Influence of quantum well inhomogeneities on absorption, spontaneous emission, photoluminescence decay time, and lasing in polar InGaN quantum wells emitting in the blue-green spectral region

Abstract

It is shown that in polar InGaN QWs emitting in the blue-green spectral region a Stokes shift between spontaneous emission (SE) and optical transition observed in contactless electroreflectance (CER) spectrum (absorptionlike technique) can be observed even at room temperature, despite the fact that the SE is not associated with localized states. Time resolved photoluminescence measurements clearly confirm that the SE is strongly localized at low temperatures whereas at room temperature the carrier localization disappears and the SE can be attributed to the fundamental transition in this QW. The Stokes shift is observed in this QW system because of the large built-in electric field, i.e., the CER transition is a superposition of all optical transitions with non-zero electron-hole overlap integrals and, therefore, the energy of this transition does not correspond to the fundamental transition of InGaN QW. Lasing from this QW has been observed at the wavelength of 475 nm, whereas the SE was observed at 500 nm. The 25 nm shift between the lasing and SE is observed because of a screening of the built-in electric field by photogenerated carriers. However, our analysis shows that the built-in electric field inside the InGaN QW region is not fully screened under the lasing conditions.

Direct evidence of photoluminescence broadening enhancement by local electric field fluctuations in polar InGaN/GaN quantum wells

“…The PL measurements were complemented by electromodulation spectroscopy together with calculations within the random QW model. 24,25,28,29) Our results show that the QW built-in electric field enhances the PL broadening related to the previously invoked mechanisms involving local inhomogeneity. Additionally, we have shown that the exciton-phonon coupling can be controlled using an external bias.…”

“…22,23) It has been shown theoretically that the electric field significantly impacts the transition broadening in InGaN=GaN QW. [24][25][26] However, there is no experimental work that quantifies how strong this effect is in the InGaN=GaN QW system. Some indication of the PL broadening enhancement due to an increased electric field has been reported for polar quantum wells.…”

“…More details concerning this model can be found elsewhere. 24,25,28,29) Figure 5 shows results of PL simulations performed for a 2.8 nm wide In 0.22 Ga 0.78 N=GaN QW for three different standard deviations of indium content: 0.50, 0.75, and 1.00%. Fluctuations in the QW width are neglected in this case for simplicity.…”

Emission and material gain spectra of polar compressive strained AlGaN quantum wells grown on virtual AlGaN substrates: Tuning emission wavelength and mixing TE and TM mode of light polarization

“…On one side, this width cannot be too small (d<2 nm) because of the weak quantum confinement, and on the other side this width cannot be too large (d>4 nm) because of small electron-hole overlap integral. Additionally it is worth noting that alloy inhomogeneities (i.e., fluctuations of QW width and content) manifest themselves much stronger in the optical properties of narrow QWs [58,60] and therefore narrow QWs (d<2 nm) will exhibit a rather broad luminescence which can be an unwanted feature of UV LEDs. Taking into account the above results and arguments we concluded that the optimal QW width for the active region in UV LEDs is in the range of 2-4 nm.…”

A comparison of the optical properties of InGaN/GaN multiple quantum well structures grown with and without Si-doped InGaN prelayers

“…This suppression of the spectral dependence of the PL decay time with increasing temperature has been reported by several groups. 49,[53][54][55][56] In these works, 49,53-55 the measured decay curves at room temperature were single exponential and independent of detection energy which was ascribed to the effects of non-radiative recombination. However, the samples discussed here are quite efficient at 300 K with IQEs of 25(62)% and 36(62)% for sample A and sample B, respectively.…”