Download REsults

Photoluminescence excitation spectra of one-dimensional
electron systems in an n-type doped quantum wire
Toshiyuki Ihara, Y. Hayamizu, M.Yoshita, H. Akiyama,
L. N. Pfeiffer and K.W. West
Institute for Solid State Physics, University of Tokyo and CREST, JST, Chiba, 277-8581, Japan
Bell Laboratories, Lucent Technologies, Murray Hill, NJ, USA
Abstract. We measured photoluminescence (PL) and PL excitation (PLE) spectra of one-dimensional (1D) excitons,
charged excitons, and an electron plasma in a high-quality n-type-modulation-doped single GaAs quantum wire with a
gate, where density of 1D electron gas (1DEG) is tuned by gate voltage. We demonstrate that the absorption line shape
of the charged excitons varies gradually from symmetric to asymmetric with electron density in the wire, which then
changes to a shoulder corresponding to an absorption onset of an electron-plasma system. The high-energy tail of the
asymmetric absorption line of the charged excitons is analogous to a power-law anomaly appearing in Fermi-edge
singularity (FES).
INTRODUCTION
Interband optical transition spectra in the presence
of Fermi Sea are expected to exhibit a power-law
singularity, which reflects the final state response of
the Fermi Sea electrons to the attractive potential of
the valence-band hole. This singularity, which is well
known as the Fermi edge singularity (FES), has been
studied in detail in 2D quantum wells (QWs) [1]. In
1D electron system, we previously reported this effect
in photoluminescence (PL) spectra of a high-quality ntype-modulation-doped quantum wire (QWR) with 1D
electron densities tuned by gate voltage [2]. In this
paper, we report our first study on PL excitation (PLE)
spectra of the QWR, which reveals many-body
absorption features of the 1D system.
increased by Si delta-doping (4 x 1011cm-2). By
applying DC gate voltage (Vg) to the n+ Al0.1Ga0.9As
layer relative to modulation-doped 2D electron gas
(2DEG) in the stem-QW, we accumulated or depleted
additional electrons in the QWR and arm-QW. MicroPL and PLE measurements on the single QWR were
performed with a cw titanium-sapphire laser with a
1m spot size. The excitation power was 20 W which
is the weakest power we achieved to measure the PLE
spectrum. The direction of PL detection was
perpendicular to the laser excitation and their
polarization are orthogonal each other to improve
signal-to-noise ratio. PL resolution was 0.15meV and
PLE resolution was 0.04meV.
SAMPLES AND EXPERIMENTS
The sample was grown by the cleaved-edge
overgrowth method with molecular beam epitaxy and
growth interrupt annealing [3]. As schematically
shown in Fig.1, the T wire consisted of a 14-nm-thick
Al0.07Ga0.93As quantum well (stem-QW) grown on a
(001) substrate, and an intersecting 6-nm-thick GaAs
QW (arm-QW) overgrown on a cleaved (110) edge of
the stem-QW. The electron density in stem-QW is
FIG. 1 The schematic structure of T-shaped QWR, stemQW, arm-QW, and a gate.
RESULTS
We measured PL and PLE spectra of QWR with
various gate voltages from 0 to 0.7V at 5K. Fig.2 (a)
shows the normalized PL (dotted lines) and PLE (solid
lines) spectra, where PL excitation energy is adjusted
between 1.569 eV and 1.575eV. At low electron
density (0V), the PLE spectrum is dominated by the
neutral exciton peak (X) that split into double peak due
to monolayer fluctuations of QWR. As electron
density become higher, the PLE peak of the neutral
excitons buleshifts and becomes weak. The peak of the
negatively charged excitons (X-) appears and becomes
strong at about 2 meV below the exciton peak. At Vg
= 0.2V, PLE shape of the X- becomes asymmetric (a
fast rise at low energies and a slow fall at high
energies). The high-energy tail corresponds to the
power-law singularity in FES for 1DEG. When the
gate voltage is above 0.35V, the PLE peak of the
charged excitons changes to a shoulder.
For comparison, we measured PL and PLE spectra
of arm-QW. Fig.2 (b) shows the normalized PL
(dotted lines) and PLE (solid lines) spectra of arm-QW
with various gate voltages from 0.2 to 0.8V. We
observed absorption peak of neutral excitons (X),
charged exciton peak (X-), and electron plasma.
Comparing these results of 1D QWR and 2D armQW, we find the following:
(a) X peak blueshifts and X- peak doesn't shift with
electron density in both 1D-QWR and 2D-QW
which means that the energy gap between X- and
X increases with electron density. This effect has
been reported in 2D-QW by other group and
studied theoretically in 2D system.
(b) FES of X- absorption peak is observed strongly in
1D-QWR, while other group has reported strong
FES of X peak in 2D-QW. By comparing our
result of 1D-QWR and 2D-QW, we wish to
mention that this difference is because X peak
becomes weak with fermi energy more quickly in
1D-QWR than in 2D-QW.
(c) The PLE line shape varies dramatically from Xpeak to 1D electron plasma absorption (0.3-0.4V)
in 1D-QWR. In 2D-QW, X- peak changes to 2D
electron
plasma
absorption
(0.4-0.6V)
continuously. Electron plasma absorption peak
blueshifts with electron density both 1D-QWR
and 2D-QW. But we observed large energy gap
between PL shoulder and PLE peak in 1D-QWR.
FIG. 2 Normalized PL (dotted lines) and PLE (solid lines)
spectra of QWR (a) and arm-QW (b) at various gate voltage.
CONCLUSION
We measured the electron density dependence of PL
and PLE spectra in an n-type-modulation-doped QWR.
We observed the energy gap between exciton and
charged exciton increases with electron density in both
1D-QWR and 2D-QW. FES of charged exciton is
observed strongly in 1D-QWR. We observed dramatic
change of the PLE line shape from charged exciton
peak to 1D electron plasma absorption that differs
from 2D-QW result.
REFERENCES
1. M. Takagiwa and T ogawa, J. Phys. Chem. Solids. 63,
1587 (2002).
2. H. Akiyama, L. N. Pfeiffer, A. Pinzuk, K. W. West, and
M. Yoshita, Solid. State. Commun. 122 169 (2002)
3. M. Yoshita, H. Akiyama, L. N. Pfeiffer and K. W. West,
Jpn. J. Appl. Phys. 40, L252 (2001).
4. G. Yusa, H. Shtrikman and I. Bar-Joseph, Phys. Rev. B
62 , 15390 (2000)