Single Threading Dislocation Research Articles

Conduction mechanism of Schottky contacts fabricated on etch pits originating from single threading dislocation in a highly Si-doped HVPE GaN substrate

This paper reports the relationship between a single dislocation structure and current leakage mechanism observed in a Pt/n+-GaN Schottky contacts with the combined use of transmission electron microscopy (TEM) and conductive atomic force microscopy (C-AFM). The contacts were fabricated on a substrate grown by hydride vapor phase epitaxy. TEM analysis clarified that large- and small-size etch pits are attributed to mixed/screw and edge dislocations, respectively. Schottky contacts formed on the surfaces of large and small etch pits (L-EPC, S-EPC), a dislocation-free flat area (FLAT), and a Schottky barrier diode with a diameter of 50 μm (SBD) were prepared and their temperature-dependent current-voltage characteristics were measured using C-AFM. Analysis of the forward bias characteristics revealed that the Schottky barrier height increased and the ideality factor approached unity with increasing temperature. Such temperature dependence of the barrier height was well explained by barrier inhomogeneity at the metal/semiconductor interface. From the analysis of the reverse bias characteristics, we found that field emission (FE) currents caused by the high donor concentration of the sample were the dominant conduction mechanism for S-EPC and FLAT. On the other hand, for L-EPC, the FE currents were overlapped by Poole-Frenkel emission (PFE) currents. For SBD, the leakage currents were successfully explained by the FE and thermionic field emission (TFE) mechanisms by assuming dislocation regions with a lowered barrier height. These findings suggest that mixed dislocations are associated with PFE and lower the Schottky barrier.

Modeling dislocation-related reverse bias leakage in GaN p–n diodes

Finite element analysis software was used to model and visualize two p-n junction models: one with a single threading dislocation (TD) and a control without one. TDs are modeled as a Gaussian distribution of trap states with a full width at half maximum value of 5 nm localized around the line in a cylindrical coordination such that the linear trap state density was 1 trap c−1-translation; this model allows the cylindrical symmetry of the c-plane GaN crystal orientation to be used to avoid more computationally intensive 3D models. In this work, a vertical p–n diode with typical doping characteristics and an equivalent threading dislocation density of was modeled in reverse bias. Our simulations show that the dislocation-mediated leakage mechanism for reverse bias leakage in GaN p–n diodes is the generation of electron–hole pairs via a trap-assisted tunneling mechanism whereby electrons from the valence band use the intermediate trap state to traverse the band gap. This mechanism results in electron–hole pairs that are swept out of the junction by the reverse bias electric field. This behavior results in a measurable leakage current within the model with behavior consistent with experimental values.

Effect of a Single Threading Dislocation on Electrical and Single Photon Detection Characteristics of 4H-SiC Ultraviolet Avalanche Photodiodes**Supported by the National Key R&D Program of China under Grant No. 2016YFB0400902, the National Natural Science Foundation of China under Grant No. 61921005, and the Natural Science Foundation of Jiangsu Province under Grant No. BK20190302

We fabricated 4H-SiC ultraviolet avalanche photodiode (APD) arrays and systematically investigated the effect of threading dislocations on electrical and single photon detection characteristics of 4H-SiC APDs. Based on a statistical correlation study of individual device performance and structural defect mapping revealed by molten KOH etching, it is determined with high confidence level that even a single threading dislocation within APD active region would lead to apparent device performance degradation, including increase of dark current near breakdown voltage, premature breakdown and reduction of single photon detection efficiency at fixed dark count rate.

Modeling dislocation-related leakage currents in GaN p-n diodes

Finite element analysis software was used to model and visualize two p-n junction models: one with a single threading dislocation (TD) and a control model without a dislocation. TDs are modeled as a Gaussian distribution of trap states with an FWHM of 5 nm localized around the r=0 line in a cylindrical coordination such that the linear trap state density was 1 trap/c-translation; this model allows the cylindrical symmetry of the c-plane GaN crystal orientation to be utilized to avoid more computationally intensive 3D models. It was discovered that the interaction of the charged dislocation region with the p-n junction had many notable effects. At zero bias, it was observed that the depletion region width (using the Depletion Approximation) and the maximum electric field were markedly reduced near the dislocation line. More significantly, an asymmetric reduction in the diffusion barrier for electrons (Vbieff=3.03eV) and holes (Vbieff=0.81eV) was observed due to the asymmetric nature of the dislocation band bending related to the doping. The asymmetric reductions in diffusion barriers persisted into VA=2.4V leakage case where asymmetric current profiles for electrons and holes were also observed. Lastly, the diffusion barrier reduction resulted in an additional Shockley-Read-Hall nonradiative recombination leakage caused by a high np-product and trap state density near the intersection of the dislocation with the junction.

