Nitride semiconductors for high power and high frequency electronic devices

Resume : Smart Cut™ technology is based on transferring a very thin single crystalline layer from a substrate onto a different handler. This method is successfully used in case of silicon and one of the results is a Silicon-On-Insulator (SOI) engineered wafer. This technology enables fabrication of low-cost, large-area, and high-quality wafers. Currently, attention is drawn to obtaining substrates with a thin layer of gallium nitride for the purpose of GaN-based electronic and optoelectronic devices. Results of HVPE-GaN growth of GaN layers on Advanced Substrates prepared with Smart Cut™ technology will be presented. The substrates consist of handler material (Sapphire or Molybdenum), a bonding layer, and a transferred, about 200-nm-thick Ga-face GaN layer. An approach analogous to growth on MOCVD-GaN/Sapphire templates engaging a technique similar to Void Assisted Separation (VAS) was applied. It is possible to obtain high-quality and crack-free HVPE-GaN crystals on such templates. The process of self-lift-off together with an influence of nitridation time in such growth was analyzed in detail. HVPE crystallization runs performed on Advanced Substrates in different conditions will be discussed. Structural properties of layers obtained on photolitographically patterned Ti masks will be presented. Results of characterization by X-ray diffraction, scanning electron microscopy, defect selective etching, photo-etching, and secondary ion mass spectrometry of the grown HVPE-GaN crystals will be included. Finally, up to 1-mm-thick and crack-free layers together with their properties will be demonstrated.

Resume : Single-crystalline aluminum nitride (AlN) is a promising substrate material not only for AlGaN epilayers with high Al content, e.g. for solid-state deep-UV optoelectronics, but also for high temperature and high power applications. This presentation gives an overview of the status of AlN substrate preparation and discusses perspectives and challenges for GaN/AlGaN-based high power electronics. AlN bulk single crystals are grown by the physical vapor transport (PVT) method at temperatures well above 2000°C. Crystals of high structural perfection (dislocation densities < 10^4 cm^-2) can be grown using AlN single crystal wafers as seeds. However, proper control of the PVT growth process is made difficult due to gradual changes of the reactor materials caused by attack of gaseous Al and unintentional incorporation of impurities (O, C, Si) into the growing crystals during growth. The latter determine the electrical, thermal, and optical properties of bulk AlN substrates. In turn, these properties can be adjusted at least partially by providing proper growth conditions or by employing doping. We will present and discuss preparation of bulk AlN substrates for power electronics which are electrically semi-insulating even at temperatures beyond 1000°C, ones that provide weak n-type conductivity (n = 1.2 10^15 cm^-3, µ = 36.5 cm^2/Vs) at room temperature by Si doping, and ones that exhibit transmission in the deep-UV wavelength range (for optical applications).

Resume : Based on Baliga’s FOM, GaN, AlN, and AlGaN-based Schottky diodes are expected to be superior to SiC by a factor of 6 to 200. Advances in native substrates have led to the possibility of vertical devices with low dislocation densities, thus approaching the materials’ ultimate performance. After effectively removing dislocations, new material challenges become apparent: epitaxy morphology on native substrates, point defect control, and surface manipulation. These influence the development of these switches as they determine: thickness and composition, carrier concentrations and mobility at the contact and drift layers, and Schottky barrier and its passivation. Onset of three main surface morphologies was found to depend on the substrate miscut with characteristic differences between AlN and GaN, as determined by surface kinetics. These differences become significant for AlGaN, determining composition and lateral uniformity. Point defect control leads to the achievement of the necessary high carrier concentrations in the back contacts while allowing for controllable low carrier concentrations in the drift layers; a carrier concentration of 2x1016 cm-3 with a mobility of 1100 cm2/Vs was obtained for GaN on sapphire. Homoepitaxial GaN on HVPE/ammonothermal GaN substrates shows a narrow PL DBX peak of 100 μeV, suggesting the higher quality of this material. Results on the performance and further limitations of Schottky diodes based on these materials achievements will be discussed.

