Strain induced deep electronic states around threading dislocations in GaN

Combining through-focus high-resolution transmission electron microscopy and hierarchical multiscale simulations consisting of density-functional theory, analytical empirical potentials, and continuum elastic theory we demonstrate the existence of a new dislocation type in GaN. In contrast with all previously identified or suggested dislocation structures in GaN, all core atoms are fully coordinated; i.e., no broken bonds occur, implying that the dislocation should be electrically inactive. However, as we show, the giant local strain-field around the dislocation core, in combination with the small lattice constant of GaN, causes deep defect states and thus electrically active edge dislocations independent on the specific core structure.     

Characterization of individual threading dislocations in GaN using ballistic electron emission microscopy

Threading dislocations (TDs) of molecular beam epitaxy grown GaN film were studied with ultrahigh vacuum ballistic electron emission microscopy in order to quantify any fixed negative charge at identifiable TDs, with approximately 3 nm spatial and approximately 10 meV local barrier resolution. In contrast to several prior studies, we find no indication of fixed negative dislocation charge at specific TD structures, with a conservative upper limit of approximately 0.25 e(-) per c-axis unit cell. We do observe evidence of positive surface charge at TDs and at GaN step edges, which may be due to local piezoelectric fields.     

Studies of threading dislocations in Nb(011) films

We obtain strain contrast in low-energy electron microscopy, by dark-field imaging of the strain-sensitive variants of a surface reconstruction. This is employed to make visible the strain fields of dislocations in Nb(011) thin single-crystal films. The strain field symmetries reveal the dislocation Burgers vectors and identify the existence of [111] a /2 and [100] a Burgers vectors for threading dislocations in these epitaxial materials. The contrast also allows interfacial and screw dislocations to be imaged.     

Carrier diffusion processes near threading dislocations in GaN and GaN:Si characterized by low voltage cathodoluminescence

We present a low temperature study of GaN epilayers by means of low voltage cathodoluminescence (CL). We show that lowering the primary electrons accelerating voltage down to 1 kV allows imaging of single threading dislocations. By using monochromatic or panchromatic low voltage CL microscopy, it is possible to extract different diffusion lengths related to free excitons, bound excitons or donor-to-acceptor pair transitions.     

Structural and optical characterization of GaN heteroepitaxial films on SiC substrates

We have estimated the threading dislocation density and type via X-ray diffraction and Williamson-Hall analysis to elicit qualitative information directly related to the electrical and optical quality of GaN epitaxial layers grown by PAMBE on 4H- and 6H-SiC substrates. The substrate surface preparation and buffer choice, specifically: Ga flashing for SiC oxide removal, controlled nitridation of SiC, and use of AlN buffer layers all impact the resultant screw dislocation density, but do not significantly influence the edge dislocation density. We show that modification of the substrate surface strongly affects the screw dislocation density, presumably due to impact on nucleation during the initial stages of heteroepitaxy.     

Electronic and optical properties of GaN/AlN quantum dots with adjacent threading dislocations

We present a theory to simulate a coherent GaN QD with an adjacent pure edge threading dislocation by using a finite element method. The piezoelectric effects and the strain modified band edges are investigated in the framework of multi-band $\bm k\cdot \bm p$ theory to calculate the electron and the heavy hole energy levels. The linear optical absorption coefficients corresponding to the interband ground state transition are obtained via the density matrix approach and perturbation expansion method. The results indicate that the strain distribution of the threading dislocation affects the electronic structure. Moreover, the ground state transition behaviour is also influenced by the position of the adjacent threading dislocation.     

Comparison study of GaN films grown on porous and planar GaN templates

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition(MOCVD)-GaN template by halide vapor phase epitaxy(HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence(PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.     

Strain-mediated phase coexistence in heteroepitaxial films

We present experimental evidence of the equilibrium coexistence between crystalline phases in heteroepitaxial films of MnAs on GaAs. The phases, which can coexist in the bulk system only at one temperature point, coexist in the epitaxial film over a wide temperature interval. An apparent contradiction with the Gibbs phase rule is resolved by the presence of strain in the film.     

Luminescent properties of GaN films grown on porous silicon substrate

GaN films have been grown on porous silicon at high temperatures (800-1050 °C) by metal organic vapor phase epitaxy. The optical properties of GaN layers were investigated by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL spectra recorded at 5 K exhibit excitonic emissions around 3.36-3.501 eV and a broad yellow luminescence at 2.2 eV. CL analysis at different electron excitation conditions shows spatial non-uniformity in-depth of the yellow and the band-edge emissions. These bands of luminescence are broadened and red- or blue-shifted as the electron beam penetrates in the sample. These behaviors are explained by a change of the fundamental band gap due to residual strain and the local thermal effect. It was found that the use of AlN buffer layer improves the crystalline quality and the luminescence property of GaN.     

Heteroepitaxial GaN films on silicon substrates with porous buffer layers

New complex buffer layers based on a porous material have been developed for epitaxial growth of GaN films on Si substrates. The characteristics of gallium nitride heteroepitaxial layers grown on silicon substrates with new buffer layers by metal-organic vapor phase epitaxy are investigated. It is shown that the porous buffer layers improve the electric homogeneity and increase the photoluminescence intensity of epitaxial GaN films on Si substrates to the values comparable with those for reference GaN films on Al₂O₃ substrates. It is found that a fianite layer in a complex buffer is a barrier for silicon diffusion from the substrate into a GaN film.     

