Post-implantation Annealing Process Research Articles

Pressure Effect on Diffusion of Native Defects and Mg Impurity in Mg‐Ion‐Implanted GaN during Ultrahigh‐Pressure Annealing

Herein, transmission electron microscopy direct observations are used to examine the time evolution of dislocation loops in Mg‐ion‐implanted GaN during annealing in N2 atmospheres of 2.0 and 0.3 GPa. It is indicated in the results that the diffusion of the native defects, for both interstitials and vacancies, is retarded during annealing at the higher pressure. Furthermore, secondary ion mass spectrometry shows that annealing at the higher pressure retards the migration of Mg and increases Mg‐acceptor concentration in the ion‐implanted region. These findings provide a design principle of the postimplantation annealing process to activate ion‐implanted Mg in GaN.

Influence of implanted Mg concentration on defects and Mg distribution in GaN

Efficient acceptor activation in gallium nitride (GaN) achieved through Mg ion-implantation depends mainly on the concentration of implanted Mg ions and the post-implantation annealing process. In this study, we conducted correlative scanning transmission electron microscopy, atom probe tomography, and cathodoluminescence (CL) measurements on Mg-implanted GaN layers with the implanted concentration ranging from 1 × 1017 cm−3 to 1 × 1019 cm−3. It was found that at the implanted concentration of ∼1 × 1018 cm−3, Mg atoms were randomly distributed with defects likely to be vacancy clusters whereas at the implanted concentration of ∼1 × 1019 cm−3, Mg-enriched clusters and dislocation loops were formed. From the CL measurements, the donor–acceptor pair (DAP) emissions from the implanted and un-implanted regions are obtained and then compared to analyze Mg activation in these regions. In the sample with Mg ∼1 × 1019 cm−3, the existence of Mg-enriched clusters and dislocations in the implanted region leads to a weaker DAP emission, whereas the absence of Mg-enriched clusters and dislocations in the sample with Mg ∼1 × 1018 cm−3 resulted in a relatively stronger DAP emission.

Effect of post-implantation annealing on Al–N isoelectronic trap formation in silicon: Al–N pair formation and defect recovery mechanisms

The effect of post-implantation annealing (PIA) on Al–N isoelectronic trap (IET) formation in silicon has been experimentally investigated to discuss the Al–N IET formation and implantation-induced defect recovery mechanisms. We performed a photoluminescence study, which indicated that self-interstitial clusters and accompanying vacancies are generated in the ion implantation process. It is supposed that Al and N atoms move to the vacancy sites and form stable Al–N pairs in the PIA process. Furthermore, the PIA process recovers self-interstitial clusters while transforming their atomic configuration. The critical temperature for the formation/dissociation of Al–N pairs was found to be 450 °C, with which we describe the process integration for devices utilizing Al–N IET technology.

Electrical activation of nitrogen heavily implanted 3C-SiC(1 0 0)

A degenerated wide bandgap semiconductor is a rare system. In general, implant levels lie deeper in the band-gap and carrier freeze-out usually takes place at room temperature. Nevertheless, we have observed that heavily doped n-type degenerated 3C-SiC films are achieved by nitrogen implantation level of ∼6×1020cm−3 at 20K. According to temperature dependent Hall measurements, nitrogen activation rates decrease with the doping level from almost 100% (1.5×1019cm−3, donor level 15meV) to ∼12% for 6×1020cm−3. Free donors are found to saturate in 3C-SiC at ∼7×1019cm−3. The implanted film electrical performances are characterized as a function of the dopant doses and post implantation annealing (PIA) conditions by fabricating Van der Pauw structures. A deposited SiO2 layer was used as the surface capping layer during the PIA process to study its effect on the resultant film properties. From the device design point of view, the lowest sheet resistivity (∼1.4mΩcm) has been observed for medium doped (4×1019cm−3) sample with PIA 1375°C 2h without a SiO2 cap.

Local Anodic Oxidation of Phosporous-Implanted 4H-SiC by Atomic Force Microscopy

In this work, local oxidation behavior in phosphorous ion-implanted 4H-SiC has been investigated by using atomic force microscopy (AFM). The AFM-local oxidation (LO) has been performed on the implanted samples, with and without activation anneal, using varying applied bias (15/20/25 V). It has been clearly shown that the post-implantation annealing process at 1650 oC has a great impact on the local oxidation rate by electrically activating the dopants and by modulating the surface roughness. In addition, the composition of resulting oxides changes depending on the doping level of SiC surfaces.

원자힘 현미경을 이용한 이온 주입된 4H-SiC 상의 국소 산화 특성

In this work, local oxidation behavior in phosphorous ion-implanted 4H-SiC has been investigated by using atomic force microscopy (AFM). The AFM-local oxidation (AFM-LO) has been performed on the implanted samples, with and without activation anneal, using an applied bias (~25 V). It has been clearly shown that the post-implantation annealing process at <TEX>$1,650^{\circ}C$</TEX> has a great impact on the local oxidation rate by electrically activating the dopants and by modulating the surface roughness. In addition, the composition of resulting oxides changes depending on the doping level of SiC surfaces.

