Graphene induced weak carrier localization in InGaN nanorods directly grown on graphene-covered Si
Graphical abstract
Introduction
Nitrides-based semiconductors have attracted much attention for their applications as light-emitting devices (LEDs) and laser diodes (LDs) in the visible and UV spectral regions [[1], [2], [3], [4]]. The bright III-nitride LEDs and LDs are typically realized on single quantum well (SQW) or multiple QW (MQW) structures. Indeed, QW-active layers can reduce the carrier capturing in nonradiative defects and, as a result, improve the light emission efficiency. Nitrides properties demonstrate their commercial success in the field of optoelectronic devices. However, several issues limit its efficiency such as the efficiency droop (reduced IQE) at high power [3]. For devices operating at longer wavelengths such as LEDs, this is further aggravated and causes the so-called “green-gap problem”. In plus, this green-gap is principally related to the Auger recombination. This last has been demonstrated to be an important non-radiative recombination mechanism in nitride LEDs and a reasonable cause of the efficiency-droop [5,6]. All these limits are may be further exacerbated by the localization of carriers. Indeed, nitrides contain statistical random composition fluctuations at the nanometer scale that spatially localize carriers [7,8]. The phenomenon has been theoretically and experimentally studied in III-V materials to report the effect of composition fluctuations on the luminescence properties. On the InGaN/GaN heterostructure, despite the work of Chi-Chang Hong et al. in 2009[9], that proves the photoluminescence (PL) enhancement in InGaN/GaN nanorods due to carrier localization, a deep experimental and quantitative study of its performences and related phenomena is required. Recently, Yang et al. [10], Auf der Maur et al. [11], Christina M. Jones et al. [12] performed simulations using atomistic tight-binding and Schrodinger-Poisson simulations in the effective-mass approximation to evaluate radiative recombination rates in fluctuating-alloy quantum wells and demonstrate the negative effect. Semi-empirical models have been used to investigate that carrier localization enhances the Auger recombination [13]. Other works on this phenomenon have been performed on III-V alloy compound using the localized state ensemble (LSE) model. Such works quantitatively describe the effect of carrier localization on the optical properties in III-V heterojunctions under different growth conditions [[14], [15], [16]]. Despite extensive studies on carrier localization's causes and consequences, the effect of the substrate and or buffer layer on the luminescence properties still lacking. To overcome the localization limits in nitrides and enhance their properties, many methods have been introduced for NRs but it needed an original AlN or GaN template or metal catalyst—which complicated the fabrication of nanostructures. Recently, researchers have suggested the growth of nitrides on graphene-based-substrates. That's, two-dimensional (2D) materials and their combination with other types of low-dimensional materials into hybrid systems open new ways to conceive optoelectronic and photodetection devices. Graphene, a two-dimensional planar configuration of sp2-bonded carbon atoms, has attracted a great interest owing to the hexagonal arrangement of C atoms, making the one-atomic layer graphene able to serve as a nearly lattice-matched buffer for the growth of wurtzite GaN and related nitrides [17] as well as a supporting matrix for other two-dimensional layered materials [[18], [19], [20]]. The GaN-based nanorods can be directly grown on graphene-covered substrates without the need of a crystalline bulk or metal catalyst [21]. Furthermore, graphene films are transferable to almost any carrier substrate, including amorphous and flexible materials [21]. Therefore, the growth of nitrides-based nanorods on a graphene buffer provides a new idea for fabrication of flexible optoelectrical devices. We suggest the growth of this last on graphene-covered-Silicon as substrate. Until now days, there is no deep qualitative and quantitative study on the graphene layer effect on the localization phenomenon. We qualitatively control the effect of graphene layer on the emission properties of InGaN/GaN nanorods elaborated on graphene-covered-Si. Temperature dependent PL spectra of InGaN/GaN nanorods show the pronounced S-shaped behavior associated with the increase of In composition, size and zones nonuniformity in NRs. The modeling results indicate a high-density localized state within the band-tails compared to the film and fully suppressed in the InGaN/GaN nanorods on graphene-covered-Si. Our suggestion could increase the quantum efficiency of nanorod devices by suppressing surface states/defects induced fluctuations, and the random localization decrease in the InGaN nanorods could improve the IQE by reducing the effect of localization on the quantum efficiency. This can be added to the fact that graphene can reduce carriers' recombination (holes and electrons) due to its high electric conductivity which increases the possibility of exploiting graphene-based materials in optoelectronics [[18], [19], [20]]. An expectation for the graphene/p-InGaN heterojunction, that the electron affinity and band gap values of p-type InGaN are changed after adding graphene monolayer. Here we focus on the bandgap evolution qualitatively and quantitatively using the LSE model. We will get deeply inside the graphene layer effects on the random fluctuation in InGaN nanorods. Hence, it gives a detailed description of special parameters that could describe the overall suggested materials and especially the bandgap changes. Our results could help to improve the growth and properties of III-V materials on graphene-based-substrates for high performance optoelectronic devices.
Section snippets
Experimental details
Generally, we proceed to the following steps towards the final growth of our structure: (i) Spin-coated poly (methyl methacrylate) (PMMA) onto graphene on Cu foil; (ii) transfer of graphene with PMMA onto Si substrates; (iii) dissolving PMMA; and (IV) MOCVD growth of InGaN nanorod.
In details: a single-layer graphene film grown on Cu foil by atmospheric-pressure chemical vapor deposition (APCVD) was transferred onto a Si (111) substrate. The poly (methyl methacrylate) (PMMA) was spin-coated onto
Results and discussion
To prove the successful growth, we performed scanning electron microscopy (SEM) and Raman measurements. Firstly, a micrograph SEM imaging in Fig. 1 clearly shows that all the NRs (yellow arrows) are vertical with a random distribution along the plan. The dimensions are not uniform with an average height and diameter of 280 nm and 55 nm, respectively. Their density of ~2.5 × 108 cm−2. However, we can observe isolated NRs and several coalescence zones (dashed red circles in the SEM and top-view
Conclusion
In summary, we have successfully grown InGaN nanorods on graphene covered Si. Two samples have been investigated experimentally and, theoretically using the LSE model for the first time. SEM images show the inhomogeneity redistribution and size of NRs with coalescence zones. Raman investigations prove the added graphene monolayer with a blue shift after the MOCVD growth. This shift has been attributed to the graphene self-doping by nitrogen and the compressive strain. Dopants and strain,
CRediT authorship contribution statement
Tarek Hidouri:Software, Conceptualization, Investigation, Writing - original draft, Writing - review & editing.Samia Nasr:Writing - review & editing, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors are grateful to King Khalid University for their continuous support during conduction of this work.
Funding
The authors declare no funding has been received.
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