Yun-Hsiang Wang

Fabrication and properties of nanoporous GaN films

Nanopore arrays with pore diameters of approximately 75nm were fabricated in GaN films by inductively coupled plasma etching using anodic aluminum oxide (AAO) films as etch masks. Nanoporous AAO films were formed on the GaN surface by evaporating an Al film onto a GaN epilayer and subsequently anodizing the aluminum. To minimize plasma-induced damage, the template was exposed to CF4-based plasma conditions. Scanning electron microscopy analysis shows that the diameter and the periodicity of the nanopores in the GaN were directly transferred from the original anodic alumina template. The pore diameter in the AAO film can be easily controlled by tuning the anodization conditions. Atomic force microscopy, photoluminescence, and micro-Raman techniques were employed to assess the etched GaN nanopore surface. This cost-effective, nonlithographic method to produce nano-patterned GaN templates is expected to be useful for growth and fabrication of nitride-based nanostructures and photonic band gap materials.

Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template

We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned surface would also serve as a template for the growth of ultraviolet-visible light-emitting III-nitride devices.

Structural and optical properties of InGaN∕GaN multiple quantum wells grown on nano-air-bridged GaN template

Structural and optical properties of InGaN∕GaN multiple quantum wells (MQWs) grown on nano-air-bridged GaN template by metal organic chemical vapor deposition were investigated. The InGaN∕GaN MQWs on nano-air-bridged GaN demonstrate much better surface morphology, revealing low defect density ∼4×108cm−2 with step flow features measured by atomic force microscopy. The photoluminescence measurement shows one magnitude higher in intensity from less defective InGaN MQWs compared to that of the control InGaN MQWs. The improvement in photoluminescence of the InGaN MQWs is benefited from the reduction of threading dislocation density in the InGaN∕GaN active layers and GaN template, revealed from cross-sectional transmission electron microscopy. High resolution x-ray diffraction analysis results show higher indium mole fraction in the MQWs when grown on nano-air-bridged GaN template, due to the strain relaxation in the nano-air-bridged GaN template. This higher indium incorporation is consistent with the redshift...

Nonlithographic nanopatterning through anodic aluminum oxide template and selective growth of highly ordered GaN nanostructures

Ordered GaN nanostructures, i.e., nanopore and nanodot arrays, have been demonstrated by combining a nonlithographic nanopatterning technique and nanoscale selective epitaxial growth. Hexagonal-close-packed nanopore arrays were fabricated in GaN surfaces and SiO2 surfaces on GaN films by inductively coupled plasma etching using anodic aluminum oxide templates as etching masks. Selective area growth through nanopores in SiO2 by metal organic chemical vapor deposition results in ordered GaN nanodot arrays with an average dot diameter and height of 60 and 100nm, respectively. The diameter and density of the GaN nanopore arrays and nanodot arrays are controlled by that of the anodic aluminum oxide template, which can be tuned in a wide range by controlling the anodization conditions. Applying anodic aluminum oxide as an etching mask provides an effective nonlithographic and free of foreign catalysts method to fabricate ordered and dense nitride nanostructures for either bottom-up or top-down technique in the ...

6.5 V High Threshold Voltage AlGaN/GaN Power Metal-Insulator-Semiconductor High Electron Mobility Transistor Using Multilayer Fluorinated Gate Stack

In this letter, the approach of partial AlGaN recess and multiple layers of fluorinated Al2O3 gate dielectric is utilized to achieve highest reported positive gate threshold voltage (VTH) without severe reduction on 2-D electron gas carrier mobility in AlGaN/GaN HEMTs. Guided by the design and verification through analytical model, proper fluorine ions incorporation is made through fabrication. The approach resulted in a high VTH of +6.5 V and competitive drain saturation current (IDMAX) of 340 mA/mm. Furthermore, low gate leakage current and high breakdown voltage of 1140 V were also demonstrated.

