| 1. |
- You, S., et al.
(author)
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Vertical GaN devices : Process and reliability
- 2021
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In: Microelectronics and reliability. - : Elsevier Ltd. - 0026-2714 .- 1872-941X. ; 126
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Journal article (peer-reviewed)abstract
- This paper reviews recent progress and key challenges in process and reliability for high-performance vertical GaN transistors and diodes, focusing on the 200 mm CMOS-compatible technology. We particularly demonstrated the potential of using 200 mm diameter CTE matched substrates for vertical power transistors, and gate module optimizations for device robustness. An alternative technology path based on coalescence epitaxy of GaN-on-Silicon is also introduced, which could enable thick drift layers of very low dislocation density. © 2021
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| 2. |
- Valcheva, E., et al.
(author)
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Elimination of nonuniformities in thick GaN films using metalorganic chemical vapor deposited GaN templates
- 2001
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 90:12, s. 6011-6016
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Journal article (peer-reviewed)abstract
- Thick hydride vapor phase epitaxial GaN layers are grown on metalorganic chemical vapor deposited GaN template layers as well as directly on sapphire, with the aim of investigating the effect of the template on the strain relaxation and spatial distribution of free carriers in the overgrown GaN films. Spatially resolved cross-sectional micro-Raman spectroscopy, cathodoluminescence, and transmission electron microscopy show improved crystalline quality for growth on metalorganic chemical vapor deposited GaN templates. The highly doped and highly defective near-substrate layer composed of columnar domains, typically present in hydride vapor phase epitaxial GaN films grown directly on sapphire, is absent in the layers grown on templates. Consequently, this results in elimination of the nonuniformities of free electron distribution, a lower residual free carrier concentration (<1017 cm-3), and improved strain relaxation. © 2001 American Institute of Physics.
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| 3. |
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| 4. |
- Valcheva, E., et al.
(author)
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Impact of MOCVD-GaN 'templates' on the spatial non-uniformities of strain and doping distribution in hydride vapour phase epitaxial GaN
- 2001
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In: Materials Science & Engineering. - 0921-5107 .- 1873-4944. ; 82:1-3, s. 35-38
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Journal article (peer-reviewed)abstract
- Thick HVPE-GaN layers are grown on Si-doped and undoped MOCVD-GaN 'template' layers as well as directly on sapphire, with the aim to investigate the effect of the MOCVD template on the strain relaxation and spatial distribution of free carriers in the overgrown HVPE films. Spatially resolved cross-sectional micro-Raman measurements, cathodoluminescence and transmission electron microscopy show improved crystalline quality resulting in elimination of the non-uniformities of electron distribution, a low free carrier concentration (< 1017 cm-3) as well as a significant strain relaxation effect. © 2001 Elsevier Science B.V.
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| 5. |
- Darakchieva, Vanya, 1971-, et al.
(author)
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Anisotropic strain and phonon deformation potentials in GaN
- 2007
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195217-
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Journal article (peer-reviewed)abstract
- We report optical phonon frequency studies in anisotropically strained c -plane- and a -plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E1 (TO), E1 (LO), and E2 phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials c E1 (TO), c E1 (LO), and c E2 are determined. A distinct correlation between anisotropic strain and the A1 (TO) and E1 (LO) frequencies of a -plane GaN films reveals the a A1 (TO), b A1 (TO), a E1 (LO), and b E1 (LO) phonon deformation potentials. The a A1 (TO) and b A1 (TO) are found to be in very good agreement with previous results from Raman experiments. Our a A1 (TO) and a E1 (LO) phonon deformation potentials agree well with recently reported theoretical estimations, while b A1 (TO) and b E1 (LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented. © 2007 The American Physical Society.
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| 6. |
- Darakchieva, Vanya, et al.
(author)
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Effect of anisotropic strain on phonons in a-plane and c-plane GaN layers
- 2007
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In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 300:1, s. 233-238
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Journal article (peer-reviewed)abstract
- We have studied phonons in two types of anisotropically strained GaN films: c-plane GaN films grown on a-plane sapphire and a-plane GaN films grown on r-plane sapphire. The anisotropic strain in the films is determined by high-resolution X-ray diffraction (HRXRD) in different measuring geometries and the phonon parameters have been assessed by generalized infrared spectroscopic ellipsometry (GIRSE). The effect of strain anisotropy on GaN phonon frequencies is presented and the phonon deformation potentials aA1 (TO), bA1 (TO), cE1 (TO) and cE1 (LO) are determined. © 2006 Elsevier B.V. All rights reserved.
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| 7. |
- Fröjdh, Christer, 1952-, et al.
