Reduction Of Threading Dislocation Density Research Articles

The effect of InGaN/GaN MQW hydrogen treatment and threading dislocation optimization on GaN LED efficiency

We report on the effect of GaN buffer threading dislocation (TD) optimization and InGaN/GaN quantum well (QW) hydrogen ( H 2 ) treatment on the efficiency of GaN light emitting diodes (LEDs) operating in the spectral range from 400 to 500 nm. A tenfold reduction of the TD density in the GaN buffer increased the efficiency of blue LEDs operating at high current density, while in green LEDs it had very little effect. The reduced TD density also increased the compressive strain in the InGaN QWs, and caused blue shift to the electroluminescence (EL) peak wavelength. The H 2 treatment of the QWs increased strain inside the MQW stack. It was possible to apply the H 2 treatment only to UV LEDs, as the increased strain in blue and green LEDs caused relaxation of the MQW stack. Although this resulted in smooth surface morphology of the MQW stack, it did not lead to any increase in the efficiency of the UV LEDs.

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.

Multistep method for threading dislocation density reduction in MOCVD grown GaN epilayers

AbstractA new multistep MOCVD method for growing GaN is used to suppress threading dislocations in GaN epilayers on c ‐plane sapphire. A nucleation island density of as low as 2.5 × 107 cm–2 is reported. Developed subsequent overgrowth prevents the formation of new islands and stimulates the inclination of threading dislocations inside nucleation islands before their coalescence. GaN epilayers with a threading dislocation density of 5.0 × 107 cm–2 are grown by the method. Nucleation island morphology and threading dislocation density are analyzed by atomic force microscopy. Transmission electron microscopy is used to support the results for the threading dislocation density and to evaluate the epitaxial relationship of the GaN films. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Threading dislocation density reduction in two‐stage growth of GaN layers

AbstractA theoretical approach is proposed to reduce the density of threading dislocations (TDs) in (0001) oriented growth of GaN layers. The approach can be realized by repeated stages of growth surface roughening (stage I) and flattening (stage II). Fundamentals of the approach include the formation of a dislocation redirection layer with inclined TDs at stage I and enhanced reactivity among inclined TDs in a dislocation reaction layer at stage II. A reaction‐kinetics model is applied for the quantitative prediction of TD density reduction which can be achieved as a result of the two‐stage growth technique. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synchrotron white beam X-ray topography, transmission electron microscopy and high-resolution X-ray diffraction studies of defects and strain relaxation processes in wide band gap semiconductor crystals and thin films

A review is presented of recent research into imaging on different length scales of defect structures in wide band gap semiconductors. This includes: synchrotron white beam X-ray topography (SWBXT) imaging of defects in SiC wafers and epilayers; high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) studies of strain relaxation in AlN and GaN epilayers on on-axis and vicinal SiC and sapphire substrates; TEM studies of fault structures in GaN/AlN films on SiC substrates; and TEM studies of threading dislocation density reduction driven by growth mode modification in AlN films grown on sapphire substrates. In detail, studies of closed-core and hollow-core screw dislocations, basal plane dislocations and stacking faults (SFs) in 4H- and 6H-SiC substrates and films are presented. The mechanism of improved mismatch strain relaxation associated with the growth of GaN (AlN) films on vicinal substrates is discussed. This is attributed to the combined effects of mutual tilt between the epilayer and the substrate, which helps to relax out-of-plane mismatch, and by the generation of geometric partial misfit dislocations (GPMDs), which serves both to relax in-plane mismatch and accommodate stacking differences between the epilayer and substrate at some proportion of the steps at the substrate/film interface. In addition, the formation mechanisms of intersecting SF structures observed in GaN/AlN/SiC epilayers are presented. These fault structures comprise SFs that fold back and forth from the basal plane (I 1 Basal Plane Faults; BSFs) to the prismatic plane (Prismatic Stacking Faults; PSFs). Finally, the threading dislocation density reduction in AlN/sapphire films is modeled based on overgrowth of the dislocation outcrops by macrosteps which appear during a transition from step flow to 2D layer-by-layer growth.

Reduction of threading edge dislocation density in n-type GaN by Si delta-doping

In this study, the defect structure of periodic Si delta-doping ( δ-doping) GaN films grown by low-pressure metalorganic chemical vapor deposition has been investigated by high-resolution X-ray diffraction. Rocking curves of five planes were investigated: (0 0 0 2), ( 1 0 1 ¯ 3 ) , ( 1 0 1 ¯ 2 ) , ( 1 0 1 ¯ 1 ) and ( 2 0 2 ¯ 1 ) , respectively. Pseudo-Voigt function was used to simulate the rocking-curve of every plane. The effects of Si δ-doping on the different types of dislocations were discussed. It was demonstrated that Si δ-doping significantly reduces the threading dislocations with a pure edge character, and induces no changes in the threading dislocations with a screw component. The results are consistent with AFM results.