Modelling temperature dependence on AlGaN/GaN power HEMT device characteristics

This paper reports extensive modelling and analysis of the temperature dependence on the device characteristics of the AlGaN/GaN high electron mobility transistors (HEMTs). A physics-based model is proposed in this study in order to correctly predict the gate flat-band Schottky barrier height, energy band Fermi-level (EC–EF) at the AlGaN/GaN interface, two-dimensional electron gas sheet density, gate threshold and (ID–VG) at sub-threshold voltages, and drain current–voltage (ID–VD) characteristics under various high-temperature conditions. The analytical results are then verified by comparing with the laboratory measurement as well as the numerical results obtained from the Sentaurus TCAD simulation. The proposed model is found to be useful for power electronic device designers on the prediction of the AlGaN/GaN HEMT device performance under high-temperature operation.

High-temperature studies of multiple fluorinated traps within an Al2O3 gate dielectric for E-Mode AlGaN/GaN power MIS-HEMTs

Normally-off AlGaN/GaN MIS-HEMT devices with multiple fluorinated ALD-Al2O3 layers as the gate dielectric have been reported to achieve a high threshold voltage for normally-off operations with satisfactory performance for both on and off states at room temperature. However, a large swing in gate threshold voltage is found when devices operate at elevated temperatures. Hence, further study of the gate dielectric on the distribution of fluorinated trap states in the energy band are required to assess the gate function at higher temperatures. Through the use of the charge analytical model and Poole–Frenkel trap emission theory, the gate voltage stressing measurement was carried out to accurately find the effective trap state distribution within the Al2O3 energy bandgap created by fluorinated treatments. For the samples fabricated and used in the investigation, we found that a higher population of fluorinated trap states located deeper than 1.1 eV corresponding to emission levels above 200 °C would allow more trapped charges to remain in the dielectric at high temperature for better threshold voltage retention. We also discovered that a higher fluorine treatment power on the gate dielectric could yield a higher trap state density at deeper levels, resulting in better temperature stability.

Threshold voltage instability in AlGaN/GaN HEMTs

In this study, the threshold voltage instability of the Schottky gate HEMT, the p-GaN gate HEMT, and the recessed MIS gate HEMT is investigated. The annealed p-GaN gate HEMT and Schottky gate HEMT are relatively stable. The threshold voltage shift of the recessed MIS gate HEMT, which is attributed to the trapping of electrons in the gate dielectric, can be as large as 2.5 V depending on the stress conditions. The activation energies of the trapping and de-trapping are extracted from the time constant spectra for the recessed MIS gate HEMT.

The physical mechanism on the threshold voltage temperature stability improvement for GaN HEMTs with pre-fluorination argon treatment

In this paper, a normally-off AlGaN/GaN MIS-HEMT with improved threshold voltage (VTH) thermal stability is reported with investigations on its physical mechanism. The normally-off operation of the device is achieved from novel short argon plasma treatment (APT) prior to the fluorine plasma treatment (FPT) on Al2O3 gate dielectrics. For the MIS-HEMT with FPT only, its VTH drops from 4.2 V at room temperature to 0.5 V at 200 °C. Alternatively, for the device with APT-then-FPT process, its VTH can retain at 2.5 V at 200 °C due to the increased amount of deep-level traps that do not emit electrons at 200 °C. This thermally stable VTH makes this device suitable for high power applications. The depth profile of the F atoms in Al2O3, measured by the secondary ion mass spectroscopy, reveals a significant increase in the F concentration when APT is conducted prior to FPT. The X-ray photoelectron spectroscopy (XPS) analysis on the plasma-treated Al2O3 surfaces observes higher composition of Al-F bonds if APT was a...

High output swing monolithic inverter with E-D mode MIS-HEMTs for GaN power integrated circuits

AlGaN/GaN power HEMT combined with partial AlGaN barrier recess and multiple CHF3 based fluorine plasma treatments onto ALD deposited Al2O3 gate dielectrics is able to bring a high gate threshold voltage (VTH) for the enhancement mode operations without much degradation on the maximum drain saturation current (IDMAX). This work reports integration of both the enhancement and depletion (E-D) modes AlGaN/GaN MIS-HEMTs to construct a monolithic inverter. Experimental data show that the proposed power inverter provides large noise margin allowances of 4.9V and 3.2V, large output swing of 9.66V and satisfactory propagation switching delay time of 35ns. The configuration is found to be suitable for GaN power integrated circuits.