Formation Of Threading Dislocations Research Articles

Control of Threading Dislocation Formation in La-doped BaSnO3 Films Grown by Hybrid Molecular Beam Epitaxy

Control of Threading Dislocation Formation in La-doped BaSnO3 Films Grown by Hybrid Molecular Beam Epitaxy

Strain relaxation in GaN/AlN superlattices on GaN(0001) substrate: Combined superlattice-to-substrate lattice misfit and thickness-dependent effects

The relaxation of built-up strain in the heteroepitaxial GaN/AlN superlattices (SLs) leads to defect-related undesirable changes of the optical and electrical properties of SL-based devices. In the present study, the influence of lattice misfit between the GaN/AlN SL and GaN(0001) substrate on the mechanism of strain relaxation in the SLs is comprehensively analyzed. A strain/thickness-dependent effect on the densities of threading dislocations (TDs) and cracks in the SLs is observed. At the initial stage of growth of the SLs with relatively small lattice misfit, the formation of TDs dominates over the post-grown cracking triggered by the thermal stress during the sample cooling. Cracking during the growth is the main mechanism of strain relaxation for the SLs with large lattice misfit. It is established that for the GaN/AlN SLs with thicknesses >115 ± 15 nm, which corresponds to a SL relaxation degree >40 ± 5%, the density of defects drastically decreases. This result offers an essential guidance for excluding the strong influence of the substrate on generation of a large number of defects in the SLs. The quantitative interpretation and discussions of the presented results are important toward the design of a high-quality heteroepitaxial GaN/AlN SLs.

On the origin of threading dislocations during epitaxial growth of III-Sb on Si(001): A comprehensive transmission electron tomography and microscopy study

Electron tomography and complementary (scanning) transmission electron microscopy (STEM) are applied to investigate the origin of threading dislocations in the large lattice misfit, heteroepitaxial system of III-Sb on vicinal Si(001). Buried AlSb islands of the initial wetting layer are revealed at the interface toward the substrate in the three-dimensionally reconstructed data. Locations of island coalescence are retrieved from the tomogram. Complementary (S)TEM measurements reveal the location of threading dislocations and the presence of antiphase boundaries at the same specimen area. The number density of threading dislocations emanating from the interface and their distribution are unexpected. It is shown that the presence of threading dislocations is not simply correlated to sites of AlSb-islands coalescence or to the film closure during the transition from a 3D to a 2D growth. Moreover, an interaction with antiphase boundaries is suggested by the presented observations. Consequently, the contemporary notion of threading dislocation formation is refined and, eventually, it is suggested that measures to avoid antiphase domains and such to reduce threading dislocations have to be balanced for future strategies to epitaxially grow sphalerite structure III-V semiconductors on Si or Ge.

Invited; Synchrotron Radiation Based X-ray Microdiffraction of Advanced Semiconductor Materials