Resume : In this study, we demonstrate high voltage Schottky barrier diodes grown on highly doped n-type GaN substrate (~1x1E19 cm-3) obtained by ammonothermal method. Such Ammono-GaN crystals are characterized by dislocations density as low as 1E4 cm-2. In our SBDs, thick GaN layers (~150 μm) grown by Hydride Vapor Phase Epitaxy (HVPE) growth technique on Ammono-GaN substrate reproduce this low dislocation density ensuring simultaneously high uniformity, purity and smoothness of GaN film. Moreover, our diodes have been designed in a vertical current flow config-uration, what should also significantly improve the current spreading in the structure leading to more efficient performance and lower power dissipation. First, prior to SBD device fabrication, the HVPE grown GaN layers have been examined elec-trically by means of Hall effect studies. Typically measured values of electron concentration and mo-bility for our structures were in range of 2-8x1E16 cm-3 and 1100-800 cm2/Vs, respectively. The aver-age HVPE-GaN resistivity was around 3x1E-1 Ω*cm. The electrical studies have shown the huge potential in such vertical GaN-based Schottky diodes with the breakdown voltage as high as 700 V and typically around 400 V. Additionally, our results are quite impressive regarding breakdown voltage when one can take into account the fact that there was no any edge termination in our structures such as field plates, guard rings or implantation.

Resume : P-type dopant implantation and activation adds additional complexity to the synthesis of p-type GaN. Advanced annealing processes, such as the Multicycle Rapid Thermal Annealing (MRTA) are required for Mg activation. The MRTA process consists of two separate, successive steps. Both steps are conducted at elevated nitrogen pressure of 25 atm. First, the implanted GaN is annealed conventionally at temperatures at which the capped GaN is stable on the order of 10’s of minutes. This step allows partial restoration of the GaN lattice damaged by implantation, preparing a more stable crystal structure for the next step. In the second step, the annealing temperature is repeatedly pulsed to temperatures above 1000˚ C, accumulating the time GaN is exposed to these high temperatures. In this paper, we introduce a modified MRTA process allowing the realization of a PIN diode using Mg implantation in GaN. The modified MRTA process includes additional step of conventional annealing after multiple rapid heating cycles. It is shown that this step is necessary to remove stresses and defects introduced by the rapid heating and cooling of the GaN. The new modified MRTA process consists of two conventional annealing steps 1 and 3, with the step 2 of rapid heating and cooling pulses in between, thus it is named as symmetrical multicycle rapid thermal annealing (SMRTA). The improvement in crystal quality provided by the SMRTA will be a key enabling step for future Mg-implanted devices.

Resume : High electron mobility transistors (HEMTs) based on III-nitrides grant higher 2D electron gas (2DEG) densities due to polarization fields intrinsic to the wurtzite structure [1]. Recently investigations were directed from Ga-polar towards N-polar (Al,Ga,In)N heterostructures. The N-polar layout is advantageous for enhancement mode and highly scaled transistors [2, 3]. However, unoptimized N-polar HEMTs suffer from large-signal dispersion and are sensitive to light due to donorlike traps within the device structure [3]. The negative impact of the traps can be mitigated by implementing an elaborate design combined with Si doping. We present results obtained by positron annihilation spectroscopy in N-polar S.I. GaN/AlGaN:Si/AlGaN/GaN HEMTs with graded backbarrier design and different Si doping where the valence band is below the Fermi level. These data are compared to measurements of a simplified HEMT structure where traps formed at the S.I. GaN/AlGaN interface. As shown in [4], polar structures affect the spatial confinement of the positron state. A dramatic change in the positron trapping at the GaN/AlGaN interface takes place when the Si doping of the AlGaN layer is above 5e18 cm-3. Interestingly, no interface traps are detected with positrons in Ga-polar structures. [1] L. Bjaalie et al., New J. Phys. 16 (2014). [2] M. H. Wong et al., Semicond. Sci. Technol. 28 (2013). [3] S. Rajan et al., J. Appl. Phys. 102 (2007). [4] I. Makkonen et al., Phys. Rev. B 82 (2010).

