Defect incorporation in In-containing layers and quantum wells: experimental analysis via deep level profiling and optical spectroscopy

Abstract

Recent studies demonstrated that the performance of InGaN/GaN quantum well (QW) light emitting diodes (LEDs) can be significantly improved through the insertion of an InGaN underlayer (UL). The current working hypothesis is that the presence of the UL reduces the density of non-radiative recombination centers (NRCs) in the QW itself: during the growth of the UL, surface defects are effectively buried in the UL, without propagating towards the QW region. Despite the importance of this hypothesis, the concentration profile of defects in the quantum wells of LEDs with and without the UL was never investigated in detail. This paper uses combined capacitance-voltage and steady-state photocapacitance measurements to experimentally identify the defects acting as NRCs and to extract a depth-profile of the traps, thus proving the incorporation upon indium-reaction. Specifically: (i) we demonstrate that LEDs without UL have a high density (9.2 × 1015 cm−3) of defects, compared to samples with UL (0.8 × 1015 cm−3); (ii) defects are located near midgap (E C-1.8 eV, corresponding to E i-E T ∼ 0.3 eV), thus acting as efficient NRCs; (iii) crucially, the density of defects has a peak within the QWs, indicating that traps are segregated at the first grown InGaN layers; (iv) we propose a model to calculate trap distribution in the QW, and we demonstrate a good correspondence with experimental data. These results provide unambiguous demonstration of the role of UL in limiting the propagation of defects towards the QWs, and the first experimental characterization of the properties of the related traps.

Imaging Nonradiative Point Defects Buried in Quantum Wells Using Cathodoluminescence

“…The strong impact of type-I PDs on CL intensity and IQE indicates they possess energy levels near the middle of the InGaN bandgap, since such midgap states are known to be efficient non-radiative recombination centres. 2,39 Our CL-derived type-I PD densities are in good agreement with previous results from macroscale measurements by Piva et al 37 (Fig. 4), which corroborates that we are spatially resolving individual PDs.…”

Defects and Reliability of GaN‐Based LEDs: Review and Perspectives

“…An example of the implementation of this technique was provided by Piva et al, [ 49 ] where the authors studied the efficacy of the insertion of a superlattice underlayer (SL UL), aimed at blocking the growth of defects toward the active region in an InGaN‐based LED. As shown in Figure , the authors evaluated traps concentration with the formula reported above and found a higher quantity of defects within the active region of the sample without the UL.…”

“…In fact, it was not possible to achieve the same results by replacing InGaN with GaN grown at lower temperature, and thus with low defects concentration, which indicates that the presence of indium atoms has a determining impact on defect generation. [ 19,49,75 ]…”

“…An example of the implementation of this technique was provided by Piva et al, [49] where the authors studied the efficacy of the insertion of a superlattice underlayer (SL UL), aimed at blocking the growth of defects toward the active region in an It shows an increase in the traps concentration during the stress test, in particular the increase in concentration of a midgap defect at 2.15 eV. Reproduced with permission.…”

“…In fact, it was not possible to achieve the same results by replacing InGaN with GaN grown at lower temperature, and thus with low defects concentration, which indicates that the presence of indium atoms has a determining impact on defect generation. [19,49,75] The use of a UL can be beneficial for blue LEDs; on the other hand, the situation is different for near-UV LEDs, where significant light absorption taking place in the InGaN UL could affect the external efficiency of the devices. Reducing the indium content in the UL is not a good solution because the incorporation of "surface defects" directly depends on the total amount of In atoms.…”