| 1. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Activation of defects in GaNP by post-growth hydrogen treatment
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Effect of post-growth hydrogen treatment on defects and their role in carrier recombination in molecular beam epitaxial GaNP alloys is examined by means of photoluminescence and optically detected magnetic resonance. We present direct experimental evidence for effective activation of several different defects in carrier recombination by the hydrogen treatment. Among them, two defect complexes are identified to contain a Ga interstitial (Gai). None of the activated Gai complexes was previously observed in GaNP. Possible mechanisms for the hydrogen-induced defect activation are discussed.
|
|
| 2. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Carrier and spin injection from ZnMnSe to CdSe quantum dots
- 2008
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Optical carrier/exction and spin injection processes from a ZnMnSe dilute magnetic semiconductor (DMS) to CdSe quantum dots (QD’s) are studied in detail by means of spinpolarized magneto- photoluminescence (PL) and PL excitation spectroscopies. Efficiency of carrier/exciton transfer is found to be practically independent of width (Lb) of a ZnSe barrier layer inserted between the DMS and QD’s. This is tentatively explained in terms of photonexchange energy transfer. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with large Lb, pointing towards increasing spin loss.
|
|
| 3. |
|
|
| 4. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Effect of growth conditions on grown-in defect formation and luminescence efficiency in GaInNP epilayers grown by molecular-beam epitaxy.
- 2008
-
Ingår i: Physica status solidi (c)Special Issue: E-MRS 2007 Spring Meeting – Symposium F and Conference on Photonic Materials. - Weinheim : Wiley-VCH Verlagsgesellschaft. ; , s. 460-463
-
Konferensbidrag (refereegranskat)abstract
- A detailed study of the impact of different growth conditions (i.e. ion bombardment, nitrogen flow and In content) on the defect formation in Ga(In)NP epilayers grown on GaP substrates by solid-source molecular beam epitaxy is performed. Reduced nitrogen ion bombardment during the growth is shown to significantly reduce formation of defects acting as competing recombination centers, such as a Ga interstitial defect and other unidentified defects revealed by optically detected magnetic resonance. Further, high nitrogen flow is found to be even more effective than the ion bombardment in introducing the defects. The incorporation of In by 5.1% is, on the other hand, found not to affect the introduction of defects. The results provide a useful insight into the formation mechanism of the defects that will hopefully shed light on a control of the defect introduction in the alloys by optimizing growth conditions.
|
|
| 5. |
|
|
| 6. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Effect of nitrogen ion bombardment on defect formation and luminescence efficiency of GaNP epilayers grown by molecular-beam epitaxy
- 2006
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:10, s. 101904-
-
Tidskriftsartikel (refereegranskat)abstract
- Radiative efficiency of GaNP epilayers grown on GaP substrates by solid-source molecular beam epitaxy is significantly improved by reduced nitrogen ion bombardment during the growth. Based on the results of temperature-dependent photoluminescence (PL) and optically detected magnetic resonance studies (ODMR), the observed improvements are attributed to reduced formation of defects, such as a Ga interstitial related defect and an unidentified defect revealed by ODMR. We demonstrate that these defects act as competing recombination centers, which promote thermal quenching of the PL intensity and result in a substantial (34×) decrease in room-temperature PL intensity. © 2006 American Institute of Physics.
|
|
| 7. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Efficiency of optical spin injection and spin loss from a diluted magnetic semiconductor ZnMnSe to CdSe nonmagnetic quantum dots
- 2008
-
Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 77:3, s. 035437-
-
Tidskriftsartikel (refereegranskat)abstract
- Magneto-optical spectroscopy in combination with tunable laser spectroscopy is employed to study optical spin injection from a diluted magnetic semiconductor (DMS) ZnMnSe into nonmagnetic CdSe quantum dots (QDs). Observation of a DMS feature in the excitation spectra of the QD photoluminescence polarization provides clear evidence for optical spin-injection from the DMS to the QDs. By means of a rate equation analysis, the injected spin polarization is deduced to be about 32% at 5 T, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process including crossing the heterointerfaces and energy relaxation within the QDs. © 2008 The American Physical Society.
