Littérature scientifique sur le sujet « Vapour Liquid Silod (VLS) growth mechanism »

In this work we report on 3C-SiC heteroepitaxial growth on 4H-SiC(0001) substrates which were patterned to form mesa structures. Two different deposition techniques were used and compared: vapour-liquid-solid (VLS) mechanism and chemical vapour deposition (CVD). The results in terms of surface morphology evolution and the polytype formation using these growth techniques were studied and compared. It was observed both 4H lateral growth from the mesa sidewalls and 3C enlargement on top of the mesas, the former being faster with CVD and VLS. Only VLS technique allowed elimination of twin boundaries for proper orientation of the mesa sidewalls.

We report on the synthesis of SiC nanowires (NWs) using iron as catalyst. The NWs were grown on silicon substrate by vapour-liquid-solid (VLS) mechanism with propane and silane as precursors, both 3% diluted in hydrogen, and hydrogen as carrier gas. The growth temperature was 1250°C, to reach the eutectic values of the Si-Fe alloy and to permit the VLS mechanism. The as-grown SiC nanowires were characterized by scanning and transmission electron microscopy. The nanowires are from 30 to 100 nm in diameter and several μm in length, with <111> growth direction.

Selective epitaxial growth in buried patterns was studied using the vapour-liquid-solid mechanism in Al-Si melt in order to obtain p+-doped SiC localized layers on 4H-SiC substrate. Homogeneous deposition with step bunched morphology was obtained by adding propane at room temperature before growth at 1100°C. Patterns as large as 800 µm and as narrow as 10 µm were completely filled in this way. The deposition kinetics demonstrates that the process is self limited and mainly depends on the initial amount of Si in the liquid. The deposit is highly p-type doped and the p-n junction is demonstrated.

Starting from 3C-SiC(111) layers grown by Vapour-Liquid-Solid mechanism, homoepitaxial growth by Chemical Vapour Deposition was carried out on top of these seeds. The effect of the growth temperature and of the C/Si ratio in the gas phase was investigated on the surface morphology, the roughness and the defect density. It was found that the initial highly step-bunched surface of the VLS seeds could be greatly smoothen using appropriate conditions. These conditions were also found to reduce significantly the defect size and/or density at the surface.

In this work, the growth by Vapour-Liquid-Solid (VLS) mechanism of 3C-SiC on silicon substrate is reported. Firstly, a germanium layer is deposited on the substrate. Then the temperature of the sample is increased above Ge melting point in order to form a SiGe liquid phase by reaction with the substrate. Upon reaching the target temperature (1100-1300°C) the VLS growth starts with the injection of propane in the reactor. Both Raman spectrometry and X-Ray diffraction analyses evidenced the formation of 3C-SiC on every sample. However, this SiC deposit, a few micrometers thick, is always found to be polycrystalline though textured. In parallel, the presence of an epitaxial Si-Ge alloy, whose composition depends on the growth temperature, was systematically detected between Si and SiC.

In this work we report on SiC epitaxial growth by vapour-liquid-solid (VLS) mechanism on on-axis 4H-SiC(0001) substrates which were previously patterned to form mesa structures. The liquid phase was set to Al70Si30. At 1100°C, it led to very high homoepitaxial lateral growth (140 µm/h) with pronounced spiral growth and in plane anisotropy of growth rate. Upon temperature increase to 1200 °C, this spiral growth was suppressed and the lateral growth was further increased up to 180 µm/h. The in-plane versus out-of-plane anisotropy of growth rate was found to be as high as 60 at this temperature and 46 at 1100°C.

The growth kinetics of 3C-SiC heteroepitaxial layers on α-SiC substrates by Vapour-Liquid-Solid (VLS) mechanism in Ge-Si melts was investigated. Various parameters were studied such as temperature, melt composition, propane flux and substrate nature (polytype, polarity and misorientation). It was found that the growth rate increases with increasing temperature, propane flux, Si content of the melt and misorientation of the substrate. The calculated activation energy (from 4.7 to 6.6 kcal/mole depending on the substrate type) is very small suggesting that the limiting process is the diffusion of the dissolved carbon inside the melt. The carbon solubility inside the melt mainly affects the carbon dissolution kinetics from the gas phase. The results also suggest that surface effects are important through the layer polarity and crystalline quality.

