This paper presents a method with an accurate control of threshold voltages (Vth) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment. Using this method, the Vth of AlGaN/GaN HEMTs can be continuously shifted from -4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 degC. At the same time, the shift in Vth shows a good thermal stability and is not affected by the post-gate annealing. The enhancement-mode HEMTs show a performance (transconductance, cutoff frequencies) comparable to the D-mode HEMTs. Experimental results confirm that the threshold-voltage shift originates from the incorporation of F ions in the AlGaN barrier. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate-leakage current, in both forward and reverse bias regions. A physical model of the threshold voltage is proposed to explain the effects of the fluoride-based plasma treatment on AlGaN/GaN HEMTs

Control of Threshold Voltage of AlGaN/GaN HEMTs by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode

Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency device applications, owing to highly desirable III-nitride physical properties. However, these devices are still hounded by issues not previously encountered in their more established Siand GaAs-based devices counterparts. Metal–insulator–semiconductor (MIS) structures are usually employed with varying degrees of success in sidestepping the major problematic issues such as excessive leakage current and current instability. While different insulator materials have been applied to GaN-based transistors, the properties of insulator/III-N interfaces are still not fully understood. This is mainly due to the difficulty of characterizing insulator/AlGaN interfaces in a MIS HEMT because of the two resulting interfaces: insulator/AlGaN and AlGaN/GaN, making the potential modulation rather complicated. Although there have been many reports of low interface-trap densities in HEMT MIS capacitors, several papers have incorrectly evaluated their capacitance–voltage (C–V) characteristics. A HEMT MIS structure typically shows a 2-step C–V behavior. However, several groups reported C–V curves without the characteristic step at the forward bias regime, which is likely to the high-density states at the insulator/ AlGaN interface impeding the potential control of the AlGaN surface by the gate bias. In this review paper, first we describe critical issues and problems including leakage current, current collapse and threshold voltage instability in AlGaN/GaN HEMTs. Then we present interface properties, focusing on interface states, of GaN MIS systems using oxides, nitrides and high-κ dielectrics. Next, the properties of a variety of AlGaN/GaN MIS structures as well as different characterization methods, including our own photo-assisted C–V technique, essential for understanding and developing successful surface passivation and interface control schemes, are given in the subsequent section. Finally we highlight the important progress in GaN MIS interfaces that have recently pushed the frontier of nitride-based device technology.

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Insulated gate and surface passivation structures for GaN-based power transistors

The recent advances in epitaxial SiC films' growth on Si are overviewed. The basic classical methods currently used for SiC films' growth are discussed and their advantages and disadvantages are explored. The basic idea and the theoretical background for a new method of the synthesis of epitaxial SiC films on Si are given. It will be shown that the new method is significantly different from the classical techniques of thin-film growth where the evaporation of the atoms onto the substrate surface is exploited. The new method is based on the substitution of some atoms in the silicon matrix by the carbon atoms to form the molecules of silicon carbide. It will be shown that the following process of SiC nucleation happens gradually without destroying the crystalline structure of the silicon matrix, and the orientation of a grown film is imposed by the original crystalline structure of the silicon matrix (not only by the substrate surface as in conventional methods of film growth). A comparison of the new method with other epitaxy techniques will be given.The new method of solid-phase epitaxy based on the substitution of atoms and on the creation of dilatation dipoles solves one of the major problems in heteroepitaxy. It provides the synthesis of low-defective unstrained epitaxial films with a large difference between the lattice parameters of the film and the substrate without using any additional buffer layers. This method has another unique feature distinguishing it from the classical techniques of SiC films' growth—it allows the growing of SiC films of hexagonal polytypes. A new kind of phase transformation in solids owing to the chemical transformation of one substance into another will be described theoretically and revealed experimentally. This type of phase transformation, and the mechanism of a broad class of heterogeneous chemical reactions between gas and solid phases, will be illustrated by an example of the growth of SiC epitaxial layers due to the chemical interaction of CO gas with the monocrystalline silicon matrix. The discovery of this mechanism yields a new kind of template: namely, substrates with buffer transition layers for wide-gap semiconductor growth on silicon. The properties of a variety of heteroepitaxial films of wide-gap semiconductors (SiC, AlN, GaN and AlGaN) grown on a SiC/Si substrate by solid-phase epitaxy will be reported. Grown films contain no cracks and have a quality sufficient to manufacture micro- and opto-electronic devices. Also, the new abilities in the synthesis of large (150 mm diameter) low-defective SiC films on Si substrates will be demonstrated.

https://iopscience.iop.org/article/10.1088/0022-3727/47/31/313001/pdf

Theory and practice of SiC growth on Si and its applications to wide-gap semiconductor films

Integrated high efficiency lateral field effect rectifier and HEMT devices of GaN or analogous semiconductor material, methods for manufacturing thereof, and systems which include such integrated devices. The lateral field effect rectifier has an anode containing a shorted ohmic contact and a Schottky contact, and a cathode containing an ohmic contact, while the HEMT preferably has a gate containing a Schottky contact. Two fluorine ion containing regions are formed directly underneath both Schottky contacts in the rectifier and in the HEMT, pinching off the (electron gas) channels in both structures at the hetero-interface between the epitaxial layers.

