Research and Academics

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Research Interests

Experimental work

MBE growth, characterization, and fabrication of III-V Nitride heterostructures and nanostructure based devices
Charge, heat, and spin transport in bulk and nanostructured semiconductors
Hot-electron and hot-phonon effects in semiconductor nanostructures
Applications to electronic and optical devices
Graphene, Nanowire and Nanocrystal physics and device applications

Theoretical work

Physics of charge, heat, and spin transport in bulk and nanostructured semiconductors
Basic semiconductor physics, electron-phonon coupling in nanostructures, hot-phonon effects
Effect of Dielectric Mismatch on charge transport in semiconductor nanostructures
Electronic and optical properties of nanostructured materials (graphene, nanomembranes, nanowires, nanocrystals, and quantum-mechanical behavior)

In our group, the experimental and theoretical work is intertwined and drives each other.

Here are the links to our group:

Semiconductor data

I found this on the web: A small reminder to anyone who spends too much time thinking about semiconductors. (Note that this is a LINK to someone else’s page!).

Current Projects

The focus of our current research is in three areas –

1) Physics of MBE growth and charge transport in III-V Nitride semiconductor heterostructures, and their applications to solid-state devices such as High Electron Mobility Transistors (HEMTs), Photodetectors, and Photovoltaics.

2) Charge transport and optical properties of bottom-up grown semiconductor quantum wires and quantum wire-solids, and using their unique properties in fabricating novel devices.

3) Physics of charge transport in Graphene and its device applications.

If physics turns you on, but you would like to see the beautiful laws of physics applied to make useful semiconductor devices, our research group will offer you the right environment for your interests. Please read the following three sections for more details, and click here if you want to see our publications and here for conference presentations on these topics.

a) Molecular Beam Epitaxy growth of III-V Nitride Heterostructures & their Device Applications

The III-V nitride semiconductor family (InN, GaN, AlN & their alloys) has ushered in a new revolution in solid-state devices. Since the mid-90s when researchers figured out how to grow these semiconducting materials and dope them controllably, the range of applications of these wide-bandgap semiconductors has expanded at a rate unmatched by any other semiconducting material. Within the short span of a decade, the so-called ‘infant’ in the semiconductor family has emerged as a giant – both in semiconductor physics and in technological applications creating and driving markets. Refer to this link for a roadmap of where GaN-based semiconductors are headed in the next few years. In short, there is no other semiconductor currently of comparable stature that can deliver short-wavelength optical emitters and detectors (blue, UV, deep UV) for lighting, optical data storage, and similar applications. Since the nitrides cover the entire visible spectrum, they are ideally suited for white-light sources, and are expected to lead to all-semiconductor based Solid State Lighting. The large bandgap offers a very large breakdown voltage, and coupled with the large electron mobilities in 2DEGs formed at AlGaN/GaN heterojuctions, the nitrides are very attractive for RF power amplifiers. Furthermore, the ability to withstand large temperatures and harmful radiation makes it very attractive for digital applications in harsh environments as well. Thus, in addition to optical emitters and detectors, the nitrides have tremendous potential in electronic devices and sensors as well. Refer to this link for a nice figure summarizing various applications of the III-V nitride semiconductors. Victor Hugo had said that nothing is more powerful than an idea whose time has come. Making nitride based devices is such an idea, whose time has not only come, but is here to stay for a long time.

In our research group, we focus on the MBE growth of III-V nitrides, electronic and optical characterization, and we fabricate devices using the materials. MBE allows for the growth of GaN, InN, and AlN with atomic layer precision, and gives us the freedom to create heterostructures – quantum wells, superlattices, and quantum dots. MBE is a tool complementary to MOCVD; in our opinion MBE is well suited for research whereas MOCVD is currently preferred for production. One of the current projects is the growth of AlGaN/GaN, and AlN/GaN heterostructures for HEMT and HBT applications. We study the charge transport and the effect of defects on the electronic and optical properties of the MBE grown materials, and correlate the physical properties with the performance of the devices.

