Deep ultra-violet (DUV) light emitting diodes (LED) are expected to be the next generation of UV sources, offering significant advantages such as compactness, low consumption and long lifetimes. Yet, improvements of their performances are still required and the potential of AlyGa1-yN quantum dots as DUV emitters is investigated in this study. Using a stress induced growth mode transition, quantum dots (QD) are spontaneously formed on Al0.7Ga0.3N/AlN heterostructures grown on sapphire substrates by molecular beam epitaxy. By increasing the QD Al composition, a large shift of the QD photoluminescence in the UV range is observed, going from an emission in the near UV for GaN QD down to the UVC region for Al0.4Ga0.6N QD. A similar behavior is observed for electroluminescence (EL) measurements performed on LED structures and an emission ranging from the UVA (320-340 nm) down to the UVC (265-280 nm) has been obtained. The main performances of Al0.7Ga0.3N based QD LED are presented in terms of electrical and optical characteristics. In particular, the emission dependence on the input current density, including the emitted wavelength, the optical power and the external quantum efficiency are shown and discussed.
The fast development of nitrides has given the opportunity to investigate AlGaN as a material for ultraviolet detection. Such AlGaN based camera presents an intrinsic spectral selectivity and an extremely low dark current at room temperature.
Firstly, we will present results on focal plane array of 320x256 pixels with a pitch of 30μm. The peak responsivity is around 280nm (solar-blind), 310nm and 360nm. These results are obtained in a standard SWIR supply chain (readout circuit, electronics).
With the existing near-UV camera grown on sapphire, the short wavelength cutoff is due to a window layer improving the material quality of the active layer. The ultimate shortest wavelength would be 200nm due to sapphire substrate. We present here the ways to transfer the standard design of Schottky photodiodes from sapphire to silicon substrate. We will show the capability to remove the silicon substrate, and etch the window layer in order to extend the band width to lower wavelengths.
The achievement of deep ultraviolet (UV) focal plane arrays (FPA) is required for both solar physics [1] and micro electronics industry. The success of solar mission (SOHO, STEREO [2], SDO [3]…), has shown the accuracy of imaging at wavelengths from 10 nm to 140 nm to reveal effects occurring in the sun corona. Deep UV steppers at 13 nm are another demanding imaging technology for the microelectronic industry in terms of uniformity and stability. A third application concerns beam shaping of Synchrotron lines [4]. Consequently, such wavelengths are of prime importance whereas the vacuum UV wavelengths are very difficult to detect due to the dramatic interaction of light with materials.
The fast development of nitrides has given the opportunity to investigate AlGaN as a material for UV detection. Camera based on AlGaN present an intrinsic spectral selectivity and an extremely low dark current at room temperature. We have previously presented several FPA dedicated to deep UV based on 320 x 256 pixels of Schottky photodiodes with a pitch of 30 μm [4, 5]. AlGaN is grown on a silicon substrate instead of sapphire substrate only transparent down to 200 nm.
After a flip-chip hybridization, silicon substrate and AlGaN basal layer was removed by dry etching. Then, the spectral responsivity of the FPA presented a quantum efficiency (QE) from 5% to 20% from 50 nm to 290 nm when removing the highly doped contact layer via a selective wet etching. This FPA suffered from a low uniformity incompatible with imaging, and a long time response due to variations of conductivity in the honeycomb. We also observed a low rejection of visible. It is probably due to the same honeycomb conductivity enhancement for wavelength shorter than 360 nm, i.e., the band gap of GaN.
We will show hereafter an improved uniformity due to the use of a precisely ICP (Inductively Coupled Plasma) controlled process. The final membrane thickness is limited to the desertion layer. Neither access resistance limitation nor long response time are observed. QE varies from 5% at 50 nm to 15% at 6 nm (85% more when taking into account the filling factor). Consequently, we can propose prototypes concerning not only “solar blind” camera optimized for narrow band in the near UV range (between 280 nm and 260 nm), but also devices with spectral range extended in the deep UV (290 nm to 10 nm). Both detectors are available for an optical budget evaluation.
The potential of GaN for X-ray detection in the range from 5 to 40 keV has been assessed. The absorption coefficient has been measured as a fonction of photon energy. Various detectors have been fabricated including MSM and Schottky diodes. They were tested under polychromatic X-ray illumination and under monochromatic irradiation from 6 to 22 keV in the Soleil synchrotron facility. The vertical Schottky diodes perform better as their geometry is better suited to the thick layers required by the low absorption coefficient. The operation mode is discussed in terms of photoconductive and photovoltaic behaviors. Some parasitic effects related to the electrical activation of defects by high energy photons and to the tunnel effect in lightly doped Schottky diodes have been evidenced. These effects disappear in diodes where the doping profile has been optimized. The spectral response is found to be very consistent with the spectral absorption coefficient. The sensitivity of GaN Schottky diodes is evaluated and found to be on the order of 40 photons per second. The response is fast nd linear.
