At present times, the world’s attention on energy-saving technologies is continuously rising due to the increased awareness of the governments on how our lifestyle is damaging the Earth. Lighting is one of the most important aspects which fall within this framework. The realization of the first high-efficiency blue LED, with the possibility to reduce the waste of energy while increasing the luminous intensity has revolutionized many technologies (i.e. automotive, displays, lighting, portable devices). The materials used to make the blue LED are GaN and its alloys (InGaN and AlGaN), a compound which can emit in the whole visible spectrum depending on the alloy composition. As a consequence of that achievement, strong effort was put into the development of these new materials for optoelectronic applications. However, despite all the progress that have been made, at present times we are still lacking an all-nitride based RGB emitter. Indeed, most of the currently available displays (LCDs) use phosphors to convert the blue light produced by nitride LEDs into white color with lots of drawbacks in terms of energetic efficiency, scalability, switching response and color gamut. Nowadays, the LCD technology is reaching its limit and new emitters are being studied. Among them, micro-LEDs is one of the most promising alternatives. This new technology should allow a reduction in pixel size of two order of magnitude, an increase in brightness, lifetime and response times and a substantial reduction in production costs. At present times, the lack of an efficient GaN-based red emitter is the main reason which hinders the realization of high-efficiency micro-LEDs. Therefore, being able to overcome this obstacle will result in a significant progress in the display technology. The low efficiency of long wavelength nitride LEDs is attributed to different reasons. First, the low crystal quality of the material due to the low growth temperature of InGaN required to have a high indium incorporation. Second, the high electric fields in the structure (i.e. quantum confined Stark effect, QCSE) originating from the piezoelectric effect due to the large lattice mismatch between GaN and InGaN. Recently, some groups have found that using an AlGaN capping layer on top of the GaN/InGaN quantum well has positive effects on emission properties compared to the typical structures used (i.e. GaN/InGaN/GaN quantum wells). The results reported in literature demonstrate that this is one of the most interesting approaches to obtain a high-efficiency red LED. The aim of this work is to improve the understanding of the AlGaN capping layer and its effects on emission properties and to obtain a sample with red emission. In order to do this, I have studied the influence of various growth parameters on the emission properties (i.e. peak wavelength, emission line width and intensity) and optimized them to obtain a long-wavelength emission. Moreover, using theoretical simulations I have studied the effect of AlGaN thickness and composition on emission wavelength and recombination probability. The results of this thesis showed that the AlGaN capping layer increases the emission wavelength due to the larger polarization field in the QW. Based on the literature, a larger QCSE is expected to reduce the emission intensity due to the larger wavefunction separation. However, it was observed an increase in emission intensity for the GaN/InGaN/AlGaN QWs compared to GaN/InGaN/GaN at the same wavelength. I proposed an explanation based on the increased confinement of electrons due to the larger potential barrier. This can increase the wavefunction overlap and reduce the interaction of electrons with point defects, resulting in an increased emission intensity. Understanding that the electric field can be seen as an allied rather than as an enemy may open alternative paths to improve the efficiency of long-wavelength nitride-based LEDs.

Al giorno d’oggi l’attenzione mondiale sulle tecnologie a basso impatto energetico è in aumento a causa della consapevolezza di come il nostro stile di vita stia danneggiando la Terra. Il settore dell’illuminazione è una di queste tecnologie. La realizzazione del primo LED blu ad alta efficienza, con la possibilità di aumentare l’efficienza energetica, ha rivoluzionato molte tecnologie. I materiali usati per la realizzazione dei LED blu sono il GaN e i suoi composti (InGaN e AlGaN) che sono in grado di emettere in tutto lo spettro visibile a seconda della composizione. Di conseguenza, lo sviluppo di questi materiali è stato fortemente supportato. Tuttavia manca tuttora un emettitore RGB composto interamente da nitruri. La grande maggioranza dei display commercialmente disponibili (LCD) usano fosfori per convertire in bianco la luce blu prodotta dai LED a base di nitruri, con grandi svantaggi in temini di efficienza energetica, scalabilità, tempi di switching e gamma cromatica. La tecnologia degli LCD sta raggiungendo i suoi limiti tecnologici e si stanno studiando nuovi emettitori. Tra questi, i micro-LED sono una delle alernative più promettenti. Questa nuova tecnologia dovrebbe permettere una riduzione nella dimensione dei pixel di due ordini di grandezza, un aumento dei luminosità, tempi di vita, tempi di risposta e una notevole riduzione dei costi di produzione. Attualmente la mancanza di un emettore rosso basato sul GaN è la principale ragione che ostacola la realizzazione di micro-LED ad alta efficienza. Perciò, riuscire a superare questo ostacolo permetterà uno sviluppo significativo della tecnologia dei display. La bassa efficienza ad alte lunghezze d’onda dei LED a base nitruri è attribuita a diversi fattori. In primo luogo alla bassa qualità cristallina del materiale causata dalla bassa temperatura di crescita dell’InGaN necessarial per avere un’alta incorporazione di indio. In secondo luogo, gli alti campi elettrici nella struttura (quantum confined Stark effect, QCSE) causato dall’effetto piezoelettrico dovuto al grande mismatch reticolare tra GaN e InGaN. Di recente, alcuni gruppi hanno osservato che uno strato di AlGaN sopra la quantum well di GaN/InGaN ha un effetto positivo sulle proprietà di emissione rispetto alle strutture GaN/InGaN/GaN. Questo risultato dimostra come questo sia uno dei più interessanti approcci per ottenere un LED rosso ad alta efficienza. Lo scopo di questa tesi è migliorare la conoscenza degli effetti dell’AlGaN sull’emissione e ottenere un campione con emissione nel rosso. A questo scopo, Ho studiato l’influenza dei parametri di crescita sull’emissione (lunghezza d’onda, larghezza di riga e intensità) e li ho ottimizzati per avere emissione ad alte lunghezze d’onda. Inoltre, con delle simulazioni ho studiato gli effetti dello spessore e della composizione dello strato di AlGaN sulla lunghezza d’onda di emissione e la probabilità di ricombinazione. I risultati hanno mostrato che l’AlGaN aumenta la lunghezza d’onda di emissione a cause dell’aumento della polarizzazione all’interno della QW. In base alla letteratura, un aumento del QCSE porta a una riduzione dell’intensità dell’emissione a causa della maggior separazione delle funzioni d’onda. Tuttavia è stato osservato un aumento dell’intensità dell’emissione. Ho quindi proposto una spiegazione basata sull’aumento di confinamento degli elettroni a causa del’aumento della barriera di potenziale. Questo può aumentare la sovrapposizione delle funzioni d’onda e ridurre l’interazione degli elettroni con i difetti circostanti, portando all’aumento dell’intensità dell’emissione. Comprendere che il campo elettrico può essere visto come un alleato piuttosto che come un nemico può aprire nuovi percorsi per aumentare l’efficienza dei LED a base nitruri con emissione ad alte lunghezze d’onda.

