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Doping effects on the structural and optical properties of GaN
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Today there is a strong drive towards higher efficiency light emitters and devices for power electronics based on GaN and its ternary compounds. Device performance can be improved in several ways on the material level. Development of bulk GaN to substitute sapphire and SiC as substrate materials can allow lower defect density epitaxial GaN layers to be grown. Using nonpolar homoepitaxial layers alleviates the problem of polarization fields present in polar GaN epilayers. This thesis advances the field by attacking outstanding problems related to doping and its influence on structural and optical properties of GaN. Optical and structural investigations were performed on bulk GaN grown by halide vapor phase epitaxy (HVPE) and on polar and nonpolar epitaxial GaN grown by metal organic chemical vapor deposition (MOCVD), doped with different impurities: Mg, Si, O or C. Optical characterization was done using photoluminescence (PL), time-resolved photoluminescence (TRPL), and cathodoluminescence (CL) in-situ scanning electron microscope, whereas structural properties were studied by means of transmission electron microscopy (TEM) and atom probe tomography (APT).

A correlation between Mg doping levels and stacking fault (SF) concentration in highly Mg-doped c-plane homoepitaxial GaN layers is found. Increasing Mg concentrations, from 2×1018 cm-3 to 5×1019 cm-3, coincides with increasing density of small, 3-10 nm-sized, SFs. Emission lines ascribed to SFs are observed in CL in all the studied samples. The observed SF-related luminescence can be explained by a model where Mg atoms interacting with the nearby SF changes the confinement for holes and leads to a pronounced defectrelated luminescence. Non-polar m-plane homoepitaxial GaN layers with Mg concentration of 2×1018 cm-3 and 3×1019 cm-3 exhibits high density of basal SFs as well as a number of prismatic SFs. Instead of normally observed in nonpolar GaN SF-related broad lines several sharp lines are detected in the 3.36-3.42 eV region. Their relation to donor-acceptor pair recombination (DAP) was dismissed by calculating the DAP energies and fitting with the measured spectra. The sharp lines are tentatively explained by some impurities bound to point defects or SFs. The origin of two Mg related acceptor bound exciton (ABE) peaks in the emission spectra is also proposed: narrower ABE1 peak at 3.466 eV is identified as coming from a substitutional Mg atom. Broader emission at 3.454 eV is deemed to be coming from a Mg acceptor atom perturbed by a nearby SF. Additionally, Mg cluster formation in the highest doped sample ([Mg] = 1×1020 cm-3) was revealed by APT.

Simultaneous doping by Si and O was studied for HVPE grown bulk GaN. Doping with O concentration from 1017 cm-3 leads to a decrease in the Si concentration to less than 1016 cm-3. Si incorporation is believed to be suppressed by the competing Ga-vacancy-O incorporation process. Bandgap narrowing by 6 meV due to high doping was observed. Donor bound exciton (DBE) lifetime was obtained from TPRL experimental data and it is found to decrease with increasing doping. In non-polar m-plane homoepitaxial GaN Si doping influences the SF-related luminescence. At moderate Si concentrations excitons are bound to the impurity atoms or impurity-SF complex. Proximity of impurity atoms changes the potential for SF creating localization for charge carriers resulting in SF-related emission. At dopant concentrations higher than the Mott limit screening destroys the carrier interaction and, thus, the exciton localization at impurity-SF complex.

Finally, C-doped HVPE grown bulk GaN layers were studied by TEM, CL, and TRPL. Enhanced yellow line (YL) luminescence was observed with increasing C doping. Stability of YL in a wide temperature range (5-300 K) confirms that YL is due to a deep defect, likely CN-ON complex. Low-temperature CL mapping reveals a pit-like structure with different luminescence properties in different areas. DBE emission dominates in CL spectra within the pits while in pit-free areas, in contrast, two ABE lines typical for Mg-doped GaN are observed.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 54 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1554
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-100760DOI: 10.3384/diss.diva-100760ISBN: 978-91-7519-483-7 (print)OAI: oai:DiVA.org:liu-100760DiVA: diva2:663449
Public defence
2013-12-12, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-11-11 Created: 2013-11-11 Last updated: 2016-08-31Bibliographically approved
List of papers
1. Luminescence related to high density of Mg-induced stacking faults in homoepitaxially grown GaN
Open this publication in new window or tab >>Luminescence related to high density of Mg-induced stacking faults in homoepitaxially grown GaN
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 7, 075324- p.Article in journal (Refereed) Published
Abstract [en]