X-ray diffraction (XRD) has been recognized as a well-established method by which lattice structures of crystalline materials can be clarified precisely. High penetrating power of X-ray allows us to reveal inner structures and buried interfaces usually implemented in materials and devices. Conventional XRD with a typical probe size of over 100 μm clarifies only averaged structures having good statistics without any untypical ones, known as ensemble average[1]. However, in general, the property and the performance of materials and devices become governed by the localized specific structures when miniaturized to a nanometer scale. In such a case, an X-ray beam with a probe size much smaller than the conventional one is truly required.X-ray microdiffraction (XRMD) is one of the most effective methods to establish both the advantages of XRD analysis and high special resolution to exceed the ensemble average analysis. The state-of-the-art XRMD system at the beam line of the synchrotron radiation facility, Super Photon ring 8 GeV (SPring-8), in Japan, realizes an X-ray probe with a size of a few hundred nanometer [2]. Thus XRMD gives us precise and rich information especially on the crystalline texture and the local strain distribution on a submicron scale.1. Crystallinity of Ge thin linesLine-shaped Ge films with nanometer-sized widths on Si substrates are promising for applying to next generation three dimensional high-mobility channels in metal-oxide-semiconductor field effect transistors. One of the concerns is the introduction of threading dislocations in the Ge films; 60° dislocations critically alter the crystallinity of Ge films. Aspect ratio trapping with the use of selective epitaxy of Ge on SiO2-patterned Si substrates is one of effective ways to reduce threading dislocations in Ge thin line channels [3]. However, there is insufficient understanding of strain relaxation and local structures in the Ge thin lines with such anisotropic geometry.XRMD has been employed to investigate the crystallinity of the line-shaped Ge films with line widths of 100-1000 nm [4]. It has been revealed that crystal domains with a size of about 100 nm in the Ge thin lines were tilted at different angles within the range of 0.6 degree. The tilt angle range was larger in thinner Ge lines after the growth and reduced by annealing for all line widths. The domaining was found to be mostly caused by the introduced 60° dislocations. The formation of a single large domain with a specific tilt angle was also achieved near the surfaces of Ge thin lines with widths of 100 and 200 nm, which was attributed to the suppression of threading dislocation formation by the SiO2side walls.2. Local strain mapping for AlN thick filmsAlN with a 6.2-eV-wide bandgap is potentially prospective as a substrate material for deep ultraviolet optoelectronics devices. Due to the lack of bulk AlN crystals, thick film growth with hydride vapor phase epitaxy is known to be promising for the production of large scale AlN wafers. Crystalline quality of AlN films directly grown on α-Al2O3substrates is poor with high dislocation density and residual strain due to lattice and thermal mismatches. In contrast, epitaxial lateral overgrowth using trench-patterned templates was proven to be effective in obtaining high-quality films [5].Local strain distribution in thick AlN films grown on a periodic trench-patterned AlN/α-Al2O3template (see Fig. 1(a)) has been investigated by XRMD [2]. Figure 1(a) reveals nanometer-scale voids that form tunnels in the AlN film and are periodically arranged at 6-µm-intervals corresponding to the periodicity of trench pattern. XRMD experiment with the X-ray incidence into the cross-sectional plane of sample (see Fig. 1(b)) was conducted for taking reciprocal space maps (RSMs) from the two-dimensional (2D) scan area at 1- and 2-μm intervals in the X and Z axes, respectively. 2D distribution maps of strains along the Z and X directions obtained from the RSMs are shown in Figs. 2(a) and 2(b), respectively. It should be noticed that both strains are considerably relaxed around the void region, as illustrated in Fig. 2(c). This result demonstrates that XRMD clearly discloses the local strain distribution in the thick AlN film and the voids are effective in relaxing the strain induced during the growth process.

A visualization of threading dislocations formation and dynamics in mosaic growth of GaN-based light emitting diode epitaxial layers on (0001) sapphire

A clear visualization of the origin and characteristics of threading dislocations (TDs) of GaN-based light emitting diode epitaxial layers on (0001) sapphire substrates have been carried out. Special experimental set up and chemical etchant along with field emission scanning electron microscopy are employed to study the dynamics of GaN TDs at different growth stages. Cross-sectional transmission electron microscopy analysis visualized the formation of edge TDs is arising from extension of coalescences at boundaries of different tilting-twining nucleation grains “mosaic growth.” Etch pits as representatives of edge TDs are in agreement with previous theoretical models and analyses of TDs core position and characteristics.

Laser operation of Ga(NAsP) lattice-matched to (001) silicon substrate

The lattice-matched growth of the direct band gap material Ga(NAsP) is a seminal concept for the monolithic integration of III/V laser on a silicon substrate. Here, we report on the growth, characterization, and lasing properties of Ga(NAsP)/(BGa)(AsP) multi quantum well heterostructures embedded in (BGa)P cladding layers which were deposited on an exactly oriented (001) Si substrate. Structural investigations confirm a high crystal quality without any indication for misfit or threading dislocation formation. Laser operation between 800 nm and 900 nm of these broad area device structures was achieved under optical pumping as well as electrical injection for temperatures up to 150 K. This “proof of principle” points to the enormous potential of Ga(NAsP) as an optical complement to Si microelectronics.

