AlGaN/GaN HEMT栅槽低损伤刻蚀技术

对AlGaN/GaN HEMT栅槽低损伤刻蚀技术进行研究,通过加入小流量的具有钝化缓冲作用的C2H4,对Cl2/Ar/C2H4的工艺条件进行了优化,有效地降低了栅槽刻蚀造成的AlGaN表面损伤和器件退化,同时防止反应生成物淀积在栅槽表面,改善了肖特基结特性,提高了栅极调控能力,实现凹栅槽的低损伤刻蚀.     

AlGaN/GaN-based diodes and gateless HEMTs for gas and chemical sensing

The characteristics of Pt/GaN Schottky diodes and Sc/sub 2/O/sub 3//AlGaN/GaN metal-oxide semiconductor (MOS) diodes as hydrogen and ethylene gas sensors and of gateless AlGaN/GaN high-electron mobility transistors (HEMTs) as polar liquid sensors are reported. At 25/spl deg/C, a change in forward current of /spl sim/6 mA at a bias of 2 V was obtained in the MOS diodes in response to a change in ambient from pure N/sub 2/ to 10% H/sub 2// 90% N/sub 2/. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same conditions. The mechanism appears to be formation of a dipole layer at the oxide/GaN interface that screens some of the piezo-induced channel charge. The MOS-diode response time is limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack, even at 25/spl deg/C. Gateless AlGaN/GaN HEMT structures exhibit large changes in source-drain current upon exposing the gate region to various polar liquids, including block co-polymer solutions. The polar nature of some of these polymer chains lead to a change of surface charges in gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. The nitride sensors appear to be promising for a wide range of chemicals, combustion gases and liquids.     

Electrical and structural characterizations of non-polar AlGaN/GaN heterostructures

A non-polar AlGaN/GaN structure is a strong candidate for the high-voltage device that can operate in enhancement-mode compared to the depletion-mode operation that is practically unavoidable for a standard polar AlGaN/GaN structure. Growth of non-polar GaN is non-trivial and a two-step nucleation scheme was developed to produce high-quality non-polar a-plane AlGaN/GaN structures on r-plane sapphire. The anisotropic nature of non-polar GaN requires a modification to a typical polar GaN-based transistor fabrication process. A KOH wet etch proceeded by a dramatically different mechanism compared to the standard polar c-face AlGaN/GaN structure. This device with Pt/Au Schottky gate displayed a barrier height of 0.76 eV and an ideality factor of 4 at 20 °C.     

AlGaN/GaN肖特基二极管中金属电极的制备方法研究

本文主要研究AlGaN/GaN二极管制备工艺中Ohmic Contact金属和Schottky Contact金属电极的制备,主要讨论了金属电极材料的选取,退火温度和退火时间对接触电阻的影响,最后得出了Ohmic Contact的比接触电阻和TiN的淀积参数。     

Isolated photosystem I reaction centers on a functionalized gated high electron mobility transistor

In oxygenic plants, photons are captured with high quantum efficiency by two specialized reaction centers (RC) called Photosystem I (PS I) and Photosystem II (PS II). The captured photon triggers rapid charge separation and the photon energy is converted into an electrostatic potential across the nanometer-scale (~6 nm) reaction centers. The exogenous photovoltages from a single PS I RC have been previously measured using the technique of Kelvin force probe microscopy (KFM). However, biomolecular photovoltaic applications require two-terminal devices. This paper presents for the first time, a micro-device for detection and characterization of isolated PS I RCs. The device is based on an AlGaN/GaN high electron mobility transistor (HEMT) structure. AlGaN/GaN HEMTs show high current throughputs and greater sensitivity to surface charges compared to other field-effect devices. PS I complexes immobilized on the floating gate of AlGaN/GaN HEMTs resulted in significant changes in the device characteristics under illumination. An analytical model has been developed to estimate the RCs of a major orientation on the functionalized gate surface of the HEMTs.     

