Q' Zhuang, Physics Department, Lancaster University, UK

1
MBE Grown InAsN/InAs Narrow Band-gap Material
for Mid-infrared Optoelectronics

• Q. Zhuang, Physics Department, Lancaster
  University, UK
• A. Godenir, Physics Department, Lancaster
  University, UK
• A. Krier, Physics Department, Lancaster
  University, UK
• M. Hopkinson, EEE, Sheffield University, UK

2
Outline

• Motivation why introduce dilute nitride
• InAsN/InAs - a promising material for
  mid-infrared
• MBE growth of dilute nitride InAsN
• Prospects of InAsN
• HRXRD characterization
• Photoluminescence
• Localization states
• N configurations
• Conclusions

3
Initiative to propose dilute nitride

• Problems on 1310 nm VCSELs
• Best 1310 nm active region materials are
  lattice-matched to InP
• Best DBR Al(Ga)As/GaAs is lattice matched to GaAs
• Why GaAs ?
• Stands on the shoulders of proven 850 nm VCSEL
  technologies
• Mature DBR mirror technology AlGaAs/GaAs DBR
• Mature manufacturing infrastructure
• Readily available oxide confinement technology
• Need a suitable active region emitting at 1310
  nm
• Introducing N into InGaAs can obtain this
  wavelength

4
Adding N to GaAs for longer wavelength

• Big bowing effect due to the large
  electronegativity reduces the band-gap energy
• Shrinkage of the lattice constant
• 1310 nm emission can be easily obtained
• semi-metal above critical certain N level (gt3)

Band-gap energy vs lattice constant
a) S. Sakai, et al, J. J. Appl. Phys. 32,
4413(1993) b) M. Kondow, et al., J. J. Appl.
Phys. 35, 1273(1996)
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Band-gap reduction for In(Ga)As

• Two-level repulsion model
• Band-gap energy reduction decreases for higher In
  composition
• 180meV each 1 for GaAs
• 25meV each 1 for InAs

InGaAs bandgap energy shift with different N
- J. Y. Duboz et al., Appl. Phys. Lett., 66,
085313 (2002)
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InGaAsN/GaAs QWs

• Active region material InGaAsN/GaAs QWs

7
Longer wavelength applications

• 1.55 ?m lasers ? introducing Sb to InGaAs/GaAs
  QWs
• Sb acts as surfactant
• Improves material quality
• Enhances N incorporation
• Extend emission wavelength
• Lasers emitting at 1.5 ?m have been reported (a)
• Mid-infrared lasers ? highly strained
  InAsN/InGaAs QWs on InP
• Using matured InP growth and device processing
• Laser operating at 2.38?m at RT has been
  demonstrated (b)
• Longer wavelength application ? InAsN/InAs

a) J. Harmand et al, Appl. Phys. Lett., 77, 2482
(2000) b) D. K. Shih , Electronics Lett., Vol.
37, 1342(2001)
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InAsN ? promising material for mid-infrared

• Big bowing effect for N incorporation to InAs,
  reduces band gap energy
• BAC 40 meV each
• Tight-binding 20 meV each
• Lattice constant is reduced
• pushes emission wavelength to mid-infrared region
• QWell critical thickness can be increased
• Potential applications for mid-infrared
  photodetectors or LED and lasers

Bandgap energy with different N
- M. Kuroda et al., 278, 254(2005)
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InAsN/InAs dilute nitride photodetectors

• InAs/InAsN/InAs pin photodetectors are to be
  fabricated to investigate performance at 4.6 µm

The variation of critical thickness and response
wavelength on N composition at 300 K
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Growths of InAsN/InAs dilute nitride

• RF-plasma assisted MBE
• Growth temperature 400 450oC
• Growth rate 1 µm/hr
• Varying N plasma flux to vary N composition
• Annealing is needed to improve the material
  optical and structural properties
• Investigation of structural properties and
  optical properties of InAsN grown on InAs
• Bulk InAsN with varying N composition ? grown at
  Sheffield
• HRXRD measurement
• PL characterization

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HRXRD simulation (M2774)

• (004) X-ray measurement on InAsN/InAs bulk
  material
• Narrow epitaxial peak indicates good crystalline
  material
• Simulation based Vegards law gives N composition
  of 0.06

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InAsN epilayers with different N

• Second peak is from epitaxial InAsN
• Broaden epitaxial peak for higher N composition
• N composition assume all N go to substitutional
  sites
• - M227 0.06
• - M2775 0.48
• - M2806 0.98

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PL spectra from epilayers of InAsN
4K PL

• Longer emission wavelength for higher N
• 3.46 µm wavelength is demonstrated for M2806
• Emission efficiency decreases for higher N
• Severe degradation for
• un-optimized high N alloy

2.99
3.29
3.46
M2774 0.06, M2775 0.48, M2806 0.98
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PL emission features for M2775
4 K PL emission at various exciting power -
Blue-shift with higher exciting power
PL emission at different temperatures -
Blue-shift with increasing temperature
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N localization states M2775

• Deviation from Varshni behaviour
• Is attributed to the localization states caused
  by compositional fluctuation
• At very low T, the excitons are trapped in the
  deepest localization states
• With increasing temperature, the trapped excitons
  with enough thermal energy can overcome the
  barrier to occupy higher energy sates ?
  blue-shift
• Derived N 0.53 BAC (or 1.06, Tight-binding
  model)

PL peak energy variation with temperatures
- Varshni parameters of ? ? are 2.2?10-4 64
for InAsN, - M. Osinski, Optoelectronics Review,
11, 321(2003)
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N composition determination

• N determination M2775
• From PL measurements 0.53 (BAC), (1.06,
  Tight-binding)
• From X-ray measurement 0.48
• Disagreement between these two measurements
• Possible reasons
• PL measurement, emission originates from valence
  band to localization states deduce N by taking
  energy at 100K
• Existing of different N configurations N
  interstitial deviates Vegards law

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Effect of N-defects on strain - theory

• The lattice strain caused by the substitutional
  NAs and N-As complex are given by equation 1 and
  2 respectively

- NuoFu Chen et al, Phys. Rev. B 54, 8516 (1996)
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Effects of N configurations to lattice constant

• Mismatch caused by substitutional NAs, (red),
  calculated with equ. 1
• Mismatch due to As-N split interstitial, (black),
  calculated with equ. 2
• Mismatch based on NAs and Vegards law (green)

The existing of N interstitial compensates strain
and consequently presents lower N composition by
X-ray simulation
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(115) HRXRD simulation M2774

• (004) simulation result doesnt fit (115)
  measurement
• N from (004) simulation 0.06
• N from (115) simulation 0.27
• Possible reason existing of different N
  configurations, particularly the interstitial
  As-N

(115) X-ray measurement simulation for M2774
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Conclusions

• InAsN with N composition of 0.98 was grown on
  InAs by plasma-assisted MBE technology
• PL emission at 3.46 ?m has been obtained
• PL characterizations exhibits strong N
  localization states in the materials
• Tow different N configurations could exist
• NAs substitutional and As-N split interstitial
• X-ray measurement is difficult to give accurate N
  composition due to the existing of different N
  configurations