Carrier quantum capture and escape mechanisms in semiconductor quantum structures

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In order to pursue the interesting mechanism behind the high luminescence efficiency of InGaN quantum-well (QW) light-emitting diodes (LEDs) the electroluminescence (EL) and photoluminescence (PL) efficiencies have been investigated as a function of external bias voltage (field). Following results are obtained:(1) EL efficiency for blue multiple QW LEDs with and without an n-type InGaN electron reservoir layer (ERL) has been studied as a function of temperature and current. With an addition of the ERL the EL efficiency is greatly improved due to decreased forward bias. The reduced EL efficiency of the main EL band observed under high injection below 100 K is significantly improved under low injection by decreasing forward bias accompanying disappearance of a short-wavelength satellite band, indicating important roles of carrier escape.(2) PL efficiency for green single QW LED has been studied as a function of bias. Under direct excitation the PL intensity can be decreased when the forward bias exceeds over +2V, indicating the PL reduction due to carrier escape processes. Furthermore, under indirect excitation where carriers are mostly photogenerated in the barriers enhancement of the PL efficiency due to efficient carrier capture can occur at +2〜3V, but over +3.25V the PL efficiency drastically decreases. This result explains the reason why the EL efficiency of the diodes is decreased under the higher forward bias conditions.(3) PL efficiency for the blue QW diodes with and without the ERL has been studied as a function of bias under indirect excitation with various excitation powers. Comparison of the PL efficiency for the well and the ERL allows us to observe that PL quenching by the reverse bias is stronger under weak power and shallow excitation depth. This can be attributed to the fact that hole escape processes limit the radiative recombination efficiency within the well regions.