The optimization and performance of opaque Galium Nitride (GaN) photocathodes deposited directly on novel Microchannel Plates (MCPs) are presented in this paper. The novel borosilicate glass MCPs, which are manufactured with the help of Atomic Layer Deposition, can withstand higher temperatures enabling direct deposition of GaN films on their surfaces. The quantum efficiency of MBE-grown GaN photocathodes of various thickness and buffer layers was studied in the spectral range of ~200-400 nm for the films grown on different surface layers (such as Al2O3 or buffer AlN layer) in order to determine the optimal opaque photocathode configuration. The MCPs with the GaN photocathodes were activated with surface cesiation in order to achieve the negative Electron Affinity for the efficient photon detection. The opaque photocathodes enable substantial broadening of the spectral sensitivity range compared to the semitransparent configuration when the photocathodes are deposited on the input window. The design of currently processed sealed tube event counting detector with an opaque GaN photocathode are also described in this paper. Our experiments demonstrate that although there is still development work required the detection quantum efficiencies exceeding 20% level should be achievable in 200-400 nm range and <50% in 100-200 nm range for the event counting MCP detectors with high spatial resolution (better than 50 μm) and timing resolution of <100 ps and very low background levels of only few events cm-2 s-1.
Polarization-induced electric fields in AlGaN quantum wells have important effects on avalanche breakdown of AlGaN
quantum-well photodiodes. When the polarization-induced fields within the AlGaN well layers have the same direction
as applied electric field, they can help enhance impact ionization rate and decrease threshold voltage of avalanche
breakdown of AlGaN avalanche photodiodes. However, according to previous research on avalanche breakdown of
AlGaN photodiodes, no distinct breakdown threshold was observed from current-voltage curve. Instead, a soft avalanche
breakdown was observed across applied voltage ranging from zero to a few volts while electroluminescence spectra
show a threshold of about 10 V for avalanche breakdown. In this work, by considering impact ionization of defect levels
and carrier screening effect, impact ionization coefficients are calculated as functions of applied voltage and the soft
breakdown is well explained. It is also found that strong carrier screening effect will decrease impact ionization rate in a
certain range of voltage thus affecting device performance.
Gallium nitride opaque and semitransparent photocathodes provide high ultraviolet quantum efficiencies from 100 nm
to a long wavelength cutoff at ~380 nm. P (Mg) doped GaN photocathode layers ~100 nm thick with a barrier layer of
AlN (22 nm) on sapphire substrates also have low out of band response, and are highly robust. Opaque GaN photocathodes
are relatively easy to optimize, and consistently provide high quantum efficiency (70% at 120 nm) provided the
surface cleaning and activation (Cs) processes are well established. We have used two dimensional photon counting
imaging microchannel plate detectors, with an active area of 25 mm diameter, to investigate the imaging characteristics
of semitransparent GaN photocathodes. These can be produced with high (20%) efficiency, but the thickness and conductivity
of the GaN must be carefully optimized. High spatial resolution of ~50 μm with low intrinsic background (~7
events sec-1 cm-2) and good image uniformity have been achieved. Selectively patterned deposited GaN photocathodes
have also been used to allow quick diagnostics of optimization parameters. GaN photocathodes of both types show great
promise for future detector applications in ultraviolet Astrophysical instruments.
A promising sensing technology utilizing AlGaN/GaN high electron mobility transistors (HEMTs) has been
developed to analyze a wide variety of environmental and biological gases and liquids. The conducting 2DEG channel
of GaN/AlGaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. Examples of
detecting mercury ions, perkinsus, lactic acid, carbon dioxide, and vitellogenin are discussed in this paper.
InGaN alloy fluctuations have been exploited in many nitride optoelectronic devices. This work reports on the
application of InGaN alloy fluctuations in a packaged vacuum electronic device utilizing an InGaN photocathode as
the detector element. The resulting image intensifier is the first ever InGaN imaging detector.
Exploitation of the particular InGaN properties of alloy fluctuations has several positive consequences for
photocathodes. One, it is advantages because of the possibility of extending the spectral response to the longer
wavelengths with lower average indium concentrations.
Two, in achieving a longer wavelength response, this lessens the strain at the sapphire-AlN-InGaN
interface because a lower average In percentage can be used. Thirdly, the larger bandgap InGaN matrix material will
have a lower amount of thermionic emission coupled with this longer wavelength photoresponse. Finally, an InGaN
alloy with visible response holds the promise in that it can be grown directly on a sapphire window as opposed to the
compression bonding of GaAs as originally reported by Antypas and Edgecumbe.
Nitride based photocathodes for image intensifiers are of interest because of the wide span of wavelengths covered by
the bandgap of the AlGaInN alloy system. The potential bandgap range for this alloy system is from 6.2 eV for AlN to
0.7 eV for InN. Coupled with microchannel plate technology, this alloy system potentially offers low noise and high gain
image intensifiers over a wide wavelength range. Results from L-3 EOS work in this area are presented beginning with a
brief summary of unpublished early work carried out from 1992 - 1997 on AlGaN image intensifiers. The early work
wrestled with the dual issues of sealing image intensifiers along with improving the quality of the AlGaN epitaxy layer.
This is followed by our current results on a GaN image intensifier sealed with a photocathode from SVTA. Imagery
using 375nm LED illumination is shown. The quantum efficiency at 300nm was estimated to be 16% measured in
transmission mode. This QE was achieved with a 0.15μm thick Mg doped GaN active layer.