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.

Mid-10 cm−2 threading dislocation density in optimized high-Ge content relaxed graded SiGe on Si for III-V solar on Si

ABSTRACTWe present a framework for obtaining high quality relaxed graded SiGe buffers on Si for III-V integration. By avoiding dislocation nucleation in Si1−xGex layers of x>0.96, we have achieved a relaxed Si0.04Ge0.96 platform on Si(001) offcut 2° that has a threading dislocation density of 7.4×105 cm−2. This 2° offcut orientation was determined to be the minimum necessary for APB-free growth of GaAs. Furthermore, we found that we could compositionally grade the Ge content in the high-Ge portion of the buffer at up to 17 %Ge μm−1 with no penalty to the dislocation density. The reduction in both threading dislocation density and buffer thickness exhibited by our method is an especially significant development for relatively thick minority-carrier devices which use III-V materials such as multi-junction solar cells.

Effects of Al-containing Intermediate III-nitride Strained Multilayers on the Threading Dislocation Density and Optical Properties of GaN Films

We report the influence of intermediate AlN/GaN, AlGaN/GaN or AlN/AlGaN strained multilayer structures on the threading dislocation (TD) density and optical properties of GaN films. A series of GaN films were deposited at 1050°C on (0001) sapphire substrates using intermediate AlN/GaN, AlGaN/GaN or AlN/AlGaN strained multilayer structures. Etching pit density (EPD) counts and transmission electron microscopic (TEM) studies show that the implementation of AlN/GaN and AlGaN/GaN in GaN films enables the blocking of TDs in the films very efficiently. Photoluminescence (PL) measurements exhibit the increment of near band edge emission intensity in the GaN films having Al0.5Ga0.5N/GaN or AlN/GaN intermediate strained multilayer structures. Cross-sectional TEM observations show that TD annihilation and de-multiplication processes play important roles in the TD density reduction in the GaN films having Al-containing strained multilayer structures.

Behaviors of Al xGa 1− xN (0.5⩽ x⩽1.0 )/GaN short period strained-layer superlattices on the threading dislocation density reduction in GaN films

GaN films were grown on c-Al 2O 3 substrates with the insertion of Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN short period strained-layer superlattices (SPSLSs) at elevated temperatures. It appears that the insertion of an Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN SPSLS having certain thickness and pair combinations is helpful to reduce the etching pit density (EPD) in GaN film for more than one order of magnitude. Cross-sectional transmission electron microscopic (XTEM) observations confirm the efficiency of Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN intermediate SPSLSs on blocking threading dislocation (TD) propagation in GaN films. The presence of intermediate Al x Ga 1− x N (0.5⩽ x⩽1.0)/GaN SPSLS in a GaN film is believed to encourage TD density reduction in the film through TD annihilation and de-multiplication processes involving interactions between two edge-type TDs.

Epitaxial lateral overgrowth of GaN on selected-area Si(1 1 1) substrate with nitrided Si mask

Epitaxial lateral overgrowth (ELO) of GaN by metalorganic vapor phase epitaxy was carried out on Si substrates using new ELO-mask patterns. The groove or SiO 2 mask pattern employed was a stripe in combination with a larger period grid (100 μm×100 μm): the former is for the ELO of the GaN to reduce the threading dislocation (TD) density and the latter is for limiting the area of growth and to suppress the formation of cracks in the grown GaN. Crack-free GaN layers with a reduced TD density were successfully obtained for both types of Si substrate, i.e., the grooved Si substrate and the SiO 2-masked Si substrate. The properties of GaN layers grown on these Si substrates are compared.

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.

Recent Advances in III-Nitride Devices Grown on Lithium Gallate

Recent advances in the growth of III-nitride materials and devices include: (i) The reduction of the near-surface threading dislocation density in GaN on lithium gallate (LGO) to ≈2 × 107 cm—2. (ii) The demonstration of GaN, 50 × 130 μm, metal–semiconductor–metal (MSM) photodiodes with extremely low leakage current, 0.11 pA at 2 V and 7.9 pA at 60 V, and UV photoresponse at 308 nm and 20 V of 0.105 A/W. (iii) State of the art MSM devices have been successfully removed from the LGO substrate and attached to silicon wafers with no degradation in current characteristics. (iv) Demonstration of very thin, 0.7 μm HFET structures, grown at a rapid rate of 0.9 μm/h, with near state of the art room temperature 2DEG mobilities of 1365 cm2/Vs at a sheet charge of 9 × 1012 cm—2. (v) The elimination of substrate impurity diffusion by inclusion of gettering buffers has also been demonstrated.

