The carrier capture and relaxation of Type II ZnTe/ZnSe quantum dots(QDs) were investigated with ultrafast photoluminescence upconversion. We found that carrier relaxation of QDs under Volmer-Weber(VW) growth mode exhibits faster decay and rise than that of QDs under Stranski-Krastanow(SK) growth mode due to the wetting layer in SK growth mode provides as a pathway for carriers to diffuse and migrate from large(small) to small (larger) QDs. The wetting layer level was found by analyze the decay time of PL with different wavelength and temperature. The PL of VW mode and SK mode by using 532nm Nd-YAG laser also prove the existence of wetting layer. We interpret our results of VW mode in terms of Auger process with large carrier density.

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Ce mémoire porte sur les mécanismes de relaxation et de fuite des excitons dans des systèmes de boîtes quantiques(BQs) d’InAs/InP. Les systèmes sont composés d’un sub- strat volumique d’InP, appelé matrice (M), d’un puits quantique d’InAs, nommé couche de mouillage (CM), et des familles de BQs d’InAs. La distinction entre les familles est faite par le nombre de monocouche d’épaisseur des boîtes qui sont beaucoup plus larges que hautes. Une revue de littérature retrace les principaux mécanismes de relaxation et de fuite des excitons dans les systèmes. Ensuite, différents modèles portant sur la fuite thermique des excitons des BQs sont comparés. Les types de caractérisations déjà produites et les spécifications des croissances des échantillons sont présentés. L’approche adoptée pour ce mémoire a été de caractériser temporellement la dynamique des BQs avec des mesures d’absorbtion transitoire et de photoluminescence résolue en temps (PLRT) par addition de fréquences. L’expérience d’absorption transitoire n’a pas fait ressortir de résultats très probants, mais elle est expliquée en détails. Les mesures de PLRT ont permis de suivre en température le temps de vie effectif des excitons dans des familles de BQs. Ensuite, avec un modèle de bilan détaillé, qui a été bien explicité, il a été possible d’identifier le rôle de la M et de la CM dans la relaxation et la fuite des excitons dans les BQs. Les ajustements montrent plus précisément que la fuite de porteurs dans les BQs se fait sous la forme de paires d’électrons-trous corrélées. / This thesis focuses on the mechanisms of relaxation and leakage of excitons in systems of quantum dots (QDs) InAs / InP. The systems are composed of a substrate of InP volume, called matrix (M), of a quantum well of InAs, named wetting layer (CM), and of QD families of InAs. The distinction between the families can be explained by the number of monolayer-thick boxes that are wider than high. A literature review highlights the main relaxation mechanisms and leakage of excitons in systems. Then, different models on the thermal leakage of the QD excitons are compared.Then, a presentation of the different types of characterizations already and of the specifications on the samples growths. The approach used for this thesis is to temporarily characterize the dynamic of the QDs with transient absorption and upconversion. The transient absorption experiment’s results are not very convincing, but are minutely explained. PLRT measures were used to follow in temperature the excitons effective lifetime in the QDs families. Then, with a detailed balance model, which has been well explained, it was possible to identify the role of theMand CM in relaxation and leakage of excitons in QDs. As shown by the adjustement, the escape of carriers in the QDs is made in a correlated electron-hole pairs form.

Ce mémoire porte sur les mécanismes de relaxation et de fuite des excitons dans des systèmes de boîtes quantiques(BQs) d’InAs/InP. Les systèmes sont composés d’un sub- strat volumique d’InP, appelé matrice (M), d’un puits quantique d’InAs, nommé couche de mouillage (CM), et des familles de BQs d’InAs. La distinction entre les familles est faite par le nombre de monocouche d’épaisseur des boîtes qui sont beaucoup plus larges que hautes. Une revue de littérature retrace les principaux mécanismes de relaxation et de fuite des excitons dans les systèmes. Ensuite, différents modèles portant sur la fuite thermique des excitons des BQs sont comparés. Les types de caractérisations déjà produites et les spécifications des croissances des échantillons sont présentés. L’approche adoptée pour ce mémoire a été de caractériser temporellement la dynamique des BQs avec des mesures d’absorbtion transitoire et de photoluminescence résolue en temps (PLRT) par addition de fréquences. L’expérience d’absorption transitoire n’a pas fait ressortir de résultats très probants, mais elle est expliquée en détails. Les mesures de PLRT ont permis de suivre en température le temps de vie effectif des excitons dans des familles de BQs. Ensuite, avec un modèle de bilan détaillé, qui a été bien explicité, il a été possible d’identifier le rôle de la M et de la CM dans la relaxation et la fuite des excitons dans les BQs. Les ajustements montrent plus précisément que la fuite de porteurs dans les BQs se fait sous la forme de paires d’électrons-trous corrélées. / This thesis focuses on the mechanisms of relaxation and leakage of excitons in systems of quantum dots (QDs) InAs / InP. The systems are composed of a substrate of InP volume, called matrix (M), of a quantum well of InAs, named wetting layer (CM), and of QD families of InAs. The distinction between the families can be explained by the number of monolayer-thick boxes that are wider than high. A literature review highlights the main relaxation mechanisms and leakage of excitons in systems. Then, different models on the thermal leakage of the QD excitons are compared.Then, a presentation of the different types of characterizations already and of the specifications on the samples growths. The approach used for this thesis is to temporarily characterize the dynamic of the QDs with transient absorption and upconversion. The transient absorption experiment’s results are not very convincing, but are minutely explained. PLRT measures were used to follow in temperature the excitons effective lifetime in the QDs families. Then, with a detailed balance model, which has been well explained, it was possible to identify the role of theMand CM in relaxation and leakage of excitons in QDs. As shown by the adjustement, the escape of carriers in the QDs is made in a correlated electron-hole pairs form.