Surface acoustic wave excitation on SF6 plasma-treated AlGaN/GaN heterostructure

In this work, we introduce a new modified approach to the formation of interdigital transducer (IDT) structures on an AlGaN/GaN heterostructure. The approach is based on a shallow recess-gate plasma etching of the AlGaN barrier layer in combination with “in-situ” SF₆ surface plasma treatment applied selectively under the Schottky gate fingers of IDTs. It enables one to modify the two-dimensional electron gas (2DEG) density and the surface field distribution in the region of the IDTs, as is needed for the excitation of a surface acoustic wave (SAW). The measured transfer characteristics of the plasma-treated SAW structures revealed the excitation of SAW at zero bias voltage due to fully depleted 2DEG in the region of the IDTs. High external bias voltages are not necessary for SAW excitation. SIMS depth distribution profiles of F atoms were measured to discuss the impact of SF₆ plasma treatment on the performance of the AlGaN/GaN-based IDTs.     

Low-Energy Defectless Dry Etching of the AlGaN/AlN/GaN HEMT Barrier Layer

A method of defectless dry etching of an AlGaN barrier layer is proposed, which consists in repeated plasmachemical oxidation of AlGaN and removal of the oxide layer by means of reactive ion etching in inductively coupled BCl₃ plasma. Using the proposed etching technology, AlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) with a buried gate have been successfully fabricated for the first time. It is shown that the currents of obtained HEMTs are independent of the number of etching cycles, while the gate operating point shifts toward positive voltages up to obtaining transistors operating in the enhancement mode.     

Performance of AlGaN/GaN high electron mobility transistors at nanoscale gate lengths

The DC and RF performance of AlGaN/GaN high electron mobility transistors with nanoscale gate lengths is presented. The layer structures were grown by either metal organic chemical vapor deposition or rf plasma-assisted molecular beam epitaxy. Excellent scaling properties were observed as a function of both gate length and width and confirm that these devices are well suited to both high speed switching and power microwave applications.     

Laser ablated ZnO layers for ALGaN/GaN HEMT passivation

ZnO layer in a role of passivation of the AlGaN/GaN-based high electron mobility transistors (HEMTs) is presented. The thin layer is deposited by pulsed laser deposition technique. It is fully compatible with the process technology of high electron mobility transistors prepared on AlGaN/GaN heterostructures due to its physical properties similar to the GaN. We have succeeded to (1) suppress the gate leakage current; (2) increase the maximum of the drain current and the electron drift mobility, and (3) ensure the threshold voltage to be unaltered by employment of the thin ZnO layer to the channel area of the HEMT.     

AlGaN/GaN high electron mobility transistors with implanted ohmic contacts

Selective area silicon implantation for source/drain regions was integrated into the fabrication of molecular beam epitaxy-grown AlGaN/GaN HEMTs. Dopant activation was achieved by rapid thermal annealing at 1100 °C in flowing N₂ ambient for 120 s with an AlN encapsulation. Linear transmission line measurements showed that the resistance of the overlay Ti/Al/Ni/Au ohmic contacts was reduced by 61% compared to the control sample. After the Schottky Ni/Au gate formation, the typical DC characteristics displayed a higher current drive, smaller knee voltage and better gate control properties for HEMTs with implanted source and drain regions.     

Nitride-based Schottky diodes and HFETs fabricated by photo-enhanced chemical wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. It was found that the maximum etch rates were 510, 1960, 300, and 0 nm/mm for GaN, Al_0.175Ga_0.825N, Al_0.23Ga_0.77N, and Al_0.4Ga_0.6N, respectively. It was also found that we could achieve a high Al_0.175Ga_0.825N to GaN etch rate ratio of 12.6. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. It was found that we could achieve a saturated I_D larger than 850 mA/mm and a maximum g_m about 163 mS/mm from PEC wet etched HFET with a 0.5 μm gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.     

