Search

This work reports a Mg (130 nm)/Al (50 nm) bilayer to realize high-reflectivity ohmic contact for n-GaN. The contact resistivity of $3.75\times 10 ^{ {-4}} \Omega \cdot $ cm $^{ {2}}$ with Mg/Al contact was achieved after annealing at 250 °C in ambient Ar for 1 min. The specific contact resistivity was found to change moderately when the annealing temperature was below 400 °C, but the ohmic characteristics deteriorated significantly when the annealing temperature was above 450 °C. X-ray photoemission spectroscopy revealed that the Ga 3d (Ga-N) peak decreased by 0.28 eV with the increase of the annealing temperature from 250 to 450 °C, which can increase the metal-semiconductor contact barrier height, leading to deteriorated current-voltage characteristics. The reflectivity of the samples was found to decrease with the increase of the annealing temperature, and decreased greatly when the temperature exceeded 350 °C. When the annealing temperature is below 300 °C, the reflectivities of the samples are greater than 90% for wavelengths from 350 to 550 nm. Mg/Al is therefore a promising candidate to serve as a reflective n-GaN electrode for GaN-based flip-chip LEDs to improve the light extraction efficiency.

Semi-polar micro-LEDs have gain increasing interests due to the advantages of polarization control and quantum efficiency improvement. In this work, a novel semi-polar (20–21)-plane micro-LEDs array has been designed and manufactured. In comparison with c-plane micro-LEDs, semi-polar micro-LEDs indicate better electrical and optical performance. The relative EQE of semi-polar micro-LEDs remains at 62% under the injected current density of 775.6 A/cm2, which indicates a reduced efficiency droop due to less polarization in MQWs. It has been further proved by a significant reduction of 55% in emission peak blue-shift under the injected current density from 11.1 A/cm2 to 775.6 A/cm2. In addition, the carrier recombination dynamics and spatial light distribution of semi-polar micro-LEDs with different pixel sizes have been studied. Fast recombination lifetime in smaller size semi-polar micro-LEDs indicates a promising way to be used as a high modulation bandwidth light source. Stable and uniform light distribution in a wider range of spatial azimuths further supports for the semi-polar micro-LEDs as a strong candidate for the applications of high-resolution display and high-speed visible light communication.

Current-voltage ( I-V ) characteristics of AlGaN ultraviolet light-emitting diodes (UV-LEDs) under the temperatures ranging from 50 K to 300 K are analyzed. The abnormal diode characteristics of UV-LEDs below the turn-on voltage indicate the existence of space-charge-limited (SCL) current transport in the presence of deep trap states, which induce a non-linear current leakage effect. The SCL current is gradually overwhelmed by Ohmic leakage as the temperature increases, which is mainly due to the thermally generated carrier concentration and traps deactivation. A modified three-diode circuit model is suggested with an additional series resistor and diode to emulate the forward-bias I-V characteristics of UV-LEDs. An excellent fit to the I-V curves of UV-LEDs is achieved, which illustrates the impact of deep trap states on the electrical characteristics of UV-LEDs.

Hybrid white light emitting diodes (LEDs) have been developed utilizing micro-LEDs radiatively pumping down-conversion phosphor materials. The cyan InGaN/GaN multiple quantum wells LEDs with 95 μm in square are fabricated into 4 × 4 pixels as one unit in 4 in wafer by photolithography and inductively coupled plasma etching techniques. The relatively low turn on voltage and low leakage of cyan micro-LEDs indicate good electrical performance. A significant reduction in the efficiency droop characteristics of cyan micro-LED has been observed in comparison with that of standard green LED. Finally, based on a systemic optimization for white emission indexes, high-quality hybrid white light emission has been demonstrated by the combination of blue and cyan micro-LEDs with two types of phosphors, which have a high color rendering index up to 87 at the correlated color temperatures of 6500 K. It also demonstrates a way to adjust the color component by modifying the output power of cyan micro-LED in hybrid LED, showing a promising candidate in a large range of applications including micro-displays, bioinstrumentation, photolithography, and lab-on-chip systems in the future.

