Optical Pumping: a Possible Approach Towards a Sige Quantum Cascade Laser
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Institut de Physique de l’ Universit´ede Neuchˆatel Optical Pumping: A Possible Approach towards a SiGe Quantum Cascade Laser E3 40 30 E2 E1 20 10 Lasing Signal (meV) 0 210 215 220 Energy (meV) THESE pr´esent´ee`ala Facult´edes Sciences de l’Universit´ede Neuchˆatel pour obtenir le grade de docteur `essciences par Maxi Scheinert Soutenue le 8 octobre 2007 En pr´esence du directeur de th`ese Prof. J´erˆome Faist et des rapporteurs Prof. Detlev Gr¨utzmacher , Prof. Peter Hamm, Prof. Philipp Aebi, Dr. Hans Sigg and Dr. Soichiro Tsujino Keywords • Semiconductor heterostructures • Intersubband Transitions • Quantum cascade laser • Si - SiGe • Optical pumping Mots-Cl´es • H´et´erostructures semiconductrices • Transitions intersousbande • Laser `acascade quantique • Si - SiGe • Pompage optique i Abstract Since the first Quantum Cascade Laser (QCL) was realized in 1994 in the AlInAs/InGaAs material system, it has attracted a wide interest as infrared light source. Main applications can be found in spectroscopy for gas-sensing, in the data transmission and telecommuni- cation as free space optical data link as well as for infrared monitoring. This type of light source differs in fundamental ways from semiconductor diode laser, because the radiative transition is based on intersubband transitions which take place between confined states in quantum wells. As the lasing transition is independent from the nature of the band gap, it opens the possibility to a tuneable, infrared light source based on silicon and silicon compatible materials such as germanium. As silicon is the material of choice for electronic components, a SiGe based QCL would allow to extend the functionality of silicon into optoelectronics. The aim of this thesis is to provide possible opportunities to achieve lasing in silicon by using intersubband transitions. Electroluminescence from SiGe quantum cascade structures was demonstrated in 2002. The present thesis summarizes complementary research regarding electrically pumped SiGe QC structures. On the basis of these results, the problems arising from the electri- cal pumping at high current densities in SiGe QCs are discussed. The main problem can be found in the free carrier absorption in the strongly doped region providing the elec- trical contacts. Furthermore, the accurate growth of strained SiGe layers is challenging with respect to the alignment of levels. The latter one is of special importance for the proper injection of carriers. To overcome those problems, another approach was chosen: the optical pumping. Here, carriers are excited to the upper laser state by using an ex- ternal laser source. This provides several advantages: (i) simple three level structures are sufficient which are either realized by double quantum wells or simple step QWs (ii) no need of doped contact layers and a lower doping in the active region which reduces free carrier losses; (iii) the growth conditions are less stringent as the injection into the upper laser state is provided by a tunable external source. The realization of this approach is presented in the main part of this thesis. The optical pumping set-up was tested using AlInAs/GaInAs double quantum well structures. From those samples, Raman lasing was obtained which is attributed to a lack of population inversion. Theoretical analysis and femto-second time-resolved measurements were em- ployed to clarify the origin of this behavior. For approaching optical pumping in the SiGe material system, three level systems ob- tained by a double quantum well and a step quantum well were investigated. Theoretical considerations and absorption measurements were performed in order to characterize the iii designs. The step QW was concluded to be the most suitable design due to its design freedom and the few growth interfaces. Lasing, however, could not be achieved what is probably due to unsatisfactory material quality. Promising results on the latest sample batch are summarized in an appendix. iv R´esum´e Le laser `acascade quantique a attir´eun large int´erˆeten tant que source infrarouge depuis sa premi`erer´ealisation en 1994 en utilisant l’AlInAs/InGaAs. Ses applications principales sont dans la spectroscopie pour la d´etection des gaz, ainsi que pour les t´el´ecommunications `atravers l’atmosph`ere. Ce type de source optique diff`erede mani`erefondamentale d’une diode laser semi-conducteur conventionnelle, car la transition radiative est bas´eesur des transitions inter-sous-bandes qui ont lieu entre des ´etatsconfin´esdans des puits quantiques. Comme la transition laser est maintenant ind´ependante de la nature de la bande interdite (directe ou indirecte), ce concept ouvre la voie `al’utilisation du silicium, ou d’autre mat´eriauxcomme le