(author)
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New sensors for dental X-ray imaging
- 1999
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In: Nuclear Instruments and Methods in Physics Research Section A. - 0168-9002 .- 1872-9576. ; 434:1, s. 24-29
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Journal article (peer-reviewed)
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| 8. |
- Pecz, B., et al.
(author)
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GaN heterostructures with diamond and graphene
- 2015
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In: Semiconductor Science and Technology. - : IOP PUBLISHING LTD. - 0268-1242 .- 1361-6641. ; 30:11, s. 114001-
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Journal article (peer-reviewed)abstract
- The full performance of GaN devices for high power applications is not exploited due to their self-heating. Possible solutions are the integration of materials with high heat conductivity i.e., single crystalline diamond and graphene layers. We report the growth of single crystalline (0001)-oriented GaN thin films on (100), (110) and (111) diamond single crystals studied by transmission electron microscopy (TEM) in cross-sections. As for graphene, we show a high quality GaN layer that was deposited on patterned graphene layers and 6H-SiC. The atomic structures of the interfaces in the heterostructure are studied using aberration-corrected scanning TEM combined with energy dispersive x-ray and electron energy-loss spectroscopy.
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| 9. |
- Arnaudov, B, et al.
(author)
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Hall effect data analysis of GaN n(+)n structures
- 2002
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In: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 872-876
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Journal article (peer-reviewed)abstract
- We develop a model for analysis of Hall effect data of GaN structures composed of sublayers with different thicknesses and contacts placed on the top surface, We analysed the contributions of the conductivity of every sublayer of a planar sample taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from the upper layer. Correction factors, which reduce the contribution of the underlying layers to the measured whole sample conductivity, are obtained from the equations relevant to the respective equivalent circuit.
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| 10. |
- Arnaudov, B., et al.
(author)
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Magnetic-field-induced localization of electrons in InGaN/GaN multiple quantum wells
- 2003
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 83:13, s. 2590-2592
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Journal article (peer-reviewed)abstract
- A study was performed on the magnetic-field-induced localization of electrons in InGaN/GaN multiple quantum wells (MQW). A stepwise behavior of both the Hall coefficient and magnetoresistivity was observed. The peculiarities were explained by a magnetic-field-induced localization of electrons in a two-dimensional (2D) potential relief of the InGaN MQW.
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| 11. |
- Arnaudov, B, et al.
(author)
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Multilayer model for Hall effect data analysis of semiconductor structures with step-changed conductivity
- 2003
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:4
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Journal article (peer-reviewed)abstract
- We present a multilayer model for analysis of Hall effect data of semiconductor structures composed of sublayers with different thicknesses and contacts placed on the top surface. Based on the circuit theory we analyze the contributions of the conductivity of every sublayer and derive general expressions for the conductivity and carrier mobility of a multilayer planar sample. The circuit analysis is performed taking into account the fact that the sample sublayers are partially connected in parallel to each other by series resistances formed in areas lying below the contacts from each upper layer. In order to solve the inverse problem of determining the electrical parameters of one of the sublayers, a procedure for analysis of the Hall effect data is proposed. The model is simplified for a structure composed of two layers with the same type of conductivity, and is used to determine the electrical parameters of GaN films grown on relatively thick GaN buffers.
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| 12. |
- Darakchieva, Vanya, et al.
(author)
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Lattice parameters of GaN layers grown on a-plane sapphire : Effect of in-plane strain anisotropy
- 2003
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:5, s. 703-705
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Journal article (peer-reviewed)abstract
- The lattice parameters of GaN layers grown on a-plane sapphire were investigated. The hydride vapor phase epitaxy and metalorganic vapor phase epitaxy were used for the determination of parameters. The strain anisotropy was found to have different values in the films and obtained values of parameters were grouped around two values.
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| 13. |
- Evtimova, S, et al.
(author)
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Effect of carrier concentration on the microhardness of GaN layers
- 2003
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In: Journal of materials science. Materials in electronics. - 0957-4522 .- 1573-482X. ; 14:10-12, s. 771-772
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Journal article (peer-reviewed)abstract
- The paper presents a microhardness study of thick crack-free hydride vapor phase epitaxial GaN layers (not intentionally doped), and of thin metal-organic vapor phase epitaxial (MOVPE) GaN layers (undoped and Si-doped), grown on sapphire. A Vickers indentation method was used to determine the microhardness under applied loads up to 2 N. An increase in the microhardness was observed with decreasing carrier concentration and increasing mobility. A dip at an indentation depth of about 0.75 mum is observed in the microhardness profile in the MOVPE films, and is correlated with peculiarities in the spatially resolved cathodoluminescence spectra. The relationship between the mechanical and electrophysical parameters is discussed. (C) 2003 Kluwer Academic Publishers.
