Yury Toomasovich Rebane

Defect-related tunneling mechanism of efficiency droop in III-nitride light-emitting diodes

The quantum efficiency of GaN-based light-emitting diodes (LEDs) is investigated at temperatures 77–300 K. It is found that the efficiency droop is due to a decrease in the internal quantum efficiency (IQE) in the low-energy part of the emission spectrum. The efficiency starts to decrease at a temperature independent forward voltage of Umax≈2.9 V. At this voltage tunneling current through the LED-structure begins to dominate. It is suggested that the external quantum efficiency droop is related to reduction of the IQE due to tunneling leakage of carriers from the quantum well (QW) to defect states in barriers, and to reduction of the injection efficiency by excess tunneling current under QW through deep defect states in barriers.

Light Emitting Diode with Charge Asymmetric Resonance Tunneling

We suggest a system of two wells connected with Charge Asymmetric Resonance Tunneling (CART) as a basic element of light emitting diode (LED) structure for semiconductors with different masses of electrons and holes. The system consists of an emitter of electrons, an emitter of holes and an active layer. The hole emitter is coupled with the active in such a way that holes can be freely supplied into the active layer without a barrier. The electron emitter is coupled to the active layer via a barrier. The barrier design uses the charge asymmetric resonance tunneling phenomenon which allows to make the barrier transparent for electrons and blocking for holes. Advantages of this design are: the increased capture efficiency of the electrons into the active layer due to direct resonance tunneling of the electrons from the electron emitter on bound electron level in the active quantum well, the suppression of electron leakage into the hole emitter, the elimination of the parasitic light generated outside the active layer, and the electron emitter acts also as a good current spreading layer. First results of experimental investigation and theoretical modeling of the CART LED devices are reported.

Efficiency droop and incomplete carrier localization in InGaN/GaN quantum well light-emitting diodes

A direct correlation between efficiency droop and broadening of emission spectrum of InGaN/GaN quantum wells (QWs) with increasing current density is found. A model of incomplete carrier localization in InGaN/GaN QWs is proposed. At low injection, the strong carrier localization and high-energy cutoff of emission spectrum results from fast carrier energy relaxation due to carrier hopping between localized tail states in QWs. At high level injection, the energy relaxation rate decreases due to the partial filling of tail states and high energy slope of the spectrum starts to determine by Boltzmann occupancy of tail states. This results in the incomplete carrier localization and the efficiency droop.

Mechanism of the GaN LED efficiency falloff with increasing current

The quantum efficiency of GaN LED structures has been studied at various temperatures and biases. It was found that an efficiency falloff is observed with increasing current density and, simultaneously, the tunnel component of the current through the LED grows and the quasi-Fermi levels reach the mobility edge in the InGaN active layer. It is shown that the internal quantum efficiency falloff with increasing current density is due to the carrier leakage from the quantum well as a result of tunnel transitions from its band-tail states to local defect-related energy levels within the energy gaps of the barrier layers.

Efficiency droop in GaN LEDs at high current densities: Tunneling leakage currents and incomplete lateral carrier localization in InGaN/GaN quantum wells

The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode p-n structures is studied. The influence exerted by two basic processes on the emission efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion n region and a corresponding local decrease in the height of the injection p barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.

Degradation and transient currents in III-nitride LEDs

Effect of degradation processes on transient currents in LEDs has been studied. It has been found that transient currents are several orders of magnitude higher than steady-state currents. The transient current time dependencies are non-exponential and show a distribution of relaxation times in the range of 1-100 microseconds. The charge associated with the transient currents is Q ~3x10-10 C which corresponds to high number of carrier traps Nt ~ 2x109 in the investigated chips. For one-year old chips an increase of charge and trap number by ~ 25% has been found compared to the fresh chips. Two probable reasons have been suggested to explain the observed increase of number of carrier traps: first one is related to increase of the number of trap sites at dislocations, and second one is a gradual phase separation process in quantum wells resulting in degradation of their quality.

Nature of V-Shaped Defects in GaN

GaN films with thicknesses up to 3 mm were grown in two custom-made halide vapor phase epitaxy (HVPE) reactors. V-shaped defects (pits) with densities from 1 to 100 cm-2 were found on the surfaces of the films. Origins of pit formation and the process of pit overgrowth were studied by analysing the kinematics of pit evolution. Two mechanisms of pit overgrowth were observed. Pits can be overgrown intentionally by varying growth parameters to increase the growth rate of pit facets. Pits can overgrow spontaneously if a fast-growing facet nucleates at their bottom under constant growth conditions.

Effect of the electric field on the intensity and spectrum of emission from InGaN/GaN quantum wells

Comparative study of the photoluminescence (PL) from quantum wells (QWs) in forward-biased p-GaN/InGaN/n-GaN structures and electroluminescence from these structures has been carried out. It is shown that, upon application of a forward bias, a characteristic red shift of the spectral peak is observed, together with a broadening of the PL line and simultaneous burning-up of the PL. This results from a decrease in the field strength in the space charge region of the p-n junction and suppression of the tunneling leakage of the carrier from band-tail states in the active InGaN layer. An analysis of the results obtained demonstrated that the tunneling strongly affects the quantum efficiency and enabled evaluation of the internal quantum efficiency of the structures. It is shown that nonequilibrium population of band-tail states in InGaN/GaN QWs depends on the injection type and is controlled by the capture of carriers injected into a QW, in the case of optical injection, and by carrier tunneling “below” the QW under electrical injection.

III-nitride efficient LEDs

A III-nitride blue LED structure based on the system of two wells with charge asymmetric resonance tunneling (CART), which allows enhancing the number of the electrons captured into the active region with the quantum well, was systematically studied. The barrier design uses the charge asymmetric resonance-tunneling phenomenon, which allows making the barrier transparent for electrons and blocking for holes. The growth and post-growth processes were optimized to achieve an efficient CART LED. The output power of 4 mW at the operating current of 20 mA has been achieved, corresponding to the external efficiency of 6%. Results presented in this report include the optimization of the quantum well growth parameters, bowing parameter for InGaN alloys grown on GaN, dry etching of III-nitride materials, Ohmic contacts to p- and n- type GaN, electrostatic discharge (ESD) problems related with the reliability of LEDs. The results presented include also modulation-technique LED characterization to tune the maximum radiative-recombination efficiency in accordance with the common operating current density.

Misfit dislocations and radiative efficiency of InxGa1-xN/GaN quantum wells

Abstract We report results of calculations of radiative efficiency of In x Ga 1− x N quantum wells embedded in wurtzite GaN epilayer. It was found that misfit dislocations with density up to ∼10 5–6 cm −1 could improve the quantum efficiency of the In x Ga 1− x N wells by more than 10 times because they reduce the quantum well built-in electric field. At higher densities, the misfit dislocations suppress the quantum efficiency of the wells since they produce an additional channel of nonradiative recombination.