Proton irradiation effects on AlN/GaN high electron mobility transistors

We have investigated the effect of proton irradiation on reliability of InAlN/GaN high electron mobility transistors (HEMTs). Devices were subjected to 5-15 MeV proton irradiations with a fixed dose of 5 × 10 15 cm -2 , or to a different doses of 2 × 10 11 , 5 × 10 13 or 2 × 10 15 cm -2 of protons at a fixed energy of 5 MeV. During off-state electrical stressing, the typical critical voltage for un-irradiated devices was 45 to 55 V. By sharp contrast, no critical voltage was detected for proton irradiated HEMTs up to 100 V, which was instrument-limited. After electrical stressing, no degradation was observed for the drain or gate current-voltage characteristics of the proton-irradiated HEMTs. However, the drain current decreased ~12%, and the reverse bias gate leakage current increased more than two orders of magnitude for un-irradiated HEMTs as a result of electrical stressing.

The effects of proton irradiation doses on dc characteristics of reference, electrically stressed under off-state conditions and stressed/annealed AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. The HEMTs were irradiated with protons at a fixed energy of 5 MeV and doses ranging from 1013 to 1015 cm−2. As expected, in all three types of HEMTs, more degradation of the device dc characteristics was observed for higher doses due to the larger displacement damage in two-dimensional electron gas channel of the HEMTs. The electrically stressed HEMTs after proton irradiation showed more degradation compared with reference and stressed/annealed HEMTs. After proton irradiation at a dose of 1015 cm−2, the drain saturation current and maximum transconductance of stressed HEMTs were decreased by 28.5% and 15%, respectively, compared to 24% and 11.5%–12%, respectively, for reference and stressed/annealed devices. The dc characteristics of stressed/annealed HEMTs after proton irradiation showed similar degradation trends compared with those of reference HEMTs, confirming that annealing is effective in removing defects created by the off-state stressing. In some cases, the authors also annealed stressed/irradiated HEMTs after the proton irradiation step. The drain current and transconductance of stressed/irradiated HEMTs were slightly increased after subsequent thermal annealing at 450 °C for 10 min, while reverse gate leakage current after annealing was decreased more than an order of magnitude. The interface trap density of stressed HEMTs after proton irradiation at a dose of 1015 cm−2 increased from 3.05 × 1012 to 1.37 × 1013/cm2 V and were reduced to 6.01 × 1012/cm2 V following thermal annealing. Our results are consistent with the notion that off-state stressing creates defects that have a common origin with those created by proton irradiation, and thus, irradiation of off-state stressed devices leads to more defects than in unstressed devices that are subsequently irradiated. Annealing the stressed devices prior to irradiation makes them behave the same as unstressed HEMTs when both are irradiated with protons.

We investigated the influence of proton irradiation on the AlGaN/GaN high-electron-mobility transistor (HEMT) devices. Unlike previous studies on the degradation behavior upon proton irradiation, we observed improvements in their electrical conductivity and carrier concentration of up to 25% for the optimal condition. As we increased the proton dose, the carrier concentration and the mobility showed a gradual increase and decrease, respectively. From the photoluminescence measurements, we observed a reduction in the near-band-edge peak of GaN (~ 366 nm), which correlate on the observed electrical properties. However, neither the Raman nor the X-ray diffraction analysis showed any changes, implying a negligible influence of protons on the crystal structures. We demonstrated that high-energy proton irradiation could be utilized to modify the transport properties of HEMT devices without damaging their crystal structures.

InP-based high electron mobility transistors (HEMTs) will be affected by protons from different directions in space radiation applications. The proton irradiation effects on InAlAs/InGaAs hetero-junction structures of InP-based HEMTs are studied at incident angles ranging from 0 to 89.9° by SRIM software. With the increase of proton incident angle, the change trend of induced vacancy defects in the InAlAs/InGaAs hetero-junction region is consistent with the vacancy energy loss trend of incident protons. Namely, they both have shown an initial increase, followed by a decrease after incident angle has reached 30°. Besides, the average range and ultimate stopping positions of incident protons shift gradually from buffer layer to hetero-junction region, and then go up to gate metal. Finally, the electrical characteristics of InP-based HEMTs are investigated after proton irradiation at different incident angles by Sentaurus-TCAD. The induced vacancy defects are considered self-consistently through solving Poisson’s and current continuity equations. Consequently, the extrinsic transconductance, pinch-off voltage and channel current demonstrate the most serious degradation at the incident angle of 30°, which can be accounted for the most severe carrier sheet density reduction under this condition.