We investigate the electrical and optical properties of graphene–Ag nanodot compounds as transparent conductive layers (TCLs) for gallium nitride (GaN)-based ultraviolet light-emitting diodes (UV-LEDs) with different Ag thicknesses. As the thickness of Ag increases from 2 to 5 nm on the 365-nm UV-LEDs, the transmittance decreases from 82.5% to 65.2% and ohmic contact resistance decreases from 0.2 to ${1.3}{\times} {10}^{-5} \Omega \text {cm}^{2}$ . At a wavelength of about 320 nm, the TCLs exhibit high transparency, which is also confirmed by the simulation data of the finite-difference time-domain solution. The simulation and experimental results suggest that Ag with a thickness of 5 nm can potentially obtain both low ohmic contact resistance and high transmittance at the wavelength of about 320 nm, which is important for medical applications. We also study the nanodot migration and coalescence mechanism of 2- and 5-nm Ag samples by scanning electron microscopy, as well as the influence of the existence of graphene. The 5-nm Ag sample has smaller and denser Ag nanodots and thus exhibits better electrical properties. In addition, the Ag nanodots after the second annealing procedure with graphene covering are larger than those without graphene covering.

This study developed an LED thermal resistance and thermal interface material evaluation method using LEDs with an internal sensor unit by both steady-state and transient thermal measurements. The problem of the measurement delay time caused by the current switching, which exist in conventional methods can be avoided in both steady-state and transient thermal measurements. Slight error in junction temperature measurement caused by the self-illumination effect can be identified without the current switching issue. The error is compensated in steady-state thermal measurement and is insignificant in transient measurement. Compared with conventional methods, this method uses low-cost equipment, while realizing improved accuracy.

GaN pyramid arrays have been successfully synthesized by selective photo-assisted chemical etching in a K2S2O8/KOH solution. A detailed analysis of time evolution of surface morphology has been conducted, which describes an etching process of GaN pyramids. Room temperature cathodoluminescence images indicate that these pyramids are composed of crystalline GaN surrounding dislocations, which is caused by the greater recombination rate of electrons and holes at dislocation than that of crystalline GaN. The Raman results show a stress relaxation in GaN pyramids compared with unetched GaN. The optical property of both unetched GaN and GaN pyramids has been studied by photoluminescence. The formation mechanism and feature of GaN pyramids are also rationally explained.

Hybrid white micro-pillar structure light emitting diodes (LEDs) have been manufacture utilizing blue micro-LEDs arrays integrated with 580 nm CIS ((CuInS2-ZnS)/ZnS) core/shell quantum dots. The fabricated hybrid white micro-LEDs have good electrical properties, which are manifested in relatively low turn-on voltage and reverse leakage current. High-quality hybrid white light emission has been demonstrated by the hybrid white micro-LEDs after a systemic optimization, in which the corresponding color coordinates are calculated to be (0.3303, 0.3501) and the calculated color temperature is 5596 K. This result indicates an effective way to achieve high-performance white LEDs and shows great promise in a large range of applications in the future including micro-displays, bioinstrumentation and visible light communication.

In order to guarantee slab thickness precision of Medium Density Fibreboard (MDF), an adaptive fuzzy Dynamic Surface Control (DSC) was investigated. Firstly, a mathematical model for hydraulic servo system of the continuous hot-pressing machine with input saturation was proposed, and the auxiliary system was introduced to compensate the effect of input saturation. Due to the uncertain function in the model, a fuzzy logic system was introduced to approximate it. Subsequently, a slab thickness tracking controller with less complexity was developed with the help of the DSC approach. Simultaneously, an on-line adaptive law was applied to estimate the adjustable parameter vector in the fuzzy system. Finally, experimental results validated that the proposed method can ensure the MDF slab thickness precision in the hot-pressing process.