The effect of Mg doping on stacking fault (SF) formation in c-plane GaN grown by metal-organic chemical-vapor deposition has been studied for Mg concentration between 2 x 10(18) cm(-3) and 5 x 10(19) cm(-3). Transmission electron microscopy studies demonstrate a direct correlation between the increasing Mg content and the number of small (3-10-nm long) SFs present. The energy dispersive x-ray analysis (EDX) line profile of a SF shows that the Mg-impurity atom resides at a distance approximately 5 nm from the SF. Cathodoluminescence (CL) mapping reveals that the Mg-doped regions radiate at energies corresponding to known SF emission peaks. SF-related peaks in CL spectra show metastability, which may be attributed to transfer processes involving Mg acceptors and nearby associated SFs.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70330 (URN)10.1103/PhysRevB.84.075324 (DOI)000293830600018 ()
Note
Funding Agencies|Swedish Energy Agency||Swedish Research Council (VR)||Swedish Governmental Agency for Innovation Systems (VINNOVA)||Available from: 2011-09-02 Created: 2011-09-02 Last updated: 2017-12-08
2. Optical and structural studies of homoepitaxially grown m-plane GaN
Open this publication in new window or tab >>Optical and structural studies of homoepitaxially grown m-plane GaN
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2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 17, 172108- p.Article in journal (Refereed) Published
Abstract [en]

We report on cathodoluminescence (CL) and transmission electron microscopy (TEM) studies of m-plane Mg-doped GaN layers grown by metal-organic vapor phase epitaxy (MOVPE). The layers contain basal plane and prismatic stacking faults (SFs) with a density of ~106 cm-1. Broad emission peaks commonly ascribed to SF were found to be insignificant in these samples. However, a set of quite strong and sharp lines were detected in the same spectral region 3.36-3.42 eV. The observed peaks are tentatively explained as excitons bound to some point defects by analogy with p-type GaAs, since donor-acceptor pair (DAP) recombination was ruled out by the CL mapping experiments.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
Keyword
GaN, m-plane, Mg-doping, stacking faults, cathodoluminescence
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-75427 (URN)10.1063/1.4706258 (DOI)000303340300039 ()
Note
funding agencies|Swedish Energy Agency||Swedish Research Council (VR) Linnaeus Environment LiLi-NFM at Linkoping||Carl Trygger Foundation||Swedish Governmental Agency for Innovation Systems (VINNOVA)||National Science Foundation, Division of Materials Research||Available from: 2012-02-29 Created: 2012-02-29 Last updated: 2017-12-07Bibliographically approved
3. Luminescence of Acceptors in Mg-Doped GaN
Open this publication in new window or tab >>Luminescence of Acceptors in Mg-Doped GaN
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2013 (English)In: Japanese Journal of Applied Physics, ISSN 0021-4922, E-ISSN 1347-4065, Vol. 52, no 8Article in journal (Refereed) Published
Abstract [en]

Recent photoluminescence (PL) data for Mg-doped GaN at 2 K are discussed, with reference to published theoretical calculations of the electronic level structure. It is concluded that the typical PL peaks at 3.466 eV (acceptor bound exciton ABE1) and the broader 3.27 eV donor-acceptor pair (DAP) PL are the expected standard PL signatures of the substitutional Mg acceptor. Additional broader peaks at 3.455 eV (ABE2) and 3.1 eV are suggested to be related to the same acceptors perturbed by nearby basal plane stacking faults. The low temperature metastability of PL spectra is assigned to a nonradiative metastable deep level.

