Luminescence Spectra Of Superbright Blue and Green InGaN/AlGaN/GaN Light-Emitting Diodes

Abstract

Electroluminescence spectra of superbright blue and green LEDs based on epitaxial InxGa1−xN/AlyGa1−yN/GaN heterostructures with thin quantum well active layers [1] were studied at currents J = 0.01-20 mA. Spectral maxima of blue and green LEDs are ωmax = 2.58-2.75 eV and ωmax = 2.38-2.45 eV, dependent on the active layer In content. The low energy tails of the spectra are exponential with the parameter E0 = 42-50 meV almost independent of the temperature. The high energy tails of the spectra are exponential with a temperature dependent parameter E1= 20-40 meV. Both parameters (E0, E1) are current independent at J > 0.5 mA. The spectral band can be described by taking into account quantum size effects, impurities and electron-phonon interactions in active layers. A structure in the spectra was detected which can be described by the influence of light interference in the GaN layer on the sapphire substrate. Light intensity was a linear function of the drive current over the interval J = 1-20 mA, and was slightly temperature dependent. In the blue LEDs, the efficiency fall off at low currents (J < 0.7 mA) had a I ~ J4-5 dependence at room temperature. The green LEDs showed no such dependence. The influence of tunnel effects on the efficiency at low currents is discussed. Tunnel radiation spectra with maxima moving with the voltage were detected at low currents in III-N structures.

The Emission Properties of Light Emitting Diodes using InGaN/AlGaN/GaN Multiple Quantum Wells

“…A dependence of h ω max of the energy eV (V-voltage) is shown in Figure 4. In a comparatively wide range of voltage this function is linear, but the slope of the line is << 1, (in contrast with the tunnel band reported in [3] [4] [5] [6]). Filling of the tail states in the active layer causes this shift.…”

“…There is an exponential part at low currents, J < 10 -7 A at 300 K, a steep exponential growth in the range V = 2.3-2.7 V, a linear part at higher currents, J > 20 mA. Low currents can be understood as a tunnel component; tunnel currents in these LEDs play some role at J 3-4 orders of magnitude lower than that for SQW-based LEDs [3] A good approximation of the J(V) curves for MQW LEDs was made when not only a series resistance R s at the linear part at higher J was taken into account, but also the quadratic part: J ~ (V-V 1 ) 2 . The fit of the curve J(V) at J > 0.1 mA by the equation:…”

“…We have not seen room-temperature spectra of GaN-based LEDs at such low currents in the literature. The maxima of the spectra of the blue LEDs move with the current in the range h ω max = 2.57-2.67 eV, in contrast to blue SQW LEDs in which the blue maximum does not shift with the current [3] [4] [5]. There is no additional band in the yellow-green region moving with the voltage at low currents.…”

“…The parameter E 0 had the value E 0 ≈ 50-60 meV, and changed only slightly with the current, as occurred in the spectra of SQW LEDs [3] [4] [5]. The high-energy sides also have an exponential form, I ~ exp(-h ω /E 1 ).…”

“…A model of radiative recombination in 2D-structures with band tails caused by potential fluctuations was successfully applied to luminescence spectra of LEDs with single quantum wells (SQW) [1] [2] [3] [4] [5] and radiative recombination by tunneling was detected at low currents [3] [4] [5] [6] [7] [8].…”