GaN-based SSD structure for THz applications

[EN]Switching diode (SSD) structure on SiC, designed using Monte Carlo simulations, for the fabrication of nano-scale SSDs to reach THz emission as a result of Gunn oscillations. Crack-free epistructure with good epi-characteristics and uniformity on 2- inch SiC substrate was achieved. High carrier density of 2 ×1018 cm-3 resulted in a low contact resistance of 0.35 Ω.mm.NRF2017-NRF-ANR003 GaNGUN project. Ministerio de Economía y Competitividad (Spain) and FEDER (European Union) through project TEC2017-83910-R and Consejería de Educación de la Junta de Castilla y León (Spain) and FEDER (European Union) through project SA254P18

Non-linear thermal resistance model for the simulation of high power GaN-based devices

[EN]We report on the modeling of self-heating in GaN-based devices. While a constant thermal resistance is able to account for the self-heating effects at low power, the decrease of the thermal conductance of semiconductors when the lattice temperature increases, makes necessary the use of temperature dependent thermal resistance models. Moreover, in order to correctly account for the steep increase of the thermal resistance of GaN devices at high temperature, where commonly used models fail, we propose a non-linear model which, included in an electro-thermal Monte Carlo simulator, is able to reproduce the strongly non-linear behavior of the thermal resistance observed in experiments at high DC power levels. The accuracy of the proposed non-linear thermal resistance model has been confirmed by means of the comparison with pulsed and DC measurements made in devices specifically fabricated on doped GaN, able to reach DC power levels above 150 W mm−1 at biases below 30 V.NRF2017-NRFANR003 GaNGUN project, the Spanish MINECO and FEDER through project TEC2017-83910-R and the Junta de Castilla y León and FEDER through project SA254P18

Origin of the two-dimensional hole gas and criteria for its existence in the III-nitride heterostructures

The existence of the two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) in the same III-nitride heterostructure is advantageous for the development of complementary nitride electronics. However, it is still unclear whether the buried-2DHG and the top 2DEG can coexist in the same III-nitride heterostructure. This study has addressed this long-standing question. Using charge distribution model, a systematic analysis is done and proposed surface acceptor states as the origin of the two-dimensional hole gas (2DHG). Using this centralized analysis, factors affecting the formation of both surface and buried-2DHG in the nitride heterostructures are presented. Furthermore, it is proved that the buried-2DHG is absent in III-nitride heterostructures, particularly under the 2DEG. In the absence of buried-2DHG at the GaN/AlXGa1-XN interface, a hole trap is observed, which not only balances the charge distribution but also reduces the electric field in the GaN channel layer.Published versio

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN

Source of two-dimensional electron gas in unintentionally doped AlGaN/GaN multichannel high-electron-mobility transistor heterostructures—experimental evidence of the hole trap state

Multichannel high electron mobility transistor (MC-HEMT) heterostructures are one of the choices for improved power performance of GaN HEMTs. By comparing the experimentally obtained two-dimensional electron gas (2DEG) concentration of unintentionally doped (UID) AlGaN/GaN MC-HEMTs with simulated 2DEG concentration, we hypothesized that hole trap(s) exist at the buried GaN/AlGaN interfaces, which act as sources of 2DEG in UID MC-HEMT heterostructures. Furthermore, these hole traps stop the Fermi level from cutting the valence band at GaN/AlGaN interfaces, which in turn precludes the generation of parallel two-dimensional hole gas (2DHG) in the MC-HEMT. However, no experimental report is present as a proof for the existence of such a hole trap in MC-HEMT heterostructures. In this study, a capacitance-conductance method on single and dual channel HEMTs revealed traps with higher time constant of 19-28.7 μs exclusively for the dual channel HEMT heterostructure. These traps are observed at the buried GaN/AlGaN interface of the dual channel HEMT; hence, they are attributed to possible hole traps at this interface. By conducting systematic deep level transient spectroscopy measurements, the existence of hole traps is confirmed at the buried GaN/AlGaN interface with an activation energy of 717 meV and a capture cross section of 1.3 × 10−14 cm2. This experimental evidence of the existence of hole traps at the GaN channel/AlGaN interface further supports our claim that these hole traps act as the source of 2DEG in UID MC-HEMTs and that buried parallel 2DHG channels do not exist in MC-HEMTs.Ministry of Education (MOE)Published versionThe authors gratefully acknowledge the funding support from the Ministry of Education, Singapore (Award No. RG128/22)

Effects of Si doping well beyond the Mott transition limit in GaN epilayers grown by plasma-assisted molecular beam epitaxy

The effects of Si doping well beyond the Mott transition limit on the structural, electrical, and optical properties of plasma assisted molecular beam epitaxy grown GaN layers were studied. Si doping up to a doping density of 1.0 × 1020 cm-3 resulted in rough surface morphology and degraded crystal quality. It also showed higher tensile strain, but did not result in cracking. Irrespective of the surface morphology and structural quality, the sheet resistance systematically decreased with increased carrier concentration up to and beyond the doping density of 1.0 × 1020 cm-3. PL study revealed three distinctive characteristics with Si doping: first, yellow luminescence is absent in Si doped samples - an indication of occupied VGa-ON and CN states in the bandgap; second, a distinctive luminescence peak is observed next to the band edge luminescence (BEL) for the samples doped beyond 2.1 × 1019 cm-3 - probably an indication of localization of some of the electrons either at donors or at excitons bound to defects; third, blue shift of the BEL is not matching with the calculated Moss-Burstein shift for doping densities beyond 2.1 × 1019 cm-3 - an indication of some of the electrons not occupying higher levels of conduction band, which is consistent with the second observation of localization of electrons near the donors or excitons bound to surface defects.National Research Foundation (NRF)Submitted/Accepted versionThis work was partially supported by the NRF2017-NRFANR003 GaNGUN project

