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Doping of high-Al-content AlGaN grown by MOCVD
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The high-Al-content AlxGa1-xN, x > 0.70, is the principal wide-band-gap alloy system to enable the development of light-emitting diodes operating at the short wavelengths in the deep-ultraviolet, λ < 280 nm. The development of the deep-ultraviolet light-emitting diodes (DUV LEDs) is driven by the social and market impact expected from their implementation in portable units for water disinfection and based on the damaging effect of the deep-ultraviolet radiation on the DNA of various microorganisms. Internationally, intense research and technology developments occur in the past few years, yet, the external quantum efficiency of the DUV LEDs is typically below 1%.

One of the main material issues in the development of the DUV LEDs is the achievement of n- and ptype doped layers of high-Al-content AlxGa1-xN with low resistivity, which is required for the electrical pumping of the diodes. The doping process, however, becomes significantly more complex with increasing the Al content and the resistivity value can be as high as 101-102 Ω cm for n-type AlN doped by silicon, and 107-108 Ω cm for p-type AlN doped by magnesium.

The present study is therefore focused on gaining a better understanding of the constraints in the doping process of the high-Al-content AlxGa1-xN alloys, involving mainly the silicon dopant. For this purpose, the epitaxial growth of the high-Al-content AlxGa1-xN and AlN by the implementation of the distinct hot-wall MOCVD is developed in order to achieve layers of good structural and morphological properties, and with low content of residual impurities, particularly oxygen and carbon. Substitutional point defects such as ON and CN may have a profound impact on the doping by their involvement in effects of n-type carrier compensation. The process temperature can be set from 1000 °C and up to 1400 °C in the present study, which is a principal advantage in order to optimize the material properties of the high-Al-content AlxGa1-xN and AlN. The epitaxial growth of the high-Alcontent AlxGa1-xN and AlN is largely performed on 4H-SiC substrates motivated by (i) the lattice mismatch of ~ 1% along the basal plane (the smallest among other available substrates including Si and sapphire), (ii) the good thermal conductivity of 3.7 W cm-1 K-1, which is essential to minimize the self-heating during the operation of any light-emitting diode, and (iii) the limited access to true-bulk AlN wafers. The Si doping is investigated over a large range of [Si] ~ 1×1017 cm-3 - 1×1020 cm-3. Only the high doping range of [Mg] ~ (1-3)×1019 cm-3 is targeted motivated by the large thermal ionization energy of this common acceptor (from 200 meV in GaN to about 630 meV in AlN). The material characterization involves extensive implementation of atomic force microscopy (AFM), x-ray diffraction (XRD), cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), capacitancevoltage measurements, as well as measurements of the conductivity of the layers by contactless microwave-based technique. The possibility to perform electron paramagnetic resonance (EPR) measurements on the Si-doped high-Al-content AlxGa1-xN is essential in order to establish any effect of self-compensation of the shallow donor state of silicon through the related so-called DX state. The EPR measurements corroborate the study of the incorporation kinetics of silicon and oxygen at various process temperatures and growth rates.

The outcome of this study is accordingly summarized and presents our understanding for (i) the complex impact of silicon and oxygen on the n-type conductivity of Al0.77Ga0.23N, which is the alloy composition at which a drastic reduction of the n-type conductivity of high-Al-content AlxGa1-xN is commonly reported (paper 1); (ii) the strain and morphology compliance during the intentional doping by silicon and magnesium, and its correlation with the resistivity in the highly doped layers of Al0.82Ga0.18N alloy composition (paper2); (iii) the n-type conductivity of highly-Si-doped Al0.72Ga0.28N layers as bound by the process temperature (paper 3); and (iv) the shallow donor or DX behavior of the Si dopant in conductive AlxGa1-xN layers, 0.63 ≤ x ≤ 1 (paper 4). It is noted that the measured n-type conductivity in reference layers of Al0.77Ga0.23N, alternatively Al0.72Ga0.28N, alloy composition is on par with the state-of-the-art values, i.e. ≤ 0.05 Ω cm, and 0.012 Ω cm, respectively. A room-temperature resistivity of 7 kΩ cm is measured in Mg-doped layers of Al0.85Ga0.15N alloy composition, which is superior to the state-of-art values (paper 5). The performance of the transport properties of the high-Al-content AlxGa1-xN layers is expected to improve with improvement of their material quality. This can be achieved by improvement of the crystalline quality of the AlN-on-SiC template and by the implementation of true-bulk AlN substrates. The AlN heteroepitaxial growth at the process temperatures of 1100-1200 °C is therefore investigated (paper 6). The lattice constants, structural and optical properties of true-bulk, homoepitaxial and heteroepitaxial AlN material grown at high process temperatures of up to 1400 °C is further reported (paper 7).

