Reduction Of Threading Dislocations Research Articles

Two-step Defect Reduction of GaAs/Si Epitaxy by Selective Aspect Ratio Trapping

AbstractWe report on the MOCVD growth of GaAs on patterned Si utilizing the Aspect Ratio Trapping (ART) method to reduce threading dislocations resulting from lattice mismatch. Defect-free GaAs was obtained from growth in sub-micron trenches formed in SiO2 on Si (001) substrates. Material quality has been characterized by cross-sectional and plan-view TEM and XRD. It was found that when growing GaAs above the trenched region, coalescence-induced threading dislocations (TDs) and planar defects were introduced at the coalescence junction interfaces. These defects were found to be unrelated to the misfit defects (MDs) on GaAs/Si interface that originated during initial epitaxial growth. Causes of coalescence defect formation were experimentally investigated by employing a two-step defect reduction scheme. It is concluded that by further optimizing growth conditions during coalesce layer growth, low defect-density GaAs material can be obtained on Si substrate.

Threading dislocation reduction in (0 0 01) GaN thin films using SiN x interlayers

The ability of in situ SiN x interlayers to lower the density of threading dislocations (TDs) has been studied for the growth of c-plane (0 0 0 1) GaN epilayers on sapphire by organometallic vapour-phase epitaxy (OMVPE). The TD density in the films may be reduced by up to a factor of 50 to 9×10 7 cm −2 and depends on the SiN x coverage and the conditions of the overgrowth. The TD reduction method relies on the formation of facetted islands on the SiN x -treated GaN surface and the formation of dislocation half-loops between bent-over TDs during the lateral overgrowth. Dislocations that are not annihilated at the interfacial region during the interlayer overgrowth may bend over at the islands’ inclined side facets and annihilate at their coalescence boundaries. Thus, the TD density was reduced at the expense of greater film thickness by increasing the SiN x coverage and delaying intentionally the coalescence of the GaN islands.

Threading dislocation reduction by SiGeC domains in SiGe∕SiGeC heterostructure: Role of pure edge dislocations

The authors previously reported an unusual phenomenon of strain relaxation accompanied by a reduction in threading dislocation density (TDD) on a Si0.77Ge0.23 layer grown on top of alternating layers of Si0.77Ge0.23∕Si0.76Ge0.23C0.01 [Appl. Phys. Lett. 88, 041915 (2006)]. In this letter, the mechanism by which SiGeC domains, formed during annealing at 1000°C, assist in TDD annihilation process is further investigated. A g∙b analysis of transmission electron microscope images showed the formation of pure edge dislocations from the reaction of two 60° misfit dislocations, which glide and/or slip with the assistance of the localized interfacial strain of the SiGeC domains. TDD reduction in this structure is thus due to the annihilation of threading dislocation arms during misfit dislocation combination.

The formation of SGOI structures with low dislocation density by a two-step oxidation and condensation method

A modified process of the Ge condensation method, which is a technique to form SiGe on insulator (SGOI) substrates for strained SOI-MOSFETs, is proposed to reduce threading dislocation density and the effectiveness of the proposed technique is demonstrated. The generation of threading dislocations, which is attributable to the rising behaviour of fragments of misfit dislocations formed at the SiGe/SOI interfaces, is suppressed by controlling the oxygen pressure during the ramping up period in the Ge condensation process. The threading dislocation density of SGOI substrates formed by the new method, which is evaluated by the enhanced secco-etching method, has been as low as 1 × 103 cm−2 for the 230 nm thick SGOI layer with a Ge content of 20% and 75% relaxation.

Deviations from ideal nucleation-limited relaxation in high-Ge content compositionally graded SiGe∕Si

The authors report the sudden rise in threading dislocation density in Ge-rich relaxed graded SiGe layers grown at higher growth temperatures (T>550°C). They attribute this rise in threading dislocation density in relaxed Ge to dislocation nucleation. This observation is contrary to conventional graded buffers in Si-rich material, where higher growth temperatures result in reduced threading dislocation densities (TDDs). Additionally, a coupling effect between the effective strain during graded buffer growth and the growth rate was observed, as evidenced by increased TDD values at reduced growth rates. They conclude that reduced growth rates allow more time for the surface to evolve (i.e., roughen) during growth, thereby trapping mobile dislocations and necessitating the nucleation of additional dislocations to continue relaxing the structure.

