Epitaxial Lateral Overgrown Research Articles

Crystal Defects and Strain of Epitaxial InP Layers Laterally Overgrown on Si

Defects in epitaxial laterally overgrown (ELO) InP layers are examined by high-resolution X-ray diffraction and synchrotron X-ray back-reflection and transmission topography. X-ray diffraction maps produce information about the overall crystal quality of the epitaxial layers in the InP ELO sample. The topographs show small angle boundaries, and the associated dislocations are located at the boundaries between the crystallites; allowing for their relative tilt, the maximum value for this is 0.06°. No defects inside the crystallites can be seen in the topographs, except for a small bending of 0.04° at most, of the ELO lattice planes. The section topographs show deformed X-ray interference fringes resulting from the large strain of the silicon lattice below the seeding areas.

Luminescence of highly excited nonpolar a ‐plane GaN epilayers

Luminescence transients have been studied in nonpolar a-plane fully coalesced epitaxial laterally overgrown (ELOG) GaN film grown over r-plane sapphire substrate under intense photoexcitation conditions. By increasing the excitation energy density (I) from 0.3 to 4.5 mJ/cm2, three regimes of carrier relaxation were revealed. These regimes are characterized by a specific time profile of the luminescence transient. For a low excitation density, I ≤ 0.35 mJ/cm2, the luminescence transient is nonexponential with an enhancement of the decay rate on the late stage of the transient due to carrier capture to nonsaturated deep traps. For an excitation density increased to about I ≅ 1 mJ/cm2, an establishment of the deep-trap saturation regime with a single luminescence decay time is observed. A further increase in the excitation density above 3 mJ/cm2 leads to an essentially nonexponential luminescence decay. Here, the initial stage of rapid recombination is attributed to laterally stimulated emission, while the late recombination stage is contributed by capture to the saturated deep traps and enhanced bimolecular emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Near‐field photoluminescence study in violet light emitting InGaN single quantum well structures

Spatial distribution of photoluminescence (PL) spectra has been assessed in violet light emitting InGaN single-quantum-well (SQW) structures by employing a scanning near-field optical microscope (SNOM) at room temperature. Clear correlation has been found between PL intensity and PL peak wavelength under illumination collection (I-C) mode, where strong PL was observed in the area emitting at lower energy bands. However, dark spots having diamter of about a few hundreds nm appeared also in lower energy region. Moreover, multi-mode SNOM-PL with a spatial resolution of 200 nm revealed that the difference of PL-intensity image between I-C mode and illumination (I) mode is not so large comparing with a blue light emitting InGaN-SQW. These findings suggest that threading dislocations act as nonradiative recombination centers in the violet sample unlike blue ones, and that the diffusion length to such centers is larger than that to localized centers leading to radiative recombination. Furthermore, nonradiative recombination centers originating from point defects have been spatially identified in the violet sample grown on epitaxially laterally overgrown (ELO) GaN template. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Computational analysis of lateral overgrowth of GaAs by liquid-phase epitaxy

Two-dimensional computational analysis of lateral overgrowth of GaAs by liquid-phase epitaxy has been carried out. The evolution of the growth interfaces is obtained by using growth velocity calculated from solute concentration field in the vicinity of the growth layer. Effects of interface kinetics and crystal surface tension (Gibbs–Thomson effect) were taken into account. The shapes of grown epilayers are in good agreement with experimental observations. The comparison of numerical results under different growth conditions demonstrates the importance of interface kinetics and Gibbs–Thomson effects on shape and aspect ratio of epitaxial laterally overgrown (ELO) layers.

Correlation of strain, wing tilt, dislocation density, and photoluminescence in epitaxial lateral overgrown GaN on SiC substrates

Epitaxial lateral overgrown (ELOG) gallium nitride (GaN) on SiC is being studied as a possible substrate for blue laser diodes. A defect density below 2.2×107cm−2 in the wings, compared to 2×109cm−2 in the windows, was achieved. Interaction of the overgrown GaN with the SiO2 mask causes a few degree wing tilt and a transition region of high defect density between windows and wings. Diminished PL, strong tensile stress, and a defect correlated line at around 3.4eV emerge in this up to two-micron-wide transition region. By changing the mask material from SiO2 to SiN we were able to reduce the wing tilt drastically to below 0.7°. This eliminates the defective transition region and extends the low strain and the low defect density area of the ELOG wings. The methods used to study strain, wing tilt, and threading dislocations in the ELOG samples are microphotoluminescence (μPL), transmission electron microscopy, x–ray diffraction, and scanning electron microscope. We also demonstrate the use of the first momentum of the μPL spectra as an effective means to measure strain distribution.