On the origin of threading dislocations in GaN films

A series of GaN films were grown by metalorganic vapor phase epitaxy on nitrided sapphire using an initial annealed low-temperature nucleation layer (LT-NL), without employing any conventional threading dislocation (TD) reduction methods. Film thicknesses ranging from the LT-NL to 500 nm were used. The island network morphology was investigated at each growth stage using atomic force microscopy. Data from cathodoluminescence studies showed initially uniform luminescence, followed by the gradual development of bright (low TD) regions which had lateral sizes different from the island sizes at all times and which continued to increase in size after coalescence. The formation of low-energy arrays of a-type TDs also continued after island coalescence. X-ray diffraction, transmission electron microscopy (TEM) and AFM data indicated that the highest (a+c)-type TD densities were found in the LT-NL, but subsequently decreased due to TD loop formation (promoted by island facets) and reaction to produce a-type TDs. a-type TD densities were also high in the LT-NL but subsequently increased slightly, due to the reaction of (a+c)-type TDs. A very sharp dynamical ‘correlation’ peak was also observed in XRD of the LT-NL, related to TDs with an a-component. Furthermore, defect formation was observed within the LT-NL using high-resolution TEM. These data are consistent with TD formation predominantly in the LT-NL, followed by TD movement at elevated growth temperatures. Initially, coalesced films had a high TD density with a spatially random TD arrangement, but progressively altered into a lower TD density, spatially clustered arrangement during growth. This type of microstructure may mistakenly be interpreted as arising from island coalescence.

Threading dislocation evolution in patterned GaN nanocolumn growth and coalescence overgrowth

Threading dislocation (TD) evolution during patterned GaN nanocolumn (NC) growth and coalescence overgrowth with metal-organic chemical vapor deposition is studied based on the comparisons of NC and coalescence overgrowth samples of different NC cross-section diameters and spacing sizes. From the measurement results of depth-dependent x-ray diffraction and cross-section transmission electron microscopy, it is found that the TD density in an NC depends on the patterned hole size for NC growth. Also, the TD formation at the beginning of coalescence overgrowth is related to the NC spacing size. Although the TD density at the bottom of the overgrown layer is weakly dependent on NC and spacing sizes, at its top surface, the TD density strongly relies on NC size. Among the overgrowth samples of different NC diameters and spacing sizes with a fixed NC diameter/spacing ratio, the one with the smallest size and spacing leads to the lowest TD density, the largest lateral domain size, and the highest photoluminescence efficiency. Also, the optical and crystal qualities at the surfaces of all the overgrowth samples are superior to those of a GaN template.

Morphology and microstructure evolution of AlxGa1−xN epilayers grown on GaN/sapphire templates with AlN interlayers observed by transmission electron microscopy

Morphology and microstructure evolution of Al0.3Ga0.7N epilayers grown on GaN/sapphire templates with low-temperature (LT) AlN interlayers (IL) by means of metal organic chemical vapor deposition have been investigated by transmission electron microscopy and atomic force microscopy. It is found that the IL improves the surface morphology and suppresses edge-type threading dislocations (TDs). When the IL thickness is 20nm, there is the lowest density of the edge-type TD with 8.7×108cm−2. However, the edge-type TD density increases somewhat as IL thickness increases to 40nm. It is believed that two mechanisms determine the microstructure evolution of the AlxGa1−xN epilayers. One is the TD suppression effect of LT-AlN ILs that ILs can provide an interface for edge-type TD termination. Another is the TD introduction effect of ILs that new edge-type TDs are produced. Due to the lattice mismatch between AlN, GaN, and AlxGa1−xN, the strain in AlxGa1−xN epilayers is modified by inserting the AlN IL, and thus changes the formation of the edge-type TDs.

High-performance 1.3 µm InAs/GaAs quantum-dot lasers with low threshold current and negative characteristic temperature

A high-growth-temperature GaAs spacer layer (HGTSL) is shown to significantly improve the performance of 1.3 µm multilayer InAs/GaAs quantum-dot (QD) lasers. The HGTSL inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics and hence improved performance of QD lasers. To further reduce the threshold current density and improve the room-temperature characteristic temperature (T0), the high-reflection (HR) coating and p-type modulation doping have been incorporated with the HGTSL technique. A very low continuous-wave room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A cm−2 are achieved for a three-layer device with a 1 mm HR/HR cavity, while a very low threshold current density of 48 A cm−2 and a negative T0 are achieved in the p-doped lasers.

Improved performance of 1.3μm multilayer InAs quantum-dot lasers using a high-growth-temperature GaAs spacer layer

The use of a high-growth-temperature GaAs spacer layer is demonstrated to significantly improve the performance of 1.3μm multilayer self-assembled InAs∕InGaAs dot-in-a-well lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics. Incorporation of these spacer layers allows the fabrication of multilayer quantum-dot devices emitting above 1.3μm, with extremely low room-temperature threshold current densities and with operation up to 105°C.

