Density Of Extended Defects Research Articles

Bandlike and localized states of extended defects in n-type In0.53Ga0.47As

In0.53Ga0.47As p + n diodes with different densities of extended defects have been analyzed by detailed structural and electrical characterization. The defects have been introduced during Metal-Organic Vapor Phase Epitaxy (MOVPE) growth by using a lattice-mismatched layer on a semi-insulating InP or GaAs substrate. The residual strain and indium content in the n-type In0.53Ga0.47As layer have been determined by high-resolution X-ray diffraction, showing nearly zero strain and a fixed indium ratio of 0.53. The deep levels in the layer have been characterized by Deep Level Transient Spectroscopy. The mean value of electron traps at 0.17 ± 0.03 eV below the conduction band minimum EC is assigned to the “localized” states of α 60° misfit dislocations; another broad electron trap with mean activation energies between EC− 0.17 ± 0.01 and 0.39 ± 0.04 eV, is identified as threading dislocation segments with “band-like” states. A high variation of the pre-exponential factor KT by 7 orders of magnitude is found for the latter when changing the filling pulse time, which can be explained by the coexistence of acceptor-like and donor-like states in the core of split dislocations in III-V materials. Furthermore, two hole traps at EV+ 0.42 ± 0.01 and EV+ 0.26 ± 0.13 eV are related to the double acceptor of the Ga(In) vacancy (VGa/In3-/2-) and 60° β misfit dislocations, respectively. Finally, the dislocation climbing mechanism and the evolution of the antisite defects AsGa/In are discussed for n-type In0.53Ga0.47As.

Radiative versus non-radiative recombination in high-efficiency mid-IR InSb/InAs/In(Ga,Al)As/GaAs metamorphic nanoheterostructures

Nanostructures with a submonolayer InSb type-II insertion inside a InAs/InGaAs type-I quantum well (QW) have been grown by molecular beam epitaxy on GaAs (0 0 1) substrates via a convex-graded InAlAs metamorphic buffer layer (MBL). Selection of optimal growth conditions and design of the MBL-virtual substrate system enables one to increase mid-infrared photoluminescence (PL) and internal quantum efficiency (IQE) of the nanoheterostructures. The maximum low temperature IQE of about 90% has been obtained owing to the residual strain engineering which has resulted in both reduction of the extended defect density in the QW, likely responsible for Shockley–Read–Hall non-radiative recombination, and suppression of the Auger recombination channels in the InAs QW and the barriers. Temperature dependence of the integrated PL intensity was analyzed to determine an activation energy of an additional high-temperature non-radiative process (~49 meV) related presumably to hole delocalization through acceptor states in the strained InAs QW.

Characterization of high-quality kerfless epitaxial silicon for solar cells: Defect sources and impact on minority-carrier lifetime

We investigate the types and origins of structural defects in thin (<100 µm) kerfless epitaxial single crystal silicon grown on top of reorganized porous silicon layers. Although the structural defect density is low (has average defect density < 104 cm−2), localized areas with a defect density > 105 cm−2 are observed. Cross-sectional and systematic plan-view defect etching and microscopy reveals that the majority of stacking faults and dislocations originate at the interface between the porous silicon layer and the epitaxial wafer. Localised dislocation clusters are observed in regions of collapsed/deformed porous silicon and at decorated stacking faults. In localized regions of high extended defect density, increased minority-carrier recombination activity is observed. Evidence for impurity segregation to the extended defects (internal gettering), which is known to exacerbate carrier recombination is demonstrated. The impact of the defects on material performance and substrate re-use is also discussed.

Spatially-resolved optical and structural properties of semi-polar mathrm{(11}bar{2}mathrm{2)} AlxGa1\u2212xN with x up to 0.56

Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar mathrm{(11}bar{2}mathrm{2)} AlxGa1−xN epilayers with AlN contents up to x = 0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.

Effect of buried extended defects on the radiation tolerance of ZnO

Interaction between defects of different types often determines their evolution in semiconductors under ion irradiation. Here, we investigate the role of buried pre-existing extended defects in the damage accumulation in ZnO single crystals implanted at room temperature and at 15 K with 80 keV N ions. For the room temperature implants but not for the 15 K ones, the defect accumulation around the projected range is strongly suppressed while a pronounced build-up occurs in the region of the pre-existing defects. This is attributed to thermally activated migration of point defects from the implanted volume and subsequent trapping/annihilation by the pre-existing defects. The obtained results imply that a defect engineering scheme involving formation of a high density of extended defects outside the active volume of a ZnO-based device could be beneficial to enhance the radiation tolerance.

Structural impact on the nanoscale optical properties of InGaN core-shell nanorods

III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar {1–100} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar {1–100} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.

