FORSCHUNG & ENTWICKLUNG
Professor Dr. Michael Heuken lehrt und forscht im Bereich Technologie der Verbindungshalbleiter
(CST, Compound Semiconductor Technology) an der RWTH Aachen University. Er arbeitet grundlagen- und anwendungsorientiert im Bereich der Abscheidung und Charakterisierung von Verbindungshalbleitern und organischen Halbleitermaterialien sowie auf dem Gebiet elektronischer und optoelektronischer Bauelemente. Im Folgenden stellen wir einige seiner wissenschaftlichen Arbeiten vor.
Integration of 650 V GaN Power ICs on 200 mm Engineered Substrates
GaN power ICs on engineered substrates of Qromis substrate technology (QST) are promising for future power applications, thanks to the reduced parasitics, thermally matched substrate of poly-AlN, high thermal conductivity, and high mechanical yield in combination with thick GaN buffer layers. In this article, we will elaborate in detail on epitaxy, integration, and trench isolation. Electrical characterizations show that the GaN buffer bears a breakdown voltage of > 650 V under the leakage criterion of 10 μA /mm 2 at 150 °C. The fabricated 36 mm power HEMTs with LGD of 16 μm show a high threshold voltage of 3.1 V and a low OFF-state drain leakage of <1 μA /mm until 650 V. The horizontal trench isolation breakdown voltage exceeds 850 V. The device dispersion is well controlled within 20% over full temperature and bias range. Finally, GaN power ICs on this platform are demonstrated.
Highly Responsive Flexible Photodetectors Based on MOVPE GrownUniform Few-Layer MoS2
Two-dimensional (2D) transition metal dichalco-genides (TMDCs) are seen as promising candidates forflexibleelectronic and optoelectronic devices due to their high tensilestrength and favorable optical properties. Molybdenum disulfide(MoS2) is a benchmark material for TMDCs, which has alreadybeen studied extensively. Here, we report on highly responsiveflexible few-layer MoS2 photodetectors based on MoS2 synthesizeduniformly for full coverage of 2 in. sapphire wafers usingmetalorganic vapor-phase epitaxy (MOVPE). Device performanceis studied by electro-optical characterization. Electrostatic gatingallows tuning both the responsivity between 150 and 920 A/W andthe specific detectivity between almost 1012and 1010Jones. Themeasured spectrally resolved responsivities of the detectors suggestapplications in the blue-light range, with opportunities forfine-tuning the most sensitive wavelength through gating, as shownthrough optical simulations. Finally, theflexible devices were bent to demonstrate their suitability forflexible electronics infields offuture Internet of Things and medical devices.
High-Intensity CsPbBr3 Perovskite LED using Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) as Hole Transport and Electron-Blocking Layer
The majority of highly efficient perovskite light-emitting diodes (PeLED) contain PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrenesulfonate) as hole transport layer (HTL). However, the hygroscopic and acidic nature of PEDOT:PSS may lead to deterioration of PeLED performance. Moreover, due to its inferior electron-blocking properties, an additional electron-blocking layer (EBL) is required to establish charge balance and consequently obtain superior emission characteristics in typically electron-rich PeLED structures. In this work, PTAA (poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)) serving both as HTL and EBL is employed to substitute PEDOT:PSS in PeLED. The perovskite CsPbBr3 is chosen as emissive layer (EML) material due to its high color purity and photoluminescence (PL) quantum yield. Dense CsPbBr3 films are fabricated on PTAA-coated ITO substrates by employing a one-step spin-coating approach based on nonstoichiometric perovskite precursor solutions. To suppress non-radiative recombination, a small amount of methylammonium bromide (MABr) is incorporated in the CsPbBr3 lattice. The resulting films exhibit excellent coverage and PL intensity. PeLED containing pure CsPbBr3 films as EML show a green emission with a peak at 520 nm, maximum luminance of 11,000 cd/m2, an external quantum efficiency (EQE) of 3.3 % and a current efficiency (CE) of 10.3 cd/A. Further enhancement to 21,000 cd/m2, 7.5 % and 27.0 cd/A is demonstrated by PeLED with MABr-doped CsPbBr3 layers.