Carrier-screening effect in an AlGaN quantum-well avalanche photodiode was investigated. The avalanche photodiode
is a p-i-n diode consisting of three periods of Al0.1Ga0.9N/Al0.15Ga0.85N multiple quantum wells (MQWs) as the active
region. Avalanche electroluminescence (EL) spectra were measured at different reverse bias voltages. By measuring the
quantum Stark induced red-shift of the EL peak at different bias values , it was found that carrier screening decreases the
local electric field across the AlGaN quantum wells, resulting in a reduced red shift of the EL peak, while enhancing the
EL intensity. The carrier screening was found to be strong at the onset of avalanche breakdown of the diode and become
weaker with increasing of the applied electric field. This is explained by considering the strong polarization-induced
internal field in the barriers of the quantum wells. The polarization-induced electric field in the barriers has a direction
opposite to the applied field thus producing a potential barrier to block the carrier transport. This leads to accumulation
of carriers in the quantum wells, thus producing the screening effect. The direction of electric field in the barriers inverts
when the applied bias increases to be larger than an inverse threshold. Carriers can then be smoothly transported and
carrier screening disappears. Our study shows that carrier-screening effect can play an important role in III-Nitride
heterojunction devices due to the existence of strong polarization fields.
Epitaxial growth of p-type GaN-based UV photocathode by RF plasma assisted molecular beam epitaxy (MBE) on
sapphire, fused silica, and alumina substrates was investigated. The electrical measurements indicted the growth of
highly p-type GaN films as thin as 0.1 μm on c-plane sapphire with a thin AlN nucleation layer. Polycrystalline p-type
GaN was obtained for growth on fused silica and alumina. Negative electron affinity (NEA) photocathodes were
fabricated by cesium activation of the p-type GaN films in vacuum. Quantum efficiency for UV detection on different
substrates was then characterized. To study the integration of UV photocathodes with MCPs, direct deposition of p-type
GaN films on glass MCPs were done at low growth temperatures by MBE. The detection efficiency of polycrystalline p-
GaN photocathodes in reflection mode was much less than the high quality p-type GaN films on sapphire, however, it
was comparable to the detection efficiency of the latter measured in the semitransparent mode. This indicates the
potential for fabrication of improved photocathodes with higher gain and better spatial and temporal resolutions.
Recent progress in Gallium Nitride (GaN, AlGaN, InGaN) photocathodes show great promise for future detector applications
in Astrophysical instruments. Efforts with opaque GaN photocathodes have yielded quantum efficiencies up to
70% at 120 nm and cutoffs at ~380 nm, with low out of band response, and high stability. Previous work with semitransparent
GaN photocathodes produced relatively low quantum efficiencies in transmission mode (4%). We now have
preliminary data showing that quantum efficiency improvements of a factor of 5 can be achieved. We have also performed
two dimensional photon counting imaging with 25mm diameter semitransparent GaN photocathodes in close
proximity to a microchannel plate stack and a cross delay line readout. The imaging performance achieves spatial resolution
of ~50μm with low intrinsic background (below 1 event
sec-1 cm-2) and reasonable image uniformity. GaN photocathodes with significant quantum efficiency have been fabricated on ceramic MCP substrates. In addition GaN
has been deposited at low temperature onto quartz substrates, also achieving substantial quantum efficiency.
GaN / Al1-xGaxN-based hetero-structures have demonstrated a versatility in RF electronic applications which
is practically unmatched by any other material system. There are many device structures under consideration
for use in RF and Power amplifiers, suitable for both commercial and military applications.
In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC
substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN
epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth
was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled
showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating
Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM
measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance
mapping, optical reflectance, and high-resolution X-ray diffraction.
Several of the wafers were fabricated into HEMT devices, and the results on these devices are also
This paper reports temperature-dependent DC and small-signal RF characteristics of a 0.4-mm-radius sapphire-based GaN p-i-n diode between -60°C and 175°C. Deep levels approximately 1 eV below the conduction band were observed in both persistent
photo-conductance and photo-capacitance measurements. Self-heating effects were also observed and modeled with the measured thermal resistance and time constant. Based on these characteristics, an equivalent-circuit model was constructed, which accurately predicted the temperature-dependent DC and RF characteristics of the diode.
We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/InGaN superlattices. As we show highly conductive heterostructures can be obtained by inserting a graded doped layer, which reduces the barrier height while maintaining high sheet carrier density. For optimized p-type AlGaN/GaN SL, an eight fold reduction of the barrier height and a 1.5 times increase in sheet hole density is obtained compared to typical SL. The optimized structure yields 13 orders of magnitude improvement in vertical conductivity (σV) compared to typical SL, and 35 times improvement in lateral conductivity (σL) compared to bulk p-GaN. For optimized p-type GaN/InGaN SL, an improvement of more than 10 orders of magnitude in σV compared to typical SL is obtained with σL better than that of bulk p-InGaN. We also investigate n-type SLs as current spreading layers. A significant improvement in current distribution is obtained for the optimized SLs.
GaN /AlGaN transistors are being developed for a variety of RF electronic and high temperature elctronics applications that will replace GaAs and Silicon devices and circuits for commercial and military applications. AlGaN/ GaN based HEMT device structure shows significant potential to meet these needs. In this paper, we present a GaN/AlGaN based HEMT design with modeling results, that includes AlN buffer layer followed by AlGaN layers on lattice matched semi-insulating SiC substrates. These devices were grown using RF Plasma Assisted MBE Technique. This approach has demonstrated very uniform epitaxial layers. Key to high quality HEMT structures is the ability to grow high quality AlN Buffer layers. Details of the electrical and optical characteristics of the HEMT layers and devices are presented and a short overview of semi-insulating SiC crystal growth is given.