Light-emitting Diodes Active Region Research Articles

Electrostatic Field Effect Light-Emitting Diode

Gallium Nitride (GaN) based light-emitting diodes (LEDs) suffer from the persistent issue of high resistivity of the p-type layer, due to inefficient dopant activation at room temperature. Here, a novel Electrostatic Field Effect LED (EFELED) is demonstrated to solve this issue by introducing significantly more holes into the LED active region for the first time. An increase in drive current is observed through application of positive biases to a capacitor layer on top of a 436 nm InGaN/GaN LED. With a positive bias of 5 V applied to the capacitor, the EFELED showed more than a two-fold increase in optical output from electroluminescence (EL) measurements, as well as over 115% enhanced external quantum efficiency (EQE). Modulation of holes can also be achieved through the novel capacitor integration in the EFELED, which leads to light output control. This proposed EFELED provides an alternative method to efficiently improve hole injection for the active region through energy band bending, which addresses the fundamental p-type doping issue for LED devices.

Microscopic Observation of Low Efficiency in Green Light-Emitting Diodes

The low efficiency of green light-emitting diodes (LEDs), a phenomenon known as the green gap, is a key obstacle hindering the application of LEDs as next-generation light sources to pioneer a plethora of new applications in the optical, medical, and communication sectors. Based on a microscopic photoluminescence analysis of green GaN-based multiple quantum wells, we find that In-enriched emission clusters on the submicrometer scale, previously thought to be efficient luminescent centers, do not emit light effectively. Such emission clusters can localize an excessively large amount of carriers, leading to the subsequent occurrence of vigorous nonradiative recombination processes. We also observe that the effective volume of the LED active region is significantly reduced, possibly because the generation of these In-enriched clusters via metastable phase separation significantly degrades the surrounding crystal quality. The microscopic analysis of luminescent clusters gives insight into the low efficiency of green LEDs, which may guide future directions for the development of LEDs.

Efficiency of True-Green Light Emitting Diodes: Non-Uniformity and Temperature Effects.

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been measured in a wide range of operating currents at various temperatures from 13 K to 300 K. Unlike blue LEDs, the efficiency as a function of current is found to have a multi-peak character, which could not be fitted by a simple ABC-model. This observation correlated with splitting of LED emission spectra into two peaks at certain currents. The characterization data are interpreted in terms of non-uniformity of the LED active region, which is tentatively attributed to extended defects like V-pits. We suggest a new approach to evaluation of temperature-dependent light extraction and internal quantum efficiencies taking into account the active region non-uniformity. As a result, the temperature dependence of light extraction and internal quantum efficiencies have been evaluated in the temperature range mentioned above and compared with those of blue LEDs.

Calcium impurity as a source of non-radiative recombination in (In,Ga)N layers grown by molecular beam epitaxy

Ca as an unintentional impurity has been investigated in III-nitride layers grown by molecular beam epitaxy (MBE). It is found that Ca originates from the substrate surface, even if careful cleaning and rinsing procedures are applied. The initial Ca surface coverage is ∼1012 cm−2, which is consistent with previous reports on GaAs and silicon wafers. At the onset of growth, the Ca species segregates at the growth front while incorporating at low levels. The incorporation rate is strongly temperature dependent. It is about 0.03% at 820 °C and increases by two orders of magnitude when the temperature is reduced to 600 °C, which is the typical growth temperature for InGaN alloy. Consequently, [Ca] is as high as 1018 cm−3 in InGaN/GaN quantum well structures. Such a huge concentration might be detrimental for the efficiency of light emitting diodes (LEDs) if one considers that Ca is potentially a source of Shockley-Read-Hall (SRH) defects. We thus developed a specific growth strategy to reduce [Ca] in the MBE grown LEDs, which consisted of burying Ca in a low temperature InGaN/GaN superlattice (SL) before the growth of the active region. Finally, two LED samples with and without an SL were fabricated. An increase in the output power by one order of magnitude was achieved when Ca was reduced in the LED active region, providing evidence for the role of Ca in the SRH recombination.

