TECNOLOGÍA TRADICIONAL, MEDIA Y MECANIZADA EN LA

A DX centre is a complex metastable state associated with Si dopant atoms which can trap free electrons. Although the mechanisms of DX centre formation have been the subject of intense debate (Bourgoin 1989, Mooney 1990) the widely accepted picture of the DX centre was described by Chadi and Chang (1988,1989) and is shown in Fig. 3.8 for GaAs.

Fig. 3.8 The Chang and Chadi Model for the Silicon DX Centre in GaAs (Chang and Chadi 1989)

D^(SLR state) DX' (LLR state)

In order to explain DX related properties such as metastability, the persistent photo-conductivity (PPC) effect - the fingerprint of the DX centre carrier trapping process - and the energetics of DX centre formation we first need to consider the structural nature of the DX system. The local structure of a defect such as the DX centre is determined by the electron configuration i.e. if the electron configuration changes then the structure of the defect changes. In Chadi and Chang's (1988) model of the DX centre, the system is stable in either the positive D+ or negative DX" state

where D and DX represent the shallow donor and the DX centre respectively. As such the system has two distinct spatial configurations: the small lattice relaxation (SLR) D+ state; and the large lattice relaxation (LLR) DX" state. To help us to understand the properties of these separate states, we will begin by considering the most basic molecular system - the hydrogen molecule. The Pauli exclusion principle means the hydrogen molecule possesses two states, the bonding and the anti-bonding state. These two states possess different configurations: the higher energy anti­ bonding state has a larger mean core separation, and the lower energy

bonding state posses a smaller mean core separation (Fig. 3 . 9 ) 4 . These two

configurations lead to a difference between the electron absorption and em ission energies. This difference is known as the Franck-Condon energy. It arises because the time taken for the molecule to adopt a new configuration is long compared to the speed of an electronic transition thus their remains some residual elastic energy from the deformed molecular configuration as the molecule slowly relaxes.

Fig. 3.9 Bonding and anti-bonding state for a hydrogen molecule

anti-bonding state

bonding state

flCÛ

Core distance

The SLR defect is very similar to the hydrogen molecule having a low energy (ground) state and a high energy (excited) state. The energy- configuration curves for the SLR state are shown in the diagram below (Fig. 3.10a). Here the configuration co-ordinate represents the structural configuration in some unspecified way and it is assumed that the total defect energy obeys Hooke's law i.e. the total defect energy depends quadratically on the molecular configuration. In the SLR state the

structural configuration is such that photo-stim ulated electronic transitions between the two states are permitted, thus the high energy state is essentially unstable and as such has a short lifetime. In the DX system this SLR state represents the ionised donor (D+).

3.10 Schematic illustration of the total energy as a function of Fig

configuration for the general SLR and LLR states and the ALGai-xAs DX" LLR state lx>0.22). SLR state e x c i t e d s t a t e g r o u n d s t a t e LLR state e x c i t e d s t a t e g r o u n d s t a t e DX- LLR State i n C B t h e r m a l

_i

However, for the LLR defect , the structural differences between the two states are large enough that the elastic energy of the ground state increases the total energy above that of the excited state at the same configuration. This prohibits vertical (optical) absorption and emission transitions between the ground state and excited state parabolas. If we consider an electron photo-excited into the excited state (Fig. 3.10b) then

after subsequent energy minimisation the energy-configuration curves are such that photo-relaxation into the ground state requires an initial

increase in the energy of the system. This energy barrier (Eg) is called the thermal emission barrier and implies that the LLR state is metastable and thus can act as a trap. Applying the general LLR model to the DX centre (Lang and Logan 1977) results in the schematic diagram (Fig. 3.10c) illustrating the relevant energies associated with the DX" state. In addition the diagram show s, the (thermal) capture and (optical and thermal) em ission processes. Population of the DX centre requires thermal excitation to overcome Eg, and it is this mechanism that is responsible for the PPC effect (see §3.6.4), preventing the recapture of electrons at low temperatures. Various authors have determined the energies associated with the Si doped AlxGai-xAs DX centre, they are: Eg ~ 150 meV, Et~150 meV and Eg~300 meV (Schubert and Ploog 1984).