Last news

Resume : We have achieved to measure a film thickness of GaN-on-GaN homoepitaxial layer nondestructively. It was found out that the absorption coefficient of GaN in near infrared region strongly depends on its carrier concentration. Based on this finding, we succeed in nondestructive films thicknesses measurement using Fourier transform infrared spectroscopy (FT-IR). The homoepitaxial layer consists of an n--layer of 13 um with Si of 9x10^15 cm^-3 which was grown on n-type GaN substrate of 2x10^18 cm^-3. The thicknesses from FT-IR were good agreed with the results of secondary ion mass spectrometry (SIMS) analysis. FT-IR is known as one of suitable method for thick film thickness measurement; furthermore, these results indicate that the GaN epi-layer, usually low carrier concentration, is optically different material from the n-type substrate in near infrared range. Therefore, the absorption coefficients of GaN substrates, which has the donor concentration of 1x10^17-7.8x10^19 cm^-3, were measured using spectroscopic ellipsometry and transmittance measurements in near infrared range up to 2.5 um. The optical constants had different in near infrared range, although they were almost same value in ultraviolet range. The wavelength dependence of the absorptions can be explained to the free carrier absorption, which was proportional to the 3.0 power of wavelength. These results can be applied to the nondestructive inspection for vertical GaN power devices.

Resume : GaN/AlGaN ultraviolet light emitting diode (UV-LED) structure with a porous AlGaN reflector structure has been demonstrated. In the UV-LED, the n+-AlGaN/undoped-AlGaN stack structure was transformed into a porous-AlGaN/undoped-AlGaN stack structure through a doping-selective electrochemical etching process. The LED epitaxial layer consisted of a 30-nm-thick GaN buffer layer grown, a 2.0-µm-thick unintentionally doped GaN layer (u-GaN, 51016 cm-3), twelve pairs of n+-Al0.085GaN:Si/u-Al0.085GaN stack structure (n+-AlGaN, 11019 cm-3), a 30 nm-thick undoped-Al0.04GaN layer, a 3.0-µm-thick n-Al0.04GaN layer (21018 cm-3), ten pairs of GaN/Al0.04GaN (3 nm/12 nm) multiple-quantum wells (MQWs), a 30 nm-thick p-type Al0.04GaN:Mg layer (11018 cm-3), and a 10 nm-thick p-type GaN:Mg layer (21018 cm-3). GaN/Al0.04GaN UV-LED structure with a porous Al0.085GaN reflector was fabricated through the selective electrochemical (EC) etching process. The Si-doped AlGaN/undoped-AlGaN stack structure inside the device was transformed into the porous-AlGaN/undoped-AlGaN stack structure functioning as an embedded reflector. The Si-heavily doped n+-AlGaN:Si layer was transformed into a porous AlGaN layer through the doping-selective electrochemical etching process in a 0.5 M nitride acid solution at a positive external bias voltage of 12 V. After the EC-etching process, a high refractive index n-type AlGaN:Si layer was transformed into a low refractive index porous AlGaN layer. The EC-treated porous AlGaN reflector with an 8.5% Al content didn’t absorb the electroluminescence (EL) emission light from the GaN/AlGaN active layer. The EL emission light at 361nm from the GaN/AlGaN active layer could be reflected by the bottom porous-AlGaN reflector exempted from the light absorption of the bottom unintentionally doped GaN layer and GaN buffer layers. High light reflectance of the porous-AlGaN reflector was formed at the bottom of the GaN/AlGaN active layer so that the light extraction efficiency could be improved. The angle-dependent reflectance spectra of the UV reflector was measured by varying the detected angles from 10o to 50o. The UV reflector was measured as the values of 374 nm for central wavelength and 35 nm for the band-width at 10o detected angle. When the detected angle increased to 50o, the UV reflector was measured at 361nm for central wavelength and 14 nm for the band-width. The cut-off wavelength of the reflectance spectra was observed at 349 nm due to the material absorption of the n+-AlGaN/u-AlGaN stack structure with 8.5% Al content. At a detected angle of 10o, the peak reflectivity of the porous-AlGaN reflector was about 93% at 374 nm with 35nm-width smooth stopband in the reflectance spectrum. In the EC-UV LED structure, a 3.24 μm-thick UV-LED epitaxial structure was grown on n+-AlGaN/u-AlGaN stack structure. After the EC etching process, the porous-AlGaN/u-AlGaN stack structure was formed below the UV-LED structure functioning as an embedded reflector. From the angle-dependent reflectance spectra, the cut-off wavelength of the EC-UV-LED was obtained at 360nm due to the light absorption in the UV-LED structure above the porous-AlGaN reflector. The reflectivity of the porous-AlGaN reflector (93%, 374 nm) was higher than that of the EC-UV-LED structure (83%, 381 nm). The phenomenon was caused by the light absorption and the light reflection on the top UV-LED structure in the EC-UV-LED structure. The EL emission intensity of the EC-UV-LED structure was stronger than that of the UV-LED structure. At 20 mA, the peak wavelengths of the EL spectra were measured at 361.9 nm for the UV-LED and 363.2 nm for the EC-UV-LED, respectively. In the EC-UV-LED structure, the peak EL emission wavelength had a slightly redshifted phenomenon caused by the formation of the embedded porous-AlGaN reflector structure. Therefore, the light output power of the EC-UV-LED structure was enhanced because of the high light reflectance on the bottom porous reflector structure. GaN/AlGaN ultraviolet light emitting diodes with the EC-treated porous-AlGaN reflectors were fabricated. The reflectivity of the porous AlGaN reflector (93% at 374 nm) was higher than that of the EC-UV-LED (83% at 381 nm) with the top LED active layer. The cut-off wavelengths in the reflectance spectra were obtained at 349 nm for UV reflector and at 360 nm for EC-UV-LED structure related to the light absorption of the AlGaN and the GaN layers. In the treated UV-LED structure, the photoluminescence emission wavelength was measured at 362 nm with a 106o divergent angle covered by the porous-AlGaN reflector. The light output power of the treated UV-LED structure was higher than that of the non-treated UV-LED structure due to the high light reflectance on the embedded porous AlGaN reflector. The light output power of the EC-UV-LED structure was higher than that of the UV-LED structure because of the high light reflectance of the embedded porous-AlGaN reflector. The UV-LED structure with the high reflectance porous-AlGaN reflector has potential for the future high efficiency UV optoelectronic device applications.

