Strain-stress study of AlxGa1-xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1-xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1-xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1-xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1-xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1-xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1-xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain is also discussed on the optical phonon energies of the epitaxial layers. The techniques discussed here can be used to study other similar materials.Comment: 14 pages, 5 figures, 2 table

Strain-stress study of AlxGa1−xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed

Surfactants and digital alloys for strain relief in III-nitride distributed Bragg reflectors and related heterostructures via metal organic vapor phase epitaxy

III-Nitride based semiconductors have emerged as one of the promising materials for electronic and opto-electronic devices, including but not limited to, solid state emitters, photodetectors, and transistors. Despite commercial success, several issues ranging from material growth to device fabrication remain unresolved and continue to hinder the efficiency of these devices. One such issue includes strain management in III-Nitride heterostructures. The binary alloys in the (Al,In,Ga)N family are characterized by a large lattice and thermal mismatch which leads to defect formation and cracking within heterostructures. These defects are detrimental to device fabrication and operation. This work investigates growth based techniques to manage strain in III-Nitride heterostructures and thereby reduce defect formation.;In particular, this work focuses on the development surfactant assisted growth and digital alloys as strain relieving techniques to minimize cracking in Aluminum Gallium Nitride (AlxGa1- xN) alloys and related heterostructures via Metal Organic Vapor Phase Epitaxy. Indium has been investigated as a surfactant in the growth of AlN/GaN Distributed Bragg Reflectors (DBRs) and has been shown to reduce the cracking by a factor of two. Using variable temperature x-ray diffraction studies, indium has been shown to influence the thermal expansion coefficients of the AlN layers. The digital growth technique has been investigated as a viable method for achieving high quality, crack free AlxGa 1-xN films. Alloys with an AlN mole fraction ranging from 0.1 to 0.9 have been grown by adjusting the periodicity of these short period superlattice structures. High resolution x-ray diffraction has been used to determine the superlattice period along with the a- and c-lattice parameter of the structure. High aluminum content digital AlxGa1-xN alloys have been employed in DBRs for high reflectivity, \u3e94%, crack-free structures. The characterization of these structures via scanning electron microscopy, atomic force microscopy, and x-ray diffraction is presented along with the results from the integration of the DBR with visible wavelength LEDs

A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content

Abstract With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11–22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x ≈ 0.57–0.85) and dopant concentration (3 × 1018–1 × 1019 cm−3) in various series of Al x Ga1-x N layers grown on (0001) and (11–22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0–5.4 eV as well as various deep impurity transition peaks in the range 2.7–4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.</jats:p

Ternary III-Nitride Semiconductors for Thermoelectricity and Light Emitters

III-Nitride semiconductors have significant applications for lasers and energy-efficient technologies including solid state lighting. Specifically, the use of InGaN alloy is of great interest as visible light emitting diodes (LEDs) active region. Conventional LEDs employ InGaN quantum wells (QWs) grown on GaN templates, which lead to large QW strain from the lattice mismatch between InGaN QW and GaN substrate / barriers. Our works have pursued the design of InGaN QWs with large optical matrix element to address the charge separation issue, resulting in 3X enhanced efficiency for green-emitting LEDs. In addition to employing large overlap QWs design, my research work has extended the approach by using ternary InGaN substrate for realizing QWs with reduced strain and polarization fields in the QWs. For green- and red-emitting InGaN QWs on ternary substrate, the spontaneous emission rates were found as ~3 times of the conventional approach.In contrast to the progress in visible nitride emitters, advances have only been realized for ultraviolet (UV) LEDs recently. The pursuit of efficient UV lasers has been limited to 1) growth challenges of high quality AlGaN gain media; 2) lack of understanding in gain characteristics of the QW employed for UV laser. My work has pointed out the first time about the physical challenge of the AlGaN QWs, which is related to the valence subbands crossover in high Al-content AlGaN QWs gain media. The valence subbands crossover is of key importance to realize large transverse-magnetic polarized gain for deep-UV lasers. We have also proposed the novel AlGaN-delta-GaN QW structure, which led to large transverse-electric polarized gain for mid-UV lasers.Furthermore, the high power density requirements in III-Nitride devices lead to the demand of solid state cooling technology, particularly for nitride-based thermoelectric materials that can be integrated with GaN devices. Our works presented the high thermoelectric figure of merit Z*T value from lattice-matched AlInN alloy grown by metalorganic chemical vapor deposition (MOCVD), which represent the record Z*T value reported for any III-nitride semiconductors. In addition, we have proposed the novel nanostructure engineering of three-layer superlattice for ~2-times enhanced thermoelectric properties for solid state cooling applications

