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We proposed a method to determine the internal quantum efficiency (IQE) of GaInN-based light-emitting diode (LED). For the accurate determination thereof, we carefully reviewed a conventional carrier rate equation and then proposed a set of advanced formulae, which can comprehensively explain the carrier dynamics in a modern GaInN-based LED. Based on our proposed formula, this convenient method to determine the IQE is presented. Next, to identify the proposed carrier rate equations and recombination dynamics, we investigated carrier lifetime. We also discuss the physical origins of IQE degradation in a modern GaInN-based LED such as efficiency droop and green gap. [ABSTRACT FROM AUTHOR]

We have systematically investigated acceptor (Mg) and donor (Si) profiles at GaN tunnel junction interfaces with the aim of a low resistivity under reverse bias. We found that an overlap between Mg and Si at the tunnel junction interface was effective in helping to obtain a lower resistivity of the GaN tunnel junctions, which contradicts the typical picture of conventional semiconductor-based tunnel junctions. We demonstrated a LED with the GaN tunnel junction prepared in a single growth run by metalorganic vapor phase epitaxy, showing a differential resistivity of 2.4 × 10−4 Ω cm2 at 5 kA cm−2. [ABSTRACT FROM AUTHOR]

This paper describes the status and prospects of gallium nitride-based vertical-cavity surface-emitting lasers (VCSELs) with semiconductor-based distributed Bragg reflectors. These optoelectronic devices, which emit laser light from the violet to green region, are expected to be a superior light source for the next-generation of displays and illumination, such as retinal scanning displays and adaptive headlights. The development status and prospects are discussed in comparison with already commercialized gallium arsenide-based infrared VCSELs. [ABSTRACT FROM AUTHOR]

We investigated polarization doping for hole generation in abrupt and graded GaN/Al0.7Ga0.3N interfaces on Al0.99Ga0.01N templates. The abrupt interface exhibited hole generation, whereas the graded interface exhibited electron generation. In the graded AlxGa1−xN (x = 0.65–0), a graded part with an AlN mole fraction ranging from 0.2 to 0 showed a large relaxation. Theoretical estimation revealed that this part contained positive polarization charges, accumulating electrons. Via Mg doping in the graded AlGaN layer, we obtained a high hole concentration of 3 × 1013 cm−2. These results indicate that understanding the relaxation conditions in the graded layer is indispensable for polarization doping. [ABSTRACT FROM AUTHOR]

We systematically investigated metalorganic vapor phase epitaxy (MOVPE) growths of AlN layers with trimethylgallium (TMGa) supply on sapphire substrates at 1100–1250 °C. We found that Ga incorporations into the AlN layers contributed to smooth surfaces covered with step terraces at the early stage of the Al(Ga)N growth. In addition, a GaN mole fraction leading to the smooth surfaces was found to be around 2–3% at the beginning of growth. The Ga supply during the AlN layer growth at 1150 °C provided very smooth Al0.99Ga0.01N layers on sapphire substrates. [ABSTRACT FROM AUTHOR]

We demonstrate the effectiveness of a hybrid multiple-quantum-wells (MQWs) structure in InGaN-based orange light-emitting diodes (LEDs) grown by metalorganic vapor phase epitaxy. The hybrid MQWs-LED is composed of orange InGaN double QWs and a blue-green InGaN single QW. Using the hybrid MQWs structure, the orange LEDs exhibited electroluminescence spectra with narrow full widths at half maximum of 51 nm at 20 mA. The light output power and external quantum efficiency of the InGaN-based orange LEDs were 0.23 mW and 0.6%, respectively, at 20 mA. [ABSTRACT FROM AUTHOR]

The room-temperature continuous-wave operation of a 1.5λ-cavity GaN-based vertical-cavity surface-emitting laser with an n-type conducting AlInN/GaN distributed Bragg reflector (DBR) was achieved. A peak reflectivity of over 99.9% was obtained in the n-type conducting AlInN/GaN DBR so that the current was injected through the DBR for the operation. The threshold current was 2.6 mA, corresponding to the threshold current density of 5.2 kA/cm2, and the operating voltage was 4.7 V. A lasing spectrum with a peak wavelength of 405.1 nm and a full-width at half maximum of 0.08 nm was also observed. [ABSTRACT FROM AUTHOR]