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 43 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1597
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-106733DOI: 10.3384/diss.diva-106733ISBN: 978-91-7519-332-8 (print)OAI: oai:DiVA.org:liu-106733DiVA: diva2:718339
Public defence
2014-06-10, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-05-20 Created: 2014-05-20 Last updated: 2014-05-20Bibliographically approved
List of papers
1. The complex impact of silicon and oxygen on the n-type conductivity of high-Al-content AlGaN
Open this publication in new window or tab >>The complex impact of silicon and oxygen on the n-type conductivity of high-Al-content AlGaN
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2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 13, 132113- p.Article in journal (Refereed) Published
Abstract [en]

Issues of major relevance to the n-type conductivity of Al0.77Ga0.23N associated with Si and O incorporation, their shallow donor or deep donor level behavior, and carrier compensation are elucidated by allying (i) study of Si and O incorporation kinetics at high process temperature and low growth rate, and (ii) electron paramagnetic resonance measurements. The Al0.77Ga0.23N composition correlates to that Al content for which a drastic reduction of the conductivity of AlxGa1−xN is commonly reported. We note the incorporation of carbon, the role of which for the transport properties of AlxGa1−xN has not been widely discussed.

Keyword
aluminium compounds, electrical conductivity, gallium compounds, III-V semiconductors, impurity states, oxygen, paramagnetic resonance, silicon, wide band gap semiconductors
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-91731 (URN)10.1063/1.4800978 (DOI)000317240200047 ()
Available from: 2013-04-30 Created: 2013-04-30 Last updated: 2017-12-06Bibliographically approved
2. Strain and morphology compliance during the intentional doping of high-Al-content AlGaN layers
Open this publication in new window or tab >>Strain and morphology compliance during the intentional doping of high-Al-content AlGaN layers
2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 8, 082106- p.Article in journal (Refereed) Published
Abstract [en]

This study presents analysis of the residual strain and related surface morphology of high-Al-content Al0.82Ga0.18N layers doped by silicon up to the level of 3×1019 cm-3, and delineates an interplay between  thermodynamic and kinetic processes underlying the epitaxial growth of the layers. It particularly points to the development of certain facet structure (nanopipes) within the doped layers, which is apparent at the high Si doping levels. The formation of nanopipes is considered as a matter of consequence for the performance of the transport properties of the layers. It is anticipated to give rise to facets with SiN-related coverage, outcompeting the  incorporation of Si at substitutional donor sites in the lattice of the Al0.82Ga0.18N layers. We do not find evidence for kinetic stabilization of preferential crystallographic facets when a dopant flow of bis(cyclopentadienyl)magnesium (Cp2Mg), instead of silane (SiH4), is implemented in the doping process.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106723 (URN)10.1063/1.4894173 (DOI)000342753500017 ()
Available from: 2014-05-20 Created: 2014-05-20 Last updated: 2017-12-05Bibliographically approved
3. Highly Si-doped Al0.72Ga0.28N layers: n-type conductivity bound by the process temperature
Open this publication in new window or tab >>Highly Si-doped Al0.72Ga0.28N layers: n-type conductivity bound by the process temperature
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2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Establishing n- and p- type conductivity via intentional doping in epitaxial layers is fundamental to any semiconductor material system and its relevant device applications. Process parameters such as temperature, precursor gas-flow-rates and pressure may all control intentional doping in metal-organic chemical vapour deposition (MOCVD) of semiconductor materials. The incorporation of impurities such as carbon and oxygen may also be affected by the same process parameters, and the concentration of these impurities has a direct impact on the electrical, optical and thermal properties of epitaxial layers, as has been observed in the MOCVD of technologically-important III-V semiconductor materials such as AlGaAs and GaN.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106724 (URN)
Available from: 2014-05-20 Created: 2014-05-20 Last updated: 2014-05-20Bibliographically approved
4. On the behavior of the silicon donor in conductive AlxGa1-xN (0.63≤x≤1) layers
Open this publication in new window or tab >>On the behavior of the silicon donor in conductive AlxGa1-xN (0.63≤x≤1) layers
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2015 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 252, no 6, 1306-1310 p.Article in journal (Refereed) Published
Abstract [en]

We have studied the silicon donor behavior in intentionally silicon doped AlxGa1-xN (0.63≤x≤1) grown by hot-wall metal-organic chemical vapor deposition. Efficient silicon doping was obtained for lower Al contents whereas the conductivity drastically reduces for AlGaN layers with Al content in the range x~0.84-1. Degradation of the structural quality and compensation by residual O and C impurities were ruled out as possible explanations for the reduced conductivity. By combining frequency dependent capacitance-voltage and electron paramagnetic resonance measurements we show that the Si donors are electrically active and that the reduced conductivity can be explained by the increased activation energy caused by the sharp deepening of the Si DX state..