MOCVD growth and electrical studies of p-type AlGaN with Al fraction 0.35

Cataloged from PDF version of article.We present a study on the high performance p-type AlxGa1-xN (x = 0.35) layers grown by low-pressure metalorganic chemical vapor deposition on AIN template/sapphire substrate. The influence of growth conditions on the p-type conductivity of the AlxGa1-xN (x = 0.35) alloy is investigated. From the Hall effect and I-V transmission line model measurements, a p-type resistivity of 3.5 Omega cm for AlxGa1-xN (x = 0.35) epilayers are achieved. To the best of our knowledge, this is the lowest resistivity ever measured for the uniform p-type AlGaN with Al fraction higher than 0.3. The Mg and impurities (O, C and H) of the atom concentration in the epi-layers are analyzed by means of SIMS depth profiles, which reveal the dependence of impurities incorporation on the III elements and growth temperature. (c) 2006 Elsevier B.V. All rights reserved

Correlating X-ray microanalysis and cathodoluminescence data from III-nitride semiconductors

Research in group III- nitride semiconductors has seen major developments during the last couple of decades. One of the materials that satisfy requirements for optoelectronic devices in the ultra-violet (UV) spectral range and high power, high frequency electronic devices is the AlGaN. Performance and reliability of these devices will strongly depend on the electronic properties of epitaxial layers which are critically affected by structural defects and unintentional and intentional doped impurities. This thesis presents research on III- nitride semiconductors, in particular AlGaN and GaN materials. It is focused on characterization of AlGaN materials and the effects of n- and p-type doping, AlN content, occurrence of defects and crystal orientation on its quality. Different electron microscopy techniques are used to investigate luminescence, composition and doping properties of semiconductor structures and their correlation with surface features. The main techniques used for the characterization consisted of cathodoluminescence spectroscopy (CL) for the probing of luminescence properties, secondary electron (SE) and backscattered electron (BSE) imaging for investigation of the sample morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The type of growth method and choice of substrate have a great influence on the surface morphology and luminescence homogeneity of the AlGaN layer, with compositional inhomogeneity of the MBE samples confirmed only on sub μm level but having lower emission intensity compared to MOCVD samples. The thesis presents detailed steps of a procedure to quantify trace elements and investigates the associated challenges. The whole process of measurement optimization for Mg and Si dopants is described and final recipe on how to measure the concentration of major (alloy) and minor Si/Mg (dopant) elements is presented. A systematic study of polar and semipolar n-type doped AlGaN/AlN layers grown on sapphire by (MOCVD) with varied Si/group-III ratios in the gas phase was accomplished. The AlN incorporation was higher in the polar samples and the highest values of Si incorporations were observed for the polar samples with the highest Si/III ratios, while saturation of Si incorporation was seen for the semipolar samples at higher Si/III ratios. CL point spectra showed how changes in the relative intensity of the NBE peaks and impurity transitions depend strongly on the growth conditions and surface orientations. The semipolar samples showed better compositional homogeneity. A study was also performed on AlGaN:Mg samples to study the impurity transitions and luminescence properties of non LED epilayer samples grown on MOCVD AlN/sapphire templates and more complicated LED structures with different numbers of MBE-grown layers. MBE samples showed superior quality to other combinations of MBE and MOCVD structures, mainly due to problems associated with the transfer of sample between different reactors and the introduction of impurities that will form different defects within the material. Finally, some proposals for future work are presented.Research in group III- nitride semiconductors has seen major developments during the last couple of decades. One of the materials that satisfy requirements for optoelectronic devices in the ultra-violet (UV) spectral range and high power, high frequency electronic devices is the AlGaN. Performance and reliability of these devices will strongly depend on the electronic properties of epitaxial layers which are critically affected by structural defects and unintentional and intentional doped impurities. This thesis presents research on III- nitride semiconductors, in particular AlGaN and GaN materials. It is focused on characterization of AlGaN materials and the effects of n- and p-type doping, AlN content, occurrence of defects and crystal orientation on its quality. Different electron microscopy techniques are used to investigate luminescence, composition and doping properties of semiconductor structures and their correlation with surface features. The main techniques used for the characterization consisted of cathodoluminescence spectroscopy (CL) for the probing of luminescence properties, secondary electron (SE) and backscattered electron (BSE) imaging for investigation of the sample morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The type of growth method and choice of substrate have a great influence on the surface morphology and luminescence homogeneity of the AlGaN layer, with compositional inhomogeneity of the MBE samples confirmed only on sub μm level but having lower emission intensity compared to MOCVD samples. The thesis presents detailed steps of a procedure to quantify trace elements and investigates the associated challenges. The whole process of measurement optimization for Mg and Si dopants is described and final recipe on how to measure the concentration of major (alloy) and minor Si/Mg (dopant) elements is presented. A systematic study of polar and semipolar n-type doped AlGaN/AlN layers grown on sapphire by (MOCVD) with varied Si/group-III ratios in the gas phase was accomplished. The AlN incorporation was higher in the polar samples and the highest values of Si incorporations were observed for the polar samples with the highest Si/III ratios, while saturation of Si incorporation was seen for the semipolar samples at higher Si/III ratios. CL point spectra showed how changes in the relative intensity of the NBE peaks and impurity transitions depend strongly on the growth conditions and surface orientations. The semipolar samples showed better compositional homogeneity. A study was also performed on AlGaN:Mg samples to study the impurity transitions and luminescence properties of non LED epilayer samples grown on MOCVD AlN/sapphire templates and more complicated LED structures with different numbers of MBE-grown layers. MBE samples showed superior quality to other combinations of MBE and MOCVD structures, mainly due to problems associated with the transfer of sample between different reactors and the introduction of impurities that will form different defects within the material. Finally, some proposals for future work are presented

