Modification of magnetic semiconductors by phosphorus doping

Dilute ferromagnetic semiconductors (DFMSs) represent a class of alloys that combine semiconductor properties with magnetism in a single material. Their magnetic properties arise from transition metal ions introduced into the semiconductor parent lattice. This combination of electronic and magnetic properties results in entirely new physical properties that are of interest in fundamental science and, importantly, hold the promise of a wide range of prototype devices relevant to the spintronics industry. Just as the introduction Mn in GaAs induces significant changes to the band structure of the parent compound, recent developments demonstrated that the band structure of the (Ga,Mn)As matrix itself can be further tuned by doping with In or Bi. In the quest for more efficient spintronic materials and a better understanding of topological effects in the band structure of (Ga,Mn)As, we study this band structure in on the quaternary (Ga,Mn)(P,As) compound. 

Mn atoms substituting Ga in the GaAs host crystal, Mn_Ga, provide magnetism but also act as acceptors with an impurity binding energy of intermediate strength, 0.11 eV. This results in a large hole density, which is believed to play a crucial role in the hole-mediated ordering of Mn spins. On the other hand, Mn atoms occupying interstitial sites of the crystal lattice, Mn_I, act as double donors in GaAs and, together with the native As_Ga donors, partially compensate Mn_Ga acceptors, thus resulting in effective reduction of the hole concentration in the (Ga,Mn)As films and, in turn, in decreasing their Curie temperature (T_C). 

With P doping, strained (Ga,Mn)(P,As) films grown on GaAs, have been shown to host ferromagnetism with perpendicular magnetic anisotropy, ideal to observe the anomalous Hall effect. Mn and P co-doping also allow the tuning of the magnetic and electrical properties of this system independently.

Here, by combining Raman spectroscopy and optical ellipsometry, we are able to track the energy gap of (Ga,Mn)(P,As) at different levels of P doping, with and without Mn. Upon introduction of Mn, a strong couple of plasmon-phonon peak emerges in the Raman spectrum, along with an enhancement of the spectra absorption below the band edge. The shape of the band edge is consistent with a picture in which valence band dispersion near the edge is significantly altered, as in the impurity band picture.

Embedding P ions into the GaAs crystal lattice leads to an increase in the band gap, while Mn impurities lead to a decrease in it. A significant impact of Mn interstitial impurities on the structural and magnetic properties of the films was observed. We observe an enhancement of the charge density in the presence of Mn of the optical gap as well. The results suggest the presence of in-gap Mn impurity states and are not consistent with the observed blue shift of the Fermi level in (Ga,Mn)As, (In,Ga,Mn)As and (Ga,Mn)(Bi,As) epitaxial layers due to the Moss-Burstein effect.