Impact of surface treatment under the gate on the current collapse of unpassivated AlGaN/GaN heterostructure field-effect transistors

Unpassivated GaN/AlGaN/GaN/SiC heterostructure field-effect transistors were fabricated on intentionally undoped and 5 × 1018 cm−3 modulationdoped material structures. The influence of surface treatment before gate metallization on the gate leakage and drain current collapse of the devices was observed. In the case of a short HCl treatment (∼5 s), a relatively small gate leakage (10−4A mm−1) and a simultaneously negligible current collapse (<5%). This effect is qualitatively similar in devices prepared on the undoped and doped heterostructures. It is assumed that a thin interfacial oxide layer under the gate might be responsible for a lower leakage current and a larger current collapse of the devices

Comparative study on unpassivated and passivated AlGaN/GaN HFETs and MOSHFETs

In this comparative study we investigate AlGaN/GaN-based unpassivated and passivated HFETs and MOSHFETs with regards to DC-, RF-, and power-performance. For optimal comparability, all devices emanate from the same wafer consisting of a SiC-substrate, a 3 μm GaN- and a 30 nm Al0.28Ga0.72N-layer. Devices are processed simultaneously to a large extend. Passivated devices are coated with a 10 nm thick SiO2-layer between the electrodes, MOSHFETs contain a 10 nm thick SiO2-layer serving as gate-insulator underneath the gate and as conventional passivation-layer between the electrodes. Unpassivated devices serve as reference. We present empirical evidence that MOSHFETs outperform both the conventional and the passivated HFETs with respect to DC-, RF-, and power-performance, and we point out the different mechanisms responsible for the behaviour of the devices

Local increase in compressive strain (GaN) in gate recessed AlGaN/GaN MISHFET structures induced by an amorphous AlN dielectric layer

We fabricated and characterized metal insulator semiconductor (MIS) structures by applying amorphous AlN thin layers as a dielectric in gate recessed AlGaN/GaN heterostructure field effect transistors (HFETs). Micro photoluminescence measurements performed on MISHFET devices reveal a local non-uniform distribution of strain in the source—gate recess—drain region. Furthermore, a reduction of compressive stress up to 0.3 GPa in GaN after gate after the deposition of 4 and 6 nm thin AlN layers in the gate recessed structures, respectively. recessing was experimentally determined. The local stress increases by ∼0.1 GPa and ∼0.2 GPa Additionally, an increase in sheet charge density in the devices under investigation from Therefore, strain engineering by applying amorphous AlN layers in gate recessed MISHFETs ∼3.8×1012 cm−2 to ∼6.2×1012 cm−2 was evaluated by capacitance–voltage measurements. can significantly improve their device characteristics.Keywords

Monolithic Integration of Ultrafast Photodetector and MESFET in the GaN Material System

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors integrated with metal–semiconductor–field-effect-transistors (MESFETs) integrated in coplanar strip lines in the GaN/AlN/SiC material system. We recorded electrical transients of the single photodetector as short as 0.9 ps wide by optoelectric pump–probe measurements using 360-nm-wavelength and 100-fs-duration laser pulses. Electric photoresponse transients of the photodetector with 6-mV peak amplitude were amplified by the MESFET, resulting in 4-ps-wide and 35-mV peak amplitude signals. This monolithically integrated optoelectronic circuit is presented as a potential candidate for high-speed ultraviolet optoelectronic

Vortex Dynamics in Ferromagnetic Nanoelements Observed by Micro-Hall Probes

In this work we measure the nucleation and annihilation of magnetic vortices in Pacman-like (PL) micromagnets prepared from Permalloy ($Ni_{81}Fe_{19}$, Py) at 77 K. Lateral dimensions of explored objects are ≤1 μm with thickness of about 40 nm. The micromagnets are located directly on the high-sensitive micro-Hall probe based on GaAs/AlGaAs heterostructure by lift-off process. Experiments show good agreement of the magnetization reversal with the micromagnetic simulation. Other shapes of micromagnets are also considered to obtain more precise picture of the vortex dynamics

Conditioning nano-LEDs in arrays by laser-micro-annealing: The key to their performance improvement

A local so-called laser-micro-annealing (LMA) conditioning technology, which is suitable for the fabrication of a large range of hybrid nano-optoelectronic devices, was applied to III-nitride-based nano-light emitting diodes (LEDs). The LEDs with a diameter of ∼100 nm were fabricated in large area arrays and designed for hybrid optoelectronic applications. The LMA process was developed for the precise local conditioning of LED nano-structures. Photoluminescence measurements reveal the enhancement of nano-LED properties, which is in very good agreement with a simple model introduced based on the reduction of the defect layer depth by the LMA process. The experimental data confirm the reduction of the defect layer depth from ∼17 nm to ∼5 nm determined. In consequence, an increase in work currents up to 40 nA at 5 V bias after the LMA procedure as well as high electroluminescence (EL) and output optical power up to 150 nW in the ∼440–445 nm emission wavelength range corresponding to ∼75% wall-plug efficiency were achieved. Additionally, the LEDs' electroluminescence intensities reach the desired values by conditioning the contact/annealed regions of individual LEDs accordingly. Furthermore, the LMA process affects the long-term stability of the electroluminescence (EL) intensity of single nano-LED devices. A study of the EL during 5000 h in the continuous wave operation testing mode revealed a moderate ∼15% decrease in the intensity in comparison to ∼50% for their non-LMA counterparts. Finally, Raman measurements indicate that the “work” temperature for nano-LED conditioned structures decreases

High resolution physical analysis of ohmic contact formation at GaN-HEMT devices

A low ohmic contact resistance is a key request on AlGaN/GaN HEMT devices especially when dealing with higher frequency applications. In such a GaN material system very high annealing temperatures of up to 900°C are frequently necessary to achieve low ohmic resistance. It is known that high annealing temperatures could reduce electrical performance and reliability of the devices. Therefore, low contact resistances at reduced annealing temperatures are beneficial. In the present work a special SiCl4 plasma treatment was applied prior to the ohm metallisation resulting in sufficiently low ohmic resistances at a significantly reduced anneal temperature of 750°C. High resolution physical analysis by transmission electron microscopy (TEM) and time of flight secondary mass spectroscopy (TOFSIMS) have been performed to understand the structural and chemical differences between the treated ohmic contacts compared to contacts processed under standard conditions. An advanced preparation technique was developed to get access to the reaction interfaces for TOFSIMS analysis from backside. It could be shown that the SiCl4 treated samples reveal less impurities/contaminants on the ohmic metal–GaN/AlGaN interface and a different metal semiconductor alloying. The diffusion of metals like Al and Au into the semiconductor substrate was detected at the front of the TiN forming reaction