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Carrier dynamics in semiconductor quantum dots
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis presents results of time-resolved photoluminescence experiments conducted on several different self-assembled InGaAs/GaAs and InAs/GaAs semiconductor quantum dot (QD) structures. Depending on the application in mind, different structural, electronic or optical properties have a different weight of importance.

Fast carrier capture and relaxation is critical for QD based lasers, for example. In this thesis, the influence of surplus carriers, introduced through modulation-doping, is studied. It is shown that carrier capture is essentially unaffected whereas the intradot relaxation mechanisms, at least at low carrier concentrations, are fundamentally different. The phonon mediated cascade relaxation found in the undoped reference sample is replaced by efficient scattering with the built-in carriers in the case of the doped structures.

Moreover, spin relaxation also depends on presence of extra carriers. During energy relaxation via carrier-carrier scattering, the spin polarization is preserved whereas in the undoped sample the strong interaction of relaxing carriers with LO phonons causes spin relaxation. The decay of the ground state spin polarization proceeds at the same rate for doped and undoped structures and is shown to be caused by acoustic phonons, even up to 300 K. While optimizing QD growth for specific applications, it is imperative to evaluate the influence of nonradiative recombination, which is most often detrimental.

While misfit dislocations, deliberately introduced in the substrate, lead to the formation of laterally ordered, uniform dots, these samples are found to suffer from strong nonradiative recombination. Structures with different barrier thicknesses and numerical simulations indicate defects in the vicinity of the QDs as main origin of fast carrier trapping.

On the other hand, it is shown that direct dot doping, compared to barrier doping or undoped structures, causes only minor degradation of the optical properties. Directly doped dots even exhibit a significantly weaker photoluminescence quenching with temperature, making them prospective for devices operating at room temperature.

Finally, the superior proton radiation hardness of QD structures compared to quantum wells is demonstrated, which is due to the three-dimensional confinement. The increase of photoluminescence intensity at low to moderate doses is interpreted as an enhanced carrier transfer into the dots via the defects introduced into the material by the protons.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , xiii, 99 p.
Series
Trita-ICT/MAP, 2006:1
Keyword [en]
quantum dots, photoluminescence, time-resolved, spectroscopy, semiconductor
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-4019OAI: oai:DiVA.org:kth-4019DiVA: diva2:10412
Public defence
2006-06-13, C1, Electrum, Isafjordsgatan 22, Kista, 10:15
Opponent
Supervisors
Note
QC 20100920Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-20Bibliographically approved
List of papers
1. Carrier dynamics in modulation-doped InAs/GaAs quantum dots
Open this publication in new window or tab >>Carrier dynamics in modulation-doped InAs/GaAs quantum dots
2005 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 8, 085316- p.Article in journal (Refereed) Published
Abstract [en]

Photoexcited carrier dynamics was studied in n and p modulation-doped self-assembled InAs/GaAs quantum dots by means of time-resolved photoluminescence with excitation and detection energies varied through barrier, wetting layer, and quantum dot states. Carrier transfer to the ground state of the dots was found to occur within 5 to 6 and 12 ps for the doped and undoped samples, respectively. The experiments suggest that in all samples the carrier capture into the highest quantum dot levels proceeds by phonon emission. The significant difference in the transfer times is attributed to different relaxation mechanisms for the subsequent process of intradot carrier relaxation. For the doped samples, the presence of built-in carriers in the dots leads to efficient electron-hole scattering, while in the undoped structure scattering by phonons is identified as the main relaxation channel. Additionally, experimental results show decreased carrier lifetimes in the doped structures, which is attributed to nonradiative recombination at doping-induced recombination centers in the vicinity of the quantum dot layers.

