The band structures of wurtzite and zincblende III-nitrides are aligned by the electron affinities and the band gaps calculated using a unified hybrid density-functional theory. Based on the Anderson’s electron-affinity rule, the conduction (and valence) band offsets of 1.60 (1.15), 2.47 (0.30), and 4.07 (1.45) eV have been extracted for wurtzite GaN/InN, AlN/GaN, and AlN/InN interfaces, where the conduction (and valence) band offsets of 1.85 (0.89), 1.32 (0.43), and 3.17 (1.32) eV have been procured for zincblende GaN/InN, AlN/GaN, and AlN/InN interfaces, respectively. The valence band edges of both wurtzite and zincblende ternary III-nitrides could be linearly interpolated because the dominant anion compositions at the valence band maximum have a weak dependence on the cation mole fractions. Contrarily, the large bowings on the conduction band edges are attributed to the cation-like nature. Both wurtzite and zincblende AlGaN behaves differently from InGaN and AlInN because (1) the conduction band edges at Γ-valley are composed of anion orbitals, which account for the linear relationship between the conduction band edges and the cation mole fractions, and (2) the conduction band edges of zincblende AlxGa1-xN shift from Γ- to X-valley when x > 0.65, which results in an anion-to- cation transition and leads to a large conduction-band-edge bowing.
The formation energies, activation energies, and self-compensation effects of silicon (Si), germanium (Ge), carbon (C), beryllium (Be), and magnesium (Mg) in wurtzite (wz-) and zincblende (zb-) GaN are explored through a unified hybrid density-functional theory. The common donors (Si and Ge) are promising donors for both wz- and zb-GaN due to small activation energies (< 30 meV). The popular acceptor alternatives (C and Be) have smaller activation energies of 490 and 134 meV in zb-GaN relative to that of 590 and 205 meV wz-GaN, respectively. However, neither C nor Be is expected to outperform Mg as the former suffers from considerable activation energy, and a strong self-compensation effect limits the latter. Mg's activation energy in zb-GaN is 153 meV, which is lower than that of 226 meV in wz-GaN. For the selfcompensation effects, C, Si, and Ge favor the interstitial incorporation in wz-GaN than zb-GaN, while Be and Mg behave oppositely. This is attributed to the coherence between the orbital symmetry and the geometrical symmetry of the interstitial site.
Optical properties of InGaN/GaN multi-quantum-well (MQWs) grown on sapphire and on Si(111) are reported. The tensile strain in the MQW on Si is shown to be beneficial for indium incorporation and Quantum-confined Stark Effect reduction in the multi-quantum wells. Raman spectroscopy reveals compressive strains of -0.107% in MQW on sapphire and tensile strain of +0.088% in MQW on Si. Temperature-dependent photoluminescence shows in MQW on sapphire a strong (30 meV peak-to-peak) S-shaped wavelength shift with decreasing temperature (6 K to 300K), whereas MQW on Si luminescence wavelength is stable and red-shifts monotonically. Micro-photoluminescence mapping over 200 by 200 μm2 shows the emission wavelength spatial uniformity of MQW on Si is 2.6 times higher than MQW on sapphire, possibly due to a more uniform indium incorporation in the multi-quantum-wells as a result of the tensile strain in MQW on Si. A positive correlation between emission energy and intensity is observed in MQW on sapphire but not in those on Si. Despite the lower crystal quality of MQW on Si revealed by atomic force microscopy, it exhibits a higher internal quantum efficiency (IQE) than MQW on sapphire from 6 K to 250 K, and equalizes at 300 K. Overall, MQW on Si exhibits a high IQE, higher wavelength spatial uniformity and temperature stability, while providing a much more scalable platform than MQW on sapphire for next generation integrated photonics.
Here we propose a new wide band gap logic circuitry providing emerging power electronics with reliable logic control capabilities with 500 MHz+ switching speeds and withstanding 300V+. Particularly, a three-stage ring oscillator composed of NMOS (μe = 1000 cm2/V-s) and PMOS (μh = 250 cm2/V-s) cubic phase GaN devices (with VT of 0.77 V and –0.84 V, respectively) is simulated. The propagation delay is minimized by optimizing the width-to-length ratio (W/L) between the NMOS and PMOS devices. Transient response of the simulation illustrates the ability of the CMOS inverter to operate at a maximum frequency of 1.22 GHz with a full voltage swing between VDD of 2.5 V and 0 V. The proposed cutting-edge p-channel GaN high hole mobility transistor (HHMT) solves one of the most longstanding problems in power electronics and constitutes the basis of an innovative reduced total life cycle cost that will serve as the cornerstone of the next generation of integrated, scalable, and reliable power systems.
