Single and molecular ion irradiation-induced effects in GaN: experiment and cumulative MD simulations (original) (raw)
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Atomistic simulation of damage production by atomic and molecular ion irradiation in GaN
Journal of Applied Physics, 2012
Gallium nitride (GaN) has emerged as one of the most important semiconductors in modern technology. GaN-based device technology was mainly pushed forward by invention of p-type doping and the successful fabrication of light emitting diodes (LEDs) and laser diodes (LDs). Intensive studies in the last 20 years on GaN have significantly advanced the understanding of the properties and have expanded the range of practical applications. Beside basic lighting, current applications of GaN include high-power and high temperature electronics, microwave, optoelectronic devices, and so on. The successful production of optical devices demands efficient tuning of charge carrier lifetime where defect engineering plays a vital role. During growth, varying the level of recombination centers is difficult, whereas ion irradiation can do this job efficiently on a final product. On the other hand, during doping, undesirable defects can also be produced and epitaxial GaN is known to have a highly defective structure. Thus, having both positive and negative aspects, it is very important to have a detailed understanding of irradiation-induced defects. To explain experimental findings, atomic level understanding is necessary, but it is not always possible to have an atomistic view of defect dynamics in experiments. Some damage build-up studies by single ions have been reported in the literature, but not many by molecular ions. In this thesis, the irradiation of GaN by single and molecular ions by the means of atomistic simulations was studied. Detailed analysis mainly of what kind of defects, their distribution, reason of defect formation and time evolution have been studied and compared with experiments.
Effects of defect clustering on optical properties of GaN by single and molecular ion irradiation
Journal of Applied Physics, 2013
The effects of irradiation by F, P and PF 4 on optical properties of GaN were studied experimentally and by atomistic simulations. Additionally, the effect of Ag was studied by simulation. The irradiation energy was 0.6 keV/amu for all projectiles. The measured photoluminescence (PL) decay time was found to be decreasing faster when irradiation was done by molecular ion compared to light ion irradiation. The PL decay time change is connected with the types of defect produced by different projectiles. Simulation results show that light ions mainly produce isolated point defects while molecular and heavy ions produce clusters of points defects. The total amount of defects produced by the PF 4 projectile was found to be very close to the sum of all defects produced in five individual cascades started by one P and four F single ions. This and the similar depth profiles of damage produced by molecular and light ion irradiation suggest that defect clusters are one of the important reasons for fast PL decay. Moreover, the simulations of irradiation by Ag ions, whose mass is close to the mass of the PF 4 molecule, showed that the produced defects are clustering in even bigger conglomerates compared to PF 4 case. The latter has a tendency to split in the pre-surface region, reducing on average the density of the collision cascade.
Defect clustering in irradiation of GaN by single and molecular ions
Vacuum, 2014
Atomistic simulations were used to study irradiation effects on photoluminescence (PL) decay time of GaN. Irradiations were done by single (F, P) and molecular ions (PF 4 ). Equal energy per mass (0.6 keV/ amu) was used for all projectiles. Irradiation by the molecular ion shows faster PL decay time in comparison with the single ion. The simulation results show that single ions produce isolated point defects, whereas molecular ions produce big clusters of points defects. The total amount of defects produced by a PF 4 projectile and five individual cascades started by one P and four F single ions (P þ 4 Â F) were very close and their defect depth profile follows the same pattern. These findings suggest that defect clusters are one of the important reasons for fast PL decay.
Enhanced diffusion as a mechanism for ion-induced damage propagation in GaN
Journal of Vacuum Science & Technology B, 2001
Although GaN is a chemically inert, thermally stable material, it has demonstrated sensitivity to ion damage generated by dry etch processes such as reacting ion etching and inductively coupled plasma etching. Recombination-enhanced diffusion is an important mechanism which has been observed in other III-V semiconductor systems. In this study we examine the possibility of enhanced diffusion in GaN using quantum well ͑QW͒ probe structures. The deeper QWs ͑750 and 1000 Å deep͒ showed a steady decrease in relative photoluminescence ͑PL͒ intensity with time, providing evidence of the cooperative effects of channeling and defect diffusion in deep etch damage propagation in GaN. In contrast, shallow QWs ͑150 and 250 Å from the surface͒ showed a slight decrease followed by a gradual increase in relative PL intensity with time which was explained by defect annihilation. Exposure to above band gap illumination, used to simulate and enhance carrier generation during etch, appears to speed defect annihilation in high defect concentration regions resulting in an increase in QW luminescence, where as in lower defect concentration areas, above band gap illumination does not appear to significantly alter QW luminescence. We attribute this difference in behavior to a difference in diffusion constant. The diffusion constant in less damaged regions may be much lower than that of the highly damaged material.