Resume : AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have recently been under intense research and are becoming attractive devices for high voltage and high-power applications at RF operating conditions. GaN is a wide band gap (~3.4 eV at room temperature) semiconductor with a promising combination of material properties including a high electric breakdown field, good electron mobility, high saturation velocity, relatively high thermal conductivity, and is stable at high operating temperatures; all of which contribute to making these devices very suitable for RF devices where high power and high frequency operation are needed. Development and fabrication of reliable AlGaN/GaN HEMTs has significantly advanced in recent years to enable the production of high quality, commercially available devices in a wide variety of high power and high frequency applications. To further study these devices, however, it is important to investigate the reliability physics associated with AlGaN/GaN HEMTs – especially under the transient operating regime where these devices are predicted to excel over existing technologies. In this work, we present finite element simulation results of the transient temperature, stress, and deformation response of an AlGaN/GaN HEMT built on SiC substrates. The modeling technique involves a small-scale electro-thermal model coupled to a large-scale mechanics model to determine the resulting stress distribution within a device. This technique allows for detailed analysis of the electrical and thermal contributions to stress during both DC and AC operation. The electrical characteristics of the modeled device are compared to experimental measurements of comparative devices, and the bias dependent heating is compared to existing simulation data from literature for validation. The results show critical regions around the gate Schottky contact undergo drastically different transient stresses during pulsed operation. Specifically, stress profiles within the AlGaN layer around the gate footprint undergo highly tensile electro-thermal stresses while stresses within the channel between the gate and drain contact undergo highly tensile electrical stress and compressive thermoelastic stress. Factors such as the operational frequency as well as the thermal boundary resistance at contacts in the device also play a major role in the thermal and deformation response of these devices. Implications on devices reliability will be discussed.

Resume : High performance group III nitride semiconductor based LEDs and laser diodes most fabricated by MOVPE. In situ monitoring in MOVPE is the key process in device manufacturing. These information can be feedback to growth condition and find its mechanism. So, in situ monitoring is expected key technology to improve the device performance. An in situ monitoring by x-ray can be measured to the lattice constant, nano-structure thin film, crystalline quality and so on. In this study, we examined the novel in situ XRD system for MOVPE growth of GaN. We analyzed of GaInN on GaN by in situ XRD monitoring. We also discuss the growth of GaInN/GaN superlattice on GaN and application of solar cell optimization using long period superlattice by in situ XRD monitoring. As the results, in situ XRD measures the critical thickness of introduction of a+c misfit dislocation in GaInN on GaN system by measuring the FWHM of GaInN XRD peak at only one time experiment. Accordingly, if we employ in situ XRD under various growth conditions, the optimization of the growth conditions will become easier because it would be possible to determine at which misfit dislocation increases by only one growth procedure.

Resume : We report a method to distinguish between Ga vacancy (VGa)-related and carbon (C)-related hole traps with similar thermal emission activation energies (0.86-0.88 eV) in MOCVD n-GaN [1]. This method combines minority carrier transient spectroscopy (MCTS) using above-band-gap light with optical deep level transient spectroscopy (ODLTS) using below-band-gap light for Schottky diodes. We used two MOCVD n-GaN on free standing n+-GaN which have the carbon (C) concentration of below ~1x1016 cm-3 (LC) and ~6x1016 cm-3 (HC), respectively. Isothermal MCTS measurements using ~355 nm UV and isothermal ODLTS measurements using ~470 nm blue LED were performed at 300 K for fabricated Schottky diodes. A peak is observed in MCTS and ODLTS at the same thermal emission time constant for the LC sample, while a peak is observed in the MCTS spectrum at the slightly different time constant from that in the ODLTS spectrum for the HC sample. The same time constant is found for LC and HC in ODLTS. This is ascribed to the difference between VGa-related and C-related hole traps in photoionization energy [2,3]. In the LC sample, VGa-related hole traps are dominant with C-related hole traps in concentration below detection sensitivity. There are C-related hole traps in addition to VGa-related hole traps in the HC sample. We will further report the energy levels of these hole traps and their trap concentrations. [1] Y. Tokuda, CS MANTECH, 19 (2014). [2] A. R. Arehart, A. Corrion, C. Poblenz, J. S. Speck, U. K. Mishra, and S. A. Ringel, Appl. Phys. Lett. 93, 112101 (2008). [3] A. Armstrong, A. R. Arehart, D. Green, U. K. Mishra, J. S. Speck, and S. A. Ringel, J. Appl. Phys. 98, 053704 (2005).