Integrated HEMT and Lateral Field-Effect Rectifier Combinations, Methods, and Systems

A method of fabricating AlGaN/GaN enhancement-mode heterostructure field-effect transistors (HFET) using fluorine-based plasma immersion or ion implantation. The method includes: 1) generating gate patterns; 2) exposing the AlGaN/GaN heterostructure in the gate region to fluorine-based plasma treatment with photoresist as the treatment mask in a self-aligned manner; 3) depositing the gate metal to the plasma treated AlGaN/GaN heterostructure surface; 4) lifting off the metal except the gate electrode; and 5) high temperature post-gate annealing of the sample. This method can be used to shift the threshold voltage of a HFET toward a more positive value, and ultimately convert a depletion-mode HFET to an enhancement-mode HFET (E-HFET).

/pdf/enhancement-mode-iii-n-devices-circuits-and-methods-56v56e56uw.pdf

Enhancement-Mode III-N Devices, Circuits, and Methods

Copper (Cu) gate AlGaN/GaN high electron mobility transistors (HEMTs) with low gate leakage current were demonstrated. For comparison, nickel/gold (Ni/Au) gate devices were also fabricated with the same process conditions except the gate metals. Comparable extrinsic transconductance was obtained for the two kinds of devices. At gate voltage of -15 V, typical gate leakage currents are found to be as low as 3.5/spl times/10/sup -8/ A for a Cu-gate device with gate length of 2 /spl mu/m and width of 50 /spl mu/m, which is much lower than that of Ni/Au-gate device. No adhesion problem occurred during these experiments. Gate resistance of Cu-gate is found to be about 60% as that of NiAu. The Schottky barrier height of Cu on n-GaN is 0.18 eV higher than that of Ni/Au obtained from Schottky diode experiments. No Cu diffusion was found at the Cu and AlGaN interface by secondary ion mass spectrometry determination. These results indicate that copper is a promising candidate as gate metallization for high-performance power AlGaN/GaN HEMT.

Copper gate AlGaN/GaN HEMT with low gate leakage current

The time-to-breakdown (tBD) of Al2O3 deposited by ozone-based atomic layer deposition (ALD) on dry-etched n-type GaN was evaluated by constant-voltage-stress time-dependent dielectric breakdown (TDDB) measurements. The influence of dry etching was not observed in the TDDB and current–voltage (I–V) measurements at room temperature. The tBD of the ALD-Al2O3 film was estimated to be more than 40,000 years at 3 MV/cm and room temperature. However, the tBD estimated at 250 °C was around 102–103 s.

https://iopscience.iop.org/article/10.7567/JJAP.52.08JN19/pdf

Reliability Evaluation of Al2O3Deposited by Ozone-Based Atomic Layer Deposition on Dry-Etched n-Type GaN

An Al1− x Si x O y mixed oxide has been deposited on GaN by plasma-enhanced atomic layer deposition. The band diagrams between the mixed oxide and GaN for various Si atom fraction x values are determined by hard X-ray photoelectron spectroscopy for the first time. The band gap of the mixed oxide increases with increasing x. This dependence has a large bowing parameter of 1.5 eV. We have successfully obtained conduction band offset (ΔE C) and valence band offset (ΔE V) as a function of x: ΔE C (eV) = 1.6 + 0.4x + 1.2x 2 and ΔE V (eV) = 1.7 + 0.34x + 0.36x 2. These relationships enable us to design GaN metal–oxide–semiconductor devices using the Al1− x Si x O y mixed oxide.

https://iopscience.iop.org/article/10.7567/JJAP.56.04CG07/pdf

Band offset of Al1− x Si x O y mixed oxide on GaN evaluated by hard X-ray photoelectron spectroscopy

The reduction of gate leakage current in AlGaN/GaN metal–insulator–semiconductor (MIS) heterostructure field effect transistors (HFETs) is influenced by the resistance of the insulating layers and even more so by the increase in AlGaN layer resistance. Electrical performance of AlGaN/GaN HFETs with and without electron beam evaporated SiO X gate insulator layers indicates that the AlGaN layer resistance is increased seven orders of magnitude by oxide evaporation at negative gate bias. Also, the transconductance frequency dispersion of the MIS-HFET can be explained with a model in which the insulator is more conductive than the AlGaN layer, though they are both highly resistive. These results indicate that the deposition of the oxide reduces electron leakage through the AlGaN layer in the direction from the AlGaN surface to the GaN layer.

Gate leakage and electrical performance of AlGaN/GaN MIS-type HFET with evaporated silicon oxide layer

The thermal stability of copper (Cu)-gate AlGaN/GaN high electron mobility transistors (HEMTs) was investigated by annealing the devices at 300 °C and 500 °C for 1 h, and at 700 °C for 30 min, respectively. The current–voltage performance of the devices such as drain current, transconductance, threshold voltage and gate leakage current has no obvious degradation up to an annealing temperature of 500 °C and a time of 1 h. Up to 500 °C and 1 h annealing, no Cu diffusion was found at the Cu and AlGaN interface by secondary ion mass spectrometry determination. At an annealing temperature of 700 °C and a time of 30 min, the devices failed due to Cu diffusion into the 2DEG channel. These results indicate that the Schottky contact and the device performance of a Cu-gate AlGaN/GaN HEMT are stable up to an annealing temperature of at least 500 °C and a time of 1 h. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Daigo Kikuta

Papers

Copper gate AlGaN/GaN HEMT with low gate leakage current

Reliability Evaluation of Al2O3Deposited by Ozone-Based Atomic Layer Deposition on Dry-Etched n-Type GaN

Band offset of Al1− x Si x O y mixed oxide on GaN evaluated by hard X-ray photoelectron spectroscopy

Gate leakage and electrical performance of AlGaN/GaN MIS-type HFET with evaporated silicon oxide layer

Thermal stability investigation of copper‐gate AlGaN/GaN high electron mobility transistors