Macroscopic defects in GaN/AlN multiple quantum well structures grown by MBE on GaN templates (original) (raw)
Related papers
Cracks in GaN/AlN multiple quantum well structures grown by MBE
Journal of Physics: Conference Series, 2008
Due to the large lattice constant mismatch and thermal expansion coefficient difference between GaN and AlN, large strain is generated inside the GaN/AlN multiple quantum wells, which causes cracks in the structure. We investigated such cracks by optical microscopy and AFM. The crack density was studied with buffer and cap layer thickness, the number of quantum well periods, and the temperature reduction rate after growth as parameters. It was found that the crack density increased exponentially, with the number of periods above 4. Besides, a very thin, 100 nm, GaN buffer layer and ~ 300 nm GaN cap layer greatly reduced the crack density.
Journal of Physics D: Applied Physics, 2011
The influence of template type and residual strain of the buffer layer on the structural properties of GaN/AlN superlattices (SLs) was studied using high resolution x-ray diffraction. Using sapphire substrates, an effective thinning of the GaN quantum wells and the corresponding thickening of the AlN barriers were observed in SL structures grown on thin, strained AlN templates as compared with SL structures grown on thick, relaxed GaN templates. Moreover, a bimodal strain relaxation of SL structures in dependence of template type was observed. The SLs grown on AlN templates relax predominantly by the formation of misfit dislocations, while the SLs grown on GaN templates relax predominantly by cracking of the layers. We explain these effects by the influence of residual strain in the buffer/template systems used for the growth processes of SL layers. A correlation is made between the strain state of the system and the cracking processes, the dislocation density, the radius of curvature and the layer thickness.
2006
Both cracked and crack-free GaN/Al 0.55 Ga 0.45 N multiple quantum wells (MQWs) grown on GaN template by metalorganic chemical vapor deposition have been studied by triple-axis X-ray diffraction, grazing-incidence X-ray reflectivity, atomic force microscope, photoluminescence spectroscopy and low-energy positron annihilation spectroscopy. The experimental results show that cracks generation not only deteriorates the surface morphology, but also leads to a period dispersion and roughens the interfaces of MQWs. The mean density of dislocations in MQWs, determined from the average full-width at half-maximum of v-scan of each satellite peak, has been significantly enhanced by the cracks generation. Furthermore, the measurement of annihilation-line Doppler broadening reveals a higher concentration of negatively charged vacancies in the cracked MQWs. The combination of these vacancies and the high density of edge dislocations are assumed to contribute to the highly enhanced yellow luminescence in the cracked sample. #
Interfacial and defect structures in multilayered GaN/AlN films
Journal of Physics-condensed Matter, 2002
A structural assessment of various interfaces formed between successive GaN/AlN layers epitaxially grown on (0001) sapphire, as well as between n-type and p-type GaN is presented, using transmission and high-resolution electron microscopy. The structure of the interfaces between the thick GaN and the thin AlN interlayers (ILs) is investigated in terms of misfit difference and elastic fit of the two crystal lattices. Dense threading dislocations are observed to originate from the substrate/buffer layer interface and their interaction with the successive AlN ILs is studied. Furthermore, basal inversion domain boundaries localized in the n-GaN/p-GaN interface are evaluated.
Strain effects in GaN/AlN multi-quantum-well structures for infrared optoelectronics
2009
We report on the effect of Al mole fraction in AlGaN substrates on the strain state and on the intersubband (ISB) absorption properties of GaN/AlN multi-quantum-well (MQW) structures. The MQWs present the same strain state in all the samples irrespective of the Al concentration in the substrate. As a consequence, the ISB transition energy was found to be independent of Al content in the substrate. In situ characterization during growth reveals a progressive relaxation in the GaN quantum wells and a combination of elastic and plastic relaxation in AlN barriers.
physica status solidi (c), 2005
Subpicosecond time-resolved photoluminescence (TRPL) has been used to compare the room temperature carrier dynamics in Al 0.1 Ga 0.9 N/Al 0.3 Ga 0.7 N multiple quantum well (MQW) structures simultaneously deposited on a high quality free standing HVPE GaN substrate (dislocation density ~ 1x10 7 cm -2 ) and 1 µm MOCVD GaN template on sapphire. The PL lifetime of ~ 500 ps in the MQW on GaN substrate is about 5 times longer than that for the MQW on GaN template, with a concomitant increase in CW PL intensity. This behavior is attributed primarily to an increase in nonradiative lifetime associated with a 100 times reduction in dislocation density in the GaN substrate. The observation that the PL lifetime in the MQW falls short of the ~ 900 ps dominant decay time in the GaN substrate may be indicative of generation of additional defects and dislocations due to substrate surface preparation, strain relaxation, and nonoptimal growth temperature associated with the difference in heating of the thin GaN template on sapphire and the thick GaN substrate. An extended PL rise time of greater than 20 ps for the MQW emission when above barrier pumping is employed implies that both wells and barriers are of high quality.
Strain accommodation and interfacial structure of AlN interlayers in GaN
Crystal Research and Technology, 2009
The strain accommodation mechanisms at AlN interlayers in GaN, grown by radio-frequency plasma assisted molecular beam epitaxy, are studied using transmission electron microscopy techniques and atomistic modelling. Interlayers of various thicknesses grown within GaN epilayers deposited on both sapphire and silicon substrates have been employed. Interlayers of thickness below 6 nm do not exhibit line defects although local roughness of the upper interlayer interface is observed as a result of the Al adatom kinetics and higher interfacial energy compared to the lower interface. Above 6 nm, introduction of a-type misfit and threading dislocations constitutes the principal relaxation mechanism. Due to strain partitioning between AlN and GaN, threading dislocations adopt inclined zig-zag lines thus contributing to the relief of alternating compressive-tensile elastic strain across the AlN/GaN heterostructure. The observed dislocation configurations are consistent with a model of independent motion by climb or ancillary glide in response to their localized three-dimensional strain environment. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
The present work concerns the microstructural characterization of multiple InGaN/GaN Quantum Well (QW) structures grown onto GaN on sapphire templates by Metalorganic Chemical Vapor Phase Epitaxy (MOVPE), using electron microscopy techniques (Transmission Electron Microscopy-TEM and Scanning Electron Microscopy-SEM). The TEM characterization showed V-shaped defects, including V-pits and trench defects on the surface of the as-grown sample. Post-growth annealing experiments were performed in order to determine whether a structural degradation of the structure has taken place. After After annealing, a very small density of V-defects was observed, having a beneficial impact onoptoelectronics performance. The presence of sharp interfaces between the InGaN QWs and the GaN barriers gave us an indication of their good structural quality.
Journal of Applied Physics, 2004
InGaN/GaN multiquantum-well ͑MQW͒ structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kühne, and Richter.
Optical and structural study of deformation states in the GaN/AlN superlattices
Article, 2017
We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20- period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed.