Gold free contacts to AlGaN/GaN heterostructures

Marcin Hajlasz

Research output: ThesisPhD Thesis - Research UT, graduation UTAcademic

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Abstract

Transistors and diodes based on AlGaN/GaN are suitable candidates for high-voltage and high-speed electronics due to the GaN material properties such as wide bandgap, large breakdown field, high electron saturation velocity and good thermal conductivity. When thin AlGaN layer is grown epitaxially on GaN, it is typically under tensile stress. Consequently, we observe piezoelectric polarization, which is additional to the spontaneous polarization in these materials. The polarization induced sheet charge density forces free electrons to try to compensate for it. These charges accumulate at the heterojunction of AlGaN and GaN layers to create a Two-Dimensional Electron Gas. The electrons are confined to the interface due to band bending in the GaN, and in order to make a contact to them, it is necessary to pass through the AlGaN layer.
Due to the complexity of AlGaN/GaN devices, they are not yet fully understood. This is especially true for the electrical contacts, where the relatively well characterized and described Au-containing metal stacks have to be replaced by Au-free stacks to allow production at scale. The mass production of GaN devices, required for consumer applications of GaN, makes it necessary to use existing Si fabrication facilities, where it is not allowed to use gold, as very low concentrations lead to strong performance degradation of Si devices. Therefore, it is crucial to investigate alternative metal stacks, such as for example TiW, as a replacement for gold. The aim of this thesis is to broaden the understanding of Ohmic and Schottky contact to AlGaN/GaN by means of material characterization techniques and novel electrical test structures.
The research on the Ohmic contacts in this thesis aims at providing insight about the electrical transport mechanism from the metal to the 2DEG in the AlGaN/GaN heterostructures. An in-depth study of the nature of these contacts, with emphasis on the nature of the charge transport into the 2DEG, the reactions occurring at the interfaces between the metals and the GaN is presented. For this purpose, accurate measurement structures were designed and new characterization methods have been used. For the Schottky gate structures, metals and alloys with the required high work function were investigated. An in-depth study of the nature of the contact was made. Phenomena specific to the AlGaN/GaN, such as metal stack strain influence on the Schottky barrier and barrier height inhomogeneity was investigated.
Original languageEnglish
Awarding Institution
  • University of Twente
Supervisors/Advisors
  • Gravesteijn, Dirk J, Supervisor
Award date25 May 2018
Place of PublicationEnschede
Publisher
Print ISBNs978-90-365-4538-9
DOIs
Publication statusPublished - 25 May 2018

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gold
metals
electric contacts
theses
polarization
electrons
tensile stress
free electrons
heterojunctions
low concentrations
high voltages
inhomogeneity
transistors
thermal conductivity
breakdown
diodes
high speed
degradation
saturation
fabrication

Cite this

Hajlasz, Marcin. / Gold free contacts to AlGaN/GaN heterostructures. Enschede : University of Twente, 2018. 109 p.
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title = "Gold free contacts to AlGaN/GaN heterostructures",
abstract = "Transistors and diodes based on AlGaN/GaN are suitable candidates for high-voltage and high-speed electronics due to the GaN material properties such as wide bandgap, large breakdown field, high electron saturation velocity and good thermal conductivity. When thin AlGaN layer is grown epitaxially on GaN, it is typically under tensile stress. Consequently, we observe piezoelectric polarization, which is additional to the spontaneous polarization in these materials. The polarization induced sheet charge density forces free electrons to try to compensate for it. These charges accumulate at the heterojunction of AlGaN and GaN layers to create a Two-Dimensional Electron Gas. The electrons are confined to the interface due to band bending in the GaN, and in order to make a contact to them, it is necessary to pass through the AlGaN layer.Due to the complexity of AlGaN/GaN devices, they are not yet fully understood. This is especially true for the electrical contacts, where the relatively well characterized and described Au-containing metal stacks have to be replaced by Au-free stacks to allow production at scale. The mass production of GaN devices, required for consumer applications of GaN, makes it necessary to use existing Si fabrication facilities, where it is not allowed to use gold, as very low concentrations lead to strong performance degradation of Si devices. Therefore, it is crucial to investigate alternative metal stacks, such as for example TiW, as a replacement for gold. The aim of this thesis is to broaden the understanding of Ohmic and Schottky contact to AlGaN/GaN by means of material characterization techniques and novel electrical test structures.The research on the Ohmic contacts in this thesis aims at providing insight about the electrical transport mechanism from the metal to the 2DEG in the AlGaN/GaN heterostructures. An in-depth study of the nature of these contacts, with emphasis on the nature of the charge transport into the 2DEG, the reactions occurring at the interfaces between the metals and the GaN is presented. For this purpose, accurate measurement structures were designed and new characterization methods have been used. For the Schottky gate structures, metals and alloys with the required high work function were investigated. An in-depth study of the nature of the contact was made. Phenomena specific to the AlGaN/GaN, such as metal stack strain influence on the Schottky barrier and barrier height inhomogeneity was investigated.",
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Gold free contacts to AlGaN/GaN heterostructures. / Hajlasz, Marcin.