Electron Microscopy Investigation of the Role of Surface Steps in the Generation of Dislocations during MOCVD Growth of GaN on 4H-SiC

Through the use of specially-prepared on-axis SiC substrates with patterned mesa tops completely free of atomic-scale surface steps, we have previously reported the growth of highquality GaN heteroepitaxial films with greatly reduced threading dislocation densities on the order of 107/cm2. In these films, we reported a defect substructure in which lateral a-type dislocations are present in the nucleation layer but do not bow into threading dislocations during the subsequent GaN growth. This study focuses further on the role of SiC substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam imaging (both to eliminate Moiré effects and to observe narrow dislocation images) from plan-view transmission electron microscopy (TEM), we identify dislocations generated on stepped and unstepped mesas and compare their geometries. We observe that misfit dislocations nucleated on an unstepped SiC mesa are confined to one set of a-type Burgers vectors of the form g=1/3 [2110] _ _ , straight and well-ordered so that they are less likely to interact with each other. On the other hand, misfit dislocation structures on a stepped SiC mesa surface are not nearly as well-ordered, having bowed structure with threading dislocations that appear to nucleate at SiC surface steps.

In situ pendeoepitaxy of GaN using heteroepitaxial AlGaN∕GaN cracks

Pendeoepitaxy on patterned templates has been proven to be efficient for reducing threading dislocation densities in GaN thin films. In this letter, we report on in situ crack-assisted pendeoepitaxy of GaN using spontaneously formed cracks in AlGaN∕GaN heterostructures. Our approach involves the growth of an AlGaN∕GaN template followed by in situ thermal etching and deposition of an amorphous silicon nitride mask in a low pressure metal organic chemical vapor deposition system. Microwirelike GaN seeds are then formed along the crack lines during the initial stage of GaN overgrowth, which act as nucleation stripes for epitaxial lateral overgrowth. Transmission electron microscopy revealed that the lateral overgrowth of the wirelike GaN seeds effectively bends threading dislocations toward ⟨11¯00⟩ directions on the amorphous silicon nitride mask. The threading dislocation density by this method has been reduced from 2×109cm−2 in control samples to 2×108cm−2 in some parts and 5×107cm−2 in other parts of the GaN layer as determined by plan-view transmission electron microscopy which is very encouraging.

Near ultraviolet InGaN/GaN MQWs grown on maskless periodically grooved sapphire substrates fabricated by wet chemical etching

In the paper, we describe the fabrication and characteristics of near ultraviolet InGaN/GaN multiple quantum wells (MQWs) grown on maskless periodically grooved sapphire, which was obtained by wet chemical etching. Hot acid etching combining with atomic force microscopy (AFM), transmission electron microscopy (TEM) were adopted to characterize the films quality. Compared with the MQWs grown on planar sapphire, the sample grown on grooved sapphire fabricated by wet chemical etching shows better crystalline quality, remarkable reduction of threading dislocations was achieved. Meanwhile, the sample grown on grooved sapphire shows more than three times larger electroluminescence (EL) intensity than the sample on planar substrate. The results show the method grown on grooved sapphire fabricated by wet chemical etching can very effectively reduce threading dislocations and get better films quality with higher optical properties.

Dislocation reduction in GaN grown on porous TiN networks by metal-organic vapor-phase epitaxy

We report on the effectiveness of porous TiN nanonetworks on the reduction of threading dislocations (TDs) in GaN grown by metal-organic vapor-phase epitaxy (MOVPE). The porous TiN networks were formed by in situ annealing of thin-deposited Ti films deposited ex situ on GaN templates within the MOVPE growth chamber. Different annealing parameters in relation to surface porosity of TiN networks were investigated. Transmission electron micrographs indicated dislocation reduction by factors of up to 10 in GaN layers grown on the TiN nanonetwork, compared with a control sample. TiN prevented many dislocations present in the GaN templates from penetrating into the upper layer. Microscale epitaxial lateral overgrowth of GaN above TiN also contributed to TD reduction. The surface porosity of the TiN network had a strong impact on the efficiency of TD reduction. X-ray-diffraction and time-resolved photoluminescence measurements further confirmed the improved GaN quality.

Reduction of threading dislocations in AlGaN layers grown on AlN∕sapphire templates using high-temperature GaN interlayer

Crack-free AlGaN layers were grown on AlN∕sapphire templates by low-pressure metalorganic chemical vapor deposition. Reduction of threading-dislocation (TD) density is achieved by inserting a high-temperature GaN interlayer between the AlGaN and AlN layers. Structural characterization reveals that such an interlayer can efficiently block the TDs propagating from the underlying AlN layer, and reduce the TD density in the subsequent AlGaN layer by one order of magnitude with an optimum thickness of 25 nm. It is also clarified that the decrease of edge TDs is the dominant contribution to this reduction.

