On the Practical Limitations for the Generation of Gunn Oscillations in Highly Doped GaN Diodes

Planar Gunn diodes based on doped GaN active layers with different geometries have been fabricated and characterized. Gunn oscillations have not been observed due to the catastrophic breakdown of the diodes for applied voltages around 20-25 V, much below the bias theoretically needed for the onset of Gunn oscillations. The breakdown of the diodes has been analyzed by pulsed I-V measurements at low temperature, and it has been observed to be almost independent of the geometry of the channels, thus allowing to discard self-heating effects as the origin of the device burning. The other possible mechanism for the device failure is impact-ionization avalanche due to the high electric fields present at the anode corner of the isolating trenches. However, Monte Carlo simulations using the typical value of the intervalley energy separation of GaN, ε_(1-2)=2.2 eV, show that impact ionization mechanisms are not significant for the voltages for which the experimental failure is observed. But recent experiments showed that ε_(1-2) is lower, around 0.9 eV. This lower intervalley separation leads to a much lower threshold voltage for the Gunn oscillations, not far from the experimental breakdown. Therefore, we attribute the devices failure to an avalanche process just when Gunn domains start to form, since they increase the population of electrons at the high electric field region, thus strongly enhancing impact ionization mechanisms which lead to the diode failure

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

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)

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)

Source of two-dimensional electron gas in unintentionally doped AlGaN/GaN multichannel high-electron-mobility transistor heterostructures

Unintentionally doped (UID) AlGaN/GaN-based multichannel high electron mobility transistor (MC-HEMT) heterostructures have been demonstrated on the SiC substrate using plasma-assisted molecular beam epitaxy. The MC-HEMT heterostructure with a GaN channel thickness of 100 nm resulted in a cumulative two-dimensional electron gas (2DEG) concentration of 4.3 × 1013 cm-2 across six GaN channels. The sample showed sheet resistances of 170 ω/sq. and 101 ω/sq. at room temperature and 90 K, respectively. The source of 2DEG in the buried GaN channels of the heterostructure was investigated. The C-V measurements conducted on UID MC-HEMTs excluded the possibility of the valence band being the source of 2DEG and the consequent formation of two-dimensional hole gas at the buried GaN-channel/AlGaN-barrier interfaces. A comparison of the experimentally obtained 2DEG concentration with the simulated data suggests the presence of donor-like trap states, situated at 0.6 to 0.8 eV above the valence band at the buried GaN-channel/AlGaN-barrier interfaces, which act as the source of 2DEG in UID MC-HEMT heterostructures.National Research Foundation (NRF)Published versionThis publication is made possible by the Singapore National Research Foundation under the NRF2017-NRF-ANR003 GaNGUN project

On the practical limitations for the generation of Gunn oscillations in highly doped GaN diodes

Planar Gunn diodes based on doped GaN active layers with different geometries have been fabricated and characterized. Gunn oscillations have not been observed due to the catastrophic breakdown of the diodes for applied voltages around 20-25 V, much below the bias theoretically needed for the onset of Gunn oscillations. The breakdown of the diodes has been analyzed by pulsed I - V measurements at low temperatures, and it has been observed to be almost independent of the geometry of the channels, thus allowing to discard self-heating effects as the origin of the device burning. The other possible mechanism for the device failure is an impact-ionization avalanche due to the high electric fields present at the anode corner of the isolating trenches. However, Monte Carlo simulations using the typical value of the intervalley energy separation of GaN, ϵ1-2 = 2.2 eV, show that impact ionization mechanisms are not significant for the voltages for which the experimental failure is observed. But recent experiments showed that ϵ1-2 is lower, around 0.9 eV. This lower intervalley separation leads to a much lower threshold voltage for the Gunn oscillations, not far from the experimental breakdown. Therefore, we attribute the device's failure to an avalanche process just when Gunn domains start to form, since they increase the population of electrons at the high electric field region, thus strongly enhancing impact ionization mechanisms that lead to the diode failure.This work was supported in part by MCIN/AEI/10.13039/501100011033 under Grant PID2020-115842RB-I00

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

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.National Research Foundation (NRF)This work was partially supported by the 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