(Invited) Electron Channeling Contrast Imaging: Potential for Future Metrology in Semiconductor Industry

For future scaling of CMOS technology beyond the 10 nm node it is necessary to integrate high mobility semiconductors including (Si)Ge and III/V compounds on silicon substrates. The lattice mismatch between these materials and the Silicon platform can lead to the formation of defects which will cause a degradation of the final device performance. Therefore, a metrology technique is needed to monitor the presence and density of defects in blanket layer as well as in patterned structures such as fins. There are several techniques for defect metrology including optical methods, X-ray diffraction or transmission electron microscopy, however, expected defect densities (<105 cm-2) make them ineffective for this task. Electron channeling contrast imaging (ECCI) is scanning electron microscopy (SEM) technique used for the visualization and characterization of crystalline defects including dislocations or stacking faults. Distortions of the crystal lattice of a material due to the presence of such defects cause the variation of the backscattered electron intensity allowing for the visualization of the defects. Application of ECCI proved to be a consistent method for assessing the density and Burgers vector of different defect types in various materials with reliability comparable to that of transmission electron microscopy (TEM). Moreover, TEM limitations related to the destructive sample preparation necessity, limited viewed area accessible for each sample and possible difference in the behavior of thin samples compared to bulk ones brings SEM based ECCI as excellent candidate for non-destructive investigation of large sample areas with low defect densities. For such an analysis, areas exceeding 105 μm2 in size need to be examined to ensure proper statistics. Therefore, automated acquisition of a set of tiles that can be stitched into a single image with a resolution allowing for the detection of single threading dislocations is required. Important parameter for high contrast ECCI of crystalline defects is the sample orientation with respect to the electron beam. The sample should be oriented close to the Bragg condition. This is facilitated by the acquisition of Electron Channeling Patterns and simultaneous sample tilting and/or rotation. For the majority of samples based on silicon wafers only require a couple of degrees tilt from the zone axis. Hence, the technique can be applied to 300 mm wafers easily.The presented technique allows the visualizing and analyzing the density and distribution of threading dislocations in blanket SiGe layers of different defect densities, strain relaxed buffers as well as on (Si)Ge and III/V fin structures. Therefore we are able to demonstrate that automated image acquisition of ECCI has big potential as possible metrology tool for future semiconductor devices process.

Direct Determination of Burgers Vectors of Threading Mixed Dislocations in 4H-SiC c-Plane Wafers Grown by PVT Method

Threading Screw Dislocations (TSDs) have long been observed in 4H-SiC crystals grown by the Physical Vapor Transport (PVT) method. Recent studies [1] show that a significant proportion of these threading dislocations are actually mixed type dislocations with a Burgers vectors nc+ma (where n and m are integers). With the help of Synchrotron White Beam X-ray Topography, it is straightforward to distinguish these mixed type threading dislocations from pure screw dislocations simply by recording different reflections from an axially cut sample (cut parallel to growth direction) and applying the g.b =0and g.bxl =0 invisibility criteria (g – diffraction vector; b – Burgers vector). However, axial wafers are only cut from the boules slated for research purposes and are usually unavailable for boules from which commercially preferred c-plane oriented wafers are sliced. In this study, Synchrotron Monochromatic X-ray topography was carried out on a 100mm 4H-SiC c-cut commercial wafer and contrast variation was observed for each single threading screw or mixed dislocation image on six different types of 11-28 reflections. The way the contrast varied in the different reflections was also found to be different and actually depends on the nature of the dislocation itself. Ray Tracing Simulation was then carried out to confirm this observation. This simulation method is capable of simulating dislocation contrast in various geometries and it is based on the fact that orientation contrast is the predominant contributor to contrast from threading dislocations in the grazing incidence and back reflection geometries. According to the simulation results and their comparison to the observed contrast, it is in principle possible to determine the Burgers vector of any single threading dislocation using this methodology. [1] F. Wu, H. Wang, S. Byrappa, B. Raghothamachar, M. Dudley, E. K. Sanchez, D. Hansen, R. Drachev, S. G. Mueller, and M. J. Loboda, Mater. Sci. Forum 717-720, 343(2011).[2] X. R. Huang, M. Dudley, W. M. Vetter, W. Huang, W. Si, and C. H. Carter, Jr., J. Appl. Cryst. 32, 516 (1999). Figure 1