Resume : Development of AlGaN/GaN-based normally-off high-electron mobility transistors (HEMTs) faces several issues. Among them is a lack of a true pinch-off, low threshold voltage for low on-state resistance devices, high gate leakage of Schottky gates contact, or a threshold voltage drift for HEMTs with a metal-oxide-semiconductor (MOS) gate structures. Inversion-type enhancement-mode GaN-based field-effect transistors is not considered here as an alternative as it features low channel mobility and freedom to manipulate the threshold voltage remains limited. In our approach we propose normally-off AlGaN/GaN MOS HEMTs, where the gate oxide/semiconductor interface is obtained by plasma oxidation of a thin AlN cap layer, and subsequently overgrown by Al2O3 using atomic-layer-deposition at low-temperature. Consequently, inherent MOS-interface is formed with potentially low density of defects and surface donors. In this case there is a possibility to increase the threshold voltage technologically by oxide thickness increasing. Moreover, overgrown Al2O3 kept forward gate leakage current low with a capability of full channel opening. On the other hand, AlN was left intact at the access regions providing 2nd quantum well and low access resistances despite of only 3-nm thick AlGaN barrier. Devices are further optimized by tailored pre- and post-metallisation annealing steps in oxygen or nitrogen atmosphere. Finalized MOS HEMTs grown on Si with in-situ passivation show true pinch -off with < 10-8 A/mm drain leakage current, threshold voltage > 1 V, maximal drain current > 0.5 A/mm, a low hysteresis, and mitigated trapping effects. Support of HipoSwitch EU project no. 287602 and VEGA no. 2/0105/13 are acknowledged

Resume : A nitride semiconductor with a wurtzite structure is a very promising material for high performance devices in comparison with the conventional semiconductors. The exotic characteristic is a crystallographic polarity. This polarity leads to the polarization in the crystal as pointed out in 1988. This polarization influences the characteristics of the epitaxial growth and all the device performances. Up to now, the growth and devices with group-III polarity have been mainly reported because in the N-polar growth, "{1" "1" ̅"0" "1" ̅"}" and "{1" "1" ̅"0" "2" ̅"}" facets appearing at the edges of the growth islands form a rough surface. In this paper, the N-polar epitaxial growth and its merit are reviewed. For epitaxial growth, the step-flow growth mode has to be promoted. This mode can be realized by using the following optimum conditions; the relatively low V/III ratio for enhancing the migration of Ga adatoms on the surface, relatively high reactor-pressure for forming {11(_)00}, and introduction of off-cut substrates for controlling the step distance. N-polar GaN with the same characteristics of quality, the dislocation density and the p-type carrier concentration as Ga-polar GaN has been successfully grown. The N-polar growth has an advantage in the growth of In-rich materials because one N atom is captured with three Ga atoms at the growth front, while that is captured with only one Ga atom in Ga-polarity. In device applications, the polarization field can improve

Resume : Si substrates are attractive for the growth of GaN because of their large size, quality and availability. However, due to the large thermal expansion coefficient mismatch between GaN and Si, the grown layers can crack during the cooling down after growth. This cracking can be avoided by using strain-balancing layers such as AlN or AlGaN interlayers, AlN/AlGaN superlattices,… These approaches are quite complex and require a perfect control of the stress during growth. A simpler approach to avoid cracks is to use mesa-patterned Si substrates. Thanks to the stress relaxation at the mesa free- edges, the cracking of the nitride layers can be limited. The cracking and the final stress of the layers critically depend on the mesa design. Also, depending on the application which is targeted, the pattern has to be adapted. The mesa design optimization can benefit from a mask-less approach using laser lithography. We show that by using such method we can design square mesas with very well controlled sizes from 20x20 µm2 to 500x500 µm2. 2-3 µm-thick GaN layers on 200 nm-thick AlN buffer layers were grown on these mesa-patterned Si substrates by metal-organic chemical vapor deposition. Structural and morphological characterizations show that high quality layers, without cracks, are obtained.

Resume : InAlN layers are of a great interest for high electron mobility transistors (HEMT), distributed Bragg reflectors (DBR), and ultraviolet light-emitting diodes (UV LED). Their physical properties are expected to be more interesting in comparison to AlGaN for HEMT applications; this is mainly due to the possibility of obtaining layers which are lattice matched to GaN (LM-GaN) at around 18% Indium content. On the other hand, changing the Indium content may allow to tune the band gap from UV to deep IR. The main issue for InAlN is that degradation takes place even during the growth of nearly LM-InAlN/GaN heterostructures. Previous works have analyzed the influence of various growth parameters, e.g. metal precursor fluxes, ammonia flux, and temperature. However, the mechanisms that govern such degradation are not yet well understood. Recently, two possible explanations for this degradation have been proposed: 1) the influence of the dislocations inside the GaN buffer layer, and 2) an intrinsic formation of pinholes at the coalescence of the growth hillocks inside AlInN layers. In the following, we report characterization studies related to a series of InAlN layers which thickness was around 65nm. The main parameter investigated during the growth was the growth pressure. From, conventional transmission electron microscopy (TEM), and atomic force microscopy (AFM), it was shown that the degradation mechanisms are complex and may critically depend on the growth kinetics.