domingo, 25 de julio de 2010

Defects in Dilute Nitrides, Gerald Soto, CRF 2010-1, (3er Parcial).

Dilute nitrides, derived from conventional III-V semiconductors such as Ga(In,Al)P and Ga(In)As by insertion of N into the group-V sublattice, have gained increasingly high interest during the last few years. They exhibit unusual and fascinating new physical properties, such as a giant band-gap bowing that allows widely extended band structure engineering. The high interest has also been driven by potential technological advantages provided by the novel dilute nitrides in lattice matching to GaAs and Si substrates. A combination of the remarkable fundamental properties with the technological advantages has provided an unprecedented opportunity to tailor material properties for desired device applications in optoelectronics and photonics, such as improved solid-state lasers for fiber-optic communications, multi-junctional solar cells, integration of efficient III- V optoelectronics with microelectronics based on silicon.

Unfortunately, epitaxial growth of dilute nitrides remains as a great chal-lenge. The required non-equilibrium growth conditions together with the disparity between N and the replaced group-V atoms are known to favor formation of various defects. As the optical quality of dilute nitrides has been shown to deteriorate with increasing N composition, even in the best available materials that are free of structural defects, there is currently a great need in identifying growin point defects and in assessing their role in non-radiative carrier recombination. In fact, Below we shall provide a brief review of our recent results from optically detected magnetic resonance (ODMR) studies of grownin non-radiative defects in Ga(In)NAs and Ga(Al,In)NP, in an effort to provide chemical identi¯cation and experimental signatures of defects and to evaluate their role in carrier recombination. Among them, defect complexes involving intrinsic defects such as antisites and self-interstitials have been positively identifyed and effects of growth conditions, chemical compositions and post-growth processing on formation of the defects have been studied. Non-radiative defects can be monitored using the ODMR technique because a magnetic resonance induced increase in efficiency of dominant non-radiative recombination channels can lead to a corresponding decrease in free carrier concentration available for radiative recombination and thus to a decrease in photoluminescence (PL) intensity.

the issue of defects is one of main problems we are currently facing that hinders a rapid progress of dilute nitrides for various device applications in optoelectronics and photonics. For example, it has been shown that about 50% of threshold current in the best available 1.3 um GaInNAs lasers has been found to be due to defect-related non-radiative recombination channels. Many key parameters of GaInNAs for solar-cell applications have also been found to be limited by defects. Identifying and understanding of the relevant defects in dilute nitrides and designing strategies to eliminate them are therefore crucial to the success of these materials for optoelectronic device applications.

Experimental evidence for the existence of AsGa antisites in Ga(In)NAs alloys grown by gas source molecular beam epitaxy (GS-MBE) has been provided by the ODMR measurements. The participation of an As atom in the defect was concluded from the experimentally resolved hyperfine (hf) structure, i.e. a group of four ODMR lines, characteristic of the hf interaction between an unpaired electron spin S = 1/2 and the nuclear spin I = 3/2 of the 75 As atom (100% natural abundance) | Fig. 1. The hf splitting parameter, A = 737 x 10^4 cm^1, and the g-value of the unpaired electron localized at the defect, g = 2, were determined by fitting experimental data with the effective spin Hamiltonian

The first and second terms in Eq. (1) are the electron Zeeman and hyperfine interaction terms, respectively; ¹B denotes the Bohr magneton. The obtained A value is about 20% smaller than that known for the isolated AsGa in GaAs, suggesting that the revealed defect is a complex involving AsGa. The microscopic structure of the complex does not depend on the N composition in the GaNxAs1¡x layers for x = 1-3%, as the defect parameters do not change with x.

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