Increase of free carrier lifetime in nonpolar a-plane GaN grown by epitaxial lateral overgrowth

Carrier recombination dynamics in epitaxial a-plane GaN and fully coalesced epitaxial laterally overgrown (ELOG) a-plane GaN films has been studied by means of time-resolved photoluminescence under high photoexcitation. The results were compared with conventional c-plane GaN films grown under similar conditions. In a-plane GaN epilayers, the total efficiency of electron-hole plasma spontaneous luminescence decreases 20 times, whereas the luminescence decay time reduces from τLU=42 to τLU⩽10ps in comparison with c-plane GaN films. Meanwhile, an essential increase in total emission efficiency (by more than two orders of magnitude) and an increase of the decay time up to τLU=430ps have been observed for an ELOG a-plane sample in comparison with a-plane GaN films. This confirms a significant reduction of the nonradiative recombination rate for nonequilibrium carriers. Assuming a saturation of the nonradiative deep-level transitions, the room-temperature free-carrier lifetime of τ=910ps for ELOG a-plane GaN sample was obtained, which indicates on an excellent quality of the a-plane ELOG GaN films.

Anisotropic structural characteristics of (112̄0) GaN templates and coalesced epitaxial lateral overgrown films deposited on (101̄2) sapphire

a-plane GaN templates and coalesced epitaxial lateral overgrown (ELOG) films on r-plane sapphire substrates were investigated by x-ray diffraction (XRD). The a-plane GaN templates were found to have [0001]-oriented stripe-features, which is related to anisotropic mosaicity. For the mosaic blocks, the mosaicity reached the largest and the smallest values along the [11̄00] and the [0001] directions. The ELOG procedure with the SiO2 mask stripes perpendicular to the [0001] direction limits the preferable growth along this direction, and thereby enhances the [11̄00] growth. This leads to large-area, featureless, a-plane GaN films for which the wing tilt and not the fine mosaic block size becomes the major XRD line-broadening mechanism.

Structural Defects related issues of GaN-based Laser Diodes

ABSTRACTStructural defects affecting the lifetime of GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers have been investigated. Almost all of the threading dislocations that appeared in the wing regions have edge character, whereas the dislocations at the coalescence boundaries have both edge character and mixed character. The origins of the threading dislocations in the wing regions are the lateral extension of dislocations from the seed regions that contingently bend upwards to the epi-surface. Thus, edge dislocations are most considerable threading dislocations in GaN-based LDs on ELO GaN layers, since the laser stripes are fabricated in the wing regions. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations were observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. Although the threading dislocations in the LD stripes do not multiply during device operation, our degradation experiments revealed that the lifetime of the GaN-based LDs depends on the dislocation density. The degradation rate was almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.

Defects in degraded GaN-based laser diodes

We investigate degraded GaN-based laser diodes (LDs) fabricated on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy. The dislocation density in the wing region of the ELO is of the order of 106/cm2 and there are approximately ten threading dislocations in the laser stripe. Neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations were observed in the devices, which were degraded within approximately one hundred hours under 30 mW continuous operation at 25 °C. We can, therefore, conclude that degradation in GaN-based LDs is not responsible for the recombination enhanced dislocation motion that is usually observed in zincblende structure-based LDs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural characterization of epitaxial lateral overgrown GaN on patterned GaN/GaAs(0 0 1) substrates

Epitaxial lateral overgrown (ELO) cubic GaN (c-GaN) on SiO 2 patterned GaN/GaAs(0 0 1) substrates by metalorganic vapor phase epitaxy was investigated using transmission electron microscopy and X-ray diffraction (XRD) measurements. The density of stacking faults (SFs) in ELO c-GaN was ∼6×10 8 cm −2, while that in underlying GaN template was ∼5×10 9 cm −2. XRD measurements showed that the full-width at half-maximum of c-GaN (0 0 2) rocking curve decreased from 33 to 17.8 arcmin, indicating the improved crystalline quality of ELO c-GaN. The mechanism of SF reduction in ELO c-GaN was also discussed.