GaN Power Schottky Diodes with Drift Layers Grown on Four Substrates

We have examined the performance of gallium nitride (GaN) high-power Schottky diodes fabricated on unintentionally doped (UID) metalorganic chemical vapor deposition (MOCVD) films grown simultaneously on four substrates ranging in threading dislocation density from 5 × 103 cm - 2 to 1010 cm - 2. The substrates were an intentionally doped and a UID freestanding hydride vapor phase epitaxy substrate, an MOCVD GaN template grown on a sapphire wafer, and a bulk GaN substrate grown via an ammonothermal method. Capacitance–voltage (C–V) results showed the carrier concentration was ∼2 × 1016 cm−3 for films grown on each of the four substrates. With that doping level, the theoretical breakdown voltage (Vb) is ∼1600 V. However, measured Vb for the devices tested on each of the four substrates fell short of this value. Also, the breakdown voltages across each of the four substrates were not substantially different. This result was especially surprising for films grown on bulk GaN substrates, because of their superior crystal quality, as determined from their x-ray rocking curve widths. Simple probability calculations showed that most of the diodes tested on the bulk substrate did not cover a single threading dislocation. Although optimization of edge-termination schemes is likely to improve Vb, we believe that point defects, not threading dislocations, are the main reason for the reduced performance of these devices.

Coincident Electron Channeling and Cathodoluminescence Studies of Threading Dislocations in GaN

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black-white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

Misfit dislocation formation via pre-existing threading dislocation glide in (112¯2) semipolar heteroepitaxy

Cathodoluminescence (CL) was used to study the onset of mechanical stress relaxation in low indium composition semipolar (112¯2) InxGa1−xN lattice-mismatched layers grown on bulk GaN substrates. Monochromatic CL of short interfacial misfit dislocation (MD) segments showed a single threading dislocation (TD) associated with each MD segment—demonstrating that the initial stage of MD formation in semipolar III-nitride heterostructures proceeded by the bending and glide of pre-existing TDs on the (0001) slip plane. The state of coherency as determined by panchromatic CL is also compared to that determined by x-ray diffraction analysis based on crystallographic epilayer tilt and Matthew-Blakeslee’s critical thickness calculations.

Simulations of Dislocations in CdZnTe/SL/Si Substrates

Discrete dislocation dynamics simulations were employed to study the primary mechanisms by which superlattices (SLs) block threading dislocations (TDs) from reaching the surface of a composite CdZnTe/SL/Si substrate. We found that the difference in strain (composition) across layer boundaries played a key role as to whether a TD might veer away or continue across. We also found that, while the distance between interfaces had little effect on the motion of a single TD, it could have an impact on the dislocation tangle as a whole and should be considered carefully in SL design. We discuss our results in relation to the design of SLs for TD blocking.

Electron-beam-induced current and cathodoluminescence study of dislocation arrays in 4H-SiC homoepitaxial layers

The electrical and optical properties of dislocation arrays (DAs) in 4H-SiC homoepitaxial layers were studied by using electron-beam-induced current (EBIC) and cathodoluminescence (CL) techniques. EBIC observations show that under electron-beam irradiation, the DAs are easily dissociated to form rhombic stacking faults (SFs), whereas the single threading dislocations are not. CL results demonstrate that a new peak (417 nm) appears at the formed SFs, which is the same as the phenomenon observed from dissociating basal plane dislocations. The dissociation mechanisms of DAs are proposed based on the assumption that small basal segments exist. The dissociation velocity of each dislocation in the DAs is discussed according to its recombination activity.

Dislocation cross-slip in heteroepitaxial multilayer films

We simulated dislocation dynamics in heteroepitaxial multilayer thin film systems, considering the case where threading dislocations emerging from the substrate replicate themselves into the thin film during the film growth process. In the regime where the thin film layer thickness is tens of nanometers, the strain hardening mechanism involves the glide of single threading dislocation segments in the thin film instead of by dislocation pile-ups. We studied the dislocations’ evolution behavior and their interactions since these then became significant to the strain hardening of the multilayer structure. Cross-slip of threading dislocation segments in multilayer structure was found to be more prevalent compared to a single-layered thin film. This can result in a more complex pattern of interfacial dislocations and may have a significant contribution to the interactions between threading and interfacial dislocations. The simulation was carried out using the level set method incorporating thin film growth.

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

Abstract 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.

Dislocations and elastic anisotropy in heteroepitaxial metallic thin films

The influence of elastic anisotropy on the critical thickness for the plastic relaxation of epitaxial layers is examined with the help of a coupled discrete-continuum simulation. The latter incorporates a rigorous treatment of the boundary conditions and of mismatch stresses, as well as the elastic properties of a single threading dislocation. Numerical experiments conducted on model Cu/Cu, Cu/Au and Cu/Ni systems with a (001) interface show that, through several distinct effects, elastic anisotropy induces a significant increase in the critical thickness with respect to the values predicted by Matthews et al. The isotropic model of a comparison of the anisotropic critical thicknesses for (001) and (111) interfaces shows that Cu-(111) films on Ni substrates are about 50% 'harder' than (001) films. This feature is discussed in relation to the strength of thin metallic films.

