In this paper we argue that the quenching of external quantum efficiency (EQE) with increase of current typically observed for AlInGaN LEDs is caused by reduction of injection efficiency. It is shown as a result of numerical simulations that the current blocking AlGaN layer is inefficient at high current density due to piezoelectric field of GaN/AlGaN interface. The results of numerical simulation are in good agreement with experimental dependence of EQE on pumping. A new design of LED heterostructure is proposed, for which the EQE quenching is not expected.
Carbon disulfide (CS 2 ) photolysis was investigated in the gas phase using an argon fluoride (ArF) laser at 193 nm. The coaxial time-of-flight (TOF) distributions of CS radicals produced in the photolysis have been measured. Photochemical fragments have been observed with translational energies below 3 kcallmol. The vibrational distribution of the CS fragments was also probed by laser induced fluorescence (LIF), and these measurements confirm that significant amounts of CS radicals are produced in vibrational levels greater than v" = 6. From a computer simulation of the experimental LIF data, a vibrational distribution was also obtained. Vibrational levels up to v" = 12 were found to be populated in a bimodal distribution, which peaks at v" = 4, and extends to v" = 12. There was a significant amount of rotational excitation of nascent CS produced in high vibrational levels of the ground state. The disjoint translational energy and CS vibrational energy distributions can be used to obtain an estimate of the Sf P) to S( I D ) ratio of 0.66.
The work is devoted to explanation of the decrease of external quantum efficiency (QE) with increase of pumping density typically observed for AlInGaN heterostrucures. It is shown as a result of numerical modeling that while the increase of QE at low pumping density is due to the competition between radiative and non-radiative recombination, the decrease of QE at large pumping density is caused by decrease of injection efficiency for the holes into the active area. A modified LED heterostructure is suggested, for which the effect of QE decreasing is not expected.1 Introduction Significant success in producing of AlInGaN-based effective LEDs make possible creation on their basis of sources of white light. However, for similar applications, besides high efficiency, high output optical power is required also. At the same time, quantum efficiency (QE) of group III-nitride based heterostructures is known to decrease substantially with increase of pumping [1]. Record values of QE as much as 50% were obtained at current I ∼ 1 mA and output power q ∼ 1 mW, but for q ∼ 100 mW and I ∼ 100 mA external QE is twice lower [2]. The QE dependence on pumping for a typical commercial LED heterostructure has a small part of growth at small values of current density followed by substantial decreasing for larger current density, the maximum is achieved at J ∼ 1 A/cm 2 [3, 4]. The increase of QE at small currents is usually attributed to competition between radiative and nonradiative recombination processes in the active area [5] while the decrease of QE has no satisfactory explanation. Attempts of an explanation of decrease of QE by filling of quantum wells of active area [6] are not confirmed by short-wave shift of the maximum at electroluminescence spectra which is not observed experimentally up to current density J ∼ 100 A/cm 2 [7,8]. Small values of current density at which decreasing of QE is observed do not allow to consider also as its basic reason an overheat of active area of the diode. That was confirmed by measurements of optical characteristics of LED in a pulse mode [10].In the given work an attempt of an explanation of the observed decrease of QE by features of charge transport in AlInGaN based LED heterostructure is made. As it is known total electroluminescence efficiency (external QE) can be represented as a product of three factors [11]:
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