Yu. S. Lelikov

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.

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.

Tunnel injection and power efficiency of InGaN/GaN light-emitting diodes

The results of studying the influence of the finite tunneling transparency of injection barriers in light-emitting diodes with InGaN/GaN quantum wells on the dependences of the current, capacitance, and quantum efficiency on the p-n junction voltage and temperature are presented. It is shown that defectassisted hopping tunneling is the main transport mechanism through the space charge region (SCR) and makes it possible to lower the injection barrier. It is shown that, in the case of high hopping conductivity through the injection barrier, the tunnel-injection current into InGaN band-tail states is limited only by carrier diffusion from neutral regions and is characterized by a close-to-unity ideality factor, which provides the highest quantum and power efficiencies. An increase in the hopping conductivity through the space charge region with increasing frequency, forward bias, or temperature has a decisive effect on the capacitance-voltage characteristics and temperature dependences of the high-frequency capacitance and quantum efficiency. An increase in the density of InGaN/GaN band-tail states and in the hopping conductivity of injection barriers is necessary to provide the high-level tunnel injection and close-to-unity power efficiency of high-power light-emitting diodes.

Effect of localized tail states in InGaN on the efficiency droop in GaN light-emitting diodes with increasing current density

The mechanism of the internal quantum efficiency droop in InGaN/GaN structures with multiple quantum wells at current densities of up to 40 A cm−2 in high-power light-emitting diodes is analyzed. It is shown that there exists a correlation between the efficiency droop and the broadening of the high-energy edge of the emission spectrum with increasing current density. It is also demonstrated that the efficiency is a spectrum-dependent quantity and the emission of higher energy photons starts to decrease at higher current densities. The effect of tunneling and thermally activated mechanisms of thermalization of carriers captured into shallow band-tail states in the energy gap of InGaN on the efficiency and the emission spectrum’s shape is considered. Analysis of the results obtained suggests that the efficiency droop occurs at high current densities because of the relative rise in the contribution from nonradiative recombination via defect states as a result of the increasing occupancy of deep band-tail states in InGaN. It is shown that power efficiency close to the theoretical limit can be obtained in the case of low-voltage tunnel injection into localized band-tail states in the InGaN active region.

Measurement of the absorption coefficient for light laterally propagating in light-emitting diode structures with In0.2Ga0.8N/GaN quantum wells

A procedure for measuring the absorption coefficient for light propagating parallel to the surface of a GaN-based light emitting diode chip on a sapphire substrate is suggested. The procedure implies the study of emission from one end face of the chip as the opposite end face is illuminated with a light emitting diode. The absorption coefficient is calculated from the ratio between the intensities of emission emerging from the end faces of the sapphire substrate and the epitaxial layer. From the measurements for chips based on p-GaN/In0.2Ga0.8N/n-GaN structures, the lateral absorption coefficient is determined at a level of (23 ± 3)cm−1 at a wavelength of 465 nm. Possible causes for the discrepancy between the absorption coefficients determined in the study and those reported previously are analyzed.

Quantum efficiency and formation of the emission line in light-emitting diodes based on InGaN/GaN quantum well structures

The spectra of electroluminescence, photoluminescence, and photocurrent for the In0.2Ga0.8N/GaN quantum-well structures are studied to clarify the causes for the reduction in quantum efficiency with increasing forward current. It is established that the quantum efficiency decreases as the emitting photon energy approaches the mobility edge in the In0.2Ga0.8N layer. The mobility edge determined from the photocurrent spectra is Eme = 2.89 eV. At the photon energies hv > 2.69 eV, the charge carriers can tunnel to nonradiative recombination centers with a certain probability, and therefore, the quantum efficiency decreases. The tunnel injection into deep localized states provides the maximum electroluminescence efficiency. This effect is responsible for the origin of the characteristic maximum in the quantum efficiency of the emitting diodes at current densities much lower than the operating densities. Occupation of the deep localized states in the density-of-states “tails” in InGaN plays a crucial role in the formation of the emission line as well. It is shown that the increase in the quantum efficiency and the “red” shift of the photoluminescence spectra with the voltage correlate with the changes in the photocurrent and occur due to suppression of the separation of photogenerated carriers in the field of the space charge region and to their thermalization to deep local states.

Optical Properties of Blue Light-Emitting Diodes in the InGaN/GaN System at High Current Densities

The current-voltage and brightness-voltage characteristics and the electroluminescence spectra of blue InGaN/GaN-based light-emitting diodes are studied to clarify the cause of the decrease in the emission efficiency at high current densities and high temperatures. It is found that the linear increase in the emission intensity with increasing injection current changes into a sublinear increase, resulting in a decrease in efficiency as the observed photon energy shifts from the mobility edge. The emission intensity decreases with increasing temperature when the photon energy approaches the mobility edge; this results in the reduction in efficiency on overheating. With increasing temperature, the peak of the electroluminescence spectrum shifts to lower photon energies because of the narrowing of the band gap. The results are interpreted taking into account the fact that the density-of-states tails in InGaN are filled not only via trapping of free charge carriers, but also via tunneling transitions into the tail states. The decrease in the emission efficiency at high currents is attributed to the suppression of tunneling injection and the enhancement of losses via the nonradiative recombination channel “under” the quantum well.

On the laser detachment of n-GaN films from substrates, based on the strong absorption of IR light by free charge carriers in n+-GaN substrates

The physical and technological basics of the method used to lift off lightly and moderately doped n-GaN films from heavily doped n+-GaN substrates are considered. The detachment method is based on the free-charge-carrier absorption of IR laser light, which is substantially higher in n+-GaN films.

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.

On the laser lift-off of lightly doped micrometer-thick n -GaN films from substrates via the absorption of IR radiation in sapphire

The intense absorption of CO2 laser radiation in sapphire is used to separate GaN films from GaN templates on sapphire. Scanning of the sapphire substrate by the laser leads to the thermal dissociation of GaN at the GaN/sapphire interface and to the detachment of GaN films from the sapphire. The threshold density of the laser energy at which n-GaN started to dissociate is 1.6 ± 0.5 J/cm2. The mechanical-stress distribution and the surface morphology of GaN films and sapphire substrates before and after laser lift-off are studied by Raman spectroscopy, atomic-force microscopy, and scanning electron microscopy. A vertical Schottky diode with a forward current density of 100 A/cm2 at a voltage of 2 V and a maximum reverse voltage of 150 V is fabricated on the basis of a 9-μm-thick detached n-GaN film.