Modeling of misfit and threading dislocations in epitaxial heterostructures

The processes of mechanical stress relaxation in lattice-mismatched heteroepitaxial films usually proceed via misfit dislocation formation at the film/substrate interface and are typically accompanied by the generation ofa high density of threading dislocations in the bulk of the film. Threading dislocations are deleterious for a wide variety of modem electronic and optoelectronic devices. Relaxation in stressed epitaxial films also gives rise to the characteristic cross-hatch surface morphology of the growing heterostructures. This article presents a short review of recent mesoscopic models describing the evolution of dislocation structures in heteroepitaxial films. Dislocation-assisted cross-hatch formation in (001)-oriented fcc film is explained theoretically and discussed on the basis of experimental results. A specific stress relaxation mechanism via threading dislocation inclination in (0001)-oriented wurtzite crystal structure films is considered and analyzed in detail. Finally, a 'reaction-kinetics' approach to threading dislocation reduction in growing films is developed.

Defect reduction in (11¯00) m-plane gallium nitride via lateral epitaxial overgrowth by hydride vapor phase epitaxy

This letter reports on extended defect density reduction in m-plane (11¯00) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase epitaxy. Several dielectric mask patterns were used to produce 10 to 100 μm-thick, partially and fully coalesced nonpolar GaN films. X-ray rocking curves indicated the films were free of wing tilt. Transmission electron microscopy showed that basal plane stacking fault (SF) and threading dislocation (TD) densities decreased from 105cm−1 and 109cm−2, respectively, less than 3×103cm−1 and ∼5×106cm−2, respectively, in the Ga-face (0001) wing of the LEO films. SFs persisted in ⟨0001⟩-oriented stripe LEO films, though TD reduction was observed in the windows and wings. Band-edge cathodoluminescence intensity increased 2 to 5 times in the wings compared to the windows depending on the stripe orientation. SFs in the low TD density wings of ⟨0001⟩-stripe films did not appear to act as nonradiative recombination centers.

Facet-controlled three-step growth of high-quality GaN on sapphire substrates by mass-production-type metalorganic vapor phase epitaxy

Three-step growth using facet-controlled GaN (FC-GaN) technique was demonstrated to successfully grow high-quality GaN on sapphire substrates by mass-production-type metalorganic vapor phase epitaxy. The FC-GaN consists of an island-like structure with intentionally formed inclined { 1 0 1 ¯ 1 } facets. Growth time, temperature, pressure and carrier gas were employed as parameters to control island and facet formation of the FC-GaN. The dependence of full width at half maximum (FWHM) of X-ray rocking curve (XRC) on growth time and temperature of FC-GaN was investigated. FWHM of XRC has a minimum peak versus growth temperature of FC-GaN. Cross-sectional transparent electron microscopy analysis revealed that dislocations were bent where the { 1 0 1 ¯ 1 } facets formed, and the threading dislocation (TD) density at GaN surface was reduced. Correlation between FC-GaN morphology and TD reduction efficiency was discussed.

Maskless lateral epitaxial overgrowth of high-aluminum-content AlxGa1−xN

We have demonstrated maskless lateral epitaxial overgrowth of Al0.96Ga0.04N on sapphire for dislocation reduction. 600 nm and 1 μm thick AlN layers were grown on sapphire via metalorganic chemical vapor deposition. Parallel, periodic trenches were then etched in the AlN and Al0.96Ga0.04N was regrown laterally from the unetched mesas. Significant threading dislocation reduction was observed for “wing” material, growing laterally, compared to “seed” material, growing vertically from the unetched mesa, as observed by atomic force microscopy, and transmission electron microscopy. Crystallographic wing tilt of ∼0.23° was measured by x-ray diffraction.

High Voltage AlGaN/GaN Heterojunction Transistors

The use of AlGaN / GaN HEMTs and HBTs for switching power supplies is explored. With its high electron velocities and breakdown fields, GaN has great potential for power switching. The field-plate HEMT increased breakdown voltages by 20% to 570V by reducing the peak field at the drain-side edge of the gate. The use of a gate insulator is also investigated, using both JVD SiO 2 and e-beam evaporated SiO 2 to reduce gate leakage, increasing breakdown voltages to 1050V and 1300V respectively. The power device figure of merit (FOM) for these devices: [Formula: see text], is the highest reported for switching devices. To reduce trapping effects, reactively sputtered SiN x, is used as a passivant, resulting in a switching time of less than 30 ns for devices blocking over 110V with a drain current of 1.4A under resistive load conditions. Dynamic load results are also presented. The development of HBTs for switching applications included the development of an etched emitter HBT with a selectively regrown extrinsic base. This was later improved upon with the selectively regrown emitter devices with current gains as high as 15. To improve breakdown in these devices, thick GaN layers were grown, reducing threading dislocation densities in the active layers. A further improvement included the use of a bevelled shallow etch and a lateral collector design to maximize device breakdown.