We demonstrated low-resistivity GaInN-based tunnel junctions using graded GaInN layers. A systematic investigation of the samples grown by metalorganic vapor phase epitaxy revealed that a tunnel junction consisting of a 4 nm both-sides graded GaInN layer (Mg: 1 × 1020 cm−3) and a 2 nm GaN layer (Si: 7 × 1020 cm−3) showed the lowest specific series resistance of 2.3 × 10−4 Ω cm2 at 3 kA/cm2 in our experiment. The InN mole fraction in the 4 nm both-sides graded GaInN layer was changed from 0 through 0.4 to 0. The obtained resistance is comparable to those of standard p-contacts with Ni/Au and MBE-grown tunnel junctions. [ABSTRACT FROM AUTHOR]

We have achieved room-temperature CW operations of GaN-based vertical cavity surface emitting lasers (VCSELs) with periodic gain structures (PGSs). The PGS-VCSEL consisted of 4.5λ-thick optical cavity length and two GaInN 5-quantum-well (QW) active regions separated with a Mg-doped GaN intermediate layer. The uniform carrier injection into the two active regions was also investigated using light-emitting diodes (LEDs). It is found that the use of an optimum Mg concentration in the intermediate layers improves the uniform carrier injection in the two active regions. From these results, we realized the CW operation of VCSELs with PGSs grown on AlInN/GaN distributed Bragg reflectors (DBRs). The VCSEL under CW operation showed a threshold current density of 16.5 kA/cm2 and its operation wavelength was 409.9 nm. [ABSTRACT FROM AUTHOR]

The electrical properties of Si-doped AlGaN layers (AlN molar fractions: 0.03–0.06) with the donor concentrations (ND) from 8.8 × 1017 to 4.5 × 1020 cm−3 were investigated by variable-temperature Hall effect measurement using the van der Pauw method. A minimum resistivity of 3.6 × 10−4 Ω cm was obtained for Si-doped AlGaN with a smooth surface at room temperature. We found that the activation energy of the Si donor is affected by the Coulomb interaction in the AlGaN layer with ND values from 8.8 × 1017 to 2.5 × 1020 cm−3. In several AlGaN layers, the free-electron concentration did not vary with sample temperature, as expected in the case of degeneracy. The localization of GaN in the AlGaN layer was speculated as a cause of degeneracy of samples. [ABSTRACT FROM AUTHOR]

We investigated the critical thickness dependence on the GaN barrier layer thickness in a GaInN/GaN superlattice (SL). The characterization was done by combining an in situ X-ray diffraction (XRD) system attached to a metalorganic vapor phase epitaxy rector and ex situ analyses such as scanning electron microscopy and transmission electron microscopy. The critical thickness required for the introduction of a + c-type misfit dislocations (MDs) in the GaInN/GaN SL was determined by analyzing the full width at half maximum of the in situ XRD spectrum from a GaInN/GaN SL as a function of SL periods, and we successfully found the critical thicknesses of specific different SLs. [ABSTRACT FROM AUTHOR]

We report the development of high-performance AlGaN/AlN heterostructure-field-effect-transistor-type (HFET) photosensors with a p-type GaN optical gate and detection wavelengths that are restricted to 220–280 nm. These photosensors employ a two-dimensional electron gas induced at the hetero-interface between Al0.6Ga0.4N and Al0.5Ga0.5N as a highly conductive channel. In addition, a p-type GaN optical gate is employed to deplete a channel. Consequently, we obtained a high photosensitivity of over 4 × 103 A/W and an externally low dark current density of approximately 5 × 10−10 A/mm at a source–drain voltage of 3 V. We also determined that the detection range of light wavelength in these HFET photosensors can be controlled by controlling the AlN molar fraction in the AlGaN channel layer. The results are very promising for the development of completely solar-blind high-performance photosensors with high photosensitivity. [ABSTRACT FROM AUTHOR]

High-performance AlGaN/AlGaN hetero-field-effect-transistor (HFET)-type photosensors with high photosensitivity were fabricated using p-type GaN comprising three-dimensional island crystals. The p-type GaN layers were grown on AlGaN layers at a high AlN molar fraction, and the area of p-type GaN comprising three-dimensional island crystals increased as the thickness of the p-type GaN film decreased, resulting in a reduced p-type GaN coverage ratio. The p-type GaN layers comprising three-dimensional island crystals and showing low coverage ratios were then used to fabricate HFET-type photosensors with high photosensitivity. A high light sensitivity of 1.5 × 104 A/W was obtained at a source–drain voltage (VSD) of 0.5 V for a photosensor with a p-type GaN thickness of 20 nm. Moreover, the dark current was suppressed to 10−10 A/mm and the photosensor achieved an extremely high photocurrent to dark current density ratio. [ABSTRACT FROM AUTHOR]

