Crossover from Excitons to an Electron–Hole Plasma in a
High-Quality Single T-Shaped Quantum Wire
Masahiro Yoshita*, Yuhei Hayamizu*, Hidefumi Akiyama*,
Loren N. Pfeiffer†, and Ken W. West†
*Institute for Solid State Physics, University of Tokyo and CREST, JST,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
†
Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA
Abstract. We studied evolution of photoluminescence (PL) spectral with increasing excitation powers in an
unprecedentedly high-quality quantum wire. The PL spectra obtained suggest that the transition from excitons to a dense
electron–hole (e–h) plasma in the wire is a gradual crossover via bi-excitons. We also found that the continuum band
edge of excitons shows no energy shift with increasing e–h densities and never crosses to the low-energy edge of the
plasma PL at higher e–h densities, which is in contrast to a prevailing picture of the exciton Mott transition.
INTRODUCTION
RESULTS AND DISCUSSION
Many-particle effects in the one-dimensional (1D)
electron–hole (e–h) system of quantum wires where
Coulomb correlations among the carriers become more
significant have recently attracted much interest. One
intriguing question is whether the transition from a
dilute exciton gas to a dense 1D e–h system is
described by a picture of the exciton Mott transition,
which is accepted as a plausible picture in higher
dimensions. In this work, we study the evolution of
photoluminescence (PL) spectra from a single
quantum wire with increasing excitation powers during
a transition from excitons to a dense 1D e–h system.
PL spectra of the T wire for various excitation
powers at 4 K are shown in Fig. 1. At excitation
powers below 5 × 10-3 mW, a dominant peak at 1.582
eV and a tiny peak on low-energy tail of the dominant
peak appeared, and were respectively ascribed to free
and localized excitons in the wire. Both the position
and the intensity of the free-exciton PL peak were
almost unchanged over 20 µm along the wire, and its
linewidth was 1.3 meV, which indicate high spatial
uniformity and quality of the wire. As the excitation
power was increased to 10-2 mW, where the estimated
e–h density in the wire was 2.6 × 104 cm-1, a new PL
peak appeared 3 meV below the free-exciton PL peak
and increased its intensity superlinearly. We ascribe
this peak to bi-excitons. At excitation powers above
0.1 mW where the e–h density was 1.7 × 105 cm-1, the
low-energy side PL peak became dominant. We
ascribe the PL in this power regime to an e–h plasma.
Note that the plasma PL peak is symmetrically
broadened and shows no peak shift with respect to the
bi-exciton peak at lower excitation powers. The
evolution of the PL spectra from the wire indicates that
the transition from excitons to an e–h plasma in the
wire is a gradual crossover via bi-excitons. Moreover,
the spectral features of the e–h plasma PL suggest that
there are strong internal Coulomb correlations, in
particular, bi-excitonic correlations in the 1D e–h
plasma [3].
EXPERIMENT
A single T-shaped quantum wire (T wire) was
fabricated by the cleaved-edge overgrowth method
with molecular beam epitaxy [1]. The quantum-wire
electronic states were formed at a T-intersection of a
14-nm-thick (001) Al0.07Ga0.93As quantum well (stem
well) and a 6-nm-thick (110) GaAs quantum well (arm
well). Details of the sample structure and fabrication
procedures are presented elsewhere [2]. PL spectra of
the wire at various excitation powers under point
excitation were measured by using micro-PL setup.
FIGURE 1. Normalized PL spectra of the T wire for
various excitation powers at 4 K. Thin solid and dotted
curves respectively show PL lines of the free-exciton peak
and the low-energy side peak separated by line-shape
analysis.
Figure 2 shows the evolution of PL spectra from
the T wire obtained at an elevated temperature of 30 K.
In Fig. 2, a small PL peak at 1.589 eV (denoted as
excited) due to an exciton excited state and a
continuous PL band with an onset at 1.593 eV
(denoted as onset) due to higher excited states of the
excitons and 1D continuum states are clearly observed.
A noticeable point in the PL spectra is that neither the
excited peak nor the onset edge shows no energy shift
from their initial positions. Even at a pair density of
3.9 × 105 cm-1 where the e–h plasma has already
formed, these PL features are still observed. In the PL
peaks from the wire ground state (denoted as ground
in Fig. 2), on the other hand, low-energy edges of the
plasma PL (marked by closed triangles) seen at high
densities show red shift with increasing excitation
powers possibly due to band-gap renormalization.
However, these edges do not continuously connect to
the band edge of excitons (open triangle) at the lowest
density. Instead, they converge to the energy position
of bi-excitons. Therefore, the level-crossing of these
two edges expected in the exciton Mott transition does
not occur. This result suggests that a new picture is
necessary for the observed crossover from excitons to
a plasma via bi-excitons in a quantum wire.
FIGURE 2. Normalized PL spectra of the T wire for various
excitation powers at 30 K. Numbers in parentheses are
estimated e–h densities in the wire.
CONCLUSIONS
From micro-PL spectroscopy of the single quantum
wire with increasing excitation powers, we found that
the evolution from excitons to a dense e–h plasma in
the wire is a gradual crossover via bi-excitons and that
there are strong bi-excitonic correlations in the 1D e–h
plasma. We also found that the low-energy edge of the
plasma peak at high densities converges to the energy
position of the bi-exciton, but never crosses to the
continuum band edge of the 1D free excitons.
REFERENCES
1. L. Pfeiffer, K. W. West, H. L. Stormer, J. P. Eisenstein,
K. W. Baldwin, D. Gershoni, and J. Spector, Appl. Phys.
Lett. 56, 1697-1699 (1990).
2. Y. Hayamizu, M. Yoshita, S. Watanabe, H. Akiyama, L.
N. Pfeiffer, and K. W. West, Appl. Phys. Lett. 81, 49374939 (2002).
3. M. Yoshita, Y. Hayamizu, H. Akiyama, L. N. Pfeiffer, K.
W. West, K. Asano, and T. Ogawa, unpublished.