Neutral donor states, exciton transitions from

Identification of residual and doped donors have been identified in expitaxial GaAs using the photolumines-cence technique in the presence of applied magnetic fields. Transitions occur between excited initial and final states of the neutral-donor-bound-exciton complexes. The magnetic field compresses the wave function which sharpens the optical transitions. The magnetic field also separates the different donors when viewed from the neutral-donor-bound-exciton transitions. These two effects make possible the identification of donors when the donor concentration is in the mid lOlScm" range. 

The donors were Identified from the collapse of excltons bound to neutral donors. The optical transitions involve the collapse of exclton complexes whose initial state consists of an exciton bound to the first nonrlgld rotational state of the neutral donor and whose terminal state consists of the 2p state of the neutral donor. The most intense transitions in this process are the spin conserving transitions. The measurements were made in an applied magnetic field of 36kG. The transitions to the 2p, state are shown in Figure 6. The dashed curve for sample A shows the spin conserving transitions for the Si donor in this Si doped... 

The values of gh derived for the U and I9 lines are very close to the gh = -1.24 obtained for the hole involved in the exciton bound to ionized donor and to the g factor of the hole in li free exciton state. " On the other hand, the expected g/ values of the holes involved in the acceptor bound exciton transitions differ significantly from the g values of the holes involved into excitons bound to ionized or neutral donors. This is similar to the situation found in CdS. Therefore we conclude that both U and I9 transitions should be assigned to the (Do,2Ci(r7)) complex rather than to the (y4o(T7) A(T7)). 

Rg. 7 Energy levels and transitions of the P neutral donor (D ), donor bound exciton (D°X), and ionized donor (D" ) from Ref. 42. (a) The Zeeman splittings of the (D ) and (D°X) states are shown from magnetic field B = 0 to B = 84.53 mT, along with the dipole-allowed optical transitions, (b) Photoconductive readout spectrum without any DO hyperpolarization, (c) The specific optical transitions (lines 4, 5, and 6) and nuclear magnetic resonance transitions (RF, RF, and RF+) used to hyperpolarize, manipulate, and read out the nuclear spins, (d) Sketches of the spins and charge densities of D+, D° and D°X. From K. Saeedi etal.. Science, 2013,342, 830. Reprinted with permission from AAAS. 

In crystals which are composed of two different partner molecules, CT excitations and with them CT excitons are frequently the predominant lowest excitation states and are thus responsible for the lowest-energy transitions in the singlet system. We will illustrate this using the example of the weak donor-acceptor complex anthracene/pyromellitic acid dianhydride, (A/PMDA) (Fig. 6.14). The ground state is neutral and nonpolar, with only a small charge-transfer fraction. The lowest optical excitation starts from the ground state of the donor D, anthracene, (from its highest occupied orbital or HOMO) and leads to the lowest unoccupied orbital (LUMO) of the acceptors A, PMDA, within the mixed stack DADADA. A polar ex-... 

---