WS2 monolayer based light emitting devices fabricated by scalable deposition techniques
Transition metal dichalcogenides (TMDC) have become attractive candidates for 2D electronics and optoelectronics. While several concepts for light emitting devices have been reported, many of them realized using exfoliated TMDC flakes of micrometer size, only few approaches tackle the challenge of upscaling to relevant device sizes. We demonstrate a light emitting diode based on WS2 monolayers in a scalable design. The devices are fabricated by combining two industrially relevant deposition processes in a vertical p-n architecture: Metal organic CVD (MOCVD) is used to realize the optically active WS2 monolayers, while ZnO deposited by spatial atomic layer deposition (sALD) is employed as an electron injection layer on the cathode side. Organic layers spin-coated on an ITO covered glass substrate provide hole injection and transport. The resulting devices exhibit rectifying behavior and red electroluminescence from an area of 6 mm2.
Analysis of an AlGaN/AlN Super-Lattice Buffer Concept for 650-V Low-Dispersion and High-Reliability GaN HEMTs
In this article, an optimized carbon-doped AlGaN/AlN super-lattice (SL) buffer structure for GaN-based high electron mobility transistors, grown on 200-mm Si wafers is demonstrated. The resulting transistor structure features:
1) maximum vertical breakdown strength as high as 2.72 MV/cm,
2) vertical breakdown voltages (BVs) above 1.2 kV,
3) lateral BVs above 2.2 kV,
4) reduction in buffer traps, which is expected to result in low-dynamic RON, and
5) more than 50 years of extrapolated lifetime at 150 °C under 650-V bias.
These were achieved by optimizing growth parameters by systematically varying the SL growth temperature, SL carbon-doping, ammonia flow, and SL pair count with adjusting the total buffer thickness. The detailed analysis shows fundamental improvements compared to a conventional carbon-doped (Al)GaN staircase buffer with the same thickness and comparable growth time.
Strain relaxation, extended defects and doping effects in InxGa1-xN/GaN heterostructures investigated by surface photovoltage
We have analysed electrical properties of extended defects and interfaces in fully strained and partially relaxed InxGa1-xN/GaN heterostructures by means of Kelvin probe force microscopy and surface photovoltage spectroscopy. The study highlights the role of indium incorporation and Si doping levels on the charge state of extended defects including threading dislocations, V defects and misfit dislocations. Surface potential maps reveal that these defects are associated with a different local work function and thus could remarkably alter electron-hole recombination mechanisms of InxGa1-xN/GaN layers locally. Surface photovoltage spectra clearly demonstrate the role of misfit dislocations and high Si-doping level on interface recombination process. The interplay between Si doping level and In% on the electronic properties of the extended defects has been also clarified.
Showerhead-Assisted Chemical Vapor Deposition of Perovskite Films for Solar Cell Application
In the last years, perovskite solar cells have attracted great interest in photovoltaic (PV) research due to their possibility to become a highly efficient and low-cost alternative to silicon solar cells. Cells based on the widely used Pb-containing perovskites have reached power conversion efficiencies (PCE) of more than 20 %. One of the major hurdles for the rapid commercialization of perovskite photovoltaics is the lack of deposition tools and processes for large areas. Chemical vapor deposition (CVD) is an appealing technique because it is scalable and furthermore features superior process control and reproducibility in depositing high-purity films. In this work, we present a novel showerhead-based CVD tool to fabricate perovskite films by simultaneous delivery of precursors from the gas phase. We highlight the control of the perovskite film composition and properties by adjusting the individual precursor deposition rates. Providing the optimal supply of precursors results in stoichiometric perovskite films without any detectable residues.
MOVPE of Large-Scale MoS2 /WS2 , WS2 /MoS2 , WS2 /Graphene and MoS2 /Graphene 2D-2D Heterostructures for Optoelectronic Applications
Most publications on (opto)electronic devices based on 2D materials rely on single monolayers embedded in classical 3D semiconductors, dielectrics and metals. However, heterostructures of different 2D materials can be employed to tailor the performance of the 2D components by reduced defect densities, carrier or exciton transfer processes and improved stability. This translates to additional and unique degrees of freedom for novel device design. The nearly infinite number of potential combinations of 2D layers allows for many fascinating applications. Unlike mechanical stacking, metal-organic vapour phase epitaxy (MOVPE) can potentially provide large-scale highly homogeneous 2D layer stacks with clean and sharp interfaces. Here, we demonstrate the direct successive MOVPE of MoS2 /WS2 and WS2 /MoS2 heterostructures on 2” sapphire (0001) substrates. Furthermore, the first deposition of large-scale MoS2 /graphene and WS2 /graphene heterostructures using only MOVPE is presented and the influence of growth time on nucleation of WS2 on graphene is analysed.