Resume : AlGaN/GaN HEMTs require a semi-insulating buffer to compensate a high background donor concentration, and to prevent parasitic effects, such as parallel conduction. Iron and Carbon are typical impurities used for such purpose, since they can behave as deep acceptors in GaN layers. The former (Fe) brings as drawback a well-known memory effect, which consists in the segregation of Fe atoms through the GaN layers, requiring thick undoped layers to keep the 2-DEG away from such traps. The latter (C), although easier to incorporate in the GaN layers and free of any memory effect, could cause current collapse. In this study we have investigated the effect of differently strained epitaxial layers on Fe-segregation. HEMT structures were grown on Sapphire substrates by MOCVD. By growing the Fe-doped layers in unusual growth conditions (low temperature, or as AlGaN), or by adding a 200nm thick interlayer (AlN, 5x AlN/GaN, or C-doped GaN) between the doped and undoped epitaxial layers, we have succeeded in limiting the Fe segregation within 200 nm of undoped GaN layer instead of the typically required 800 nm, as proven by SIMS. While the morphology and the crystal quality of the HEMT structure has been affected to a very low extent, the electrical characteristics have dramatically benefited from such layers. Higher carrier mobility and lower sheet resistance distinguish such strain-engineered epitaxial layers. Further benefits are expected by the device performance, such as reduced soft-subthreshold behavior (soft-breakdown) and dispersion effects.