Some 2D imagers based on AlGaN materials have been developed in the framework of a CNES founded research
program to sustain visible blind imagers devoted to solar physics. We have already presented several prototypes of focal
plane arrays extending the range of detection from near UV to deep UV [1, 2]. It consists in an array of 320x256 pixels
of Schottky photodiodes with a pitch of 30 μm. AlGaN is grown on a silicon substrate instead of sapphire substrate only
transparent down to 200 nm. The use of honeycomb structure has straightened the membrane after hybridization,
maintained membrane integrity but decreases the filling factor. After a preliminary study to optimize substrate and
AlGaN window layer elimination, 12 focal plane arrays have been fabricated in order to achieve aging and reliability
tests based on thermal cycling. Technological analyses such as cross-section, profilometry, microscopy and electrical
measurements are presented without showing any ageing effect. We present here the final results with a complete
evaluation of quantum efficiency on all the spectral range of interest. A large intrinsic absorption in AlGaN takes place
in the 100 nm range where the quantum efficiency decreases down to 1%. Several growth parameters are identified as a
key component to avoid cracks in the epitaxial structure and surface electrical traps affecting the quantum efficiency.
Photodetectors designed for the Extreme Ultraviolet (EUV) range with the Aluminum Gallium Nitride
(AlGaN) active layer are reported. AlGaN layers were grown by Molecular Beam Epitaxy (MBE) on
Si(111) wafers. Different device structures were designed and fabricated, including single pixel
detectors and 2D detector arrays. Sensitivity in different configurations was demonstrated, including
front- and backside illumination. The latter was possible after integration of the detector chips with
dedicated Si-based readouts using high-density In bump arrays and flip-chip bonding. In order to avoid
radiation absorption in silicon, the substrate was removed, leaving a submicron-thin membrane of
AlGaN active layer suspended on top of an array of In bumps. Optoelectrical characterization was
performed using different UV light sources, also in the synchrotron beamlines providing radiation
down to the EUV range. The measured cut-off wavelength of the active layer used was 280 nm, with a
rejection ratio of the visible radiation above 3 orders of magnitude. Spectral responsivity and quantum
efficiency values
We report on the fabrication and characterization of solar blind Metal-Semiconductor-Metal (MSM) based
photodetectors for use in the extreme ultraviolet (EUV) wavelength range. The devices were fabricated in the AlGaN-on-
Si material system, with Aluminum Gallium Nitride (AlGaN) epitaxial layers grown on Si(111) by means of Molecular
Beam Epitaxy. The detectors' IV characteristics and photoresponse were measured between 200 and 400 nm. Spectral
responsivity was calculated for comparison with the state-of-the-art ultraviolet photodetectors. It reaches the order of 0.1
A/W at the cut-off wavelength of 360 nm, for devices with Au fingers of 3 μm width and spacing of 3 μm. The rejection
ratio of visible radiation (400 nm) was more than 3 orders of magnitude. In the additional post-processing step, the Si
substrate was removed locally under the active area of the MSM photodetectors using SF6-based Reactive Ion Etching
(RIE). In such scheme, the backside illumination is allowed and there is no shadowing of the active layer by the metal
electrodes, which is advantageous for the EUV sensitivity. Completed devices were assembled and wire-bonded in
customized TO-8 packages with an opening. The sensitivity at EUV was verified at the wavelengths of 30.4 and 58.4 nm
using a He-based beamline. AlGaN photodetectors are a promising alternative for highly demanding applications such as
space science or modern EUV lithography. The backside illumination approach is suited in particular for large, 2D focal
plane arrays.
The fast development of nitrides has given the opportunity to investigate AlGaN as a material for ultraviolet detection.
Camera based on this alloy present an intrinsic spectral selectivity and an extremely low dark current at room
temperature. We present here an extension from near UV (360 nm-260 nm) to deep UV (10 nm-200 nm) in a packaging
common to the SWIR supply chain. It concern both readout circuit and camera electronics. Such camera are now
available for on UV optical budget evaluation. The vacuum UV wavelengths are a very difficult range for detection due
to the strong interaction of light with materials. Nevertheless, such wavelengths are of prime importance for solar
observation. We present a prototype of focal plane arrays to extend the range of detection from near UV to deep UV. It
is based on 320 x 256 pixels of Schottky photodiodes with a pitch of 30 μm. AlGaN is grown on a silicon substrate
instead of sapphire substrate only transparent down to 200 nm. After a flip-chip hybridization, silicon substrate is
thinned and removed by dry etching. The use of a honeycomb structure straightens the membrane after hybridization and
allows the membrane integrity. The results show that the dry etching process doesn't affect the readout circuit properties.