We investigated electron and hole accumulations at GaN/AlInN/GaN interfaces by Hall effect measurement. The InN mole fraction and temperature dependences on the sheet carrier densities at the interfaces reveal that electrons and holes were induced by large positive and negative polarization charges to satisfy the charge neutrality conditions, respectively. On the basis of the above results, we then designed and demonstrated a low-resistity 10-pair Si-doped n-type AlInN/GaN distributed Bragg reflector (DBR) by using high Si doped and graded layers at the GaN/AlInN interfaces. The low-resistity n-type AlInN/GaN DBR will reduce the resistance and the internal loss in blue vertical-cavity surface emitting lasers. [ABSTRACT FROM AUTHOR]

We investigated InGaN-based amber light-emitting diodes (LEDs) with AlN/(Al)GaN barrier layers grown by metalorganic vapor-phase epitaxy. Tensilely strained AlN/Al0.03Ga0.97N barriers improved the crystalline quality of compressively strained InGaN quantum wells. We found that strain compensation among wells and barriers improves the external quantum efficiency of high-In-content InGaN-based amber LEDs. The amber LEDs with AlN/Al0.03Ga0.97N barriers have shown an electroluminescence (EL) intensity approximately 2.5-fold that of LEDs with the AlN/GaN barriers at 20 mA. [ABSTRACT FROM AUTHOR]

We demonstrated a room-temperature (RT) continuous-wave (CW) operation of a GaN-based vertical-cavity surface-emitting laser (VCSEL) using a thick GaInN quantum well (QW) active region and an AlInN/GaN distributed Bragg reflector. We first investigated the following two characteristics of a 6 nm GaInN 5 QWs active region in light-emitting diode (LED) structures. The light output power at a high current density (∼10 kA/cm2) from the 6 nm GaInN 5 QWs was the same or even higher than that from standard 3 nm 5 QWs. In addition, we found that hole injection into the farthest QW from a p-layer was sufficient. We then demonstrated a GaN-based VCSEL with the 6 nm 5 QWs, resulting in the optical confinement factor of 3.5%. The threshold current density under CW operation at RT was 7.5 kA/cm2 with a narrow (0.4 nm) emission spectrum of 413.5 nm peak wavelength. [ABSTRACT FROM AUTHOR]

We investigated a V-based electrode for the realization of low ohmic-contact resistivity in n-type AlGaN with a high AlN molar fraction characterized by the circular transmission line model. The contact resistivity of n-type Al0.62Ga0.38N prepared using the V/Al/Ni/Au electrode reached 1.13 × 10−6 Ω cm2. Using this electrode, we also demonstrated the fabrication of UV light-emitting diodes (LEDs) with an emission wavelength of approximately 300 nm. An operating voltage of LED prepared using a V/Al/Ni/Au electrode was 1.6 V lower at 100 mA current injection than that prepared using a Ti/Al/Ti/Au electrode, with a specific contact resistance of approximately 2.36 × 10−4 Ω cm2 for n-type Al0.62Ga0.38N. [ABSTRACT FROM AUTHOR]

The dependence of the critical thickness for the introduction of misfit dislocations in a GaInN/GaN heterostructure system on the dislocation density in the underlying GaN layer was investigated using in situ X-ray diffraction (XRD), ex situ scanning electron microscopy, and transmission electron microscopy analyses. The critical thickness for the introduction of misfit dislocations in the GaInN layer was found to significantly depend on the dislocation density in the underlying GaN layer. Notably, on the basis of the in situ XRD results, a reliable critical thickness for obtaining misfit-dislocation-free growth was determined. [ABSTRACT FROM AUTHOR]

In this study, we investigated the application of a novel in situ X-ray diffraction (XRD) monitoring method to control GaN crystal growth using a low-temperature (LT)-deposited buffer layer. We found that this method is useful in controlling the annealing of the LT-buffer layer, which strongly depends on the crystallinity of GaN. Accordingly, if we employ in situ XRD grown on GaN using LT-buffer layer on sapphire substrate, the optimization of the annealing conditions will become easier because it would be possible to determine by only one growth procedure. Therefore, we expect that this method will serve as a new and helpful optimization tool for crystal growth. [ABSTRACT FROM AUTHOR]