Electronic structure in narrow-gap quantum dots

Electronic Structure in Narrow-Gap Quantum Dots

S. J. Prado¹, G. E. Marques¹, and C.Trallero-Giner²

¹Departamento de Física, Universidade Federal de São Carlos,

13565-905, São Carlos, SP, Brazil

²Departamento de Física Teórica, Universidade de La Habana

10400, C. Habana, Cuba.

Received February, 1999

In this work we calculated the electronic structure of spherical quantum dots based on zincblend semiconductor compounds. The strong conduction-valence band coupling in this class of semi-conductors induces a strong mixing of the electronic states which requires a theoretical model to properly take into acount these effects. We have used a full 8 x 8 Kane Hamiltonian in order to include the strong admixture and study the set of symetries associated with these electronic states and their angular momentum in this central force problem. As an application, we have calculated the electronic structure in narrow-gap HgCdTe, InSb and CdTe quantum dots.

I Introduction

The synthesis and characterization of new structural materials showing potential application as optical devices has been the subject of a large number of investigations in recent years. Almost all of them have studied semiconductor structures such as quantum wells(QW), quantum well wires(QWW) and quantum dots(QD), presenting quantum confinement in one, two and three dimensions, respectively.

The main interest in this work will be the electronic structure of QD which show a completely discrete energy spectrum for all carriers and, for this reason, they are frequently referred to as "artificial atoms''. Synthesis on colloids and on vitreous matrices are techniques which have shown high degree of reproducibility and control. The crystallites have uniform spherical shapes where the diameter size can be tuned during the synthesis or by thermal treatment of the vitreous matrices producing narrow distribution of QD sizes within 5% rms [1].

In these nanocrystallite structures the number of carriers in the dots can be controlled externally[2] and the full spatial confinement, besides producing sharp energy levels, also has strong influence on their linear [3] and nonlinear[4] optical properties.

In this short work we show some results for the electronic structure of quantum dots with spherical symmetry within the envelope-function approximation where we solve analytically the full k.p Hamiltonian.

II The k.p representation

A large number of band structure calculation has been used in solids however the k.p, first introduced by Kane[5] and Luttinger[6] has proved to be the most general effective mass approach for heterostructures where the states are expanded in a finite set of Bloch states close to an extremum, k₀, of the bulk band structure inside the Brillouin zone of the crystal.

The basic k.p Hamiltonian procedure used by us is well described by Weiler[7] and used in a modified version by [7] in QD's. We are going to use the standard k.p version, with only a special ordering of states in two sets with spin up and down, as c⁺, hh⁺, lh⁺, so⁺, c^-, hh^-, lh^-, so^-. All operators appearing in the standard k.p Hamiltonian are given in terms of , and , that will be transformed into spherical coordinates operators in the form

Our model consider the QD with a radius R and infinite barrier at the surface of the QD, since outside the crystallites the materials have amorphous structures for QD's grown on colloidal or vitreous matrices. Thus, the potential is V = 0 for r £ R and V = ¥ for r > R is a realistic model. Therefore, the solutions of

can be found in the form,

where the term is an eigenfunction obtained as a solution, for each carrier type, of the diagonal elements of and the kets {| j >} with ( j = 1,8) are the Bloch functions at the G-point, with the proper symmetry of the crystallite. The coefficients A_n,l are the normalization constants, are Bessel spherical functions, as shown in Fig.1, whose zeroes for each level n and angular momentum l, are found as m_n^l and, finally, Y_l^m(q,f) are the spherical harmonic functions.

The number, in the Fig. 1, labels the first twelve spherical Bessel functions in order of increasing energy. The are associated to each root of , which has quantum numbers (n, l) for each energy level having degeneracy (m = 2l+1) in the quantum number m. The root are only function of (n, l) and, in a spectroscopic notation they represent the (nl)-atomic like states, and are labeled: 1S:l = 0; 1P:l = 1; 1D:l = 2; 2S:l = 0; 1F:l = 3; 2P:l = 1; 1G:l = 4; 2D:l = 2; 1H:l = 5; 3S:l = 0; 2F:l = 3; 1I:l = 6.

Within the model used in this work every element of matrix, , in the spherical QD's, can be calculated analytically since every part in y_n,l,m(r, q, f) is a well known function.

III Results

We will apply our model to calculate the QD electronic structure, as a function of the radius, in three different materials. We have chosen to make the comparison of the energy levels of QD's for CdTe, InSb, Hg_0.8Cd_0.2Te due to the size of their band gap energy. The Table below show all values of all k.p parameters used in the calculation.

In Fig. 2 we show the conduction band states, for each material of Table I, as a function of the inverse of the radius of the QD. We display the first four states, being two for l = 0 and two for l = 1. It becomes very clear that the coupling between the conduction and the valence bands is the strongest one for HgCdTe material, as should be expected due to its very small band gap energy. This effect is somewhat similar for InSb and becomes small, but not negligible at all, for CdTe QD's.

Figure 2.
Comparison of the effect due to conduction-valence band coupling on the first four energy levels of electrons in quantum dots of the three materials listed in Table I.

In Fig. 3 we show the calculated G₈ valence band energy spectra for l = 0 and l = 1, as a function of the inverse of the radius of the QD, for all three materials of Table I. Here again the two material with narrow gap somewhat show similar dispersions for the mixed lh and hh states. For CdTe the energy levels corresponding to l = 0 are further away from those with l = 1.

Figure 3.
Comparison of the effect due to valence band mixing on the first four energy levels of light- and heavy-holes in quantum dots of the three materials listed in Table I.

Finally, we may notice the complexity of the valence band electronic structure in spherical quantum dots. It seems that our full k.p Hamiltonian model can describe well these states and some degeneracy are lifted for some quantum numbers. We intend to further explorer and study these effects on the optical spectrum of these structures.

Authors acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo(FAPESP) for partial financial support of this work.

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Publication Dates

• Publication in this collection
  23 Feb 2001
• Date of issue
  Dec 1999

History

• Received
  Feb 1999