Honggang Liu

Batch-fabricated high-performance graphene Hall elements

Hall elements are by far the most widely used magnetic sensor. In general, the higher the mobility and the thinner the active region of the semiconductor used, the better the Hall device. While most common magnetic field sensors are Si-based Hall sensors, devices made from III-V compounds tend to favor over that based on Si. However these devices are more expensive and difficult to manufacture than Si, and hard to be integrated with signal-processing circuits for extending function and enforcing performance. In this article we show that graphene is intrinsically an ideal material for Hall elements which may harness the remarkable properties of graphene, i.e. extremely high carrier mobility and atomically thin active body, to create ideal magnetic sensors with high sensitivity, excellent linearity and remarkable thermal stability.

Kinetic study of GeO disproportionation into a GeO2/Ge system using x-ray photoelectron spectroscopy

GeO disproportionation into GeO2 and Ge is studied through x-ray photoelectron spectroscopy. Direct evidence for the reaction 2GeO → GeO2 + Ge after annealing in ultra-high vacuum is presented. Activation energy for GeO disproportionation is found to be about 0.7 ± 0.2 eV through kinetic and thermodynamic calculations. A kinetic model of GeO disproportionation is established by considering oxygen transfer in the GeO network. The relationship between GeO disproportionation and GeO desorption induced by GeO2/Ge interfacial reaction is discussed, and the apparent contradiction between GeO desorption via interfacial redox reaction and GeO disproportionation into Ge and GeO2 is explained by considering the oxygen vacancy.

Enhanced Performance of GaN-Based Light-Emitting Diodes by Using Al Mirror and Atomic Layer Deposition-TiO2/Al2O3 Distributed Bragg Reflector Backside Reflector with Patterned Sapphire Substrate

GaN-based light-emitting diodes (LEDs) coated with an Al mirror and a three-pair TiO2/Al2O3 distributed Bragg reflector (DBR) by atomic layer deposition (ALD) grown on a patterned sapphire substrate (PSS) were proposed and realized for the first time. A 43.1% enhancement in light output power (LOP) was realized at 60 mA with the LED coated with an Al mirror and a three-pair ALD-grown TiO2/Al2O3 DBR compared with the LED without a backside reflector, as well as a 10.7% enhancement compared with the LED with a conventional Al mirror and a three-pair TiO2/SiO2 DBR reflector.

Neutral base recombination in InP∕GaAsSb∕InP double-heterostructure bipolar transistors: Suppression of Auger recombination in p+ GaAsSb base layers

We report on the tradeoff between current gain β and the base sheet resistance RSH in metalorganic chemical vapor deposition-grown NpN InP∕GaAs1−xSbx∕InP double-heterojunction bipolar transistors (DHBTs) with heavy base carbon-doping levels resulting in hole concentrations NB ranging from 4×1019 to 12×1019∕cm3. In contrast to Ga0.47In0.53As and GaAs–based transistors, which both display gain variations proportional to 1∕(NB×WB)2 due to Auger recombination at high doping levels, neutral base recombination in InP∕GaAsSb∕InP DHBTs is not limited by Auger processes, and the measured current gain is proportional to 1∕(NB×WB). We show that GaAs1−xSbx base layers offer a growing lifetime advantage over Ga0.47In0.53As with increasing doping levels. Potential explanations for the observed suppression of Auger recombination in the InP–GaAsSb system are proposed.

High RF Performance Enhancement-Mode Al 2 O 3 /AlGaN/GaN MIS-HEMTs Fabricated With High-Temperature Gate-Recess Technique

In this letter, we report high-performance enhancement-mode (E-mode) Al2O3/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) fabricated with high-temperature low-damage gate recess technique. The high-temperature gate recess is implemented by increasing the substrate temperature to 180 °C to enhance the desorption of chlorine-based etching residues during the dry etching of AlGaN barrier. High-crystal-quality Al2O3 gate dielectric was grown by atomic-layer deposition using O3 as the oxygen source to suppress hydrogen-induced weak bonds. The fabricated E-mode MIS-HEMTs exhibit a threshold voltage of 1.6 V, a pulsed drive current of 1.13 A/mm, and very low OFF-state standby power of