X-ray diffraction studies of epitaxial laterally overgrown (ELOG) GaN layers on sapphire substrates

GaN layers grown by metalorganic vapor phase epitaxy on sapphire substrates in epitaxial lateral overgrowth (ELOG) mode were studied using the high-resolution X-ray diffraction technique in double and triple axis modes (including reciprocal lattice mapping), both in reflection and transmission geometries. GaN layer grown on unmasked sapphire was examined in the same way for a comparison. We have found that in the fully overgrown ELOG structures: (i) hexagonal unit cells are deformed, so the lattice parameters a ⊥ (perpendicular to the stripes) and a 30 (inclined by 30° to the stripes) are smaller by up to 0.1% and 0.02%, respectively, as compared to those in the GaN buffer, (ii) curvature radii of the samples are of 1.2–1.4 m in the direction parallel to the stripes and of 1 m in the direction perpendicular to the stripes, which is consistent with the strain observed, and (iii) sapphire miscut along the 〈11.0〉 direction (i.e. perpendicular to the stripes) causes an additional inclination of ELOG lattice planes with respect to those in the buffer. A qualitative explanation of these findings is presented taking into account increase of GaN grain size in the ELOG layers.

Distribution of Horizontal Dislocations in ELO-GaN

Transmission electron microscopy (TEM) observation was performed for microstructure in epitaxial lateral overgrown (ELO) GaN layers with attention to the behavior of horizontal dislocations (HDs). The ELO-GaN layers were grown by metalorganic vapor phase epitaxy (MOVPE). The facet planes of the ELO-GaN layer were controlled by the two-step ELO technique. In the specimen with a narrow mask-window of 3 μm, HDs were generated over the mask-window. In the case of a wider mask-window of 5 μm, HDs were generated also near the mask-edge. Plan-view TEM observation showed that HDs were formed in the shape of waves. The Burgers vector of the HDs was perpendicular to the mask-edge. From the curvature of HDs, the internal stress in the region over the mask-window of ELO-GaN was roughly estimated to be 0.01–0.04 GPa.

Influences of mask width, fill factor, HCl addition and C doping on wing tilts in the epitaxial laterally overgrown GaN films by hydride vapor phase epitaxy

Crystal tilts in the epitaxial lateral overgrown (ELO) GaN region over SiO2 mask by hydride vapor phase epitaxial away from the opening region were investigated by scanning electron microscope and the four-circle x-ray diffraction method. The increased mask width and ratio of stripe opening width to stripe period (fill factor) lead to increased wing tilts. The introduction of additional HCl and C3H8/H2 can decrease wing tilts and improve the ELO GaN surface morphologies significantly. Moreover, it has been observed that a high growth rate will bring more defects and large wing tilts into the GaN materials.

Quantum 1/f noise in epitaxial lateral overgrown GaN: piezoelectric effect

Low-frequency noise has been investigated for epitaxial lateral overgrown (ELO) gallium nitride (ELO-GaN). The noise parameter α evaluated was about 2. The coherent piezoelectric quantum 1/f noise theory was applied to model the experimentally obtained α and excellent correlation was achieved. The high value of α indicates the significant contribution of the piezoelectric interaction and of the transversal acoustic phonon scattering mechanism.

Raman mapping, photoluminescence investigations, and finite element analysis of epitaxial lateral overgrown GaN on silicon substrates

Epitaxial lateral overgrown (ELO) GaN grown by metalorganic vapor phase epitaxy on Si substrates was characterized using Raman mapping, photoluminescence (PL) experiments and finite element (FE) analysis. Stress in the structures was determined from the E2 phonon frequency and compared to FE results. Low temperature PL spectra are dominated by donor bound exciton (DBE) emission at (3.457–3.459) eV. PL spectra reveal a peak at ∼3.404 eV in window regions attributed to structural defects in the GaN. Differences in crystalline quality between window and overgrown regions of ELO GaN were investigated.

