Jason Kaszpurenko Journal Club Feb. 3, 2011

Jason Kaszpurenko

Journal Club

Feb. 3, 2011

Formation of Mn-derived impurity band in III-Mn-V alloys by valence

band anticrossingAlberi, et all, Phys Rev B 2008

Overview

Motivation Anticrossing Bandgap calculations Mobility Conclusion

Motivation

There are a wide variety of theories to describe DMS and DMO: For an overview please see a recent Nature Materials Review article

A quick summary is that there are a lot of theories and even though they are accepted they don't apply to all situations and have conflicting underlying principles

The authors want to take a look at some of the things that traditional theories don’t explain

Motivation

Motivation is that the traditional model accounts for a lot of things but there are some hole transport properties that it doesn't adequately explain

Using the Mott theory for Metal-to-Insulator Transition theory the valence bands of the GaAs and the impurity band of Mn are supposed to intersect as Mn concentration goes up

Motivation

DMFT and first-principle band-structure calculations support a separate impurity band for all doping levels

Experimental evidence shows weak localization

So in experiment there is a small impurity band 110 meV above the valence band

What to do?

MIT is controversial when it comes to magnetic dopants in semiconductors because there is strong localization and p-d hybridization

Some studies have shown that the p state undergoes an anticrossing interaction with the extend p states causing the conduction and valence band to restructure

So what is anticrossing?

Anticrossing (and crossing) occur in two level systems

These transitions occur below the conduction band and can create new allowed energy levels which can be probed

These states do not have to be bound Originally I thought it was when an optical transition

had occurred within a cavity, resonated and was reabsorbed

Not completely sold on this being the picture for this case

Bandgap

One of the first things to appear from the Valence Band Anticrossing (VBAC) model is that the Mn band increases with Mn dopant concentration while a proportionally large decrease occurs in the valence band

Band gap calculations support this result when compared to other peoples work

Bandgap

(a) Photomodulated reflectance spectra of Ga1−x−yMnxBeyAs demonstrate that the valence to conduction-band transition increases in energy with increasing Mn concentration but shows no movement with Be concentration, suggesting an anticrossing interaction between the GaAs and Mn states. The dashed line marks the photomodulated reflectance transition of the underlying GaAs substrate while the arrows indicate the transitions of the film.

(b) Experimentally determined movement of the band-gap energy of Ga1−xMnxAs as well as the theoretical trend predicted by the VBAC model E− valence band edge to conduction band

(c) Movement of the valence and impurity band-edge energies of Ga1−xMnxAs as a function of Mn concentration.

Mobility

Transport measurements were calculated: It should be noted that there are a lot of issues in doing this.

In this case when adding the dopant it decreased the mobilities of GaAs from 10-50 cm^2/V*s to 1-5 cm^2/V*s

Mobility

The order of magnitude mobility change isn't associated with spin-disorder scattering because it is weakly dependent on temperature

The next culprit investigated was the effective mass of the holes

The justification for this becomes when you calculate the effective mass for the GaAs (0.47me) and plug those values into the Ga1-xMnxAs the carrier concentrations become far to great

30me is the calculated value for the effective mass which is definitely on the large side but is consistent with values from VBAC

Mobility

Dispersions of the impurity band in Ga0.99Mn0.01As and Ga0.96Mn0.04As, illustrating the increase in the width of the impurity band with increasing Mn concentration

These effective masses are consistent with another groups experimental findings

Can we explain the MIT?

The MIT transition can still occur if the lifetime of the hole is broadened

In lower dopant concentrations the holes move via hopping and have lower lifetimes

In high dopant concentration the states start to hybridize and the system becomes a conductor (confused about this)

Impurity bandwidth solid lines and heavy-hole lifetime broadening dashed lines for Ga1−xMnxAs and Ga1−xMnxP as a function of Mn concentration.

Conclusion

A model was proposed to augment an existing models approach on the origins of ferromagnetism to explain certain shortcomings

This model predicts the bandgap accurately More interestingly the effective mass

becomes incredibly large The authors propose that this model can be

applied to other systems as well

Questions?

Thank you