Influence of GaN buffer layer under InGaN/GaN MQWs on luminescent properties

doi:10.1016/j.jcrysgro.2018.11.025

Journal of Crystal Growth

Volume 507, 1 February 2019, Pages 246-250

https://doi.org/10.1016/j.jcrysgro.2018.11.025 Get rights and content

Highlights

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Presence of In in buffer layer is not necessary for PL improvement.
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Thicker low temperature buffer layer slightly improves luminescence intensity.
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N2 carrier gas in comparison with H2 increases V-pit size and luminescence intensity.
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Increased V-pit size by buffer layer was found to be the reason of MQW PL enhancement.
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Zn was determined as important contaminant in InGaN/GaN MQW region.

Abstract

Although InGaN layers or InGaN/GaN superlattices are commonly used as efficiency improving buffers for LED structure production, there is still a controversy and active discussion about the mechanisms improving the luminescence properties of InGaN QWs grown above such buffers. In this manuscript it is shown that presence of In in the buffer layer is not the primary reason for photoluminescence improvement which can be also achieved by introduction of GaN buffer layer grown at lower temperature under nitrogen atmosphere. SIMS analysis suggests that low temperature buffer layer does not influence the impurity incorporation and hence the PL improvement is caused by suppressed contamination of MQW region grown above the low temperature buffer. AFM images for two samples that differ mostly in morphology however supports another explanation in which formation of larger V-pits is the main reason for the luminescence improvement.

Introduction

It was observed that with increased number of InGaN/GaN quantum wells (QWs) their photoluminescence (PL) efficiency is increased. Usually PL improvement was observed until 10 InGaN/GaN QWs [1], but some works have reported PL improvement even for 30 QW structure [2]. The nonlinear improvement of PL efficiency with increased QW number was a motivation to develop buffer layers containing indium which were also reported to enhance the PL of InGaN/GaN multiple QW (MQW) structures [3], [4], [5], [6], [7], [8]. The most common types of such buffers are InGaN layers [3], [4], [5] or InGaN/GaN superlattices [6], [7], [8]. Although these buffer layers are already commonly used for LED structure production, there is still controversy and active discussion about the mechanisms improving the luminescence properties of InGaN QWs grown above such buffers. Several possible mechanisms are suggested, such as strain relaxation [3], [7], [8], enhanced impurity incorporation [5] or increased size of V-pits [4], [6]. During the growth of InGaN layers, hexagonal V-pits form around threading dislocations reaching the sample surface (Fig. 2). Their depth and diameter are determined by the thickness of the layer grown at sufficiently low temperature [9] and/or after switching the H₂ carrier gas to N₂ [10], [11], [12]. It was suggested that sufficiently wide V-pits represent an effective barrier for carriers in QWs and may prevent excitons from approaching the threading dislocation, thus preventing their non-radiative recombination [13].

Although different mechanisms of PL improvement were suggested, previous works assumed the presence of In in buffer layers to be a necessary condition for the PL enhancement.

The defect band (DB) emission was not studied before. Majority of published works focus on QW emission, since the luminescence of DB is very low under excitation conditions used for LEDs or laser diodes. Recently, new application of InGaN/GaN MQW structure as fast scintillators has emerged [14]. Scintillators consisting of InGaN/GaN MQWs, see Fig. 1, present a promising technology for radiation detection and imaging owing to their high efficiency of blue light emission upon irradiation. The fast electron-hole recombination in MQWs takes place within few nanoseconds, however the temporal resolution of our samples is deteriorated by an additional spectral component, the already mentioned DB which has much longer decay time. Unfortunately, it becomes dominant in the emission spectrum at very low excitation power densities that are typical for some scintillator operation.

Convincing arguments exist to assume that the DB arises from within the MQW active region [2], in contrast to other weaker spectral components that are emitted from impurities of the GaN buffer, denoted in the literature as yellow band and blue band. The DB central energy is sensitive to the MQW thickness and composition; in particular, DB is found systematically 300–400 meV below the narrower spectral emission of the MQW exciton. In addition, depth-resolved cathodoluminescence measurements corroborate the spatial overlap of DB emission with the MQW structure [2]. The exact source of DB has not been conclusively identified yet.

In this paper, we investigated several modifications of the GaN buffer layer below the MQWs that do not contain In, with the aim to elucidate mechanism of InGaN/GaN PL enhancement, to judge whether presence of In is necessary condition for the PL improvement and also to eliminate the harmful DB luminescence.

Section snippets

Experimental

We prepared all discussed structures on sapphire substrates with Aixtron 3x2″ CCS MOVPE system, equipped by Laytec epicurveTT in-situ monitoring. All samples were grown on c-oriented Al₂O₃ substrates, baked out and nitrified by NH₃ prior to the growth of 25 nm thick nucleation layer. TMGa precursor was used for the growth of 3.4 µm thick GaN high-temperature buffer layer (HTB). TEGa was subsequently used as gallium source for the low-temperature buffer layer (LTB), for the active region of the

SIMS analysis

Since we consider chemical contamination can be one of possible source for DB emission (beside native defects or their complexes), we performed a secondary ion mass spectroscopy (SIMS) analysis on a MQW sample of type A (i.e. one without LTB). Fig. 3a shows that some elements, notably oxygen, exhibit propensity to accumulate in the InGaN layers. Increased oxygen incorporation was also reported earlier [15] and was explained by more favorable stoichiometric conditions for incorporation of oxygen

Conclusions

In this manuscript it is shown that the presence of In in the buffer layer is not the primary reason for the PL improvement which can be also achieved by introduction of GaN buffer layer grown at lower temperature under nitrogen atmosphere. In a series of samples where a low-temperature buffer layer is added to separate the MQW structure from the high-temperature buffer beneath it, we confirmed that the luminescence efficiency of the exciton recombination improved and simultaneously the

Acknowledgments

The authors acknowledge support from MEYS project NPU LO1603 – ASTRANIT and from the Czech Science Foundation project no. 16-15569S. Partial support of EC project H2020-TWINN-2015 no. 690599 (ASCIMAT) is also gratefully acknowledged.

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These quasi barriers in V-pits help to separate carriers confined in InGaN QWs from dislocations which are situated in the center of V-pits suppressing thus the non-radiative recombination rate. These V-pits open during the epitaxy at lower temperature and with N2 used as a carrier gas [5]. It was shown that the optimal diameter of V-pits to reach the maximum efficiency of luminescence is around 200 nm [6,7].
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Influence of GaN buffer layer under InGaN/GaN MQWs on luminescent properties

Highlights

Abstract

Introduction

Section snippets

Experimental

SIMS analysis

Conclusions

Acknowledgments

J. Cryst. Growth

Solid-State Electron.

J. Cryst. Growth

Physica E

J. Cryst. Growth

J. Cryst. Growth

On the correlations between the excitonic luminescence efficiency and the QW numbers in multiple InGaN/GaN QW structure

J. Appl. Phys.

Effects of an InGaN prelayer on the properties of InGaN/GaN quantum well structures

Phys. Status Solidi C

Burying non-radiative defects in InGaN underlayer to increase InGaN/GaN quantum well efficiency

Appl. Phys. Lett.