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Scientific Report 2007-2009
Condensed matter physics and biophysics
C31. Electronic properties of novel semiconductor materials
investigated by optical spectroscopy under intense magnetic fields
The use of magnetic fields combined with optical spectroscopy
techniques is a most powerful means to address
the fundamental electronic properties of solids and,
specifically, of semiconductor materials. Magnetic fields
with relatively high intensity (B¡20 T) can be reached in
small-scale laboratories, whilst large facilities are nowadays
available worldwide to researchers for using fields
up to 45 T (continuous) and up to 100 T (pulsed). As
well-known from atomic physics, magnetic fields remove
eigenstate degeneracy or uncover hidden symmetries. In
bulk and nanostructured semiconductors, the electronic
states in a magnetic field are arranged in Landau levels
consisting of discrete eigenstates. These Landau orbits
are the quantum mechanical analogue of classical cyclotron
orbits and allow determining fundamental band
structure parameters, such as the effective mass of charge
carries. However, in optical experiments, the concomitant
presence of (positively charged) holes and electrons
leads to the formation of Coulomb-like bound pairs, referred
to as excitons (the analogue of the hydrogen atom
in solids). In semiconductors, excitons can be stable up
to room temperature and dominate the emission properties
of most materials and nanostructures. Thus, several
model calculations have been developed in order to reproduce
the field dependence of the recombination (or
absorption) spectra of magneto-excitons.
Figure 1: (a) PL spectra at T=90 K for different magnetic
fields B and two hydrostatic pressures on a GaAs1-
xNx sample (x=0.10%). FE and Ci indicate the free-exciton
and N complex-related recombinations, respectively. (b) Dependence
of the free-exciton diamagnetic shift ?Ed on magnetic
field for different pressures in a GaAs1-xNx sample with
x=0.10%. The dashed lines are a fit to the data by means of
the model reported in [1]. The exciton reduced mass is the
only fitting parameter.
To this regard, magneto-photoluminescence (m-PL)
experiments are conveniently used whenever the fundamental
properties of novel semiconductor materials
or nanostructures are being investigated. This is the
case of dilute nitrides, such as Ga(As,N), which feature
surprising physical properties and qualitatively new
alloy phenomena, e.g., a giant negative bowing of the
band gap energy and a large deformation of the conduc-
Figure 2: Energies of the Landau level, LLn, transitions
measured in an InN sample treated with hydrogen. The value
of the band gap energy at B=0 T, E(0), and the value of the
carrier reduced mass, , are used as fit parameter.
tion band structure. This latter has been successfully
investigated by combining a magnetic field (B up to
12 T) with hydrostatic pressure (P up to 10 kbar). P
allows tuning the relative energy position between the
conduction band minimum and nitrogen-cluster levels,
while B permits to determine the electron effective
mass for each relative alignment between those states
(see Fig. 1). In this manner, it was discovered that
the whole electronic properties of Ga(As,N) are indeed
determined by a hierarchical distribution of N cluster
energy levels [1]. Intriguing behaviors in other technologically
relevant semiconductors have been revealed
by m-PL under very intense fields (B up to 30 T).
In Ga(As,Bi), an alloy of interest for spintronics and
telecommunications, the exciton reduced mass value
reveals an unexpected influence of Bi complexes on
both the valence and conduction bands of the crystal
[2]. In InN, a material having great importance for
photovoltaics and transport applications, the Landau
levels were measured for the first time by m-PL up to
30 T [3] in samples, whose electron concentration was
tuned on-demand by post-growth hydrogen irradiation
(see Fig. 2) [4]. This shed new light on the influence
of native as well as of purposely incorporated hydrogen
donors on the transport properties of InN.
References
1. G. Pettinari et al., Phys. Rev. Lett. 98, 146402 (2007).
2. G. Pettinari et al., Appl. Phys. Lett. 92, 262105 (2008).
3. G. Pettinari et al., Phys. Rev. B 79, 165207 (2009).
4. G. Pettinari et al., Phys. Rev. B 77, 125207 (2008).
Authors
A. Polimeni, G. Pettinari, M. Capizzi
http://chimera.roma1.infn.it/G29
Sapienza Università di Roma 84 Dipartimento di Fisica