My Research Work

I am a Ph.D student working with Dr. Debdeep Jena. My current research work is focused on InN growth by MBE and its characterization including: XRD, TEM, photoluminescence, and transport studies. I also work on the optical characterization of hot-phonon effects in nitrides.

1. InN: growth, characterization and related devices

The III-Nitride semiconductors (AlN, GaN, and InN) and their alloys have been receiving considerable attention since mid 1990s because they are suitable for optoelectronic devices in UV and visible region, and high power, high speed electronic devices (GaN applications). InN, the least studied semiconductor in nitride family, has attracted much interest in recent years. Recent research studies show that InN has a narrow bandgap energy ~0.65 eV, which is much smaller than the previously reported value of 1.9 eV. As a result, InN and its alloys are suitable materials for opto-electronic devices working at IR region. Since band gap energies of the nitride semiconductor family (AlN, GaN, and InN) cover the whole solar spectrum, they are also considered as possible candidates for high efficiency multijunction solar cells.Furthermore, the small electron effective mass, high room temperature electron mobility, and large electron saturation and overshoot velocities make InN an attractive material for high speed electronic devices.

At University of Notre Dame, we have used a Veeco Gen 930 MBE system to grow nitride semiconductors. Structural characterization including in-situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM) are used to characterize the structural quality of InN. The Hall effect measurement is used to determine carrier concentrations and mobilities. A theoretical model of scattering mechanisms in InN is studied. The optical band gap energy of InN is measured to be 0.67 eV using absorption spectroscopy and photoluminescence spectroscopy. Isotype InN-GaN heterojunction Schottky diodes have been fabricated by growing InN on n-type GaN. Current-voltage, capacitance-voltage, and photocurrent spectroscopy measurements have been performed. The conduction band offset between InN and GaN has been determined by these measurements.

2. Hot-phonon effects in Nitrides characterized by optical measurements

At the heterojuction formed by nitride semiconductors, a high density 2DEG could be formed due to the contribution of spontaneous and piezoelectric polarization. This property makes GaN as a prime candidate for high power microwave applications. The drift velocity of electrons in GaN has been observed in some report to be considerably lower than predicted by Monte Carlo simulations. It has been proposed that the accumulation of nonequilibrium phonons so-called “hot phonons” is responsible for the lowering of the drift velocity.

In our study, we have directly measured electron temperatures in a doped GaN layer at low to moderate electric fields (with in 1kV/cm). We extract the hot-electron temperatures from the high-energy tails of the photoluminescence spectra for this purpose. A strong evidence of the existence of hot-phonons by comparing the measured field-induced hot electron temperatures with theoretical models for energy loss mechanisms is found and a hot-phonon lifetime of 3 to 4 ps is extracted. To further expand the capability of this measurement, a micro-photoluminescence setup is built.

Band offset at the InN/GaN heterojunction

Albert (Kejia) Wang, Chuanxin Lian, Ning Su, Debdeep Jena, and John Timler
submitted to Appl. Phys. Lett.

Structural and transport properties of InN grown on GaN by MBE

Albert (Kejia) Wang, Thomas Kosel, and Debdeep Jena
submitted to physica status solidi

Resonant terahertz generation from InN thin films

Xiaodong Mu, Yujie J. Ding, Kejia Wang, Debdeep Jena, and Yuliya B. Zotova
Opt. Lett. 32, 1423 (2007)

Hot phonons in Si-doped GaN

Juozapas Liberis, Mindaugas Ramonas, Olegas Kiprijanovic, Niti Goel , John Simon, Kejia Wang, Huili (Grace) Xing, and Debdeep Jena

Appl. Phys. Lett., 89, 202117 (2006)

Effect of dislocation scattering on the transport properties of InN grown on GaN substrate by Molecular Beam Epitaxy

Albert (Kejia) Wang, Yu Cao, John Simon, Jing Zhang, Alexander Mintairov, James Merz, Douglas Hall, Thomas Kosel, and Debdeep Jena
Appl. Phys. Lett., 89, 162110, 2006

Polarization-induced 3-dimensional electron slabs in graded AlGaN layers

John Simon, Albert (Kejia) Wang, Huili Xing, Debdeep Jena, and Siddharth Rajan

Materials Research Society Symposium Proceedings, v 892, 2006, p 417-42

Carrier transport and confinement in polarization-induced 3-d electron slabs: Importance of alloy scattering

John Simon, Albert (Kejia) Wang, Huili Xing, Siddharth Rajan, and Debdeep Jena

Appl. Phys. Lett., 88, 042109, 2006.

Optical study of hot-electron transport in GaN: Signatures of the hot-phonon effect

Kejia Wang, John Simon, Niti Goel, and Debdeep Jena

Appl. Phys. Lett., 88, 022103, 2006.

"Structual and transport properties of InN grown on GaN by MBE"
Kejia (Albert) Wang and Debdeep Jena
7th Int'l Conference of Nitride Semiconductors (ICNS-7) Conference, Sept 16-21, MGM Grand Hotel, Las Vegas, NV, USA, 2007

"Conduction band offset and Schottky barrier formation at InN/GaN heterojunctions"
Kejia (Albert) Wang, Chuanxin Lian, Ning Su, John Timler, and Debdeep Jena
49th Electronic Materials Conference (EMC), June 2007 20-22, Notre Dame, IN, USA, 2007

"Structural, electrical and optical properties of InN grown on GaN substrate by molecular beam epitaxy"

Kejia (Albert) Wang, Yu Cao, John Simon, Jing Zhang, Kai Sun, Tom Kosel, Alexander Mintairov, James Merz, Douglas Hall, and Debdeep Jena

48th Electronic Materials Conference (EMC), June 28-30, Pennsylvania State University, Pennsylvania, USA , 2006.

Related conference talks and posters

"High-mobility ultrashallow pseudomorphic AlN/GaN heterojunctions by MBE"

Yu Cao, David Deen,Kejia Wang, Debdeep Jena
Electronic Materials Conference (EMC) 2007, Notre Dame, (June 2007)

"Observation of strong many-body effects in thin InN layers grown on GaN"
X. Mu, Y. J. Ding, Kejia Wang, Debdeep Jena, and J. Khurgin
Lasers and Electro-Optic Society (LEOS Annual meeting) 2006, Montreal, (Nov 2006)

"Polarization-induced 3-Dimensional slabs in Graded AlGaN layers"

John Simon, Siddharth Rajan, Kejia Wang, Huili Xing and Debdeep Jena,

Materials Research Society (MRS) Fall Meeting, Boston USA, 2005.

"PL study of Electric-Field induced Hot Carriers in an n-GaN MESFET: observation of hot phonon effect"

Albert (Kejia) Wang, John Simon, Niti Goel, and Debdeep Jena,

14th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS-14) Chicago, July 24-29, 2005 .

"Design and Fabrication of Photodetectors Based on III-V Compound Semiconductors"

Wang Kejia, Su Ning, Teo Siew Lang, Akkipeddi Ramam, Dong Jianrong, Chua Soo Jin,

International Conference on Materials for Advanced Technologies (ICMAT) Singapore, 2003