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.
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.
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