Masanobu Hiroki

Systematic study of insulator deposition effect (Si3N4, SiO2, AlN, and Al2O3) on electrical properties in AlGaN/GaN heterostructures

To systematically examine the effect of insulator deposition on the electrical properties in AlGaN/GaN heterostructures, the Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3) have been deposited on Al0.3Ga0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (Ns) was observed for all the insulators with the order of Ns(Al2O3) > Ns(AlN) ~ Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al2O3) < R(AlN) < R(Si3N4) < R0 ~ R(SiO2) (R0: R without insulators). This order is the same as that of Ns except for SiO2, where the 2DEG mobility largely degraded due to the diffusion of Si atoms into nitride layers. The increase in Ns was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) the surface potential barrier (B), and (ii) the interface charge (NInt) between an insulator and AlGaN. B (eV) was estimated to be 1.7 (Si3N4), 2.2 (AlN), 2.7 (Al2O3), and 3.6 (SiO2), exhibiting a positive correlation between B and the bandgap of the insulator. NInt (1013 cm-2) was estimated to be ~0 (Si3N4), 0.1 (SiO2), 0.3 (AlN), and 0.5 (Al2O3); thus, the interface was found to be positively charged for AlN and Al2O3, whereas it was found to be almost neutral for Si3N4 and SiO2. Thus, the insulator deposition effect has been shown to be significant and to vary among insulators. The analysis shown here offers a guideline for understanding and designing the electrical properties in AlGaN/GaN heterostructures, where insulators are deposited as surface passivation and/or gate insulators.

Fabrication of an InAlN/AlGaN/AlN/GaN Heterostructure with a Flat Surface and High Electron Mobility

We fabricated a novel heterostructure comprising InAlN/AlGaN/AlN/GaN by metal organic vapor phase epitaxy. Owing to the flat surface of the AlGaN underlayer, the obtained surface is flatter [root mean square (RMS) roughness of 0.27 nm] than that for the conventional InAlN/AlN/GaN heterostructure (RMS roughness of 0.53 nm). The electron mobility in the new structure is 1360 cm2 V-1 s-1 with NS of 1.85×1013 cm-2, which is higher that in the conventional one. The insertion of the AlGaN layer into the conventional structure is effective for improving surface morphology and electron mobility.

Suppression of self-heating effect in AlGaN/GaN high electron mobility transistors by substrate-transfer technology using h-BN

We fabricated AlGaN/GaN high electron mobility transistors (HEMTs) on h-BN/sapphire substrates and transferred them from the host substrates to copper plates using h-BN as a release layer. In current–voltage characteristics, the saturation drain current decreased by about 30% under a high-bias condition before release by self-heating effect. In contrast, after transfer, the current decrement was as small as 8% owing to improved heat dissipation: the device temperature increased to 50 °C in the as-prepared HEMT, but only by several degrees in the transferred HEMT. An effective way to improve AlGaN/GaN HEMT performance by a suppression of self-heating effect has been demonstrated.

Flat Surfaces and Interfaces in AlN/GaN Heterostructures and Superlattices Grown by Flow-Rate Modulation Epitaxy

AlN and AlGaN with high Al composition were grown by flow-rate modulation epitaxy (FME); a growth method in which group-III sources and group-V sources are supplied alternately. The AlN growth efficiency in FME was three times higher than those by simultaneous-supply. Vapor-Solid relation was almost linear in AlGaN grown by FME with N2 carrier gas. In contrast to rough surfaces of AlN/GaN heterostructures grown by simultaneous-supply, the surfaces by FME were flat. Intensities of X-ray diffraction satellite peaks indicate that AlN/GaN superlattices by FME (FME-SLs) have more abrupt and flat interfaces than those grown by simultaneous-supply. The threading dislocations were effectively reduced by about half on FME-SLs. These results indicate FME is an effective growth method for the fabrication of high quality AlN/GaN heterostructures and SLs with flat interfaces.

High Temperature Characteristics of Insulated-Gate AlGaN/GaN Heterostructure Field-Effect Transistors with Ultrathin Al2O3/Si3N4 Bilayer

The device performance of AlGaN/GaN-based metal–insulator–semiconductor heterostructure field-effect transistors (MIS-HFETs) with an ultrathin (1 nm/0.5 nm) Al2O3/Si3N4 bilayer has been investigated at elevated temperatures up to 200°C. The devices exhibited excellent transconductance characteristics with high maximum transconductances and ultralow gate current leakages under reverse gate bias conduction at both room and high temperatures due to the employment of an ultrathin bilayer with large dielectric constants and the large conduction band offset between Al2O3 and nitrides. The excellent characteristics observed at high temperatures might indicate the very high interfacial quality between nitrides and bilayer insulator. The results in this report demonstrate that Al2O3/Si3N4 bilayer insulator is a superior candidate for nitride-based MIS-HFET devices operating at high temperatures.

