Growth of InN Films by Cluster Beam Epitaxy and RF Plasma-assisted MBE

1
Growth of InN Films byCluster Beam Epitaxy and
RF Plasma-assisted MBE

• T.C. Chen, C. Thomidis, J. Abell, T. Xu and
  Theodore D. Moustakas
• Department of Electrical and Computer Engineering
• Boston University
• Support AFOSR MURI (Monitored by Dr. T.
  Steiner)
• ONR (Monitored by Dr. C. Wood)

AFOSR InN Workshop 2 Kailua-Kona, Hawaii 9-13
January, 2005
2
Outline

• Discussion of the Cluster Beam Epitaxy method
• - Formation and characterization of
  nitrogen clusters
• Growth of InN films by RF plasma MBE
• - Nucleation
• - Film adherence
• - Formation of QDs
• Growth of InN films by the Cluster Beam Epitaxy
  method
• - Nucleation
• -Structure
• -Film adherence
• Physical Properties
• -Transport
• - Optical absorption
• - Electron effective mass
• Conclusion

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Methods used for the growth of InN

• RF plasma MBE
• Gas-cluster beam epitaxy

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What is Cluster Beam Epitaxy?

• Neutral clusters of several thousand
  atoms/molecules are formed by supersonic
  expansion through a small nozzle at high
  stagnation pressure.
• The clusters are singly or doubly ionized by
  electron impact
• High mass to charge (M/Q) ratio
  (potential for high growth rate).
• Controlled energy is added to the ionized
  clusters by use of acceleration potentials.
• The energy of the individual atoms is low even
  if the total energy of the cluster is high.
• Control of the energy of impinging flux atoms
  on the substrate can enhance
• adatom migration and dopant
  incorporation.
• The ionized clusters deposit high energy density
  into a small volume of the target material
• High chemical reactivity effects (Chemistry at
  104 to 105 K).
• Deposition of films at low temperatures .
• Non-linear sputtering and implantation
  effects.

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Interaction of Ionized Clusters with a Target
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Difference between impacts of cluster and monomer
ions
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N2 Cluster Distribution determined by
time-of-flight measurements

• Nitrogen clusters with average size of 2000
  molecules have been formed (stagnation
  pressuregt14 Atm)
• Clusters with 2000 molecules, accelerated at
  20kV, disintegrate into molecules with energy of
  10 eV,
• which is sufficient to break them into
  nitrogen atoms (required energy 9.5eV).
• - Use a magnetic filter to remove light
  clusters, which produce very energetic molecules

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Growth of InN films by RF-plasma MBE

• Substrate (0001) Sapphire
• Nitridation ( at 250 C to 550 C)
• Nucleation
• - An InN buffer grown at 3000 C
• A nitrogen polar GaN-template grown at 7500 C
• InN Films
• - Growth temperature 550-560C
• - Growth rate 1.2 mm/h

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AFM image of (0001) sapphire after nitridation at
250 C with the RF-plasma source

• RF power 400 W
• Substrate scratches due to
• polishing were not affected
• by the nitridation process

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Initial growth of InN films on (000-1) GaN
templates
1mm
These data suggest that threading dislocations
should occur primarily at domain boundaries
11
SEM images of GaN and InN films grown
sequentiallyusing the RF-Plasma Source
GaN
InN

• Films tend to delaminate when they are more
  than 23 mm thick due to their compressive
  stress.
• - Delamination occurs upon exposure to the
  atmosphere
• - The delaminated films are very strong and
  can be used for the characterization of the
• properties of stress free InN films

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Structure of InN Films nucleated with a low
temperature InN-buffer
RHEED of InN Buffer
RHEED of InN Film

• Both RHEED and XRD indicate that the films are
  single
• crystals.

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SEM images of an InN film grown on LT InN buffer

• Films as thick as 6mm have been grown on a InN
  buffer
• However, these films tend to delaminate during
  growth

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InN Quantum Dots/Islands
(50 s)

• Dot Density 2 x 109 dots/cm2
• Mean Height 15 nm
• Mean Diameter 115 nm

10 mm x 10 mm AFM Height Image
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Growth of InN films by Cluster Beam Epitaxy

• Substrate (0001) Sapphire
• Nitridation ( at 250 C)
• - Scratches due to mechanical polishing are
  removed (Substrate smoothing)
• - Substrate surface morphology depends
  strongly on cluster acceleration voltage
• Nucleation
• - An InN buffer grown at 3000 C
• InN Films
• - Growth temperature 550-560C

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Growth of InN films with the N2-Cluster Source

• This method has the potential to address the
  issue of delamination of the films from the
  substrate due to the energetic nature of the
  clusters
• InN films using only the cluster source
• InN films using both sources together
• The nucleation steps were done with the
  cluster source and the InN film
• was grown with RF plasma source

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AFM images of sapphire substrates before and
after nitridation using nitrogen clusters of
various energies.

• The bare substrates have
• scratches due to mechanical
• polishing.
• The nitrogen clusters remove
• the scratches.
• - The smoother surface
• morphology is obtained at
• 15 KV acceleration voltage

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XRD data of a thin InN film grown by the Cluster
Beam Epitaxy method
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SEM images of an InN film grown with both
nitrogen sources

• The cluster source was used for the nitridation
  and InN buffer steps.
• The RF plasma source was used to grow the high
  temperature InN film
• These films adhere well to the substrate.

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Transport data of InN Films

• The grown InN films are auto-doped n-type with
  carrier concentration higher than 3x1018/cm3.
• The best RT electron mobility obtained is 1130
  cm2/V.s

21
Optical absorption constant of a 900nm thick InN
film

• The energy gap of InN films, determined by
  transmission measurements, was found to be 0.75eV

22
IR Reflectivity data of InN Films(determination
of TO Phonon and electron effective mass)
Sample Name N (cm-3) eh TO Phonon Frequency (cm-1) Plasma Frequency (cm-1) m
E104 Free standing 7.20E18 9 477 891 0.091
E107 2.26E19 9 470 1400 0.115
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Effective Mass vs. Energy Gap for Direct Band-gap
Semiconductors
k.p method
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Conclusions

• InN films were grown in the same MBE system using
  either an RF Plasma source or a Cluster Source
  for the activation of nitrogen
• The films grown by the plasma source tend to
  delaminate as they become thicker.
• XRD of the films grown by the cluster source
  have shown, in addition to the (0002)
• diffraction, an additional small peak
  attributed to (10-11) diffraction of InN.
• Hybrid films in which the nucleation layers
  were deposited with the cluster source
• and the rest of the film with the plasma
  source are the most promising.
• The films are auto-doped n-type with carrier
  concentration larger than
• 3x1018 cm-3. The best RT electron
  mobility is 1130 cm2/vs
• Transmission measurements show the energy gap of
  0.75eV
• The electron effective mass was determined by IR
  reflectivity measurements to be 0.09m0. The
  result is in qualitative agreement with the
  predictions of the k.p method..