The sublimation of GaN is a powerful alternative etching technique to avoid the electrical traps usually induced by dry etching. It is selective towards Al containing alloys such as AlGaN and towards dielectric materials like silicon oxide or silicon nitride so that patterns can be defined to fabricate devices based on GaN/AlGaN heterostructures. In the present work, we report on the fabrication of enhancement mode p-GaN/Al(Ga)N/GaN high electron mobility transistors (HEMTs) with selective area sublimation under vacuum of the p-GaN cap layer used to define the gate. Furthermore, we show that sublimation can be combined with the regrowth of AlGaN, which is a key to increase the maximum drain current in the transistors and enables the co-integration of enhancement mode devices with depletion mode ones.
By combining nanomasking with thermally resistant materials and sublimation in a molecular beam epitaxy reactor, porous (In)GaN layers can be obtained. The advantages and disadvantages of this technique compared to classical electrochemistry methods will be discussed. The porosity can be adjusted from 0 to 1 and the pore depth can be controlled by the sublimation temperature and time. Preferential sublimation occurs at the dislocation position which strongly enhance the photoluminescence properties. As the porosification process by sublimation does not depend on the doping, fully porous light emitting diodes can be demonstrated.
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.
Since III-nitride semiconductor-based ultraviolet (UV) light-emitting diodes (LEDs) are compact and efficient, they can be suggested as a substitute for conventional arc-lamps. However, reported UV LEDs focused on a narrow range of UV spectrum contrary to conventional arc-lamps. Here, we introduce GaN quantum dots (QDs) grown on different facets of hexagonal truncated pyramid structures on a conventional sapphire substrate. These structures include semipolar facets as well as a polar facet, which obtain intrinsically different piezoelectric fields and growth rates of QDs. Consequently, we demonstrated a plateau-like broadband UV emitter ranging from UV-C to UV-A from the GaN QDs.
In recent decades, literatures about visible vertical cavity surface emitting lasers (VCSELs) have been reported. However, due to high optical loss in the cavity, lasing from deep ultraviolet (DUV) VCSEL was still rarely achieved. The optical loss in nitride DUV microcavity was analyzed in detail. DUV nitride vertical Fabry–Pérot microcavity with active layer of AlGaN-based quantum dots and double-side HfO2 / SiO2 distributed bragger reflectors was fabricated. Optical losses with of the order of 103 cm − 1 were deduced from the Q value of the cavity modes. The main origination of optical loss in DUV cavity was calculated and ascribed to the interface scattering. The interface roughness appearing after laser lift-off process and overlap between rough interface and standing optical wave were two key parameters that contributed to interface scattering loss. We believe that our results will provide useful information for improving DUV VCSEL devices.
Group-III-nitride nanophotonics on silicon is a booming field, from the near-IR to the UV spectral range. The main interest of III-nitride nanophotonic circuits is the integration of active structures and laser sources. Laser sources with a small footprint can be obtained with microresonators formed by photonic crystals or microdisks, exhibiting quality factors up to a few thousand down to the UV-C. So far, single microdisk laser devices have been demonstrated, mostly under optical pumping. Combining microdisk lasers under electrical injection with passive devices represents a major challenge in realizing a viable III-nitride nanophotonic platform on silicon. As a first step to realize this goal, we have separately demonstrated electroluminescence from microdisks and side-coupling of microdisks to bus waveguides with outcoupling gratings in the blue spectral range. We have developed the fabrication of electrically injected microdisks with a circular p-contact on top of the disk that is connected to a larger pad via a mechanically stable metal microbridge. Blue electroluminescence is observed under current injection at room temperature. We also demonstrated high Q factor (Q > 2000) WGMs in the blue spectral range from microdisks side-coupled to bus waveguides, as measured from the luminescence of embedded InGaN quantum wells. The WGM resonances are clearly observed through outcoupling gratings following propagation in partially etched waveguides to remove quantum well absorption. Small gaps between microdisks and bus waveguides of around 100 nm are necessary for efficient coupling in the blue spectral range, which represents a major fabrication challenge. We will discuss the progress brought by these building blocks to generate future III-nitride photonic circuits.
A fraction of a SiNx mono-layer is formed on a GaN layer by exposing the surface to a Si flux. When the sample is heated under vacuum at high temperature (900°C), we observe the sublimation of GaN in the regions uncovered by the thermally resistant SiNx mask. This selective area sublimation (SAS) process can be used for the formation of nanopyramids and nanowires with a diameter down to 4 nm. Also, if InGaN quantum wells are included in the structures before sublimation, InGaN quantum disks with quasi identical sizes in the 3 dimensions of space can be formed using SAS.
The high-gain photomultiplier tube (PMT) is the most popular method to detect weak ultra-violet signals which attenuate quickly in atmosphere, although the vacuum tube makes it fragile and difficult to integrate. To overcome the disadvantage of PMT, an AlN/GaN periodically–stacked-structure (PSS) avalanche photodiode (APD) has been proposed, finally achieving good quality of high gain and low excessive noise. As there is a deep г valley only in the conduction band of both GaN and AlN, the electron transfers suffering less scattering and thus becomes easier to obtain the threshold of ionization impact. Because of unipolar ionization in the PSS APD, it works in linear mode. Four prototype devices of 5-period, 10-period, 15-period, and 20-period were fabricated to verify that the gain of APD increases exponentially with period number. And in 20-period device, a recorded high and stable gain of 104 was achieved under constant bias. In addition, it is proved both experimentally and theoretically, that temperature stability on gain is significantly improved in PSS APD. And it is found that the resonant enhancement in interfacial ionization may bring significant enhancement of electron ionization performance. To make further progress in PSS APD, the device structure is investigated by simulation. Both the gain and temperature stability are optimized alternatively by a proper design of periodical thickness and AlN layer occupancy.
