Pil Sung Park

Large area single crystal (0001) oriented MoS2

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, the potential for exciting physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area single crystal Molybdenum Disulfide (MoS2) thin films. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) oriented sapphire yielded highly oriented single crystal MoS2 films. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.

Suppression of electron overflow and efficiency droop in N-polar GaN green light emitting diodes

In this letter, we experimentally demonstrate direct correlation between efficiency droop and carrier overflow in InGaN/GaN green light emitting diodes (LEDs). Further, we demonstrate flat external quantum efficiency curve up to 400 A/cm2 in a plasma assisted molecular beam epitaxy grown N-polar double quantum well LED without electron blocking layers. This is achieved by exploring the superior properties of reverse polarization field of N-face polarity, such as effective carrier injection and higher potential barriers against carrier overflow mechanism. The LEDs were found to operate with a low (∼2.3 V) turn-on voltage.

Polarization-engineered GaN/InGaN/GaN tunnel diodes

We report on the design and demonstration of polarization-engineered GaN/InGaN/GaN tunnel junction diodes with high current density and low tunneling turn-on voltage. Wentzel–Kramers–Brillouin calculations were used to model and design tunnel junctions with narrow band gap InGaN-based barrier layers. N-polar p-GaN/In0.33Ga0.67N/n-GaN heterostructure tunnel diodes were grown using molecular beam epitaxy. Efficient interband tunneling was achieved close to zero bias with a high current density of 118 A/cm2 at a reverse bias of 1 V, reaching a maximum current density up to 9.2 kA/cm2. These results represent the highest current density reported in III-nitride tunnel junctions and demonstrate the potential of III-nitride tunnel devices for a broad range of optoelectronic and electronic applications.

Low resistance GaN/InGaN/GaN tunnel junctions

Enhanced interband tunnel injection of holes into a p-n junction is demonstrated using p-GaN/InGaN/n-GaN tunnel junctions with a specific resistivity of 1.2 × 10−4 Ω cm2. The design methodology and low-temperature characteristic of these tunnel junctions are discussed, and insertion into a p-n junction device is described. Applications of tunnel junctions in III-nitride optoelectronics devices are explained using energy band diagrams. The lower bandgap and polarization fields reduce tunneling barrier, eliminating the need for ohmic contacts to p-type GaN. This demonstration of efficient tunnel injection of carriers in III-nitrides can lead to a replacement of existing resistive p-type contact material in light emitters with tunneling contact layers requiring very little metal footprint on the surface, resulting in enhanced light extraction.

Demonstration of forward inter-band tunneling in GaN by polarization engineering

We report on the design, fabrication, and characterization of GaN interband tunnel junction showing forward tunneling characteristics. We have achieved very high forward tunneling currents (153 mA/cm2 at 10 mV, and 17.7 A/cm2 peak current) in polarization-engineered GaN/InGaN/GaN heterojunction diodes grown by plasma assisted molecular beam epitaxy. We also report the observation of repeatable negative differential resistance in interband III-Nitride tunnel junctions, with peak-valley current ratio of 4 at room temperature. The forward current density achieved in this work meets the typical current drive requirements of a multi-junction solar cell.

Simulation of Short-Channel Effects in N- and Ga-Polar AlGaN/GaN HEMTs

We have carried out 2-D simulation of N-polar and Ga-polar AlGaN/GaN HEMTs to investigate short-channel effects in highly scaled devices. N-polar HEMTs were found to have better drain-induced barrier lowering (DIBL) suppression than Ga-polar HEMTs. The short-channel effects were found to originate from the 2-D potential distribution in the channel and space-charge-limited current through the buffer. The inverted structure of the N-polar HEMT was found to provide better suppression of short-channel effects under idealized theoretical assumptions that were used in the model presented.

Interface charge engineering at atomic layer deposited dielectric/III-nitride interfaces

Interface charges at atomic layer deposited Al2O3/III-nitride interfaces were investigated for III-nitride layers of different polarity. A large positive sheet charge density is induced at the Al2O3/III-nitride interface on all the orientations of GaN and Ga-polar AlGaN, and this sheet charge can be significantly altered using post-metallization anneals. It is proposed that the charges are caused by interfacial defects that can be passivated and neutralized through a H2 based anneal. Tailoring of the interface charge density described here can be used to improve critical device characteristics such as gate leakage and electron transport, and for lateral electrostatic engineering.

Interface Charge Engineering for Enhancement-Mode GaN MISHEMTs

We demonstrate an efficient approach to engineer the dielectric/AlGaN positive interface fixed charges by oxygen plasma and post-metallization anneal. Significant suppression of interface fixed charges from 2 × 1013 to 8 × 1012 cm-2 was observed. Experimental and theoretical electron mobility characteristics and the impact of remote impurity scattering were investigated. The reduction in oxide/semiconductor interface charge density leads to an increase of electron mobility, and enables a positive threshold voltage.

GdN Nanoisland-Based GaN Tunnel Junctions

Tunnel junctions could have a great impact on gallium nitride and aluminum nitride-based devices such as light-emitting diodes and lasers by overcoming critical challenges related to hole injection and p-contacts. This paper demonstrates the use of GdN nanoislands to enhance interband tunneling and hole injection into GaN p-n junctions by several orders of magnitude, resulting in low tunnel junction specific resistivity (1.3 × 10(-3) Ω-cm(2)) compared to the previous results in wide band gap semiconductors. Tunnel injection of holes was confirmed by low-temperature operation of GaN p-n junction with a tunneling contact layer, and strong electroluminescence down to 20 K. The low tunnel junction resistance combined with low optical absorption loss in GdN is very promising for incorporation in GaN-based light emitters.

Unipolar vertical transport in GaN/AlGaN/GaN heterostructures

In this letter, we report on unipolar vertical transport characteristics in c-plane GaN/AlGaN/GaN heterostructures. Vertical current in heterostructures with random alloy barriers was found to be independent of dislocation density and heterostructure barrier height. Percolation-based transport due to random alloy fluctuations in the ternary AlGaN is suggested as the dominant transport mechanism. This hypothesis is supported by simulations using drift-diffusion transport model incorporating statistical fluctuations of Al-composition and confirmed through experiments showing that non-random or digital AlGaN alloys and polarization-engineered binary GaN barriers can eliminate percolation transport and reduce leakage significantly. The understanding of vertical transport and methods for effective control proposed here will greatly impact III-nitride unipolar vertical devices.