Effect of hole transport on performance of infrared type-II superlattice light emitting diodes

The effect of hole transport on the performance of infrared light emitting diodes (LED) was investigated. The active area of the LEDs comprised two type-II superlattices with different periods and widths connected in series. Electroluminescence spectra of the devices with different positions of long wave and mid wave superlattice sections were mostly contributed by the superlattice closest to the p-contact. The experimental results indicate that due to suppressed vertical hole transport, the recombination of electrically injected electrons and holes in a type II superlattice LED active region takes place within a few superlattice periods near p-barrier. Possible reason for the effect is reduction of hole diffusion coefficient in an active area of a superlattice LED under bias.

Looking for Auger signatures in III-nitride light emitters: A full-band Monte Carlo perspective

Recent experiments of electron emission spectroscopy (EES) on III-nitride light-emitting diodes (LEDs) have shown a correlation between droop onset and hot electron emission at the cesiated surface of the LED p-cap. The observed hot electrons have been interpreted as a direct signature of Auger recombination in the LED active region, as highly energetic Auger-excited electrons would be collected in long-lived satellite valleys of the conduction band so that they would not decay on their journey to the surface across the highly doped p-contact layer. We discuss this interpretation by using a full-band Monte Carlo model based on first-principles electronic structure and lattice dynamics calculations. The results of our analysis suggest that Auger-excited electrons cannot be unambiguously detected in the LED structures used in the EES experiments. Additional experimental and simulative work are necessary to unravel the complex physics of GaN cesiated surfaces.

Origin of electrons emitted into vacuum from InGaN light emitting diodes

The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

Electroluminescence properties of InGaN/GaN multiple quantum well light emitting diodes

Light emitting diodes (LED) and laser diodes (LD) based on III-Nitrides emitting in visible spectrum have seen an interesting advance these last few years. Here, we present a simulation study of a multi quantum well MQW InGaN/GaN light emitting diode LED, where the diode active region is formed by a series of several periods of InGaN quantum wells (QWs) and GaN quantum barriers (QBs). We will, first, present free carriers distribution within LED active region and device electroluminescence under forward polarisation. We will, secondly, see indium element content of InxGa1-xN wells and number of InGaN/GaN periods impacts on LED optical properties.

Two channels of non-radiative recombination in InGaN/GaN LEDs

Abstract The results of low-frequency noise study in blue InGaN/GaN light emitting diodes (LEDs) allow to suppose that there are two non-radiative recombination channels in InGaN/GaN LEDs: one of them is related to the extended defect system piercing the LED active region and other is related with the presence of point defects. The generation of new defects under current density more than 10 A/cm 2 was found. This process is reversible up to operation current density of about 200 A/cm 2 .

Effect of threading defects on InGaN∕GaN multiple quantum well light emitting diodes

Photoelectrochemical etching was used to measure the threading defect (TD) density in InGaN multiple quantum well light-emitting diodes (LEDs) fabricated from commercial quality epitaxial wafers. The TD density was measured in the LED active region and then correlated with the previously measured characteristics of these LEDs. It was found that the reverse leakage current increased exponentially with TD density. The temperature dependence of this dislocation-related leakage current was consistent with a hopping mechanism at low reverse-bias voltage and Poole-Frenkel emission at higher reverse-bias voltage. The peak intensity and spectral width of the LED electroluminescence were found to be only weakly dependent on TD density for the measured TD range of 1×107–2×108cm−2.

Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes

The low-temperature breakdown of the electroluminescence intensity (ELI) of blue/violet InGaN-based light-emitting diodes (LEDs) is shown to be independent of the structural details of the LED active region. Instead, the presence of an electron blocking layer (EBL) plays a decisive role. The authors attribute the ELI collapse to the low-temperature hole-blocking properties of the EBL. However, removing the EBL leads to a much reduced ELI because of a disproportional increase of electron overflow processes, which shows that the presence of an EBL in blue/violet InGaN-based LEDs is still essential. Optimization of the EBL by means of Mg doping is discussed.