In this paper, the issues related to in-situ doping of silicon carbide (SiC) semiconductor during epitaxial growth are reviewed. Some of these issues can find solution by using an original approach called vapour-liquid-solid (VLS) mechanism. In this technique, the SiC seed is covered by a Sibased melt and is fed by propane in order to growth the epitaxial film. Using Al-Si melts and temperatures as low as 1100°C, very high p type doping was demonstrated, with a record value of 1.1021 at.cm-3. It leads to very low contact resistivity and even to metallic behaviour of the SiC deposit even at low temperature. Using Ge-Si melts, non intentionally low doped n type layers are grown. By forming Si-containing liquid droplets on a SiC seed, one can extrapolate this VLS growth to selective epitaxial growth (SEG). Such approach was successfully applied for both Al and Ge-based systems in order to form p+ and n doped areas respectively.

L'utilisation récente d'une voie originale de croissance cristalline basée sur les mécanismes vapeur-liquide-solide (VLS) à partir d'un bain Ge-Si a permis des améliorations importantes de la qualité cristalline des couches minces hétéroépitaxiales de SiC-3C sur substrats sur substrat α-SiC(0001). Ce travail a pour but d'approfondir les connaissances sur cette technique de croissance, d'améliorer le procédé et de déterminer les propriétés du matériau élaboré. La première partie est dédiée à la compréhension et la maîtrise des différents mécanismes impliqués dans la croissance de SiC-3C par VLS. Cela a notamment permis la détermination des paramètres limitant la taille des échantillons et la démonstration des avantages à utiliser des alliages fondus contant 50 at% de Ge au lieu de 75 at%. Une étude de la croissance latérale sur substrats patternés a donné des indications intéressantes pouvant être intégrées dans le modèle d'élimination des macles. L'incorporation intentionnelle et non intentionnelle de dopants de type n et p pendant la croissance VLS a été suivie. Pour le dopage n, nous avons démontré l'existence d'un lien clair entre l'impureté N et la stabilisation du polytype SiC-3C. En outre, nous avons réussi à abaisser le dopage résiduel n des couches en dessous de 1x1017 cm-3. Pour le dopage p, le meilleur élément n'est pas le Ga mais l'Al, même s'il doit être ajouté à un alliage de type Si-Ge pour éviter l'homoépitaxie. Enfin, ces couches ont été caractérisées optiquement et électriquement par différentes techniques. Les mesures C-V et G-V ont permis d'estimer une concentration très faible (7×109 cm-2) de charges fixes dans l'oxyde SiO2 ainsi qu'une densité d'états d'interface aussi basse que 1.2×1010 cm-2eV-1 à 0.63 eV sous la bande de conduction. Ces valeurs record sont une très bonne base pour le développement d'un composant de type MOSFET en SiC-3C
Recently, the use of an original growth approach based on vapour-liquid-solid (VLS) mechanism with Ge-Si melts has led to significant improvement of the crystalline quality of the 3C-SiC thin layers heteroepitaxially grown on α-SiC(0001) substrate. This work tries to deepen the knowledge of such specific growth method, to improve the process and to determine the properties of the grown material. The first part was dedicated to the understanding and mastering of the various mechanisms involved in 3C-SiC growth by VLS mechanism. This led to the determination of the parameters limiting sample size and the demonstration of the benefits of using 50 at% Ge instead of 75 at% Ge melts. A study of lateral enlargement on patterned substrates gave some interesting hints which can be integrated in the model of twin defect elimination. The incorporation of non intentional and intentional n- and p-type dopants during VLS growth was studied. For n-type doping, a clear link between N impurity and 3C polytype stability was demonstrated. Besides, high purity layers with residual n-type doping below 1x1017 cm-3 were achieved. For p-type doping, the best element was shown to be Al and not Ga, even if it has to be alloyed with Ge-Si melts to avoid homoepitaxial growth. Finally, these layers were characterised by several optical and electrical means like Raman spectroscopy, low temperature photoluminescence, deep leveltransient spectroscopy and MOS capacitors measurements. Very low concentrationsof fixed oxide charges estimated about 7×109 cm-2 and interface states densities Dit equal to 1.2×1010 cm-2eV-1at 0.63 eV below the conduction band have been achieved. These record values are a very good base toward 3C-SiC MOSFET