AlGaN/GaN Schottky Barrier Photodetector With Multi-${rm Mg}_{rm x} {rm N} _{rm y} $/GaN Buffer

AlGaN/GaN heterostructure Schottky barrier photodetector (PD) with multi-Mg_xN_y/GaN buffer was proposed and fabricated. Compared with AlGaN/GaN heterostructure PD prepared on conventional low-temperature GaN buffer, it was found that we can reduce dark leakage current by more than three orders of magnitude. It was also found that we can use the multi-Mg_xN_y/GaN buffer to suppress photoconductance gain, enhance UV-to-visible rejection ratio, reduce noise level and enhance the detectivity.     

Single- and few-walled carbon nanotubes grown at temperatures as low as 450 degrees c: electrical and field emission characterization

Single-wall (SW-) and few-walled (FW-) carbon nanotubes (CNTs) were synthesized on aluminum/ cobalt coated silicon at temperatures as low as 450 degrees C by plasma enhanced chemical vapor deposition technique (PECVD). The SWCNTs and FWCNTs grow vertically oriented and well separated from each other. The cold field emission studies of as-grown SWCNTs and FWCNTs showed low turn-on field emission threshold voltages, strongly dependent of the nanotubes morphology. Current-voltage curves of individual CNTs, measured by conductive atomic force microscopy (CAFM), showed an electrical resistance of about 90 Komega, that is attributed mainly to the resistance of the contact between the CNTs and the conductive CAFM tip (Au and Pt).     

The control of two-dimensional-electron-gas density and mobility in AlGaN/GaN heterostructures with Schottky gate

The effects of Schottky gate on behavior of two-dimensional electron gas (2DEG) density and two-dimensional electron mobility (2DEM) in AlGaN/GaN heterostructures with different Al mole fraction in AlGaN barrier and its different thickness have been studied. The sheet carrier concentration, NS, was determined self-consistently from the coupled Schrödinger and Poisson equations, by assuming a real model for heterostructures and by using Numerov's method.The most dominant scattering mechanisms have been considered to calculate 2DEM with using more accurate numerical calculation and considering all intra-subband, inter-sub-band scattering.The results of our analysis clearly indicate that increasing the gate voltage leads to an increase in the 2DEG density and 2DEM. Also it shows that increasing the gate voltage for higher positive voltage; decrease the 2DE Mobility where the 2DEG density is saturated. These behaviors depend on barrier thickness and Al mole fraction.     

The study of Pt Schottky contact on porous GaN for hydrogen sensing

This article reports the studies of Pt Schottky contact on porous n-type GaN for hydrogen sensing. A simpler and improved electroless etching method has been developed to generate porous GaN in which high uniformity of the porous area could be achieved. Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contacts onto the porous GaN sample. For comparative study, a standard hydrogen sensor was also prepared by depositing Pt Schottky contacts on the as-grown sample. Hydrogen detection was carried out at room temperature and 100 °C. This Pt/porous GaN sensor exhibited a significant change of current upon exposure to 2% H₂ in N₂ gas as compared to the standard Pt/GaN sensor. Morphological studies by scanning electron microscopy revealed that Pt contact deposited on porous GaN having a very rough surface morphology with pores distributed all over the contact layer. Therefore, the increase of current could be attributed to the unique microstructure at porous Pt/porous GaN interface which allowed higher accumulation of hydrogen and eventually led to stronger effect of the H-induced dipole layer.     

Influence of annealing effects on the electrical and microstructural properties of Se Schottky contacts on n-type GaN

The electrical and microstructural properties of Se/n-gallium nitride (GaN) Schottky diode have been investigated by current–voltage (I–V), capacitance–voltage (C–V) and transmission electron microscopy (TEM) measurements as a function of annealing temperature. The Se/n-GaN Schottky contact exhibited an excellent rectification behavior. The barrier height of the as-deposited Se/n-GaN Schottky contact is 0.94 eV (I–V) and 1.55 eV (C–V), respectively. However, the barrier height of Se/n-GaN Schottky diode decreases to 0.90 eV (I–V) and 1.33 eV (C–V) upon annealing at 200 °C. Cheung’s and modified Norde functions are employed to determine the barrier height and series resistance. TEM results reveal that the Se film becomes fully crystallized for the contact annealed at 200 °C compared to the as-deposited contact without the reaction between Se film and GaN substrate. It is observed that the barrier height of Se/n-GaN Schottky diode decreases with increasing annealing temperature. This could be associated with the decrease in series resistance caused by the phase transformation from high resistance amorphous Se to low resistance crystalline Se. Further, the interface states density is found to be increased with the increasing annealing temperature. The Schottky emission mechanism is found to dominate the reverse leakage current in Se/n-GaN Schottky diodes irrespective of annealing temperatures.     

