Polarity in GaN and ZnO: Theory, measurement, growth, and devices

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade

Characterization of capture cross sections of interface states in dielectric/III-nitride heterojunction structures

We performed, for the first time, quantitative characterization of electron capture cross sections sigma of the interface states at dielectric/III-N heterojunction interfaces. We developed a new method, which is based on the photo-assisted capacitance-voltage measurements using photon energies below the semiconductor band gap. The analysis was carried out for AlGaN/GaN metal-insulatorsemiconductor heterojunction (MISH) structures with Al2O3, SiO2, or SiN films as insulator deposited on the AlGaN layers with Al content (x) varying over a wide range of values. Additionally, we also investigated an Al2O3/InAlN/GaN MISH structure. Prior to insulator deposition, the AlGaN and InAlN surfaces were subjected to different treatments. We found that sigma for all these structures lies in the range between 5 x 10(-19) and 10(-16) cm(2). Furthermore, we revealed that sigma for dielectric/AlxGa1-xN interfaces increases with increasing x. We showed that both the multiphonon-emission and cascade processes can explain the obtained results. Published by AIP Publishing

Schottky contact investigation on reactive ion etched 6H α-SiC

Reactive Ion Etching (RIE) was performed on monocrystalline 6H α-SiC samples with CF4/H2-based gas mixtures. Schottky contacts on RIE etched α-SiC were compared with reference Schottky contacts on non-etched α-SiC. © 1997 Elsevier Science S.A

Schottky contact investigation on reactive ion etched 6H α-SiC

Reactive Ion Etching (RIE) was performed on monocrystalline 6H α-SiC samples with CF4/H2-based gas mixtures. Schottky contacts on RIE etched α-SiC were compared with reference Schottky contacts on non-etched α-SiC. © 1997 Elsevier Science S.A

Reactive ion etching of β-SiC in CCl2F2/O2

For the first time, the reactive ion etching (RIE) of β-SiC in CCl2F2/O2 gas mixture is reported. The addition of oxygen to the CCl2F2 does not appear to enhance the etch rate, however, the RF power and the DC bias conditions prove to be the dominant factors for controlling the etch rate. In addition, fine line structures with vertical walls and smooth etched surfaces are achieved

Reactive ion etching of β-SiC in CCl2F2/O2

For the first time, the reactive ion etching (RIE) of β-SiC in CCl2F2/O2 gas mixture is reported. The addition of oxygen to the CCl2F2 does not appear to enhance the etch rate, however, the RF power and the DC bias conditions prove to be the dominant factors for controlling the etch rate. In addition, fine line structures with vertical walls and smooth etched surfaces are achieved

Electrical overstress in AlGaN/GaN HEMTs: study of degradation processes

We study degradation mechanisms in 50 mum gate width/0.45 mum length AlGaN/GaN HEMTs after electrical overstresses. One hundred nanosecond long rectangular current pulses are applied on the drain contact keeping either both of the source and gate grounded or the source grounded and gate floating. Source-drain pulsed I-V characteristics show similar shape for both connections. After the HEMT undergoes the source-drain breakdown, a negative differential resistance region transits into a low voltage/high current region. Changes in the Schottky contact de I-V characteristics and in the source and drain ohmic contacts are investigated as a function of the current stress level and are related to the HEMT dc performance. Catastrophic HEMT degradation was observed after I-stress = 1.65 A in case of the 'gate floating' connection due to ohmic contacts burnout. In case of the 'gate grounded' connection, I-stress = 0.45 A was sufficient for the gate failure showing a high gate susceptibility to overstress. Backside transient interferometric mapping technique experiment reveals a current filament formation under both HEMT stress connections. Infrared camera observations lead to conclusion that the filament formation together with a consequent high-density electron flow is responsible for a dark spot formation and gradual ohmic contact degradation. (C) 2003 Elsevier Ltd. All rights reserved

InAIN/GaN HEMTs: A first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In0.2Al0.8N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In0.2Al0.8N/GaN heterojunction were 2 x 10(13\) cm(-2) and 260 cm(2)V(-1)s(-1), respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (pc = 2.4 X 10(-5) Omega(.)cm(2)) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-mu m gate-length HEMT were 0.64 A/mm drain current at V-GS = 3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAIN/GaN REMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density