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| 14. |
- Hemmingsson, Carl, 1964-, et al.
(author)
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Growth of bulk GaN in a vertical hydride vapour phase epitaxy reactor
- 2006
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In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 40:4-6 SPEC. ISS., s. 205-213
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Journal article (peer-reviewed)abstract
- Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence. © 2006 Elsevier Ltd. All rights reserved.
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| 15. |
- Hemmingsson, Carl, et al.
(author)
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Hydride vapour phase epitaxy growth and characterization of thick GaN using a vertical HVPE reactor
- 2007
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In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 300:1, s. 32-36
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Journal article (peer-reviewed)abstract
- Growth of 2-inch diameter bulk GaN layers with a thickness up to 2 mm is demonstrated in a vertical hydride vapour phase growth reactor. Morphology, dislocations, optical and electrical properties of the material have been investigated using atomic force microscopy, optical microscopy, decorative etching in hot H3PO4, Hall measurements and low-temperature photoluminescence. Atomic force microscopy reveals a two-dimensional step flow growth mode with step bunching for layers with a thickness of 250 µm. As the growth proceeds, the morphology is changed to a hill and valley structure. The EPD was determined to 5×105 cm-2 for a 2 mm thick layer. The Hall mobility and the carrier concentration were determined. For a 1.7 mm thick layer at 300 K the mobility and the carrier concentration is 520 cm2/V s and about 4×1017 cm-3, respectively. Low-temperature photoluminescence spectra measured on a 350 µm thick freestanding layer show the DBE line at 3.4707 eV with a full-width half-maximum of 1 meV, confirming a stress free GaN layer. © 2006 Elsevier B.V. All rights reserved.
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| 16. |
- Hemmingsson, Carl, et al.
(author)
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Modeling, optimization, and growth of GaN in a vertical halide vapor-phase epitaxy bulk reactor
- 2008
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In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 310:5, s. 906-910
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Journal article (peer-reviewed)abstract
- In this work we are presenting growth results of thick gallium nitride (GaN), numerical modeling and optimization of a vertical hot-walled halide vapor-phase epitaxy reactor. Using a simulation model, the growth rate and thickness uniformity of the GaN layers have been predicted and optimized. The simulation results have been correlated with experiments to verify the model. Using constant precursor flows, the average growth rate over a 2 in substrate was increased with a factor of four by only optimizing the composition of N2 and H2 in the carrier gas and the carrier gas flow rates. With a simple sticking model, assuming Ga mass transport-limited growth, the growth rate and thickness uniformity could be estimated. Photoluminescence mapping of the grown layer shows that the layers have excellent optical properties and a high degree of uniformity. © 2007 Elsevier B.V. All rights reserved.
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| 17. |
- Kasic, A., et al.
(author)
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Characterization of crack-free relaxed GaN grown on 2″ sapphire
- 2005
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:7, s. 73525-
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Journal article (peer-reviewed)abstract
- We demonstrate the growth of high-quality and virtually strain-free bulklike GaN by hydride vapor-phase epitaxy in a vertical atmospheric-pressure reactor with a bottom-fed design. The 300‐μm-thick GaN layer was grown on a 2″ (0 0 0 1) sapphire substrate buffered with a ∼ 2‐μm-thick GaN layer grown by metal-organic chemical-vapor deposition. During the cool down process to room temperature, cracking was induced in the sapphire substrate, thereby allowing the bulklike GaN layer to relax without provoking cracking of itself. The crystalline quality and the residual strain in the 2″ GaN wafer were investigated by various characterization techniques. The lateral homogeneity of the wafer was monitored by low-temperature photoluminescence mapping. High-resolution x-ray diffraction and photoluminescence measurements proved the high crystalline quality of the material grown. The position of the main near-band-gap photoluminescence line and the phonon spectra obtained from infrared spectroscopic ellipsometry show consistently that the 2″ crack-free GaN is virtually strain-free over a diameter of approximately 4 cm.
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| 18. |
- Malinauskas, T., et al.
