The influence of the conditions during growth of InAs/GaAs quantum-dot structures on GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving high quantum dot densities, homogeneity and narrow sizes distribution. These are requirements for obtaining the necessary intermediate band in intermediate band solar cells. The growth temperature, InAs deposit, As flux, growth rate and III/V flux ratio were varied. Atomic force microscopy and a modular program for SPM data analysis were used to study the morphological properties of the QDs. The effect of several operations during image processing was analysed and a suitable routine was defined. The optimal temperature for the growth was found to be 480°C although 420°C was sufficient to induce 2D-3D transition. Varying the InAs thickness, a self-limiting size effect was detected at 2.2 ML, while increasing As-flux to 6E−6 Torr led to big clasters formation. Finally, low growth rate (0.1-0.5 ML/s) was necessary to keep good homogeneity and the optimal III/V flux ratio was found to be in the range 1/10 - 1/5.
La presente tesi si focalizza sullo studio di punti quantisti InAs/GaAs su substrato GaAs(001) per applicazioni su celle solari a banda intermedia. Un’alta densità di punti quantistici, una buona omogeneità e una distribuzione stretta di dimensioni laterali e altezze sono requisiti necessari per ottenere la banda intermedia in questa specifica applicazione. I campioni sono cresciuti per mezzo di epitassia da fasci molecolari, variando sistematicamente i parametri di crescita quali temperatura, quantità di InAs depositato, flusso di As, velocità di crescita e rapporto III/V di flussi. Lo scopo di questo studio è analizzare l’influenza di questi parametri su densità, distribuzioni di dimensioni e omogeneità dei punti quantistici. I profili 3D della superficie dei campioni sono estratti tramite microscopia a forza atomica; successivamente le informazioni sono estrapolate dalle immagini per mezzo di uno specifico programma di analisi dopo aver valutato l’effetto di diverse correzioni durante l’image processing e dopo aver definito un’opportuna routine. È stato dimostrato che la temperatura ottimale di crescita è 480°C, sebbene 420°C siano sufficienti per indurre la transizione 2D-3D. Variando lo spessore di InAs, a 2.2 ML è stato notato un effetto auto-limitante per quanto riguarda le dimensioni, mentre aumentando il flusso di As fino a 6E−6 Torr si induce la coalescenza di più punti quantistici. Infine, basse velocità di crescita (0.1-0.5 ML/s) sono necessarie per mantenere una buona omogeneità e il rapporto ottimale III/V è nell’intervallo 1/10-1/5.
Systematic investigation into the influence of growth conditions on InAs/GaAs quantum dot densities and sizes for intermediate band solar cell application
VITELLI, STEFANO
2009/2010
Abstract
The influence of the conditions during growth of InAs/GaAs quantum-dot structures on GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving high quantum dot densities, homogeneity and narrow sizes distribution. These are requirements for obtaining the necessary intermediate band in intermediate band solar cells. The growth temperature, InAs deposit, As flux, growth rate and III/V flux ratio were varied. Atomic force microscopy and a modular program for SPM data analysis were used to study the morphological properties of the QDs. The effect of several operations during image processing was analysed and a suitable routine was defined. The optimal temperature for the growth was found to be 480°C although 420°C was sufficient to induce 2D-3D transition. Varying the InAs thickness, a self-limiting size effect was detected at 2.2 ML, while increasing As-flux to 6E−6 Torr led to big clasters formation. Finally, low growth rate (0.1-0.5 ML/s) was necessary to keep good homogeneity and the optimal III/V flux ratio was found to be in the range 1/10 - 1/5.File | Dimensione | Formato | |
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2010_07_Vitelli.pdf
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https://hdl.handle.net/10589/1981