Temperature dependent lateral and vertical conduction mechanisms in AlGaN/GaN HEMT on thinned silicon substrate
This paper presents the influence of Si substrate thinning by grinding and dicing on the mechanical and electrical properties of AlGaN/GaN HEMT grown on 6-inch Si substrate. By experimentally removing 96% of the Si substrate, an increase in the lateral breakdown voltage by 13% is achieved for a wide temperature range. A decreasing vertical breakdown voltage with substrate thinning shows the necessity of substrate replacement to maintain the vertical isolation. Poole-Frenkel and space charge limited conduction mechanisms are identified for the vertical leakage in forward and reverse bias down to thicknesses of 200 μm, respectively. The increase in chip bow and the resulting increase in tensile strain in the epitaxial layers resulting from the substrate removal are correlated to a 40% decrease in 2DEG charge carrier concentration along with a 27% increase in threshold voltage. These findings are explained by strain-induced gate barrier and polarization charge variations.
An Improved Model of Electrical Contact Resistance of Pad-Probe Interaction during Wafer Test
Semiconductor wafer test requires a stable electrical contact resistance between each individual I/O pad and the probe needle. Due to the strong sensitivity of thin layers of IC's to mechanical stress, low-force probe cards are mostly used. Those probes face the challenge of a randomly increased and instable electrical contact resistance during continuing insertions, whose root cause is not fully understood yet. Therefore, we firstly validated the contact behavior of a single probe on different pad metal alloys and layer thicknesses with respect to Holm's theory. We could confirm the force and material dependency of the contact resistance for Au and Al pad metallization's but we did observe a very strong pad metal thickness influence. Further on we evaluated the effective contact area related to probe tip diameter and surface topography, which has a significant effect on the probability for an increased and instable electrical contact resistance. As proven in high-resolution 3D microscopy strong variances regarding probe tip shape and roughness within a probe card were observed. Based on the experimental findings we could develop an improved model how the roughness and pad material built-up is related to the alpha-spot formation and thus the contact resistance stability.
Elemental Distribution and Structural Characterization of GaN/InGaN Core-Shell Single Nanowires by Hard X-ray Synchrotron Nanoprobes
Improvements in the spatial resolution of synchrotron-based X-ray probes have reached the nano-scale and they, nowadays, constitute a powerful platform for the study of semiconductor nanostructures and nanodevices that provides high sensitivity without destroying the material. Three complementary hard X-ray synchrotron techniques at the nanoscale have been applied to the study of individual nanowires (NWs) containing non-polar GaN/InGaN multi-quantum-wells. The trace elemental sensitivity of X-ray fluorescence allows one to determine the In concentration of the quantum wells and their inhomogeneities along the NW. It is also possible to rule out any contamination from the gold nanoparticle catalyst employed during the NW growth. X-ray diffraction and X-ray absorption near edge-structure probe long- and short-range order, respectively, and lead us to the conclusion that while the GaN core and barriers are fully relaxed, there is an induced strain in InGaN layers corresponding to a perfect lattice matching with the GaN core. The photoluminescence spectrum of non-polar InGaN quntum wells is affected by strain and the inhomogeneous alloy distribution but still exhibits a reasonable 20% relative internal quantum efficiency.
GaN Micropillar Schottky Diodes with High Breakdown Voltage Fabricated by Selective‐Area Growth
Herein, selective‐area growth (SAG) of lightly n‐doped GaN micropillars on masked GaN‐on‐sapphire templates is investigated. Using the micropillar SAG approach, the maximum GaN drift layer thickness in Schottky diodes on foreign substrates is increased. Thus, cost‐efficient vertical power devices with large breakdown voltages (VBD) based on heteroepitaxy are enabled. The influence of different hard‐mask materials and SAG temperatures (TSAG) on growth selectivity, morphology, and net doping concentration (ND–NA) is investigated. By using an AlOx hard mask and TSAG = 1045 °C, 3.7 μm high GaN micropillars are grown in circular mask openings. Quasi‐vertical Schottky diodes on these pillars exhibit low ND–NA = 5.2 × 1016 cm−3, VBD = 393 V, and a critical electric field EC = 2.63 MV cm−1.