We study the design of index assignment based multiple description quantizers for multiple-antenna systems with slow Rayleigh fading. A time-interleaver is employed at the transmitter to provide independent channel instances for the multiple descriptions, and a maximum a posteriori (MAP) detector is employed at the receiver to jointly decode the multiple descriptions. The concatenation of index assignment, binary phase-shift keying modulators, space-time orthogonal block coder, multiple-antenna channel, and MAP detector is modeled as a discrete memoryless channel. We optimize multiple description vector quantizers using an upper bound of the channel transition probability achieved by the MAP detector. The proposed scheme of combining multiple description coding and space-time orthogonal block coding is shown to be superior to each individual coding scheme.

We present the detailed dc and radio-frequency characteristics of an Al0.3Ga0.7N/GaN/In0.1Ga0.9 N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In0.1Ga0.9N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In0.1Ga0.9N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF4-based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs

We examine the performance of multiple description coding (MDC) with and without the use of automatic repeat request (ARQ) protocols for packet network communication, in comparison with layered coding (LC). The rate-distortion lower bound of MDC and LC are incorporated into an E-model based performance measure, which accounts for the additional costs of excess rates and delay incurred from using ARQ. The results show that the relative merits of the schemes depend on the values of the packet loss rates and round-trip-time (RTT). LC is superior for small RTT and unaided MDC is superior for large RTT. For moderate RTT, LC is preferred for small packet loss rates and MDC aided by ARQ is preferred for large packet loss rates. Copyright © 2007 John Wiley & Sons, Ltd.

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

Based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing (RTA), enhancement mode (E-mode) AlGaN/GaN HEMTs with low on-resistance and low knee-voltage were fabricated. The fabricated E-mode AlGaN/GaN HEMT with 1 μm-long gate exhibits a threshold voltage of 0.9 V, a knee-voltage of 2.2 V, a maximum drain current density of 310 mA/mm, a peak g m of 148 mS/mm, a current gain cutoff frequency f T of 10.1 GHz and a maximum oscillation frequency f max of 34.3 GHz. 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. Two orders of magnitude reducation in gate leakage current was observed in the fabricated E-mode HEMTs compared to the conventional D-mode HEMTs without fluoride-based plasma treatment.

We present the design, fabrication, and characterization of AlGaN-GaN double-channel HEMTs. Two carrier channels are formed in an AlGaN-GaN-AlGaN-GaN multilayer structure grown on a sapphire substrate. Polarization field in the lower AlGaN layer fosters formation of a second carrier channel at the lower AlGaN-GaN interface, without creating any parasitic conduction path in the AlGaN barrier layer. Unambiguous double-channel behaviors are observed at both dc and RF. Bias dependent RF small-signal characterization and parameter extraction were performed. Gain compression at a high current level was attributed to electron velocity degradation induced by interface scattering. Dynamic IV measurement was carried out to analyze large-signal behaviors of the double-channel high-electron mobility transistors. It was found that current collapse mainly occurs in the channel closer to device surface, while the lower channel suffers minimal current collapse, suggesting that trapping/detrapping of surface states is mainly responsible for current collapse. This argument is supported by RF large-signal measurement results.

The correlation between the resistivity of an undoped GaN/Al2O3 interfacial layer and in-situ reflectance spectrum in metalorganic chemical vapor deposition and the mechanism of this correlation were investigated. The first minimum reflectance during the initial high-temperature GaN growth was found to be a good indicator of the resistivity of the GaN buffer. The background electron concentration and mobility were both higher in the samples with higher indicative reflectance at that point. The resistivity of the GaN buffer layer was predominantly determined by an ∼0.25-µm-thick layer near the GaN/Al2O3 interface. Atomic force microscope (AFM) and high-resolution x-ray diffraction (HRXRD) results showed that the samples with higher indicative reflectance had smaller sized but higher density nuclei before the high-temperature GaN growth and lower screw threading dislocation (TD) density in the initially grown GaN. The difference in the background electron concentration and mobility of the interfacial layer was related to the relatively higher concentration of the O and Al diffused from Al2O3, which is also dependent on the size and density of the nuclei. These differences were found not to affect the structural and electrical properties or the surface morphology of AlGaN/GaN high electron-mobility transistors (HEMTs, except for the buffer conduction) when the GaN buffer is thick enough (e.g., ∼2.5 µm).