Review—Ionizing Radiation Damage Effects on GaN Devices
ECS Journal of Solid State Science and Technology, 2015
Gallium Nitride based high electron mobility transistors (HEMTs) are attractive for use in high power and high frequency applications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20-30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN-based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or 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. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 10 14 cm −2 or electrons at fluences near 10 16 cm −2 , both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances.
2007
Epitaxial GaN layers grown by MOCVD on c-plane sapphire substrates are irradiated with 150 MeV Ag ions at a fluence of 5 · 10 12 ions/cm 2 . Samples used in this study are 2 lm thick GaN layers, with and without a thin AlN cap-layer. Surface morphology is studied using contact mode atomic force microscopy (AFM). Irradiated samples show qualitatively different morphologies as well as quantitative changes. Different kinds of morphology are attributed to specific type of dislocations using the existing models available in the literature. The residual strain and sample quality have been analysed before and after irradiation using high resolution X-ray diffraction (HRXRD). The Lorentzian shape analyses of the experimental scans complement the AFM results. Optical properties are studied by spectrophotometer used in the transmission mode. A sharp band-edge in the as grown samples was observed at $3.4 eV. The band-edge absorption broadened due to irradiation and these results have been discussed in view of the damage created by the incident ions which compliment HRXRD results. In general the effect of irradiation induced-damages are analysed as a function of material properties. A possible mechanism responsible for the observations has been discussed.
RBS/Channeling and TEM Study of Damage Buildup in Ion Bombarded GaN
Acta Physica Polonica A, 2011
A systematic study on structural defect buildup in 320 keV Ar-ion bombarded GaN epitaxial layers has been reported, by varying ion fluences ranged from 5 × 10 12 to 1 × 10 17 at./cm 2 . 1 µm thick GaN epitaxial layers were grown on sapphire substrates using the metal-organic vapor phase epitaxy technique. Rutherford backscattering/channeling with 1.7 MeV 4 He beam was applied for analysis. As a complementary method high resolution transmission electron microscopy has been used. The later has revealed the presence of extended defects like dislocations, faulted loops and stacking faults. New version of the Monte Carlo simulation code McChasy has been developed that makes it possible to analyze such defects on the basis of the bent channel model. Damage accumulation curves for two distinct types of defects, i.e. randomly displaced atoms and extended defects (i.e. bent channel) have been determined. They were evaluated in the frame of the multistep damage accumulation model, allowing numerical parameterization of defect transformations occurring upon ion bombardment. Displaced atoms buildup is a three-step process for GaN, whereas extended defect buildup is always a two-step process.
Radiation damage formation and annealing in GaN and ZnO
Proceedings of SPIE - The International Society for Optical Engineering, 2011
The radiation damage formation upon low temperature ion implantation and neutron irradiation has been compared for GaN and ZnO. Both materials exhibit strong dynamic annealing effects during implantation, even at 15 K, leading to high amorphisation thresholds. The damage build-up with fluence was found to proceed in a similar way for GaN and ZnO, both showing two saturation regimes below the amorphisation level where, over wide fluence regions, the damage level increases only very slowly. For low fluences the damage accumulation rate is similar for both materials. For higher fluences, on the other hand, GaN shows considerably higher damage levels and finally collapses into an amorphous structure while ZnO remains single crystalline up to the highest fluence of 7×10 16 Ar/cm 2 .
Swelling or erosion on the surface of patterned GaN damaged by heavy ion implantation
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2010
Wurtzite undoped GaN epilayers (0 0 0 1) was implanted with 500 keV Au + ions at room temperature under different doses, respectively. Ion implantation was performed through photoresist masks on GaN to produce alternating strips. The experimental results showed that the step height of swelling and decomposition in implanted GaN depended on ion dose and annealing temperature, i.e., damage level and its evolution. This damage evolution is contributed to implantation-induced defect production, and defect migration/accumulation occurred at different levels of displacement per atom. The results suggest that the swelling is due to the formation of porous structures in the amorphous region of implanted GaN. The decomposition of implanted area can be attributed to the disorder saturation and the diffusion of surface amorphous layer.
Defect formation in GaN epitaxial layers due to swift heavy ion irradiation
Radiation Effects and Defects in Solids, 2011
GaN epitaxial layers were irradiated with 200 MeV Ag ions at various fluences. These samples were characterized ex situ by resistivity/Hall, XRD, and transmission electron microscopy (TEM). The resistivity of irradiated layers increased by eight orders of magnitude after irradiation with a fluence of 5 × 10 12 ions/cm 2 . The increase in peak width (FWHM) with the incident ion fluence showed a reduction in the crystallinity of epitaxial layers. Cross-sectional TEM images confirmed that at the highest fluence (5 × 10 12 ions/cm 2 ), electronic energy loss caused structural defect formation in the GaN layer.