Strain plays an important role in the performance and reliability of AlGaN/GaN high electron mobility transistors (HEMTs). However, the impact of strain on the performance of proton irradiated GaN HEMTs is yet unknown. In this study, we investigated the effects of strain relaxation on the properties of proton irradiated AlGaN/GaN HEMTs. Controlled strain relief is achieved locally using the substrate micro-trench technique. The strain relieved devices experienced a relatively smaller increase of strain after 5 MeV proton irradiation at a fluence of 5 × 1014 cm−2 compared to the non-strain relieved devices, i.e., the pristine devices. After proton irradiation, both pristine and strain relieved devices demonstrate a reduction of drain saturation current (Ids,sat), maximum transconductance (Gm), carrier density (ns), and mobility (μn). Depending on the bias conditions the pristine devices exhibit up to 32% reduction of Ids,sat, 38% reduction of Gm, 15% reduction of ns, and 48% reduction of μn values. In contrast, the strain relieved devices show only up to 13% reduction of Ids,sat, 11% reduction of Gm, 9% reduction of ns, and 30% reduction of μn values. In addition, the locally strain relieved devices show smaller positive shift of threshold voltage compared to the pristine devices after proton irradiation. The less detrimental impact of proton irradiation on the transport properties of strain relieved devices could be attributed to reduced point defect density producing lower trap center densities, and evolution of lower operation related stresses due to lower initial residual strain.

AlGaN/GaN high electron mobility transistors (HEMTs) are promising candidates for high power and high current applications with its high mobility and high breakdown field. Epitaxial films for device substrate are typically grown on sapphire or silicon carbide, SiC, with dislocation densities in the order of 108 - 1010cm-2 due to their thermal coefficient and lattice mismatches. The thinner GaN buffer layers on SiC have been reported to have higher threading dislocation density in the GaN, but lead to higher off-state breakdown voltages. However, thick GaN buffer layers on sapphire and their impact on dc performance have not been widely studied. In this study, the impact of GaN buffer layer quality on dc and gate-lag pulse performance of AlGaN/GaN HEMTs was studied. Transmission electron microscopy (TEM) was used to measure the dislocation density in the GaN buffer layer of the HEMT samples. Dc characteristics, including drain I-Vs, transfer characteristics, and gate pulsed drain I-V characteristics of the HEMTs, were measured to establish a correlation between the AlGaN/GaN HEMTs’ dc and gate-pulsed performance and GaN buffer layer quality. AlGaN/GaN HEMT structures grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates were acquired from two different vendors. Both types of wafers had 24% Al concentration in the AlGaN barrier layer. One type of wafer had a 5 µm GaN buffer and the other had a 2 µm GaN buffer layer. HEMT fabrication was started with mesa patterning by conventional optical lithography. The mesa-etching was achieved using a unaxis shuttle-lock reactive ion etcher with inductively coupled plasma module (ICP) for device isolation with a Cl2/Ar plasma. Ohmic metallization of electron-beam evaporated Ti/Al/Ni/Au (25 nm/125 nm/45 nm/100 nm) was patterned by lift-off and subsequently rapid thermally annealed at 850⁰C in a flowing N2 ambient for 45 seconds. Schottky gates of e-beam evaporated Ni/Au (20 nm/80 nm) with 100 μm width and 1 μm length were obtained by standard lift-off patterning. DC current–voltage (I-V) characteristics were measured using an HP4156 parameter analyzer, and gate-lag pulse measurements were measured using an Agilent 8114A pulse generator. Figure 1 shows the bright field cross-sectional transmission electron microscopy (BF-XTEM) pictures of AlGaN/GaN surface for the top layers of (a) HEMT structure with 2 µm GaN buffer layer, (b) HEMT structure with 5 μm GaN buffer layer as well as the GaN/sapphire interface for the (c) HEMT structure with 2 μm GaN buffer layer, and (d) HEMT structure with 5 μm GaN buffer layer. Dislocations are seen to originate near the GaN/sapphire interface and propagate up through the AlGaN/GaN interface to the surface of both HEMT wafers. However, it is clear that the HEMT structure with 2 μm GaN buffer layer exhibited a much higher defect density. The calculated defect density for 2 μm GaN buffer layer substrate had an order of magnitude higher defect density. However, 5μm GaN buffer layer wafer showed that it was under high strain with larger height difference across the 2 inch with lower radius of curvature. There was no major difference in HEMTs’ dc performance was observed. In the other hand, Figure 2 (a) and (b) shows that gate lag measurement on fabricated HEMTs on 5 μm GaN buffer layer wafer showed no degradation and on 2 μm GaN buffer layer wafer showed the otherwise. The effects of defect density on HEMT dc and radio frequency performance were studied. HEMTs were fabricated on 2 and 5 μm of GaN buffer layers with the same composition of active layers. Despite there was no significant difference was seen in dc characteristics, significant reduction in pulse measurements was observed on thinner GaN layer HEMTs. These results suggest that the defects created in GaN buffer layer do not largely influence device dc performance but are closely related to high frequency performance. Figure 1

