Dislocations in LD and LED semiconductor heterostructures

Behavior of misfit and threading dislocations in HgCdTe/CdTe and HgCdTe/CdZnTe heterojunctions has been investigated by the dislocation-pit etching technique. It is confirmed that by lattice matching the linear pit density caused by misfit dislocations on (1̄10) cleaved plane was decreased to 500 cm−1, which is about 1% of that obtained using CdTe substrates. It has been found that the density of the threading dislocations is not decreased by lattice matching, and nearly coincides with the dislocation density of the substrate.

GaInSb is a promising Sb‐based channel material for high electron mobility transistors (HEMTs) targeting ultra‐high frequency operation. In this study, we fabricate double‐sided Te δ‐doping InSb/Ga 0.22 In 0.78 Sb composite channel HEMT structures employing GaSb buffer and AlInSb graded buffer structure. Te δ‐doping time on the lower side is optimized so as not to induce electron buffer leakage, resulting in a 25% increase in sheet electron density ( N s ). The effects of GaSb buffer and AlInSb‐graded buffer on threading dislocation (TD) behavior are quantitatively evaluated using a dislocation pair‐annihilation model, which provides a unified framework for assessing the impact of buffer design on TD reduction. As a result, TD density is reduced by 79%, yielding a 38% enhancement in electron mobility ( μ ) compared to a conventional buffer structure. Eventually, the optimized HEMT structure achieves μ = 19,800 cm 2 V −1 s −1 , N s = 3.35 × 10 12 cm −2 , and a low sheet resistance ( R s ) of 94.3 Ω/□, demonstrating its promise for terahertz‐range transistor applications.

Epitaxially integrated III–V semiconductor lasers for silicon photonics have the potential to dramatically transform information networks, but currently, dislocations limit performance and reliability even in defect-tolerant InAs quantum dot (QD)-based lasers. Despite being below the critical thickness, QD layers in these devices contain previously unexplained misfit dislocations, which facilitate non-radiative recombination. We demonstrate here that these misfit dislocations form during post-growth cooldown due to the combined effects of (1) thermal-expansion mismatch between the III–V layers and silicon and (2) mechanical hardening in the active region. By incorporating an additional sub-critical thickness, indium-alloyed “misfit dislocation trapping layer,” we leverage these mechanical hardening effects to our advantage, displacing 95% of misfit dislocations from the QD layer in model structures. Unlike conventional dislocation mitigation strategies, the trapping layer reduces neither the number of threading dislocations nor the number of misfit dislocations. It simply shifts the position of misfit dislocations away from the QD layer, reducing the defects' impact on luminescence. In full lasers, adding a misfit dislocation trapping layer both above and below the QD active region displaces misfit dislocations and substantially improves performance: we measure a twofold reduction in lasing threshold currents and a greater than threefold increase in output power. Our results suggest that devices employing both traditional threading dislocation reduction techniques and optimized misfit dislocation trapping layers may finally lead to fully integrated, commercially viable silicon-based photonic integrated circuits.

Strain management is the key to achieving high-quality aluminum gallium nitride (AlGaN) materials for fabricating AlGaN-based ultraviolet-B (UV-B) light-emitting diodes (LEDs). In this work, a kinetic defect evolution model to capture the strain relaxation process for AlGaN growth of UV-B LEDs was demonstrated. The relaxation started from the inclination of threading dislocations (TDs) dominantly over the coexistence of TD inclination, surface roughening, misfit dislocation (MD) generation, enhanced phase separation, etc. as the thickness of n-AlGaN increased. The critical thickness for strain relaxation by TD inclination, MD generation, and the theoretical MD density were obtained. Finally, flip-chip UV-B LEDs with properly engineered buffer strain were fabricated, which showed a maximum light output power of 49.1 mW under 500 mA and a high external quantum efficiency of 3.00% at 310.2 nm, without a high-reflective Rh electrode or any complex packaging process. This work improves our understanding of the relaxation mechanism and prompts further consideration of the strain management in AlGaN-based UV-B LEDs.