Gruppe III-Nitrid basierte UVC LEDs und Laser mit transparenten AlGaN:Mg Schichten und Tunneldioden, hergestellt mittels MOVPE

In this work, AlGaN-based light-emitting diodes (LEDs) and lasers with emission wavelengths in the deep ultraviolet (UVC) spectral range are produced, analyzed, and optimized. Here, the focus is on the UV transparency of the structures, enabling high light extraction efficiency for UVC LEDs and being a necessary condition for UVC laser diodes, however at the same time challenging due to low electrical conductivity. AlN and AlGaN layers as well as heterostructures for devices are grown by metalorganic vapor phase epitaxy. A systematic analysis of the influence of individual layer properties on the emission properties of LEDs and lasers is provided. Defect reduced (ELO) AlN layers on sapphire and AlN substrates serve as basis for the epitaxial growth of AlN and AlGaN layers. By analyzing the influence of substrate offcut on surface morphology, atomically smooth AlN layers are reproducibly obtained on both types of substrates for offcut angles < 0.17°. For the realization of n-type AlGaN:Si cladding layers, the influence of growth parameters such as temperature, gas phase composition and growth rate was separately analyzed. Highly conductive, uniform and smooth AlGaN:Si layers were obtained by the implementation of a superlattice concept with 10 s growth interruptions to increase the diffusion length of metal adatoms. Despite high compressive strain, pseudomorphic laser structures with three-fold quantum wells were obtained with emission wavelength at 270 nm by the choice of Al0.7Ga0.3N waveguide composition, whereas lower aluminum contents lead to partial strain relaxation. In addition, the formation of V-pits acting as scattering centers in the waveguide was successfully reduced by increasing the growth temperature from 900 ℃ to 1080 ℃. Finally, the influence of these individual optimization steps on laser properties was analyzed. Optically pumped UVC lasers with laser threshold, spectral linewidth reduction, and TE polarized emission above threshold were shown near 270 nm. By reducing the surface roughness, the laser thresholds were reduced by a factor of seven. Electrical injection mechanisms were experimentally analyzed by electroluminescence measurements on transparent UVC LEDs with waveguide system, and combined with simulations of optical modes and the corresponding losses. By the variation of composition and layer thickness of waveguide and cladding layers an optimized heterostructure design for UVC laser diodes with 200 nm thick Al0.76Ga0.24N:Mg cladding layers was found. This design simultaneously enables efficient carrier injection and sufficient mode confinement with low optical losses of 40 cm-1. As an unconventional alternative to resistive AlGaN:Mg layers, tunnel junctions (TJ) in reverse bias configuration were implemented into the UVC LED heterostructure for efficient injection of holes. By the initial optimization of individual TJ components, such as doping concentrations at the TJ interface or the composition of an interlayer, the first demonstration of functional TJ-LEDs with AlGaN tunnel homojunction was achieved, as well as the first demonstration of AlGaN-based TJ-LEDs grown by metalorganic vapor phase epitaxy. Based on these devices, the interlayer thickness was varied to exploit polarization charges at the interface in order to reduce the space charge region width and enhance tunneling probabilities. Using 8 nm thick GaN interlayers, a reduction of the operation voltage by 20 V was achieved, as well as TJ-LEDs with external quantum efficiencies of 2.3% and emission powers of 6.6 mW at 268 nm and 0.26 mW at 232 nm.In dieser Arbeit werden AlGaN-basierte Leuchtdioden (LEDs) und Laser mit Emissionswellenlängen im tiefen ultravioletten (UVC) Spektralbereich hergestellt, charakterisiert und optimiert. Dabei liegt die UV-Transparenz der Strukturen im Fokus, die hohe Lichtextraktionseffizienz für UVC LEDs ermöglicht und eine notwendige Bedingung für UVC Laserdioden darstellt, gleichzeitig aber aufgrund geringer elektrischer Leitfähigkeit herausfordernd ist. AlN