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Radiation-induced defects in GaN bulk grown by halide vapor phase epitaxy
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-9840-7364
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
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2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 10, 102103- p.Article in journal (Refereed) Published
Abstract [en]

Defects induced by electron irradiation in thick free-standing GaN layers grown by halide vapor phase epitaxy were studied by deep level transient spectroscopy. In as-grown materials, six electron traps, labeled D2 (E-C-0.24 eV), D3 (E-C-0.60 eV), D4 (E-C-0.69 eV), D5 (E-C-0.96 eV), D7 (E-C-1.19 eV), and D8, were observed. After 2MeV electron irradiation at a fluence of 1 x 10(14) cm(-2), three deep electron traps, labeled D1 (E-C-0.12 eV), D5I (E-C-0.89 eV), and D6 (E-C-1.14 eV), were detected. The trap D1 has previously been reported and considered as being related to the nitrogen vacancy. From the annealing behavior and a high introduction rate, the D5I and D6 centers are suggested to be related to primary intrinsic defects.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014. Vol. 105, no 10, 102103- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-111755DOI: 10.1063/1.4895390ISI: 000342758700028OAI: oai:DiVA.org:liu-111755DiVA: diva2:759769
Note

Funding Agencies|Swedish Research Council (VR); Swedish Energy Agency

Available from: 2014-10-31 Created: 2014-10-31 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Investigation of deep levels in bulk GaN
Open this publication in new window or tab >>Investigation of deep levels in bulk GaN
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The first gallium nitride (GaN) crystal was grown by hydride vapor phase epitaxy in 1969 by Maruska and Tietjen and since then, there has been an intensive development of the field, especially after the ground breaking discoveries concerning growth and p-type doping of GaN done by the 2014 year Nobel Laureates in Physics, Isamu Akasaki, Hiroshi Amano and Shuji Nakamura. GaN and its alloys with In and Al belong to a semiconductor group which is referred as the III-nitrides. It has outstanding properties such as a direct wide bandgap (3.4 eV for GaN), high breakdown voltage and high electron mobility. With these properties, GaN is a promising material for a variety of applications in electronics and optoelectronics. The perhaps most important application is GaN-based light-emitting-diodes (LED) which can produce a highbrightness blue light. Since the bandgap of GaN can be controlled by alloying it with aluminium (Al) or indium (In) for a larger or smaller bandgap, respectively, GaN is very important for optoelectronic applications from infrared to the deep ultraviolet region. There are other semiconductors with bandgap similar to GaN such as SiC, and the first commercially blue light emitting LEDs where manufactured in SiC, however, SiC has an indirect bandgap with a low efficiency of emitting photons, and today, the SiC based LEDs have been completely replaced by the considerable more efficient GaN based LEDs.

One problem, which has hampered the development of GaN based devices, is the lack of native substrate of GaN. Due to that, most of the GaN based devices are fabricated on foreign substrates such as SiC or Al2O3. Growing on a foreign substrate results in high threading dislocation (TD) densities (~109 cm-2) and stress in the GaN layer due to lattice mismatch and difference of thermal expansion coefficient between GaN and the substrate. The high TD density and the stress influence the performance of the devices.

Another important aspect related to GaN which has attracted manystudies is how defects affect the efficiency of GaN-based devices.Therefore, it is necessary to understand the properties and to identifythem. When we know there properties, one can estimate how they willinfluence the behavior of devices, and thereby, optimize the performance of the device for its application. Basically, a fundamental knowledge of defect properties, and how to introduce them in a controlled manner, or to avoid them, is important in order to optimize the performance of devices. Defects can be introduced both intentionally and unintentionally into semiconductors during the growth process, during processing of the device or from the working environment, for example, devices working in a radioactive ambient are more likely to have defects induced by irradiation.