SAW characteristics of GaN layers with surfaces exposed by dry etching

We evaluated the possibility of monolithic integration of electron devices and surface acoustic wave (SAW) devices on GaN. We removed top n+ GaN layers of n+ GaN/unintentionally-doped GaN structures by inductively coupled plasma (ICP) etching and fabricated SAW filters on the exposed unintentionally doped GaN layers. We found that the device characteristics are almost the same as those of devices fabricated on as-grown GaN layers, although the surface morphology of GaN layers is degraded due to the ICP etching. The results indicate that SAW devices and electron devices can be monolithically integrated on GaN-based semiconductor structures.

High-temperature characteristics in normally off AlGaN/GaN heterostructure field-effect transistors with recessed-gate enhanced-barrier structures

High-temperature characteristics in normally off AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed-gate enhanced-barrier structures were examined up to 300 °C, where a high threshold voltage (Vth) of +3.0 V and a high drain current density (Id) of 610 mA/mm were obtained at room temperature (RT). Interestingly, Id did not degrade significantly up to 300 °C with a small positive shift in Vth from +3.0 to +3.5 V. A model has been proposed that channel electrons should experience a potential step when they pass the nonrecessed/recessed boundary region in recessed-gate structures, which should be related to the observed high-temperature characteristics.

Mechanism of superior suppression effect on gate current leakage in ultrathin Al2O3/Si3N4 bilayer-based AlGaN/GaN insulated gate heterostructure field-effect transistors

On the basis of the thin barrier surface (TSB) model, the mechanism of gate current leakage under reverse gate–source bias in nitride-based heterostructure field effect transistors (HFETs) and metal–insulator–semiconductor (MIS) HFETs with an ultrathin (1 nm/0.5 nm) Al2O3/Si3N4 bilayer has been investigated. The simulations show that the electron tunneling through the Schottky barrier is the dominant mechanism for gate current in conventional HFETs due to the high density of donor like defects on the surface. An Al2O3/Si3N4 bilayer insulator can substantially reduce the donor like surface defect density and then significantly suppress the gate current leakage in nitrides-base MIS-HFET devices.

Comparison of AlGaN/GaN Insulated Gate Heterostructure Field-Effect Transistors with Ultrathin Al2O3/Si3N4 Bilayer and Si3N4 Single Layer

Device performances have been compared between two types of AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) with Al2O3/Si3N4 bilayers and a Si3N4 single layer. Al2O3/Si3N4 bilayer-based MIS-HFETs have much lower gate current leakage than Si3N4-based MIS devices by more than 3 orders of magnitude under reverse gate biases. An ultralow gate leakage of 1×10-11 A/mm at -15 V has been achieved in the Al2O3/Si3N4 bilayer-based MIS devices though higher maximum drain-source current has been obtained in the Si3N4-based MIS devices. A maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been achieved in the ultrathin Al2O3/Si3N4 bilayer-based MIS-HFET device with a gate length of 1.5 µm, which is much higher than that of less than 130 mS/mm in the Si3N4-based MIS devices. The reduction in the transconductance of Al2O3/Si3N4 bilayer-based devices was much smaller than that in the Si3N4-based MIS devices due to the employment of ultrathin bilayers with a large dielectric constant.This work demonstrates that an Al2O3/Si3N4 bilayer insulator is a superior candidate for nitride-based MIS-HFET devices.

Al2O3/Si3N4 insulated gate channel-doped AlGaN/GaN heterostructure field-effect transistors with regrown ohmic structure: Low gate leakage current with high transconductance

An advanced structure of AlGaN/GaN heterostructure field-effect transistors (HFETs) has been proposed and fabricated, which is characterized by the following structural features: (i) a metal-insulator-semiconductor (MIS) structure using an Al2O3/Si3N4 bilayer gate insulator to reduce the gate leakage current, (ii) a thin AlGaN barrier with a doped channel to simultaneously obtain the high transconductance and high drain current, and (iii) a regrown ohmic structure to reduce the contact resistance. The fabricated devices have been proved to exhibit attractive characteristics such as low gate leakage current, low contact resistance, high drain current, and high transconductance. An HFET with a gate length of 0.1 µm has exhibited a gate leakage current density of below 10-4 A/mm even at a gate voltage of +3 V. It has also exhibited a low contact resistance of 0.3 Ωmm, a high maximum drain current density of 1.23 A/mm, and a high transconductance of 280 mS/mm, which is the highest transconductance ever reported in the category of MIS-HFETs. The cutoff frequency and maximum oscillation frequency, measured with the pad capacitances included, were 52 and 75 GHz, respectively. The proposed structure has thus been proved to be effective in further improving the device performance in GaN-based HFETs.