Electronic, structural and chemical properties of GaAs/ZnSe heterovalent interfaces as dependent on MBE growth conditions and ex situ annealing

A study of electronic, structural and chemical properties of GaAs/ZnSe heterovalent interfaces (HI) in dependence on molecular beam epitaxy (MBE) growth conditions and post-growth annealing was performed. Initial GaAs surface reconstructions ((2 × 4)As or c(4 × 4)As) and ZnSe growth mode (MBE or migration-enhanced epitaxy (MEE)) were varied for different undoped and n-doped heterovalent structures. Although all the structures have low extended defect density (less than 106 cm−2) and rather small (less than 5 nm) atomic interdiffusion at the HI, the structural, chemical and electronic properties of the near-interface area (short-distance interdiffusion effects, dominant chemical bonds, and valence band offset values) as well as electrical properties of the n-GaAs/n-ZnSe heterovalent structures were found to be influenced strongly by the MBE growth conditions and post-growth annealing.

High-Speed Nonpolar InGaN/GaN LEDs for Visible-Light Communication

Free-standing nonpolar GaN substrates provide an excellent platform for the fabrication of high-speed blue and green light-emitting diodes (LEDs), which are attractive for visible-light communication, plastic optical fiber communication, and short-range under water optical communication. Nonpolar LEDs on free-standing GaN exhibit a large electron-hole wave function overlap, low extended defect density, and favorable thermal properties. Here, we demonstrate high-speed nonpolar InGaN/GaN LEDs with a peak emission wavelength between 455 and 465 nm on free-standing nonpolar GaN substrates. A large frequency modulation bandwidth of 524 MHz is demonstrated at a current density of 10 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Limits to mobility in InAs quantum wells with nearly lattice-matched barriers

The growth and the density dependence of the low temperature mobility of a series of two-dimensional electron systems confined to un-intentionally doped, low extended defect density InAs quantum wells with Al$_{1-x}$Ga$_{x}$Sb barriers are reported. The electron mobility limiting scattering mechanisms were determined by utilizing dual-gated devices to study the dependence of mobility on carrier density and electric field independently. Analysis of the possible scattering mechanisms indicate the mobility was limited primarily by rough interfaces in narrow quantum wells and a combination of alloy disorder and interface roughness in wide wells at high carrier density within the first occupied electronic sub-band. At low carrier density the functional dependence of the mobility on carrier density provided evidence of coulombic scattering from charged defects. A gate-tuned electron mobility exceeding 750,000 cm$^{2}$/Vs was achieved at a sample temperature of 2 K.

(Invited) GaN/Si Buffer Development for RF and Power Applications

Gallium nitride transistors grown on Si(111) are becoming commercially available for applications such as microwave communication at 2-6 GHz and power conversion in the 600V range. Three-terminal off-state breakdown voltage of 1.35 kV and a low specific on-resistance of 1.3 mohm-cm2 have been achieved on 200 mm diameter GaN/Si wafers with 4.5-µm-thick device epilayer thickness.[1] However additional improvements to materials and processes are desirable in order to drive further market penetration. For power applications, breakdown voltage can be increased by tuning the resistivity of the buffer layers.Transistor structures incorporating GaN or Al(0.08)Ga(0.92)N buffer layers were grown and fabricated into isolation test structures for leakage characterization. For GaN buffer layers, the leakage current was significantly decreased and the isolation blocking voltage was increased by increasing the carbon doping concentration. AlGaN buffer layers exhibited smaller increase in leakage current at elevated temperature relative to GaN buffer layers. Room-temperature two-terminal breakdown voltage (defined at 1 µA/mm off-state leakage for 5 µm isolation spacing) was 100 V higher for the AlGaN-based buffer relative to the GaN buffer (1000V and 900V, respectively). At 150°C, the two-terminal breakdown voltage was 150V higher for the AlGaN buffer compared to the GaN, indicating a lower temperature coefficient for the AlGaN-based buffer. The effect of buffer composition on leakage current at high temperature was magnified at higher voltage, where the AlGaN-based buffer sustained over 1200 V at 100 µA/mm compared to 1000V for GaN-based buffer. RF devices can suffer from parasitic capacitance resulting from mobile charge at the substrate interface. Crystalline rare-earth oxides (cREO) can be deposited on Si(111) to form a single-crystal nucleation layer for AlN or GaN [2] and avoid charge accumulation at the substrate interface. GaN transistor structures were grown on cREO/Si(111) and characterized by transmission electron microscopy, X-ray diffraction, and spreading resistance profile measurements.The film morphology and growth modes were comparable to a baseline GaN/Si process, with an offset in the density of extended defects which indicated the need for additional optimization. In contrast to a baseline process which resulted in p-type doping at the substrate interface, spreading resistance measurements indicate that the cREO / substrate interface was depleted, consistent with the cREO layer acting as a barrier for the group-III elements at the onset of growth. The benefits of this reduced charge density for device performance improvements and material optimization are discussed.