SiGe, compatibles avec le silicium, pour obtenir une source infrarouge accordable. Comme le Silicium est le mat´eriau privil´egi´epour les dispositifs ´electroniques, un laser `acascade quantique bas´esur le SiGe permettrait de rajouter la fonctionnalit´eopto´electronique au silicium. Le but de cette th`eseest d’essayer d’utiliser les transitions inter-sous-bandes pour r´ealiser un laser bas´esur la technologie du silicium. L’´electroluminescence de structures `acascade quantique bas´eessur le SiGe a ´et´emontr´ee en 2002. Cette th`esepr´esente une ´etude compl´ementaire concernant les structures `a cascade quantiques pomp´ees´electriquement. Sur la base de ces r´esultats, les probl`emes soulev´espar l’injection des forts courants n´ecessaires l’injection ´electrique dans les struc- tures `acascades bas´eessur le SiGe sont discut´es. Un des probl`emesprincipaux est l’absorption par les porteurs libres dans les r´egions fortement dop´eesutilis´eespour l’injection du courant dans la structure. Une difficult´esuppl´ementaire est la croissance pr´ecise des couches contraintes de SiGe qui puissent maintenir un bon alignement des niveaux ´electroniques, indispensable pour une injection efficaces dans les ´etats d´esir´es. Pour r´esoudreces probl`emes,une autre approche a ´et´eutilis´ee: le pompage optique, dans lesquel les ´electrons sont amen´essur l’´etat excit´eapr`esl’absorption d’un photon. Cette approche a plusieurs avantages. Premi`erement une structure simple `atrois niveaux est suffisante, qui peut ˆetrer´ealis´ee`al’aide d’un puit coupl´eou d’un puit avec une marche de potentiel. Deuxi`emement les couches qui oeuvrent de contacts ´electriques peuvent ˆetre ´evit´ees, ainsi que les pertes optiques induites. Finalement les exigences sur la pr´ecision de la croissance sont un peu moindres vu que l’injection peut ˆetre ”accord´ee” par un changement de la longueur d’onde de pompage. La r´ealisation de cette approche a repr´esent´ela partie principale de cette th`ese. L’exp´erience de pompage optique a ´et´etest´een utilisant un puit coupl´eAlInAs/GaInAs. Les r´esultats obtenus l’aide de ces ´echantillons ont montr´eque le m´ecanisme de gain obtenu provenait v d’un effet Raman ´electronique plutˆotque d’une inversion de population. Une analyse th´eorique ainsi que des mesures en temps r´esoluont ´et´eutilis´eespour bien identifier le m´ecanismed’op´erations du laser. Pour r´ealiser la mˆemeapproche dans les mat´eriaux`abase de SiGe, des syst`emes`atrois niveaux obtenus dans des puits coupl´eset dans un puit `amarche de potentiel ont ´et´e ´etudi´es. Des consid´erations th´eoriques, ainsi que des mesures d’absorption on ´et´eutilis´ees pour caract´eriser diff´erents options conceptuelles. Le puit `amarche de potentiel s’est r´ev´el´eˆetre l’approche la plus fructueuse `acause de la libert´ede concept qu’il offrait ainsi que la pr´esenced’un nombre moindre d’interfaces. Malheureusement, aucun effet laser n’a ´et´eobserv´e.Des r´esultats en absorption prometteurs sont r´esum´esdans un appendice. vi Contents Abstract iii R´esum´e v 1 Introduction 1 1.1 Motivation . 1 1.2 Scope and organization of this thesis . 5 2 Intersubband Transitions and Quantum Cascade Lasers 6 2.1 Intersubband versus Interband Transitions . 6 2.1.1 Intersubband Transitions in SiGe . 7 2.2 The principle of Quantum Cascade Lasers . 9 2.2.1 Applications for Quantum Cascade Lasers . 10 3 Theory 13 3.1 Si and Ge Crystal . 13 3.2 Valence band Structure . 15 3.2.1 k · p model for a single band . 16 3.2.2 6-band k · p model . 17 3.2.3 Strain Effects . 19 3.2.4 Band Offsets . 21 3.2.5 Valence Band Structure in a QW . 23 3.3 Optical matrix elements and selection rules . 25 3.4 Conclusion for SiGe Quantum Well Structures . 27 4 MBE growth and Characterization 28 4.1 MBE growth . 28 4.2 Characterization . 30 5 Measurement Setups 33 5.1 Absorption Setup . 33 5.2 Optical Pumping Setup . 35 vii CONTENTS 6 Electrically pumped SiGe quantum cascade structures 37 6.1 Active Layer Design based on strain compensated bound-to-continuum transitions . 37 6.2 Intersubband Broadening . 40 6.3 Optical Loss in Waveguide Structures . 42 6.3.1 Waveguide Design . 42 6.3.2 Contact Design . 45 6.4 SiGe Resonant Tunnelling Structures . 47 6.4.1 Basic principle . 47 6.4.2 Transport measurements . 50 6.4.3 Magneto-Tunnelling . 54 6.4.4 Barrier Thickness Dependence . 58 6.4.5 Injection Barrier Thickness . 60 6.5 Conclusion on electrically pumped structures . 61 7 III-V optically pumped Intersubband Lasers 63 7.1 Introduction . 63 7.2 Theoretical models for a three-level system . 65 7.2.1 Population Inversion Gain . 66 7.2.2 The Raman Process . 67 7.3 Experiment . 71 7.3.1 Sample Characterization . 71 7.3.2 Laser Characteristics . 74 7.3.3 Time resolved pump-probe spectroscopy . 78 8 SiGe Structures for Optical Pumping 82 8.1 Theoretical Framework . 82 8.1.1 Double Quantum Well .