InGaN Active Layer Research Articles

Evidence of exciton recombination at very high temperature in InGaN

Time-resolved electroluminescence measurements are carried out on the blue light emitting diodes with InGaN active layer at temperatures from 30 to 530 K. The decay mechanisms of the ultraviolet optical pulses corresponding to the band-to-band recombinations are investigated. The exciton-related recombination is found mainly responsible for this band-edge radiative recombination from 250 to 425 K. A thermal equilibrium model, in which the exciton dissociation process is taken into account, is used to fit the experimental results. The fitted exciton binding energy is about 48.3 meV. This high exciton binding energy is attributed to the indium-related localization effect in InGaN. Moreover, it is also found that the nonradiative lifetimes in these samples are quite long. This is ascribed to the suppression of the nonradiative recombination centers because of the incorporation of indium in GaN material.

Radiative recombination process in InGaN active layers of GaN-based light emitting diodes

We present a theoretical analysis of radiative recombination process in active layers of blue/green InGaN-based light emitting diodes (LEDs) in the framework of a quantum disk model. Taking the structural and compositional inhomogeneity and the finite subband-states effects into account we modify the optical absorption and energy relaxation equations for quantum-disk systems. The carrier relaxation dynamic process and related time-dependent photoluminescence spectra are calculated numerically. Our results show that the quantum-disk model can interpret the main optical properties of InGaN-based LEDs reasonably.

MBE grown InGaN quantum dots and quantum wells: effects of in-plane localization

InGaN/GaN heterostructure samples were grown by molecular beam epitaxy using ammonia as a nitrogen precursor. The growth of InGaN/GaN self-assembled quantum dots was monitored in situ by reflection high energy electron diffraction intensity oscillations. Atomic force microscopy scans showed a very high density of InGaN islands, ∼1×10 11 cm −2, well above the dislocation density. This could explain the increased radiative efficiency of these samples compared to homogeneous quantum wells. Light emitting diodes (LEDs) with InGaN active layers buried in GaN were realized. Electroluminescence and photocurrent spectra of these LEDs evidence a strong Stokes shift that can be attributed to high localization of carriers in InGaN layers.

CW Operation of (In,Ga)N MQW Laser Diodes on FIELO-GaN Substrates

The continuous-wave operation at room temperature has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on FIELO-GaN substrates with a backside n-contact. This was made possible by introducing an important new concept of reducing threading dislocations (FIELO) during the growth of GaN substrate. InGaN active layers grown on FIELO-GaN have been characterized to be far more superior than those grown on conventional sapphire substrate in terms of growth mode and resultant indium compositional fluctuation. The laser diode fabricated shows internal quantum efficiency as high as 98%.

Investigation into the Role of Low-Temperature GaN in n-GaN/InGaN/p-GaN Double-Heterostructure Light-Emitting Diodes

The role of two-step low-temperature GaN (LT-GaN) layers was investigated by cathodoluminescence, high resolution double crystal X-ray diffraction, transmission electron microscopy, atomic force microscopy, and current-voltage measurements. It was shown that the introduction of the LT-GaN layer prevents In from evaporating from InGaN during the high-temperature growth of p-GaN. The trasmission electron microscopic (TEM) results showed that the LT-GaN hampers dislocation propagation from the InGaN active layer into the p-GaN, leading to reduction in the dislocation density in the p-GaN. The use of the two-step LT-GaN resulted in an increase in the output power of light-emitting diodes and a decrease in the operating forward voltage.

Changes in electrical characteristics associated with degradation of InGaN blue light-emitting diodes

Because of the high concentration of threading dislocations, the reverse current-voltage (I–V) characteristics for either homo- or heterojunctions made on GaN-based materials grown on sapphire often show a strong electric field dependence (called a soft breakdown characteristic), which can be described by a power law I=Vn, with n between 4 to 5. We find a significant increase of reverse currents associated with the early degradation of emission in InGaN blue single-quantum-well light-emitting diodes (LEDs) subjected to aging tests (injected current of 70 mA over a total time of about 300 h). The formation of dislocations might be due to the relaxation of strain in the thin InGaN active layer during the aging tests.