Keyword
ELECTRON-CAPTURE, EXCITED-STATES, RELAXATION; INAS, PHONON, PHOTOLUMINESCENCE, SPECTROSCOPY, TIME, TRANSITIONS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-5878 (URN)10.1103/PhysRevB.72.085316 (DOI)000231564600096 ()2-s2.0-33644956550 (Scopus ID)
Note
QC 20100901Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-01Bibliographically approved
2. Carrier spin dynamics in modulation-doped InAs/GaAs quantum dots
Open this publication in new window or tab >>Carrier spin dynamics in modulation-doped InAs/GaAs quantum dots
2006 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 100, no 5, 054310- p.Article in journal (Refereed) Published
Abstract [en]

Photoexcited electron and hole spin relaxation was studied in modulation-doped and undoped InAs/GaAs quantum dots by means of time-resolved photoluminescence. After excitation into the barriers or the wetting layer, the electron spin polarization is preserved during the capture and relaxation in the dots, especially in the p-doped structures, and decays with a characteristic time of about 100 ps. Spin state admixture in combination with electron interaction with acoustic phonons is suggested as the spin relaxation mechanism. Rapid spin polarization decay during carrier relaxation in undoped quantum dots is attributed to electron-optical phonon interaction. For carrier excitation directly into the dots, no significant spin polarization was observed, which points to the mixed nature of hole levels in quantum dots. The hole spin polarization randomizes on a much shorter time scale and is not detected in the experiment.

Keyword
Electrons, Polarization, Relaxation processes, Semiconducting gallium arsenide, Semiconducting indium compounds, Wetting
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5879 (URN)10.1063/1.2337776 (DOI)000240602500103 ()2-s2.0-33748846722 (Scopus ID)
Note
QC 20100902. Uppdaterad från In press till Published (20100902)Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-02Bibliographically approved
3. Changes in luminescence intensities and carrier dynamics induced by proton irradiation in In_xGa_1-xAs/GaAs quantum dots
Open this publication in new window or tab >>Changes in luminescence intensities and carrier dynamics induced by proton irradiation in In_xGa_1-xAs/GaAs quantum dots
Show others...
2002 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 23, 235314- p.Article in journal (Refereed) Published
Abstract [en]

The effects of proton irradiation (1.5 MeV) on photoluminescence intensities and carrier dynamics were compared between InGaAs/GaAs quantum dots and similar quantum well structures. A significant enhancement in radiation tolerance is seen with three-dimensional quantum confinement. Measurements were carried out in different quantum dot structures varying in dot surface density (4x10(8)-3x10(10) cm(-2)) and substrate orientation [(100) and (311)B]. Similar trends were observed for all quantum dot samples. A slight increase in photoluminescence emission intensity after low to intermediate proton doses is observed in InGaAs/GaAs (100) quantum dot structures. The latter is explained in terms of more efficient carrier transfer from the wetting layer via radiation-induced defects.

Keyword
enhanced radiation hardness, energy-relaxation, deep levels, damage, lasers, temperature, dependence, wells, gaas, degradation
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-5880 (URN)10.1103/PhysRevB.66.235314 (DOI)000180279400081 ()
Note
QC 20100920Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-20Bibliographically approved
4. Recombination properties of Si-doped InGaAs/GaAs quantum dots
Open this publication in new window or tab >>Recombination properties of Si-doped InGaAs/GaAs quantum dots
2006 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 17, no 21, 5373-5377 p.Article in journal (Refereed) Published
Abstract [en]

The recombination properties of directly doped InGaAs/GaAs quantum dots (QDs) for application in quantum dot infrared photodetectors (QDIPs) have been investigated by time-resolved photoluminescence. Compared with undoped and barrier-doped samples, the overall effect of direct dot doping is found to be small, resulting in only slight deterioration of dot homogeneity. Low-temperature photoluminescence decay times decrease very little, indicating that direct doping does not cause a significant increase of nonradiative recombination. In addition, directly doped quantum dots show a significantly weaker quenching of the photoluminescence intensity with temperature. At the same time, barrier doping causes the formation of more and smaller dots, which results in high photoluminescence intensity at low temperatures but an early onset of thermal carrier emission from the dots. The results suggest that direct QD doping is more prospective for realizing room-temperature operation in QDIPs.