Resume : Recently, there have been many studies to combine two different device functions (or components) into a single device structure to obtain multiple functions or high performance. Some of the examples are electrically and optically readable light-emitting memories (LEMs) [1] and optically readable InGaN/GaN resistive random access memory (RRAM) [2]. However, these devices have some limitations in terms of device complexity and performance because the former needs three electrodes to control both light-emitting diode (LED) and RRAM devices while the latter needs defective active layers for both LED and RRAM functions simultaneously. In this study, we demonstrated new types of LEM devices that combine GaN LEDs and Pt/ZnO/Pt-based unipolar RRAM structures. The RRAM unit was integrated with n- contact electrode to avoid increasing device complexity, playing a role of an on/off switch for LED depending on the electrical status of RRAM, so we could avoid either increasing device complexity or performance degradation. Our devices exhibited I-V characteristics similar to those of typical RRAM when the forward voltage is above the threshold voltage of the LED. The current level was over 1 mA at the LRS and below 0.1 µA at the HRS at an operating voltage of 4 V, leading to a high on/off ratio of ~10^4. More details will be presented at the conference. [1] C.-W. Chang; et al. Sci. Rep. 2014, 4, 5121 [2] K. Zheng; et al. IEEE Trans. Electron Devices 2016, 63, 6, 2328-2333

Resume : Given the high resistivity of p-type GaN, a low optical absorbance current spreading contact is required in order to uniformly inject current into III-nitride based optoelectronic devices. One promising transparent current spreading contact for this application is homoepitaxially grown n-type GaN. With n-GaN tunnel junction contacts it can be possible to increase the efficacy and device design space for III-nitride optoelectronics. It has been recently demonstrated that molecular beam epitaxy (MBE) can be used to regrow conductive n-GaN while preventing the re-passivation of thermally activated p-GaN. In this work, we present MBE regrown n-type GaN tunnel junction contacts to p-type GaN and InGaN. At the regrowth interface both the Mg concentration in the p-GaN and the Si concentration is delta doped to decrease the depletion with and improve carrier transport at the junction. By varying the Si dopant concentration in the n+ region of the junction between 9(10)19 to 4(10)20 cm-3, we could improve both the turn-on voltage and series resistance of GaN pn diodes. To further reduce operating voltages a p+ InGaN cap layer was introduced to the diode structure tunneling barrier. Currently we have achieved series resistances of 3(10)-3 Ω cm2 and a turn on voltage increase of 0.2 V compared to a reference Pd/Au metal contact.

Resume : Aluminium nitride (AlN) is a promising candidate for III-V nitrides based UV optoelectronic applications. It is characterized by its ultra-wide bandgap (6.28eV), high thermal conductivity (3.4Wcm-1k-1), excellent electrical breakdown field (1.17X107Vcm-1) and high electron mobility (1100cm2V-1S-1); However, the applications of AlN is hindered by the absence of large-size high quality AlN substrates. In this work large AlN crystals were grown by physical vapour transport (PVT) method on various 2 inch substrates, including AlN(0001)/SiC (silicon carbide), AlN(0001)/Al2O3 (sapphire) and few-layer-graphene. Apart from the various substrates, growth parameters such as temperature, pressure, source-seed distance, source packing density, crucible structure were all examined systematmically. Authors found that the flow rate of nitrogen has a strong impact on AlN crystal c/a-axis growth rate ratio. When nitrogen flow rate rises from 50 to 200sccm, the c/a-axis growth rate ratio decreases from 50 to 0.1, In the same time, morphology of AlN crystal changes from 1D needles to 2D large size plates. Authors also found that crystal expansion requires steady and uniform supersaturation. Computational fluid dynamics (CFD) simulation shows that large nitrogen flow rate contributes to AlN supersaturation uniformity and increase AlN source supply stability. Optimized growth parameters are applied in various substrates including AlN/SiC, AlN/Al2O3, and few-layer-graphene. The initial nucleation, element analysis, crystal orientation and quality of AlN grown on those substrates are characterized and compared by different techniques, as well as the AlN nucleation mechanism, defect generation-elimination, and bulk crystal expansion kinetics are discussed in the work.

