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Quantum well infrared photodetectors (QWIPs) have been demonstrated to be a suitable candidate for IR detection applications. These detectors attracted increasing interest due to their design flexibility and broad spectral absorption from short wave (SWIR) to long wave infrared (LWIR) and high uniformity. In this paper, we demonstrate device design, growth, and characterization of a p-type AlxGa1-xN/GaN quantum well infrared photodetector (QWIP) for near IR absorption with 1.55 μm peak grown by metal organic chemical vapor deposition (MOCVD). Utilizing a p-QWIP allows for normal incidence light absorption due to the strong band mixing between heavy and light holes at k ≠ 0 which eliminates the need for light couplers such as grating and facilitates the fabrication of large focal plane arrays (FPAs). We developed MOCVD growth conditions to achieve nm-thick and smooth interfaces in QWIP. Sample characterizations including atomic force microscopy (AFM) show uniform surface morphology with RMS roughness ∼0.5 nm. Scanning transmission electron microscopy (STEM) was used to characterize layer thicknesses and interface roughness. We demonstrate energy band diagram simulation of an AlxGa1-xN/GaN p-QWIP by considering polarization chargers to determine the accurate band offset and adjust the absorption wavelength (ISBT energies). Our results show the feasibility of MOCVD-grown p-type AlxGa1-xN/GaN QWIP for IR absorption and open a pathway for further research and growth development on III-Nitride p-QWIPs, allowing growth and fabrication of large focal plane arrays.

We review the recent progress to achieve air stable III-nitride photocathodes for applications as photon detectors. High conductivity p-type films are critically important to realize high quantum efficiency (QE) photocathodes with effective negative electron affinity (NEA) and downward surface band bending with narrow surface depletion width. Initial reports of III-nitride photocathodes utilize a Cs-surface activation to achieve effective NEA. To attain air stable, Cs-free photocathodes, novel energy band engineering has been shown using Si delta-doping and an n+-GaN cap on the surface of a Ga-polar p-GaN layer. Improvement of QE has been achieved utilizing the N-polarity of III-nitrides due to advantageous depletion and polarization charges at the surface. High QE > 25% has been demonstrated by improvements in p-type conductivity with improved Mg-dopant incorporation in N-polar hillock structures and by control over unintentional impurity incorporation and distribution. Further, the importance of achieving high p-type conductivity films is further shown through reviewing recent simulations of GaN photocathodes with varied band bending and hole concentrations. Through comparison of experimental photoemission with Monte Carlo simulations, band structure parameters such as electron effective mass in the conduction band valleys have been elucidated.

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.

In this work, physical compact modelling of Gallium Nitride High Electron Mobility Transistor (GaN HEMT) based pressure sensor for high temperature applications is presented. The model builds around device placement at arbitrary positions on circular membranes, which itself acts as a transducer between pressure and surface strain. The presented model is integrated with the compact Advanced SPICE Model for High Electron Mobility Transistors (ASM-HEMT), as the core HEMT model. In this part, the effects of internal strain, external strain and temperature on the HEMT characteristic pinch-off voltage is modelled as a function of epitaxial design (barrier design, device dimensions, substrate type and thickness). In addition, the device response to pressure as a function of position on the membrane is also modelled. The analytical equations to describe the mechanical strain situation at the wafer surface, was integrated in the compact model on basis of Finite Element Method (FEM) simulations. Lattice-related effects like thermal expansion and elastic constants degradation with temperature is also modelled, based on physical equations fitted to experimental published data. The model was verified against experimental ${C}$ - ${V}$ measurements and published experimental data of GaN HEMTs used as pressure sensors, with different substrates (Si, sapphire), different epitaxial design, and different sensor geometry. The presented compact model allows the simulation of GaN based pressure sensors as a part of a compact circuit simulation. The model developed here is used in Part II to propose an optimized design of temperature-insensitive GaN-based pressure sensor, for high temperature applications. [ABSTRACT FROM AUTHOR]

It is known that the commercialized white light-emitting diodes (WLEDs) depend on phosphor conversion, which is limited by the unbalanced carrier injection in multiple quantum wells (MQWs) and low down-conversion efficiency. To solve these problems, hybrid organic/inorganic III-nitride WLEDs were designed and fabricated based on Rubrene/(InGaN/GaN) MQWs p-n junction. In such new structure, Rubrene can act as yellow-red emitter and play an important role in hole transport layer owing to its high carrier mobility. By optimizing the hybrid LED structure, the charge balance and recombination zone in radiative region were significantly improved. In this work, the optimal hybrid WLED with color coordinates of (0.31, 0.33) shows a maximum current efficiency of 15.22 cd/A with excellent long lifetime stability. The hybrid WLEDs based on organic/inorganic p-n junction were demonstrated with a good performance, which provides an attractive route to achieve high performance phosphor-free WLED sources for microdisplay or large-area WLED applications.