Novel layer release and transfer technology of single-crystalline GaN semiconductors is attractive for enabling many novel applications including flexible photonics and hybrid device integration. To date, light emitting diode (LED) research has been primarily focused on rigid devices due to the thick growth substrate. This prevented fundamental research in flexible inorganic LEDs, and limited the applications of LEDs in the solid state lighting (due to the substrate cost) and in biophotonics (i.e. optogenetics) (due to LED rigidness). In the literature, a number of methods to achieve layer transfer have been reported including the laser lift-off, chemical lift-off, and Smartcut. However, the release of films of LED layers (i.e. GaN semiconductors) has been challenging since their elastic moduli and chemical resistivity are much higher than most conventional semiconductors. In this talk, we are going to review the existing technologies and new mechanical release techniques (i.e. spalling) to overcome these problems.
Gallium Nitride (GaN) materials are the backbone of emerging solid state lighting. To date, GaN research has been primarily focused on hexagonal phase devices due to the natural crystallization. This approach limits the output power and efficiency of LEDs, particularly in the green spectrum. However, GaN can also be engineered to be in cubic phase. Cubic GaN has a lower bandgap (~200 meV) than hexagonal GaN that enables green LEDs much easily. Besides, cubic GaN has more isotropic properties (smaller effective masses, higher carrier mobility, higher doping efficiency, and higher optical gain than hexagonal GaN), and cleavage planes. Due to phase instability, however, cubic phase materials and devices have remained mostly unexplored. Here we review a new method of cubic phase GaN generation: Hexagonal-to-cubic phase transition, based on novel nano-patterning. We report a new crystallographic modelling of this hexagonal-to-cubic phase transition and systematically study the effects of nano-patterning on the GaN phase transition via transmission electron microscopy and electron backscatter diffraction experiments. In summary, silicon-integrated cubic phase GaN light emitters offer a unique opportunity for exploration in next generation photonics.
In this work, we show that a 2D cleave layer (such as epitaxial graphene on SiC) can be used for precise release of GaNbased light emitting diodes (LEDs) from the LED-substrate interface. We demonstrate the thinnest GaN-based blue LED and report on the initial electrical and optical characteristics. Our LED device employs vertical architecture: promising excellent current spreading, improved heat dissipation, and high light extraction with respect to the lateral one. Compared to conventional LED layer release techniques used for forming vertical LEDs (such as laser-liftoff and chemical lift-off techniques), our process distinguishes itself with being wafer-scalable (large area devices are possible) and substrate reuse opportunity.
This work presents a new type of polarization-free GaN emitter. The unique aspect of this work is that the ultraviolet and
visible emission originates from the cubic phase GaN and the cubic phase InGaN/GaN multi-quantum-wells,
respectively. Conventionally, GaN emitters (e.g. light emitting diodes, laser diodes) are wurtzite phase thus strong
polarization fields exist across the structure contributing to the “droop” behavior – a phenomenon defined as “the
reduction in emitter efficiency as injection current increases”. The elimination of piezoelectric fields in GaN-based
emitters as proposed in this work provide the potential for achieving a 100% internal efficiency and might lead to droopfree
light emitting diodes. In addition, this work demonstrates co-integration of GaN emitters on cheap and scalable
CMOS-compatible Si (100) substrate, which yields possibility of realizing a GaN laser diode uniquely – via forming
mirrors along the naturally occurring cubic phase GaN-Si(100) cleavage planes.
Thin, lightweight and flexible electronics are being regarded as an important evolutionary step in the development of novel technological products. Interestingly, this trend has emerged in a wide range of industries; from microelectronics to photovoltaics and even solid state lighting. Historically, most attempts to enable flexibility have focused on the introduction of new material systems that, so far, severely compromise the performance compared to state-of-the-art products. The few approaches that do attempt to render contemporary high-performance materials flexible rely on layer transfer techniques that are complicated, expensive and material-specific. In this paper, we review a method of removing surface layers from brittle substrates called Controlled Spalling Technology that allows one to simple peel material or device layers from their host substrate after they have been fabricated. This allows one to fabricate high-performance electronic products in a manner of their choosing, and make them flexible afterwards. This technique is simple, inexpensive and largely independent of substrate material or size. We demonstrate the power and generality of Controlled Spalling by application to a number of disparate applications including high-performance integrated circuits, high-efficiency photovoltaics and GaN-based solid state lighting.
