Growth and Characterization of IV-VI Semiconductor Multiple Quantum Well Structures

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Growth and Characterization of IV-VI
Semiconductor Multiple Quantum Well Structures
 Patrick J. McCann, Huizhen Wu, and Ning
Dai School of Electrical and Computer
Engineering Department of Physics and
Astronomy University of Oklahoma Norman, OK
73019 Electronic Materials Conference Santa
Barbara, CA June 27, 2002
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Outline

• IV-VI Semiconductors
• Biomedical Applications
• MBE Growth and Characterization
• Square and Parabolic MQWs
• Summary

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IV-VI Semiconductors (Pb-Salts)

• Unique Features
• High Dielectric Constants ? Defect Screening
• Can be Grown on Silicon ? Low Cost, Integration
  Possibilities
• Symmetric Band Structure ? High Electron and
  Hole Mobilities
• Applications
• Thermoelectric Coolers (Low Lattice Thermal
  Conductivity)
• Infrared Detectors (Silicon Integration
  Possible)
• Spintronics (Quantum Dots with Magnetic
  Impurities)
• Tunable Mid-IR Lasers (Medical Diagnostics, etc.)

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IV-VI Laser Materials
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Breath Analysis with IV-VI Lasers
IV-VI Laser
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Asthma Diagnosis
Laser Focus World, June 2002, P. 22 Roller et
al., Optics Letters 27, 107 (2002).

• High exhaled NO indicates airway inflammation.
• People with asthma suffer from chronic airway
  inflammation.
• Quantum cascade mid-IR lasers have not been able
  to do such measurements even though several
  attempts have been made.

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IV-VI Epitaxial Layers

• High quality layers can be grown on silicon
• McCann et al., Journal of Crystal Growth 175/176,
  1057 (1997).
• Strecker et al., Journal of Electronic Materials
  26, 444 (1997).
• Room temperature cw photoluminescence
• McCann et al., Applied Physics Letters 75, 3608
  (1999).
• McAlister et al., Journal of Applied Physics 89,
  3514 (2001).
• Optical devices on silicon
• Through-the-substrate inter-chip optical
  interconnects (PC Magazine, January 21, 2002).
• Modulators for free-space optical communication.
• Infrared imaging arrays.

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MBE Growth on Silicon and BaF2
IV-VI MBE Chamber at OU Sources PbSe, Sr, Se,
PbTe, BaF2, CaF2, Ag, Bi2Se3
Si(111) (7?7) after oxide desorption
BaF2 (111) substrate (1?1) at 500 C

• SiO2 desorption at 700C allows epitaxial growth
  of nearly lattice-matched CaF2 on Si
• CaF2 growth on Si is layer-by-layer
• BaF2 growth on CaF2 is layer-by-layer
• PbSrSe growth on low surface energy BaF2 is
  initially 3D (island)
• PbSrSe layer eventually becomes 2D after growth
  of more than 1 µm

After growth of 2 nm CaF2
After growth of 6 Å of PbSrSe on BaF2
After growth of 600 nm BaF2
After growth of 3 µm of PbSrSe on BaF2
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PbSe/PbSrSe MQWs
HRXRD
4 nm to 100 nm

• MQWs on Si have high crystalline quality
• MQWs on BaF2 substrates have higher crystalline
  quality due to better thermal expansion match

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Photoluminescence
Near-IR (980 nm) cw diode laser pumping (low
intensity, 250 mW)

• Strong Quantum Size Effect
• Strong CW Emission at 55C
• Interference Fringes Dominate Spectra
• Spacings depend on index of refraction and
  epilayer thickness
• Strong optical resonance indicates stimulated
  emission processes

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Mid-IR Emitter on Silicon
Si Substrate
IV-VI MQW
Near-IR (980 nm) cw diode laser (250 mW)
Emission through Silicon Substrate Promising
optical interconnect architecture
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Optical Heating of Epilayers
H. Z. Wu et al., J. Vac. Sci. and Technol. B 19,
1447 (2001).
InGaAs (972 nm) diode laser pump current
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IR Transmission
LQW20.6 nm
Differential Transmission Fourier Transform
Infrared Spectroscopy Subtract transmission
spectra collected at two different
temperatures Peaks yield interband transition
energies H. Z. Wu et al., Applied Physics
Letters 78, 2199 (2001).
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Quantum Size Effects
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Removal of L-Valley Degeneracy

• Direct gap is at the L-point in k-space
• Four Equivalent L-valleys
• Symmetric conduction and valence bands
• Potential variation in 111 direction
• One L-valley is normal to the (111) plane in
  k-space
• Three L-valleys are at oblique angles
• Two different effective masses for electrons (and
  holes) in the PbSe MQWs

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Interband Transitions
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Energy Levels
Oblique
Normal
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PL Emission
??
Oblique Valleys
Density of States

• Lowest energy level has a low density of
  states
• Lower threshold for population inversion
• Stimulated emission at low excitation rates
• Four-level laser design

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Lasing Thresholds

• IV-VI Mid-IR VCSELs
• Bulk Active Region
• Optical pumping threshold 69 kW/cm2
• Z. Shi et al., Appl. Phys. Lett., 76, 3688
  (2000)
• MQW Active Region
• Optical pumping threshold 10.5 kW/cm2
• C. L. Felix et al., Appl. Phys. Lett. 78,
  3770 (2001)

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Parabolic MQWs
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Parabolic MQW Analysis

• Measured bandgaps in strained PbSe (caused by
  lattice mismatch with PbSrSe) compared to 77 K
  bandgap for bulk PbSe allows determination of
  deformation potentials Dd 6.1 eV and Du
  -1.3 eV.
• Energies for the higher confined states in 100 nm
  sample allows determination of band
  non-parabolicity parameters ?c ?v
  1.9?10-15 cm2

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Summary

• IV-VI semiconductors are versatile materials for
  a variety of applications.
• A mid-IR laser spectroscopy application for
  asthma diagnosis has been developed.
• PbSe-based MQW structures have attractive
  properties for improved mid-IR laser technology.
• L-valley degeneracy removal.
• Energy level structure in MQWs on (111)-oriented
  substrates enables low population inversion
  thresholds.