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Optical and structural studies of homoepitaxially grown m-plane GaN
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284, USA.
Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284, USA.
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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. Vol. 100, no 17, 172108- p.
Keyword [en]
GaN, m-plane, Mg-doping, stacking faults, cathodoluminescence
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-75427DOI: 10.1063/1.4706258ISI: 000303340300039OAI: oai:DiVA.org:liu-75427DiVA: diva2:506671
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
In thesis
1. The Effect of Mg Doping on Optical and Structural Properties of GaN
Open this publication in new window or tab >>The Effect of Mg Doping on Optical and Structural Properties of GaN
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Mg is the most commonly used p-type dopant for GaN, however the impact of Mg incorporation on structural, morphological and optical properties of GaN is still not fully understood. Another research challenge is to understand and improve the properties of nonpolar GaN as it allows the fabrication of more efficient optoelectronic devices due to the absence of polarization fields.

Thus, the aim of this thesis was to explore the effect of Mg doping on polar c-plane GaN in Paper 1 and nonpolar m-plane GaN in Paper 2. The samples were grown by metal-organic vapor phase epitaxy with varying Mg content on free-standing GaN substrates. The studies were done by transmission electron microscopy (TEM) and low temperature cathodoluminescence (CL) with the aim to correlate the optical and structural data obtained by these techniques.

Polar c-plane GaN:Mg layers exhibit such structural defects as stackingfaults (SF) of a small size (5-10 nm). The basal plane SF (BSF)density was estimated to be ~ 105 − 106 cm−1 increasing with Mgconcentrations. Comparison between as-grown and annealed sampleshas not shown significant difference in structural or optical properties.Characteristic broad emission lines observed in CL in the rangeof 3.29 − 3.41 eV have been attributed to SF-related emissions byanalogy with nonpolar undoped GaN films grown heteroepitaxially.Acceptor bound exciton (ABE) emission and SF-related peaks havedemonstrated metastability. CL mapping performed on the TEMsamples at the energies corresponding to SF-related peaks has confirmedthat the origin of these lines is associated with Mg-doped GaNlayers. In nonpolar m-plane GaN:Mg layers similar BSFs have been observed. In addition more extended BSFs and prismatic SFs wereidentified at the interface with the GaN substrate. For the m-planesamples with Mg concentration of ~ 1019 cm−3 a number of fine CLlines have been detected in the region of 3.3-3.4 eV. Their shape andappearance were unlike the SF-related emissions in the case of c-planeGaN discussed in Paper 1. These peaks are not likely to be associatedwith donor-acceptor pair (DAP) recombination as has been proved byestimation of the separated DAP energies and by CL mapping experiment.A tentative explanation is given to these peaks as being relatedto excitons bound to some low symmetry acceptor defect centers.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. 33 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1520
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-75428 (URN)9789175199504 (ISBN)
Supervisors
Available from: 2012-02-29 Created: 2012-02-29 Last updated: 2017-12-15Bibliographically approved
2. Doping effects on the structural and optical properties of GaN
Open this publication in new window or tab >>Doping effects on the structural and optical properties of GaN
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:nbn:se:liu:diva-100760 (URN)10.3384/diss.diva-100760 (DOI)978-91-7519-483-7 (ISBN)
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

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