Tài liệu Electron và phonon trong dây lượng tử có cấu trúc lõi – vỏ

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TH¤NG TIN VÒ NH÷NG §ãNG GãP MíI CñA LUËN ¸N Tên luận án: Chuyên ngành: Electron và phonon trong dây lượng tử có cấu trúc lõi – vỏ Vật lý lý thuyết và vật lý toán Nghiên cứu sinh: Người hướng dẫn khoa học: Mã số: 62 44 01 03 Lê Thanh Hải PGS. TS. Nguyễn Như ðạt Cơ sở ñào tạo: Viện Vật lý, Viện Hàn lâm Khoa học và Công nghệ Việt Nam LuËn ¸n nghiªn cøu lý thuyÕt ¶nh h−ëng cña líp vá (®é dµy, sù chªnh lÖch h»ng sè ®iÖn m«i cña vËt liÖu vá vµ lâi), vµ cña líp bäc kim lo¹i ®Õn tr¹ng th¸i cña electron, hµm ®iÖn m«i ®iÖn tö, t¸n s¾c cña plasmon vµ phonon, t−¬ng t¸c electron - LO phonon, vµ tèc ®é t¸n x¹ cña electron bëi LO phonon trong d©y l−îng tö b¸n dÉn cã cÊu tróc lâi - vá . C¸c kÕt qu¶ míi luËn ¸n ®· ®¹t ®−îc: 1. NhËn ®−îc hµm ®iÖn m«i ®éng cho hÖ electron trong d©y l−îng tö b¸n dÉn kiÓu lâi-vá trong khu«n khæ lý thuyÕt ph¶n øng tuyÕn tÝnh vµ gÇn ®óng pha ngÉu nhiªn. TÇn sè plasmon ®−îc x¸c ®Þnh nh− lµ nghiÖm cña hµm ®iÖn m«i. §· chØ ra r»ng tr¹ng th¸i cña electron, hµm ®iÖn m«i ®iÖn tö vµ tÇn sè plasmon phô thuéc vµo b¸n kÝnh d©y, ®é dµy líp hµng rµo, sù chªnh lÖch h»ng sè ®iÖn m«i cña vËt liÖu lâi vµ vËt liÖu vá, nång ®é electron vµ nhiÖt ®é cña hÖ. 2. ThiÕt lËp ph−¬ng ph¸p x©y dùng c¸c mode phonon quang däc trong c¸ch tiÕp cËn m«i tr−êng ®iÖn m«i liªn tôc vµ thu nhËn ®−îc ph−¬ng tr×nh t¸n s¾c cho c¸c kiÓu phonon kh¸c nhau trong d©y l−îng tö b¸n dÉn kiÓu lâi – vá kh«ng cã hoÆc cã líp bäc kim lo¹i máng bªn ngoµi. Trong d©y l−îng tö cã thÓ tån t¹i 4 kiÓu phonon tïy thuéc c¸c th«ng sè cña vËt liÖu chÕ t¹o lâi vµ vá cña d©y. 3. TÝnh tèc ®é t¸n x¹ cña electron bëi LO phonon trªn c¬ së Hamiltonian t−¬ng t¸c electron - LO phonon thu nhËn ®−îc. 4. KÕt qu¶ tÝnh sè cho d©y GaAs/AlxGa1-xAs cho thÊy: • ¶nh h−ëng cña kÝch th−íc d©y, ®é dµy líp vá, sù chªnh lÖch h»ng sè ®iÖn m«i gi÷a miÒn hµng rµo vµ miÒn lâi ®èi víi hµm ®iÖn m«i tÜnh cña d©y thÓ hiÖn râ 1 trong miÒn vÐc t¬ sãng nhá. §èi víi d©y cã líp vá dµy h÷u h¹n, hµm ®iÖn m«i t¨ng khi b¸n kÝnh lâi vµ/hoÆc ®é dµy líp vá gi¶m. Trong miÒn vÐc t¬ sãng lín, hµm ®iÖn m«i tÜnh cña d©y gÇn nh− kh«ng thay ®æi khi thay ®æi c¸c yÕu tè kÓ trªn. • Sù chªnh lÖch h»ng sè ®iÖn m«i gi÷a vËt liÖu lâi vµ vËt liÖu vá cµng lín th× tÇn sè plasmon thay ®æi cµng nhiÒu. TÇn sè plasmon t¨ng khi kÝch th−íc lâi gi¶m. §é thay ®æi tÇn sè plasmon cã thÓ ®¹t ®Õn cì vµi meV, hoµn toµn cã thÓ quan s¸t ®−îc b»ng thùc nghiÖm. • §é dµy cña líp vá thay ®æi còng nh− sù cã mÆt cña líp kim lo¹i bäc bªn ngoµi lµm tÇn sè cña phonon thay ®æi, ®Æc biÖt ®èi víi phonon kiÓu 1, nh−ng d¸ng ®iÖu cña ®−êng cong t¸n s¾c kh«ng bÞ ¶nh h−ëng nhiÒu. Tuy nhiªn, thÕ tÜnh ®iÖn g©y bëi dao ®éng quang däc thay ®æi râ rÖt trong miÒn líp vá, nhÊt lµ ®èi víi phonon kiÓu 2. §iÒu ®ã dÉn ®Õn t−¬ng t¸c electron - LO phonon thay ®æi. §é dµy líp vá t¨ng th× tèc ®é t¸n x¹ cña electron gi¶m. MÆt kh¸c, líp bäc kim lo¹i bªn ngoµi lµm thay ®æi nhiÒu sù t¸n x¹ cña electron trong d©y cã ®é dµy líp vá cµng nhá. Trong tr−êng hîp nµy, tèc ®é t¸n x¹ cña electron cã thÓ gi¶m ®Õn 40% so víi d©y kh«ng cã líp bäc kim lo¹i. §èi víi d©y cã líp vá dµy v« h¹n, tèc ®é t¸n x¹ cña electron gi¶m khi b¸n kÝnh lâi d©y gi¶m. V× vËy, ®Ó gi¶m tèc ®é t¸n x¹ cña electron, cÇn cã sù lùa chän thÝch hîp kÝch th−íc d©y vµ líp bäc kim lo¹i bªn ngoµi. 