Transcript Presentazione di PowerPoint - Istituto Nazionale di Fisica

Recombination Dynamics in Nitride Heterostructures:
role of the piezoelectric field vs carrier localization
A.Vinattieri, M.Colocci, M.Zamfirescu
Dip.Fisica- INFM-LENS, Firenze
In collaboration with
F.Rossi, N.Armani, C.Ferrari
IMEM-CNR, Parma
A.Reale, A.Di Carlo, P.Lugli
INFM-Dip.Fisica, Univ.Roma Tor-Vergata
Motivation
Unscreened
potential profile
Why GaN, AlN, InGaN are interesting ?
Strength by photoluminescence experiments in different excitation conditions.
High excitation density, easily reached in Cathodoluminescence experiments,
can induce complete screening of the internal fields for the narrower wells.
Time-resolved Photoluminescence experiments show the recovery of the built-in
field
Tunable bandgap in the UV-visible spectral range
High radiative efficiency (Blue-visible lasers)
Radiation hardness (UV detectors)
as the photoinjected carriers recombine.
Therefore the main features observed in PL spectra are nicely described by
the interplay between polarization field, charge screening and radiative and nonradiative recombinations.
Major problems
Different aspects of the recombination dynamics can be addressed by different
Experimental conditions: excitation density, stationary or pulsed excitation, lattice
temperature
Poor material quality even for epitaxially grown material
High dislocation density ( typically 109-1010 cm-2)
Highly strained material when grown on Sapphire and SiC: Huge builtin electric field (MV/cm)
The built-in field causes the Quantum -Confined Stark Eggect,
dramaticalyy reducing the oscillatorstrength. Thereforeemission shifths
to lowe energy and radiative rate bencomes smaller.
PL-time integrated spectra and
CL results
PL spectra and decays can be nicely reproduced in the framework of a model
screened by
optical pumping
the electronic states in the nanostructure with a rate equation model to account
for time-dependent effects of charge re-arrangement.
Peak Energy (eV)
Sample 03
#MQW04
TR-PL for different
detection energy
36 Q W
45
3.07
40
3.065
EM
35
Sample 04
3.06
PL temporal responses evaluated in a spectral window of 8 meV as a function of
the detection energy.
The wider QW (Sample 02) presents the more pronounced spectral shift,
together with a weaker PL intensity.
Both phenomena are a consequence of the fast destruction of the screening of
internal polarization field. The oscillator strength of the optical transitions is
quickly reduced because of wavefunction separation.
0
500
1000
Delay (ps)
1500
2000
An inverse Quantum Confined Stark
Effect is observed
No major change in the PL FWHM is
observed, except at early times
The faster decay on the high energy side of
the PL spectra is due to two different
processes; (i) free carrier relaxation
towards lower energies ;(ii) Red- shift of
the peak energy due to the recovery of the
built-in field.
Temperature Analysis
S Shape of PL shift
Peak energy presents S-like shift because
of trapping-detrapping mechanism,
activated by temperature.Such process is
nicely described by a thermally populated
inhomogeneously broadened band ( s
broadening parameter).
Non-resonant excitation exhibit nonmonotonic
dependence of PL intensity. At high
temperature detrapping in the barriers
contribute to increase QW carriers, and thus
recombinations.
#MQW02
FWHM (meV)
Sample 02
#MQW01
TR-PL – peak shift
TR-PL decays vs detection energy
Temperature Analysis – PL intensity
High Resolution TEM characterization
by coupling a self-consistent solution of Schrödinger and Poisson equations to
determine
3.075
When the well is wider, the Stark shift
becomes more important. Increasing the
excitation density a blue shift is
observed
Experimental setup
To investigate the effect of built-in field on the carrier dynamics we can “ probe”
the oscillator
Where do carriers localize? Roughness due to interface fluctuations . Alloy
inhomogeneities: indium clusterization as seen in NSOM PL spectra
Conclusions:
•Recombination Dynamics is ruled by charge
accumulation in the well and loss of carriers from the
ground level induced by both radiative and nonradiative recombinations processes.
• The measured PL decays show a time dependence
that is controlled by an interplay between radiative
and non-radiative recombination processes.
•The PL decays are affected by carrier trapping and
detrapping mechanisms , depending on the lattice
temperature