We have investigated the interfacial structure of epitaxial (Ba,Sr)TiO3 films grown on (111)-oriented SrTiO3 single-crystal substrates using transmission electron microscopy (TEM) techniques. Compared with the (100) epitaxial perovskite films, we observe dominant dislocation half-loop with Burgers vectors of a⟨110⟩ comprised of a misfit dislocation along ⟨112⟩, and threading dislocations along ⟨110⟩ or ⟨100⟩. The misfit dislocation with Burgers vector of a⟨110⟩ can dissociate into two ½a⟨110⟩ partial dislocations and one stacking fault. We found the dislocation reactions occur not only between misfit dislocations, but also between threading dislocations. Via three-dimensional electron tomography, we retrieved the configurations of the threading dislocation reactions. The reactions between threading dislocations lead to a more efficient strain relaxation than do the misfit dislocations alone in the near-interface region of the (111)-oriented (Ba0.7Sr0.3)TiO3 films.

Modeling of threading dislocation reduction in growing GaN layers

Free-standing GaN substrates are of great importance for short wavelength Laser Diodes (LDs) and high performance Light Emitting Diodes (LEDs) applications. One of the technical barriers for the applications is relatively high threading dislocation density (TDD) in GaN substrates known as non-radiative centers. In this paper, we analyzed the influence of TDD to Hall effects in Si-doped GaN free-standing substrates grown by Hydride Vapor Phase Epitaxy(HVPE). To verify the role of threading dislocations as deep acceptor level and carrier scattering lines, we compared active carrier concentration and Si doping concentration[nSi] of each GaN substrate with different DD. In addition, we measured O doping concentration[nO] of unintentionally doped(UID) GaN substrate that had similar DD with the Si-doped GaN substrate to consider the influence of doped O known as main background impurity, and compared the Hall characteristics of UID GaN with that of the Si-doped GaN. From the results, it was experimentally postulated that threading dislocations function as trap centers disturbing the movement of n-carriers in GaN substrates. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the nanostructure of AlGaN∕GaN strained-layer superlattice (SLS) cladding in the GaN-based violet laser diode (LD) and the AlInGaN-based ultraviolet (UV) light emitting diode (LED) with a scanning transmission electron microscope (STEM). In the p-SLS cladding, comprising 34 pairs of p-Al0.1Ga0.9N∕p-GaN:Mg layers in the GaN-based LD, the Al0.1Ga0.9N and GaN layers were distinguished as dark and bright bands ∼6nm wide in the high-angle annular dark-field (HAADF) STEM images. Threading dislocations (TDs) were observed. Among TDs that came from the underlying layer, some run outside through the SLS, and the others disappeared within the SLS, which discloses a role of the SLS in suppressing defect propagation. A HAADF-STEM image of the TD with a dark line along the center of a bright contour was found. The dark line, which was darker than the surrounding matrix, is striking. One of the probable explanations for the dark line that may be considered is local segregation of light atoms (Mg or Al) in Cottrell atmosphere around the dislocation core. In the HAADF-STEM image of the UV LED wafer, the AlInGaN and AlInGaN:Si layers in the MQW were definitely resolved, appearing as dark and bright bands. HAADF-STEM also distinguished between the AlGaN and GaN layers in the p-SLS cladding in the UV LED wafer.

Heterojunction bipolar transistors (HBTs) with GaAs0.825P0.175 bases and collectors and In0.40Ga0.60P emitters were integrated monolithically onto Si substrates. The HBT structures were grown epitaxially on Si via metalorganic chemical vapor deposition, using SiGe compositionally graded buffers to accommodate the lattice mismatch while maintaining threading dislocation density at an acceptable level (∼3 × 106 cm−2). GaAs0.825P0.175 is used as an active material instead of GaAs because of its higher bandgap (increased breakdown voltage) and closer lattice constant to Si. Misfit dislocation density in the active device layers, measured by electron-beam-induced current, was reduced by making iterative changes to the epitaxial structure. This optimized process culminated in a GaAs0.825P0.175/In0.40Ga0.60P HBT grown on Si with a DC current gain of 156. By considering the various GaAsP/InGaP HBTs grown on Si substrates alongside several control devices grown on GaAs substrates, a wide range of threading dislocation densities and misfit dislocation densities in the active layers could be correlated with HBT current gain. The effect of threading dislocations on current gain was moderated by the reduction in minority carrier lifetime in the base region, in agreement with existing models for GaAs light-emitting diodes and photovoltaic cells. Current gain was shown to be extremely sensitive to misfit dislocations in the active layers of the HBT—much more sensitive than to threading dislocations. We develop a model for this relationship where increased base current is mediated by Fermi level pinning near misfit dislocations.