(2021). Bandgap and Intrinsic Electric Field Engineering in Nitrides: Towards Efficient Red LEDs. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).

Bandgap and Intrinsic Electric Field Engineering in Nitrides: Towards Efficient Red LEDs

VICHI, STEFANO
2021

Abstract

At present times, the world’s attention on energy-saving technologies is continuously rising due to the increased awareness of the governments on how our lifestyle is damaging the Earth. Lighting is one of the most important aspects which fall within this framework. The realization of the first high-efficiency blue LED, with the possibility to reduce the waste of energy while increasing the luminous intensity has revolutionized many technologies (i.e. automotive, displays, lighting, portable devices). The materials used to make the blue LED are GaN and its alloys (InGaN and AlGaN), a compound which can emit in the whole visible spectrum depending on the alloy composition. As a consequence of that achievement, strong effort was put into the development of these new materials for optoelectronic applications. However, despite all the progress that have been made, at present times we are still lacking an all-nitride based RGB emitter. Indeed, most of the currently available displays (LCDs) use phosphors to convert the blue light produced by nitride LEDs into white color with lots of drawbacks in terms of energetic efficiency, scalability, switching response and color gamut. Nowadays, the LCD technology is reaching its limit and new emitters are being studied. Among them, micro-LEDs is one of the most promising alternatives. This new technology should allow a reduction in pixel size of two order of magnitude, an increase in brightness, lifetime and response times and a substantial reduction in production costs. At present times, the lack of an efficient GaN-based red emitter is the main reason which hinders the realization of high-efficiency micro-LEDs. Therefore, being able to overcome this obstacle will result in a significant progress in the display technology. The low efficiency of long wavelength nitride LEDs is attributed to different reasons. First, the low crystal quality of the material due to the low growth temperature of InGaN required to have a high indium incorporation. Second, the high electric fields in the structure (i.e. quantum confined Stark effect, QCSE) originating from the piezoelectric effect due to the large lattice mismatch between GaN and InGaN. Recently, some groups have found that using an AlGaN capping layer on top of the GaN/InGaN quantum well has positive effects on emission properties compared to the typical structures used (i.e. GaN/InGaN/GaN quantum wells). The results reported in literature demonstrate that this is one of the most interesting approaches to obtain a high-efficiency red LED. The aim of this work is to improve the understanding of the AlGaN capping layer and its effects on emission properties and to obtain a sample with red emission. In order to do this, I have studied the influence of various growth parameters on the emission properties (i.e. peak wavelength, emission line width and intensity) and optimized them to obtain a long-wavelength emission. Moreover, using theoretical simulations I have studied the effect of AlGaN thickness and composition on emission wavelength and recombination probability. The results of this thesis showed that the AlGaN capping layer increases the emission wavelength due to the larger polarization field in the QW. Based on the literature, a larger QCSE is expected to reduce the emission intensity due to the larger wavefunction separation. However, it was observed an increase in emission intensity for the GaN/InGaN/AlGaN QWs compared to GaN/InGaN/GaN at the same wavelength. I proposed an explanation based on the increased confinement of electrons due to the larger potential barrier. This can increase the wavefunction overlap and reduce the interaction of electrons with point defects, resulting in an increased emission intensity. Understanding that the electric field can be seen as an allied rather than as an enemy may open alternative paths to improve the efficiency of long-wavelength nitride-based LEDs.
SANGUINETTI, STEFANO
Nitruri; Quantum well; MOVPE; Luminescenza; QCSE
Nitrides; Quantum well; MOVPE; Luminescence; QCSE
FIS/03 - FISICA DELLA MATERIA
English
17-feb-2021
SCIENZA E NANOTECNOLOGIA DEI MATERIALI
33
2019/2020
open
(2021). Bandgap and Intrinsic Electric Field Engineering in Nitrides: Towards Efficient Red LEDs. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/304382
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