The dark current is negligible and the measured noise is the readout noise due to the large capacitance of the photodiode.
The spectral responsivity of this focal plane array presents a quantum efficiency from 10% to 20% from 50 nm to
290 nm after the removing of the highly doped contact layer.
We present several prototypes to extend the range of AlGaN focal plane arrays from near UV to deep UV range
(200 nm - 4 nm). Arrays include 320x256 pixels with a pitch of 30 μm and are based on Schottky photodiodes. AlGaN
is grown on a silicon substrate. After a flip-chip hybridization, silicon substrate is thinned and removed by dry etching.
The tricky point is to maintain the membrane integrity. By using a honeycomb structure in the Si substrate, after
hybridization, we were able to keep the membrane plane and rigid, avoid the crack expansion, and thus maintain the
membrane integrity. The structure includes an Al.35Ga.65N active layer grown on a thick Al.55Ga .45N window layer, with
a graded AlGaN layer in between. The high quality materials are grown by MBE. The Al.55Ga.45N window layer is also
thinned by dry etching down to the gradual layer and desertion layer where a higher internal electric field takes place.
The results show that the dry etching process doesn't affect the readout circuit properties. The dark current is negligible
and non uniformity in etching slightly contributes into a constant offset. The measured noise factor, a bit more than 100
electrons rms, is due to reset noise in the integration capacitance and in other parasitic capacitances. With a peak
response at 300 nm of 35%, the responsivity is 1% at 266 nm and in the deep UV range. The spectral responsivity
measured on a synchrotron line at a wavelength of 2nm reaches more than 200% due to multiple photoexcitation.
We report on the results of fabrication and optoelectrical characterization of Gallium Nitride (GaN) based Extreme
UltraViolet (EUV) photodetectors. Our devices were Schottky photodiodes with a finger-shaped rectifying contact,
allowing better penetration of light into the active region. GaN layers were epitaxially grown on Silicon (111) by Metal-
Organic-Chemical Vapor Deposition (MOCVD). Spectral responsivity measurements in the Near UltraViolet (NUV)
wavelength range (200-400 nm) were performed to verify the solar blindness of the photodetectors. After that the
devices were exposed to the EUV focused beam of 13.5 nm wavelength using table-top EUV setup. Radiation hardness
was tested up to a dose of 3.3·1019 photons/cm2. Stability of the quantum efficiency was compared to the one measured
in the same way for a commercially available silicon based photodiode. Superior behavior of GaN devices was observed
at the wavelength of 13.5 nm.
We report on the fabrication of Schottky-diode-based Extreme UltraViolet (EUV) photodetectors. The devices were
processed on Gallium Nitride (GaN) layers epitaxially grown on 4 inch Silicon (111) substrates by Metal-Organic
Chemical Vapor Deposition (MOCVD). Cutoff wavelength was determined together with the spectral responsivity
measurements in the Near UltraViolet (NUV) range (200nm to 400nm). Absolute spectral responsivity measurements
were performed in the EUV range (5nm to 20nm) with the synchrotron radiation using the facilities of Physikalisch-
Technische Bundesanstalt (PTB), located at Berliner Elektronenspeicherring-Gesellschaft fuer Synchrotronstrahlung
(BESSY). The described work is done in the framework of the Blind to Optical Light Detectors (BOLD) project
supported by the European Space Agency (ESA).
The fast development of nitrides has given the opportunity to investigate AlGaN as a material for ultraviolet detection.
Such camera present an intrinsic spectral selectivity and an extremely low dark current at room temperature. It can
compete with technologies based on photocathodes, MCP intensifiers, back thinned CCD or hybrid CMOS focal plane
arrays (FPA) for low flux measurements. AlGaN based cameras allow UV imaging without filters or with simplified
ones in harsh solar blind conditions. Few results on camera have been shown in the last years, but the ultimate
performances of AlGaN photodiodes couldn't be achieved due to parasitic illumination of multiplexers, responsivity of p
layers in p-i-n structures, or use of cooled readout circuit. Such issues have prevented up to now a large development of
this technology. We present results on focal plane array of 320x256 pixels with a pitch of 30μm for which Schottky
photodiodes are multiplexed with a readout circuit protected by black matrix at room temperature. Theses focal plane
present a peak reponsivity around 280nm and 310nm with a rejection of visible light of four decades only limited by
internal photoemission in contact. Then we will show the capability to outdoor measurements. The noise figure is due to
readout noise of the multiplexer and we will investigate the ultimate capabilities of Schottky diodes or Metal-
Semiconductor-Metal (MSM) technologies to detect extremely low signal. Furthermore, we will consider deep UV
measurements on single pixels MSM from 32nm to 61nm in a front side illumination configuration. Finally, we will
define technology process allowing backside illumination and deep UV imaging.