The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties. The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature. An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD. Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %. In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+).  Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton. In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements. / Arbetet som presenteras i denna avhandling rör studier av kvantprickars optiska egenskaper. En kvantprick är en halvledarkristall som endast är några tiotals nanometer stor. Den ligger oftast inbäddad inuti en större kristall av ett annat halvledarmaterial och pga. den begränsade storleken får en kvantprick mycket speciella egenskaper. Bland annat så kommer elektronerna i en kvantprick endast att kunna anta vissa diskreta energinivåer liknande situationen för elektronerna i en atom. Följaktligen kallas kvantprickar ofta för artificiella atomer. För halvledarmaterial gäller det generellt att det inte endast är fria elektroner i ledningsbandet, som kan leda ström utan även tomma elektrontillstånd i valens­bandet, vilka uppträder som positivt laddade partiklar, kan leda ström. Dessa kallas kort och gott för hål. I en kvantprick har hålen såsom elektronerna helt diskreta energinivåer. Precis som är fallet i en atom, så kommer elektroniska övergångar mellan olika energi­nivåer i en kvantprick att resultera i att ljus emitteras. Energin (dvs. våglängden alt. färgen) för detta ljus bestäms av hur energinivåerna i kvant­pricken ligger, för elektronerna och hålen, och genom att analysera ljuset kan man således studera kvantprickens egenskaper. Studierna i den här avhandlingen visar att växelverkan mellan en kvantprick och den omgivande kristallen, som den ligger inbäddad i, har stor inverkan på kvantprickens optiska egenskaper. T.ex. visas att man kan kontrollera antalet elektroner, som kommer att finnas i kvantpricken genom att modifiera hur elektronerna kan röra sig i omgivningen. Dessa rörelser modifieras här genom att variera temperaturen och genom att lägga på ett magnetiskt fält. Ett magnetiskt fält, vinkelrätt mot en elektrons rörelse, kommer att böja av dess bana och dess chans att nå fram till kvantpricken kan således minskas. Elektronen kan då istället fastna i andra potentialgropar i kvantprickens närhet. Genom att öka temperaturen, vilket ger elektronerna större energi, kan deras chans att nå fram till kvantpricken å andra sidan öka. En annan effekt, som studerats, är möjligheten att kontrollera spinnet hos elektronerna i en kvantprick. Även i dessa studier visar det sig att växelverkan med omgivningen spelar stor roll och kan användas till att kontrollera elektronens spin. Mekanismen som föreslås är att om elektronerna hinner före hålen till kvantpricken, så hinner de överföra sitt spin till atomkärnorna i kvantpricken. På detta sätt kan man få atomkärnornas spin polariserat, vilket resulterar i ett inbyggt magnetfält, i storleksordningen 1.5 Tesla, som i sin tur hjälper till att upprätthålla en hög grad av spinpolarisering även hos elektronerna. För att få elektronerna att hinna först, måste deras rörelser i omgivningen kontrolleras. I en ytterligare studie undersöktes den process där en elektronisk övergång i kvantpricken inte enbart resulterar i emission av ljus, utan även i att en annan partikel tar över en del av energin och blir exciterad. Dessa processer avspeglas i att en del av det ljus som emitteras har lägre energi. Detta ljus är också mycket svagt, ca 1000 ggr lägre intensitet, och möjligheten att kunna mäta detta är helt beroende på hur ljusstarka kvantprickarna är. De prover som använts i denna studie består av pyramidstrukturer, ca 7.5 mikrometer stora, med kvantprickar inuti. Denna geometri ger ca 1000 ggr bättre ljusutbyte jämfört med traditionella strukturer, vilket möjliggjort studien.