In this paper, we took a brief investigation on the properties of two-dimensional electron gas (2DEG) in AlGaN/InGaN/GaN heterostructures. Band profiles and the 2DEG distribution were calculated by self-consistently solving the Schrodinger and Poisson equations. Our calculated results indicate that the 2DEG concentration can be considerably increased with the incorporation of an InGaN layer. Quantum confinement, and thereby the 2DEG distribution are very sensitive to the strain of the incorporated InGaN layer. Hence one can design the 2DEG by incorporating proper InGaN layer into the conventional AlGaN/GaN heterostructures. These novel features are attributed to the strong polarization effect in AlGaN/InGaN/GaN heterostructures.

Pt/Au Schottky contacts were fabricated on modulation-doped Al0.22GaN0.78/GaN heterostructures. Some different surface treatments were used before the deposition of the metal contacts to prevent the formation of the native oxide layer on the samples’ surfaces. Comparing the barrier height and the ideal factor of the Schottky contacts, we found that the treatment of the boiling (NH4)2S solution can remove the native oxide layer effectively. Detailed X-ray photoelectron spectroscopy measurements were also made to investigate the chemical reactions on the surface of samples after different treatments.

The specific contact resistivity (ρ C ) and interfacial reaction between Au/Pt/Al/Ti metallic multi-layers and Si-doped n-type Al x Ga 1-x N (n-AlGaN) layers in modulation-doped Al 0.22 Ga 0.78 N/GaN heterostructures have been investigated. By means of the measurements based on the transmission line model, the ρ C as low as 1.6 × 10 -4 Ωcm 2 is obtained. Based on the X-ray diffraction analysis, it is found that N atoms in n-AlGaN layer diffuse out and a much amounts of N-vacancies are formed in n-AlGaN layer near the interface after the sample is annealed at temperatures higher than 500 °C. It induces the heavy n-type doped region in n-AlGaN near the interface, and thus leads to the decrease of the ρ C . With increasing the annealing temperature, more N atoms in n-AlGaN layer diffuse out and react with Ti atoms. Ti 2 N phase is formed at the interface after the sample is annealed at 800 °C. In this case, the ρ C further decreases.

An InGaN/GaN superlattice (SL) with Mg-doped barriers was designed and inserted into the InGaN-based blue light-emitting diodes (LEDs) as a hole gathering layer (HGL) to promote hole injection into the active region. The fabricated LEDs with the SL HGL show 36.4% increase in light output power at an injection current of 200 mA. Meanwhile, the efficiency droop is also mitigated effectively, as compared to the traditional LEDs. The improved performance is attributed to increased hole injection efficiency and decreased electron leakage into the p-type region.

Monolithic integrated 1.9-GHz low noise amplifier (LNA) using 1 μm-gate high-linearity and low noise composite-channel Al0.3Ga0.7N/Al0.05Ga0.95N/GaN HEMT (CC-HEMT) device is designed, fabricated and characterized. The LNA exhibits a noise figure of 1.3 dB, an associated gain of 17 dB, an input return loss of −5.5 dB and an output return loss of −15 dB at 1.9 GHz. The IIP3 and IIP5 of the LNA are 15 and 12 dBm at 1.9 GHz. The LNA with 1 × 100 μm2 device shows high-dynamic range with decent gain and noise figure. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1360–1362, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22459

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin In/sub x/Ga/sub 1-x/N(x=0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm/sup 2//V/spl middot/s and a sheet resistance of 420 /spl Omega//sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 14.5 GHz, and a peak power gain cutoff frequency (f/sub max/) of 45.4 GHz were achieved on a 1/spl times/100 /spl mu/m device. The off-state source-drain leakage current is as low as /spl sim/5 /spl mu/ A/mm at V/sub DS/=10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz.