Indium Gallium Nitride (InGaN) based PV have the best fit to the solar spectrum of any alloy system and emerging LED lighting based on InGaN technology and has the potential to reduce energy consumption by nearly one half while enabling significant carbon emission reduction. However, getting the maximum benefit from GaN diode -based PV and LEDs will require wide-scale adoption. A key bottleneck for this is the device cost, which is currently dominated by the substrate (i.e. sapphire) and the epitaxy (i.e. GaN). This work investigates two schemes for reducing such costs. First, we investigated the integration of Zinc Oxide (ZnO) in InGaN-based diodes. (Successful growth of GaN on ZnO template layers (on sapphire) was illustrated. These templates can then be used as sacrificial release layers for chemical lift-off. Such an approach provides an alternative to laser lift-off for the transfer of GaN to substrates with a superior cost-performance profile, plus an added advantage of reclaiming the expensive single-crystal sapphire. It was also illustrated that substitution of low temperature n-type ZnO for n-GaN layers can combat indium leakage from InGaN quantum well active layers in inverted p-n junction structures. The ZnO overlayers can also double as transparent contacts with a nanostructured surface which enhances light in/out coupling. Thus ZnO was confirmed to be an effective GaN substitute which offers added flexibility in device design and can be used in order to simultaneously reduce the epitaxial cost and boost the device performance. Second, we investigated the use of GaN templates on patterned Silicon (100) substrates for reduced substrate cost LED applications. Controlled local metal organic chemical vapor deposition epitaxy of cubic phase GaN with on-axis Si(100) substrates was illustrated. Scanning electron microscopy and transmission electron microscopy techniques were used to investigate uniformity and examine the defect structure in the GaN. Our results suggest that groove structures are very promising for controlled local epitaxy of cubic phase GaN. Overall, it is concluded that there are significant opportunities for cost reduction in novel hybrid diodes based on ZnO-InGaN-Si hybridization.
Gallium Nitride (GaN) is a unique material system that has been heavily exploited for photonic devices thanks to
ultraviolet-to-terahertz spectral tunability. However, without a cost effective approach, GaN technology is limited to
laboratory demonstrations and niche applications. In this investigation, integration of GaN on Silicon (100) substrates is
attempted to enable widespread application of GaN based optoelectronics. Controlled local epitaxy of wurtzite phase
GaN on on-axis Si(100) substrates is demonstrated via metal organic chemical vapor deposition (MOCVD). CMOScompatible
fabrication scheme is used to realize [SiO2-Si{111}-Si{100}] groove structures on conventional 200-mm
Si(100) substrates. MOCVD growth (surface treatment, nucleation, initiation) conditions are studied to achieve
controlled GaN epitaxy on such grooved Si(100) substrates. Scanning electron microscopy and transmission electron
microscopy techniques are used to determine uniformity and defectivity of the GaN. Our results show that
aforementioned groove structures along with optimized MOCVD growth conditions can be used to achieve controlled
local epitaxy of wurtzite phase GaN on on-axis Si(100) substrates.
Resonant tunneling diode (RTD) is an electronic device embodying a unique quantum-interference phenomenon:
negative differential resistance (NDR). Compared to other negative resistance devices such as (Esaki) tunnel and
transferred-electron devices, RTDs operate much faster and at higher temperatures. III-nitride materials, composed of
AlGaInN alloys, have wide bandgap, high carrier mobility and thermal stability; making them ideal for high power high
frequency RTDs. Moreover, larger conduction band discontinuity promise higher NDR than other materials (such as
GaAs) and room-temperature operation. However, earlier efforts on GaN-based RTD structures have failed to achieve a
reliable and reproducible NDR. Recently, we have demonstrated for the first time that minimizing dislocation density
and eliminating the piezoelectric fields enable reliable and reproducible NDR in GaN-based RTDs even at room
temperature. Observation of NDR under both forward and reverse bias as well as at room and low temperatures attribute
the NDR behaviour to quantum tunneling. This demonstration marks an important milestone in exploring III-nitride
quantum devices, and will pave the way towards fundamental quantum transport studies as well as for high frequency
optoelectronic devices such as terahertz emitters based on oscillators and cascading structures.