|
|
| 8. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Evidence for a phosphorus-related interfacial defect complex at a GaP/GaNP heterojunction
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : The American Physical Society. - 1098-0121 .- 1550-235X. ; 81, s. 115334-
-
Tidskriftsartikel (refereegranskat)abstract
- Optically detected magnetic resonance (ODMR) studies of molecular beam epitaxial GaNP/GaP structures reveal presence of a P-related complex defect, evident from its resolved hyperfine interaction between an unpaired electronic spin (S=1/2) and a nuclear spin (I = ½) of a 31P atom. The principal axis of the defect is concluded to be along a <111> crystallographic direction from angular dependence of the ODMR spectrum, restricting the P atom (either a PGa antisite or a Pi interstitial) and its partner in the complex defect to be oriented along this direction. The principal values of the electronic g-tensor and hyperfine interaction tensor are determined as: g┴=2.013, g║=2.002, and A┴=130´10-4 cm-1, A║=330´10-4 cm-1, respectively. The interface nature of the defect is clearly manifested by the absence of the ODMR lines originating from two out of four equivalent <111> orientations. Defect formation is shown to be facilitated by nitrogen ion bombardment under non-equilibrium growth conditions and the defect is thermally stable upon post-growth thermal annealing.
|
|
| 9. |
|
|
| 10. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Formation of grown-in defects in molecular beam epitaxial Ga(In)NP : effects of growth conditions and post-growth treatments
- 2008
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103, s. 063519-
-
Tidskriftsartikel (refereegranskat)abstract
- Effects of growth conditions and post-growth treatments, such as presence of N ions, N2 flow, growth temperature, In alloying, and postgrowth rapid thermal annealing (RTA), on formation of grown-in defects in Ga(In)NP prepared by molecular beam epitaxy are studied in detail by the optically detected magnetic resonance (ODMR) technique. Several common residual defects, such as two Ga-interstitial defects (i.e., Gai-A and Gai-B) and two unidentified defects with a g factor around 2 (denoted by S1 and S2), are closely monitored. Bombardment of impinging N ions on grown sample surface is found to facilitate formation of these defects. Higher N2 flow is shown to have an even more profound effect than a higher number of ions in introducing these defects. Incorporation of a small amount of In (e.g., 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In; however, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai-related defects formed during the growth. In the alloys where the Gai-A and Gai-B defects are absent in the as-grown samples (i.e., GaNP grown at a low temperature of 460 °C), the concentrations of the two Gai defects are found to increase after postgrowth RTA. This indicates that the defects originally introduced in the as-grown alloys have been transformed into the more thermally stable Gai-A and Gai-B during RTA. On the other hand, when the Gai-A and Gai-B are readily abundant (e.g., at higher growth temperatures (>=500 °C), RTA leads to a slight reduction of the Gai-A and Gai-B ODMR signals. The S2 defect is also shown to be thermally stable upon the RTA treatment.
|
|
| 11. |
|
|
| 12. |
- Dagnelund, Daniel, 1980-
(författare)
-
Magneto-optical studies of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work aims at a better understanding of magneto-optical properties of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures. The thesis is divided into two parts. The first part gives an introduction of the research fields, together with a brief summary of the scientific results included in the thesis. The second part consists of seven scientific articles that present the main findings of the thesis work. Below is a short summary of the thesis.Dilute nitrides have been of great scientific interest since their development in the early 1990s, because of their unusual fundamental physical properties as well as their potential for device applications. Incorporation of a small amount of N in conventional Ga(In)As or Ga(In)P semiconductors leads to dramatic modifications in both electronic and optical properties of the materials. This makes the dilute nitrides ideally suited for novel optoelectronic devices such as light emitting devices for fiber-optic communications, highly efficient visible light emitting devices, multi-junction solar cells, etc. In addition, diluted nitrides open a window for combining Si-based electronics with III-V compounds-based optoelectronics on Si wafers, promising for novel optoelectronic integrated circuits. Full exploration and optimization of this new material system in device applications requires a detailed understanding of their physical properties.Papers I and II report detailed studies of effects of post-growth rapid thermal annealing (RTA) and growth conditions (i.e. presence of N ions, N2 flow, growth temperature and In alloying) on the formation of grown-in defects in Ga(In)NP. High N2 flow and bombardment of impinging N ions on grown sample surface is found to facilitate formation of defects, such as Ga interstitial (Gai) related defects, revealed by optically detected magnetic resonance (ODMR). These defects act as competing carrier recombination centers, which efficiently decrease photoluminescence (PL) intensity. Incorporation of a small amount of In (e.g. 