2007/2008
This thesis is devoted to the study of the growth of III-V nanowires (NWs) by catalyst assisted and catalyst free molecular beam epitaxy (MBE). The nanostructures have been routinely characterized by scanning electron microscopy (SEM) and, to a minor extent by transmission electron microscopy (TEM). X-ray photoemission spectroscopy (XPS), scanning photoemission microscopy (SPEM), extended X-ray absrorption fi ne structure analysis (EXAFS), photoluminescence (PL) and trans- port measurements have given an important contribution on specifi c topics. The first section of this thesis reports on GaAs, InAs, and InGaAs NWs growth by Au assisted MBE. A substrate treatment is proposed that improves uniformity in the NWS morphology. Thanks to a careful statistical analysis of the NWs shape and dimensions as a function of growth temperature and duration, evidence is found of radial growth of the NWs taking place together with the axial growth at the tip. This eff ect is interpreted in term of temperature dependent diff usion length of the cations on the NWs lateral surface. The control of the NWs radial growth allowed to grow core shell InGaAs/GaAs NWs, displaying superior optical quality. A new procedure is proposed to protect NWs surface from air exposure. This procedure allowed to perform ex-situ SPEM studies of electronic properties of the NWs. The second part of this thesis is devoted to Au-free NWs growth. GaAs and InAs NWs were successfully grown for the first time using Mn as catalyst. Incorporation of Mn in the NW is studied using EXAFS technique. It is shown that Mn atoms are incorporated in the body of GaAs NWs. Use of low growth temperature is suggested in order to improve the Mn incorporation inside GaAs NWs and obtain NWs with magnetic properties. Finally, growth of GaAs and InAs NWs on cleaved Si subtrate is demonstrated without the use of any outside metal catalyst. Two kinds of nanowires have been obtained. The experimental findings suggest that the two types of nanowires grow after di fferent growth processes.
Questa tesi e' dedicata allo studio della crescita di nanofili di semiconduttori III- V tramite epitassia da fasci molecolari (MBE) assistita da catalizzatore e senza l'uso di catalizzatori. Le nanostrutture sono state caratterizzate sistematicamente tramite microscopia elettronica a scansione (SEM), e in maniera minore microscopia elettronica in trasmissione (TEM). Altre tecniche come la spettroscopia di fotoemissione da raggi x (XPS), la microscopia da fotoemissione in scansione (SPEM), la spettroscopia di assorbimento x (in particolare la extended X-ray absorpition fine structure analysis (EXAFS)) la fotoluminescenza (PL), e il trasporto elettrico hanno dato importanti contributi su problematiche specifiche. La prima parte di questa tesi riguarda la crescita di nanofili di GaAs, InAs e InGaAs tramite MBE assistita da oro. Viene proposto un trattamento del substrato che migliora nettamente l'omogeneita' morfologica dei nanofili. Grazie ad un'attenta analisi statistica della forma e delle dimensioni dei nanofili in funzione della temperatura e del tempo di crescita e' stata dimostrata la crescita radiale dei nanofili, che avviene insieme alla crescita assiale che ha luogo alla punta del nanofilo. Le osservazioni sperimentali sono state interpretate in termini di dipendenza dalla temperatura della lunghezza di diffusione dei cationi sulle super ci laterali dei nanofili. Il controllo della crescita radiale ha permesso di crescere nanofili di InGaAs/GaAs core shell, costituiti cioe' da una anima centrale di InGaAs (core) e uno strato esterno di GaAs (shell) , che hanno dimostrato eccellente qualita' ottica. Viene quindi proposta una nuova procedura per proteggere la super ficie dei nanofili durante l'esposizione all'aria. Grazie a questa e' stato possibile realizzare ex-situ uno studio SPEM delle proprieta' elettroniche dei nanofili. La seconda parte della tesi riguarda la crescita di nanofili senza l'uso di oro. Viene per la prima volta dimostrata la possibilita' di crescere nanofili di GaAs e InAs usando il manganese come catalizzatore. L'incorporazione del Mn come impurezza nei nanofili e' stata studiata tramite EXAFS. Le misure hanno dimostrato che atomi di Mn sono effettivamente incorporate nel corpo dei nano fili. La crescita delle nanostrutture a temperatura piu' bassa potrebbe migliorare qualitativamente l'incorporazione del Mn e permettere la crescita di nanofili con proprieta' magnetiche. Viene in fine dimostrata la crescita di nanofili di GaAs e di InAs senza l'utilizzo di materiali diversi da quelli costituenti il semiconduttore. Tale risultato e' ottenuto su superfici sfaldate di silicio. Sono state osservate nanostrutture di due tipi, che sulla base dei dati sperimentali sembrano essere dovuti a due diversi meccanismi di crescita.
XXI Ciclo
1977