Impact of SF6 plasma treatment on performance of AlGaN/GaN HEMT

Selective plasma treatment of an AlGaN/GaN heterostructure in the RF discharge of the electronegative SF₆ gas was studied. Shallow recess-gate etching of AlGaN (∼5 nm) was performed in CCl₄ plasma through a photoresist mask. Subsequently, recess-gate etching followed in situ by SF₆ plasma. The plasma treatment provides the following advantages in the technology of AlGaN/GaN high-electron mobility transistors (HEMT): It (1) simplifies their technology; (2) ensures sufficient selectivity; and (3) enables the technologist to set the threshold voltage of the HEMTs controllably. At the same time, the treatment can (1) provide the AlGaN/GaN heterostructure with surface passivation; (2) modify the 2DEG in any area of a HEMT channel; and (3) make it possible to convert a HEMT operation from depletion mode to enhancement mode. The treatment also improved significantly the DC and RF parameters of HEMTs studied.     

氧气等离子体处理对AlGaN肖特基接触的影响

研究了不同的干法刻蚀以及氧气等离子体处理条件对AlGaN表面特性的影响。在合适的条件下,氧气等离子体处理可以使AlGaN表面发生氧化,并使肖特基接触的反向漏电流降低两个数量级,反向击穿电压也有显著提高。该方法简单易行,可应用于制备高性能的AlGaN／GaN HEMT器件。     

GaN nanorod Schottky and p-n junction diodes

Conductive atomic force microscopy has been used to characterize single GaN nanorod Schottky and p-n junction diodes. The ideality factor, reverse breakdown voltage, and the Schottky barrier height of individual nanorod diodes were compared to those from conventional thin-film diodes. Large-area contacts, enabling diodes with arrays of GaN nanorods in parallel, were also fabricated and their electrical characteristics investigated. The defect-free nature of the GaN nanorods and enhanced tunneling effects due to nanoscale contacts have been invoked to explain the electrical behavior of the nanorod diodes.     

Dopant-free GaN/AlN/AlGaN radial nanowire heterostructures as high electron mobility transistors

We report the rational synthesis of dopant-free GaN/AlN/AlGaN radial nanowire heterostructures and their implementation as high electron mobility transistors (HEMTs). The radial nanowire heterostructures were prepared by sequential shell growth immediately following nanowire elongation using metal-organic chemical vapor deposition (MOCVD). Transmission electron microscopy (TEM) studies reveal that the GaN/AlN/AlGaN radial nanowire heterostructures are dislocation-free single crystals. In addition, the thicknesses and compositions of the individual AlN and AlGaN shells were unambiguously identified using cross-sectional high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM). Transport measurements carried out on GaN/AlN/AlGaN and GaN nanowires prepared using similar conditions demonstrate the existence of electron gas in the undoped GaN/AlN/AlGaN nanowire heterostructures and also yield an intrinsic electron mobility of 3100 cm(2)/Vs and 21,000 cm(2)/Vs at room temperature and 5 K, respectively, for the heterostructure. Field-effect transistors fabricated with ZrO(2) dielectrics and metal top gates showed excellent gate coupling with near ideal subthreshold slopes of 68 mV/dec, an on/off current ratio of 10(7), and scaled on-current and transconductance values of 500 mA/mm and 420 mS/mm. The ability to control synthetically the electronic properties of nanowires using band structure design in III-nitride radial nanowire heterostructures opens up new opportunities for nanoelectronics and provides a new platform to study the physics of low-dimensional electron gases.