Scalable Fabrication of Ambipolar Transistors and Radio‐Frequency Circuits Using Aligned Carbon Nanotube Arrays

6.8 \times 10-8 W/mm at VGS = 0 V and VDS = 30 V. At 4 GHz and in pulse-mode operation, the output power density and power-added efficiency were measured to be 5.76 W/mm and 57%, both of which are the highest for GaN-based E-mode MIS-HEMTs reported to date.

High-performance GaN-based light-emitting diodes on patterned sapphire substrate with a novel hybrid Ag mirror and atomic layer deposition-TiO2/Al2O3 distributed Bragg reflector backside reflector

Electronic devices based on carbon nanotubes (CNTs) have attracted signifi cant attention for potential radio frequency (RF) applications. [ 1–3 ] It has been shown that intrinsic current-gain and power-gain cutoff frequencies ( f T and f max ) above 1 THz should be possible, but experimental demonstration using fi eldeffect transistors (FETs) based on individual CNTs has suffered from excessive parasitic effects and impedance mismatch problems. [ 2,3 ] In order to overcome these limitations, great efforts have been concentrated on FETs made from aligned arrays of CNTs. [ 4,5 ] Since all of the published A-CNT RF circuits were designed to work in the linear region, it is often stated that it is necessary to use dense arrays of all-semiconducting nanotubes to achieve high performance. [ 2 ] While impressive progress has been made in achieving high-density arrays of CNTs, [ 6–12 ] for example in several cases the required density of tens of nanotubes per micrometer has been realized, it is still challenging to eliminate all metallic CNTs without damaging semiconducting CNTs and thus severely degrading the performance of the CNTarray FETs. [ 11–16 ] Therefore most of the published RF integrated circuits based on CNT array on quartz only can operated at a relative low frequency far below 1 GHz owing to the low cutoff frequency of FETs. Here we argue that rather than trying to avoid the requirement of pure semiconducting CNTs, we may indeed make a good use of natural growing CNTs for RF applications. We demonstrate that perfect ambipolar modulation may be achieved using FETs based on as-grown CNTs arrays on quartz near the minimum current point (MCP). Using the ambipolar region rather than the linear region of these FETs, RF circuits including frequency multipliers and mixers are batchfabricated and shown to retain their function up to 40 GHz, outperforming all previously reported carbon-based RF circuits. The core device in our RF circuits is a FET fabricated on a horizontally aligned large-diameter (about 2.4 nm on average) SWCNT arrays. The carrier mobility in the CNT is known to increase rapidly with increasing tube diameter, making the largediameter CNTs more suitable for RF applications. [ 17 ] The geometry of the RF FET is illustrated in Figure 1 a, where the source (S) electrodes are connected to the common ground and the double parallel gate (G) fi ngers and drain (D) electrodes are used for the input and output ports, respectively. Figure 1 b is an optical image showing arrays of fabricated FETs, and Figures 1 c and d are, respectively, low-magnifi cation optical microscope and high-magnifi cation scanning electron microscope (SEM) images showing the detailed electrode structure of the FETs. The transfer characteristics of the fabricated FET shows ambipolar behavior (Figure 1 e) in the sense that the branch at positive gate voltage (n-branch) and that at negative gate (p-branch) are symmetric about the MCP. Owing to the participation of metallic CNTs, as well as the large diameter of the CNTs, [ 10 ] the transfer characteristics shown in Figure 1 e have a current on/off ratio of less than 1.5, which is comparable to that of graphene top-gated FETs. [ 18,19 ] Although low current on/ off ratio would preclude applications in digital logic, it is acceptable in analogous RF systems where the device operates in a narrow range of voltage around a fi xed point. It should be noted that Ti contacts and a high-effi ciency top gate were used here to improve the symmetry between nand p-branches in transfer characteristics owing to the suitable work function of Ti and high gate effi ciency of Y 2 O 3 insulator. [ 20,21 ] When V ds is fi xed at 1 V, as in Figure 1 e, the drain current I ds is modulated by gate voltage V gs and varies between 1.31 and 0.94 mA, which corresponds to a channel resistance ranged from 780 to 1060 Ω . The transfer characteristics shown in Figure 1 e are very similar to those measured from a graphene FET, and the corresponding output characteristics (Figure 1 f) show a saturation tendency at large bias, which is diffi cult to realize in graphene FETs. [ 22,23 ]