This thesis is focused on electrical characterization of defects in bulk GaN grown by halide vapor phase epitaxy (HVPE) by using deep level transient spectroscopy. Other measurement techniques like currentvoltage measurement (IV), capacitance-voltage measurement (CV) and Hall measurement were also been used. Defects related to the growth process and the polishing process are discussed in Paper 1. In Paper 2 and Paper 3, we focus on intrinsic defects in GaN introduced intentionally by electron irradiation. This type of defects are important since they can be unintentionally introduced during growth of the material, in the fabrication process of devices or if it is exposed to a radioactive environment. By electron irradiation, we can in a controlled manner introduce intrinsic defects for studies and by varying the electron beam energy and doses we can judge the nature of them. After electron irradiation, we observed several electrically active defects. These defects were characterized by DLTS to get important parameters such as activation energy, trap concentration, trap profile and capture cross-section. Especially, from temperature-dependent capacitance transient studies, we have determined the mechanism of the electron capturing process for some of them.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 52 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1696
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-112262 (URN)10.3384/lic.diva-112262 (DOI)978-91-7519-169-0 (ISBN)
Presentation
2014-12-18, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Available from: 2014-11-20 Created: 2014-11-20 Last updated: 2015-09-22Bibliographically approved
2. Electronic properties of intrinsic defects and impurities in GaN
Open this publication in new window or tab >>Electronic properties of intrinsic defects and impurities in GaN
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With its outstanding properties such as a wide direct bandgap (3.4 eV), high electron mobility and high breakdown voltage, GaN and its alloys with In and Al are considered as one of the most important semiconductors for optoelectronic devices and high-power and high-frequency transistors. The most important application of GaN today is high-brightness blue LEDs, which is used for white LEDs. With the discovery of GaN-based blue LED, Isamu Akasaki, Hiroshi Amano and Shuji Nakamura were awarded the Nobel Prize in 2014. Intrinsic defects and impurities are important in semiconductors since they influence the electronic properties. An impurity is one or several foreign atoms in the host crystal while an intrinsic defect is an imperfection in the host’s crystal lattice. Normally, impurities and intrinsic defects can be introduced either intentionally or unintentionally into semiconductors during the growth process, during processing of the device or from the working environment. Especially for GaN, due to the lack of native substrates, most of the GaN-based device structures are fabricated on foreign substrates such as silicon carbide (SiC) or sapphire (Al2O3). Growth on foreign substrates gives rise to high threading dislocation densities, and they can give rise to electronically active intrinsic defects that influence the performance of the device.

This thesis is focused on electrical characterization of intrinsic defects and impurities in GaN grown by halide vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOCVD). In the first part of the thesis, impurities and intrinsic defects in freestanding thick HVPE grown GaN and Mg-doped MOCVD grown GaN is studied. In thick HVPE grown GaN, six electron traps were detected, where two of them were introduced by the polishing process. For three of the traps, the temperature dependence of the electron capture cross section was studied. From their electron capture properties, it was suggested that the traps are  associated with point defects. In Mg-doped MOCVD grown GaN, one hole trap of high concentration was observed. The hole emission rate is enhanced by increasing electric field and by study the emission process in detail by simulation, it is suggested that the emission process is governed by both the Poole-Frenkel effect and phonon-assisted tunneling.

In the second part, intrinsic defects in GaN introduced intentionally by electron irradiation with different fluences have been studied. In electron irradiated HVPE grown GaN, three electron-irradiation-induced electron traps appeared after 2 MeV electron irradiation at a fluence of 1 × 1014 cm2. Due to the annealing behavior, two of the levels were suggested to be related to primary intrinsic defects. In addition, the temperature dependence of the electron capture cross sections for three levels in electron-irradiated GaN was studied. The temperature dependence of one of them showed that the electron capturing is governed by a cascade capturing process, whereas no temperature dependence was observed for the other levels. The thermal stability of electron traps introduced by 2 MeV electron irradiation was studied. Isochronal annealing shows that most of the defects, which has been associated to nitrogen vacancies, annealed out already at 550 K and by using isothermal annealing the activation energy of one of the process was determined. By minority carrier spectroscopy and deep level transient spectroscopy, hole and electron traps in as-grown and 2 MeV ntype electron irradiated GaN were studied. One hole trap was observed in the as-grown material. By electron irradiation, it was observed that the concentration increases. Simultaneously, the concentration of two electron traps increases. Due to the low introduction rate of one of the electron traps, it is suggested that the defect is associated with a primary defect decorating extended structural defects. The high introduction rate of the hole trap suggests that the defect is associated with a primary intrinsic defect or a complex.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 55 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1701
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-121710 (URN)10.3384/diss.diva-121710 (DOI)978-91-7685-950-6 (ISBN)
Public defence
2015-10-26, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-10-02 Created: 2015-10-02 Last updated: 2015-10-07Bibliographically approved

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