Observation of a blue shift in the optical response at the fundamental band gap in Ga1−xMnxAs

Abstract

We report the observation of a sharp band-edge response in spectrally resolved differential reflectivity experiments on GaMnAs, in contrast to linear optical experiments in which large band-tail effects are known to dominate. The differential reflectivity response exhibits a blue shift relative to results in GaAs and LT-GaAs, consistent with the valence-band model of ferromagnetism. Our results demonstrate the utility of nonlinear optical techniques for studying the electronic structure of III-Mn-V diluted magnetic semiconductors.Diluted magnetic semiconductors (DMSs), which result from doping traditional semiconductors with magnetic impurities, have been the subject of an intensive research effort in recent years due to their potential for applications in spin-sensitive electronics, 1-3 photonics, 4-6 and opticallyaddressable nonvolatile memory. 7-9 This potential stems from the hole-mediated nature of the ferromagnetic coupling between the local magnetic impurities, which permits external control of magnetic characteristics using electrical gates or optical excitation. 10-13 GaMnAs has become the prototype DMS system, representing a model for understanding the nature of ferromagnetic coupling in the III-Mn-V DMS as well as for engineering the magnetic characteristics through tailored growth and processing. 14-16 A considerable body of research spanning the past decade has provided insight into the nature of exchange coupling between the Mn and hole spins, 17-21 the character of the magnetic anisotropy, 22-25 and the role of defects and compensation in determining the magnetic characteristics. 14,16,26,27 Despite these advances, unanswered questions remain with regard to the electronic structure and the appropriate model of ferromagnetic coupling. An active debate in the recent literature has focused on the position of the Fermi level, in particular whether the holes that mediate ferromagnetic coupling between the local Mn moments exist in the GaAs valence band or in a detached impurity band. 16,18,20,21,[27][28][29][30][31] As the physical mechanism of ferromagnetic coupling differs in the two limits, determining the correct picture is of central importance. Considerable input into the debate has been provided through recent experiments, with evidence supporting both points of view. [16][17][18][19][20][21]27,29 Nonlinear optical spectroscopy provides an alternative approach to investigating the position of the Fermi level in GaMnAs. In pump-probe spectroscopy, a femtosecond optical pulse (pump pulse) is used to excite electron-hole pairs in the semiconductor. These carriers modify the optical response of a subsequent, weaker pulse (probe pulse), providing a way to detect the presence of the pump-injected carriers. If the Fermi level in GaMnAs exists in the valence band, one expects to observe a blue shift in the band-edge optical response relative to undoped GaAs (the so-called Moss-Burstein shift) 32 since states that are full prior to the arrival of the pump pulse cannot lead to a signal. Alternatively, if t...

Band structure evolution and the origin of magnetism in (Ga,Mn)As: From paramagnetic through superparamagnetic to ferromagnetic phase

“…It provides an improvement in the accuracy over our earlier photoreflectance studies performed at room temperature. [26][27][28][29] LT-PR offers also an advantage over other optical methods applied to (Ga,Mn)As [30][31][32][33][34] where the optical response from the gap may be quenched by defects and is marred by the hole plasma. Importantly, the method allows to probe the valence-to-conduction band optical transitions regardless of the Fermi level location within the gap.…”

“…Hence with LT-PR one obtains a significantly higher energy resolution around the band gap, a merit unavailable by earlier spectroscopic measurements. [30][31][32][33][34] Additional advantage is that the LT-PR directly probes (Ga,Mn)As in the ferromagnetic phase -the case of x  1.6% layer. Fig.…”

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

“…[11][12][13][14][15] Realizing such applications requires a good understanding of the fundamental properties of these materials; however, the large density of defects associated with their growth at low temperatures (essential for the substitutional incorporation of magnetic dopants such as Mn) severely complicates the theoretical treatment of exchange coupling and ferromagnetic order [16][17][18] . These defects also impede the interpretation of transport and linear optical experiments on these materials [19][20][21][22][23][24] .…”

Direct observation of spin-orbit splitting and phonon-assisted optical transitions in the valence band by internal photoemission spectroscopy

“…[1][2][3][4] Many fundamental studies and device applications make use of optical transitions between different energy bands, [5][6][7][8][9][10][11] among which the valence-to-conduction band (VB to CB) transitions are typically used to determine band parameters, 9 e.g., the interband Van Hove singularities. 10 Despite the well-known electronic structure, understanding of VB-related physics is still very limited.…”

“…10 Despite the well-known electronic structure, understanding of VB-related physics is still very limited. Nearly all of what is currently understood about VBs come from optical absorption [5][6][7] and interband transition-based experiments, 9,10 the latter of which encounters a fundamental difficulty when band tailing perturbed obscuring effects dominate. 12 This limitation makes it impossible to measure the VB singularity on the basis of free-carrier effects.…”