Rapid evaluation of dislocation densities in n-type GaN films using photoenhanced wet etching

We describe a technique based on photoelectrochemical wet etching that enables efficient and accurate evaluation of dislocation densities in n-type GaN films. The etching process utilizes dilute aqueous KOH solutions and Hg arc lamp illumination to produce etched GaN “whiskers” by selectively etching away material around threading dislocations. The etched whiskers, each corresponding to a single threading dislocation, can be effectively imaged by plan-view scanning electron microscopy. The distribution and density of dislocations are then readily observed over very large sample areas. Transmission electron microscope and atomic force microscope studies of the GaN samples confirm the accuracy of the dislocation density obtained by the wet etching.

Computer simulation of threading dislocation density reduction in heteroepitaxial layers

Abstract In the heteroepitaxial growth of films with large misfit with the underlying substrate, the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggests that, after an entanglement region near the film-substrate interface, there is a fall-off in TD density that is inversely proportional to the film thickness h (applicable to densities in the approximate range 107–109 cm−2), followed by saturation or weak decay of the TD density with further increase in film thickness. In this paper, we build upon a recent framework for understanding TD reduction in terms of their effective lateral motion with increasing film thickness, coupled with the probability that certain pairs of TDs annihilate, reacting to form a single TD, or undergoing a change in line direction. A computer simulation approach is presented that augments a recent theoretical prediction of both the 1/h scaling behaviour and the saturation of TD densities. Specific cases are considered involving TD distributions typical of surface nucleation, island periphery nucleation and combinations thereof. In addition, an analysis is made of the propensity for various TDs to cluster together. Results are presented which suggest that longrange fluctuations in the net Burgers vector content of the local TDs are a cause for saturation behaviour.

Multiplication of dislocations in Si1-xGex layers on Si(001).

A mechanism describing how dislocations in epitaxial layers multiply is presented. It is shown that a single threading dislocation can give rise to an array of dislocation sources, where each source generates a separate dislocation loop perpendicular to the primary misfit dislocation. The conditions controlling the activation of these sources are discussed in detail. Most notably, it is demonstrated that a minimum layer thickness, referred to as the ``multiplication thickness,'' is required to accommodate the cross-slip processes that are necessary for source activation. An experimental value of 0.67 \ensuremath{\mu}m for the multiplication thickness ${\mathit{h}}_{\mathit{x}}$ is measured from a single ${\mathrm{Si}}_{0.87}$${\mathrm{Ge}}_{0.13}$ layer on Si(001), and a general expression for ${\mathit{h}}_{\mathit{x}}$ is developed. The dislocation patterns produced by this mechanism are also considered, as are the implications of such a mechanism in light of the established view of strain relaxation.

A Multiplication Mechanism for Misfit Dislocations

ABSTRACTA new mechanism which describes how misfit dislocations in epitaxial layers multiply is presented. This work demonstrates how a single threading dislocation can give rise to an array of dislocation sources, where each source generates a single dislocation loop perpendicular to the primary misfit dislocation. As a threading dislocation with pure screw character glides through an epilayer, certain processes occur which lead to the production of a single dislocation half-loop, and the regeneration of the original threading dislocation. The regenerated threading dislocation continues to propagate on its primary glide plane, which allows the process to repeat itself at some later time. The result of this sequential process is an array of half-loops perpendicular to the primary misfit dislocation. The shape and symmetry of the arrays also contains information regarding how the mechanism operates. The proposed mechanism is related to misfit dislocation arrays in a single Si0.87Ge0.13 layer on Si(001).

Bonding by atomic rearrangement of InP/InGaAsP 1.5 μm wavelength lasers on GaAs substrates

A technique, namely bonding by atomic rearrangement has been invented to realize high quality heteroepitaxy for lasers and optoelectronics. High performance lasers of 1.5 μm wavelength have been fabricated on GaAs substrates using this method. The laser has the same threshold current and quantum efficiency as lasers on InP substrates. No performance degradation has been observed. The transmission electron microscopic results show that the heteroepitaxy is excellent, without a single threading dislocation or stacking fault.

Relaxed GexSi1−x films grown by rapid thermal processing chemical vapor deposition

High quality, epitaxial, relaxed GexSi1−x layers have been grown by rapid thermal processing chemical vapor deposition. Relaxation is believed to be due primarily to the high deposition temperature of 1000 °C and occurred through the formation of an asymmetric misfit dislocation network aligned along the 〈110〉 directions and confined to the interface. The only other defects observed were single threading dislocations at the ends of misfit dislocations.