The growth mechanism of pentacene-fullerene heteroepitaxial films

The growth of pentacene on C₆₀ film has been studied in real-time by low energy electron microscope. The standing-up phase overgrows on the lying-down phase at room temperature with increasing film thickness. At intermediate temperature we observed two distinct types of nucleation: an earlier nucleation of lying-down phase and a delayed nucleation of standing-up phase on the bare C₆₀ surface between islands of lying-down phase. Further thermal activation control enabled us to tune the types of nucleation and the standing-up phase without co-presence of the lying-down phase could be achieved above ∼70 °C.     

Role of oxygen at screw dislocations in GaN

Here we report the first direct atomic scale experimental observations of oxygen segregation to screw dislocations in GaN using correlated techniques in the scanning transmission electron microscope. The amount of oxygen present in each of the three distinct types of screw dislocation core is found to depend on the evolution and structure of the core, and thus gives rise to a varying concentration of localized states in the band gap. Contrary to previous theoretical predictions, the substitution of oxygen for nitrogen is observed to extend over many monolayers for the open core dislocation.     

Ni/Au/n-GaN肖特基二极管可导位错的电学模型

可导线性位错被普遍认为是GaN基器件泄漏电流的主要输运通道,但其精细的电学模型目前仍不清楚.鉴于此,本文基于对GaN肖特基二极管的电流输运机制分析提出可导位错的物理模型,重点强调:1)位于位错中心的深能级受主态(主要Ga空位)电离后库仑势较高,理论上对泄漏电流没有贡献; 2)位错周围的高浓度浅能级施主态电离后能形成势垒高度较低的薄表面耗尽层,可引发显著隧穿电流,成为主要漏电通道;3)并非传统N空位,认为O替代N所形成的浅能级施主缺陷应是引发漏电的主要电学态,其热激活能约为47.5 meV.本工作亦有助于理解其他GaN器件的电流输运和电学退化行为.     

Solutions for heteroepitaxial growth of GaN and their impact on devices

GaN technology relies on highly mismatched heteroepitaxial growth, mainly on sapphire or SiC substrates, and therefore suffers from 10⁹ to 10¹⁰ threading dislocations per cm². The origin and the deteriorating influence of the extremely high dislocation densities are analyzed with regard to the specific circumstances of GaN technology. Various attempts to cope with heteroepitaxial growth are discussed, from the use of nucleation layers to the growth on GaN single bulk crystals. Special focus is put on the impact of the approaches on the device performance.     

Dynamics of step bunching in heteroepitaxial growth on vicinal substrates

When a heteroepitaxial film is grown on a vicinal substrate, the terrace steps at the growth front may bunch together to relieve strain, resulting in a rough surface. On the other hand, proper manipulation of the growth kinetics may suppress the inherent bunching instability, thus preserving step-flow growth. Here we show that the step dynamics in the early stages of growth can already determine whether the bunching instability is truly suppressed, prior to bunching actually taking place in the unstable regime. We determine the critical film thickness above which steps will bunch and exploit its scaling properties and usefulness for extracting intrinsic energy parameters. Experimental studies of SrRuO(3) films grown on vicinal SrTiO(3) substrates clearly establish the existence of the critical film thickness in step bunching.     

Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120° to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.     

Strain relief of heteroepitaxial bcc-Fe(001) films

The strain relief of heteroepitaxial bcc-Fe(001) films, deposited at 520-570 K onto MgO(001), has been investigated by scanning tunneling microscopy. In accordance with real-time stress measurements, the tensile misfit strain is relieved during coalescence of flat, mainly 2-3 monolayers (ML) high Fe islands at the high thickness of approximately 20 ML. To accommodate the misfit between merging strain-relaxed islands, a network of 1/2[111] screw dislocations is formed. A strong barrier for dislocation glide--which is typical for bcc metals--is most likely responsible for the big delay in strain relief of Fe/MgO(001), since only the elastic energy of the uppermost layer(s) is available for the formation of an energy-costly intermediate layer.     

Langevin equation for self-organized morphologies of thin heteroepitaxial films

The self-organization of nanostructures on strained epitaxial films is expressed as a Langevin equation obtained from an atomistic model of the growth kinetics. The transition rules are based on the incorporation of strain effects into environment-dependent detachment barriers. Comparisons are made with a previous approach based on continuum elasticity to provide an atomistic interpretation of the governing equation for the morphological evolution of strained films.     

Electrophysical properties and energetical spectrum of heteroepitaxial germanium films

An epitaxial growth of Ge films from molecular beam is characterized by thermodynamical nonequilibrium. This leads to formation of numerous structural defects connected with local levels in the band gap (mainly acceptor-type). Depending on the deposition temperature the density of such levels may change in wide range (10¹⁶–10¹⁸ cm^?3). This determines various mechanisms of impurity conduction. The tails of the density of states in the band gap are also connected with the defect structure of the films. Stresses in heteroepitaxial Ge films result in the splitting of the valence band atk = 0 and in a change of the band gap. Thus, these stresses have influence on the electrical and optical properties of the films.