WBSL History

The Wide Bandgap Semiconductor Laboratory has had a great history of productivity and innovation. It originally focused on the growth of III-nitrides and the study of its fundamental properties. Many significant discoveries have been made about the basic properties and growth control of III-nitrides. More recently, this lab has moved on to study III-nitride device production and characterization. Although material and growth properties are still being investigated, we are now able to implement our knowledge of III-nitride growth into the creation of devices such as light emitting diodes, lasers, and transistors.

Professor Moustakas started the lab in 1987 with the purchase of the GeNII MBE system. Since its conception, the lab has grown to have obtained an Eiko MBE, two custom-cuilt HVPE systems (horizontal and vertical), a sputtering machine, an electron beam evaporator, 2 wet chemistry benches, an ICP etching system, a Hall Effect machine, an RTA machine, and various optical measurement tools such as lasers, monochromators, and an optical microscope. The lab currently has 11 graduate students, one post-doc, and a lab manager.

Important discoveries made in the Wide Bandgap Semiconductor Laboratory

1987	Nucleation steps of GaN	We discovered nitridation of the sapphire substrate is necessary to reduce the lattice mismatch between sapphire and GaN. By changing the top layer of the sapphire from Al₂O₃ to AlN, a lattice structure more like GaN is created.
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1987	Low temp buffer	A low temperature GaN buffer layer is the next step in creating GaN by MBE. This is necessary to reduce the dislocations in the crystal.
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1991	Cubic, zinc-blende form	We first showed that GaN can also be grown with a zinc-blende structure.
		Related paper : "Epitaxial Growth of Zincblende and Wurtzitic Gallium Nitride Thin Films on (001) Silicon," T. Lei et. al. Appl. Phys. Lett., 59, 944 (1991).

1993	P-type GaN, no post-growth processing	We demonstrated the first highly doped p-type GaN without post-processing. Most highly doped GaN requires post-growth processing such as an anneal, theorized to release hydrogen, thus activating more magnesium dopants. However, by MBE, we demonstrated a carrier concentration of 6e18cm^-3 without post-processing.
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1994	1^st Blue LED by MBE	We demonstrated the 1^st Blue LED grown by Molecular Beam Epitaxy. Our lab continues to show that MBE is a competitive, viable, option for nitride-based device growth.
		Related paper : "Blue-violet Light Emitting Gallium Nitride p-n Junctions Grown by Electron Cyclotron Resonance-Assisted Molecular Beam Epitaxy," R.J. Molnar, R. Singh, and T.D. Moustakas, Appl. Phys. Lett., 66, 268 (1995).

1995	Metal contacts to GaN	Our lab took a large step in developing how to make electrical contacts to GaN. We showed that the work function of the metals involved in the contact plays a role in determining the semiconductor-metal barrier height. This is due to the ionic nature of GaN.
		Related paper : "Metal Contacts to Gallium Nitride," J.S. Foresi and T.D. Moustakas, Appl. Phys. Lett., 62, 2859 (1993).

1996	InGaN Phase separation	We showed phase separation in InGaN. We preposed this is due to spinoidal decomposition. Due to the large mismatch between In and Ga(11%), there is a natural tendency for InN or GaN to agglomerate to relieve stress.
		Related paper : "Phase Separation in InGaN Thick Films and Formation of InGaN/GaN Double Heterostructures in the Entire Alloy Composition," R. Singh and T.D. Moustakas, Appl. Phys. Lett. 70, 1089 (1997).

1997	Ordering in AlGaN alloys	We first showed ordering in AlGaN alloys.
		Related paper : "Atomic Ordering and Phase Separation in AlGaN Alloys." T.D. Moustakas, R. Singh, D. Korakakis, D. Doppalapudi, H.M. Ng, A. Sampath, E. Iliopoulos and M. Misra. (INVITED) Mat. Res. Soc. Symp. Proc. 482,193 (1998)

1998	Ordering in InGaN alloys	We first showed ordering in InGaN alloys.
		Related paper : "Phase Separation and Ordering in InGaN Alloys”, D. Doppalapudi, S.N. Basu and T.D. Moustakas Mat. Res. Soc. Symp. Vol. 512, p.431 (1998).

2000	1^st e-beam pumped VCSEL	We demonstrated the first Vertical Cavity Surface Emitting Laser(VCSEL). This was accomplished by using a high reflectivity AlGaN/GaN DBR below a GaN/InGaN active region. The top of the cavity was made by a Ag layer, through which the laser was e-beam pumped.
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2000	Complex Ordering in AlGaN	We showed complex ordering in AlGaN alloys.
		Related paper : "Complex Ordering in Ternary Wurtzite Nitride Alloys” E.Iliopoulos, K.F. Ludwig, Jr, and T.D. Moustakas, Journal of Physics and Chemistry of Solids 64, 1525 (2003)

2002	Kinetics of AlGaN growth	We investigated the kinetics of growth of AlGaN alloys.
		Related paper : "Growth Kinetics of AlGaN Films by Plasma Assisted Molecular Beam Epitaxy” E. Iliopoulos, T.D. Moustakas Appl. Phys. Lett. 81, 295 (2002).

2003	1^st GaN-based Permeable-Base Transistor (PBT)	We demonstrated the first permeable-base transistor (also called static-induction transistor) in the GaN material system.
		Related paper : "Design and Fabrication of GaN-based Permeable-base Transistor” J. S. Cabalu, L. Gunter, I. Friel, A. Bhattacharyya, Y. Fedyunin, K. Chu, E. Bellotti, C. Eddy, and T. D. Moustakas Mat. Res. Soc. Symp. Proc. 798, 85 (2004).

2004	Efficient p-doping by PA-MBE	We learned how to efficiently dope GaN p-type by plasma assisted MBE.
		Related paper : "Efficient P-Type Doping of GaN films by Plasma-assisted Molecular Beam Epitaxy” A. Bhattacharrya, W. Li, J. Cabalu, T. D. Moustakas, D. J. Smith, and R. L. Hervig Appl. Phys. Lett. 85, 4956 (2004).

2005	1^st GaN/AlGaN MQW optical modulator	We demonstrated the first optical modulator based on GaN/AlGaN multiple quantum wells.
		Related paper : “Ultraviolet Electroabsorption Modulator based on AlGaN/GaN Multiple Quantum Wells” I. Friel, C. Thomidis, T. D. Moustakas, J. of Appl. Phys.97 2005.