the surfaces of both Zn- and O-polar ZnO films as mixture of c and $c/2$ planes with a ratio of about 50:50. The ZnO film with a ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ interface layer shows a degradation in the crystal quality as evidenced by a broadening of the x-ray rocking curves. The CBED results for the O-plasma-preexposed samples reveal Ga-polar GaN and O-polar ZnO for the O-plasma-preexposed samples, which directly confirms polarity inversion from cation to anion polar. On the other hand, Zn-polar ZnO CBED patterns are obtained from ZnO films grown on Zn-preexposed Ga-polar GaN, which indicates the same cation polarity for a ZnO/GaN interface without the formation of an interface layer. It is noted that no planar or faceted inversion domain boundaries are formed to invert the polarity (from Ga polar to O polar). This indicates that we can control the polarity by engineering interfaces.

Functionalizing the GaN( [formula omitted])-(1×1) surface I. The chemisorption of aniline

Chemisorption of aniline (C 6H 5NH 2) on the GaN(0 0 0 1)-(1×1) Ga-polar surface has been studied using mainly X-ray-excited Auger electron, electron energy loss and ultraviolet photoemission spectroscopies (XAES, ELS and UPS, respectively). The XAES data show adsorption near room temperature with a total sticking probability of ∼0.05 and a saturation coverage of ∼0.28 phenyl rings per surface lattice site. The ELS data show removal of the characteristic surface-state band centered at ∼3.4 eV and the appearance of a π→ π ∗ loss at 6.5 eV due to CC bonds. In contrast, benzene (C 6H 6) does not chemisorb under these conditions. The UPS data, which have been analyzed with the aid of density functional theory molecular orbital calculations, indicate that adsorption occurs with a phenyl–NH group forming a Ga–N–Ga bridge. Adsorption causes a decrease in electron affinity ( δχ≈−0.55 eV) due to a surface dipole layer, and measurement of δχ and of changes in band bending have been used to construct a partial energy level diagram for the aniline-covered surface.

Structure Analysis of GaN Thin Film with Inversion Domains by High Voltage Atomic Resolution Microscopy

The atomic structures and surface morphologies of three types of GaN films were investigated by high voltage atomic resolution microscopy (HVARM). By HVARM, each atomic column of Ga and N could clearly be resolved and the polarity of the film and inversion domains could be directly determined. The GaN film was grown on a sapphire substrate by molecular beam epitaxy (MBE) after nitridation of the sapphire surface. Inversion domains (IDs) crossed the whole film to the surface and made small pyramids on the surface. The small pyramids had Ga-polarity and the rest had N-polarity. A GaN film with In exposure during film growth had an almost Ga-polarity flat surface. In exposure process reduced the density of inversion domains that have a N-polarity. While a GaN film grown on an AlN buffer layer was unipolar, with a Ga-polarity. HVARM observation revealed that the density of the IDs determine the qualities and the polarity of the film.

Seeded growth of GaN at high N 2 pressure on (0 0 0 1) polar surfaces of GaN single crystalline substrates

GaN single crystals obtained by the high nitrogen pressure solution method without an intentional seeding, show strong growth anisotropy which results in their platelet shape. The attempts to enhance the growth into (0 0 0 1) directions by the increase of the integral supercooling in the solution, often lead to the growth instabilities on both Ga-polar and N-polar (0 0 0 1) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface. The results of the seeded growth on both Ga- and N-polar (0 0 0 1) surfaces in configuration enabling such a control is reported. It is shown that dominating mechanisms of the unstable growth such as the cellular growth or edge nucleation can be suppressed. Differences in nucleation and growth in dependence on surface polarity are discussed.

High pressure growth of bulk GaN from solutions in gallium

In this paper, the growth of GaN single crystals from solutions of atomicnitrogen in liquid gallium under high N2 pressure is described. GaN singlecrystals obtained by the high nitrogen pressure solution method, without anintentional seeding, show strong growth anisotropy, which results in theirplatelet shape. The attempts to enhance the growth into (0001) directions bythe increase of the integral supercooling in the solution often lead togrowth instabilities on both N-polar and Ga-polar (0001) surfaces. This can beavoided only by the precise control of the growth conditions at thecrystallization front on the particular surface.The results of the seeded growth in directions parallel and perpendicular tothe c-axis of GaN are discussed. In particular, it is shown that dominatingmechanisms of the unstable growth on (0001) polar surfaces such as thecellular growth or edge nucleation can be suppressed and the crystal can begrown in a much more stable way.Physical properties most relevant for understanding the growth of GaN crystalsare reviewed. The most important feature of GaN crystals grown by the HNPSmethod is that they are almost free of dislocations and therefore used assubstrates give a possibility to grow perfect epitaxial structures.

Silicon on GaN(0001) and (0001̄) surfaces

Surface reconstructions and adatom kinetics of silicon on GaN(0001) and (0001̄) surfaces are studied by scanning tunneling microscopy, electron diffraction, and first-principles calculations. For silicon coverage near 0.5 monolayer, a 2×2 structure is observed, and is interpreted in terms of a model consisting of a Ga adatom on a monolayer of 3 Ga+1 Si and a SiGa atom in the third layer. For higher silicon coverage, disordered 2×2 domains and “1×1” domains are found to coexist. After annealing above 300 °C the “1×1” regions become dominant and a 4×4 structure is seen near step edges. It is concluded that the silicon adatoms tend to reside in subsurface sites on the Ga-polar surface. Surface morphology in the presence of Si is smooth for the (0001) surface but rough for the (0001̄) surface. This difference is attributed to the presence of multiple Ga surface layers in the former case, which enhance surface diffusivities.

Surface superstructures and optical properties of wurtzite GaN grown on 6H-SiC

We report on the surface reconstructions on the basal planes of wurtzite GaN. Depending on the surface polarity, two distinct classes of reconstructions form. The Ga-polar surface displays the Ga fluid-1×1, 10×10, 5√3×2√13, 5×5, 4×4, and 2×2, and the (0 0 0 1 ̄ ) N-polar surface the 6×8, 6×6, √7×√7 and 2×3 with decreasing surface Ga coverage. We will show that their structure is consistent with a simple Ga-adatom-based scheme. Crystalline structure and optical properties of the GaN epilayers are examined by high resolution X-ray diffraction and photoluminescence, and the results reveal a strong relation between the threading-dislocation density and the intensities of both near-band transition at 3.427 eV and yellow luminescence at 2.16 eV.

Computational Chemistry Study on Crystal Growth of InGaN/GaN

We carried out density functional theory (DFT) calculations and tight-binding quantum chemical molecular dynamics simulations to investigate the crystal growth of InGaN layers on a GaN(0001) surface under periodic boundary conditions. The periodic DFT calculations indicate that the formation of the uniform InGaN thin films is difficult, which leads to the segregation of In atoms in the InGaN thin films. The tight-binding quantum chemical molecular dynamics simulations suggest that the Ga polarity surface of the GaN(0001) is preferable for the construction of the smooth GaN thin films than the N polarity surface of the GaN(0001). Moreover, the formation of the smooth InN thin films was found to be difficult on the GaN(0001) surface.