In this letter, we present a multi-channel in-plane-gate field effect transistor (MC-IPGFET). In the proposed device, multiple vertically stacked two-dimensional electron gases (2DEGs) are simultaneously controlled by lateral in-plane gates formed with the same multi-2DEG stack. The multi-channel heterostructure allows to increase carrier density in the channel while keeping high electron mobility. Besides, the in-plane gate geometry provides an effective control of multiple channels with a smaller intrinsic gate capacitance. As compared to single-channel IPGFETs, multi-channel structure resulted in a three-time enhancement in current density and transconductance, offering opportunities for efficient scaling up of in-plane gate devices. High current density of 4.35 A/mm along with 2.05 S/mm transconductance are achieved in an optimized device. The effective control of the multiple high-mobility channels along with the reduced intrinsic capacitance of the in-plane gate open a pathway for new device concepts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is known that the commercialized white light-emitting diodes (WLEDs) depend on phosphor conversion, which is limited by the unbalanced carrier injection in multiple quantum wells (MQWs) and low down-conversion efficiency. To solve these problems, hybrid organic/inorganic III-nitride WLEDs were designed and fabricated based on Rubrene/(InGaN/GaN) MQWs p-n junction. In such new structure, Rubrene can act as yellow–red emitter and play an important role in hole transport layer owing to its high carrier mobility. By optimizing the hybrid LED structure, the charge balance and recombination zone in radiative region were significantly improved. In this work, the optimal hybrid WLED with color coordinates of (0.31, 0.33) shows a maximum current efficiency of 15.22 cd/A with excellent long lifetime stability. The hybrid WLEDs based on organic/inorganic p-n junction were demonstrated with a good performance, which provides an attractive route to achieve high performance phosphor-free WLED sources for microdisplay or large-area WLED applications. [ABSTRACT FROM AUTHOR]

In-plane-gate field-effect transistors (IPGFETs) offer an innovative device architecture in which the channel conductivity is modulated by the electric field from the 2D electron gas in the two adjacent in-plane gates, isolated by etched trenches. The planar nature of the gate electrode yields a huge reduction in parasitic gate capacitance, which can lead to much higher frequency. Moreover, the fabrication process for these devices is extremely simple and with inherently self-aligned gates. Here, we combine for the first time the promising architecture of IPGFETs with the exceptional properties of III-Nitrides, such as large carrier density and breakdown field, to reveal their enormous potential for high-power RF devices. AlGaN/GaN IPGFETs demonstrated large drain current up to 1.4 A/mm and transconductance up to 665 mS/mm, which are, respectively, nine times- and five times-larger than the best IPGFETs demonstrated in other semiconductors. These devices presented excellent gate control with ON–OFF ratio up to 10^7 along with ultra-low capacitances down to 0.7 aF, leading to an estimated fT up to 0.89 THz. Extremely large breakdown voltage of 500 V was observed despite their nanoscale dimensions, with small leakage current below 1 nA up to 300 V. These results reveal that III-Nitride IPGFETs offer a promising pathway for future terahertz devices delivering large output powers. [ABSTRACT FROM AUTHOR]

The fabrication studies of silica/polystyrene (PS) colloidal microlens arrays with various aspect ratios were performed on the III-nitride light-emitting diodes (LEDs). The use of colloidal-based microlens arrays led to significant enhancement in light extraction efficiency for III-nitride LEDs. In varying the aspect ratios of the microlens arrays, the engineering of various PS thicknesses was employed by using high-temperature treatment and redeposition process. The effects of PS thickness on the light extraction efficiency and far-field emission patterns of InGaN quantum-well (QW) LEDs were studied. The total output powers of microlens LEDs with various PS thicknesses exhibited 1.93-2.70 times enhancement over that of planar LEDs, and the use of optimized PS layer thickness is important in leading the enhancement of the light extraction efficiency in large angular direction.

We review III-Nitride optoelectronic device technologies with an emphasis on recent breakthroughs. We start with a brief summary of historical accomplishments and then report the state of the art in three key spectral regimes as follows: 1) Ultraviolet (AlGaN-based avalanche photodiodes, single photon detectors, focal plane arrays, and light-emitting diodes); 2) Visible (InGaN-based solid state lighting, lasers, and solar cells); and 3) Near-, mid-infrared, and terahertz (AlGaN/GaN-based gap-engineered intersubband devices). We also describe future trends in III-Nitride optoelectronic devices.

