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 Electronic Materials Conference Santa Barbara, CA June 27, 2002

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

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.)

Pb 1-x Sn x Se IV-VI Laser Materials PbSrSe p-type PbSe Substrate ~~ ~ PbSe n-type PbSrSe n-type Double Heterostructure Laser

Breath Analysis with IV-VI Lasers Heat Sink IV-VI Laser

Asthma Diagnosis 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. Laser Focus World, June 2002, P. 22 Roller et al., Optics Letters 27, 107 (2002).

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.

MBE Growth on Silicon and BaF 2  SiO 2 desorption at 700°C allows epitaxial growth of nearly lattice-matched CaF 2 on Si  CaF 2 growth on Si is layer-by-layer  BaF 2 growth on CaF 2 is layer-by-layer  PbSrSe growth on low surface energy BaF 2 is initially 3D (island)  PbSrSe layer eventually becomes 2D after growth of more than 1 µm IV-VI MBE Chamber at OU Sources: PbSe, Sr, Se, PbTe, BaF 2, CaF 2, Ag, Bi 2 Se 3 In Situ RHEED Si(111) (7  7) after oxide desorption After growth of 2 nm CaF 2 After growth of 600 nm BaF 2 BaF 2 (111) substrate (1  1) at 500 °C After growth of 6 Å of PbSrSe on BaF 2 After growth of 3 µm of PbSrSe on BaF 2

PbSe/PbSrSe MQWs 4 nm to 100 nm Si Substrate BaF 2 Substrate HRXRD  MQWs on Si have high crystalline quality  MQWs on BaF 2 substrates have higher crystalline quality due to better thermal expansion match

Photoluminescence BaF 2 Substrates Near-IR (~980 nm) cw diode laser pumping (low intensity, ~250 mW)  Strong Quantum Size Effect  Strong CW Emission at 55°C  Interference Fringes Dominate Spectra –Spacings depend on index of refraction and epilayer thickness –Strong optical resonance indicates stimulated emission processes

Mid-IR Emitter on Silicon Si Substrate Near-IR (~980 nm) cw diode laser (~250 mW) Emission through Silicon Substrate – Promising optical interconnect architecture IV-VI MQW Si Substrate

InGaAs (972 nm) diode laser pump current BaF 2 Substrate Silicon Substrates Less epilayer heating with higher thermal conductivity silicon substrates Optical Heating of Epilayers H. Z. Wu et al., J. Vac. Sci. and Technol. B 19, 1447 (2001).

20.6 nm IR Transmission 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). 4 nm to 100 nm L QW =20.6 nm

Quantum Size Effects

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 Normal Oblique m N e = m O e = m N h = m O h = NormalOblique (3-Fold Degenerate)

Interband Transitions (1-1) N (1-1) O Eg (PbSe) = 150 meV (4K) (1-1) O

Energy Levels Normal Oblique

PL Emission Oblique Valleys Lowest energy level has a low density of states – Lower threshold for population inversion – Stimulated emission at low excitation rates – Four-level laser design Density of States 

Lasing Thresholds IV-VI Mid-IR VCSELs Bulk Active Region –Optical pumping threshold: 69 kW/cm 2 Z. Shi et al., Appl. Phys. Lett., 76, 3688 (2000) MQW Active Region –Optical pumping threshold: 10.5 kW/cm 2 C. L. Felix et al., Appl. Phys. Lett. 78, 3770 (2001)

Parabolic MQWs Expect Evenly-Spaced Harmonic Oscillator Eigenvalues

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: D d = 6.1 eV and D u = -1.3 eV. Energies for the higher confined states in 100 nm sample allows determination of band non-parabolicity parameters:  c =  v = 1.9  cm 2 Equally Spaced Energy Levels (Harmonic Oscillator)

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.