Metal-Semiconductor-Metal photodiodes were fabricated on epitaxially grown AlxGa1-xN on Si(111). The Aluminium
content of the layers grown by means of molecular beam epitaxy (MBE) was 50, 80 and 100%, respectively. The
processing was performed by standard microelectronic fabrication techniques like photolithography, wet and dry etching
(RIE) and physical and chemical vapor deposition (PVD,CVD). The devices were characterized under illumination in a
wavelength range from 400 to 185nm to determine the cut-off wavelength defined by the band-gap energy. Typical
figures of merit like spectral responsivity R quantum efficiency &eegr; and specific detectivity D* have been extracted from
the measurement data.
The photo-response of AlGaN based UV detectors to a 193 nm excimer laser radiation is presented. Two devices have
been tested and compared, a metal-semiconductor-metal (MSM) planar structure and a Schottky diode. These sensors
have already shown good performances in the 240-280 nm region under CW illumination and have been used for the
realization of 2D and linear arrays. Here the capability of these devices to detect the emission of a nanosecond pulsed
excimer laser is proven and the decay time and dependency on the beam's density of energy evaluated. The measured
transient response of the MSM device closely follows the nanosecond laser pulses, with a decay time shorter than 3 ns.
Conversely, the Schottky diodes showed a slower rise and decay kinetics principally limited by the coupling with the
junction capacitance. The decay curve of such a device has been analyzed on the basis of two decay mechanisms: the
second exponential decay has been found to be in the order of 40 ns. This slow kinetic has been attributed to the presence
of trap states localized at a distance from the conduction or valence band larger then the thermal energy of the carriers.
Both the realized devices do follow the Rose's law with a linear response at the lower beam fluxes (density of energy
4×10-5 - 0.2 mJ/mm2) and a transition to a sub-linear regime for higher fluxes.
The fast development of nitrides has given the opportunity to investigate AlGaN as a material for ultraviolet solar blind detection in competition with technologies based on photocathodes, MCP intensifiers, back thinned CCD or hybrid CMOS focal plane arrays. All of the them must be associated to UV blocking filters. These new detectors present both an intrinsic spectral selectivity and an extremely low dark current at room temperature. First we will present the ultimate properties of the AlGaN based devices. These spectral properties are analysed in regards to the sharp cut off required for
solar blind detection around 280nm, and we will quantify how the stringent difficulties to achieve solar blind filters can be reduced. We also investigated the electrical capabilities of Schottky diodes or Metal-Semiconductor-Metal (MSM) technologies to detect extremely low UV signal. We will especially present results from a linear array based on a CCD readout multiplexor.
The influence of carrier localization on the opto-electronic properties of GaInNAs/GaAs quantum well (QW) light emitting diodes (LED) and laser diodes (LD) grown by molecular beam epitaxy is studied. The external quantum efficiency of the LEDs at low temperature is found to be strongly affected by emission from localized states, and its evolution with the injected current is modified compared to the typical one of a QW LED. The light-current characteristics of GaInNAs LDs are measured for different temperatures between 15 and 295 K, and an anomalous behaviour of the threshold current with temperature is obtained comparing to a reference InGaAs laser. In particular, a negative or infinite T0 is obtained at very low temperatures, followed by a region of very small T0. In addition, if the temperature is further increased, a change to a higher T0 is obtained at a temperature which is in the range of the typical delocalization temperatures in GaInNAs QWs. All these features are attributed to the influence of carrier localization. The temperature induced changes in the relative carrier population of the localized states and the band edge states change the lineshape of the gain spectrum and its peak value, and consequently the threshold current of GaInNAs QW lasers.
