Density of States in Dilute GaAsN

Detail of the calculated DOS and measured scanning tunneling spectroscopy (STS) data (circles) in the conduction band. The red solid line is the raw calculated DOS before the extrinsic thermal and modulation broadening effects are incorporated (dashed line) for x = 1.2%.
Detail of the calculated DOS and measured scanning tunneling spectroscopy (STS) data (circles) in the conduction band. The red solid line is the raw calculated DOS before the extrinsic thermal and modulation broadening effects are incorporated (dashed line) for x = 1.2%.

Materials Theory Group > Transport and Atomic Structure in Semiconductor Alloys > Dilute Nitrides > Density of States in Dilute GaAsN

The electronic structure of dilute Ga(In)As1-xNx semiconductors has attracted considerable interest due to the very large band-gap bowing resulting from the incorporation of dilute concentrations of nitrogen [1]. However, despite many years research, the only experimental determination of the band dispersion relations above the conduction band edge (CBE) has been for ultra-dilute samples [2] (x = 0.2% and lower). These measurements were consistent with the predictions of the band anticrossing (BAC) model [3] in which the conduction band (CB) becomes split into two subbands due to hybridisation of the extended host semiconductor CB with a localised, resonant nitrogen energy level. The resulting band-structure showed an increased electron effective mass and marked non-parabolicity. However, measurements of the band edge electron effective mass [4,5] show a strong deviation from the predictions of the single impurity BAC model with increasing N composition x. This is believed to be due to the effect of cluster states such as N-N pairs [6-8], formed at higher nitrogen concentrations. The linear combination of isolated nitrogen states (LCINS) model [9,10], in which the reduced Hamiltonian of a distribution of interacting nitrogen states is extracted from full tight-binding (TB) calculations, has been used successfully to explain the anomalous variation of the band edge effective mass with increasing x [11].

A further problem of the BAC model is that it fails to provide the correct density of states (DOS) of the system after mixing. The assumption of a well-defined wavevector for the hybridised states leads to the unphysical prediction of an infinite number of states below a given nitrogen impurity level [12]. This problem has been addressed by deriving the band structure from the Green's function of the Anderson many-impurity model [13,14]. Although the optical properties of GaAsN semiconductors have been investigated intensively, the DOS for this material system at the higher nitrogen concentrations of interest for device applications has never been observed experimentally. In the present work, the DOS of a x = 1.2% sample has been determined experimentally using cross-sectional scanning tunneling spectroscopy (XSTS) [15]. This is then modeled using the Green's function formulation together with the calculated LCINS data for the state and interaction energies of the N environments, yielding excellent agreement to the experimental data without arbitrary fitting parameters.

In collaboration with

  • Lena Ivanova
  • Holger Eisele
  • Mario Dähne

of Institut für Festkörperphysik, Technische Universität Berlin.

Related publications
Direct measurement and analysis of the conduction band density of states in diluted GaAs1-xNx alloys
L. Ivanova, H. Eisele, M. P. Vaughan, Ph. Ebert, A. Lenz, R. Timm, O. Schumann, L. Geelhaar, M. Dahne, S. Fahy, H. Riechert, and E. P. O' Reilly
Phys. Rev. B 82, 161201(R) (2010)

Modeling and direct measurement of the density of states in GaAsN
M.P. Vaughan, S. Fahy, E.P. O’Reilly, L. Ivanova H. Eisele and M. Dahne
Physica Status Solidi (Accepted for publication 2011)

References
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[3] W. Shan et al, Phys. Rev. Lett. 82, 1221 (1999).
[4] J. Endicott et al, Phys. Rev. Lett. 91, 126802 (2003).
[5] P.N. Hai, W.M. Chen, I.A. Buyanova, H.P. Xin, and C.W. Tu, Appl. Phys. Lett. 77, 1843 (2000).
[6] P.R.C. Kent, and A. Zunger, Phys. Rev. Lett. 86, 2613 (2001).
[7] A. Lindsay, and E.P. O’Reilly, Physica E (Amsterdam) 21, 901 (2004).
[8] J.S. Kang et al, J. Phys.: Condens. Matter 16, 3257 (2004).
[9] A. Lindsay and E.P. O’Reilly, Phys. Rev. Lett. 93, 196402 (2004).
[10] E.P. O’Reilly, A. Lindsay and S. Fahy, J. Phys. Condens. Matter 16, S3257 (2004).
[11] F. Masia et al, Phys. Rev. B 73, 073201 (2006)
[12] M.P. Vaughan and B.K. Ridley, Phys. Rev. B 75, 195205 (2007).
[13] P.W. Anderson, Phys. Rev. 124, 41 (1961).
[14] J. Wu, W. Walukiewicz and E.E. Haller, Phys. Rev. B 65, 233210 (2002).
[15] L. Ivanova et al, Phys. Rev. B 82, 161201 (2010)

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