AlGaN/GaN MBE 2DEG heterostructures : interplay between surface-, interface- and device-properties

Due to their wide band gap and high bond strength, the III-N semiconductors can be used for violet, blue and green light emitting devices as well as for high temperature and high power transistors. In comparison to conventional AlGaAs/GaAs heterostructures the AlGaN/GaN material system offers new properties, which are suitable for HEMT (High Electron Mobility Transistor) device application. The aim of the work was to optimize the growth of AlGaN/GaN HEMT structure on different substrates and to study the surface and interface properties which form the knowledge basis for understanding and improving the performance of AlGaN/GaN HEMTs. The growth of III-nitride samples was performed by molecular beam epitaxy with Gallium, Aluminium, Silicon effusion cells and an UNI-bulb RF-plasma source which provides the active nitrogen. The III-nitride layers were deposited on SiC and Si substrates. The structural, surface and electronic properties of the grown layers were investigated. The optimized AlGaN/GaN HEMT structure grown on insulating SiC substrate was used for fabricating a HEMT device. The properties of different AlGaN/GaN heterostructures were studied theoretically using an energy band scheme simulation by means of a self-consistent Schrödinger-Poisson calculation. A dramatic effect of the polarization charges on the 2DEG formation at the AlGaN/GaN interface was found. The influence of the surface states is included into the calculation by introducing the measured surface potential as a boundary condition. The surface and interface electronic properties of AlGaN, GaN layers were studied experimentally. We found that the in-situ electronic surface properties of GaN layers do not depend on the surface morphology, but on the growth conditions. However, after the GaN samples have been exposed to air the surface potential turns out to be independent of growth conditions. The surface potential of a silicon nitride passivated AlGaN layer does not change compared to the as grown AlGaN surface, which implies that the 2DEG concentration at the AlGaN/GaN interface is unaffected, too. Furthermore, in HEMT devices the low conductivity of the passivation overlayer prevents the transfer of electrons from the gate to the interface layer and the formation of a virtual gate. In the case of the free surface the formation of a virtual gate can cause a collapse in the drain saturation current. Finally, studying the surface, interface and device electronic properties helps to better understand the physics and to improve the performance of a AlGaN/GaN heterostructure for HEMT applications. The provided theoretical and experimental work gives better insight into the origin and the properties of 2DEGs in AlGaN/GaN structures. To optimize the performance of AlGaN/GaN layer structures for HEMT applications it is essential to consider the interplay of surface, interface and device properties.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2003

Author dc:creator

• Kocan, Martin

Contributors dc:contributor

• Lüth, H.

• info:eu-repo/classification/ddc/620
• Aluminiumnitrid
• Galliumnitrid
• Drei-Fünf-Halbleiter
• Heterostruktur-Bauelement
• HEMT
• Molekularstrahlepitaxie
• Ingenieurwissenschaften
• MBE
• 2DEG
• AlGaN
• GaN
• Heterostructure
• Surface
• Interface

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-120887%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59071