Al(Ga)N/GaN resonant tunneling diodes (RTDs) are grown by metal-organic chemical vapor deposition. The effects of
material quality on room temperature negative differential resistance (NDR) behaviour of RTDs are investigated by
growing the RTD structure on AlN, GaN, and lateral epitaxial overgrowth GaN templates. This reveals that NDR
characteristics of RTDs are very sensitive to material quality (such as surface roughness and dislocations density). The
effects of the aluminum content of AlGaN double barriers (DB) and polarization fields on NDR characteristic of
AlGaN/GaN RTDs were also investigated by employing low dislocation density c-plane (polar) and m-plane (nonpolar)
freestanding GaN substrates. Lower aluminum content in the DB RTD active layer and minimization of dislocations and
polarization fields enabled a more reliable and reproducible NDR behaviour at room temperature.
There is a need for semiconductor-based ultraviolet photodetectors to support avalanche gain in order to realize better
performance andmore effective compete with existing technologies. Wide bandgap III-Nitride semiconductors are the
promising material system for the development of avalanche photodiodes (APDs) that could be a viable alternative to
current bulky UV detectors such as photomultiplier tubes. In this paper, we review the current state-of-the-art in IIINitride
visible-blind APDs, and present our latest results on GaN APDs grown on both conventional sapphire and low
dislocation density free-standing c- and m-plane GaN substrates. Leakage current, gain, and single photon detection
efficiency (SPDE) of these APDs were compared. The spectral response and Geiger-mode photon counting performance
of UV APDs are studied under low photon fluxes, with single photon detection capabilities as much as 30% being
demonstrated in smaller devices. Geiger-mode operation conditions are optimized for enhanced SPDE.
Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet
(UV) detectors in numerous civilian and military applications. By designing III-Nitride photodetectors that utilize
low-noise impact ionization high internal gain can be realized-GaN APDs operating in Geiger mode can achieve
gains exceeding 1×107. Thus with careful design, it becomes possible to count photons at the single photon level. In
this paper we review the current state of the art in III-Nitride visible-blind APDs and discuss the critical design
choices necessary to achieve high performance Geiger mode devices. Other major technical issues associated with the
realization of visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the
performance of these devices are outlined. The photon detection efficiency, dark count rate, and spectral response of
or most recent Geiger-mode GaN APDs on free-standing GaN substrates are studied under low photon fluxes, with
single photon detection capabilities being demonstrated. We also present our latest results regarding linear mode gain
uniformity: the study of gain uniformity helps reveal the spatial origins of gain so that we can better understand the
role of defects.
Numerous applications in scientific, medical, and military areas demand robust, compact, sensitive, and fast ultraviolet
(UV) detection. Our (Al)GaN photodiodes pose high avalanche gain and single-photon detection efficiency that can
measure up to these requirements. Inherit advantage of back-illumination in our devices offers an easier integration and
layout packaging via flip-chip hybridization for UV focal plane arrays that may find uses from space applications to
hostile-agent detection. Thanks to the recent (Al)GaN material optimization, III-Nitrides, known to have fast carrier
dynamics and short relaxation times, are employed in (Al)GaN based superlattices that absorb in near-infrared regime. In
this work, we explain the origins of our high performance UV APDs, and employ our (Al)GaN material knowledge for
intersubband applications. We also discuss the extension of this material engineering into the far infrared, and even the
terahertz (THz) region.
Hybrid green light-emitting diodes (LEDs) comprised of n-ZnO/(InGaN/GaN) multi-quantum-wells/p-GaN were grown
on semi-insulating AlN/sapphire using pulsed laser deposition for the n-ZnO and metal organic chemical vapor
deposition for the other layers. X-ray diffraction revealed that high crystallographic quality was preserved after the n-
ZnO growth. LEDs showed a turn-on voltage of 2.5 V and a room temperature electroluminescence (EL) centered at 510
nm. A blueshift and narrowing of the EL peak with increasing current was attributed to bandgap renormalization. The
results indicate that hybrid LED structures could hold the prospect for the development of green LEDs with superior
performance.
A pulsed metalorganic chemical vapor deposition (MOCVD) technique, specifically designed for high quality AlN/GaN superlattices (SLs) is introduced. Optical quality and precise controllability over layer thicknesses are investigated. Indium is shown to improve interface and surface quality. An AlN/GaN SL designed for intersubband transition at a telecommunication wavelength of ~1.5 μm, is grown, and processed for intersubband (ISB) absorption measurements.