und AlGaN Schichten sowie Heterostrukturen für Bauelemente werden mittels metallorganischer Gasphasenepitaxie hergestellt und der Einfluss einzelner Schichteigenschaften auf die Emissionseigenschaften von LEDs und Lasern systematisch analysiert. Defektreduzierte (ELO) AlN Schichten auf Saphirsubstraten sowie AlN Substrate dienen als Basis für das epitaktische Wachstum von AlN und AlGaN Schichten. Durch die Analyse des Einflusses des Substratfehlschnittes auf die Oberflächenmorphologie konnten atomar glatte AlN Schichten auf beiden Substrattypen für Fehlschnittwinkel < 0.17° reproduzierbar hergestellt werden. Die AlGaN:Si Wachstumsparameter Temperatur, Gasphasenzusammensetzung und Wachstumsrate wurden separat variiert. Leitfähige, homogene und glatte AlGaN:Si Schichten konnten durch die Umsetzung eines Übergitterkonzeptes mit je 10 s Wachstumsunterbrechung zur Erhöhung der Diffusionslänge von Metalladatomen realisiert werden. Pseudomorphe Laserstrukturen mit Dreifach-Quantenfilmen und Emissionswellenlängen von 270 nm wurden trotz stark kompressiver Verspannung mittels Al0.7Ga0.3N Wellenleitern realisiert, wogegen geringere Aluminiumgehalte zu Teilrelaxation der Verspannung führen. Zudem konnte die Ausbildung von V-Pits als Streuzentren im Wellenleiter durch Erhöhung der Wachstumstemperatur von 900 ℃ auf 1080 ℃ erfolgreich reduziert werden. Schließlich wurde der Einfluss dieser einzelnen Optimierungsschritte auf die Lasereigenschaften analysiert. Optisch gepumpte UVC Laser mit spektraler Einschnürung, Laserschwelle sowie TE polarisierter Emission nahe 270 nm wurden gezeigt. Durch Reduktion der Oberflächenrauheit konnte die Laserschwelle schrittweise um den Faktor sieben reduziert werden. Elektrische Injektion wurde mittels Elektrolumineszenz an transparenten UVC LEDs mit Wellenleitersystem experimentell analysiert und mit Simulationen optischer Moden und deren Verluste kombiniert. Durch die Variation von Zusammensetzung und Schichtdicke von Wellenleiter- bzw. Mantelschichten konnte ein optimiertes Heterostrukturdesign für UVC Laserdioden mit 200 nm dicken Al0.76Ga0.24N:Mg Mantelschichten gefunden werden, welches gleichzeitig effiziente Ladungsträgerinjektion und ausreichenden Modeneinschluss mit geringen optischen Verlusten von 40 cm-1 ermöglicht. Als unkonventionelle Alternative zu resistiven AlGaN:Mg Schichten wurden Tunneldioden (TJ) zur Löcherinjektion implementiert. Durch die anfängliche Optimierung individueller Komponenten wie der Zusammensetzung einer Zwischenschicht oder der Dotierlevel an der Grenzfläche, wurde die erste Demonstration AlGaN-basierter TJLEDs ermöglicht, die mit metallorganischer Gasphasenepitaxie gewachsen wurden. Auf dieser Basis wurde die Zwischenschichtdicke gezielt variiert, um Polarisationsladungen an der Grenzfläche zur Reduktion der Raumladungszonenbreite auszunutzen und die Tunnelwahrscheinlichkeit zu erhöhen. Mit 8 nm GaN Zwischenschichten wurde eine Spannungsreduktion um 20 V erreicht, sowie TJ-LEDs mit externer Quanteneffizienz von 2,3% und Emissionsleistung von 6,6 mW bei 268 nm und 0,26 mW bei 232 nm

Strain Analysis of GaN HEMTs on (111) Silicon with Two Transitional AlxGa1-xN Layers

We have designed and then grown a simple structure for high electron mobility transistors (HEMTs) on silicon, where as usual two transitional layers of AlxGa1−xN (x = 0.35, x = 0.17) have been used in order to engineer the induced strain as a result of the large lattice mismatch and large thermal expansion coefficient difference between GaN and silicon. Detailed x-ray reciprocal space mapping (RSM) measurements have been taken in order to study the strain, along with cross-section scanning electron microscope (SEM) images and x-ray diffraction (XRD) curve measurements. It has been found that it is critical to achieve a crack-free GaN HEMT epi-wafer with high crystal quality by obtaining a high quality AlN buffer, and then tuning the proper thickness and aluminium composition of the two transitional AlxGa1−xN layers. Finally, HEMTs with high performance that are fabricated on the epi-wafer have been demonstrated to confirm the success of our strain engineering and above analysis