Metal-semiconductor interfaces to Ga2O3 have been demonstrated to be highly sensitive to device processing conditions. Liftoff processing leads to inconsistency in ohmic contact formation and quality due to apparent chemical modification of the surface layer, which is not affected by most traditional surface cleanings but can be removed by Ga-flux polishing. Metal-first processing on as-grown material, which avoids exposure of the Ga2O3 surface to photoresist prior to metal deposition, has been shown to form reliable, low-resistance ohmic contacts. Investigation of the chemical composition of contacts to material that has been modified by liftoff and cleaned by Ga-flux polishing by x-ray photoelectron spectroscopy (XPS) reveals slight variations in the oxidation states of the Ti and Ga in the contacts that indicate the nature of the surface chemistry modification.
We present LED profiting from the bottom-tunnel junction (BTJ) construction. The BTJ design aligns the polarization fields in a desired direction in the vicinity of active region and inverts the ordering of the layer stack in the structure. This leads the situation were conductive, n-type layer is on the very top of the structure. Since current spreading in n-type material is much better than in p-type, BTJ-based light emitters open new possibilities in heterostructure design. In this talk we present new light emitting structures grown by plasma-assisted MBE based on BTJ platform and compare prospects for bottom and top tunnel junction devices.
The physics of the bottom tunnel junction (BTJ) and its improvement over standard p-up geometry in InGaN blue LEDs is quantified through pulsed power measurements. It is found that the peak external quantum efficiency (EQE) and wall-plug efficiency (WPE) for a p-down BTJ LED is about threefold that of its counterpart, the p-up top tunnel junction (TTJ) LED. This is contributed to increased radiative recombination and reduced electron overflow. Further, the peaks occur at lower current densities for the BTJ device, suggesting earlier saturation of Shockley-Read-Hall traps. In the droop regime, where electron overflow, device heating, and 3-particle interactions are significant, the performance of the BTJ is found to be consistently better than that of the TTJ, converging at large current densities where the polarization fields are screened.
In this paper, we analyze the threshold voltage stability of beta-Ga2O3 FinFETs for power applications using Al2O3 as gate insulator. In dynamic characterization measurements, when the filling bias condition is moved from off-state to on-state a positive threshold voltage shift is induced, caused by the trapping of electrons in the insulator or at the insulator interface with the semiconductor. The threshold voltage variation was found to be stable in rest condition, but illumination by 280 nm UV light was able to slowly recover the threshold voltage even below its value before the filling condition was applied, suggesting the presence of natively trapped charge into the oxide even in the as-grown device. In order to obtain more information on the role of the external illumination, monochromatic excitation in the range from 1.5 eV to 5 eV was applied to the device before a transfer characteristic measurement. Results show that photon energies lower than 2.2 eV cause a positive threshold voltage shift, caused by charge trapping during the measurement phase and not related to illumination. Photon energies between 2.2 eV and 3.5 eV promote electron detrapping, leading to a partial recovery in the threshold voltage. Finally, energies above 3.5 eV cause an additional charge trapping process. The physical origin of the photon energy difference was investigated by monochromatic light-induced current transients, and a suitable model considering the conduction band discontinuities between the gate metal and the oxide and between the oxide and the semiconductor was developed to explain the experimental data.
New approach towards efficient light emission with bottom-tunnel junctions is developed. The bottom-tunnel junction design aligns the polarization fields in a desired direction in the vicinity of quantum well, while simultaneously eliminating the need for p-type contacts, and allowing efficient current spreading. By preventing electron overshoot past quantum wells, it disables carrier recombination in undesired regions of the heterostructures, increasing injection efficiency and opening new possibilities in heterostructure design. InGaN-based buried-tunnel junction is used to construct first monolithically grown p-type-down laser diode on n-type, Ga-polar bulk GaN substrate. Unique advantages of such construction that enables to separate design of carrier injection and optical mode confinement for such laser diode structures is discussed.
The outstanding material properties of III-Nitride semiconductors, has prompted intense research efforts in order to engineer resonant tunneling transport within this revolutionary family of wide-bandgap semiconductors. From resonant tunneling diode (RTD) oscillators to quantum cascade lasers (QCLs), III-Nitride heterostructures hold the promise for the realization of high-power ultra-fast sources of terahertz (THz) radiation. However, despite the considerable efforts over last two decades, it is only during the last three years that room temperature resonant tunneling transport has been demonstrated within the III-Nitride family of semiconductors. In this paper we present an overview of our current understanding of resonant tunneling transport in polar heterostructures. In particular we focus on double-barrier III-Nitride RTDs which represents the simplest device in which the dramatic effects of the internal polarization fields can be studied. Tunneling transport within III-heterostructures is strongly influenced by the presence of the intense spontaneous and piezoelectric polarization fields which result from the non-centrosymmetric crystal structure of III-Nitride semiconductors. Advances in heterostructure design, epitaxial growth and device fabrication have led to the first unequivocal demonstration of robust and reliable negative differential conductance. which has been employed for the generation of microwave power from III-Nitride RTD oscillator. These significant advances allowed us to shed light into the physics of resonant tunneling transport in polar semiconductors which had remained hidden until now.
