Dependence of InGaN Quantum Well Thickness on the Nature of Optical Transitions in LEDs

Abstract

The design of the active region is one of the most crucial problems to address in light emitting devices (LEDs) based on III-nitride, due to the spatial separation of carriers by the built-in polarization. Here, we studied radiative transitions in InGaN-based LEDs with various quantum well (QW) thicknesses-2.6, 6.5, 7.8, 12, and 15 nm. In the case of the thinnest QW, we observed a typical effect of screening of the built-in field manifested with a blue shift of the electroluminescence spectrum at high current densities, whereas the LEDs with 6.5 and 7.8 nm QWs exhibited extremely high blue shift at low current densities accompanied by complex spectrum with multiple optical transitions. On the other hand, LEDs with the thickest QWs showed a stable, single-peak emission throughout the whole current density range. In order to obtain insight into the physical mechanisms behind this complex behavior, we performed self-consistent Schrodinger-Poisson simulations. We show that variation in the emission spectra between the samples is related to changes in the carrier density and differences in the magnitude of screening of the built-in field inside QWs. Moreover, we show that the excited states play a major role in carrier recombination for all QWs, apart from the thinnest one.

Keywords: InGaN; light-emitting diode; molecular beam epitaxy; nitrides; quantum well.

Figure 1

(a) Schematic structure of grown and simulated LEDs. (b) Example photographs of measured devices operating on intermediate single QW in low and high current density.

Figure 2

Selected measured electroluminescence spectra of InGaN LEDs utilizing QW with thickness ranging from (a) 2.6 nm, (b) 6.5 nm, (c) 7.8 nm, (d) 12 nm and (e) 15 nm for increasing drive current. The dotted lines indicate the peak emission wavelength from different states and are assigned arbitrarily.

Figure 3

Dependence of the peak emission wavelength on current density for devices with various thicknesses of QW in the whole measured range of drive current.

Figure 4

Calculated band structure of the LED with In_0.17Ga_0.83N QW with thickness (a) 2.6 nm (b) 6.5 nm and (c) 15 nm. The wave functions of the e1, e2, h1 and h2 states and density of holes and electrons are shown for low and high current density regimes.

Figure 5

The peak density of holes and electrons in thin (blue), intermediate (green) and wide (red) QW.

Figure 6

Calculated dependence of wavefunction overlap and peak wavelength emission of various states inside the QW on current density. The QW thickness varies from (a) 2.6 nm, (b) 6.5 nm, (c) 7.8 nm, (d) 12 nm and (e) 15 nm.

Figure 7

Calculated spectra for InGaN LEDs utilizing a QW with thickness of (a) 2.6 nm, (b) 6.5 nm, (c) 7.8 nm, (d) 12 nm and (e) 15 nm.