Devices based on nanostructures hold great potential to improve the performance of present light emitter technologies. In this paper, we examine a buried multiquantum well III-nitride diffusion injected light-emitting diode (DILED), where the active region is located outside the p-n junction and current injection to the active region takes place through bipolar diffusion. We study the current–voltage behavior and light emission characteristics of the DILED as a function of temperature experimentally and theoretically. We show that in contrast to conventional LEDs, the light output efficiency of the DILED increases when temperature is increased from 0 °C to 100 °C. This anomalous temperature behavior is shown to be linked to the strong temperature dependency of ionized acceptor density in the p-doped region, which increases the hole diffusion current into the active region. This highlights the fundamental difference in the operating principle of the DILED compared with conventional LEDs. In addition to optical and electrical characterization of a DILED, we also study the relation of the observed yellow-band luminescence to Shockley–Read–Hall recombination, compare the measurements to charge carrier transport simulations, and present an equivalent circuit model of the DILED structure for additional insight into the new current injection scheme. [ABSTRACT FROM AUTHOR]

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered. [ABSTRACT FROM AUTHOR]

Investigation of the reliable operation of AlGaN/GaN HFETs at elevated temperatures is attempted. In this paper, a Monte Carlo-based temperature-dependent mobility model, with incorporation of steady-state velocity overshoot, is employed in modeling the drain current-voltage characteristics of AlGaN/GaN HFETs at 300, 400, and 500 K. One of the major merits of this model is that it employs a very small set of fitting parameters. The modeled drain current-voltage characteristics have been successfully matched to the experimental characteristics at the aforementioned temperatures. While confirming that a brief measurement at these temperatures is of no reliability concern on the quality of the metal-semiconductor contacts, this matching proves that the temperature dependency of the electron drift velocity is the cause of the degradation of drain current within the aforementioned range of temperature. In producing the aforementioned match for the long-gate AlGaN/GaN HFETs, it is also shown that the accurate modeling of the temperature dependency of the low-field drift transport is more consequential than the accurate representation of the transport in the medium-to-high electric fields. [ABSTRACT FROM PUBLISHER]

We report on the physics, epitaxial growth, fabrication, and characterization of optoelectronic devices based on intersubband transitions in the AlN/GaN material system. While in 1999, only results of optical absorption experiments could be shown, photodetectors and modulators with operation frequencies beyond 10 GHz as well as optically pumped light emitters have been demonstrated recently. This is the reason for a comprehensive report on the most important properties of such devices. Beside some basic theoretical considerations, we will concentrate on the fabrication and characterization of modulators, switches, photodetectors, and light emitters. At the end of this paper, an outlook to future trends and developments in this emerging field will be given. [ABSTRACT FROM AUTHOR]

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations. [ABSTRACT FROM PUBLISHER]

The performance characteristics of green light-emitting diodes (LEDs) grown by metal-organic chemical-vapor deposition were investigated to study the dependence of the device performance on the materials and the growth conditions of p-type layer grown after the InGaN multiple-quantum-well active region. The electrical and structural qualities of Mg-doped p-In0.04Ga0.96N and p-GaN layers grown under different growth conditions were studied to optimize the growth conditions of p-type hole injection layers of green LEDs. A free-hole concentration of p=1.6times1018 cm-3 of with a resistivity of 0.33Omegamiddotcm was achieved for p-GaN:Mg layers grown at 1040degC. Lower hole concentrations and mobilities and rough surfaces were obtained when the growth temperature was decreased to 930degC in H2 ambient. In the case of p-In0.04Ga0.96N grown at 840degC in N2, a significant improvement of the hole concentration was achieved due to the reduced ionization activation energy of Mg acceptors in InGaN. Also we observed that as-grown p-GaN layers grown in N2 ambient showed p-type properties without Mg dopant activation. The electrical and optical properties of In0.25Ga0.75 N/GaN multiple-quantum-well green LEDs with such different p-layers were investigated. The electroluminescence intensity was improved for the LEDs with p-In0.04Ga0.96N layers grown at 840degC as compared to the LEDs with p-GaN layers grown at higher temperatures due to the reduced thermal damage to the active region, high hole injection, and low piezoelectric field induced in the active region. p-InGaN layers are very attractive candidates for the p-layer in green LED structures. The low temperature and N2 ambient used during the growth of InGaN layers are beneficial to protect the InGaN active region containing high-indium composition quantum-well layers in a- ddition to the advantage of providing a higher hole concentration. However, the LEDs with p-In0.04Ga0.96 N layer showed a slightly higher turn on voltage which could originate from the potential barrier for hole transport at the interface of the p-InGaN layer and the last GaN quantum-well barrier. to reduce this problem, we designed and characterized an LED structure having a graded indium composition in the p-In0.04Ga0.96N layer in order to improve hole transport into the active region. Optimized LEDs with p-InGaN layers grown in a N2 ambient showed much brighter electroluminescence due to low damage to the active region during p-InGaN layer growth [ABSTRACT FROM PUBLISHER]