Recent progress of nitride-based light emitting devices

We review the recent progress of nitride-based light-emitting diodes (LEDs) and discuss the relation between dislocation density and quantum efficiency. We also discuss how to improve the external quantum efficiency of nitride-based LEDs. Secondly, the group-III nitride laser diodes (LDs), which emit from near-ultraviolet to pure-blue, are reviewed. Reducing threading dislocations can increase the lifetime of nitride LDs. Using the epitaxial lateral overgrowth technique, a dislocations density of the order of 105 cm−2 has been obtained. The relation between the lifetime of nitride LDs and the density of dislocations is discussed. Finally, near-ultraviolet LDs and pure-blue LDs are described. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Engineering of an insulating buffer and use of AlN interlayers: two optimisations for AlGaN–GaN HEMT-like structures

The semi-insulating character of GaN epitaxial layers can be achieved by the control of the early stages of growth on the substrate. Adding two low temperature (LT) AlN interlayers is a technique enough powerful to reduce threading dislocation densities by up to one order of magnitude. A compressive strain as high as 2.8 x 10 -3 is induced in the uppermost GaN epilayer. The global structure is kept semi-insulating so that it is a perfect template for undoped AlGaN-GaN HEMTs (High Electron Mobility Transistors). HEMTs with interlayers present better two dimensional electron gas (2DEG) properties: up to 20% higher carrier density (n s ) and 40% higher mobility. Typically n s is as high as 1.7 x 10 13 cm -2 for a record mobility of 1200 cm 2 /Vs. The improvement of the mobility can be correlated to the reduction of nano-scale V-shaped defects in the AlGaN (less morphological-relaxation). The improvement of n s could be explained by the higher piezo-doping resulting from GaN extra-compression and AlGaN weaker relaxation. As a consequence, the DC transistors characteristics are improved: in 2 μm gate transistors, the maximum current and transconductance are increased by up to 80% and 20%, respectively, and could be extrapolated to values as high as 1500 mA/mm and 250 mS/mm for 0.2 μm gate devices.

Recent Progress in GaN-Based Superlattices for Near-Infrared Intersubband Transitions

A review of the recent progress in intersubband transitions in GaN-based superlattice structures grown by plasma-assisted molecular beam epitaxy (MBE) is presented. Careful control of the growth parameters resulted in reduced threading dislocation density as well as optimized interfaces for the superlattices. Intersubband absorption has been observed from a variety of configurations involving GaN single or asymmetric double quantum wells with either thick AlxGa1—xN barriers or short-period GaN/AlxGa1—xN superlattice barriers. The peak wavelength of absorption can be varied between 1.4–4.2 μm by changing the quantum well thickness. Electron scattering times were measured by the pump–probe technique and were found to be 240–330 fs at 1.55 μm. In addition, intersubband transitions have also been observed for GaN/AlN superlattices grown with a non-polar (110) orientation.

Threading dislocation reduction via laterally overgrown nonpolar (112̄0) a-plane GaN

Threading dislocation density reduction of nonpolar (112̄0) a-plane GaN films was achieved by lateral epitaxial overgrowth (LEO). We report on the dependence of morphology and defect reduction on crystallographic stripe orientation. Stripes aligned along [0001] and [1̄100], the most favorable a-plane GaN LEO stripe orientations, possessed well-behaved, symmetric morphologies. Threading dislocation reduction via mask blocking was observed by transmission electron microscopy for [1̄100] stripes which had optimal rectangular cross-sections. Cathodoluminescence studies showed increased light emission for the overgrown regions in comparison to the window regions. The extent of lateral overgrowth of these stripes was asymmetric due to the opposing polarities of the vertical c-plane sidewalls. Conversely, threading dislocations propagated into the symmetric overgrown regions of [0001] stripes which possessed coexisting inclined and vertical {101̄0} facets.

Protection of In0.25Ga0.75As/GaAs structures during lateral oxidation using an amorphous InGaP layer

Using very-low-temperature molecular beam epitaxy growth techniques, an amorphous InGaP layer was deposited to protect the surface during lateral oxidation of an underlying AlGaAs layer. For comparison, other oxidation protection layers such as SiNx and SiO2 were also studied. The oxidized structure consisted of single crystal In0.25Ga0.75As grown on the underlying AlGaAs layer, and then capped with an oxidation protection layer. The oxidation rate of the amorphous InGaP was investigated and compared to the oxidation rates of both single crystal InGaP and GaAs. In addition, the effects of the InGaP layer thickness on the threading dislocation density of the In0.25Ga0.75As layers were investigated. It was found that the amorphous InGaP layers allowed for threading dislocation reduction in the underlying In0.25Ga0.75As layers, while the dielectric protection layers caused an increase in dislocation densities. Atomic force microscopy was also used to investigate the surface after removal of the InGaP protection layers.