Resume : Prevalent droop mitigation strategies in InGaN-based LEDs require structural and/or compositional changes in the active region but are accompanied by a detrimental reduction in external quantum efficiency (EQE) due to increased Shockley-Read-Hall recombination. Understanding the optoelectronic impacts of structural modifications in InGaN/GaN quantum wells (QWs) remains critical for emerging high-power LEDs. In this work, we use a combination of electron microscopy tools along with standard electrical characterization to investigate a wide range of low-droop InGaN/GaN QW designs. We find that chip-scale EQE is uncorrelated with extended well-width fluctuations observed in scanning transmission electron microscopy. Further, we observe delayed cathodoluminescence (CL) response from designs in which calculated band profiles suggest facile carrier escape from individual QWs. Samples with the slowest CL responses also exhibit the lowest EQEs and highest QW defect densities in deep level optical spectroscopy. We propose a model in which the electron beam (i) passivates deep level defect states and (ii) drives charge carrier accumulation and subsequent reduction of the built-in field across the multi-QW active region, resulting in delayed radiative recombination. Finally, we correlate CL rise dynamics with capacitance-voltage measurements and show that certain early-time components of the CL dynamics reflect the open circuit carrier population within one or more QWs.

Resume : AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) are attracting much attention due to their wide area application fields such as water purification, sterilization, skin cure, resin courting, printing, and so on. However, the external quantum efficiency (EQE) of DUV LED is still much lower than that of InGaN-based blue LEDs. The low EQE of DUV LEDs is attributed to low light-extraction efficiency (LEE), which is suppressed to below 10% due to light absorption by p-GaN contact layer. In order to improve LEE, it is required to introduce transparent contact layer and highly-reflective electrode. However, the reflectivity of p-type reflective electrode (Ni/Al or Rh) is not sufficiently high (maximally 70%). Then, we proposed to use highly-reflective photonic crystal (HR-PhC) on the p-contact layer to increase LEE. At first, we have confirmed that more than 90% reflectivity can be obtained by introducing PhC on p-AlGaN contact layer by simulation using FDTD method. Air-hole pattern PhC with the period and the depth of the hole were 250 nm and 80 nm, respectively, was fabricated on transparent p-AlGaN contact layer of DUV LEDs. Damage-less fabrication was performed using nano-imprinting and ICP dry-etching technique. Electrode was fabricated by tilting evaporation method in order to remain air hole. EQE of the 285 nm AlGaN LED was increased from 4.5 to 6.1% by introducing HR-PhC for evaporating low reflective (25%) Ni electrode, measured under bare wafer condition. Also, EQE was increased from 7.5 to 9.5% by introducing HR-PhC in case evaporating highly-reflective (75%) Ni/Mg electrode. EQE will be further more increased if we adopt flip-chip (FC) geometry, as expected by simulation.

Resume : Doping control is a major challenge in GaN epilayers, and thus for GaN devices. Typically, GaN p-n diodes are grown assuming that maximizing the hole concentration in the p-layer will result in the best electrical performance. As a result, such devices generally have a p-layer with a uniform Mg concentration of 1-3 x 1019 cm-3. However, our work demonstrates that the electrical performance of MOCVD GaN-on-sapphire p-i-n diodes with a high/low p-type junction anode structure achieve superior electrical performance to similar devices with uniform p-layers. This enhancement is observed even when the mean hole concentration is less. Devices with a 20 nm thick [Mg] = 2 x 1020 cm-3 layer on top of a 480 nm thick [Mg] = 1018 cm-3 layer show greatly increased forward current, and reduced series resistance vs. a 500 nm thick uniformly 3-4 x 1019 cm-3 doped p-layer. Forward knee voltage and ideality factor are also reduced, from 8.8 V to 3.5 V and from n ? 25 to n ? 10 at 3.5 V, respectively. From electrical, optical, and X-ray characterization, we conclude that the high/low Mg profile improves hole injection into the drift layer, due in part to improved material quality. In low-temperature photoluminescence, LO phonon replicas and near-bandgap (~3.48 eV at 5 K) emission are observed from the high/low junction p-layer, indicating good material quality. No such features are seen in the uniform p-layer. Factors such as the built-in field in the high/low junction are also considered.