III-nitride ultraviolet (UV) light emitting diodes (LEDs) with emission wavelengths in the range of 250-280 nm have attracted considerable interest for applications such as germicidal disinfection and biological detection. However, the widely-used AlGaN quantum well (QW)-based LEDs at such wavelengths suffer from low quantum efficiencies. One main factor that limits the AlGaN QW LED efficiency at ~250-280 nm is the suffering of the severe band mixing effect caused by the valence subbands crossover, as well as the Quantum Confined Stark Effect (QCSE). Therefore, the novel AlGaN-delta-GaN QW design was proposed to address these issues in order to realize high-efficiency deep-UV LEDs.
Here, we proposed a novel Al0.9Ga0.1N-delta-GaN QW by inserting an ultra-thin delta-GaN layer into a conventional Al0.9Ga0.1N QW active region. The physics from such QW design was investigated by 6-band k·p model and the structure was experimentally demonstrated by Plasma-assisted Molecular Beam Epitaxy (PAMBE). The calculated results show that the insertion of delta-GaN layer could successfully address the band mixing issue and QCSE, leading to a significant improvement in spontaneous emission rate as compared to that of Al0.55Ga0.45N QW at 260 nm. The 5-period Al0.9Ga0.1N-delta-GaN QW with 3-nm AlN barrier was grown on AlN/sapphire substrate by MBE with ~2-monolayer delta-GaN layer, which was evidenced by the cross-sectional transmission electron microscope. The two-photon photoluminescence spectrum presented a single peak emission centered at 260 nm from the grown Al0.9Ga0.1N-deltaGaN QW with a full width at half maximum of 12 nm, which shows that the demonstrated QW would be promising for high-efficiency UV LEDs.
III-nitride based ultraviolet (UV) light emitting diodes (LEDs) are of considerable interest in replacing gas lasers and mercury lamps for numerous applications. Specifically, AlGaN quantum well (QW) based LEDs have been developed extensively but the external quantum efficiencies of which remain less than 10% for wavelengths <300 nm due to high dislocation density, difficult p-type doping and most importantly, the physics and band structure from the three degeneration valence subbands. One solution to address this issue at deep UV wavelengths is by the use of the AlGaN-delta-GaN QW where the insertion of the delta-GaN layer can ensure the dominant conduction band (C) - heavyhole (HH) transition, leading to large transverse-electric (TE) optical output. Here, we proposed and investigated the physics and polarization-dependent optical characterizations of AlN-delta- GaN QW UV LED at ~300 nm. The LED structure is grown by Molecular Beam Epitaxy (MBE) where the delta-GaN layer is ~3-4 monolayer (QW-like) sandwiched by 2.5-nm AlN sub-QW layers. The physics analysis shows that the use of AlN-delta-GaN QW ensures a larger separation between the top HH subband and lower-energy bands, and strongly localizes the electron and HH wave functions toward the QW center and hence resulting in ~30-time enhancement in TEpolarized spontaneous emission rate, compared to that of a conventional Al0.35Ga0.65N QW. The polarization-dependent electroluminescence measurements confirm our theoretical analysis; a dominant TE-polarized emission was obtained at 298 nm with a minimum transverse-magnetic (TM) polarized emission, indicating the feasibility of high-efficiency TEpolarized UV emitters based on our proposed QW structure.
We analyze amplification of terahertz plasmons in a grating-gate semiconductor hetero-structure. The device consists of a resonant-tunneling-diode gated high-electron-mobility transistor (RTD-gated HEMT), i.e. a HEMT structure with a double-barrier gate stack enabling resonant tunneling from gate to channel. In these devices, the key element enabling substantial power gain is the coupling of terahertz waves into and out of plasmons in the RTD-gated HEMT channel, i.e. the gain medium, via the grating-gate itself, part of the active device, rather than by an external antenna structure as in previous works, enabling amplification with associated power gain >> 30 dB at room temperature.
In two-dimensional electron systems with mobility on the order of 1,000 – 10,000 cm2/Vs, the electron scattering time is about 1 ps. For the THz window of 0.3 – 3 THz, the THz photon energy is in the neighborhood of 1 meV, substantially smaller than the optical phonon energy of solids where these 2D electron systems resides. These properties make the 2D electron systems interesting as a platform to realize THz devices. In this paper, I will review 3 approaches investigated in the past few years in my group toward THz devices. The first approach is the conventional high electron mobility transistor based on GaN toward THz amplifiers. The second approach is to employ the tunable intraband absorption in 2D electron systems to realize THz modulators, where I will use graphene as a model material system. The third approach is to exploit plasma wave in these 2D electron systems that can be coupled with a negative differential conductance element for THz amplifiers/sources/detectors.
