Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-µm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

ß-Ga2O3nanotube arrays for high-performance self-powered ultraviolet photoelectrochemical photodetectors.

Self-powered ultraviolet (UV) photodetectors (PDs) are critical for future energy-efficient optoelectronic systems due to their low energy consumption and high sensitivity. In this paper, the vertically alignedß-Ga2O3nanotube arrays (NTs) have been prepared on GaN/sapphire substrate by the thermal oxidation process combined with the dry etching technology, and applied in the UV photoelectrochemical photodetectors (PEC-PDs) for the first time. Based on the large specific surface area ofß-Ga2O3NTs on GaN/sapphire substrates and the solid/liquid heterojunction, the PEC-PDs exhibit excellent self-powered characteristics under 255 nm (UVA) and 365 nm (UVC) light illumination. Under 255 nm (365 nm) light illumination, the maximum responsivity of 49.9 mA W-1(32.04 mA W-1) and a high detectivity of 1.58 × 1011Jones (1.01 × 1011Jones) were achieved for theß-Ga2O3NTs photodetectors at 0 V bias. In addition, the device shows a fast rise/decay time of 8/4 ms (4/2 ms), which is superior to the level of the previously reported self-powered UV PEC-PDs. This high-performance PEC-PD has potential applications in next-generation low-energy UV detection systems.

Al0.1Ga0.9N p-i-n ultraviolet avalanche photodiodes with suppressed surface leakage current and uniform avalanche breakdown.

We report high-performance Al0.1Ga0.9N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with stable breakdown voltages (VBR) around 113.4 V, low dark current densities (JBR) below 9 × 10-4 A/cm2 and a high avalanche gain over 2 × 106. A two-step deposition method was employed to reduce passivation-induced plasma damage while maintaining high dielectric film quality. Consistent JBR for various mesa sizes at the VBR are demonstrated, which reveals the suppression of the surface leakage current. Uniform electroluminescence (EL) distributions during the avalanche multiplication processes are displayed, which confirms the elimination of edge breakdown. Pure bulk leakage current distributions and uniform body avalanche breakdown behaviors are observed for the first time in AlGaN APDs. The emission spectra of the EL at various current levels are also presented.

Enhanced light output from deep ultraviolet light-emitting diodes enabled by high-order modes on a photonic crystal surface.

The authors demonstrate the enhanced light output from 275-nm AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED) structures via the in-plane modulation of shallow photonic crystal (PC) patterns that were fabricated on the p-AlGaN contact layer surface. The employed PC lattice constants are in the range of 270-780 nm, much larger than the fundamental Bragg order lattice constant (∼95 nm). As compared to the unpatterned sample, the intensity of the top (or bottom) emission can be enhanced by up to 331% (or 246%), attributed to the high-order coherent diffraction of the internal trapped light and also the Purcell enhancement of spontaneous emission. The findings in this Letter suggest an easier way for the realization of more energy-efficient DUV LEDs which offer the advantage of high emission for various applications in disinfection and sterilization.

High Power Figure-of-Merit, 10.6-kV AlGaN/GaN Lateral Schottky Barrier Diode with Single Channel and Sub-100-µm Anode-to-Cathode Spacing.

GaN-based lateral Schottky barrier diodes (SBDs) have attracted great attention for high-power applications due to its combined high electron mobility and large critical breakdown field. However, the breakdown voltage (BV) of the SBDs are far from exploiting the material advantages of GaN at present, limiting the desire to use GaN for ultra-high voltage (UHV) applications. Then, a golden question is whether the excellent properties of GaN-based materials can be practically used in the UHV field? Here, UHV AlGaN/GaN SBDs are demonstrated on sapphire with a BV of 10.6 kV, a specific on-resistance (RON,SP ) of 25.8 mΩ cm2 , yielding a power figure-of-merit (P-FOM = BV2 /RON,SP ) of 4.35 GW cm-2 . These devices are designed with single channel and 85-µm anode-to-cathode spacing, without other additional electric field management, demonstrating its great potential for the UHV application in power electronics.

Responsibility optimization of a high-speed InP/InGaAs photodetector with a back reflector structure.

Top-illuminated PIN photodetectors (PDs) are widely utilized in telecommunication systems, and more efforts have been focused on optimizing the optical responsibility and bandwidth for high-speed and capacity applications. In this work, we develop an integrated top-illuminated InP/InGaAs PIN PD with a back reflector by using a microtransfer printing (µ-TP) process. An improved µ-TP process, where the tether of silicon nitride instead of photoresist, is selected to support an underetched III-V device on an InP substrate before transfer. According to theoretical simulations and experimental measurements, the seamless integration of the PD with a back reflector through µ-TP process makes full use of the 2nd or even multiple reflecting light in the absorption layer to optimize the maximum responsibility. The integrated device with a 5 µm square p-mesa possesses a high optical responsibility of 0.78 A/W and 3 dB bandwidth of 54 GHz using a 500 nm i-InGaAs absorption layer. The present approach for top-illuminated PIN PDs demonstrates an advanced route in which a thin intrinsic layer is available for application in high-performance systems.