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In, on the other hand, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai related defects formed during the growth.In Paper III, the first identification of an interfacial defect at a heterojunction between two semiconductors (i.e. GaP/GaNP) is presented. The interface nature of the defect is clearly manifested by the observation of ODMR lines originating from only two out of four equivalent <111> orientations. Based on its resolved hyperfine interaction between an unpaired electronic spin (S=1/2) and a nuclear spin (I=1/2), the defect is concluded to involve a P atom at its core with a defect/impurity partner along a <111> direction. Defect formation is shown to be facilitated by N ion bombardment.In Paper IV, the effects of post-growth hydrogenation on the efficiency of the nonradiative (NR) recombination centers in GaNP are studied. Based on the ODMR results, incorporation of H is found to increase the efficiency of the NR recombination via defects such as Ga interstitials.In Paper V, we report on our results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well, by the optically detected cyclotron resonance (ODCR) technique. By monitoring PL emissions from various layers, the predominant ODCR peak is shown to be related to electrons in GaAs/AlAs superlattices. This demonstrates the role of the SL as an escape route for the carriers confined within the InGaNAs/GaAs single quantum well.The last two papers are within a relatively new field of spintronics which utilizes not only the charge (as in conventional electronics) but also the quantum mechanical property of spin of the electron. Spintronics offers a pathway towards integration of information storage, processing and communications into a single technology. Spintronics also promises advantages over conventional charge-based electronics since spin can be manipulated on a much shorter time scale and at lower cost of energy. Success of semiconductor-based spintronics relies on our ability to inject spin polarized electrons or holes into semiconductors, spin transport with minimum loss and reliable spin detection.In Papers VI and VII, we study the efficiency and mechanism for carrier/exciton and spin injection from a diluted magnetic semiconductor (DMS) ZnMnSe quantum well into nonmagnetic CdSe quantum dots (QD’s) by means of spin-polarized magneto PL combined with tunable laser spectroscopy. By means of a detailed rate equation analysis presented in Paper VI, the injected spin polarization is deduced to be about 32%, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process. In Paper VII, we present evidence that energy transfer is the dominant mechanism for carrier/exciton injection from the DMS to the QD’s. This is based on the fact that carrier/exciton injection efficiency is independent of the width of the ZnSe tunneling barrier inserted between the DMS and QD’s. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with wide barriers, pointing towards increasing spin loss.
|
|
| 13. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Optically detected cyclotron resonance studies of InGaNAs/GaAs structures
- 2007
-
Ingår i: Physics of semiconductors : 28th International Conference on the Physics of Semiconductors, ICPS 2006, Vienna, Austria, 24-28 July 2006. - : American Institute of Physics (AIP). - 9780735403970 ; , s. 383-384
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We report on our recent results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well (QW), by the optically detected cyclotron resonance (ODCR) technique. By monitoring photoluminescence (PL) emissions from various layers the predominant ODCR peak is shown to be related to carriers with a 2D character and an effective mass of 0.51 m∗e. The responsible carriers are ascribed to be electrons in GaAs/AlAs superlattices (SL) that are employed in the laser structures to prevent carrier leak by sandwiching the InGaNAs/GaAs QW. This conclusion is based on the following observations: (a) the ODCR peak is only observed in the structures containing the SL; (b) the effective mass value determined by the ODCR peak is independent of In and N compositions; (c) the same ODCR peak is also observed by monitoring PL from the SL. Unfortunately no ODCR signal related to InGaNAs was observed
|
|
| 14. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Optically detected cyclotron resonance studies of InxGa1-xNyAs1-y/GaAs quantum wells sandwiched between type-II AlAs/GaAs superlattices.