Continuing advances in scaling of conventional semiconductor devices are enabling mainstream electronics to operate in
the millimeter-wave through THz regime. At the same time, however, novel devices and device concepts are also
emerging to address the key challenges for systems in this frequency range, and may offer performance and functional
advantages for future systems. In addition to new devices, advances in integration technology and novel system
concepts also promise to provide substantial system-level performance and functionality enhancements. Several
emerging devices and device concepts, as well as circuit-level concepts to take advantage of them, are discussed. Based
on unconventional semiconductor device structures and operational principles, these devices offer the potential for
significantly improved system sensitivity and frequency coverage. When combined in arrays, features such as
polarimetric detection and frequency tunability for imaging can be achieved. As examples of emerging devices for
millimeter-wave through THz sensing and imaging, heterostructure backward diodes in the InAs/AlSb/GaSb material
system and GaN-based plasma-wave high electron mobility transistors (HEMTs) will be discussed. Based on interband
tunneling, heterostructure backward diodes offer significantly increased sensitivity and extremely low noise for direct
detection applications, and have been demonstrated with cutoff frequencies exceeding 8 THz. The plasma-wave HEMT
is an emerging device concept that, by leveraging plasma-wave resonances in the two-dimensional electron gas within
the channel of the HEMT, offers the prospect for both tunable narrowband detection as well as low-noise amplification
at frequencies well into the THz. These emerging devices are both amenable to direct integration within compact planar
radiating structures such as annular slot antennas for realization of polarimetric detection and frequency tuning for
spectroscopy and imaging.
We report a technique using photo-induced coded-aperture arrays for potential real-time THz imaging at roomtemperature.
The coded apertures (based on Hadamard coding) were implemented using programmable illumination on
semi-insulating Silicon wafer by a commercial digital-light processing (DLP) projector. Initial imaging experiments
were performed in the 500-750 GHz band using a WR-1.5 vector network analyzer (VNA) as the source and receiver.
Over the entire band, each array pixel can be optically turned on and off with an average modulation depth of ~20 dB
and ~35 dB, for ~4 cm2 and ~0.5 cm2 imaging areas respectively. The modulation speed is ~1.3 kHz using the current
DLP system and data acquisition software. Prototype imaging demonstrations have shown that a 256-pixel image can be
obtained in the order of 10 seconds using compressed sensing (CS), and this speed can be improved greatly for potential
real-time or video-rate THz imaging. This photo-induced coded-aperture imaging (PI-CAI) technique has been
successfully applied to characterize THz beams in quasi-optical systems and THz horn antennas.
GaN/AlN quantum dots (QDs) have been observed to emit in deep ultra violet (UV) regime. The emission wavelength
can be tuned from 270 nm to 238 nm using GaN growth time and Ga flux. In this work, tunnel injection GaN/AlN QD
UV LEDs have been fabricated utilizing polarization doped p-n junctions grown on AlN templates on sapphire. The QD
EL emission is obtained at 250 nm whereas a second peak emission is observed at 290 nm from the p-type AlGaN.
However, the enhanced doping and carrier injection in polarization doped structure boosts the deep-UV emission
intensity by 26 times compared to non-polarization doped UV LED.
This paper reviews the first demonstrations of broadband graphene terahertz modulators as well as recent progress on
reconfigurable terahertz devices using graphene. Although atom-thick, single layer graphene is capable of efficiently
tuning terahertz absorption meanwhile introducing negligible insertion loss. Recent developments in terms of
transmission-mode and reflection-mode electro-absorption modulators are reviewed. Moreover, an application of these
devices is presented and discussed: arrays of graphene electro-absorption modulators as electrically reconfigurable
patterns for terahertz cameras.
Energy exchange between the electrons and phonons is particularly important to electron transport, and understanding
this process will be vital for the realization of future graphene-based electronics. Epitaxial growth is a very promising
approach for practical applications, as it has the ability to prepare graphene on a large scale and supported on a substrate.
However, epitaxially grown graphene is highly inhomogeneous, with variations in the sample thickness occurring over
length scale of a few micrometers. To pave the road for electronic devices based on epitaxial graphene, characterization
methods with high spatial resolution are required. In this paper, we describe transient absorption microscopy as a novel
tool to characterize graphene, and to interrogate the charge carrier dynamics. The carrier cooling exhibited a biexponential
decay that showed a significant dependence on carrier density. The fast and slow relaxation times were
assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting
value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial
heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.
This is attributed to differences in coupling between the graphene and the substrate. These results point to transient
absorption microscopy as a potentially important tool for characterizing graphene.
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