| 1. |
- Godlewski, M., et al.
(author)
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In-plane and in-depth nonuniformities in defect distribution in GaN and InGaN epilayers
- 2001
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In: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308-310, s. 102-105
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Journal article (peer-reviewed)abstract
- The in-plane and in-depth characteristics of the GaN and InGaN epilayers grown by the metalorganic chemical vapour deposition (MOCVD) on three different substrates (sapphire, SiC and bulk GaN) are evaluated. Relatively large intensity fluctuations of "edge" GaN and InGaN emissions are observed and are related to the details of the micro-structure of the GaN and InGaN films studied. The experiments indicate a nonuniform defect distribution in all types of the MOCVD films studied. In particular, the decoration of structural defects with impurities, an increased defect accumulation at the interfaces and a surprisingly small influence of the micro-structure on the in-plane homogeneity of the yellow band cathodoluminescence emission are observed. © 2001 Elsevier Science B.V. All rights reserved.
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| 2. |
- Paskova, Tanja, 1961-, et al.
(author)
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Characterization of mass-transport grown GaN by hydride vapour-phase epitaxy
- 2004
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In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 273:1-2, s. 118-128
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Journal article (peer-reviewed)abstract
- A comprehensive study of the morphological, optical and microstructural properties of mass-transport (MT) and conventionally grown GaN by hydride vapour-phase epitaxy is presented. Spatially resolved techniques have been utilized to reveal in a comparative way, the characteristics of the material grown either in predominant vertical or lateral growth modes. A strong donor-acceptor pair (DAP) emission is observed from the MT regions with a distinctive intensity contrast between the exciton and DAP emission bands from MT and nontransport regions. Secondary ion mass spectroscopy and imaging were employed to investigate the impurity incorporation into different regions. An increase of residual oxygen and aluminium impurity concentrations was found in the MT areas. In addition, positron annihilation spectroscopy showed a strong signal of Ga vacancy clusters in the MT grown material. The increase of the point defect concentrations of both Ga vacancy and oxygen impurity, most likely forming defect complexes, is related to the enhancement of the DAP emission.
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| 3. |
- 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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| 4. |
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| 5. |
- 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
-
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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| 6. |
- Buyanova, Irina A., et al.
(author)
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Strain relaxation in GaNxP1-x alloy : Effect on optical properties
- 2001
-
In: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 308-310, s. 106-109
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Journal article (peer-reviewed)abstract
- By using scanning electron microscopy and cathodoluminescence (CL), a decrease in radiative efficiency of GaNP alloy with increasing N content is seen due to the formation of structural defects. The defect formation is attributed to relaxation of tensile strain in the GaNP layer, which is lattice mismatched to GaP substrate. Several types of extended defects including dislocations, microcracks and pits are revealed in partly relaxed GaNxP1-x epilayers with x=1.9%, whereas coherently strained layers exhibit high crystalline quality for x up to 4%. According to the CL measurements, all extended defects act as competing, non-radiative channels leading to the observed strong decrease in the radiative efficiency. From CL mapping experiments, non-uniformity of strain distribution around the extended defects is partly responsible for the broadening of the photoluminescence (PL) spectra recorded in the macro-PL experiments. © 2001 Elsevier Science B.V. All rights reserved.
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| 7. |
- Paskova, Tanja, et al.
(author)
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Influence of growth rate on the structure of thick GaN layers grown by HVPE
- 2000
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In: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 208:1, s. 18-26
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Journal article (peer-reviewed)abstract
- Thick GaN films grown by hydride vapour phase epitaxy have been investigated by cathodoluminecsence, X-ray diffraction, and photoluminescence. Cross-sectional studies of thick GaN layers grown on sapphire without buffers reveal three zones: a highly disordered interface region, a columnar defective region and a good quality main region of the layer. The influence of the highly doped columnar region on the surface morphology and crystal structure of the layers has been studied. We show that the columnar region influences the material quality more strongly in thinner films. Thicker layers exhibit improved morphology with lower surface pit density and better crystal quality shown in photoluminescence and X-ray diffraction spectra. The relationship between the near-interface columnar structures and surface pits is revealed. A strong effect of the growth rate on the structure of thick layers is found. The results suggest that GaN layers with optimum crystalline quality may be obtained by varying the growth rate during growth.
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| 8. |
- Paskova, Tanja, et al.
(author)
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Mass transport growth and properties of hydride vapour phase epitaxy GaN
- 2001
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In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 188:1, s. 447-451
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Journal article (peer-reviewed)abstract
- We report a comparative study of the optical and structural properties of mass-transport and conventionally grown GaN by hydride vapor phase epitaxy. A strong donor-acceptor pair emission is observed from the mass-transport regions with a distinctive intensity contrast between the exciton and donor-acceptor bands from mass-transport and nontransport regions. Secondary ion mass spectroscopy was employed to investigate the impurity incorporation into different regions. A moderate increase of residual impurity incorporation or redistribution was found in mass-transport regions related to different growth modes.
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| 9. |
- Buyanova, Irina, 1960-, et al.
