Fatih Akyol

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.

Tunneling-based carrier regeneration in cascaded GaN light emitting diodes to overcome efficiency droop

In this work, we demonstrate visible wavelength transparent GaN/GdN/GaN tunnel junction interconnects to cascade multiple p-n junctions with low resistance (5.7 × 10−4 Ω × cm2). We model a device structure using cascaded light emitting diodes (LEDs) with tunnel junction-based carrier regeneration to create high-power LEDs operating at low current density. Experimental LED characteristics are used to model the use of these low resistance tunnel junctions in cascaded multiple active region LEDs that can effectively provide high power by operating under low current and high forward voltage. The adoption of cascaded LED structures can enable high power LEDs while maintaining high efficiency.

N-Polar III–Nitride Green (540 nm) Light Emitting Diode

We report the demonstration of a N-polar InGaN based green light emitting diode (LED) grown by N2 plasma-assisted molecular beam epitaxy (PAMBE). High quality multiple quantum well LEDs with In0.29Ga0.71N quantum wells were grown at a temperature of 600 °C by applying a new growth model. LED structures exhibited green emission, and electroluminescence measurements on the test structure showed peak emission wavelengths varying from 564.5 to 540 nm. The full width at half-maximum reduced from 74 to 63 nm as the drive current was increased to 180 A/cm2. This work is the first demonstration of an N-polar LED with emission in the green wavelength range.

Interband tunneling for hole injection in III-nitride ultraviolet emitters

Low p-type conductivity and high contact resistance remain a critical problem in wide band gap AlGaN-based ultraviolet light emitters due to the high acceptor ionization energy. In this work, interband tunneling is demonstrated for non-equilibrium injection of holes through the use of ultra-thin polarization-engineered layers that enhance tunneling probability by several orders of magnitude over a PN homojunction. Al0.3Ga0.7N interband tunnel junctions with a low resistance of 5.6 × 10−4 Ω cm2 were obtained and integrated on ultraviolet light emitting diodes. Tunnel injection of holes was used to realize GaN-free ultraviolet light emitters with bottom and top n-type Al0.3Ga0.7N contacts. At an emission wavelength of 327 nm, stable output power of 6 W/cm2 at a current density of 120 A/cm2 with a forward voltage of 5.9 V was achieved. This demonstration of efficient interband tunneling could enable device designs for higher efficiency ultraviolet emitters.

InGaN/GaN tunnel junctions for hole injection in GaN light emitting diodes

InGaN/GaN tunnel junction contacts were grown on top of an InGaN/GaN blue (450 nm) light emitting diode wafer using plasma assisted molecular beam epitaxy. The tunnel junction contacts enable low spreading resistance n-GaN top contact layer thereby requiring less top metal contact coverage on the surface. A voltage drop of 5.3 V at 100 mA, forward resistance of 2 x 10-2 ohm cm2 and a higher light output power are measured in tunnel junction LED. A low resistance of 5 x 10-4 ohm cm2 was measured in a MBE grown tunnel junction on GaN PN junction device, indicating that the tunnel junction LED device resistance is limited by the regrowth interface and not by the intrinsic tunneling resistance.

Polarity in GaN and ZnO: Theory, measurement, growth, and devices

The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

Modeling of high composition AlGaN channel high electron mobility transistors with large threshold voltage

We report on the potential of high electron mobility transistors (HEMTs) consisting of high composition AlGaN channel and barrier layers for power switching applications. Detailed two-dimensional (2D) simulations show that threshold voltages in excess of 3 V can be achieved through the use of AlGaN channel layers. We also calculate the 2D electron gas mobility in AlGaN channel HEMTs and evaluate their power figures of merit as a function of device operating temperature and Al mole fraction in the channel. Our models show that power switching transistors with AlGaN channels would have comparable on-resistance to GaN-channel based transistors for the same operation voltage. The modeling in this paper shows the potential of high composition AlGaN as a channel material for future high threshold enhancement mode transistors.

Low-resistance GaN tunnel homojunctions with 150 kA/cm2 current and repeatable negative differential resistance

We report GaN n++/p++ interband tunnel junctions with repeatable negative differential resistance and low resistance. Reverse and forward tunneling current densities were observed to increase as Si and Mg doping concentrations were increased. Hysteresis-free, bidirectional negative differential resistance was observed at room temperature from these junctions at a forward voltage ∼1.6 V. Thermionic PN junctions with GaN homojunction tunnel contact to the p-layer exhibited forward current density of 150 kA/cm2 at 7.6 V, with a low series device resistance of 1 × 10−5 Ω cm2.