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This work reports on high-reflectivity Ag nanodot (AgND)/Mg/Al Ohmic contacts suitable for both p-GaN and n-GaN. The lowest specific contact resistances are found to be 2.25 × 10−2 Ω·cm2 on p-GaN and 2.56 × 10−5 Ω·cm2 on n-GaN. Ag was deposited and converted into AgNDs by annealing, and Mg/Al was then deposited. A second annealing process at different temperatures was performed to check the thermal stability of the contacts. Both the p-GaN and n-GaN contacts were Ohmic after annealing at 300 °C or below. The AgND/Mg/Al contacts annealed at 250 °C or less showed a reflectivity of over 91% for wavelengths from 400 to 550 nm. X-ray photoelectron spectroscopy and x-ray diffraction measurements were performed to investigate the contact mechanisms. We propose that in AgND/Mg/Al on p-GaN, the effective barrier is lowered due to the presence of an Ag2O intermedia layer and the tunneling effect enables Ohmic contact. When the annealing temperature is 350 °C or higher, the Ag2O changes to β-AgGaO2 or is decomposed and the height and width of the barrier for holes increase, which causes the Ohmic contact to deteriorate. The Ohmic behavior of AgND/Mg/Al contacts on n-GaN is assumed to be mainly due to the high direct coverage ratio of Mg and the good Ohmic contact behavior of Mg/n-GaN. These results show that AgND/Mg/Al Ohmic contacts can be fabricated simultaneously on both p-GaN and n-GaN, which is a possible solution for improving the density of both GaN-based ICs and micro-light emitting diodes (LEDs). This contact scheme can also improve the light output efficiency of GaN-based LEDs.

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.

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.

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.

Recently, the emerging inorganic perovskites CsPbX 3 (X = Br, I) have received great attention in the photovoltaic community due to their excellent photoelectric property and good thermal stability. However, most of the reported CsPbX 3 films are fabricated by the uncontrolled solution method, and it is challenging to generate pinhole-free Br-rich films due to the over-rapid liquid-phase reaction feature and the solubility limitation of bromide. Herein, we develop a sequential chemical vapor deposition (CVD) method to fabricate colorful, band-gap tunable, and stable CsPb(I x Br 1-x ) 3 films. First, air-stable CsPbBr 3 precursor films are fabricated via a Br 2 -assisted CVD process. Second, a series of CsPb(I x Br 1-x ) 3 (x = 0, 0.2, 0.33, 0.5, 0.67, 0.8, 1) high-quality films with consistent compositions are then obtained by another MAI/MABr-assisted CVD process. Experimental results show that all of the CsPb(I x Br 1-x ) 3 films have the same cubic structure and (100) preferred orientation. With the increase of MAI component, the colors of films darken gradually, the main diffraction peaks at (100) and (200) planes shift to small angle, their optical bandgaps monotonously decrease from 2.34 eV to 1.67 eV, the grains of films grow up, and the porous films become more compact. Strikingly, it is found that CsPb(I x Br 1-x ) 3 films become very stable in air ambient when x is less or equal to 0.5. Therefore, we first employ a cost-effective CVD technology to modulate the anion compositions of CsPbX 3 inorganic perovskite films, which shows a high potential for the photovoltaic applications such as highly efficient tandem perovskite solar cells (PSCs) and colorful smart photovoltaic windows.

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.

Highly efficient lead halide perovskites with tunable emission performance have become new candidate materials for light‐emitting devices and displays; however, the toxicity of lead and instability of halide perovskites greatly limits their application. Herein, rapid and large‐scale synthesis of highly emissive organic–inorganic manganese halide perovskites, (C5H6N)2MnBr4 and C5H6NMnCl3, are presented by a one‐pot solution‐based method, of which (C5H6N)2MnBr4 displays a high absolute photoluminescence quantum yield (95%) in the solid‐state. The developed (C5H6N)2MnBr4 perovskite noticeably exhibits high stability. Therefore both as‐synthesized green and red emissive manganese‐based phosphors with superior optical properties are used to fabricate blue light pumped white light‐emitting diodes (WLEDs), displaying excellent quality white light with a high color rendering index value of 91 and a correlated color temperature of 5331 K. This study not only presents the robust large‐scale production synthetic approach for organic–inorganic manganese halide perovskites, but also facilitates the development of high‐performance phosphors for future lighting and display technologies.

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.

We numerically investigated the optical/electrical characteristics of blue light-emitting-diodes (LEDs) and developed some optimization strategies. Our aim is to increase the optical performance of LEDs at low current density (0.02–2 A cm−2) for micro-LEDs used in display applications. First, we investigated the effects of quantum well (QW) number in blue In0.225Ga0.775N/In0.10Ga0.90N LEDs. We found that at low current density LEDs with lower QW number have much higher internal quantum efficiency (IQE) than those with higher QW number. Second, we also numerically investigated the effects of the electron blocking layer, n-side quantum barrier (QB) doping concentration, QB material and QW width with single quantum well (SQW) structure. For SQW LEDs at low current density, the electron injection efficiency was nearly one. In terms of the QB doping concentration, we found that an appropriate n-side QB doping can ensure Shockley-Read-Hall recombination appropriately equal Auger recombination, which ultimately results in high IQE. For QB material, we choose In0.05Ga0.95N as QB material to achieve both high IQE and low forward voltage. Regarding the QW width, we found that although LEDs with wide QW had high IQE, there were also two peaks in the electroluminescence (EL) spectrum, thus 3 nm was used as an optimal QW width. The original 5-QW structure showed a lower IQE range from 50 to 80% at the current density range from 0.01 to 2 A cm−2 and a forward voltage of 3.06 V at 1 A cm−2. In contrast, our optimized LED structure achieved an IQE of 92.3% at the current density range from 0.01 to 2 A cm−2 and a forward voltage of 2.72 V at 1 A/cm2.

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

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.