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2015
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106725 (URN)10.1002/pssb.201451559 (DOI)000355756200016 ()
Note

Swedish Research Council (VR); VR Linkoping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM); Swedish Energy Agency; Knut and Alice Wallenberg Foundation (KAW); Swedish Governmental Agency for Innovation Systems (VINNOVA)

Available from: 2014-05-20 Created: 2014-05-20 Last updated: 2017-12-05Bibliographically approved
5. Mg-doped Al0.85Ga0.15N layers grown by hot-wall MOCVD with low resistivity at room temperature
Open this publication in new window or tab >>Mg-doped Al0.85Ga0.15N layers grown by hot-wall MOCVD with low resistivity at room temperature
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2010 (English)In: PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, ISSN 1862-6254, Vol. 4, no 11, 311-313 p.Article in journal (Refereed) Published
Abstract [en]

We report on the hot-wall MOCVD growth of Mg-doped AlxGa1-xN layers with an Al content as high as x similar to 0.85. After subjecting the layers to post-growth in-situ annealing in nitrogen in the growth reactor, a room temperature resistivity of 7 k Omega cm was obtained indicating an enhanced p-type conductivity compared to published data for AlxGa1-xN layers with a lower Al content of x similar to 0.70 and a room temperature resistivity of about 10 k Omega cm. It is believed that the enhanced p-type conductivity is a result of reduced compensation by native defects through growth conditions enabled by the distinct hot-wall MOCVD system.

Place, publisher, year, edition, pages
John Wiley and Sons, Ltd, 2010
Keyword
MOCVD, epitaxy, high-Al-content AlGaN, p-type semiconductors, electrical properties
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-62728 (URN)10.1002/pssr.201004290 (DOI)000284206700005 ()
Available from: 2010-12-03 Created: 2010-12-03 Last updated: 2014-05-20
6. High-quality AlN layers grown by hot-wall MOCVD at reduced temperatures
Open this publication in new window or tab >>High-quality AlN layers grown by hot-wall MOCVD at reduced temperatures
2012 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 338, no 1, 52-56 p.Article in journal (Refereed) Published
Abstract [en]

We report on a growth of AlN at reduced temperatures of 1100 C and 1200 C in a horizontal-tube hot-wall metalorganic chemical vapor deposition reactor configured for operation at temperatures of up to 15001600 C and using a joint delivery of precursors. We present a simple route - as viewed in the context of the elaborate multilayer growth approaches with pulsed ammonia supply - for the AlN growth process on SiC substrates at the reduced temperature of 1200 C. The established growth conditions in conjunction with the particular in-situ intervening SiC substrate treatment are considered pertinent to the accomplishment of crystalline, relatively thin, ∼700 nm, single AlN layers of high-quality. The feedback is obtained from surface morphology, cathodoluminescence and secondary ion mass spectrometry characterization.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
A3. Metalorganic chemical vapor deposition; B1. Nitrides; B2. Semiconducting IIIV materials
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-74117 (URN)10.1016/j.jcrysgro.2011.10.052 (DOI)
Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2017-12-08
7. Lattice parameters, structural and optical properties of AlN true bulk, homoepitaxial and heteroepitaxial material grown at high temperatures of up to 1400 °C
Open this publication in new window or tab >>Lattice parameters, structural and optical properties of AlN true bulk, homoepitaxial and heteroepitaxial material grown at high temperatures of up to 1400 °C
2016 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, no 17Article in journal (Refereed) Published
Abstract [en]

The lattice parameters and residual strain of homo- and heteroepitaxial AlN layers grown at elevated process temperatures (1200-1400 °C) by hot-wall MOCVD are studied. The average lattice parameters for the homoepitaxial AlN layers grown on true bulk AlN substrates are determined to be a = 3.1113 ± 0.0001 Å and c = 4.9808 ± 0.0001 Å are discussed in relation to previously published data. The lattice parameters measured from biaxially strained AlN layers grown on SiC are used to determine the biaxial strain relaxation coefficient to be RB = -0.556 ± 0.021. The structural and optical quality of the heteroepitaxial layers improved with increasing layer thickness and at a thickness of 1.3 μm, crack-free AlN of high crystalline quality with full widths at half maximum of the (0002) and (1012) rocking curves of 25 arc sec and 372 arc sec, respectively, were obtained. Tensile strain developed with increasing layer thickness despite the higher crystalline quality of these layers. This can be explained by the thermal mismatch between the AlN and SiC in combination with island coalescence at the initial stage and/or during the growth.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016
National Category
Natural Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-106726 (URN)10.1088/0022-3727/49/17/175108 (DOI)000374146600013 ()
Note

Funding agencies: Swedish Research Council (VR); Linkoping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM, VR); Swedish Governmental Agency for Innovation Systems (VINNOVA)

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Available from: 2014-05-20 Created: 2014-05-20 Last updated: 2017-12-05Bibliographically approved

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