- 2007
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101, s. 073705-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on our results from a systematic study of layered structures containing an InGaNAs/GaAs single quantum well (SQW) enclosed between staggered type II AlAs/GaAs superlattices (SL), by the photoluminescence (PL) and optically detected cyclotron resonance (ODCR) techniques. Besides the ODCR signal known to originate from electrons in GaAs, the predominant ODCR peak is shown to be related to carriers with a two-dimensional character and a cyclotron resonance effective mass of m*[approximate](0.51-0.56)m0. The responsible carriers are ascribed to electrons on the ellipsoidal equienergy surface at the AlAs X point of the Brillouin zone within the SL, based on results from angular and spectral dependences of the ODCR signal. No ODCR signal related to the InGaNAs SQW was detected, presumably due to low carrier mobility despite the high optical quality. Multiple absorption of photons with energy below the band gap energy of the SL and the GaAs barriers was observed, which bears implication on the efficiency of light-emitting devices based on these structures.
|
|
| 15. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Spin resonance spectroscopy of grown-in defects in Ga(In)NP alloys
- 2008
-
Ingår i: 7th International Conference on Physics of Light-Matter Coupling in Nanostructures, 2007. - : Elsevier Ltd.. ; , s. 620-625
-
Konferensbidrag (refereegranskat)abstract
- We employ the optically detected magnetic resonance (ODMR) technique to study and identify important grown-in defects in Ga(In)NP grown by molecular-beam epitaxy (MBE). Several types of defects were revealed from ODMR studies. The dominant defects were found to be related to Ga interstitials, evident form their characteristic hyperfine interaction arising from the spin interaction between the electron and the Ga nucleus. Some other as yet unidentified intrinsic defects were also found to be commonly present in the alloys. The effects of growth conditions (ion bombardment, N2 gas flow, etc.) and post-growth rapid thermal annealing on the formation of these defects were studied in detail, shedding light on the formation mechanism of defects.
|
|
| 16. |
- Furuta, T., et al.
(författare)
-
Spin-injection dynamics and effects of spin relaxation in self-assembled quantum dots of CdSe
- 2008
-
Ingår i: Journal of the Korean Physical Society. - 0374-4884 .- 1976-8524. ; 53, s. 163-166
-
Tidskriftsartikel (refereegranskat)abstract
- We study the spin injection dynamics and the effects of spin relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ZnMnSe, where spin-polarized excitons can be injected from the DMS into the QDs because of the potential difference. The degree of circular polarization, $P$, of excitonic photoluminescence (PL) at 5 T in the coupled QDs shows a rapid increase with increasing delay time after a linearly polarized pulse excitation, indicating the spin-injection dynamics. The $P$ value tends to decay gradually because of the exciton-spin relaxation in the QDs after the spin injection. The spin-polarized excitons in the QD ensemble migrate simultaneously from QDs with higher exciton energies to those with lower exciton energies. This inter-dot transfer of excitons also affects the $P$ value in the lower energy region of the QD-emission band because the excitons lose their spin polarizations due to the spin relaxation in the dots during the migration. The detailed analysis for the exciton-spin transfer is presented in a coupled QD system after spin injection.
|
|
| 17. |
- Murayama, A., et al.
(författare)
-
Dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots of CdSe coupled with a diluted magnetic semiconductor layer of Zn0.80 Mn0.20 Se
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195308-
-
Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of Zn0.80 Mn0.20 Se, where spin-polarized excitons can be injected from the DMS into the QDs. The degree of circular polarization P of excitonic photoluminescence (PL) at 5 T in the coupled QDs exhibits a rapid increase with increasing delay time, up to +0.3 at 25 ps after the pulse excitation of the DMS by a linearly polarized light. This development of a positive P value directly reflects the spin-injection dynamics from the DMS, since the intrinsic polarization of the QD excitons due to Zeeman splitting is P∼-0.1 when only the QDs are selectively excited. The P value gradually decays with time after reaching its maximum, as a result of the exciton-spin relaxation with a time constant of 800 ps in the QDs. Time-resolved circularly polarized PL spectra immediately after the pulse excitation directly show the exciton-energy dependence of the spin-injection dynamics in the QD ensemble, where two-dimensional-like QDs with higher exciton energies show higher receptivity to the spin-polarized excitons than three-dimensional-like dots with lower exciton energies. A rate equation analysis reveals all time constants responsible for the spin-injection dynamics. We deduce a time constant of 10 ps for the spin injection. The spin-injection efficiency of 0.94 is also obtained, which corresponds to the ratio between the number of the spin-polarized excitons responsible for the rise of the positive P value in the QD emission and the total number of the excitons injected from the DMS. Moreover, we observe that interdot exciton transfer significantly affects the P value within the QD emission band after the fast spin injection, in addition to the spin relaxation within the QDs. © 2007 The American Physical Society.
|
|
| 18. |
- Park, J. H., et al.