Direct imaging of the spectral emission characteristic of an InGaN/GaN-ultraviolet light-emitting diode by highly spectrally and spatially resolved electroluminescence and photoluminescence microscopy

The microscopic spectral emission characteristic of an InGaN/GaN double-heterostructure light-emitting diode is directly imaged by highly spectrally and spatially resolved scanning electroluminescence microscopy under operation as a function of injection current density. The luminescence intensity maps and especially the peak-wavelength scanning images provide access to the optical quality of the final device and yield direct images of the In fluctuations with 1 μm spatial resolution. Indium concentrations varying from 6% to 9% are found in the active InGaN region of the ultraviolet diode emitting at 400 nm. While for low injection current densities the electroluminescence is dominated by emission from the p GaN originating from the whole accessible area, the emission from the InGaN active layer increases and takes over for higher injection conditions showing a strongly localized spatial emission characteristic. Correlation of the results with low-temperature scanning photoluminescence microscopy enables the identification of the underlying recombination processes.

Effects of Carrier Localization on the Optical Characteristics of MOCVD-Grown InGaN/GaN Heterostructures

We have studied both the spontaneous and stimulated emission (SE) properties as a function of excitation photon energy for InGaN/GaN multiple quantum wells (MQWs). A significant redshift of the SE peak with decreasing excitation photon energy was observed as the excitation photon energy was tuned below a certain photon energy (“mobility edge”) for the InGaN/GaN MQWs, with similar behavior observed for the spontaneous emission. The relative position of the mobility edge with respect to the absorption edge and the spontaneous and stimulated emission peak positions indicates the emission originates from carriers localized by extremely large potential fluctuations in the InGaN active layers of the MQWs. Therefore, carrier localization in the InGaN active regions explains the observed spontaneous and stimulated emission behaviors of these materials.

Characteristics of InGaN-Based UV/Blue/Green/Amber/Red Light-Emitting Diodes

Highly efficient light-emitting diodes (LEDs) emitting ultraviolet (UV), blue, green, amber and red light have been obtained through the use of InGaN active layers instead of GaN active layers. Red LEDs with an emission wavelength of 675 nm, whose emission energy was almost equal to the band-gap energy of InN, were fabricated. The dependence of the emission wavelength of the red LED on the current (blue shift) is dominated by both the band-filling effect of the localized energy states and the screening effect of the piezoelectric field. In the red LEDs, a phase separation of the InGaN layer was clearly observed in the emission spectra, in which blue and red emission peaks appeared. In terms of the temperature dependence of the LEDs, InGaN LEDs are superior to the conventional red and amber LEDs due to a large band offset between the active and cladding layers. The localized energy states caused by In composition fluctuation in the InGaN active layer contribute to the high efficiency of the InGaN-based emitting devices, in spite of the large number of threading dislocations and a large effect of the piezoelectric field. The blue and green InGaN-based LEDs had the highest external quantum efficiencies of 18% and 20% at low currents of 0.6 mA and 0.1 mA, respectively.

Recent Developments in InGaN-Based Blue Leds and Lds

UV/blue/green/amber InGaN quantum-well structure light-emitting diodes with an external quantum efficiency of 7.5%, 11.2%, 11.6%, and 3.3% were developed. The localization in the InGaN well layer induced by the In composition fluctuations seems to be a key role of the high efficiency of those InGaN-based light-emitting diodes. When the electrons and holes are injected into the InGaN active layer of the light-emitting diodes, these carriers are captured by the localized energy states before they are cap- tured by the nonradiative recombination centers caused by the large number of threading dislocations. InGaN multi-quantum-well structure laser diodes ' with modulation doped strained-layer superlattice cladding layers grown on the epitaxially lateral overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10000 hours under-room temperature con- tinuous-wave operation. When the laser diode was formed on the GaN layer above the SiO2 mask region without any threading dislocations, the thresh- old current density was as low as 2.7 kA cm -2 . When the laser diode was formed on the window region with the high threading dislocation density, the threshold current density was as high as 4.5 to 9 kA cm -2 . A leakage current