Resume : GaN power devices are expected to have wide applications in highly efficient power controlling systems. To achieve feasible GaN power devices, manufacturing technique to grow high-quality large-diameter GaN bulk crystals at low cost is a key issue. To mitigate this issue, we have proposed halogen-free vapor phase epitaxy (HF-VPE), which utilizes the simplest reaction to synthesize GaN (Ga(g) + NH3 → GaN(s) + H2) at high growth rates for prolonged duration. Here we demonstrate thick homoepitaxial GaN growth (~100 um) by HF-VPE on native GaN substrates. The typical growth rate was about 100 um/h and HF-VPE-GaN layer was confirmed to be crack-free. However, a considerable density of large defects, i.e. hexagonal pits (~1 x 10^4 cm^-2), were observed on the as-grown surface, which seemed to cause the slight degradation of overall crystal quality in terms of X-ray rocking curve (XRC) analysis. Through the combined microscopic analyses of transmission electron microscope (TEM) and 3-dimentional atom probe tomography, we confirmed for the hexagonal pits to originate from a flaw at very initial stage of HF-VPE-regrowth. To suppress the flaw, re-optimizing growth conditions at very initial growth stages was conducted and the optimized HF-VPE-GaN layer exhibited no hexagonal pits on the grown surface. The full widths at half maximum (FWHMs) of omega-scan XRC for (0002) and (11-22) reflections obtained at the optimized HF-VPE-GaN layer were 82 and 88 arcsec, respectively, which were almost the same values of the underlying GaN substrate. Furthermore, neither dislocation increase nor particles at the HF-VPE-GaN/GaN substrate interface was confirmed by TEM observation. The details of initial growth conditions and the resultant crystal quality of the HF-VPE-GaN layer will be discussed in the presentation.

Resume : InGaN-based resonant cavity light emitting diode (RC-LED) with a top dielectric distributed Bragg reflector (DBR) and a bottom porous GaN DBR was demonstrated. In the porous GaN reflector, the Si-heavily doped GaN epitaxial layers (n+-GaN) in the 20-period n+-GaN/u-GaN stack structure are transformed into low refractive index nanoporous GaN structure through the doping-selective electrochemical wet etching process. The central wavelength of the porous DBR structure was located at 454 nm with a 42 nm linewidth and a 94.0 % peak reflectivity. The peak wavelengths and the divergent angles of electroluminescence (EL) spectra were measured at 443.0 nm/158 degrees for the non-treated LED and at 450.2 nm/70 degree for the RC-LED, respectively. The EL intensity of the RC-LED had 54 % enhanced compared to the non-treated LED. Under the pulse current injection at 20 mA, the peak wavelength and linewidth of the EL spectra were measured at 457.7 nm/1.7 nm for the resonant emission peak and at 458.3 nm/12.6 nm for the spontaneous emission peak. The lasing-like peak of the EL emission spectra was measured at 457.7 nm with a 0.6nm line-width which was embedded in the resonant emission peak. The narrow divergent angle and the narrow line-width of the RC-LED were caused by the resonant cavity effect. The resonant cavity effect is observed in the InGaN-based RC-LED structure with the embedded porous GaN DBR structure that has a potential for the high-efficiency optoelectronic device applications.

Resume : Using plasma-assisted molecular beam epitaxy, GaN nanowires (NWs) of high crystal quality can be synthesized on a wide variety of substrates, independent of the lattice mismatch. Besides the reduced defect density in comparison to heteroepitaxial layers, the NW geometry facilitates efficient light extraction and enables the growth of non-polar core-shell heterostructures on their M-plane sidewall facets. Furthermore, a core-shell architecture offers the possibility to drastically increase the total effective active area of the device by increasing the NW length. The fabrication of homogeneous core-shell heterostructures using self-assembled GaN NWs is, however, hindered by their large number density due to the mutual shadowing of the NWs from the impinging fluxes. Here, we demonstrate the self-assembled formation of ensembles of GaN NWs on TiN films with a number density suitable for the growth of homogeneous core-shell heterostructures. We use these unique NW ensembles as a template to study the radial growth of GaN NWs in the absence of shadowing effects. To this end, we systematically investigate the radial growth of GaN on the NW sidewall facets as a function of the substrate temperature and the III/V flux ratio. We find that there is only a very narrow window at a substrate temperature of about 620 °C where homogenous radial growth can be achieved. These results set thus the basis for the growth of more complex multi-shell NW heterostructures.