A lot of progress have been recently realized concerning the laser performances at 1.3 μm. However, extending the emission of (Ga,In)(N,As) lasers above 1.3 μm with good performances is still challenging, since it is reported that the threshold current density significantly increases. In order to extend the lasing wavelength above 1.3 μm, while keeping good laser characteristics, we have optimized the growth of (Ga,In)(N,As)/GaAs quantum wells (QWs) grown by molecular beam epitaxy in view of realizing laser structures. During the growth of a laser structure the QW is "self"-annealed due to the growth of the upper AlGaAs cladding layer at high temperature. It is important to know the effect of this self-annealing on the QW optical properties. For that purpose, we have realized in situ thermal annealing on QWs grown at different temperatures and with different nitrogen composition. Separate confinement hetero-structure laser diodes with a single In0.4Ga0.6As1-xNx (x=0.015, 0.021 and 0.033)/GaAs QW have been grown, combining a low growth temperature and a high in situ annealing temperature. The broad area devices have a room temperature threshold current density of 1500 A/cm2 and emit around 1.34 μm just above threshold. Furthermore, increasing the nitrogen composition extends the lasing operation up to 1.44 μm with a threshold of 1755 A/cm2 and even to 1.52μm with a 4060A/cm2 threshold.
Lasing emission is demonstrated at room temperature in the entire spectral region from 1.29 to 1.52 microns using GaInNAs/GaAs quantum well (QW) laser diodes (LD) grown by molecular beam epitaxy on GaAs substrates. The separate confinement heterostructures (SCH) is made up by AlGaAs cladding layers, a GaInNAs-based QW and GaAs barriers. To achieve lasing emission from 1.29 to 1.52 microns the In in the QW content is maintained at 40%, while the N content is varied from 1.3 to 3.3%. With this structure, the threshold current density (Jth) and external differential quantum efficiency (hd) at 1.29 microns are 685 A/cm2 and 45 %, respectively. Increasing the wavelength to reach 1.5 micron emission degrades these figures to Jth=2890 A/cm2 and hd=23% at 1.49 microns, and to Jth=4060 A/cm2 and hd=16% at 1.52 microns, which still represent a very large improvement with respect to previous reports of LDs based on the quaternary. Even though adding N to the structure decreases the internal quantum efficiency (hi), from 75% to 50%, this figure does not change with increasing wavelengths up to 1.44 microns. The differential modal gain also degrades as a result of adding N to the QW, but like the case of hi, does not change significantly with increasing wavelength. Thus, achieving long wavelength emission up to 1.55 micron emission starts to become viable, even with simple LD structures.
High-power unipolar GaAs/AlGaAs lasers emitting in the 14-15 micrometers wavelength range under optical pumping by a pulsed CO2 laser are investigated. Operation of edge lasers with side-facet pumping gas well as broad-area lasers with normal-incidence pumping is demonstrated. We show that record high optical powers can be obtained from these quantum fountain unipolar lasers. Optical powers per facet as high as 6.6 W for edge lasers and 7.8 W for broad-area lasers are achieved with TM00 mode emission. Extended tunability of the lasing wavelength, (Delta) (lambda) /(lambda) approximately equals 2.5 percent, is observed by varying the pump wavelength. Operating temperatures as high as 137 K are presently achieved. Application of quantum fountain unipolar lasers to CO2 gas detection is demonstrated.
During the last decade, the QWIPs technology has improved from start to an undeniable maturity level. High performance focal plane arrays have already been realized (ATT, Lockheed-Martin, JPL, . . .) with a spectacular format increase ranging from 128 by 128 up to 640 by 480, and images from bicolor 256 by 256 arrays have been shown last year. All these devices illustrate the high potential of the QWIP technology. In the same time, the modeling of detection mechanism has advanced to permit the present design of specific detectors and their optimization in given operating environments (near 77 K detector temperature for instance). In this communication, we summarize our recent technological studies leading to the next generation of very large infrared detector arrays. We present the QWIP ultimate performances allowed by the standard dual III - V technological processes developed at THOMSON CSF, in terms of pixel size, array filling factor or connectics. The influence of the pixel size for the grating optical coupling is analyzed. We finally include in this analysis our results for more complex devices like multispectral infrared detectors.
Quantum well infrared photodetectors (QWIPs) form a new generation of infrared detectors based on carrier confinement in ultrathin semiconductor heterostructures. The artificial energy levels in these wells can be tailored to match any optical transition in the 3 - 20 micrometer photon wavelength range by adjusting the quantum well width and the barrier composition. In this communication, we summarize our present understanding of the physics of QWIP detection: photoexcited carrier emission and capture probability, contact injection, and noise mechanisms. We also present the performances of optimized devices for the infrared Detection in the 3 - 5 micrometer and 8 - 12 micrometer wavelength ranges. We also illustrate the major advantages of this new technology for infrared staring arrays: (1) standard III-V substrates and technology, thermal stability, uniformity, large areas, low development costs, radiation hardness, (2) adjustability from 3 to 20 micrometer, (3) new functions: multispectrality, spectrophotometry, band switching, optical reading.
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