(författare)
-
Transfer dynamics of spin-polarized excitons in ZnCdMnSe/ZnCdSe double quantum wells
- 2008
-
Ingår i: Journal of the Korean Physical Society. - 0374-4884 .- 1976-8524. ; 53, s. 167-170
-
Tidskriftsartikel (refereegranskat)abstract
- We study the transfer dynamics of spin-polarized excitons in diluted magnetic semiconductor (DMS) double quantum wells (QWs) of ZnCdMnSe/ZnCdSe, where the spin-polarized excitons tunnel from the non-magnetic ZnCdSe to DMS-ZnCdMnSe wells and subsequently relax to the ground state in the DMS well with phonon emissions. The exciton-transfer time decreases when the exciton-energy difference between both the wells is equal to the one-LO-phonon energy, indicating the LO-phonon-assisted acceleration of the spin transfer. Moreover, the exciton transfer alters into the separate transfer of the electron and the heavy hole when the electron potential difference becomes larger than the LO-phonon energy. This observation demonstrates the importance of energy-relaxation dynamics in the spatial transport of spins in semiconductor quantum structures.
|
|
| 19. |
- Park, Jiho, et al.
(författare)
-
Spin-Conserving Tunneling of Excitons in Diluted Magnetic Semiconductor Double Quantum Wells
- 2008
-
Ingår i: Japanese Journal of Applied Physics. - 0021-4922 .- 1347-4065. ; 47, s. 3533-3536
-
Tidskriftsartikel (refereegranskat)abstract
- Spin polarization in carrier tunneling was studied in double quantum wells by the polarized photoluminescence-excitation spectroscopy. The double quantum wells consist of a diluted magnetic quantum well of Zn0.77Cd0.15Mn0.08Se and a nonmagnetic quantum well of Zn0.82Cd0.18Se. Efficient spin-conserving tunneling of an exciton as an entity was observed from the nonmagnetic quantum well to the magnetic well. The spin-reversing tunneling was suppressed by two orders of magnitude in high magnetic field. The spin-conserving tunneling time was determined as 20 ps by time resolved photoluminescence measurement.
|
|
| 20. |
- Vorona, Igor, 1967-, et al.
(författare)
-
Identification of point defects in Ga(Al)NAs alloys
- 2007
-
Ingår i: AIP Conference Proceedings / Volume 893. - : American Institute of Physics (AIP). ; , s. 227-228
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- By employing the optically detected magnetic resonance (ODMR) technique, two differentGai defects, namely Gai-A and Gai-B, are found and identifiedin the investigated Ga(Al)NAs epilayers grown on GaAs substrates bymolecular-beam epitaxy (MBE). This finding shows that Ga interstitials arecommon intrinsic defects in various dilute nitrides. In addition tothe Gai-related defects, “middle line” ODMR signals were observed ataround g=2 and are suggested to arise from superposition ofa defect with a single ODMR line and a defectwith an unresolved HF structure. All defects studied are shownto act as non-radiative recombination centers, and are therefore harmfulto performance of potential light-emitting devices based on the alloys.©2007 American Institute of Physics
|
|
| 21. |
- Vorona, Igor, et al.
(författare)
-
Optically detected magnetic resonance studies of point defects in Ga(Al)Nas
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:12, s. 125204-
-
Tidskriftsartikel (refereegranskat)abstract
- An optically detected magnetic resonance (ODMR) study of Ga(Al)NAs alloys grown by molecular beam epitaxy on GaAs substrates is presented. A number of grown-in defects were observed which act as nonradiative recombination centers. A detailed analysis of experimental data using a spin Hamiltonian leads to the identification of two Gai defects. A comparison with similar defects in other phosphide-based diluted nitride III-V compounds, such as GaAlNP and GaInNP, allows us to obtain additional information about the nearest surrounding of the defects. A discussion of possible models for other defects observed in the experiments is also presented. © 2006 The American Physical Society.
|
|