Metamorphic semiconductor devices such as high electron mobility transistors (HEMTs), light-emitting diodes (LEDs), laser diodes, and solar cells are grown on mismatched substrates and typically exhibit a high degree of lattice relaxation. Linearly-graded buffer layers are commonly used to accommodate the lattice mismatch in these structures, but theoretical and experimental studies suggest that non-linear compositional grading might allow lower threading dislocation densities and improved device performance. In this work we have compared the threading dislocation densities in heterostructures employing bi-parabolic, linear, and S-graded (complementary error function) buffer layers. The threading dislocation densities were calculated using the approximate Matthews, Mader, and Light model (1) for plastic flow and by assuming that the coefficient for second-order annihilation and coalescence reactions between threading dislocations is equal to the length of misfit dislocations.

The monolithic integration of III-Sb materials on Si substrates enables the development of novel functionalities and promising device applications. However, due to the large lattice mismatch to Si, direct heteroepitaxy of III-Sb results in a very large and disadvantageous dislocation density. Here, we report on the effect of strained AlSb interlayers in GaSb as dislocation filters for the epitaxy on Si(001) substrates. The epitaxial stress introduced by the lattice mismatched AlSb layer leads to threading dislocation bending at the interfaces as a strain relieving mechanism, thereby facilitating the reaction and annihilation of dislocations during their lateral glide process. Using comprehensive electron channeling contrast imaging and scanning transmission electron microscopy, we measure the dislocation density at the surface and after filter and cap layers of different thicknesses and analyze the effect of plastic strain relaxation on dislocation reduction. The reduction mechanism is discussed in the context of misfit dislocation networks forming at the respective compressively and tensile strained interfaces. With the new findings, an effective double filter layer structure with adopted layer thicknesses could be realized, which has resulted in a very low threading dislocation density of 1.7×107cm−2.

GaN and its related alloys have emerged as eminently useful materials for the fabrication of green-to-ultraviolet light-emitting diodes (LEDs), laser diodes (LD) and also high-power, high-efficiency transistor devices due to their widely tunable bandgap. The key technological challenge in the manufacturing of these high performance optoelectronic and electronic devices is the absence of GaN substrates of practical size. As a result, all epitaxial GaN films are grown on the top of a substrate with different lattice constant, such as sapphire, Si and SiC, leading to the inevitable introduction of dislocations and residual strain in the epilayers, which is known to degrade the performance and reliability of the devices. Moreover, the popularly used sapphire substrate is an insulator with low thermal conductivity and difficult to cleave beneath the GaN based LD wafers. So there is a high demand for both low dislocation GaN epilayers and freestanding substrate. In this work, we present two approaches to meet this demand. (1). The Innovative One-step Lateral Epitaxy with Serpentine Mask Structures. There are two key features to this technology, one being a unique mask structure that blocks threading dislocations using amorphous films, and the other being associated with the controlled selective nucleation directly on sapphire surface. The direction of the growth front of GaN makes two 90° turns before emerging from the serpentine channel. The growth direction at the beginning is along the substrate surface norm that subsequently turns to become parallel to the surface and finally turns towards the surface norm again when emerging from the serpentine channel. It achieves the blockage of threading dislocations from reaching the active device region. The second key feature is the selective nucleation of GaN on sapphire surfaces but not on the amorphous mask surfaces, which is the enabling factor that allows for the entire fabrication process being carried out with only one single epitaxial step, leading to significant reduction in the fabrication cost. The microstructural and optical properties of the GaN epilayer grown on serpentine masked structures will be described. Furthermore, we will summarize that this novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication including high brightness LED, laser diodes, and high-power, high-efficiency transistors. (2) Free-Standing (FS) GaN Substrate by HVPE. Recently GaN substrate has been developed to improve qualities of III-nitride devices, thermal conductivity, dislocation density, thermal and lattice mismatch, etc. due to home-epitaxial growth through various growth methods including ammonothermal, High-pressure solution growth, Na-flux and Hydride vapor phase epitaxy (HVPE). In spite of advantage of HVPE method such as high growth rate, capability of wide area wafer and already commercialized growth methods for GaN substrate with comparing other growth methods, the problems of large dislocation density, parasitic nucleation and stress controls in HVPE- GaN substrate were still remained. We will provide one simple HVPE growth method, named as the pulsed NH3-flow modulation method, to realize a crack-free GaN thick layer with high crystal quality and nearly no residual strain. A few simple self-separation methods are developed for removal of hetero-substrates to obtain FS-GaN. 2-inch FS-GaN substrates are obtained with high quality. The FWHM values are about 108 and 81 arcsec for (002) and (102) reflections. The corresponding dislocation density of the freestanding GaN is low to 3-7×106 cm-2. We will also discuss self-separation mechanisms for FS-GaN substrates grown by HVPE.