Change of structural and electrical properties of diamond with high-dose ion implantation at elevated temperatures: Dependences on donor/acceptor impurity species

We have investigated phase transformation, optical and electrical properties of diamond implanted with high dose (up to ∼1021cm−3) B, P and N acted as donor or acceptor elements in diamond, at elevated temperatures in the medium energy range (30 to several 100eV) followed by post-implantation annealing. Unlike a typical semiconductor such as Si, the high dose ion implantation over critical dose Dc to diamond below about room temperature results in phase transformation to a thermally stable graphitic phase after a post-implantation annealing process. In this study, it was found that any diamonds B-, N- and P-implanted at high doses at elevated temperatures (∼400°C) maintain diamond structures (graphite transformation does not occur) after annealing, from the results of optical absorption spectra, Raman scattering and temperature dependence of electrical properties. In the case of B and N, not only graphitization was avoided but also resistivities were reduced after annealing. In particular, in the B doped samples high electrical activation of B acceptors occurred, and heavily B doped (∼2.5×1021cm−3) diamond became a p-type degenerate semiconductor with low resistivity due to a metal-like band structure. In the N doped sample, the resistivity was ∼103 times as high as that in the B doped sample, but the value was much lower than normal diamond as an insulator. This suggests that in the N doped sample only impurity band conduction occured unlike the B dope samples. On the other hand, the resistivity of the P doped sample was ∼106 times as high as that of the N doped sample. This result most likely means that graphitization was avoided, also in the P doped sample, while the high mass number element P compared with B and N induced many radiation defects (not annealed out) in diamond acting as carrier trapping, and the resultant resistivity became very high.

Slab optical waveguides in Er&lt;sup&gt;3 +&lt;/sup&gt;-doped tellurite glass by N&lt;sup&gt;+&lt;/sup&gt; ion implantation at 1.5 MeV

Slab optical waveguides were fabricated in tung-sten-tellurite glass doped with Er3 + ions by means of nitrogen ion implantation at 1.5 MeV. A wide range of ion doses (from 5·1012 to 8·1016 ions/cm2) was used. Optical characterization, performed by dark-line spectroscopy, revealed that the waveguides were of optical barrier type: the implanted layer exhibited a decrease of the refractive index with respect to the virgin bulk glass, while the region comprised between the sample surface and the end of the ion track acted as an optical guiding structure. It was also demonstrated that a post-implantation annealing process, performed at various temperatures on the samples implanted at higher doses, contributes to the reduction of the barrier region.

Influence of Heating and Cooling Rates of Post-Implantation Annealing Process on Al-Implanted 4H-SiC Epitaxial Samples

We report on topographical, structural and electrical measurements of aluminum-implanted and annealed 4H-SiC epitaxial samples. The influence of heating-up and cooling-down temperature rates on the SiC surface roughness, the crystal volume reordering and the dopant electrical activation was particularly studied. A higher heating-rate was found to preserve the rms roughness for annealing temperatures lower than 1700°C, and to improve the sheet resistance whatever the annealing temperature due to a better dopant activation (except for 1600°C process, which induced a dark zone in the sample volume). A complete activation was calculated for an annealing at 1700°C during 30 minutes, with a ramp-up at 20°C/s. Rising the cooling-down rate appeared to increase the sheet resistance, probably due to a higher concentration of point defects in the implanted layer.

Carbon-Cap for Ohmic Contacts on Ion-Implanted 4H–SiC

A pyrolyzed photoresist film is commonly used as a protective cap of the surface of ion-implanted 4H-SiC wafers during the postimplantation annealing process with the aim to prevent Si sublimation and step bunching formation. Such a film that is called carbon-cap (C-cap) is always removed after postimplantation annealing and before any other processing step of the SiC wafer. Here, we show that this C-cap is a continuous, hard, black, mirrorlike, and planar thin film that can be patterned by a reactive ion etching O 2 -based plasma for the fabrication of ohmic contact pads on both Al + - and P + -implanted 4H-SiC. This C-cap material has an electrical resistivity of 1.5 × 10 -3 Ω cm and a good resistance against scratch. Al (1% Si) wires can be ultrasonically bonded on the C-cap pads. Such a bonding and the C-cap adhesion to the implanted 4H-SiC surface are stable for electrical characterizations in vacuum between room temperature and 450°C. The measured specific contact resistance of the C-cap on a 1 × 10 20 cm -3 p+-implanted 4H-SiC is 9 × 10- 5 Ω cm 2 at room temperature. Micro-Raman characterizations show that this C-cap is formed of a nanocrystalline graphitic phase.