Reducing the interface trap density in Al2O3/InP stacks by low-temperature thermal process

Abstract. GaN-based light-emitting diodes (LED) on a patterned sapphire substrate with a novel hybrid atomic layer deposition (ALD)-TiO2Al2O3 distributed Bragg reflector (DBR) and Ag mirror have been proposed and fabricated. Due to the excellent thickness uniformity of ALD for the proposed reflector, high reflectivity over 99.3% at an incident angle of 5 deg has been achieved. It was also found that the reflectivity of a backside reflector with an Ag mirror slightly depends on incident light wavelength and incident angle. Moreover, because of the good adhesion between TiO2/Al2O3 DBR and the Ag mirror, the fabrication process was simplified and reliable. With a 60 mA current injection, an enhancement of 5.2%, 8.9%, and 47.1% in light output power (LOP) at the 460 nm wavelength was realized for the proposed LED with Ag mirror and 3-pair ALD-TiO2Al2O3 DBR as compared with a LED with a traditional Ag mirror and 3-pair TiO2/SiO2 DBR, with Al mirror and 3-pair ALD-TiO2Al2O3 DBR, and without backside reflector, respectively. This result shows that the ALD-TiO2/Al2O3 DBR can be used to enhance the LOP greatly and improve adhesion between the sapphire substrate and the metallic mirror, and thus is very promising for fabricating high performance GaN-based LEDs.

High performance GaN-based LEDs on patterned sapphire substrate with patterned composite SiO 2 /Al 2 O 3 passivation layers and TiO 2 /Al 2 O 3 DBR backside reflector

By applying low-temperature processes below 300 °C, high-performance Al/Al2O3/InP metal–insulator–semiconductor capacitors with low interface trap density and small capacitance frequency dispersion at the accumulation regime are demonstrated. A minimum interface trap density of 1.2 × 1011 cm−2 eV−1 near the midgap is obtained. The impacts of thermal treatment on interface traps, thermal stability, and interfacial bonding configurations are studied and discussed. It is found that interface trap density could be significantly reduced by removing phosphorus and its oxides at low temperature (250–300 °C), while further increasing the thermal treatment temperature is harmful to interface quality.

Emitter-Size Effects and Ultimate Scalability of InP:GaInP/GaAsSb/InP DHBTs

GaN-based light-emitting diodes (LEDs) on patterned sapphire substrate (PSS) with patterned composite SiO(2)/Al(2)O(3) passivation layers and TiO(2)/Al(2)O(3) distributed Bragg reflector (DBR) backside reflector have been proposed and fabricated. Highly passivated Al(2)O(3) layer deposited on indium tin oxide (ITO) layer with excellent uniformity and quality has been achieved with atomic layer deposition (ALD) technology. With a 60 mA current injection, an enhancement of 21.6%, 59.7%, and 63.4% in the light output power (LOP) at 460 nm wavelength was realized for the LED with the patterned composite SiO(2)/Al(2)O(3) passivation layers, the LED with the patterned composite SiO(2)/Al(2)O(3) passivation layers and Ag mirror + 3-pair TiO(2)/SiO(2) DBR backside reflector, and the LED with the patterned composite SiO(2)/Al(2)O(3) passivation layer and Ag mirror + 3-pair ALD-grown TiO(2)/Al(2)O(3) DBR backside reflector as compared with the conventional LED only with a single SiO(2) passivation layer, respectively.