Keyword
infrared photodetectors, room-temperature, inas, gaas, photoluminescence, responsivity, growth, lasers
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-5881 (URN)10.1088/0957-4484/17/21/015 (DOI)000241856200015 ()2-s2.0-33846117095 (Scopus ID)
Note
Uppdaterad från submitted till published: 20100920 QC 20100920Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-20Bibliographically approved
5. Dislocation-induced spatial ordering of InAs quantum dots:  Effects on optical properties
Open this publication in new window or tab >>Dislocation-induced spatial ordering of InAs quantum dots:  Effects on optical properties
Show others...
2002 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 91, no 9, 5826-5830 p.Article in journal (Refereed) Published
Abstract [en]

Misfit dislocations were used to modify the surface morphology and to attain spatial ordering of quantum dots (QDs) by molecular beam epitaxy. Effects of anneal time and temperature on strain-relaxed InxGa1-xAs/GaAs layers and subsequent spatial ordering of InAs QDs were investigated. Photoluminescence (PL) and time-resolved PL was used to study the effects of increased QD positional ordering, increased QD uniformity, and their proximity to dislocation arrays on their optical properties. Narrower inhomogeneous PL broadening from the QDs ordered on dislocation arrays were observed, and differences in PL dynamics were found.

Keyword
MISFIT DISLOCATIONS, ISLANDS, SUPERLATTICES, TRANSITIONS, LASERS
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-5882 (URN)10.1063/1.1467963 (DOI)000175069000044 ()
Note
QC 20100908Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-08Bibliographically approved
6. Carrier recombination in aligned InAs/GaAs quantum dots grown in strain-relaxed InGaAs layers
Open this publication in new window or tab >>Carrier recombination in aligned InAs/GaAs quantum dots grown in strain-relaxed InGaAs layers
Show others...
2003 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 0, no 4, 1213- p.Article in journal (Refereed) Published
Abstract [en]

Alignment of InAs quantum dots was achieved by introducing misfit dislocations in a metastable InGaAs layer and subsequent annealing. Photoexcited carrier dynamics were studied using time-resolved photoluminescence. Comparison with control “non-aligned” InAs quantum dots show remarkable differences in integrated photoluminescence intensities with excitation power and photoluminescence decay time dependences on excitation intensities. Low-temperature carrier lifetimes were found to be below one hundred picoseconds for the aligned quantum dots and to be determined by non-radiative recombination processes. Samples with different barrier thicknesses between the InGaAs and the quantum dot layer allow the discussion of possible influences of carrier traps at InGaAs/GaAs interface and in the dot layer.

Keyword
dislocations, heterostructures, temperature, wells, traps
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-5883 (URN)10.1002/pssc.200303046 (DOI)000186108000031 ()
Note
QC 20100920Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-20Bibliographically approved
7. Photoexcited carrier dynamics in aligned InAs/GaAs quantum dots grown on strain-relaxed InGaAs layers
Open this publication in new window or tab >>Photoexcited carrier dynamics in aligned InAs/GaAs quantum dots grown on strain-relaxed InGaAs layers
Show others...
2003 (English)In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 18, no 4, 541- p.Article in journal (Refereed) Published
Abstract [en]

Carrier dynamics in aligned InAs/GaAs quantum dots (QDs) grown on cross-batched patterns induced by metastable InxGa1-xAs layers have been studied by time-resolved photoluminescence. The low-temperature carrier lifetimes were found to be of the order of 100-200 ps and determined by carrier trapping and nonradiative recombination. Comparisons with control "nonaligned" InAs QDs show remarkable differences in dependence of peak PL intensities on excitation power, and in PL decay times dependences on both temperature and excitation intensities. Possible origin of traps, which determine the carrier lifetimes, is discussed.

Keyword
quantum dots, dislocations, lateral alignment, time-resolved photoluminescence
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-5884 (URN)10.1016/S1386-9477(03)00232-7 (DOI)000183846300020 ()
Note
QC 20100920Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-09-20Bibliographically approved

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