Influence of Si doping on characteristics of InGaN/GaN multiple quantum wells

We have systematically studied the influence of Si doping on the characteristics of InGaN/GaN multiple quantum wells (MQWs) by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy. The twelve- period MQWs were grown by metalorganic chemical vapor deposition. Si doping in the GaN barriers was varied from 1×1017 to 3×1019 cm−3. Information on the structural quality of the MQWs as a function of Si doping was extracted from the linewidth broadening of the higher-order superlattice satellite peaks measured in HRXRD. The HRXRD measurements indicate that increased Si doping results in better interface properties of the MQWs. PL and PLE measurements show a decrease in the Stokes shift with increasing Si doping concentration. The 10 K radiative recombination lifetime was observed to decrease from ∼30 ns (for n<1×1017 cm−3) to ∼4 ns (for n=3×1019 cm−3) with increasing Si doping concentration. The reduced Stokes shift, the decrease in radiative recombination lifetime, and the increase in the interface quality indicate that Si doping results in a decrease in carrier localization at potential fluctuations in the InGaN active layers.

The roles of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes

REVIEW High-efficiency light-emitting diodes emitting amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layer instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based emitting devices. The blue and green InGaN quantum-well structure light-emitting diodes with luminous efficiencies of 5 and 30 lumens per watt, respectively, can be made despite the large number of threading dislocations (1 x 10(8) to 1 x 10(12) cm-2). Epitaxially laterally overgrown GaN on sapphire reduces the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. InGaN multi-quantum-well structure laser diodes formed on the GaN layer above the SiO2 mask area can have a lifetime of more than 10,000 hours. Dislocations increase the threshold current density of the laser diodes.

High-power UV InGaN/AlGaN double-heterostructure LEDs

Ultraviolet (UV) InGaN/AlGaN double-heterostructure (DH) light-emitting diodes (LEDs) with an external quantum efficiency of 7.5%, an output power of 5mW and an emission wavelength of 371nm were developed. High-power UV LEDs are obtained using an InGaN active layer with a thickness of 400Å instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based LEDs.

Impact ionization luminescence of InGaN/AlGaN/GaN p-n-heterostructures

The luminescence spectra of InGaN/AlGaN/GaN p-n heterostructures with reverse bias sufficient for impact ionization are investigated. The injection luminescence of light-emitting diodes with such structures was examined earlier. A strong electric field is present in the InGaN active layer of the heterostructures, and for small reverse bias the tunneling component of the current predominates. Avalanche breakdown commences at voltages Vth>8–10 V, i.e., ∼3Eg, (Eg is the width of the band gap) in the absence of lightly doped structures. The luminescence spectra have a short-wavelength edge corresponding to the width of the GaN band gap (3.40 eV) and maxima in the region 2.60–2.80 eV corresponding to the maxima of the injection luminescence spectra in the active layer. The long-wavelength edge of the spectra in the region 1.7–1.8 eV may be associated with deep recombination levels. Mechanisms of recombination of the hot electron-hole plasma in the strong electric fields of the p-n heterostructures are discussed.

Aging of InGaN/AlGaN/GaN Light-Emitting Diodes

AbstractChanges in luminescent spectra, current-voltage, and capacitance-voltage characteristics were studied versus time of working at forward currents for light-emitting diodes based on InGaN/AlGaN/GaN heterostructures. The samples of blue and green LEDs with a single quantum well InGaN active layers were studied during 102-2.103 hours at currents 30–80 mA. An increase in efficiency at the first stage and a decrease one at the second stage were detected. The main changes were detected at low currents in blue diodes for tunnel components of current and radiation. Activation of acceptors Mg because of residual H atoms are going out of complexes Mg-H and creation of donor type defects are proposed as models for two stages. A model of injection stimulated (non-thermal) underthreshold creation of defects can explain the results.