Nanoscopic Insights into InGaN/GaN Core-Shell Nanorods: Structure, Composition, and Luminescence.

Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.

Effect of Reduced Extended Defect Density in MOCVD Grown AlGaN/GaN HEMTs on Native GaN Substrates

AlGaN/GaN high-electron mobility transistor (HEMT) structures were grown by metal–organic chemical vapor deposition on SiC, hydride vapor phase epitaxy (HVPE) GaN, and ammonothermal GaN substrates to achieve HEMTs with over five orders of magnitude variation in extended defect density. This enables a direct comparison of the effect of extended defects on device performance to achieve the best possible reliability. As-grown material was characterized by atomic force microscopy, electron channeling contrast imaging, and Raman spectroscopy. Devices were characterized by Hall, dc $I$ – $V$ , and pulsed $I$ – $V$ behavior. Reduced threading dislocation density provides an increased 2-D electron gas mobility, but inhibits ohmic contact formation resulting in high contact resistance. Transistor characteristics were nominally identical, with higher OFF-state leakage in the HEMTs on ammonothermal GaN. The pulsed $I$ – $V$ response indicated significantly reduced current collapse in the HEMT on HVPE GaN due to reduced buffer trapping.

(Invited) On the Electrical Activity of Extended Defects in High-Mobility Channel Materials

The electrical activity of extended defects (dislocations; antiphase boundaries;...) in high-mobility channel materials (strained-Si; (strained)-Ge, GaAs and InGaAs) is investigated by means of simple device structures (p-n diodes and MOS capacitors). These are fabricated in substrates with a well-controlled density of predominant extended defects, enabling the determination of a specific leakage current contribution per defect. It is shown that there is a linear relationship between the area leakage current density and the threading dislocation density for a wide range of group IV p+n junctions. In the case of antiphase boundaries (APBs) of GaAs on Ge substrates, only a moderate electrical activity has been derived from these studies. On the other hand, a clear band of deep states has been demonstrated for TDs in InGaAs layers on GaAs substrates.

Molecular beam epitaxy of ZnSSe/CdSe short-period superlattices for III–V/II–VI multijunction solar cells

Results on the molecular-beam epitaxy growth of short-period alternately-strained ZnS x Se1−x /CdSe superlattices which are pseudomorphic to GaAs (001) substrates and possess effective band-gap values within the range of E g ≈ 2.5–2.7 eV are presented. Oscillations of the specular-spot intensity in reflection high-energy electron diffraction are used for in situ control of the superlattice parameters. A method to determine the SL parameters (compositions and thicknesses of the constituent layers) based on combined analysis of the grown structures by low-temperature photoluminescence and X-ray diffractometry is developed. It is found that the parameters of the grown ZnS x Se1 − x /CdSe superlattices are close to their design values and the density of extended defects in the structures is low even though the structure thickness (∼300 nm) considerably exceeds the critical thickness for bulk II–VI layers with the same lattice-constant mismatch.

Defect creation in InGaAs/GaAs multiple quantum wells–I. Structural properties

We present a systematic study of extended defect creation in InGaAs/GaAs multiple quantum well (MQW) structures. Three sets of samples, grown by molecular beam epitaxy, were characterized by high-resolution x-ray diffraction and transmission electron microscopy. First, in a temperature series, optimal deposition temperature of 505°C was found for the In composition of 20% as determined from dislocation loop (DL) density and inspection of diffuse scattering patterns. InGaAs decomposition and lateral layer thickness undulations were observed above this optimal temperature. Second, increase of MQW periodicity from ×5 to ×10 revealed a thickness-related cumulative deterioration, characterized by increased likelihood of defect intersection with continued MQW growth, as suggested by an increase of the secondary DL density from ~1.6×107cm−2 to ~6×108cm−2. Additional strained layers experienced an ever-degrading quality of the growth surface. Third, a set consisting of samples with three different MQW periods was investigated. Different stages, suggested by a model of defect creation, were obtained for these different MQW periods, allowing specification of particular type, density, and spatial distribution of extended defects for each stage of defect creation.

Sensitivity analysis explains quasi-one-dimensional current transport in two-dimensional materials

We demonstrate that the quasi-one-dimensional (1D) current transport, experimentally observed in graphene as measured by a collinear four-point probe in two electrode configurations A and B, can be interpreted using the sensitivity functions of the two electrode configurations (configurations A and B represents different pairs of electrodes chosen for current sources and potential measurements). The measured sheet resistance in a four-point probe measurement is averaged over an area determined by the sensitivity function. For a two-dimensional conductor, the sensitivity functions for electrode configurations A and B are different. But when the current is forced to flow through a percolation network, e.g., graphene with high density of extended defects, the two sensitivity functions become identical. This is equivalent to a four-point measurement on a line resistor, hence quasi-1D transport. The sensitivity analysis presents a formal definition of quasi-1D current transport, which was recently observed experimentally in chemical-vapor-deposition graphene. Our numerical model for calculating sensitivity is verified by comparing the model to analytical calculations based on conformal mapping of a single extended defect.