We have demonstrated a novel technique of doping of the III-V nitrides. Owing to the large spontaneous and piezoelectric polarization in the nitrides, graded alloy layers can be doped without impurities. This novel technique of polarization-induced doping has been used to demonstrate 3D electron slabs in AlGaN layers, and we have also demonstrated Polarization-Doped Field-Effect Transistors (PolFETs). We are actively investigating the feasibility of this doping technique for generating mobile holes, which would be a natural precursor to HBTs and many optical device applications. It is important to note that very high Al-containing nitrides (with large bandgaps) are very resistant to traditional doping techniques. We are investigating polarization doping as a possible solution to those problems.

Finally, the combination of AlN and GaN in heterostructures can enable a rich range of as-yet unrealized device technologies ranging from very high-performance HEMTs (with very high current densities and ultrafast performance), ultrafast (long-wavelength) optical switches, as well as transparent, high-sensitivity biosensors. Therefore, we are spending a substantial effort in perfecting the growth of these structures, characterizing them, and building devices out of them.

b) Hot-Phonon effects in III-V Nitride HEMTs

Since a Ga-N bond tends to be much more polar than a Ga-As or In-P bond, GaN and its alloys have much higher piezoelectric polarization than the other III-Vs. In addition, the wurtzite crystal structure of the III-V nitrides is the reason for the presence of a very large spontaneous polarization in the crystal as well. The highly polar nature of the crystal has many remarkable consequences, and makes much of the physics of III-V nitride unlike all other semiconductors.

One of the first surprises with the nitrides was the formation of a very high density 2D electron gas without the need for modulation doping. The physics of this was traced down to the difference in polarization between the pseudomorphically strained AlGaN layer grown on the GaN layer. The 2DEG now forms the heart of a GaN HEMT, and is being investigated thoroughly in terms of its transport, gate modulation, quantum confinement, and various other properties so crucial to electronic devices.

Under the application of very high electric fields (for example in the drain-source bias of a HEMT), a very large amount of energy is coupled into the HEMT channel by the 2D electron gas from the battery. A large fraction of this power has to be converted into heat and ultimately extracted from the device. Due to the extremely large electron-LO phonon coupling in the III-V nitrides, all the heat is initially dumped into the optical phonon modes near the drain end of the gate in a HEMT. However, since optical phonons have negligible group velocity, they have to decay into acoustic modes to dissipate the heat. Whereas the generation of LO phonons occur in the 10s of fs timescales, the decay into acoustic modes occurs in ~ps timescales. This leads to the unwanted accumulation of a large density of non-equilibrium optical phonons right in the path of electron motion, and can severely degrade the electron saturation velocity. This effect is known as the hot-phonon effect. Similar effects should happen in other III-Vs, but in GaAs and other compound semiconductors, it is much, much weaker than in the nitrides.

Therefore, the investigation of hot-phonon effects in III-V nitrides, possible mechanisms of degradation limiting the performance of high-power high-speed RF amplifiers is an interesting and modern topic. Once most of the parasitic resistances and capacitances in III-V nitride HEMTs have been engineered, the hot-phonon effect will pose the show-stopper in achieving high power amplification at true RF frequencies (100s of GHz). In our group, we are using various experimental tools to probe the amount of degradation of HEMTs brought about by hot phonons, and we are also designing heterostructures that provide solutions to this bottleneck. This part of the work is performed in collaboration with Dr. Xing’s group who specialize in ultra-scaled HEMT fabrication and characterization.

c) InN – MBE growth and Device Applications

In 2002, researchers at the Ioffe Institute (St. Petersburg, Russia) made the surprising discovery that the fundamental bandgap of InN is ~0.7eV and not 1.9eV as believed previously. This discovery has opened up a whole new range of applications for the III-V nitride semiconductors, that were previously thought to be beyond the reach of the nitride family. For example, the bandgap of the III-V nitrides ranges from IR (InN, 0.68eV) through visible and UV (GaN, 3.4eV) to deep-UV (AlN, 6.2eV), and all these materials and their alloys are direct-bandgap! Thus, they are well suited not only for optical emitters spanning the entire visible spectrum, they can also be used for the absorption of photons over a large wavelength range. InN, when alloyed with GaN can cover the entire solar spectrum and therefore may be attractive for solar cell applications.