Optimization of Transparent Organic Light-Emitting Diodes by Simulation-Based Design of Organic Capping Layers
In this work, we use the transfer matrix method to optimize TPBi capping layers deposited on organic light emitting diodes with respect to light extraction and transmittance. The green transparent organic light emitting diodes comprise three organic semiconductors (CBP, Ir(ppy)₃ and TPBi) forming an efficient simplified phosphorescent organic light emitting diode stack. A transparent cathode of 2 nm Cs₂CO₃, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The diode stack as well as the capping layer are deposited by organic vapor phase deposition. The refractive indices and extinction coefficients of all materials in the transparent organic light emitting diodes (glass, indium tin oxide, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance and reflectance measurements. With these spectrally resolved data, we calculate the transmittance of transparent organic light emitting diodes with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm-1000 nm) by a series of experiments. By choosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated transparent organic light emitting diodes with an optical transmittance which was strongly enhanced from 47% (reference without capping layer) to 65%, measured at 555 nm.
Hydride-free MOCVD of 2D MoS2 and 2D WS2 for optoelectronic applications (Conference Presentation)
The 2D transition metal dichalcogenides MoS2 and WS2 have attracted great interest due to their unique (opto)electronic properties. For their fabrication on an industrial scale, high-productivity MOCVD systems are most suitable because of precisely controlled precursor fluxes, advanced temperature control and superior homogeneity. Here, we report on the development of an MOCVD process for 2D WS2 and 2D MoS2 on sapphire (0001) substrates in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Molybdenum hexacarbonyl (MCO), tungsten hexacarbonyl (WCO) and di-tert-butyl sulfide (DTBS) are used as sources, respectively. A one-step process was developed to control nucleation and lateral growth of both 2D materials at 30 hPa total pressure in an N2 atmosphere. It was found that a fine-tuned S-to-metal ratio can inhibit the parasitic deposition of carbon contaminations. Investigations of the influence of deposition temperature on lateral growth of WS2 confirm previous findings for MoS2. The optimum growth temperature for MoS2 and WS2 is 845 °C. WS2 deposition experiments show that in order to achieve a fully coalesced 2D film, it is necessary to supply a sufficient amount of WCO, the limiting species during WS2 growth. Increasing the WCO flow from 1 nmol/min to 20 nmol/min raises the total substrate coverage from 2.5 % to >50 % in 10 h processes. Extending gradually the growth time to 20 h results in deposition of fully coalesced WS2 samples without carbon contaminations and only sparse bilayer nucleation. Moreover, the fully coalesced WS2 samples exhibit strong PL signals.
Comparison of MOCVD and MBE Regrowth for CAVET Fabrication
In this paper, we demonstrate the fabrication of current aperture vertical electron transistors (CAVET) realized with two different epitaxial growth methods. Templates with a p-GaN current blocking layer (CBL) were deposited by metal organic chemical vapor deposition (MOCVD). Channel and barrier layers were then regrown by either molecular beam epitaxy (MBE) or MOCVD. Scanning electron microscope (SEM) images and atomic force microscope (AFM) height profiles are used to identify the different regrowth mechanisms. We show that an AlN interlayer below the channel layer was able to reduce Mg diffusion during the high temperature MOCVD regrowth process. For the low-temperature MBE regrowth, Mg diffusion was successfully suppressed. CAVET were realized on the various samples. The devices suffer from high leakage currents, thus further regrowth optimization is needed.
Detection and Cryogenic Characterization of Defects at the SiO2/4H-SiC Interface in Trench MOSFET
We employed the thermal dielectric relaxation current (TDRC) method for the detection and cryogenic characterization of traps at the 4H-SiC/SiO2 interface in n-channel trench MOSFETs and n-MOS trench capacitors. The interface of trench devices on the (112̅0)-plane atomically differs from the interface of standard lateral devices [(0001)-plane]. In the MOSFET, two TDRC signal peaks originating from electron traps were found and characterized by parameter variation in TDRC measurements. One peak corresponds to interface states located 0.13 eV below the 4H-SiC conduction-band edge EC. The other one is attributed to the near-interface traps (NITs) with an electron emission barrier of 0.3 eV. The NIT peak shows an inverse dependence on the discharging voltage compared to regular interface states. In contrast to the MOSFET, only NITs were measured in n-type MOS capacitors. The results found in trench devices were also confirmed for lateral devices. Therefore, we show that the study of SiC MOS capacitors is not sufficient for the understanding of degradation mechanisms in SiC MOSFETs. The interface states at EC -0.13 eV detected in MOSFETs are assumed to contribute to the degradation of the apparent channel mobility and ON-resistance of SiC MOSFETs. These states are strongly reduced by annealing in NO compared to N2 .