Film quality effects associated with formation of misfit dislocations at semiconductor interfaces

The influence of the interface orientation on the strain state and structure properties of epitaxial layers is theoretically investigated for semiconductor films grown on (001) misoriented substrates. The analysis shows that for two intersected MD arrays, the film energy strongly depends on the type of the screw component of their Burgers vector. The energy is much lower if for both mutually perpendicular MD arrays the components are left-screw or right-screw. Identification of these components for 60° misfit dislocation arrays lying in a (001) interface may explain some experimental results of deteriorated epitaxial film structures. If the plastic relaxation process occurs in quasi-equilibrium; the whole multitude of vicinal (001) orientations can be classified into two types: orientations of type 1, when the conditions for threading dislocation formation are present, but the long-range stress field does not appear, and orientations of type 2, when the probability of threading dislocation formation is small, but the field is present.

Electron microscopy of GaAs grown on Si- and Ge-based substrates

A GaAs-on-Si technology is desirable to take advantage of the mobility and direct bandgap of GaAs in combination with the crystalline quality, low cost and established technology of Si. Differences in lattice constant (4.1%), thermal expansion coefficient (a factor of ~ 3), and bonding polarity between the two materials can lead to problems such as: threading dislocation formation, thermally induced stress and delamination, and antiphase domain boundaries (APBs), respectively. The lattice mismatch is responsible for the formation of (necessary) misfit dislocations which can concurrently create threading dislocations with typical densities in the range of 106 - 108cm-2. This density of electrically active defects in a device region is highly undesirable.A proposed scheme for lattice mismatch accommodation and potential threading dislocation reduction has previously been reported in which each layer of a SixGe1-x multilayer structure (MLS) is grown beyond the critical thickness with a progressively higher Ge composition than the previous layer.

Strain Relief and Defect Formation in High Dose Oxygen Implanted Silicon

ABSTRACTLattice strain and defect formation in oxygen implanted silicon (SIMOX) were investigated as a function of dose and annealing conditions by high resolution x-ray diffraction and transmission electron microscopy. X-ray rocking curves revealed the presence of two distinct strained layers. Self-interstitial clusters and oxygen interstitials gave rise to a buried strain layer in unidirectional tension centered around the projected range at low doses (l×1016 and 3×1016 cm−2. As the dose was increased to 1×1017 and 3×1017 cm−2 a supersaturation of vacancies near the surface produced a second strain layer with a unidirectional lattice contraction at the wafer surface. Annealing at 900°C, 0.5 hr. reduced this surface strain as the defects coarsened into observable cavities. The development of cavities upon annealing was used in a sequential implantation and low temperature annealing process to produce low threading dislocation density SIMOX. Possible mechanisms for threading dislocation formation are discussed.

Defect and Dopant Control During Silicon Epitaxy Using B and Ge

ABSTRACTThe control of dopants, impurities and defects for VLSI of silicon integrated circuits requires a complex set of crystal and processing conditions to be satisfied simultaneously. In order to achieve the maximum yield and highest level of electrical performance for a given device design, we have manipulated the lattice constant and boron doping levels in CVD epitaxial silicon layers co-doped with germanium. By adjusting the ratios of germane and diborane in a dichlorosilane/hydrogen CVD reactor we have obtained buried high conducting layers which are strain-free and lattice matched to the Si substrate. Degenerate boron and boron and germanium codoped epitaxial layers on (100) p-type silicon substrates were investigated. Solubility, electrical activity limits and defect structure of boron in strained and strain-free silicon epitaxial layers were investigated by spreading resistance, SIMS profiling, X-ray and transmission electron microscopy techniques. Bright field and weak-beam dark field imaging of cross-sectional and plan-view specimens were used to confirm the presence or absence of precipitates and threading dislocations. A model has been proposed to describe the mechanism of threading dislocation formation in heavily boron-doped layers. We are now in a position to strategically locate co-doped Si(B, Ge) p++ layers as recombination zones or buried field plates to suit the needs of MOS latchup control, high speed and radiation hard devices, as well as the needs of defect free p++ etch stops for thin membranes and three-dimensional silicon structures.