However, the growth of high quality InN poses a significant challenge since it is lattice-mismatched to GaN, as well as the traditional substrates used for nitride growth such as sapphire and SiC. In addition, it is also lattice mismatched with Si and GaAs, the larger lattice constant semiconductors. Therefore, to exploit its potential in photovoltaics, one has to figure out ways to grow high quality InN with high mobilities of carriers and long minority carrier lifetimes, which are inextricably linked with defects and scattering. Furthermore, no one has convincingly demonstrated p-type doping of InN, a precursor to many photovoltaic applications.

InN, by virtue of its small bandgap, also boasts the smallest electron effective mass and highest carrier mobilities among the III-V nitrides. Therefore, it is an exciting material for high-speed electronic devices as well. In our group, we are studying the growth of InN on various substrates, and are working towards demonstrating some proof-of-concept devices with the novel material. We are investigating the effect of crystal growth conditions on the fundamental electron transport properties such as electron mobilities and velocity-field characteristics, electron-phonon interactions in InN, and ways to extract various fundamental physical properties of the material.

d) Nitride Nanostructures by MBE

Recently, we have been able to grow III-V Nitride nanowires on Silicon by MBE, and are pursuing device applications of these nanostructures. Check back later for updates!

Representative publications related to III-V Nitride Research:

12) Hot phonon effect on electron velocity saturation in GaN: A second look

(J. Khurgin, Y. Ding, & D. Jena

Appl. Phys. Lett., 91, 252104, 2007)

11) Conduction band offset at the InN/GaN heterojunction

(K. Wang, C. Lian, N. Su, D. Jena, & J. Timler

Appl. Phys. Lett., 91, 232117, 2007)

10) MBE-grown Ultra-Shallow AlN/GaN HFET Technology

(H. Xing, D. Deen, Y. Cao, T. Zimmerman, P. Fay, & D. Jena

ECS Transactions, 11, 233, 2007)

9) A High-Mobility Window for Two-Dimensional Electron Gases at Ultrathin AlN/GaN Heterojunctions

(Y. Cao & D. Jena

Appl. Phys. Lett., 90, 182112, 2007)

8) Resonant Terahertz generation from InN thin films

(X. Mu, Y. J. Ding, K. Wang, D. Jena & Y. B. Zotova

Opt. Lett., 32, 1432, 2007)

7) Compositional modulation and optical emission in AlGaN epitaxial films

(M. Gao, S. Bradley, Y. Cao, D. Jena, Y. Lin, S. Ringel, H. Hwang, W. Schaff, & L. Brillson

J. Appl. Phys., 100, 103512, 2006)

6) Hot Phonons in Si-Doped GaN

(J. Liberis, M. Ramonas, O. Kiprijanovic, A. Matulionis, N. Goel, J. Simon, K. Wang, H. Xing, & D. Jena,

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

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

(K. Wang, Y. Cao, J. Simon, J. Zhang, A. Mintairov, J. Merz, D. Hall, T. Kosel, & D. Jena,

Appl. Phys. Lett., 89, 162110, 2006)

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

(K. Wang, J. Simon, N. Goel & D. Jena,

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

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

(J. Simon, K. Wang, H. Xing, D. Jena & S. Rajan,

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

2) Dipole Scattering in highly polar semiconductor alloys

(W. Zhao and D. Jena,

J. Appl. Phys. 96, 2095, 2004)

1) AlGaN/GaN polarization-doped field-effect transistor for microwave power applications

(Siddharth Rajan, Huili Xing, Steve DenBaars, Umesh K. Mishra, and D. Jena,

Appl. Phys. Lett., 84, 1591, 2004)

Link to complete list of publications.

A general introduction to the new carbon-based 2D-material called Graphene can be found here, and a technical introduction authored by one of the inventors is found here. In 2004, researchers at the University of Manchester in UK discovered than single layers of graphite (carbon atoms sp²-bonded in a honeycomb lattice) can be isolated onto separate substrates. This honeycomb carbon structure forms the backbone of Fullerenes (Buckyballs or C₆₀), Carbon Nanotubes, as well as the much more common Graphite. Thereafter, a group at Georgia Tech found that graphene can also be grown on SiC. Since then, interest in graphene has been growing. The primary interest stems from the excellent carrier transport properties in the material – very high electron and hole mobilities, and ideal electrostatics. The bandstructure of 2D graphene is different from ordinary semiconductors: the energy of electrons in graphene depends on the momentum of carriers in a linear fashion (E~k), a characteristic common to massless photons and not electrons. The bandgap of a large 2D graphene sheet is zero – but gaps can be opened if one is clever about it.