(author)
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All-optical characterization of carrier lifetimes and diffusion lengths in MOCVD-, ELO-, and HVPE- grown GaN
- 2007
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In: Journal of Crystal Growth, Vol. 300. - : Elsevier BV. - 0022-0248. ; , s. 223-227
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Conference paper (peer-reviewed)abstract
- The metrological capability of the picosecond four-wave mixing (FWM) technique for evaluation of the photoelectrical properties of GaN heterostructures grown on sapphire, silicon carbide, and silicon substrates as well as of free-standing GaN films is demonstrated. Carrier recombination and transport features have been studied in a wide excitation, temperature, and dislocation density (from ∼1010 to 106 cm-2) range, exploring non-resonant refractive index modulation by a free carrier plasma. The studies allowed to establish the correlations between the dislocation density and the carrier lifetime, diffusion length, and stimulated emission threshold, to reveal a competition between the bimolecular and nonradiative recombination, and to verify the temperature dependence of bimolecular recombination coefficient in the 10-300 K temperature range. It was shown that the FWM technique is more advantageous than the time-resolved photoluminescence technique for determination of carrier lifetimes in high quality thick III-nitride layers. © 2006 Elsevier B.V. All rights reserved.
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| 19. |
- Paskova, Tanja, et al.
(author)
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Effect of Si doping of metalorganic chemical vapor deposition-GaN templates on the defect arrangement in hydride vapor phase epitaxy-GaN overgrown layers
- 2000
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In: Journal of Applied Physics. - 0021-8979 .- 1089-7550. ; 88:10, s. 5729-5732
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Journal article (peer-reviewed)abstract
- Two different types of dislocation arrangements have been observed in hydride vapor-phase epitaxial GaN films grown on sapphire substrates using both undoped and Si-doped GaN templates grown by metalorganic chemical vapor deposition: (i) predominantly straight threading dislocations parallel to the [0001] direction in the layer grown on an undoped template, and (ii) a network of interacting dislocations of edge, screw, and mixed character in the layer grown on a Si-doped template. The two types of defect distribution result in essentially different surface morphologies, respectively: (i) low-angle grain boundaries formed by pure edge dislocations around spiral grown hillocks, and (ii) smooth surface intersected by randomly distributed dislocations. The Si doping of the GaN templates was found to enhance defect interaction in the templates and to enable a reduction of the dislocation density in the overgrown thick GaN films, although it does not lead to an improvement of the overall structural properties of the material. (C) 2000 American Institute of Physics. [S0021-8979(00)08422-X].
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| 20. |
- Paskova, Tanja, et al.
(author)
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Growth of GaN on a-plane sapphire : In-plane epitaxial relationships and lattice parameters
- 2003
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In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 240:2, s. 318-321
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Conference paper (other academic/artistic)abstract
- We have studied GaN films grown on a-plane sapphire substrates by both hydride vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOVPE). The in-plane orientation relationships between the epitaxial films and the substrate are determined to be [11-20]GaN ? [0001] sapphire and [1-100]GaN ? [1-100]sapphire in the HVPE growth, while [1-100]GaN ? [0001]sapphire and [11-20]GaN ? [1-100]sapphire are found in the MOVPE growth. The different orientation preferences are attributed to the atom termination of the sapphire surface determined by the substrate treatment used in the different growth methods. The effect of the lattice matches on the in-plane lattice parameters and strain anisotropy in the two cases is studied. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 21. |
- Paskova, Tanja, et al.
(author)
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Hydride vapour phase homoepitaxial growth of GaN on MOCVD-grown 'templates'
- 2000
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Conference paper (peer-reviewed)abstract
- We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN 'template' layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN 'template'. Additionally the effect of Si doping of the GaN buffer layers on the HVPE-GaN properties was analysed.
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| 22. |
- Paskova, Tanja, et al.
(author)
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In-plane epitaxial relationships between a-plane sapphire substrates and GaN layers grown by different techniques
- 2003
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In: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 257:1-2, s. 1-6
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Journal article (peer-reviewed)abstract
- We have studied GaN films grown on a-plane sapphire substrate by both hydride vapour phase epitaxy (HVPE) and metalorganic vapour phase epitaxy (MOVPE) using X-ray diffraction and transmission electron microscopy. The in-plane orientation relationships between the epitaxial films and the substrate are determined to be [1 1 2¯ 0]GaN?[0 0 0 1] sapphire and [1 1¯ 0 0]GaN?[1 1¯ 0 0] sapphire in the HVPE growth, while [1 1¯ 0 0] GaN?[0 0 0 1]sapphire and [1 1 2¯ 0] GaN?[1 1¯ 0 0]sapphire are found in the MOVPE growth. In a few films of both types a simultaneous presence of two domains, representing the two in-plane relationships, is observed although the preferable one in each type of films is strongly dominating. We propose that the two in-plane orientations of GaN are generally possible on the a-sapphire substrate and are related to the atom termination of the sapphire surface determined by the substrate pre-treatment used in the different growth methods. © 2003 Elsevier B.V. All rights reserved.
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