Radiative and Non-Radiative Processes in Gallium Nitride/Aluminium Nitride Superlattice Structures

1
Radiative and Non-Radiative Processes in Gallium
Nitride/Aluminium Nitride Superlattice Structures
Phonon
Spectroscopy
6th April CMMP 2004

• C E Martinez, N M Stanton, A J Kent,
• C R Staddon, S V Novikov and C T Foxon

School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
2
Phonon
Spectroscopy
Introduction

• AlxGa1-xN/GaN superlattice (SL) systems have
  recently received
• increased attention
• Extending the range of GaN based optical devices
  deep into the
• UV region of the spectrum
• Due to c-axis growth on GaN, combination of
  strains due
• to lattice mismatch results in strong internal
  electric fields

6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
3
Phonon
Spectroscopy

• Investigation into the influence of these fields
  on the radiative and nonradiative recombination
  mechanisms in GaN structures

Two complimentary techniques

• Information about non-radiative processes often
  inferred from
• results of optical measurements
• Measure directly the phonons emitted during
  non-radiative
• processes, and compare with results of time
  resolved
• PL measurements (TRPL)

6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
4
Phonon
Spectroscopy
Sample Parameters
Sample Number Well Width (nm) Barrier Width (nm) No. of Periods Calculated Internal Field (MV/cm) Calculated e1h1 Energy (eV) Observed e1h1 Energy (eV)
MS541 4.2 3.2 40 3.1 2.82 2.95
MS540 3.7 3.7 40 3.6 2.86 2.99
MS542 3.2 4.2 40 4.0 2.96 3.10
R. Cingolani et al, Phys. Rev. B 61, 2711
(2000).
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
5
Phonon
Spectroscopy

• PL emission energy is below the GaN band gap
• With decreasing well width, the PL peak emission
  energy blueshifts
• In reasonable agreement with results of
  calculations for these structures

Normalised PL spectra for all samples at power
density of 38 kW/cm2, as measured at the peak of
the PL time decays.
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
6
Phonon
Spectroscopy
The Technique
Laser _at_ 355nm, 10ns pulses, 50mm spot size
Sample _at_ 1.5K

6th April CMMP 2004
AlN/GaN 40 period superlattice
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
7
Phonon
Spectroscopy

• Observations
• Arrival of longitudinal acoustic phonons (LA) at
  35 ns
• Arrival of transverse acoustic phonons (TA) at
  70 ns
• As excitation power increases the optical to
  phonon signal ratio increases
• Delayed phonon signal at 185 ns becomes more
  intense at higher power

Phonon signals obtained from MS 541 for a range
of excitation power densities (3.9W/cm2 -
38 kW/cm2)
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
8
Phonon
Spectroscopy

• Three possible sources of delayed signals
• Phonon reflection
• Signal does not correspond to reflection arrival
  times of LA and TA phonons (110 ns and 210 ns
  respectively)
• Decay of optic (LO) phonons
• Decay product signals are characteristically
  broad and slow - the delayed signal we observe
  exhibits a fast decay
• Observation of delayed non-radiative
  recombination at higher excitation powers
  direct observation of the de-screening internal
  fields due to the quantum confined Stark effect?

6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
9
Phonon
Spectroscopy
Quantum Confined Stark Effect (QCSE)
Large internal fields tilt the bands, and
suppress radiative recombination
As excitation power density increased - number of
carriers increases
Internal fields are partially screened
Non-radiative recombination processes take over,
via phonon assisted tunnelling
Reduction of e/h spatial separation, efficiency
of radiative recombination increases
As carriers recombine, internal fields are
gradually de-screened
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
10
Phonon
Spectroscopy
Pexc 3kW/cm2
Pexc 38kW/cm2

• At low powers, photogenerated carrier density is
  not sufficient to screen the fields, no delayed
  phonon signal

• As power is INCREASED, so is the carrier
  density, and QCSE is reduced. Delayed phonon
  signal observed more pronounced in the sample
  with lowest internal field strength

• Increasing the power further delayed phonon
  signal apparent in all samples

Screening of internal fields by photogenerated
carrier density, followed by gradual de-screening
can account for the delayed phonon signal
similar effects should be visible in PL data
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
11
Phonon
Spectroscopy

• Lifetime for a given power increases as well
  width increases (60 ns narrowest wells to 160 ns
  widest wells)
• Lifetime is reduced at higher excitation powers
• Effect is more pronounced for the widest well
  sample (weakest internal field strength)
• Sudden decrease in PL decay time corresponds to
  the onset of the delayed phonon signal

The PL lifetimes as a function of power density.
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
12
Phonon
Spectroscopy
Reduction in e/h spatial separation

• At low power, no shift in PL emission peak
  observed
• At higher power, PL peak energy redshifts at
  longer times

Enhancement of radiative recombination

• Further evidence of
• re-establishment of internal fields following
  partial screening by photogenerated carriers

Progressive loss of carriers
De-screening effect - QCSE gradually restored
System returns to previous unscreened state
6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK
13
Phonon
Spectroscopy
Conclusions

• Combined direct detection of phonons and TRPL
  measurements to study the effects of internal
  electric fields on radiative and non-radiative
  recombination in AlN/GaN superlattices
• At high excitation power, photoinjected carriers
  screen the strong internal fields, and enhanced
  radiative recombination is observed
• As carriers recombine, de-screening effects
  result in an increase in recombination via
  non-radiative processes
• We observe directly these non-radiative
  processes as a delayed acoustic phonon signal in
  time of flight measurements

6th April CMMP 2004
School of Physics and Astronomy, University Park,
Nottingham NG7 2RD, UK