Resume : Three-wave X-ray multiple diffraction was used to investigate the structural properties of AlGaN films grown on low defect density AlN substrates using metallorganic chemical vapor deposition. The scans were recorded using the Renninger scheme set to the primary (0001) forbidden reflection. As the samples were rotated about the surface normal, the visibility of several three-wave X-ray combination peaks was achieved, attributed to the high crystallinity of the films. The position, shape and relative intensities of the resolved multiple diffraction peaks (MPs) were analyzed for differently aligned Renninger curves in order to distinguish normal and hybrid XRMD paths. The effects of AlN wafer curvature and AlGaN crystallographic tilt on Renninger diagrams are presented and discussed. For pseudomorphic alloys, the appearance of the AlGaN (0001) Bragg reflection was identified at sample azimuth positions away from those corresponding to MPs. The existence of this reflection is attributed to partial ordering of the solid solution, which has not been explored as a strain-induced effect in AlGaN materials. This phenomenon is unrelated to commonly reported alloy film compositional inhomogeneity, as the surface morphology was atomically smooth, free of macrostepped features. As such, the occurrence of atomic ordering is potentially a response to the residual built-in film strain, which can have significant impact on the electronic structure and optical properties of the alloys.

Resume : Pure single photon emission and its temperature/excitation-power dependence is revealed from novel GaN interface-fluctuation quantum dots. These dots are shown to exhibit significantly suppressed spectral diffusion effects and therefore have narrow linewidths compared to typical III-nitride quantum dots. Single photon emission is measured using CW laser excitation, revealing g(2)(0) values that are zero to within the experimental error. Quantum light emission is confirmed at temperatures up to ∼ 77 K, by which point a background emission (most likely related to quantum-well states) degrades the emission purity. A discussion on the extent of the background contamination is also given though comparison to extensive data taken under various ambient and experimental conditions, showing unequivocally that the uncorrelated background is the only cause of measured g(2)(0) degradation at higher temperatures. The high purity and narrow linewidths exhibited by these quantum-dot single photon emitters at lower temperatures contribute a giant leap towards the realization of III-nitride based indistinguishable single photon sources- which will be required for more advanced quantum information technologies.

Resume : One of the most important and less understood issues of nitride laser diodes (LDs) is the mechanism of their degradation. It is different from that characteristic for phosphide and arsenide devices and its further investigation and understanding is crucial for their lifetime optimization. In this work we present detailed report on factors influencing nitride LDs degradation. We compare how temperature and current affect the aging process of devices produced by two different techniques: MOCVD (Metal Organic Chemical Vapour Deposition) and MBE (Molecular Beam Epitaxy). For each laser diode we determined the degradation rate (Vd) defined i.e as change of optical power over time for laser operating at constant current. Knowing the increase in temperature resulting from the increase in current we determined the thermal resistance of investigated devices. By adjusting the temperature of a device each time while increasing the current, we singled out an impact of current on degradation rate (excluding self-heating effects). By comparing the results obtained for devices grown by different techniques we are able to observe the influence of growth method on degradation mechanism. In both cases dependence of degradation rate on current is linear whereas on temperature exponential, what stays in the agreement with the literature reports. However, we discovered that devices produced by MBE are characterized by much higher activation energy i.e. 0.4 eV for MOVPE and 1 eV for MBE. Also optimized MBE grown laser diodes show extremely long operation times, exceeding 50 000h. During this presentation we will suggest possible reason for this extremely high robustness of these devices.