Random telegraph signal (RTS) fluctuations with relative amplitude up to 50% are observed in forward and reverse gate current of unstressed and reverse-bias-stressed AlGaN/GaN high electron mobility transistors. Measurements of RTS amplitude and mean pulse widths as a function of forward gate bias indicate that the RTS is due to modulation of current along an intrinsic or stress-induced percolation path across the AlGaN-barrier by electron capture and emission on a trap within the barrier. Processes of electron capture from GaN to trap and subsequent tunneling to metal gate or electron exchange between GaN channel and the trap are considered.

Gallium nitride (GaN)-based high electron mobility transistors (HEMTs) that work in aerospace are exposed to particles radiation, which can cause the degradation in electrical performance. We investigate the effect of proton irradiation on the concentration of two-dimensional electron gas (2DEG) in GaN-based HEMTs. Coupled Schrödinger’s and Poisson’s equations are solved to calculate the band structure and the concentration of 2DEG by the self-consistency method, in which the vacancies caused by proton irradiation are taken into account. Proton irradiation simulation for GaN-based HEMT is carried out using the stopping and range of ions in matter (SRIM) simulation software, after which a theoretical model is established to analyze how proton irradiation affects the concentration of 2DEG. Irradiated by protons with high fluence and low energy, a large number of Ga vacancies appear inside the device. The results indicate that the ionized Ga vacancies in the GaN cap layer and the AlGaN layer will affect the Fermi level, while the Ga vacancies in the GaN layer will trap the two-dimensional electrons in the potential well. Proton irradiation significantly reduced the concentration of 2DEG by the combined effect of these two mechanisms.

view Abstract Citations (13) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effects of proton irradiation on a gas phase in which condensation takes place. I. Negative 26Mg anomalies and 26Al. Heymann, D. ; Dziczkaniec, M. ; Walker, A. ; Huss, G. ; Morgan, J. A. Abstract In the present paper, isotopic effects in magnesium generated in a proton-irradiated gas phase are examined, taking only (p,n), (p,d), and (p, alpha) reactions in magnesium, aluminum, and silicon into consideration. In the presence of proton radiation, the three elements are 'removed' from the gas phase by condensation. It is required that a value of Al-26/Al-27 greater than 6 times 10 to the -5th must be reached, consistent with the value deduced by Lee Papanastassiou, and Wasserburg (1976) from their studies of the Allende meteorite. The calculations show that fast aluminum condensation reduces the required proton fluence substantially, that a significant fraction of aluminum remains uncondensed when the above value of the Al-26/Al-27 ratio is reached, that a detectable MG-24 excess is very likely to occur, that detectable negative MG-28 anomalies can be generated, and that proton fluxes and irradiation times can be varied simultaneously, and over a wide range of values, without significant changes in the required proton fluence. Publication: The Astrophysical Journal Pub Date: November 1978 DOI: 10.1086/156570 Bibcode: 1978ApJ...225.1030H Keywords: Aluminum 26; Astronomical Models; Magnesium Isotopes; Meteoritic Composition; Nuclear Reactions; Proton Irradiation; Abundance; Aluminum 27; Differential Equations; Energy Spectra; Proton Energy; Proton Flux Density; Solar System; Time Dependence; Vapor Phases; Lunar and Planetary Exploration; Physics full text sources ADS |