Enschede : University of Twente, 2018. 109 p.

Research output: ThesisPhD Thesis - Research UT, graduation UTAcademic

TY - THES

T1 - Gold free contacts to AlGaN/GaN heterostructures

AU - Hajlasz, Marcin

PY - 2018/5/25

Y1 - 2018/5/25

N2 - Transistors and diodes based on AlGaN/GaN are suitable candidates for high-voltage and high-speed electronics due to the GaN material properties such as wide bandgap, large breakdown field, high electron saturation velocity and good thermal conductivity. When thin AlGaN layer is grown epitaxially on GaN, it is typically under tensile stress. Consequently, we observe piezoelectric polarization, which is additional to the spontaneous polarization in these materials. The polarization induced sheet charge density forces free electrons to try to compensate for it. These charges accumulate at the heterojunction of AlGaN and GaN layers to create a Two-Dimensional Electron Gas. The electrons are confined to the interface due to band bending in the GaN, and in order to make a contact to them, it is necessary to pass through the AlGaN layer.Due to the complexity of AlGaN/GaN devices, they are not yet fully understood. This is especially true for the electrical contacts, where the relatively well characterized and described Au-containing metal stacks have to be replaced by Au-free stacks to allow production at scale. The mass production of GaN devices, required for consumer applications of GaN, makes it necessary to use existing Si fabrication facilities, where it is not allowed to use gold, as very low concentrations lead to strong performance degradation of Si devices. Therefore, it is crucial to investigate alternative metal stacks, such as for example TiW, as a replacement for gold. The aim of this thesis is to broaden the understanding of Ohmic and Schottky contact to AlGaN/GaN by means of material characterization techniques and novel electrical test structures.The research on the Ohmic contacts in this thesis aims at providing insight about the electrical transport mechanism from the metal to the 2DEG in the AlGaN/GaN heterostructures. An in-depth study of the nature of these contacts, with emphasis on the nature of the charge transport into the 2DEG, the reactions occurring at the interfaces between the metals and the GaN is presented. For this purpose, accurate measurement structures were designed and new characterization methods have been used. For the Schottky gate structures, metals and alloys with the required high work function were investigated. An in-depth study of the nature of the contact was made. Phenomena specific to the AlGaN/GaN, such as metal stack strain influence on the Schottky barrier and barrier height inhomogeneity was investigated.

AB - Transistors and diodes based on AlGaN/GaN are suitable candidates for high-voltage and high-speed electronics due to the GaN material properties such as wide bandgap, large breakdown field, high electron saturation velocity and good thermal conductivity. When thin AlGaN layer is grown epitaxially on GaN, it is typically under tensile stress. Consequently, we observe piezoelectric polarization, which is additional to the spontaneous polarization in these materials. The polarization induced sheet charge density forces free electrons to try to compensate for it. These charges accumulate at the heterojunction of AlGaN and GaN layers to create a Two-Dimensional Electron Gas. The electrons are confined to the interface due to band bending in the GaN, and in order to make a contact to them, it is necessary to pass through the AlGaN layer.Due to the complexity of AlGaN/GaN devices, they are not yet fully understood. This is especially true for the electrical contacts, where the relatively well characterized and described Au-containing metal stacks have to be replaced by Au-free stacks to allow production at scale. The mass production of GaN devices, required for consumer applications of GaN, makes it necessary to use existing Si fabrication facilities, where it is not allowed to use gold, as very low concentrations lead to strong performance degradation of Si devices. Therefore, it is crucial to investigate alternative metal stacks, such as for example TiW, as a replacement for gold. The aim of this thesis is to broaden the understanding of Ohmic and Schottky contact to AlGaN/GaN by means of material characterization techniques and novel electrical test structures.The research on the Ohmic contacts in this thesis aims at providing insight about the electrical transport mechanism from the metal to the 2DEG in the AlGaN/GaN heterostructures. An in-depth study of the nature of these contacts, with emphasis on the nature of the charge transport into the 2DEG, the reactions occurring at the interfaces between the metals and the GaN is presented. For this purpose, accurate measurement structures were designed and new characterization methods have been used. For the Schottky gate structures, metals and alloys with the required high work function were investigated. An in-depth study of the nature of the contact was made. Phenomena specific to the AlGaN/GaN, such as metal stack strain influence on the Schottky barrier and barrier height inhomogeneity was investigated.

U2 - 10.3990/1.9789036545389

DO - 10.3990/1.9789036545389

M3 - PhD Thesis - Research UT, graduation UT

SN - 978-90-365-4538-9

PB - University of Twente

CY - Enschede

ER -

Hajlasz M. Gold free contacts to AlGaN/GaN heterostructures. Enschede: University of Twente, 2018. 109 p. https://doi.org/10.3990/1.9789036545389