Limitations for Reliable Operation at Elevated Temperatures of Al2O3/AlGaN/GaN MISHEMTs Grown by MOCVD on Silicon Substrate
In this work, the gate degradation mechanisms of GaN‐based metal insulator semiconductor high electron mobility transistors (MISHEMTs) utilizing Al2O3 grown by plasma enhanced atomic layer deposition (PEALD) is systematically investigated. By applying constant voltage stress and the time dependent dielectric breakdown methodology under variation of bias and temperature an activation energy of 1.25 eV for the time to breakdown and a 1/E model extrapolating the lifetime is found. A maximum gate operation voltage at 298 K of 4.9 V is extrapolated, which decreases to a projected voltage of 3.5 V at 598 K operation temperature, due to an accelerated defect generation. The physical origin of the time dependent breakdown (TDDB) of the Al2O3 is related to the formation of a percolation path by randomly generated defects in the oxide under stress bias. This mechanism, which also requires the presence of an initial defect density in the Al2O3, is confirmed by Monte Carlo simulations, which are in agreement with the experimental data. This article is protected by copyright. All rights reserved.
Enabling MOCVD Technology for Micro LED High Volume Manufacturing
Epitaxy wafers are key to the technology development of Micro LED displays. As Micro LED chips are extremely small, the quality of epitaxy wafers including wavelength uniformity and low defect becomes crucial for further production process. We present progress on improvements to MOCVD technology of blue and red Micro LEDs based on Planetary Reactor® technology to meet the new challenging wavelength and defect requirements.
Chemical Vapor Deposition of Organic-Inorganic Bismuth-Based Perovskite Films for Solar Cell Application
Perovskite solar cells have shown a rapid increase of performance and overcome the threshold of 20% power conversion efficiency (PCE). The main issues hampering commercialization are the lack of deposition methods for large areas, missing long-term device stability and the toxicity of the commonly used Pb-based compounds. In this work, we present a novel chemical vapor deposition (CVD) process for Pb-free air-stable methylammonium bismuth iodide (MBI) layers, which enables large-area production employing close-coupled showerhead technology. We demonstrate the influence of precursor rates on the layer morphology as well as on the optical and crystallographic properties. The impact of substrate temperature and layer thickness on the morphology of MBI crystallites is discussed. We obtain smooth layers with lateral crystallite sizes up to 500 nm. Moreover, the application of CVD-processed MBI layers in non-inverted perovskite solar cells is presented.
H2S-free Metal-Organic Vapor Phase Epitaxy of Coalesced 2D WS2 Layers on Sapphire
The 2D transition metal dichalcogenide (TMDC) tungsten disulfide (WS2) has attracted great interest due to its unique properties and prospects for future (opto)electronics. However, compared to molybdenum disulfide (MoS2), the development of a reproducible and scalable deposition process for 2D WS2 has not advanced very far yet. Here, we report on the systematic investigation of 2D WS2 growth on hydrogen (H2)-desorbed sapphire (0001) substrates using a hydrogen sulfide (H2S)-free metal-organic vapor phase epitaxy (MOVPE) process in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Tungsten hexacarbonyl (WCO, 99.9 %) and di-tert-butyl sulfide (DTBS, 99.9999 %) were used as MO sources, nitrogen (N2) was selected as carrier gas for the deposition processes (standard growth time 10 h). In an initial study, the impact of growth temperature on nucleation and growth was investigated and an optimal value of 820 °C was found. The influence of the WCO flow on lateral growth was investigated. The aim was to maximize the edge length of triangular crystals as well as the total surface coverage. Extending gradually the growth time up to 20 h at optimized WCO flow conditions yields fully coalesced WS2 samples without parasitic carbon-related Raman peaks and with only sparse bilayer nucleation. After substrate removal, a fully coalesced WS2 film was implemented into a light-emitting device showing intense red electroluminescence (EL).