In-situ stress evolution and its correlation with structural characteristics of GaN buffer grown on Si substrate using AlGaN/AlN/GaN stress mitigation layers for high electron mobility transistor applications

In-situ stress evolution as a function of thickness has been investigated and correlated with the structural properties and surface morphology of GaN buffer layer grown on AlGaN/AlN/GaN stress mitigating layers (SMLs). For comparison, GaN buffer was also grown on AlN/GaN SMLs. AlGaN/AlN/GaN SMLs exhibited efficient stress mitigation characteristics resulting in higher compressive mean stress during the growth and convex bow at the end of the growth. Horizontal screw-type misfit dislocations generated at the GaN/AlGaN and AlGaN/AlN interfaces were attributed to the stress mitigation property. The residual compressive stress in the GaN buffer was found to be lower with the AlGaN/AlN/GaN SMLs, which resulted in rough surface morphology. Increased V/III ratio used for GaN buffer growth was found to result in reduced stress relaxation at the interface leading to higher residual compressive stress and enhanced diffusion of ad-atoms. This consequently reduced the kinetic roughening and improved surface morphology. Thus, stress engineering by using AlGaN/AlN/GaN SMLs and by changing of the V/III ratio of GaN buffer, the mean stress of heterostructure was controlled and relatively smoother surface morphology was achieved, respectively. Reasonably good uniformity in electrical characteristics with a standard deviation of 7%, 1% and 8% for the sheet resistance, carrier concentration and mobility, respectively, were achieved for GaN high-electron-mobility transistor heterostructures across the 100 mm substrate.Ministry of Education (MOE)This work was supported by the funding support from the Ministry of Education, Singapore, Singapore (MOE 2017-T1-001-200)

Ga-bilayer controlled AlGaN/GaN HEMT structure grown on Si by PA-MBE

Recently, AlGaN/GaN HEMTs grown on 100 mm diameter Si using plasma assisted molecular beam epitaxy (PA-MBE) have been demonstrated [1,2]. In these structures, the growth rate is limited by the active nitrogen species available from the nitrogen plasma. Consequently, longer growth times are required to achieve thicker buffer layer necessary for the device structures. However, the narrow growth widow for the GaN layer in PA-MBE technique is affected by the fluctuation in substrate temperature and material fluxes during the long growth period.Accepted versio

Surface morphology evolution of N-polar GaN on SiC for HEMT heterostructures grown by plasma-assisted molecular beam epitaxy

The surface morphology evolution of N-polar GaN with growth time was investigated and compared with Ga-polar GaN. N-polar GaN directly grown on SiC substrates was found to have slower 3D-to-2D growth transformation and less coalescence than the Ga-polar counterpart, resulting in rougher surface morphology, whereas the AlN nucleation layer accelerated 3D-to-2D transformation, resulting in smoother surface morphology. N-polar GaN was found to have mound-type surface morphology with clustered atomic steps, unlike the regular screw-type dislocation-mediated step-flow growth observed for Ga-polar GaN. This was explained by the lower diffusion of adatoms on the N-polar surface due to its higher surface energy and higher Ehrlich-Schwoebel barrier. In addition, the increased III/V ratio in N-polar GaN growth was found to reduce the surface roughness from 2.4 nm to 1 nm. Without Si doping, the N-polar GaN high electron mobility transistor (HEMT) heterostructures grown under optimized conditions with smoother surface morphologies exhibited a sheet carrier density of 0.91 × 1013 cm−2 and a mobility of 1220 cm2 (V s)−1. With Si δ-doping, the sheet carrier density was increased to 1.28 × 1013 cm−2 while the mobility was reduced to 1030 cm2 (V s)−1. These results are comparable to the state-of-the-art data of plasma-assisted molecular beam epitaxy-grown N-polar GaN HEMT heterostructures on SiC substrates

A study on Ga – Si interdiffusion during (Al)GaN/AlN growth on Si by plasma assisted molecular beam epitaxy

The Ga–Si interdiffusion during (Al)GaN/AlN growth on Si substrate by plasma assisted molecular beam epitaxy (PA-MBE) is studied. The epilayers were grown using a combination of different III/V ratios for GaN and AlN layers. The columnar morphology of the nitrogen-rich (III/V < 1) AlN nucleation layer allows the migration of Ga metal into Si, leading to the interdiffusion. The presence of less Ga at the GaN/AlN interface and the two-step growth process of AlN with different column sizes on the top and bottom AlN completely eliminates the possibility of Ga–Si interdiffusion. Alternatively, a thin silicon nitride as a nucleation layer for the growth of (Al) GaN layers was also found to prevent the Ga–Si interdiffusion thereby circumventing the process of melt-back etching.Ministry of Education (MOE)Accepted versionThe authors gratefully acknowledge the funding support from Ministry of Education, Singapore (MOE 2017-T1-001-200)