We report broadband microwave noise characteristics of a high-linearity composite-channel HEMT (CC-HEMT). Owing to the novel composite-channel design, the CC-HEMT exhibits high gain and high linearity such as an output third-order intercept point (OIP3) of 33.2 dBm at 2 GHz. The CC-HEMT also exhibits excellent microwave noise performance. For 1-/spl mu/m gate-length devices, a minimum noise figure (NF/sub min/) of 0.7 dB and an associated gain (G/sub a/) of 19 dB were observed at 1 GHz, and an (NF/sub mi/) of 3.3 dB and a G/sub a/ of 10.8 dB were observed at 10 GHz. The dependence of the noise characteristics on the physical design parameters, such as the gate-source and gate-drain spacing, is also presented.

We report the characterization of quality (Q)-factor of RF metal-semiconductor-metal (MSM) planar interdigitated varactors fabricated by standard AlGaN/GaN HEMT process. The MSM varactors have wide tuning range and exhibit high quality-factor at both the maximum and minimum capacitance values. The fundamental limitation of the Q-factor in the medium capacitance range is also revealed. The elimination of ohmic contact resistance in the MSM varactor configuration pushed up the peak Q-factor to 92 at 0.5 GHz and 41 at 1.1 GHz. The operation of the MSM varactor is modeled by a physical equivalent circuit, with which the dependence of the Q-factor over the entire tuning voltage range can be explained.

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500/spl deg/C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from -4.0 to 0.9 V. Most importantly, a zero transconductance (g/sub m/) was obtained at V/sub gs/=0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At V/sub gs/=0 V, the off-state drain leakage current is 28 /spl mu/A/mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 /spl mu/m-long gate exhibit a maximum drain current density of 310 mA/mm, a peak g/sub m/ of 148 mS/mm, a current gain cutoff frequency f/sub T/ of 10.1 GHz and a maximum oscillation frequency f/sub max/ of 34.3 GHz.

We report an Al/sub 0.3/Ga/sub 0.7/N-Al/sub 0.05/Ga/sub 0.95/N-GaN composite-channel HEMT with enhanced linearity. By engineering the channel region, i.e., inserting a 6-nm-thick AlGaN layer with 5% Al composition in the channel region, a composite-channel HEMT was demonstrated. Transconductance and cutoff frequencies of a 1 /spl times/100 /spl mu/m HEMT are kept near their peak values throughout the low- and high-current operating levels, a desirable feature for linear power amplifiers. The composite-channel HEMT exhibits a peak transconductance of 150 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 12 GHz and a peak power gain cutoff frequency (f/sub max/) of 30 GHz. For devices grown on sapphire substrate, maximum power density of 3.38 W/mm, power-added efficiency of 45% are obtained at 2 GHz. The output third-order intercept point (OIP3) is 33.2 dBm from two-tone measurement at 2 GHz.

Digital satellite communication has been restricted to date to digital modulations having an efficiency of 2 bps/Hz; that is, filtered-QPSK with a close to constant envelope. The main limitation to higher-order digital modulations is the need for linearity of a high power amplifier in both the satellite and the terminal. Many years ago, C.M. Thomas et al. (see IEEE Trans. Commun., vol.COM-22, p.168-80, 1974) conducted a complete study of the use of higher-order linear modulations on nonlinear satellite channels. However, pulse-shaping and predistortion were not treated. We consider these items for amplitude-phase-shift-keying (APK) modulation, that is, signal points assigned on concentric circles. We consider both single- and double-link channels. Our best result is for a single-link channel and (5,11)-APK (5 points on an inner circle and 11 on the other), which, for predistortion on the nonlinear channel, loses only about 1 dB to 16-QAM transmission on the linear channel. Our results for single-link, narrow band channels are felt to be generally applicable to narrowband wireless communication channels.