Scalable Large-Area p–i–n Light-Emitting Diodes Based on WS2 Monolayers Grown via MOCVD
Transition metal dichalcogenides (TMDCs) represent a novel and sustainable material basis for ultrathin optoelectronic devices. Although various approaches toward light-emitting devices, e.g., based on exfoliated or chemical vapor deposited (CVD) TMDC monolayers, have been reported, they all suffer from limited scalability and reproducibility required for industrial fabrication. Here, we demonstrate a light-emitting device in a scalable approach by embedding metal−organic (MO-)CVD WS2 monolayers into a vertical p–i–n device architecture using organic and inorganic injection layers. Red electroluminescence is emitted from an active area of 6 mm2 starting already at a driving voltage of about 2.5 V.
Effect of Carbon Doping Level on Static and Dynamic Properties of AlGaN/GaN Heterostructures Grown on Silicon
The impact of the extrinsic carbon doping level was investigated with the aim of finding the optimal level for GaN-on-Si HFETs. A tradeoff between the crystal quality degradation by carbon doping and dynamic properties of HFET structures was observed indicating the role of vertical dislocations in the dynamic performance of the carbon-doped GaN buffer.
Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2
The influence of the main growth parameters on the growth mechanism and film formation processes during metalorganic vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-butyl sulfide as metalorganic precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation density of the polycrystalline film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Additionally, the influence of growth parameters such as reactor pressure and surface temperature is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temperature above 900°C.
Growth and Characterization of Vertical and Lateral p‐n Junctions Formed by Selective‐Area p‐GaN MOVPE on Patterned Templates
In this work, vertical and lateral GaN p‐n junctions are investigated. In particular, there is a strong demand of lateral p‐n junctions for the realization of numerous types of power devices. The comparison of I‐V characteristics of regrown and continuously‐grown p‐n diodes, formed by metal‐organic vapor phase epitaxy (MOVPE), reveals the impact of different surface treatments before regrowth. The use of tetramethylammonium hydroxide (TMAH) prove vital for growth on etch‐damaged GaN and to smoothen vertical sidewalls for selective‐area regrowth (SAR). Initial growth on patterned templates is identified to occur both vertically on the horizontal c‐plane and laterally on the vertical m‐planes allowing for an integration of the developed process in power device fabrication routes. The formation of local superelevations (LSE) near edges of masked areas is analyzed and explained.
Temperature Dependent Vertical Conduction of GaN HEMT Structures on Silicon and Bulk GaN Substrates
The vertical leakage current mechanisms of high electron mobility transistors (HEMT) grown by metalorganic chemical vapor deposition (MOCVD) on Si and GaN substrate under forward and reverse bias are analyzed at ambient temperatures from 25 °C to 200 °C. For the GaN/Si case, a thermally activated vertical conduction with two temperature regimes and activation energies of 0.06 eV and 0.43 eV is found. In contrast to that, the GaN/GaN case shows a single activation energy of 0.67 eV for the rate‐limited vertical conduction. For forward vertical bias, Poole–Frenkel (PF) conduction is identified as the dominant conduction mechanism at higher fields for both substrates. In reverse bias, space charge limited and PF conduction are identified as dominant conduction mechanism for GaN/Si and GaN/GaN, respectively. The deviation in vertical conduction mechanism is related to a significant reduction in the dislocation density by three orders of magnitude and homoepitaxially lattice matched growth for the GaN/GaN HEMT.
Enabling the Next Era of Display Technologies by Micro LED MOCVD Processing
Conventional Display Technologies are being challenged by the emergence of Micro LED Display Technology. AIXTRON identifies the increased MOCVD requirements Micro LED applications are bringing and how these can be addressed. We present progress on improvements to MOCVD processing of red, green and blue Micro LEDs based on Planetary technology.