Room temperature measurements show intersubband absorption centered at 1.49 μm. Minimal (n-type) silicon doping of
the well is shown to be crucial for good ISB absorption characteristics. The potential to extend this technology into the
far infrared and even the terahertz (THz) region is also discussed.
Research into avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors
in numerous civilian and military applications. By designing photodetectors to utilize low-noise impact ionization
based gain, GaN APDs operating in Geiger mode can deliver gains exceeding 1×107. Thus with careful design, it
becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride
visible-blind APDs, and present our latest results regarding linear and Geiger mode III-Nitride based APDs.
This includes novel device designs such as separate absorption and multiplication APDs (SAM-APDs). We also
discuss control of the material quality and the critical issue of p-type doping - demonstrating a novel delta-doping
technique for improved material quality and enhanced electric field confinement. The spectral response and Geiger-mode
photon counting performance of these devices are then analyzed under low photon fluxes, with single photon
detection capabilities being demonstrated. Other major technical issues associated with the realization of high-quality
visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance
of these devices are outlined.
The use of nanostructures in semiconductor technology leads to the observation of new phenomena in device physics.
Further quantum and non-quantum effects arise from the reduction of device dimension to a nanometric scale. In
nanopillars, quantum confinement regime is only revealed when the lateral dimensions are lower than 50 nm. For larger
mesoscopic systems, quantum effects are not observable but surface states play a key role and make the properties of
nanostructured devices depart from those found in conventional devices. In this work, we present the fabrication of GaN
nanostructured metal-semiconductor-metal (MSM) and p-i-n photodiodes (PIN PDs) by e-beam lithography, as well as
the investigation of their photoelectrical properties at room temperature. The nanopillar height and diameter are about
520 nm and 200 nm, respectively. MSMs present dark currents densities of 0.4 A/cm2 at ±100 V. A strong increase of
the optical response with bias is observed, resulting in responsivities higher than 1 A/W. The relationship between this
gain mechanism and surface states is discussed. PIN PDs yield peak responsivities (Rpeak) of 35 mA/W at -4 V and show
an abnormal increase of the response (Rpeak>100 A/W) under forward biases.
In order for solar and visible blind III-nitride based photodetectors to effectively compete with the detective
performance of PMT there is a need to develop photodetectors that take advantage of low noise avalanche gain.
Furthermore, in certain applications, it is desirable to obtain UV photon counting performance. In this paper, we
review the characteristics of III-nitride visible-blind avalanche photodetectors (APDs), and present the state-of-the-art
results on photon counting based on the Geiger mode operation of GaN APDs. The devices are fabricated on
transparent AlN templates specifically for back-illumination in order to enhance hole-initiated multiplication. The
spectral response and Geiger-mode photon counting performance are analyzed under low photon fluxes, with single
photon detection capabilities being demonstrated in smaller devices. Other major technical issues associated with the
realization of high-quality visible-blind APDs and Geiger mode APDs are also discussed in detail and solutions to the
major problems are described where available. Finally, future prospects for improving upon the performance of these
devices are outlined.
Although ZnO has recently gained much interest as an alternative to the III-Nitride material system, the development of
ZnO based optoelectonic devices is still in its infancy. Significant material breakthroughs in p-type doping of ZnO thin
films and improvements in crystal growth techniques have recently been achieved, making the development of
optoelectonic devices possible. ZnO is known to be an efficient UV-emitting material (~380 nm) at room temperature,
optical UV lasing of ZnO has been achieved, and both homojunction and hybrid heterojunction LEDs have been
demonstrated.
In this paper, processing techniques are explored towards the achievement of a homo-junction ZnO LED. First, a
survey of current ZnO processing methods is presented, followed by the results of our processing research.
Specifically, we have examined etching through an n-ZnO layer to expose and make contact to a p-ZnO layer.
Wide bandgap III-Nitride semiconductors are a promising material system for the development of ultraviolet
avalanche photodiodes (APDs) that could be a viable alternative to photomultiplier tubes. In this paper, we report the
epitaxial growth and physical properties of device quality GaN layers on high quality AlN templates for the first backilluminated
GaN p-i-n APD structures on transparent sapphire substrates. The 25 μm x 25 μm device characteristics
were measured, and compared with the same devices grown on GaN templates, under low bias and linear mode
avalanche operation where they exhibited gains near 1500 after undergoing avalanche breakdown. The breakdown
electric field in GaN was determined to be 2.73 MV/cm. The hole impact ionization coefficients were shown to be
greater than those of electrons. These APDs were also successfully operated under Geiger mode.
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