The effects of proton and electron radiation on the performances of GaAs and CuInSe/sub 2//CdZnS (CIS) thin-film solar cells are evaluated. Bare cells (i.e. with no coverglass) were exposed to several energies of electron and proton radiation, and their performances were monitored by illuminated I-V curves, dark I-V curves, and spectral response measurements at different fluence levels. These experiments reconfirm that no measurable damage occurs to CIS solar cells when exposed to 1 and 2 MeV electrons to a total fluence of 5.0*10/sup 15/ cm/sup -2/. For proton irradiations, the results indicate that CIS cells are more radiation resistant than GaAs cells, by a factor of ten, when compared at the same energy levels. Comparison of the electron and proton irradiation results to those reported in the literature shows that the thin-film GaAs solar cells are as radiation resistant as other GaAs bulk cells at 1.0 MeV protons and are more radiation resistant at both the 1.0 MeV electron and 200 keV proton energies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sc 2 O 3 -passivated AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with 40 MeV protons to a fluence corresponding to approximately 10 years in low-earth orbit (5×109 cm−2). Devices with an AlGaN cap layer showed less degradation in dc characteristics than comparable GaN-cap devices, consistent with differences in average band energy. The changes in device performance could be attributed completely to bulk trapping effects, demonstrating that the effectiveness of the Sc2O3 layers in passivating surface states in the drain-source region was undiminished by the proton irradiation. Sc2O3-passivated AlGaN/HEMTs appear to be attractive candidates for space and terrestrial applications where resistance to high fluxes of ionizing radiation is a criteria.

GaN HEMTs are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. We review data on the radiation resistance of GaN-based transistors such as AlGaN/GaN and InAlN/GaN High Electron Mobility Transistors (HEMTs) to different types of ionizin gradiation. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects are discussed. The carrier removal rates are a function of initial carrier concentration, dose and dose rate but not of hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. Neutron irradiation created more extended damage regions and at high doses leads to Fermi level pinning while 60Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation.

An anti-radiation structure of InP-based high electron mobility transistor (HEMT) has been proposed and optimized with double Si-doped planes. The additional Si-doped plane under channel layer has made a huge promotion in channel current, transconductance, current gain cut-off frequency, and maximum oscillation frequency of InP-based HEMTs. Moreover, direct current (DC) and radio frequency (RF) characteristic properties and their reduction rates have been compared in detail between single Si-doped and double Si-doped structures after 75-keV proton irradiation with dose of 5 × 1011 cm−2, 1 × 1012 cm−2, and 5 × 1012 cm−2. DC and RF characteristics for both structures are observed to decrease gradually as irradiation dose rises, which particularly show a drastic drop at dose of 5 × 1012 cm−2. Besides, characteristic degradation degree of the double Si-doped structure is significantly lower than that of the single Si-doped structure, especially at large proton irradiation dose. The enhancement of proton radiation tolerance by the insertion of another Si-doped plane could be accounted for the tremendously increased native carriers, which are bound to weaken substantially the carrier removal effect by irradiation-induced defects.

InAlN/GaN high electron mobility transistors (HEMTs) grown on SiC substrates were subjected to 5-15 MeV high energy protons with a fixed 5 × 1015 cm−2 fluence. The saturation currents and gate leakage currents of all the proton-irradiated InAlN/GaN HEMTs were degraded. Proton irradiation at lower energy was found to degrade the direct current (DC) current-voltage (I-V) characteristics more severely than higher-energy irradiation, because the energy loss component of the lower energy protons was larger than those of higher-energy protons in the vicinity of the 2-dimensional electron gas conducting channel. Our experimental results were consistent with stopping and range of ions in matter simulation results of the energy deposition profile by the protons.

Effects of high-energy proton and electron irradiation on GaN Schottky diode