In our group, we are interested in studying the charge transport properties in graphene sheets and nanoscale ribbons carved out of it, and in finding ways to use this exciting new material for device applications – both conventional and unconventional. For doing so, we are initially employing the tried and tested fabrication (E-beam lithography, etc), characterization (Magnetotransport, Hi-Field measurements), and theoretical (carrier statistics, electrostatics) used in traditional semiconductor device design to gain the knowledge necessary to exploit its unique potential.

Though it is always too early to predict what device technologies graphene may be used for in the future, it is one of the very few new materials which is a fascinating playground for generating new ideas for future device technologies. And it certainly does not hurt that it is one atomic layer thick – it can be integrated with virtually any other technology.

Representative publications related to Graphene Devices:

2) Carrier Statistics and Quantum Capacitance in Graphene Sheets and Ribbons

(T. Fang, A. Konar, H. Xing & D. Jena

Appl. Phys. Lett., 91, 092109, 2007)

1) Enhancement of carrier mobility in semiconductor nanostructures by dielectric engineering

(D. Jena & A. Konar

Phys. Rev. Lett., 98, 136805, 2007)

Link to complete list of publications.

A very good introduction to semiconductor nanowires can be found here. In our group, we are using PbSe, CdTe, and CdSe nanowires grown by Prof. Kuno’s group in the department of Chemistry and Biochemistry at Notre Dame. The method used for the growth of these nanowires is called the Solution-Liquid-Solid (SLS) technique, which is quite distinct from the more widely used Vapor-Solid-Liquid (VLS) technique. Solution growth is very well suited for scaling and mass-production, and offers perhaps the most attractive path to real commercial applications of these nanomaterials.

In our group, we are interested taking the nanowires that are grown by Prof. Kuno’s group to real-world applications. However, one has to learn to walk before one can run. Therefore, it is absolutely necessary to develop a thorough understanding of the charge transport properties through single nanowires, as well as through dense networks of nanowires on 2D surfaces. We call such networks quantum-wire solids in analogy to the closely related quantum-dot solids formed from nanocrystals.

We are investigating the transport properties of optically, thermally, and electrostatically generated carriers in ultrathin nanowires (diameters ~10nm), and are studying the effect of the environment on electron transport through these novel nanostructures. Charge transport through these quantum-wire solids involves a rich range of mechanisms such as band transport, hopping, and percolation in random networks. The device applications of these nanowires range from cheap flexible macro-electronics (they are expected to give organic materials a strong competition) to wearable transistors, and from photodetectors, to sensors and photovoltaics.

Representative publications related to Nanowire Devices:

4) Tailoring the carrier mobility in semiconductor nanowires by remote dielectrics

(A. Konar & D. Jena

J. Appl. Phys., 102, 123705, 2007)

3) Polarization-sensitive photodetectors based on solution-synthesized semiconductor nanowire-based quantum-wire solids

(A. Singh, X. Li, G. Galantai, V. Protasenko, M. Kuno, H. Xing, & D. Jena

Nano Lett., 7 (10), 2999, 2007)

2) Polarization anisotropy, frequency dependent emission, and transport properties of dielectrophoretically aligned CdSe nanowire arrays

(R. Zhou, H.-C. Chang, V. Protasenko, M. Kuno, A. Singh, D. Jena, & H. Xing

J. Appl. Phys., 101, 073704, 2007)

1) Ultrathin CdSe Nanowire FETs and their Optical Properties

(A. Khandelwal, D. Jena, J. Grebinski, K. Richter, and M. Kuno,

J. Electron. Mat., 35, 170, 2006)

Link to complete list of publications.

Contact:

Debdeep Jena

Department of Electrical Engineering

University of Notre Dame

Research Keywords: Gallium Nitride (GaN), Indium Nitride (InN), MBE, HEMTs, HBTs, Graphene, Nanowire, Devices, Transport, Scattering, Phonons, Hopping

(djena [at] nd [dot] edu)

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