Resume : Lateral GaN devices are very promising for future power applications, however their voltage-blocking performance is far away from the materials limit, which is due to the inhomogeneous distribution of the electric field within these devices. Under a high blocking voltage in OFF state, the electric field peaks at the edge of the gate, resulting in an early breakdown of the device. Slant field plates (FPs) have been shown effective to enhance the breakdown voltage, but they require a precise control over the sloped oxide in the vertical direction, which is an extremely challenging process and restricts the efficacy of the FPs in enhancing the breakdown voltage. In this work we propose for the first time slanted tri-gate FPs to address these issues which, instead of the common vertical design, rely on a lateral design to tailor their properties. We demonstrate this concept in AlGaN/GaN MOSHEMTs on silicon for which the slanted tri-gate was defined with a width varying from 350 nm at its source side to 700 nm at its drain side. Compared with counterpart planar transistors, the breakdown voltage was significantly enhanced from 880 V to 1350 V at 1 µA/mm by the slanted tri-gate, leading to the highest FOM of 1.2 GW/cm2 among GaN (MOS)HEMTs on silicon. The high VBR of 1350 V presented in this work is comparable to the best-reported VBR in the literature but with a 14-µm-smaller gate-to-drain distance, and hence a 3.6x-smaller on-resistance. These results reveal the significant potential of the slanted tri-gate FPs for the enhancement of breakdown voltage in lateral GaN power devices and open enormous opportunities for nanostructured GaN devices in future power/RF applications.

Resume : We theoretically investigate III-N tunnel junctions grown along the wurtzite c-axis. We developed a dedicated quantum electronic transport model assuming a realistic multi-band framework, the polarizations and the strain. This model coupled the 8-band k.p Hamiltonian to the non-equilibrium Green's function formalism. For moderate reverse bias this model is in excellent agreement with experimental result. We performed an investigation of the GaN/InGaN/GaN tunnel junctions with various doping concentrations and InGaN thicknesses. We first show that the transmission is dominated by quantum states localized at the interfaces of the heterojunction. Such states result from both confinement and tunneling. We also confirm, for thin InGaN layer, that current strongly increases with doping. A reduction of a factor 10 on doping generates a decrease of about 8 decades for the current. Indeed, in this case the space charge region is controlled by the doping. On the other hand, for thick InGaN layers (>8 nm), our results show an unexpected low impact of doping on current. In this latter case, the spontaneous and the piezoelectric polarizations reduce the tunnel-barrier width to the InGaN layer thickness. Almost independently of the doping, the highest currents are obtained with a thickness of 9 nm. In conclusion this work shows that interfaces control current with both polarizations and localized states. Such behaviors could be reduced by non abrupt interfaces or a decrease of the In content.

Resume : AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) are attracting much attention due to their wide area application fields such as water purification, sterilization, skin cure, resin courting, printing, and so on. However, the external quantum efficiency (EQE) of DUV LED is still much lower than that of InGaN-based blue LEDs. One of the targets for commercially available DUV LED is to achieve higher efficiency than that of mercury lamp (20%). It is required to improve poor (< 10%) light-extraction efficiency (LEE) to realize efficient DUV LED. In this work, in order to improve LEE, we introduced transparent p-AlGaN contact layer and highly-reflective electrode, and also integrated lens with flip-chip (FC) DUV LED. At first, we prepared two types FC DUV LED, i.e., LED with p-GaN contact layer and with p-AlGaN transparent contact layer and highly-reflective (70%) Rh electrode, those were supplied by DOWA ELECTRONICS MATERIALS CO., LTD. We observed drastic increase of EQE from 4 to 15.5% (by approximately 3.8 times) by taking place p-GaN to transparent p-AlGaN and using highly reflective Rh electrode. Although the applied voltage at injection current of 20 mA was increased by about 4V, wall-plug efficiency was increased from 3 to 6.5 %. Then, we bonded sapphire lens (D=3 mm) on the backside of sapphire of the FC with resin adhesive. We observed LEE increase by about 1.2-1.6 times for each sample by bonding sapphire lens to FC. By integrating sapphire lenses into FC DUV LEDs (wavelength were 285 nm), we achieved maximum EQE of 20.1% and WPE of 9.6 %, for LED with transparent p-AlGaN contact layer and highly-reflective Rh electrode.