2 SUMMARY OF NEW RESULTS IN THE DOCTORAL DISSERTATION Title: “Electron and phonon in core – shell semiconductor quantum wires” Speciality: Theoretical and Mathematical Physics Ph.D.Student: Le Thanh Hai Period of Study: Supervisor: Institution: Code: 62 44 01 03 2009 – 2014 Assoc. Prof. Dr. Nguyen Nhu Dat Institute of Physics, Vietnam Academy of Science and Technology. This thesis presents theoretical studies of the effects of the shell (the thickness, the difference of the dielectric constants between the shell and the core) and the thin outer metallic layer on electron states, the electronic dielectric function, the plasmon and phonon dispersions, the electron - LO phonon interaction, and the LO-phonon assisted electron scattering rate in core - shell semiconductor quantum wires. Main results of the thesis 1. To derive the dynamic dielectric function of the electron system in a core - shell semiconductor quantum wire within the framework of the linear response theory and the random phase approximation (RPA). The plasma oscillation frequencies are determined as its zeros. It is shown that the electron states, the electronic dielectric function and the plasmon frequencies depend on the wire radius, the shell thickness, the dielectric constants of the core and the shell materials, the electron concentration and the temperature of the system. 2. To establish a method to construct longitudinal optical phonon modes following the dielectric continuum approach and obtain the dispersion equations for various phonon types in core - shell semiconductor quantum wires without or with a thin outer metal layer. There may exist four types of LO phonons depending on the material constants of the core and the shell. 1 3. To calculate the electron scattering rates by LO phonons based on the obtained the electron - LO phonon interaction Hamiltonian. 4. Numerical calculations for GaAs / AlxGa1-xAs quantum wires show that • The influence of wire dimension, shell thickness, and the difference in dielectric constants between the core and the barrier on the static dielectric function of the wire is more evident for small wave vectors. For wires with finite shell thickness, the dielectric function increases as the radius of the core and/or the shell thickness decreases. In the domain of large wave vectors, the static dielectric function remains almost unchanged with the change of the above mentioned factors. • The greater difference between the dielectric constants of the core and the shell materials, the larger the change in the plasmon frequencies is. The plasmon frequencies increase when the core size decreases. It is found that the change of the plasmon frequencies can be of a few meV, quite observable experimentally. • Changing the shell thickness as well as adding a thin outer metal layer leads to the change of phonon frequencies, especially for type-1-phonons, but the behavior of the dispersion curves is not much affected. The electrostatic potential associated with LO oscillations, however, changes evidently in the shell, especially for type-2phonons. It results in the change of electron-LO-phonon interaction. The electron scattering rate decreases with increasing shell thickness. On the other hand, the metallic outer layer could much modify electron scattering in the wires of smaller shell thickness. In this case, scattering rates could be reduced up to 40% in comparison with that of the wires without the metal layer. For the wires with infinite shell thickness, the electron scattering rate decreases as the radius of the core decreases. Therefore, for a possible reduction of the scattering rates we could make a suitable choice of the quantum wire dimensions and an alternative outer metal layer. 2
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