Large-area MoS2 deposition via MOVPE
The direct deposition of the 2D transition metal dichalcogenide MoS2 via metal-organic vapour phase epitaxy (MOVPE) is investigated. Growth is performed in a commercial AIXTRON horizontal hot-wall reactor. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as metal-organic precursors for molybdenum and sulfur, respectively. The successful deposition of MoS2 is demonstrated via Raman spectroscopy on various substrates such as sapphire and Si as well as AlN and GaN templates. The influence of growth time on the evolution of layer morphology is investigated. Variation of carrier gas reveals that a pure nitrogen growth atmosphere and a growth temperature of 750 °C improve layer quality. Additionally, a post-deposition annealing process of the grown samples is examined. It is shown that annealing in a pure nitrogen atmosphere at temperatures between 650 °C and 750 °C strongly increases the Raman intensities.
Effect of Different Carbon Doping Techniques on the Dynamic Properties of GaN-on-Si Buffers
The effect of different carbon doping techniques on the dynamic behavior of GaN-on-Si buffer was investigated. Intentional doping using a hydrocarbon precursor was compared with the more common autodoping technique. Breakdown and dynamic behavior of processed devices indicate that extrinsic carbon doping delivers better dynamic properties for the same blocking voltage capabilities. Modeling and simulations have revealed that charge transport across the GaN buffer is the main limiting factor during the buffer discharge process.
Semi-polar {1-101} blue and green InGaN/GaN light-emitting diodes on micro-stripe patterned Si (100)
A novel III-nitride-based light emitting diode (LED) fabrication process which is based on selective-area epitaxial growth on Si {1 1 1} facets etched into Si (100) substrates is presented. A micro-stripe pattern is formed with semi-polar {1-101} crystallographic planes of GaN evolving from an epitaxial lateral overgrowth (ELOG)-like process. The {1-101} planes of GaN serve as a template for the growth of semi-polar blue and green LED structures with InGaN/GaN multiple quantum wells (MQW). A complete fabrication chain encompassing substrate etching, metalorganic vapor phase epitaxy (MOVPE), characterization, LED processing and device manufacture has been developed.
The semi-polar LED stacks are of high crystalline quality, which is manifested by homogeneous InGaN layers in the {1-101} MQW structure and smooth {1-101} LED surface planes. Although threading dislocations intersect with the semi-polar {1-101} MQW, V-shaped defects typically observed in polar c-plane MQW structures are not detected.
The blue and green semi-polar LED show only a weak polarization-related wavelength shift at large current densities consistent with the lower built-in electric fields in the semi-polar MQW. At low current densities, the green LED exhibit a strong wavelength shift due to In clustering effects. The blue LED reveal a stable emission color, which indicates a homogeneous In distribution in the wells.
Power performance at 40 GHz on quaternary barrier InAlGaN/GaN HEMT
Depletion-mode high-electron mobility transistors (HEMTs) based on a quaternary barrier In0.11Al0.72Ga0.17N/GaN heterostructure on sapphire substrate are fabricated and characterized. This structure shows a very high Hall electron mobility of 2200 cm2/V·s, which is the highest value ever reported on In-containing GaN-based HEMTs. For T-shaped gate transistor with a gate length of 75 nm, current gain (ft) and power gain (fmax) cutoff frequencies of 113 and 200 GHz are extracted from S-parameter measurements, respectively. Nonlinear characterization of a T-shaped gate device with a gate length of 225 nm gives an output power density of 2 W/mm at 40 GHz. These results clearly demonstrate the capabilities of such quaternary barrier-based devices.
MOVPE growth, optical and electrical characterization of thick Mg-doped InGaN layers
A series of Mg-doped thick InGaN layers with different Cp2Mg flows were grown on n-type GaN layers. The Mg doping effect on optical and electrical properties of InGaN:Mg was investigated through capacitance–voltage (C–V) measurements and temperature-resolved photoluminescence (PL). After annealing, p-type conductivity with acceptor concentrations about 3.5×1018 cm−3 and 9.5×1017 cm−3 were observed for the samples doped with little Cp2Mg. With the highest Cp2Mg flow, an inversion from p-type to n-type was observed by analysis of a Mott–Schottky (M–S) plot. The inversion of conductivity type was accompanied by a disappearance of InGaN band-to-band PL emission. It should be noted that annealing led to a substantial reduction of this band intensity. Thus, too high Mg doping is found to cause a strong compensation of p-type conductivity by nonradiative defects of n-type as it is seen from C–V and PL measurements.
Corporate Research & Development
Prof. Dr. Michael Heuken
Vice President Advanced Technologies
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Prof. Dr. Michael Heuken
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Director Corporate Communications