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

Electron-Beam-Driven III-Nitride Plasmonic Nanolasers in the Deep-UV and Visible Region.

Plasmonic nanolasers based on wide bandgap semiconductors are presently attracting immense research interests due to the breaking in light diffraction limit and subwavelength mode operation with fast dynamics. However, these plasmonic nanolasers have so far been mostly realized in the visible light ranges, or most are still under optical excitation pumping. In this work, III-nitride-based plasmonic nanolasers emitting from the green to the deep-ultraviolet (UV) region by energetic electron beam injection are reported, and a threshold as low as 8 kW cm-2 is achieved. A fast decay time as short as 123 ps is collected, indicating a strong coupling between excitons and surface plasmon. Both the spatial and temporal coherences are observed, which provide a solid evidence for exciton-plasmon coupled polariton lasing. Consequently, the achievements in III-nitride-based plasmonic nanolaser devices represent a significant step toward practical applications for biological technology, computing systems, and on-chip optical communication.

Back-illuminated AlGaN heterostructure solar-blind avalanche photodiodes with one-dimensional photonic crystal filter.

AlGaN heterostructure solar-blind avalanche photodiodes (APDs) were fabricated on a double-polished AlN/sapphire template based on a separate absorption and multiplication (SAM) back-illuminated configuration. By employing AlGaN heterostructures with different Al compositions across the entire device, the SAM APD achieved an avalanche gain of over 1×105 at an operated reverse bias of 92 V and a low dark current of 0.5 nA at the onset point of breakdown. These excellent performances were attributed to the acceleration of holes by the polarization electric field with the same direction as the reverse bias and higher impact ionization coefficient of the low-Al-content Al0.2Ga0.8N in the multiplication region. However, the Al0.2Ga0.8N layer produced a photocurrent response in the out of the solar-blind band. To retain the solar-blind detecting characteristic, a periodic Si3N4/SiO2 photonic crystal was deposited on the back of the AlN/sapphire template as an optical filter. This significantly improved the solar-blind characteristic of the device.

Localized surface plasmon enhanced Ga2O3 solar blind photodetectors.

Enhancement in the light interaction between plasmonic nanoparticles (NPs) and semiconductors is a promising way to enhance the performance of optoelectronic devices beyond the conventional limit. In this work, we demonstrated improved performance of Ga2O3 solar-blind photodetectors (PDs) by the decoration of Rh metal nanoparticles (NPs). Integrated with Rh NPs on oxidized Ga2O3 surface, the resultant device exhibits a reduced dark current of about 10 pA, an obvious enhancement in peak responsivity of 2.76 A/W at around 255 nm, relatively fast response and recovery decay times of 1.76 ms/0.80 ms and thus a high detectivity of ∼1013 Jones. Simultaneously, the photoresponsivity above 290 nm wavelength decreases significantly with improved rejection ratio between ultraviolet A (UVA) and ultraviolet B (UVB) regions, indicative of enhanced wavelength detecting selectivity. The plasmonic resonance features observed in transmittance spectra are consistent with the finite difference time-domain (FDTD) calculations. This agreement indicates that the enhanced electric field strength induced by the localized surface plasmon resonance is responsible for the enhanced absorption and photoresponsivity. The formed localized Schottky barrier at the interface of Rh/Ga2O3 will deplete the carriers at the Ga2O3 surface and lead to the remarkable reduced dark current and thus improve the detectivity. These findings provide direct evidence for Rh plasmonic enhancement in solar-blind spectral region, offering an alternative pathway for the rational design of high-performance solar-blind PDs.

The influence of an AlN seeding layer on nucleation of self-assembled GaN nanowires on silicon substrates.

Gallium nitride (GaN)-based nanowires (NWs) have attracted much attention for the fabrication of novel nanostructured devices. In this paper, the influence of an AlN seeding layer on the nucleation of self-assembled GaN NWs grown by plasma-assisted molecular beam epitaxy (MBE) on Si (111) substrates has been investigated. Not only is the formation of a two-dimensional compact GaN layer at the bottom of the NWs suppressed, but also a high density of vertically aligned well-separated GaN NWs originating from GaN islands are successfully obtained after introducing annealing and nitridation processes. Scanning electronic microscope and transmission electron microscope measurements show that the NWs have a high crystalline wurtzite structure nearly free of dislocations and stacking faults and the NW diameter remains constant over almost the entire length. Due to the temperature-dependent diffusion length of Ga adatoms during the nucleation process, the formation of well-separated NWs relies on the distribution and morphology of the underlying AlN seeding layer. Moreover, the SiNx layer served as mask to inhibit coalescence at the nucleation sites. The developed growth processes and the obtained results provide a viable path facilitating the use of MBE growth techniques to fabricate III-nitride NW-based materials and related devices on Si substrates.