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.
GaN/Al0.5Ga0.5N quantum dots deposited on the (11-22) plane have been grown by combining Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE). The (11-22) GaN oriented template was realized by MOVPE starting from a M-plane oriented sapphire substrate. The average dot sizes are the following: between 15 and 20 nm in the <-1-123> and <1-100> directions and a height ranging between 0.8 and 1.4 nm. Their density is ranging between 2 and 8x1010cm-2. The crystal field splitting is measured in Al0.5Ga0.5N via polarized microphotoluminescence. We study the photoluminescence properties of small quantum dots which present innovative optical properties among which are the evolution of the polarization of the emitted photons at different temperatures. We also analyze the distortion of the photoluminescence at different time delays after the excitation pulse. A redshift is found that is attributed to the complex thermally-induced delocalization of the carriers through the assembly of dots from the smaller ones to the bigger ones.
Julien Brault, Benjamin Damilano, Aimeric Courville, Mathieu Leroux, Abdelkarim Kahouli, Maxim Korytov, Philippe Vennéguès, Gaetano Randazzo, Sébastien Chenot, Borge Vinter, Philippe De Mierry, Jean Massies, Daniel Rosales, Thierry Bretagnon, Bernard Gil
(Al,Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al,Ga)N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al,Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented “polar” and (11 2 2) oriented “semipolar” materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al,Ga)N nanostructure-based LED demonstrators are presented.
Benjamin Damilano, Kaddour Lekhal, Hyonju Kim-Chauveau, Sakhawat Hussain, Eric Frayssinet, Julien Brault, Sébastien Chenot, Philippe Vennéguès, Philippe De Mierry, Jean Massies
Commercially available inorganic white light emitting diodes (LEDs) are essentially based on the combination of a blue InGaN based LED chip covered by a long wavelength emitting (yellow, red) phosphor. We propose to avoid this step of phosphor deposition by taking advantage of the fact that yellow to red emission can be achieved using InGaN alloys. By stacking an InGaN/GaN multiple quantum well (QW) emitting in the yellow, acting as a light converter, and a short wavelength blue-violet pump LED grown on top, white light emission can be obtained. Furthermore, if we extend the emission spectrum of the light converter into the red, a warm white light color is demonstrated when a pump LED is grown on top. However, the high In content InGaN QWs of the light converter have a low thermal stability and the QW efficiency tends to degrade during the growth of the pump LED. Three different solutions are explored to avoid the thermal degradation of the light converter. The monolithic LED structures were grown by molecular beam epitaxy (MBE), by a combination of both MBE and metal-organic chemical vapor phase epitaxy (MOCVD), or by a low temperature full-MOCVD process. The best results are obtained using a complete MOCVD growth process. The structure and the MOCVD growth conditions are specifically adapted in order to avoid the thermal degradation of the large In composition InGaN QWs emitting at long wavelength during the growth of the subsequent layers.
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.
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.
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.
Studies on the optical properties related to built-in internal electric field and carrier localization present in the GaN self-assembled quantum dots (QDs) are essential for the physical interest in atomic-like confined system and the visible and ultraviolet light emitting applications. We have systematically studied the optical properties of hexagonal GaN (h-GaN) and cubic GaN (c-GaN) self-assembled QDs by means of photoluminescence (PL), PL excitation (PLE), cathodoluminescence (CL), and time-resolved PL experiments. The GaN self-assembled QD samples were grown in Stranski-Krastanov mode by plasma-assisted molecular beam epitaxy. The substrates for the growth of h-GaN and c-GaN were 6H-SiC and 3C-SiC, respectively. With increasing temperature, the PL intensity of GaN quantum wells was dramatically decreased while that of GaN QDs was not changed much. From the wavelength-resolved CL images, strong carrier localization in the QD confinement was clearly observed. An apparent Stokes-like shift between PLE absorption edge and PL emission from the h-GaN QDs increases with increasing detection wavelength (so, with QD size), which is attributed to the separation of wavefunction overlap due to the built-in internal field present in the QDs. From the time-resolved PL experiments, we found that the measured lifetime of the h-GaN QDs emission increased with emission wavelength (i.e., with QD size), while that of the c-GaN QDs kept almost constant. It is concluded that the h-GaN QD emissions are strongly influenced by built-in internal electric field as well as carrier localization in the QDs.
In this work we demonstrated multicolor quantum dot IR photodetectors with normal incidence background limited performance at 70K. The devices, which were studied in this work, were composed of self-assembled InAs dots grown by MBE on InAlAs barrier lattice matched to InP substrate. Normal incidence photocurrent spectra reveal several polarized and unpolarized peaks in the range 100-350 meV due to the intersubband transitions. The detector resonsivity at normal incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in quatnum well IR photodetectors.
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