Place, publisher, year, edition, pages
Japan Society of Applied Physics, 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-98151 (URN)10.7567/JJAP.52.08JJ03 (DOI)000323883100123 ()
Note

Funding Agencies|K. A. Wallenberg Foundation||Swedish Energy Agency||Li-Li NFM Center at Linkoping University||

Available from: 2013-09-30 Created: 2013-09-30 Last updated: 2017-12-06
4. Atom probe tomography study of Mg doped GaN layers
Open this publication in new window or tab >>Atom probe tomography study of Mg doped GaN layers
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2014 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 25, no 27, 275701- p.Article in journal (Refereed) Published
Abstract [en]

Atom probe tomography studies on highly Mg-doped GaN(0001) layers with concentrations 5×1019 cm-3 and 1×1020 cm-3 were performed. Mg cluster formation was observed only in the higher doped sample whereas in the lower doped sample the Mg distribution was homogeneous. CL measurements showed that the emission normally attributed to stacking faults was only present in the lower doped layers ([Mg] = 1.5×1019 and 5×1019 cm-3), but absent in the higher-doped layer, where Mg clusters were detected. Mg clusters are proposed to produce a screening effect thereby destroying the exciton binding on the SFs thus rendering them optically inactive.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2014
Keyword
Mg-doped GaN, stacking faults, Mg clusters, atomic probe tomography, cathodoluminescence
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-100752 (URN)10.1088/0957-4484/25/27/275701 (DOI)000338715800013 ()
Available from: 2013-11-11 Created: 2013-11-11 Last updated: 2017-12-06Bibliographically approved
5. Effect of silicon and oxygen doping on donor bound excitons in bulk GaN
Open this publication in new window or tab >>Effect of silicon and oxygen doping on donor bound excitons in bulk GaN
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 16, 165213- p.Article in journal (Refereed) Published
Abstract [en]

Freestanding n-type intentionally doped GaN layers grown by halide vapor phase epitaxy (HVPE) were studied by transient photoluminescence (PL). Concentrations of silicon and oxygen were varied in the range between 10(17) and 10(18) cm(-3), as confirmed by secondary ion mass spectroscopy (SIMS). We show that a reduction of the background silicon concentration by one order of magnitude compared to the background level in undoped samples can be achieved by incorporation of oxygen during the growth. A strong band gap narrowing (BGN) of similar to 6 meV was observed with increasing doping in the studied samples. The low temperature PL recombination time for donor-bound excitons (DBEs) was found to depend significantly on donor concentration. A model assuming generation of DBEs by capturing of free excitons by neutral donors explains the experimental results at low temperature. From fitting the experimental DBE lifetime to the model, the donor concentration dependence for O and Si donors could be reproduced. An effective exciton capture cross-section was found to be similar to 9.4 x 10(-15) and 1.2 x 10(-14) cm(2) for silicon and oxygen donors, respectively.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-72660 (URN)10.1103/PhysRevB.84.165213 (DOI)000296856100006 ()
Note
Funding Agencies|Swedish Research Council (VR)||Swedish Energy Agency||Swedish Governmental Agency for Innovation Systems (VINNOVA)||Available from: 2011-12-02 Created: 2011-12-02 Last updated: 2017-12-08
6. Correlation between Si doping and stacking fault related luminescence in homoepitaxial m-plane GaN
Open this publication in new window or tab >>Correlation between Si doping and stacking fault related luminescence in homoepitaxial m-plane GaN
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2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, 192101- p.Article in journal (Refereed) Published
Abstract [en]

Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-100756 (URN)10.1063/1.4828820 (DOI)000327817000031 ()
Available from: 2013-11-11 Created: 2013-11-11 Last updated: 2017-12-06Bibliographically approved
7. Effect of C-doping on near-band gap luminescence in bulk GaN substrates grown by halide vapor phase epitaxy
Open this publication in new window or tab >>Effect of C-doping on near-band gap luminescence in bulk GaN substrates grown by halide vapor phase epitaxy
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2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Freestanding bulk C-doped GaN substrates grown by halide vapor phase epitaxy were studied by optical spectroscopy and electron microscopy. In cathodoluminescence (CL) the yellow line (YL) was more intense in samples with higher C content and stable in the temperature range 5-300 K. CL mapping in situ a scanning electron microscope revealed pitlike structure of the layers with higher YL intensity in the pits related to higher local oxygen incorporation. Near bandgap (NBG) emission studies of the pits revealed donor-bound excitons (DBE) with broad emission and no significant acceptor bound exciton (ABE) emission. Pit-free areas demonstrate two well-resolved ABEs with DBE quenched. Quenching of the DBE is explained by potential fluctuations in the vicinity of the carbon atoms in the pits-free regions lowering the ionization barrier for DBE.

Keyword
Bulk GaN, carbon doping, luminescence, bound excitons
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-100758 (URN)
Available from: 2013-11-11 Created: 2013-11-11 Last updated: 2016-08-31Bibliographically approved

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