Study on the self-absorption of InGaN quantum wells at high photon density.

In this paper, the self-absorption of InGaN quantum wells at high photon density is studied based on a rectangular ridge structure. The ridge structure was fabricated based on a standard GaN-based blue LED wafer grown on (0001) patterned sapphire substrate. The high-density photons were obtained by a high-power femtosecond laser with high excitation of 42kW/cm2 at room temperature. Based on the analysis of the photoluminescence intensities of the InGaN quantum wells, we found that the absorption coefficient of the InGaN quantum wells varies with the background photon density. The results revealed that the final absorption coefficient of the InGaN quantum well decreases with the increase of photon density, which can be 48.7% lower than its normal value under our experimental conditions.

Temperature-dependent ultraviolet Raman scattering and anomalous Raman phenomenon of AlGaN/GaN heterostructure.

Temperature-dependent ultraviolet (UV) Raman scattering from AlGaN/GaN heterostructure is investigated. Compared to the visible Raman spectrum, four new peaks at 600, 700, 780, and 840 cm-1 are observed in the UV Raman spectrum. The peak at 780 cm-1 is from the AlGaN A1(LO) mode. According to the calculated dispersion relations of the interface phonon modes in the AlGaN/GaN heterostructure, the peaks at 600 and 840 cm-1 correspond to interface phonon modes. Meanwhile, the peak at 700 cm-1 is attributed to the disorder-active mode near the 2DEG interface. Due to the near-resonant enhancement effect, the intensities of the GaN A1(LO) mode, interface phonon modes, disorder active mode and the AlGaN A1(LO) mode exhibit different temperature dependence. Furthermore, the frequencies of the interface phonon modes and the disorder active mode show anomalous temperature dependence, which can be attributed to the strong built-in electric field near the 2DEG interface.

Enhanced polarization photodetection of metallic cavity ensemble through spontaneously configured lateral electrodes.

Metallic cavities show substantial advantages in light confinement, providing opportunities to modulate the optical resonances and absorption. Here, we report on the configuration of horizontally aligned ZnO-nanowires-based metallic cavity ensemble with a light to dark current ratio of ∼1000. An enhanced polarization photodetection ratio of transverse electric (TE) to transverse magnetic (TM) was experimentally observed compared to the single ZnO nanowire photodetector. Finite difference time domain simulation was performed on the metallic cavities, showing the distinct resonance behaviors under TE and TM light. The confinement by the multi-reflection and optical resonances between the metallic claddings contribute to the high anisotropy ratio. Furthermore, the polarized light absorption in the metallic cavity was studied as well as in the naked nanowire, which reveal a significant dependence on the cavity diameter and wavelength. For the metallic cavities, the absorption ratio of TE to TM show an enhanced value in the range of 300-500 nm wavelength and 85-150 diameter and a reversed value in the range of 400-500 nm wavelength and 17-50 diameter. While for the naked nanowires, the ratio show an apparently opposite value in these two regions. The presented metallic cavities demonstrate a specific paradigm of optical engineering in nanoscale and potentially helps the development of optoelectronic devices.

Toward facile broadband photodetectors based on self-assembled ZnO nanobridge/rubrene heterointerface.

ZnO nanowire photodetectors have attracted much attention due to their excellent optoelectronic performance. However, operating speed remains a challenge, and scalability is also impeded by uncontrolled transfer methods and sophisticated fabrication process. In this paper, we have fabricated an excellent ZnO nanobridge ultraviolet photodetector array by using a simple one-step method. The faster photoresponse speed and a broader response wavelength (from UV to visible range) have been achieved by constructing a type-II ZnO/rubrene heterointerface. Performance enhancement is believed to arise from the well-matching band alignment and highly efficient separation of photogenerated electron-hole pairs at the heterointerface. Our strategy provides a simple and promising route to develop cost-effective and highly sensitive UV-vis photodetectors.

Low-threshold ultraviolet stimulated emissions from large-sized single crystalline ZnO transferable membranes.

