Interface Phonons and Polaron Effect in Quantum Wires
© The Author(s) 2010
Received: 22 June 2010
Accepted: 13 July 2010
Published: 11 August 2010
The theory of large radius polaron in the quantum wire is developed. The interaction of charge particles with interface optical phonons as well as with optical phonons localized in the quantum wire is taken into account. The interface phonon contribution is shown to be dominant for narrow quantum wires. The wave functions and polaron binding energy are found. It is determined that polaron binding energy depends on the electron mass inside the wire and on the polarization properties of the barrier material.
KeywordsQuantum wire Electron–phonon interaction Interface phonons Polaron
The electron–phonon interaction in semiconductor heterostructures is of greater interest in comparison to bulk materials. This is due to the fact that the quasi-particle space localization leads to the modifications of the energy spectrum. The all-important factor is the rise of new vibration branches of optical spectrum, namely, the interface optical phonon . In addition, the intensity of electron–phonon interaction is changed. The interaction of charge particles with polar optical phonons should exhibit the most intensity. This interaction is of considerable importance in the understanding of the properties of heterostructures based on material with high ionicity. It can lead to self-consistent bond state of a charge particle and phonons, that is, the large radius polaron .
Currently, an investigation on the part played by interface phonons has attracted considerable interest in polaron state formation study. The heterostructures of different symmetry are under investigation. The contributions to polaron binding energy both of interface and of bulk optical phonons are the same value order in the quantum dots [3–5]. Taking into account, interface phonons are essential for quantitative analysis of the polaron states. It does not lead to new qualitative effects. Alternatively, the interface phonon role dominates in polaron binding energy for quantum well case [6, 7]. In response to this fact, the strong electron–phonon interaction can be realized in the quantum wells based on non-polar material with high iconicity barrier material. In addition, from the results, it follows that profound polaron effects should be expected, e.g., in the Si/SiO 2 compounds. Although there are no polar optical phonons in the material of such quantum well, these may be produced at the heteroboundary. As a result, the strong interaction of charged particles with interface phonons becomes possible. Conversely, the essential depression of electron–phonon interaction is possible when the quantum well is made of polar material and for the barriers is taken non-polar material.
In recent years, varied technologies of semiconductor quantum wire growth with assorted barriers are progressing rapidly. The most success has been achieved for the quantum wires based on III–V compounds [8–12]. Some advances have been made in the formation of II–VI semiconductor wire structures [13, 14]. It is in these structures that the polaron states can arise. At the same time, no extended theoretical study of the polaron states in such structures is available. Proper allowance must be made for the interaction of charge particles with interface optical phonons for an understanding of this problem. In this paper, we develop a theory of polarons in the quantum wires, taking into account the interaction of charged particles with all branches of the optical phonon spectrum.
Interface Phonons in the Quantum Wire
The Polaron in the Quantum Wire
Here is n(e)-th root of m(e)-th order Bessel function. The wave function is to be obtained by solving self-consistent problem. In so doing, the total wave function from Eq. 15 is perceived to be normalized.
The energy (Eq. 22) is defined by the electron interaction with phonon modes correspond to m = 0 only. This equation (Eq. 22) contains the contribution to polaron energy for all size-quantization levels. This contribution is caused by the interaction of localized electron with confined and interface phonons. It can be used for numerical analysis of electron–phonon interaction characteristic properties. However, the electron energy and wave function can be obtained analytically on condition the unequality (Eq. 14) is satisfied.
Results and Discussion
It is this quantity from Eq. 29 which contains the adiabatic parameter (Eq. 13). Substituting material parameters  into Eq. 29 for the quantum wire ZnSe/CdSe/ZnSe leads one to expect that the strong polaron effects for these structure should be observed when the quantum wire radius ρ0 < 40 Å.
It might be well to point out that both the polaron binding energy (Eq. 28) and polaron radius (Eq. 29) depend on effective electron mass inside the quantum wire and barrier dielectric properties. This clearly demonstrates the prevailing role of the interaction of an electron with interface optical phonons. The availability of the surface phonons leads to widening the range of materials in which the strong polaron effect should be expected. The strong electron–phonon interaction may exist near the interface between polar and non-polar materials. Among other things the significant electron–phonon interaction can result from the interface phonon influence in Si/SiO 2 heterostructures.
The results obtained show that the intensity of electron–phonon interaction is determined significantly by interface optical phonons in narrow quantum wires corresponding to the condition (Eq. 13). These interface phonons are localized basically in the heteroboundary vicinity. And its field penetrates also into the barriers region. By this is meant that the interface phonons can produce the effective canal of excitation transfer in the structures with several quantum wires. Related ways should be allowed for the transport theory development in quantum nanostructures.
This work was supported by Russian Foundation for Basic Research, grant 09-02-00902-a and the program of Presidium of RAS “The Fundamental Study of Nanotechnologies and Nanomaterials” no. 27.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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