(Invited) Three-Dimensional Epitaxial Si1-XGex, Ge and SiC Crystals on Deeply Patterned Si Substrates

We show that geometric shielding of the reactive flux in chemical vapor deposition by tall neighboring structures obtained by deep substrate patterning, along with short surface diffusion lengths, can provide nearly space filling arrays of high-quality epitaxial crystals despite large mismatches of lattice parameters and thermal expansion coefficients. The density of extended defects is strongly reduced by the method, and wafer bowing and crack formation largely inhibited. The concept is shown to be valid for SiGe/Si heterostructures ranging from pure Si to pure Ge both on Si(001) and Si(111) substrates. Here, dislocations are efficiently eliminated from three-dimensional faceted crystals with high-aspect ratios on top of micron-sized Si pillars. The application to 3C-SiC/Si(001) ridges, characterized by a lattice mismatch of nearly 20%, provides significantly lower stacking fault densities compared with layers grown on planar substrates.

Effect of Mg doping on the structural and free-charge carrier properties of InN films

We present a comprehensive study of free-charge carrier and structural properties of two sets of InN films grown by molecular beam epitaxy and systematically doped with Mg from 1.0 × 1018 cm−3 to 3.9 × 1021 cm−3. The free electron and hole concentration, mobility, and plasmon broadening parameters are determined by infrared spectroscopic ellipsometry. The lattice parameters, microstructure, and surface morphology are determined by high-resolution X-ray diffraction and atomic force microscopy. Consistent results on the free-charge carrier type are found in the two sets of InN films and it is inferred that p-type conductivity could be achieved for 1.0 × 1018 cm−3 ≲ [Mg] ≲ 9.0 × 1019 cm−3. The systematic change of free-charge carrier properties with Mg concentration is discussed in relation to the evolution of extended defect density and growth mode. A comparison between the structural characteristics and free electron concentrations in the films provides insights in the role of extended and point defects for the n-type conductivity in InN. It further allows to suggest pathways for achieving compensated InN material with relatively high electron mobility and low defect densities. The critical values of Mg concentration for which polarity inversion and formation of zinc-blende InN occurred are determined. Finally, the effect of Mg doping on the lattice parameters is established and different contributions to the strain in the films are discussed.

Peculiarities of the electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration in the two-dimensional channel

The electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration are reported. We have observed anisotropy of the electron concentration and mobility measured at a low magnetic field in the [110] and \(\left[ {1\bar 10} \right]\) crystallographic directions. It has been established by analysis of Shubnikov-de Haas oscillations that the conductivity through the two-dimensional electron channel InSb/AlInSb quantum well (QW) does not depend on the crystallographic direction. However, the magnetic-field dependences of the Hall coefficient and resistivity of the structures reveal a strong influence of the crystallographic directions. This has allowed one to conclude that the anisotropy of the electron transport parameters in the QW structures measured at low magnetic fields correspond to parasitic conductivity through the Al0.09In0.91Sb buffer layer with two pronounced anisotropic contributions: the influence of metallic In nanoclusters inhomogeneously distributed within the buffer layer and conductivity of the near-interface layer with a high anisotropic density of extended defects.

On the mechanical strength of monocrystalline, multicrystalline and quasi‐monocrystalline silicon wafers: a four‐line bending test study

ABSTRACTQuasi‐monocrystalline silicon wafers have appeared as a critical innovation in the PV industry, joining the most favorable characteristics of the conventional substrates: the higher solar cell efficiencies of monocrystalline Czochralski‐Si (Cz‐Si) wafers and the lower cost and the full square‐shape of the multicrystalline ones. However, the quasi‐monocrystalline ingot growth can lead to a different defect structure than the typical Cz‐Si process. Thus, the properties of the brand new quasi‐monocrystalline wafers, based on low and high crystal defect densities, have been for the first time studied from a mechanical point of view, comparing their strength with that of both Cz‐Si monocrystalline and typical multicrystalline materials. The study has been carried out employing the four line bending test and simulating them by means of FE models. For the analysis, failure stresses were fitted to a three‐parameter Weibull distribution. High mechanical strength was found in all the cases. However, the quasi‐monocrystalline wafers characterized by large density of bulk defects, due to the noticeable density of extended defects, showed lower fracture tensions. Copyright © 2013 John Wiley &amp; Sons, Ltd.