Wide-bandgap inorganic semiconductors based ultraviolet lasers bring versatile applications with significant advantages including low-power consumption, high-power output, robustness and long-term operation stability. However, flexible membrane lasers remain challenging predominantly due to the need for a lattice matched supporting substrate. Here, we develop a simple laser liftoff process to make freestanding single crystalline ZnO membranes that demonstrate low-threshold ultraviolet stimulated emissions together with large sized dimension (> 2 mm), ultralow-weight (m/A<15 g/m2) and excellent flexibility. The 2.6 µm-thick crack-free ZnO membrane exhibits well-retained single crystallinity and enhanced excitonic emissions while the defect-related emissions are completely suppressed. The inelastic exciton-exciton scattering stimulated emissions with increased spontaneous emission rate is obtained with a reduced threshold of 0.35 MW/cm2 in the ZnO membrane transferred onto a flexible polyethylene naphthalate substrate. Theoretical simulations reveal that it is a synergetic effect of the increased quantum efficiency via Purcell effect and the improved optical gain due to vertical directional waveguiding of the membrane, which functions as a Fabry-Perot photonic resonator due to the refractive index contrast at ZnO-air boundaries. With simple architecture, efficient exciton recombination and easy fusion with waveguide system, the ZnO membranes provide an alternative platform to develop compact low-threshold ultraviolet excitonic lasers.

Photo-assisted hysteresis of electronic transport for ZnO nanowire transistors.

Recently, ZnO nanowire field effect transistors (FETs) have received renewed interest due to their extraordinary low dimensionality and high sensitivity to external chemical environments and illumination conditions. These prominent properties have promising potential in nanoscale chemical and photo-sensors. In this article, we have fabricated ZnO nanowire FETs and have found hysteresis behavior in their transfer characteristics. The mechanism and dynamics of the hysteresis phenomena have been investigated in detail by varying the sweeping rate and range of the gate bias with and without light irradiation. Significantly, light irradiation is of great importance on charge trapping by regulating adsorption and desorption of oxygen at the interface of ZnO/SiO2. Carriers excited by light irradiation can dramatically promote trapping/detrapping processes. With the assistance of light illumination, we have demonstrated a photon-assisted nonvolatile memory which employs the ZnO nanowire FET. The device exhibits reliable programming/erasing operations and a large on/off ratio. The proposed proto-type memory has thus provided a possible novel path for creating a memory functionality to other low-dimensional material systems.

High Sensitive pH Sensor Based on AlInN/GaN Heterostructure Transistor.

The AlInN/GaN high-electron-mobility-transistor (HEMT) indicates better performances compared with the traditional AlGaN/GaN HEMTs. The present work investigated the pH sensor functionality of an analogous HEMT AlInN/GaN device with an open gate. It was shown that the Al0.83In0.17N/GaN device demonstrates excellent pH sense functionality in aqueous solutions, exhibiting higher sensitivity (−30.83 μA/pH for AlInN/GaN and −4.6 μA/pH for AlGaN/GaN) and a faster response time, lower degradation and good stability with respect to the AlGaN/GaN device, which is attributed to higher two-dimensional electron gas (2DEG) density and a thinner barrier layer in Al0.83In0.17N/GaN owning to lattice matching. On the other hand, the open gate geometry was found to affect the pH sensitivity obviously. Properly increasing the width and shortening the length of the open gate area could enhance the sensitivity. However, when the open gate width is too larger or too small, the pH sensitivity would be suppressed conversely. Designing an optimal ratio of the width to the length is important for achieving high sensitivity. This work suggests that the AlInN/GaN-based 2DEG carrier modulated devices would be good candidates for high-performance pH sensors and other related applications.

Phase shifting mask modulated laser patterning on graphene.

A one-step graphene patterning method is developed in this paper. A phase shifting mask is used to modulate incident laser beam spatially and generate graphene patterns by laser heating. Periodic graphene nanoribbon and nanomesh structures are fabricated by employing 1D and 2D phase shifting masks, respectively. The noncontact, simple procedure, easy handling and economic properties of this method make it promising towards graphene-based device fabrication.

Great enhancement in the excitonic recombination and light extraction of highly ordered InGaN/GaN elliptic nanorod arrays on a wafer scale.

A series of highly ordered c-plane InGaN/GaN elliptic nanorod (NR) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The photoluminescence (PL) integral intensities of NR samples show a remarkable enhancement by a factor of up to two orders of magnitude compared with their corresponding as-grown samples at room temperature. The radiative recombination in NR samples is found to be greatly enhanced due to not only the suppressed non-radiative recombination but also the strain relaxation and optical waveguide effects. It is demonstrated that elliptic NR arrays improve the light extraction greatly and have polarized emission, both of which possibly result from the broken structure symmetry. Green NR light-emitting diodes have been finally realized, with good current-voltage performance and uniform luminescence.