Investigation on the origin of crystallographic tilt in lateral epitaxial overgrown GaN using selective etching

The crystallographic tilt of the lateral epitaxial overgrown (LEO) GaN on sapphire substrate with SiN x mask is investigated by double crystal X-ray diffraction. Two wing peaks beside the GaN 0002 peak can be observed for the as-grown LEO GaN. During the selective etching of SiN x mask, each wing peak splits into two peaks, one of which disappears as the mask is removed, while the other remains unchanged. This indicates that the crystallographic tilt of the overgrown region is caused not only by the plastic deformation resulted from the bending of threading dislocations, but by the non-uniformity elastic deformation related with the GaN/SiN x interfacial forces. The widths of these two peaks are also studied in this paper.

Microstructures of GaN Films Laterally Overgrown on Si(111) by Hydride Vapour Phase Epitaxy

We have investigated the microstructures of GaN films laterally epitaxiallyovergrown (LEO) on Si(111) substrates using hydride vapour phase epitaxy.The threading dislocation density in the LEO GaN isreduced by about two orders. Different etching angles of the twosidewalls of SiO2 masks (66o and 90o) lead tothe asymmetry of the LEO and cause the particular microstructures of LEOGaN. In micro-Raman spectra, the intensities vary weakly periodicallyabout 5 µm perpendicular to the mask stripes.The indistinct selective growth in the top surface is discussed.

Electron microscopy analyses of microstructures in ELO-GaN

Several topical results of electron microscope analyses for microstructures in epitaxial lateral overgrown (ELO)-GaN are reported. (1) Dislocations lying on (0 0 0 1), or horizontal dislocations (HDs), are generated in ELO-GaN layers overlying on a mask of a-SiO 2. The HDs have a shape of loop or semi-loop, and the morphology suggests that the HDs are generated through a multiplication mechanism with the assistance of internal stress. (2) On GaInN/GaN, a pit is formed at the end of a threading dislocation (TD) of any Burgers vector, a , c or a+ c . The density and distribution of TDs in ELO-GaN can be estimated by observing these growth pits after a subsequent deposition of a thin GaInN layer on the ELO-GaN. The critical thickness for the formation of pits has a dependency upon the concentration of In in GaInN and the Burgers vector. (3) It was demonstrated that electron backscattering diffraction pattern (EBSP) analyses can estimate a two-dimensional distribution of c-axis orientation of ELO-GaN for a wide area with an accuracy of 0.2° or better.

Raman Mapping and Finite Element Analysis of Epitaxial Lateral Overgrown GaN on Sapphire Substrates

ABSTRACTUsing micro-Raman scattering and finite element (FE) analysis, stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates using a two-step growth method were investigated. Nearly full stress relaxation at the top ELO GaN surface can be achieved by increasing the thickness of ELO GaN to about 50 μm. Reductions in stress variation between window and overgrown regions can be achieved by using a double ELO GaN growth at a much smaller ELO thickness. Increased compressive stress at the coalescence boundary of two adjacent wings of ELO GaN was related to the presence of voids in this area. In the double ELO growth, stress near the top surface was mainly attributed to the presence of voids on top of the upper dielectric mask.

半導体エレクトロニクス サブミクロン解像度極低温顕微ホトルミネセンス装置の開発とワイドギャップ半導体の欠陥検出への応用

A new photoluminescence (PL) microscope has been developed with a conventional optical system to obtain a monochromated PL image at a low temperature with a spatial resolution in sub-micron range. The objective and sample were put in the identical vacuum chamber to ensure thermal insulation between them. The spatial resolution at 15K was confirmed to be almost equal to the diffraction limit, i.e., 0.3μm, at a wavelength of 488nm. The microscope clearly visualized defective region, and polytype domain with a spatial resolution of 0.3μm in GaN. PL emissions ascribed to free excitons (EXA) and excitons bound to donors (D0X) were discriminated in μ-PL measurements at 15K on epitaxially laterally overgrown (ELO) GaN. The emission at 3.487eV ascribed to D0X became strong above the SiO2 mask within a distance of 8μm from the mask. In a μ-PL image related to excitons bound to N atoms in a 4H-SiC homoepitaxial layer, a dark line was observed at a line-shaped surface defect. Since the PL emission from donor (N)-acceptor (B) pairs was spatially uniform, N atoms were incorporated uniformly in the 4H-SiC epilayer. The dark line is probably caused by an enhancement of nonradiative recombination at the surface defect.