Room-temperature continuous-wave operation of InGaN multi-quantum-well structure laser diodes

Continuous-wave (cw) operation of InGaN multi-quantum-well structure laser diodes (LDs) was demonstrated at room temperature (RT). The threshold current and voltage of the LD were 130 mA and 8 V, respectively. The threshold carrier density was 9 kA/cm2. The lifetime of the LDs under RT cw operation was 1 s due to large heat generation. Mode hopping of the emission wavelength of the LDs was observed. The average wavelength drift due to temperature increase was 0.066 nm/K between 20 and 70 °C, because of the temperature dependence of the gain profile due to band-gap narrowing of the InGaN active layer.

Growth and Characterization of AllnGaN/InGaN Heterostructures

AbstractThe emission wavelength of the InxGa1−xN ternary system can span from the near ultraviolet through red regions of the visible spectrum. High quality double heterostructures with these InxGa1−xN active layers are essential in the development of efficient optoelectronic devices such as high performance light emitting diodes and laser diodes. We will report on the MOCVD growth and characterization of thick and thin InGaN films. Thick InxGa1−xN films with values of x up to 0.40 have been deposited and their photoluminescence (PL) spectra measured. AlGaN/InGaN/AlGaN double heterostructures (DHs) have been grown that exhibit PL emission in the violet, blue, green and yellow spectral regions, depending on the growth conditions of the thin InGaN active layer. Preliminary results of an AllnGaN/InGaN/AllnGaN DH, with the potential of realizing a near-lattice matched structure, will also be presented.

InGaN/AlGaN blue-light-emitting diodes

Highly efficient InGaN/AlGaN double-heterostructure blue-light-emitting diodes (LEDs) with an external quantum efficiency of 5.4% were fabricated by codoping Zn and Si into an InGaN active layer. The output power was as high as 3 mW at a forward current of 20 mA. The peak wavelength and the full width at half maximum of the electroluminescence of blue LEDs were 450 and 70 nm, respectively. Blue-green LEDs with a brightness of 2 cd and a peak wavelength of 500 nm were fabricated for application to traffic lights by increasing the indium mole fraction of the InGaN active layer.

Low-Temperature Growth of High Quality InxGa1−xN by Atomic Layer Epitaxy

ABSTRACTWe report on the low temperature epitaxial growth of InxGa1−xN with 0≤x≤0.27 by Atomic Layer Epitaxy (ALE). GaN and InGaN single crystal films have been grown by ALE in the temperature range between 600 and 700 °C using the rotating substrate approach. Films were deposited on sapphire substrates using TMG, EdMIn, and NH3 as precursors. Up to 27% indium content has been achieved in the InGaN films. The FWHM of the (0002) InGaN peak by double crystal X-ray diffraction of these films was as small as 5 minutes. Room-temperature photoluminescence (PL) from these films was dominated by band edge emission between 365 nm and 446 nm. AlGaN/InGaN double heterostructures were grown in a hybrid reactor, in which the AlGaN barrier layers were grown by MOCVD and the InGaN active layer by ALE. The structures showed good crystal quality, and sharp PL emission with peak intensity at 410 nm.

High-brightness InGaN/AlGaN double-heterostructure blue-green-light-emitting diodes

High-brightness InGaN/AlGaN double-heterostructure blue-green-light-emitting diodes with a luminous intensity of 2 cd were fabricated by increasing an indium mole fraction of the InGaN active layer up to 0.23. Both Zn and Si were codoped into the InGaN active layer to afford relatively stronger luminescence. The blue-green emission intensity of room-temperature photoluminescence became maximum when the electron carrier concentration of the InGaN active layer was around 1×1019 cm−3. Donor-acceptor pair recombination is a dominant emission mechanism of the InGaN active layer. The external quantum efficiency was as high as 2.4% at a forward current of 20 mA at room temperature. The peak wavelength and the full width at half-maximum of the electroluminescence were 500 and 80 nm, respectively.