Field emission enhancement in nitrogen-ion-implanted ultrananocrystalline diamond films

Enhanced electron field emission (EFE) properties for ultrananocrystalline diamond (UNCD) films grown on silicon substrate were achieved, especially due to the high dose N ion implantation. Secondary ion mass spectroscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy measurements indicated that the N ion implantation first expelled H−, induced the formation of disordered carbon (or defect complex), and then induced the amorphous phase, as the ion implantation dose increased. The postimplantation annealing process healed the atomic defects, but converted the disordered carbon to a stable defect complex, and amorphous carbon into a more stable graphitic phase. The EFE characteristics of the high dose (&amp;gt;1015ions∕cm2) ion-implanted UNCD were maintained at an enhanced level, whereas those of the low dose (&amp;lt;1014ions∕cm2) ion-implanted ones were reverted to the original values after the annealing process. Ion implantation over a critical dose (1×1015ions∕cm2) was required to improve the EFE properties of UNCD films.

Effect of implantation of nitrogen into SIMOX buried oxide on its fixed positive charge density

In order to obtain greater radiation hardness for SIMOX (separation by implanted oxygen) materials, nitrogen was implanted into SIMOX BOX (buried oxide). However, it has been found by the C-V technique employed in this work that there is an obvious increase of the fixed positive charge density in the nitrogen-implanted BOX with a 150 nm thickness and 4×1015cm-2 nitrogen implantation dose, compared with that unimplanted with nitrogen. On the other hand, for the BOX layers with a 375 nm thickness and implanted with 2×1015 and 3×1015cm-2 nitrogen doses respectively, the increase of the fixed positive charge density induced by implanted nitrogen has not been observed. The post-implantation annealing conditions are identical for all the nitrogen-implanted samples. The increase in fixed positive charge density in the nitrogen-implanted 150 nm BOX is ascribed to the accumulation of implanted nitrogen near the BOX/Si interface due to the post-implantation annealing process according to SIMS results. In addition, it has also been found that the fixed positive charge density in initial BOX is very small. This means SIMOX BOX has a much lower oxide charge density than thermal SiO2 which contains a lot of oxide charges in most cases.

Effects of heating ramp rates on the characteristics of Al implanted 4H–SiC junctions

The role of the heating rate in the postimplantation annealing process of SiC was investigated. Structural and electrical properties of an Al+ implanted junction in n-type 4H–SiC were studied for a 30min annealing at 1600°C in argon atmosphere and changing the heating rate between 7 and 40°C∕s. With the increasing of the heating rate the surface roughness increases, but the electrical performance of the devices improves. In particular, the faster ramp-up rate reduces the sheet resistance of the implanted layer of about 40% with respect to the slowest process and significantly reduces the leakage current of the diodes.

In situ characterization of phase formation during high-energy oxygen ion implantation in molybdenum

A special designed high-temperature vacuum chamber for in situ X-ray diffraction (XRD) measurements was used to study structural phase formation and transformation kinetics in molybdenum during oxygen ion implantation and post-annealing treatment. Oxygen ions with an energy of 1.5MeV were implanted in polycrystalline molybdenum up to a fluence of 3×1018/cm2 at different temperatures (160–700°C). Subsequently, implanted samples were annealed up to 700°C for in situ study during synthesis of buried oxide layers. Complementary, transmission electron microscopy (TEM) and sputter Auger electron spectroscopy (AES) were employed to obtain depth-dependent information concerning both the crystal structure and its elemental composition. The formation of different molybdenum oxides during oxygen implantation and post-implantation annealing process was observed by in situ X-ray analysis. The XRD spectra of samples implanted at 160°C show that MoO3 and/or Mo4O11 precipitates have been formed, whereas implantation in the temperature range 300–700°C preferably leads to the MoO2 phase formation.

Ion beam synthesis of semiconductor nanoparticles for Si based optoelectronic devices

Intense white (to the eye) luminescence has been obtained by multiple implantation of Si + and C + ions into thermal SiO 2 and a post-implantation annealing process. This white emission is a consequence of the convolution of three luminescence peaks centred at about 1.45 eV (infrared with a long tail in the red), 2.1 eV (yellow) and 2.8 eV (blue). These emissions have been correlated to the synthesis of nanocrystals of Si and SiC, and the existence of C-rich precipitates. Cross section TEM shows a buried layer with dark contrast, which correlates with the maximum of the C implanted profile, and likely with a high density of C-rich amorphous domains. Besides, two kinds of nanocrystalline precipitates are found, which have been identified as Si and hexagonal 6H–SiC by electron diffraction experiments. To our knowledge, these data provide the first experimental evidence on the ion beam synthesis of nanocrystalline 6H–SiC embedded in SiO 2. Correlation with previous data gives support to the assignment of the infrared, yellow and blue peaks with the Si, C-rich and SiC precipitate phases and/or its interfaces with SiO 2.

Ion-beam induced isolation of gallium arsenide layers

Epitaxial (n +−n) layers on semi-insulating GaAs samples were implanted with 60 keV He + ions at elevated temperatures. Samples were analysed to provide sheet resistivity, Hall mobility and carrier depth profiles using electrical measurement techniques and damage distributions using TEM and Rutherford backscattering and channeling. All of the data were correlated to identify the optimum conditions to achieve electrical isolation. Elevated temperature He + implants have been found to create uniform, single step isolation of GaAs layers. Isolation of the GaAs layers can be enhanced and stabilised further by a suitable post-implantation annealing process.