(author)
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Optical and electronic properties of GaNAs/GaAs structures
- 2000
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Conference paper (peer-reviewed)abstract
- We review our recent results from studies of electronic properties of GaNAs/GaAs structures with low nitrogen content, by photoluminescence (PL), PL excitation, time-resolved PL spectroscopies as well as optically detected magnetic resonance (ODMR) and cyclotron resonance (ODCR) studies. The issues to be addressed include key material-related properties and fundamental electronic parameters of the GaNAs alloy, relevant to device applications, such as identification of the dominant recombination processes in the alloy, compositional dependence of the electron effective mass and band alignment in the GaNAs/GaAs heterostructures.
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| 10. |
- Buyanova, Irina, 1960-, et al.
(author)
-
Raman Studies of GaNP Alloy
- 2002
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Conference paper (peer-reviewed)abstract
- Raman scattering (RS) spectroscopy is employed to characterize the effect of nitrogen on structural properties of GaNxP1-x alloy with nitrogen composition up to 3 %. Two-mode behavior of the alloy is clearly shown. The frequency of the GaP-like LO phonons is found to decrease with N composition as -1.13 cm-1 x. This dependence is proposed to be largely due to the biaxial strain in the GaNP epilayers, as a result of lattice mismatch to the GaP substrate. The frequency of the GaN-like phonons is found to be more sensitive to nitrogen content, increasing with the rate of +2.6 cm-1x. The addition of nitrogen is also found to cause a dramatic quenching of the two-phonon Raman scattering and an appearance of the zone edge GaP-like vibrations. These effects are suggested to reflect local distortion in the GaNP lattice induced by nitrogen, as well as possible clustering of N atoms.
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| 11. |
- Buyanova, Irina, 1960-, et al.
(author)
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Structural properties of a GaNxP1-x alloy : Raman studies
- 2001
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 78:25, s. 3959-
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Journal article (peer-reviewed)abstract
- Raman measurements in backscattering configuration are employed to characterize the effect of nitrogen on the structural properties of a GaNxP1-x alloy with x<=3%. The following effects of N incorporation on the vibrational spectra of GaNP are observed. First, frequencies of GaP-like and GaN-like longitudinal optical phonons exhibit strong compositional dependence, due to a combined effect of alloying and biaxial strain. Second, a dramatic quenching of two-phonon Raman scattering and an emergence of zone-edge GaP-like vibrations are observed. These effects are tentatively attributed to a local distortion of the GaNP lattice and/or compositional disorder in the alloy.
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| 12. |
- Gryczynski, I., et al.
(author)
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Directional two-photon induced surface plasmon-coupled emission
- 2005
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090. ; 491:1-2, s. 173-176
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Journal article (peer-reviewed)abstract
- We measured a directional surface plasmon-coupled emission (SPCE) induced by a two-photon absorption. A 60 nm thick layer of poly(vinyl alcohol) film doped with rhodamine 123 was deposited on a silvered (50 nm Ag) glass slide, which was attached to a hemicylindrical glass prism. The 820 nm excitation from a femtosecond Ti:Sapphire laser was used either in reverse Kretschmann or Kretschmann configuration. The angular distribution of two-photon induced SPCE does not depend on the used configuration. The two-photon induced SPCE can be applied to improve immunoassays and deoxyribonucleic acid detection. © 2005 Elsevier B.V. All rights reserved.
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| 13. |
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| 14. |
- Paskova, Tanja, et al.
(author)
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Donor-acceptor pair emission enhancement in mass-transport-grown GaN
- 2005
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:3, s. 033508-
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Journal article (peer-reviewed)abstract
- A dominating donor-acceptor pair (DAP) emission at about 3.27 eV is observed in the photoluminescence and cathodoluminescence spectra of intentionally undoped mass-transport (MT)-grown GaN, while only a weak presence of the DAP emission is recorded in the as-grown hydride vapor phase epitaxial GaN. A comparative study of impurity and native defect incorporation in the as-grown and MT GaN was performed, showing a significant increase of oxygen and empty clusters involving Ga vacancy and oxygen in the MT GaN. Based on the observed results as well as on doping analysis of the structure and kinetic analysis of the emission intensities, we propose an acceptorlike complex, creating a state as a semiclassical potential well near the valence-band top due to the local tensile strain caused by the empty clusters to be responsible for the dominating behavior of the DAP emission. © 2005 American Institute of Physics.
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| 15. |
- Paskova, Tanja, et al.
(author)
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Mass transport growth and optical emission properties of hydride vapor phase epitaxy GaN
- 2001
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 78:26, s. 4130-4132
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Journal article (peer-reviewed)abstract
- The optical emission properties of mass-transport regions of GaN grown by hydride vapor phase epitaxy are studied by cathodoluminescence imaging and spectroscopy. A strong donor-acceptor pair emission is observed from the mass-transport regions. Spatially resolved cathodoluminescence reveals a strong intensity contrast between the exciton and donor-acceptor bands from mass-transport and nontransport regions. Focused Auger electron and x-ray photoelectron spectroscopies were employed to investigate the impurity incorporation in the different regions. A preferential moderate increase of residual impurity incorporation or redistribution in mass-transport regions is suggested to be responsible for the observed change of the dominant radiative mechanism. © 2001 American Institute of Physics.
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