Threading dislocation morphologies and characteristics, as well as their generation conditions in InxGa1−xAs films grown by molecular beam epitaxy on GaAs (001) substrates have been investigated, mainly using cross-sectional transmission electron microscopy. The 3-μm-thick InxGa1−xAs films are mostly examined for x ranging over x=0.01, 0.05, 0.1, 0.15, 0.2, 0.3, and 0.5. Moreover, for x=0.2, epitaxial layers having film thicknesses of 0.5, 1, and 2 μm are investigated. The formation of a high density of threading dislocations which reach the film surface is detected in epilayers of x≥0.2 at a fixed film thickness of 3 μm and with film thicknesses greater than 2 μm at x=0.2. In layers of x≤0.15, such threading dislocations are rarely detected, although dislocation segments on the {111} planes threading into the upper regions from the interface are frequently observed. Most of the observed threading dislocations are 60° and pure-edge type dislocations along the <211≳ and <110≳, and [001] directions, respectively. The former type dislocations are mainly observed in layers of x≤0.15; the latter predominantly exist in layers of x≥0.3. In epilayers of x=0.2, the two types of threading dislocations mentioned above coexist. It is also discussed that the formation of the above-mentioned threading dislocations is strongly associated with misfit dislocations which are introduced in the InxGa1−xAs layers under the different growth modes, depending on x.

The “effective kinetics” of threading dislocations (TDs) in growing epitaxial films is developed on the basis of the crystallographic and geometrical considerations from Part I of the paper. A system of coupled first-order nonlinear differential equations for twenty-four families of TDs is derived and solved for several initial conditions for TD densities. Numerical solutions demonstrated two general types of asymptotic behavior with increasing film thickness: linear decrease or saturation of total TD density. This behavior agrees with experimental data on TD reduction in mismatched homogeneous buffer layers.

We investigated the effects of SiN_x interlayers on the structural and electrical properties of nonpolar <i>a</i>-plane (11-20) GaN grown on <i>r</i>-plane (1-102) sapphire substrates by metal–organic chemical vapor deposition (MOCVD). The Nomarski optical microscope images showed that the deposition conditions of the SiN_x layer could strongly affect the <i>a</i>-plane GaN surface morphology due to the different SiN_x coverage. Basal-plane stacking faults (BSFs) and threading dislocation (TD) densities were reduced in the <i>a</i>-plane GaN samples with high SiN_x coverage and multiple SiN_x-treated GaN interlayers. These results indicate that TD reduction is associated with an increase in the 3D growth step and with the blocking of TD propagation. From on-axis (11-20) X-ray rocking curve (XRC) measurements, the anisotropy of full width at half maximum (FWHM) can be attributed to the crystal mosaicity due to insertion of different SiN_x interlayers. The anisotropy of sheet resistance between the <i>c</i>-and <i>m</i>-axis was also clearly seen in a-plane GaN samples with a high density of defects, which was attributed to the BSFs as scattering centers.