Recent Advances in III-Nitride Devices Grown on Lithium Gallate

Recent advances in the growth of III-nitride materials and devices include: (i) The reduction of the near-surface threading dislocation density in GaN on lithium gallate (LGO) to ≈2 × 107 cm—2. (ii) The demonstration of GaN, 50 × 130 μm, metal–semiconductor–metal (MSM) photodiodes with extremely low leakage current, 0.11 pA at 2 V and 7.9 pA at 60 V, and UV photoresponse at 308 nm and 20 V of 0.105 A/W. (iii) State of the art MSM devices have been successfully removed from the LGO substrate and attached to silicon wafers with no degradation in current characteristics. (iv) Demonstration of very thin, 0.7 μm HFET structures, grown at a rapid rate of 0.9 μm/h, with near state of the art room temperature 2DEG mobilities of 1365 cm2/Vs at a sheet charge of 9 × 1012 cm—2. (v) The elimination of substrate impurity diffusion by inclusion of gettering buffers has also been demonstrated.

Optoelectronic device performance on reduced threading dislocation density GaAs/Si

A technique for the heteroepitaxy of GaAs/Si films having reduced threading dislocation density is presented. The important attribute of this technique is the suppression of three-dimensional Volmer–Weber island formation during initial deposition. This suppression is achieved by deposition of a stoichiometric GaAs buffer layer by a migration-enhanced epitaxy technique on silicon at 348 K to a thickness greater than the “monolithic thickness,” hm. Subsequent GaAs films deposited by conventional molecular beam epitaxy on buffer layers of thickness greater than hm possess structural and optical characteristics that exceed those for state-of-the-art GaAs/Si layers: an x-ray full width at half maximum (FWHM) of 110 arcsec with a dislocation density at the film surface of 3×106 cm−2 and a concomitant 4 K photoluminescence FWHM of 2.1 meV. The p-i-n structures suitable for use as light-emitting diodes (LEDs) that were grown on the reduced threading dislocation density GaAs/Si and by means of forward- and reverse-bias measurements, demonstrated an ideality factor of n=1.5, an increased reverse-bias breakdown electric field of 2.1×107 V/m, and an intrinsic region resistivity of 4×107 Ω cm for LEDs of increasingly smaller mesa size.

Mechanism for the Reduction of Threading Dislocation Densities in Si0.82Ge0.18 Films on Silicon on Insulator Substrates

ABSTRACTWe have made an extensive study of Si0.82Ge0.18 film relaxation on silicon on insulator (SOI) substrates having a top Si layer 40, 70, 330nm, and 10[.proportional]m thick. SiGe films were deposited with a thickness up to 1.2[.proportional]m in an ultrahigh vacuum chemical vapor deposition system at 630°C. Following growth, films were characterized by X-ray diffraction and a dislocation revealing etch. The same level of relaxation is reached for each thickness of SiGe film independent of the substrate structure. Accompanying the film relaxation is the development of a tetragonal tensile strain in the thin Si layer of the SOI substrates. This strain reached 0.22% for the 1.2[.proportional]m film on the 40nm SOI and decreases with SOI thickness. The Si thickness of the SOI substrate also effected the threading dislocation density. For 85% relaxed films the density fell from 7×106 pits/cm2 on bulk Si to 103pits/cm2 for the 40, 70, and 330nm SOI substrates. The buried amorphous layer of the SOI substrate alters the dislocation dynamics by allowing dislocation core spreading or dislocation dissociation. The reduced strain field of these dislocations reduces dislocation interactions and the pinning that results. Without the dislocation pinning, the misfit dislocations can extend longer distances yielding a greatly reduced threading dislocation density.

Reduction of threading dislocation density in GaN using an intermediate temperature interlayer

GaN thin films with a reduced threading dislocation density have been produced by organometallic vapor phase epitaxy using an intermediate temperature interlayer. A description of the growth process is presented with characterization results. Reduction of the dislocation density was obtained by insertion of a single thin interlayer grown at an intermediate temperature after the initial growth at high temperature. A large percentage of the threading dislocations present in the first GaN epilayer are found to bend in the interlayer and do not propagate in the top layer grown at higher temperature in a lateral growth mode.